Handbook of Yarn Production: Technology, Science and Economics

Handbook of yarn production Handbook of yarn production Technology, science and economics Peter R. Lord CRC Press Bo...

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Handbook of yarn production

Handbook of yarn production Technology, science and economics Peter R. Lord

CRC Press Boca Raton Boston New York Washington, DC

WOODHEAD

PUBLISHING LIMITED Cambridge England

Published by Woodhead Publishing Limited in association with The Textile Institute Woodhead Publishing Ltd Abington Hall, Abington Cambridge CB1 6AH, England www.woodhead-publishing.com Published in North America by CRC Press LLC 2000 Corporate Blvd, NW Boca Raton FL 33431, USA First published 2003, Woodhead Publishing Ltd and CRC Press LLC © 2003, Woodhead Publishing Ltd The author has asserted his moral rights. Originally published in 1979 by the author under the title The economics, science and technology of yarn production, this is a new, completely revised version of the book. This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. Reasonable efforts have been made to publish reliable data and information, but the author and the publishers cannot assume responsibility for the validity of all materials. Neither the author nor the publishers, nor anyone else associated with this publication, shall be liable for any loss, damage or liability directly or indirectly caused or alleged to be caused by this book. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming and recording, or by any information storage or retrieval system, without permission in writing from the publishers. The consent of Woodhead Publishing and CRC Press does not extend to copying for general distribution, for promotion, for creating new works, or for resale. Specific permission must be obtained in writing from Woodhead Publishing or CRC Press for such copying. Trademark notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation, without intent to infringe. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library. Library of Congress Cataloging in Publication Data A catalog record for this book is available from the Library of Congress. Woodhead Publishing ISBN 1 85573 696 9 CRC Press ISBN 0-8493-1781-9 CRC Press order number: WP1781 Typeset by Replika Press Pvt Ltd, India Printed by TJ International, Cornwall, England

Contents

Acknowledgments ....................................................................................................... ix 1

Review of yarn production ............................................................................ 1 1.1 1.2 1.3 1.4

2

Textile products and fiber production ...................................................... 18 2.1 2.2 2.3

3

Textile materials (fabrics, fibers, and filaments) ............................... 18 Natural fibers (types and production) ................................................ 22 Man-made fibers (polymer extrusion and yarn production) ............. 38 References ........................................................................................... 54

Common principles .................................................................................... 56 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 3.11 3.12

4

Historical basis ...................................................................................... 1 Present day conditions .......................................................................... 8 Future of the means of textile production ............................................ 9 Modern production systems ................................................................ 10 References ........................................................................................... 17

Introduction ......................................................................................... 56 Twist in strands ................................................................................... 56 Twist insertion ..................................................................................... 61 Confined and non-confined systems .................................................. 67 Twist evenness ..................................................................................... 68 Tension control .................................................................................... 69 Drawing ............................................................................................... 70 Consequences of roller errors on the textile product ........................ 76 Control of irregular flow in drawing or drafting ............................... 77 Doubling .............................................................................................. 83 Effects of shear .................................................................................... 84 Integration of sub-processes ............................................................... 86 References ........................................................................................... 87

Filament yarn production .......................................................................... 88 4.1 4.2

Introduction ......................................................................................... 88 Texturing filament yarns ..................................................................... 89

vi

Contents

4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10 4.11

5

Carding and prior processes for short-staple fibers ............................. 116 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 5.10 5.11 5.12

6

Introduction ....................................................................................... 155 Drawframe ......................................................................................... 155 Combing ............................................................................................ 159 Creel blending ................................................................................... 164 An industrial case study .................................................................... 165 References ......................................................................................... 167

Short-staple spinning ............................................................................... 168 7.1 7.2

8

Introduction ....................................................................................... 116 Opening line ...................................................................................... 118 Bale preparation ................................................................................ 119 The first stage of blending and opening .......................................... 121 The process of disintegration of fiber clumps ................................. 122 Condensation ..................................................................................... 123 The process of cleaning .................................................................... 125 Intimate blending .............................................................................. 129 Fiber flow .......................................................................................... 133 Carding .............................................................................................. 136 Waste control ..................................................................................... 149 Safety ................................................................................................. 153 References ......................................................................................... 154

Sliver preparation ..................................................................................... 155 6.1 6.2 6.3 6.4 6.5

7

Real twist texturing ............................................................................. 90 False twist texturing ............................................................................ 92 Draw-texturing ................................................................................... 102 Stuffer box texturing ......................................................................... 104 Air-jet texturing ................................................................................. 106 Other texturing techniques ................................................................ 110 Industrial filaments ........................................................................... 113 Silk filaments and staple yarns ........................................................ 113 Morphology and dyeing .................................................................... 114 References ......................................................................................... 114

Ring spinning .................................................................................... 168 Open-end spinning ............................................................................ 185 References ......................................................................................... 203

Long-staple spinning ................................................................................ 205 8.1 8.2 8.3 8.4 8.5

Introduction: Effects of lengthening the staple ............................... 205 Wool fibers and their preparation .................................................... 206 Worsted systems ................................................................................ 213 The woolen system ............................................................................ 220 Bast fiber spinning processes ........................................................... 231 References ......................................................................................... 232

Contents

9

Post-spinning processes ............................................................................ 234 9.1 9.2 9.3 9.4 9.5 9.6

10

Yarns of complex structure ............................................................... 260 Processes using modified twist ........................................................ 261 Compact spinning .............................................................................. 261 Air-jet spinning ................................................................................. 263 Sirospun yarns and process .............................................................. 268 Hollow spindle spinning ................................................................... 270 Self-twist spinning ............................................................................ 271 Twisted self-twist yarns and processes ............................................ 274 References ......................................................................................... 275

Quality and quality control ..................................................................... 276 11.1 11.2 11.3 11.4

12

Winding ............................................................................................. 234 Yarn joining ....................................................................................... 245 Ply yarns ............................................................................................ 250 Automation ........................................................................................ 253 Two-for-one twisting ......................................................................... 255 Customer concerns ............................................................................ 257 References ......................................................................................... 259

Staple systems and modified yarn structures ........................................ 260 10.1 10.2 10.3 10.4 10.5 10.6 10.7 10.8

11

vii

Quality ............................................................................................... 276 Quality control .................................................................................. 278 Yarn evenness .................................................................................... 291 End-breaks and quality ..................................................................... 298 References ......................................................................................... 300

Economics of staple yarn production ..................................................... 301 12.1 12.2 12.3 12.4 12.5 12.6

Yarn economics ................................................................................. 301 Productivity ....................................................................................... 303 Quality and economics ...................................................................... 306 Cost minimization ............................................................................. 308 Operational factors ............................................................................ 313 International competition .................................................................. 315 References ......................................................................................... 316

Appendices ................................................................................................ 317 1. Calculations I: Elementary theory ....................................................... 317 2. Calculations II: Worked examples ....................................................... 329 3. Advanced topics I: Air conditioning and utilities ............................... 341 4. Advanced topics II: Testing of textile materials ................................. 350 5. Advanced topics III: Staple yarn structures ........................................ 373 6. Advanced topics IV: Textured yarn structures .................................... 383 7. Advanced topics V: Blending of staple fibers..................................... 389 8. Advanced topics VI: Drafting and doubling ....................................... 407 9. Advanced topics VII: Yarn balloon mechanics ................................... 427 10. Advanced topics VIII: Topics in rotor spinning .................................. 453 Index ........................................................................................................................ 465

Acknowledgments

Grateful acknowledgments are made to the many friends and colleagues who have read various parts of this script and given very helpful and constructive criticism. These include Charles Chewning, David Clapp, Philip Dabbs, Yehia E1 Moghazi, Wally Johnson, W Oxenham, Jon Rust and W C Stuckey. Acknowledgments are also made to UMIST, NC State University, Cotton Incorporated, the many different commercial organizations in a variety of fields with whom I have been associated and the many people involved. It has been a great experience and pleasure. Much of what I have to say in this book has its origins in the many discussions and shared fieldwork experiences. This embraces too large a number of people to name individually but I take the opportunity to express my thanks to all of you. I hope that by sharing these experiences, I will repay the help that I have received during my career in this fascinating field. I would be remiss in not acknowledging the wonderful forbearance of my wife Mavis during these last years. Only someone that cared so much could have tolerated the late arrivals for meals, the relegation of important social responsibilities and the overwhelming obsession with the project. I dedicate the work to her in celebration of our 56th wedding anniversary.

1 Review of yarn production

1.1

Historical basis

1.1.1 Historical background [1] The long reach of history shows how prosperity varies as civilizations have waxed and waned. The course of prosperity has been bumpy and there are dangers in extrapolating the future based on the short-term past. Successive centuries have seen fundamental changes of varying types. Greenwood [2] outlines steps related to yarns and textiles in the first two millennia and points out the extraordinary fineness of the materials that have been made. He also discusses some of the developments that have improved the productivity of the manufacturing systems and reduced the costs over the centuries. The eighteenth century saw a financial revolution, the nineteenth saw the industrial revolution, and the twentieth saw the information revolution. The history of humanity contains many references to textile materials because they were, and still are, part of the fabric of our lives. Consequently, the history of fibers is one of the traceable threads in the story of yarn production. A second thread concerns the extraordinary developments of the industrial revolution. There were gigantic steps in productivity of both people and machines. Another thread concerns the developing economic environment that has surrounded these changes. Thus, let us first make a brief survey of the history of some important fibers.

1.1.2 A brief history of silk The origin of silk is found only in legend and fable; certainly it was used in the time of Emperor Huang Ti in China in the third millennium BC. Sanskrit literature refers to silk in India in the second century BC and the Old Testament also refers to it. When it percolated to the West, it was as valuable as gold on a weight-to-weight basis. Roman Emperor Justinian tried to monopolize the trade (unsuccessfully), smuggled silk worms to Constantinople (c. AD 550) and started sericulture there. Byzantine silks became world famous. The Moors established sericulture in Spain and so the production of silk spread. It reached northern Europe in the fifteenth century and the

2


western hemisphere in the sixteenth, although it failed at first. However, the strong luster and ability to take brilliant dyes made silk very attractive. The peak of activity was after World War I and by 1919 the price had risen to US$21/lb; that is equivalent to over $1200/lb in the currency of 2003. Once fine man-made fibers entered the market, the price and the demand for silk dropped; but there is still an important market in some areas of the world. Perhaps the early inventors of synthetic fibers were influenced by the knowledge of the manner in which silkworms, spiders, and other creatures extruded filaments. Doubtless, they were also impressed by the extraordinary properties of these naturally extruded fibers. Such inspiration was probably very important in determining the future of fiber production.

1.1.3 A brief history of bast fibers Bast fibers are derived from the stems of various plants. Cultivated flax [3] probably originated in the Mediterranean region; certainly it was used in prehistoric times. It was found in Stone Age dwellings in Switzerland, the ancient Egyptians used it, and references to it are sprinkled throughout historical writings. It has been used both for its fiber and for its seed. The fiber is used to make linen cloth, and the crushed seed yields linseed oil, long used for the preservation of leather and wood. Until the eighteenth century, linen manufacture was widespread in the domestic industry of European countries. The development of cotton processing and the great inventions of the industrial revolution dealt an almost fatal blow to this erstwhile prevailing industry. Jute fiber was largely unknown in the West until the eighteenth century, but it was in common use in Bengal before then. There was resistance to its use because blending it with hemp or flax was regarded as adulteration. In the nineteenth century, the Dutch government replaced linen coffee bags with jute and this gave an impetus to use it in the West. Research was carried out in Dundee, Scotland, which became a recognized center of yarn production. Also, much of the production was in what are now Pakistan and India. (Strangely, after partition in 1947, India had the jute processing resources, and the bulk of the corresponding agricultural producing sector was in Pakistan: Jute played a prominent role in the development of trade relations between the two countries.) It was attractive because it was strong, bulky, and cheap. However, in more recent times, the increasing use of polypropylene for cotton-bale wrapping, carpet backing, sacking, and other products has decimated the jute industry. Hemp fiber is thought to have originated north of the Himalayas and was well known in China in the second millennium BC. It was brought to the Americas in the sixteenth century and, by the twentieth century, was being grown throughout the world. The plant not only produces fibers but also narcotics. Some species of hemp produce little in the way of narcotics, but many countries make the growing of it illegal for social reasons.

1.1.4 A brief history of wool fibers The use of wool for clothing dates back to antiquity. Outstanding properties of wrinkle resistance, moisture absorption, warmth, and tendency to felt, have given it a role, not only in apparel, but also in blankets, upholstery, and floor coverings. Babylonia is translated by some as meaning ‘the land of wool’. It is known that the

Review of yarn production

3

Phoenicians traded wool fabric during the first millennium BC [3]. The Ancient Romans established wool factories to supply their army; the fame of these factories was spread by the travels of Roman soldiers. In Britain, the wool flocks were scattered by the incoming Saxons and the wool trade there then went into decline. The Normans re-established the trade and it developed for a time, although there seems to have been little progress through the dark ages; it was not until after the seventeenth century that structural changes started to occur. After many struggles over restraints in trade, wool was very important in England in the eighteenth century. Spain too was a major producer but its government had enforced rigid restrictions on the export of fleeces at about that time [4]. In times of rapid technological change, many are left behind. Mechanization in the Low Countries and Britain in the nineteenth century permitted spinners in these regions to out-produce those who had not embraced the emerging technologies. There was then a vast opening-up of the supply of raw wool from the western and southern hemispheres. It was the combination of a plentiful supply of raw material and high productivities of people and machines that produced the displacement of the centers of production and sites of the markets changed also. Now, many of the industrial companies then formed have, in turn, been overtaken by new technology and economic changes. The development of synthetic fibers and new processes has created a new situation; the market in wool has declined somewhat even in the last decade. Despite this, the world consumes about 1.5 million tons of wool per year, and its value is greater than the weight might suggest. Australia abolished its price support in the 1980s and prices globally were determined more than before by market forces. In the decade centered on 1990, prices plummeted [5], but supply is now in better balance with demand and there is hope for expansion. China is now a large consumer. A remarkable feature of wool is its ability to recover from deformation over a time, and this gives apparel made from the fibers attractive crease-shedding properties. Also, the rate at which the fiber takes up and disperses moisture is such that it gives clothes made from wool good comfort properties. These inherent properties give wool an attraction that is likely to guarantee it a place in the world market; the main question is how much of that market it will retain.

1.1.5 A brief history of cotton fibers The use of cotton fibers has been traced back to as far as 3000 BC. Yarns were found in the ruins of Mohenjo-Daro, a city in the Indus valley [2]. Cotton has been known, cultivated, and worked in India since the earliest historical periods. A Hindu Rigveda hymn (c 1500 BC) mentions cotton, and Herodotus (c 450 BC) is said to have mentioned ‘wild trees bearing fleeces as their fruit’. Ancient Egyptians were known to have grown and spun yarns in the seventh century AD. When the Spaniards arrived in America they found cotton being used to make cloth. Cotton was found in prehistoric pueblo ruins in Arizona, and cotton grave cloths from pre-Inca Peru are still in existence. Cotton has remarkable durability in the marketplace; it filled a major role in the industrial revolution and it has formed an alliance with man-made fibers in more recent times. Therefore, it is perhaps best if further discussion of the history of cotton is left to unfold with some of those events.

4


1.1.6 A brief history of man-made fibers Ideas about synthetic fiber processes were expressed by Robert Hook (1775) and René de Reaumur (1734); Louis Schwabe extruded glass fibers in 1842. Much of the early work was to develop a means of ‘liquefying’ cellulose to permit extrusion. In 1846, C F Schoenbein prepared nitrocellulose, and George Audemars patented a process for making a material related to rayon from nitrated wood in 1855. Sir Joseph Swan coagulated nitrocellulose solutions to produce fibers but he was interested mostly in producing filaments for electric light bulbs. The stage was therefore set for Count Hilare de Chardonnet to begin commercial production, in 1891, of filaments coagulated in heated air, from a nitrocellulose solution derived from mulberry leaves. Louis Henri Despeissis then developed a cuprammonium solvent for cellulose itself, from which filaments could be coagulated in sulfuric acid. Another important milestone was when C F Cross and E J Bevan patented (1892) a viscose rayon solution resulting from dissolving cellulose xanthate in dilute sodium hydroxide. In 1902, Max Mueller discovered a way to convert cellulose xanthate into regenerated cellulose and the production of viscose rayon yarn could then start at Marcus Hook in Pennsylvania, USA, in 1911. (It might be added that the pressures from environmental concerns at the end of the twentieth century have led to the closure of some plants making products of this nature.) Synthetic polymers (which are large molecules, or ‘macromolecules’) were developed as a result of research into the properties of large molecules starting in 1926. This is an early example of the commercial exploitation of organized scientific research. Wallace H. Carothers and his associates found that they could draw out filaments from long-chain polymers. Such filaments were extruded and toothbrush bristles were made from them. Soon after that, polyamide filaments for knitted hosiery entered limited production, and in 1940 wider production began of the first truly commercial filament – nylon. Nylon is a polyamide and the idea of extruded macromolecules has since expanded to include many other chemical types. Polyester was developed in the 1950s and Brunnschweiler and Hearle have collated an interesting account of this development [6]. Even after the chemical and mechanical problems (with excessive discontinuities in extrusion) had been reasonably well worked out, there were still difficulties with developing appropriate fiber crimping and finishes. There were also difficulties with static electrification, dyeability, oily soiling, pilling, dye sublimation during ironing, hole melting, and other problems in fabrics. But, by 1952, production had begun and there was a boom that lasted for five years; then the pace of market development moderated as the producers of natural fibers formed their own marketing organizations. The excesses of a multitude of small, single-knit fabric producers also caused a condition of oversupply of knitted fabric. Since that time, however, there has been a continuous expansion of the markets for all fibers. In those markets, polyester has expanded to some 10 million tons /year, with acrylic fiber running at about one-third of this. It is noteworthy that in staple production, the ratio between polyester and natural fibers still holds at roughly equal proportions. The centers of production of man-made fibers have spread throughout the world. Man-made fibers have been produced in filament and staple forms. In the early days, the cellulosic filaments were difficult to texture, and the shiny, slick surfaces were not in universal demand. Many were cut up into staple and blended with natural fibers. As synthetic filaments began to appear, means of texturing them followed closely and a successor to the silk throwing industry, aimed at the apparel industry, began to appear in advanced countries. Technical filaments rapidly penetrated the


5

industrial market because of the strength of the materials, and heavy textured nylon yarns made a large penetration of the home furnishings market, especially in carpets. Also, blends of synthetic staple and natural staple fibers began to appear. Of these, polyester/cotton and polyester/wool blends have become significant raw materials for yarn makers.

1.1.7 Historical development of the economic environment Religious persecution of the Huguenots in mainland Europe before the seventeenth century caused them to flee to England and they carried their knowledge of textile manufacturing with them. The village of Worstead in Norfolk gave its name to the worsted process at that time. The business skills of these and others led to the development of the idea that credit is as good as cash. The outcome of this was the founding of the first publicly financed companies, leading to an explosion of business ventures that swept the inventions into the industrial revolution. Others similarly exported their skills over succeeding decades and centuries with the result that technological and business environments spread through the world. Writers have referred to the industrial revolution as though it occurred in a flash. In reality, it was spread over the eighteenth and nineteenth centuries, during which time there were ups and downs in all the economies concerned. Similarly, the newly termed ‘information revolution’ will evolve over a considerable timespan. The success of the industrial revolution stems from the opening of new markets. For example, in the initial phase, cotton fibers were very expensive in markets where cotton did not grow naturally. Cotton, at the beginning of the industrial revolution, cost the equivalent of roughly US$100/lb in 2003 currency. The alternatives used by many were wool and bast fibers; the yarns were coarse and the fabrics heavy. Only the rich could afford to pay more than the equivalent of, say, $100/lb for fine cotton yarns. Cotton in Europe at the beginning of the twentieth century, cost roughly the equivalent of $10/lb in modern currency. In 2003 it is less than $1/lb. Reductions in the cost of fiber did not happen accidentally. Expanding demand, improvement in agricultural techniques, mechanization of harvesting and ginning, and, ultimately, the rise of man-made fibers all put downward pressure on fiber prices. The beginnings of the industrial revolution involved extraordinary inventiveness, availability of capital, and much human exploitation. Development of machinery did little to make the life of the mill worker easier at the turn of the century; much of the benefit went to the mill owners and traders. Many inventors were also excluded from benefit. The short-term result in the nineteenth century was that the cost of the goods produced was sharply reduced and there was a worldwide expansion of trade. In the early stages, goods were transferred from a rich economy (Group A) to others (Group B) and wealth in other forms flowed in the opposite direction as shown in Fig. 1.1. As Goods A

B

Wealth

Fig. 1.1 Historical flow of wealth

6


Cost (log scale)

the development continued, Group A expanded to include a range of nations and economies so that the simplistic model given here became less applicable, but the general idea is the same. Long-term results of the worker exploitation in the Group A economies were the rise of socialism and the associated rise in wage costs during the twentieth century (Fig. 1.2). This widened the economic gap between Groups A and B; also, there was a marked decrease in fiber prices as the market expanded. These two factors played important parts in the economic changes throughout the world. Of course, there were short-term fluctuations in these costs; therefore Fig. 1.2 shows only trend lines. It is not surprising to find that a growing number of the Group B economies desired to join the industrial nations and set up production units under their own control. This was especially so during the second half of the twentieth century. The element of widely differing wage levels became an important factor in the competition, and eventually in the net flow of textile goods. To combat the differences in wages, machinery was then developed to reduce the need for human intervention and improved to increase machine productivity as indicated in Fig. 1.3. The increases in productivity now seem to be leveling off and perhaps we should not expect the same massive changes in the present century that we have experienced in the last. The enhancement of machine productivity was not the only contributor to the reduction in costs. At the beginning of the twentieth century, it was normal to have six or seven stages of processing between carding and spinning in the cotton industry. Nowadays, some mills work with as little as one stage between those processes. The reduction in the number of stages gives at least two benefits. First, it reduces the capital cost components. Secondly, it reduces the need for transfers of the textile product from one machine to the next. Formerly, these transfers were manual and 10 Cotton fiber, $/lb

1.0 Wages, $ /hr 1850

Fig. 1.2

1900 1950 Year

2000

Costs in 1994 US$ in an ‘A’ economy

Productivity, log scale (tons/spindle year)*

10

1.0

0.1

0.01 1900

1950 Year *Estimated values

Fig. 1.3

2000

Changes in machine productivity

7

100

200 24/ l Cotton yarn

50

100 50

20 10

20

Group B

10

Group A

1950

5

(operator hrs /100 lb)

Productivity, log scale (operator hrs /100 kg)


2000 Year

Fig. 1.4

Changes in operator productivity

thus accounted for much of the need for human resources. Modern automated transfer systems are now available, which change the balance between the cost categories of labor and capital. The transfers are still a significant cost component; reducing the number of them plays a part in the economies of the modern technology. Technical developments include automation, automatic handling, and reduction of the number of process stages as well as enhanced productivity of equipment. The rate of adoption has varied throughout the world and, for the present purpose, rates appropriate to the categories mentioned earlier are indicated in Fig. 1.4 to illustrate the point. The normalized variable quoted in the diagram in metric units (known colloquially as ‘HOK’) is a measure of the productivity of the workers in the factories.1 If we multiply the wage rate by the HOK variable we get the labor cost per 100 kg of product. Labor cost per unit weight has changed less than might be expected over the last century after discounting inflation and allowing for changes in other cost components. This is despite the steep rise in wage rate shown earlier. The cost per unit weight has been low in the Group B economies and this has given them an advantage in the past. However, as the HOK shrinks, so do the effects arising from the differences in wages; other cost factors then tend to predominate. As wealth flows to the lesserdeveloped countries, the standards of living improve and wage rates rise. As wealth flows from Group A economies, there is a trend for a particular industry to decline, sometimes to the detriment of the local standard of living. In other words, there is a tendency towards equalization of wages. Of course, there are more than two categories of economy in the world and the spectrum of wage rates is wide. Nevertheless, the trend still exists, with some nations moving up the scale and others moving down; it is a fluid situation with changes certain over the coming years. Losses in manufacturing jobs due to automation also have had, and will have, a marked effect. Automation not only affects costs of production but also affects the tastes and the ability of the remaining workers to buy textile goods. The markets tend to be more widely distributed and their character changes. The cost of fibers has changed, especially in the last half of the twentieth century, and there are a number of reasons for this. There was more attention given in the second half of the century to civilian matters than in the first.

1 HOK = operator hours per 100 kilograms of product and OHP = operator hours per 100 lb of product. The acronym ‘HOK’ appears to be illogical because it is derived from a language other than English.

8


Much of the first half was spent in war and depression, and the markets did not realize sustained expansion. The economic expansion after 1950 probably played a large part in reducing costs because of improved efficiencies of the organizations and equipment. Also, man-made fibers became a larger competitor to natural fibers. A range of man-made fibers became commercially viable over a number of markets in the second half of the century. The sales were accompanied by technical service and research that developed more new markets and facilitated competition in many traditional ones. As time progressed, the production of man-made fibers spread throughout the world and is now ubiquitous. Producers of natural fibers have combined to form various organizations that emulate the research and service provided by the man-made fiber companies. But if the price of a fiber falls, as it did with wool, then the service from such organizations becomes more restricted. There have been cutbacks over a range of such organizations. In general, as fibers become more nearly perfect commodities, less is spent by fiber producers on research. Eventually, they are less able to give the same technical support to the textile producer as before. It is therefore likely that textile producers will have to look for other resources and this may have an impact on some companies’ ability to compete.

1.2

Present day conditions

1.2.1 Costs and sales In yarn production, labor costs are only part of the total cost. Livingston [7,8] states that US labor costs formed 14% of the total in 1992; the largest component was said to be that of fiber costs, which comprised approximately 50% of the total. However, the percentages vary from place to place in the world. Energy costs in yarn production vary with the product but, for example, can run at up to some 10% of the total. These costs rise with speed and count. It can be expected that power requirements will rise in Group B economies as more plants run at higher speeds and install air conditioning. Also, amortization costs rise with investments in equipment. Again, there is a trend for equalization of the costs between the various economies. Where the main flow of goods is global, shipping costs become increasingly important because they impose a premium of perhaps some 10% on the transoceanic shipper of the goods. An additional hindrance to trade is caused by tariffs and quotas. The fairly recent international actions expressed by GATT (General Agreement on Tariffs and Trade) and NAFTA (North American Free Trade Agreement), and others to follow, are likely to reduce these barriers and there are hopes for an enlarged market. There is emerging evidence that the move of parts of the textile industry from Group A to Group B regions has been hastened by the freer market. Some erstwhile Group B regions have, indeed, become Group A regions. Relative currency changes also affect the issue. Sales are not determined by price alone. Quality of the product and service also affect the issue. Most textile products go through a chain of sales transactions before reaching their final destination. These intermediate transactions are between professionals, and technical quality becomes important. Of course, the requirements of the purchasing public have also to be considered. The point is that considerable investment has to be made in appropriate testing equipment, and care in testing becomes essential to satisfy buyers. Probably the most important aspect of service is delivery of goods at the specified times. To get the best advantage, quality and service have to be managed efficiently.


9

History leads to think that the most important factor for success is to recognize expanding markets and plan accordingly. Mechanical inventions may have a relatively small effect on the future competitive position. Other technologies, such as telecommunications and computing, are likely to have a greater effect. Nevertheless, the need to operate a mill in the most economic manner is still a paramount consideration, and high productivity machinery has to be used for major installations. In addition, there is a great need to make products of a quality that will satisfy the market; this involves quality control, which becomes ever more sophisticated. Cost and quality of the product have to be carefully balanced for each market to achieve a competitive position without which the enterprise will fail.

1.2.2 World market for yarns According to Thomas et al. [9], the market for spun yarns will be dominated by cotton. At the time of writing, the share held by cotton is about two-thirds of the world market and this has been stable in recent years. Nevertheless, this is not to say that the market has remained unchanged; on the contrary, shifts in consumer demands and preferences, cost structures, and geographic migrations of the industry are powerful agencies for change. Europe has suffered losses in production capabilities whereas Asian output has soared. American output has increased but the character has changed. There is consolidation amongst the companies that might be seen as evidence of the sorts of pressures that have affected Europe. However, Europe is still the world leader in the smaller market of long-staple spinning. The production of cotton is still very strong in the USA and this is one of the reasons why the industry there has maintained stability. Production of polymers and man-made fibers and filaments has dispersed through the world and, again, the production in Asia has made remarkable strides and is affecting Western markets.

1.3

Future of the means of textile production

One reason for the reduction in HOK over the years is that the productivity of the machines has increased. Greenwood [2] quotes HOK values ranging from 12 500 in Neolithic times, through 3120 in ‘pre-fourteenth century’, to 0.63 for open-end (OE) spinning in the 1970s. In staple spinning, the mule was superseded by the ring frame and then by newer technology. The move to rotor spinning and other new technology in the USA and in some other areas has been highly significant. The productivity of the fastest machines has escalated rapidly but it is difficult to imagine how the pace can continue. There are signs that the productivity curves are flattening and they seem to be approaching maximum values asymptotically. Next let us turn to materials handling. At the beginning of the twentieth century, the whole process consisted of a myriad of steps, with human intervention at each one. Gradually the number of steps has been reduced and automatic handling has become common. Automatic handling takes several forms. It ranges from the pneumatic transfer of fiber, to the use of robots to carry packages between machines. It follows that these developments have also contributed to the reduction in labor. Again there are limits; as we approach the irreducible minimum number of stages and automate the transfer of textile material, there is little to be gained in possible labor cost

10


reductions. However, there might well be other advantages. Thus, for this reason, the HOK curves shown earlier cannot be extrapolated too far. Nevertheless, these considerations imply that improvements in the technology of production will play a diminishing role in deciding the partition of the markets.

1.4

Modern production systems

1.4.1 Some comments on the mechanics of fiber structures Some knowledge of the properties of the fiber, and of the yarn as well, is useful to understand the difference between various products. At this stage, suffice it to consider only the mechanical properties of stiffness and bulk. A coarse fiber (i.e. one having a high linear density) is stiffer than a fine one. Consequently, fabrics made from the coarser fibers often feel harsh and prickly. Thus one can understand the drive to use fine fibers that give a softer ‘hand’ to fabrics. However, if carried too far, the use of ultra fine fibers can lead to difficulties with the production of nep (a fiber fault caused by fiber knotting or tangling with itself to yield a tiny ball of fiber which may take up dye at a different rate). Next, consider fiber bulk, which is affected by fiber crimp or convolution. Crimp typifies the extent of zigzag, helical, or other non-linear shape of the fiber. The greater the crimp, the more volume it takes up and the more ‘bulky’ is the yarn. Figure 1.5 is intended to show the effect of texture in regard to bulk. Figure 1.5(a) indicates a series of parallel fibers, which could be easily compressed to form a strand of very little bulk. Figure 1.5(b) shows similar fibers which have been induced to curl into helical configurations, lying beside one another like a series of bedspring coils to occupy a much greater volume than before. A bulky yarn made up into fabric produces a material with good insulation properties. Fine, bulky fibers produce fabrics that feel warm and soft. Fine, non-bulky fibers produce silk-like fabrics. On the other hand, coarse fibers are often used to make carpets because the carpet tufts stand out from the backing and can be loaded at their ends in normal use without buckling too severely. A fine fiber would buckle and change the appearance in the loaded areas of the carpet. Thus, the use of coarse fibers helps to reduce the appearance of tread marks on the surface of the carpet.

(a)

(b)

Fig. 1.5

Volume occupied by fibers


11

1.4.2 Filament production Fiber production before the eighteenth century was an agricultural undertaking with the result that, except for silk, the fibers available were of a relatively short finite length (we call these staple fibers). Yarn production systems of antiquity were mostly staple yarn systems. In modern times the range of raw material has expanded and synthetic fibers have become available. These so-called man-made fibers are supplied in staple form and also as ‘continuous’ filament. Thus, in modern times, there is a range of yarn making technologies which did not exist earlier. This range continues to expand, perhaps at a declining rate, as economic factors other than machine productivity take precedence. Extruded filament yarn manufacture is a short, mechanical process involving only one or two steps. Yarn is extruded and drawn to approximately the right ‘size’; it then is often textured to give the final product. A schematic drawing of a simple melt extrusion system is shown in Fig. 1.6. It shows only a rudimentary polymer chip feed system. A practical system may have a complex liquid polymer feed and two or more draw zones in a single spinline. There are a variety of alternative systems within this broad category of filament production. Although the production of filament yarns appears deceptively simple, there are complexities. The processing conditions have to be very carefully monitored and controlled because heat, humidity, and mechanical stress affect the polymer in a way that affects the dyeability of the final product. Thus, it is imperative that those in charge understand the problems which can arise due to the chemistry and molecular structure of the polymers from which the fibers are made. This is in contrast to the mechanical complexities of staple processing. Staple yarn manufacture is much more complex from a mechanical standpoint; it involves many stages of processing before the products are ready for shipping. There Polymer input

Melt

Extrusion

Filaments solidify & cool during transit

Draw

Wind yarn on cone or cheese

Fig. 1.6

Simple polymer chip feed system for the production of filament yarn (Note: practical systems are more complex)

12


are a variety of staple spinning systems available, but broadly they can be categorized as short- and long-staple systems. Short-staple spinning is the logical development of the cotton spinning of history, but the range of fibers has increased dramatically in this century. Long-staple spinning has a heritage of spinning wool and bast fibers; but in recent times, the range of long-staple fibers has also increased markedly. A comparison of various systems is given later in Table 1.1.

1.4.3 Textured yarn production In yarn production, polymer is supplied either directly from the chemical reactor or as polymer chip. The polymer is fed to an extruder in which a rotating screw or auger transports the input material through the extruder barrel and pressurizes it; as the polymer passes through the barrel it is melted or maintained in the molten condition. The extruder changes the form of the molten polymer, from a relatively slowly moving mass to the high speed thin jets of polymer which form the yarn. It is metered and filtered before passing through the spinneret, which contains one tiny hole for each filament. The emerging filaments cool rapidly and solidify; they are also ‘drawn’ by taking them up at a faster rate than that of the supply. Drawing is a very important part of the process because it stabilizes the molecular structure and strengthens the yarn by improving the molecular orientation. The main idea in most texturing systems is to heat set the filaments into some sort of crimped or convoluted form, such that each filament is held as separate from its neighbors as possible. In this way the yarn contains the many air pockets needed to produce insulation properties, permeability, and softness. Furthermore, the yarn now occupies a greater volume, which is also very important since the purpose of most textile materials is to cover some underlying strata; the greater the bulk, the better the cover. Also the yarn becomes more extensible and this, too, is an added attraction. It is possible to get various combinations of stretch and bulk. For filaments (such as rayon) that cannot be heat set, it is possible to tangle the fibers to lock them mechanically. Table 1.1

Typical process schedules

Fiber processes

Nylon filament

Nylon tow

Polyester staple

Cotton

Wool

Extrusion Drawing Winding

Extrusion Drawing Stretch-break

Extrusion Cutting Crimping Baling

Harvesting Ginning

Shearing Sorting Scouring Baling

Baling

Mill processes

Texturing Winding

Drawing Drawing Roving Ring spinning Winding

Opening Carding Drawing Drawing Roving Ring spinning Winding

Opening Cleaning Carding Drawing Drawing Roving Ring spinning Winding

Opening Cleaning Scouring Carding Drawing Drawing Roving Ring spinning Winding

Typical yarns Typical uses

Textured Hosiery

Staple Carpet

Staple Apparel Household

Staple Apparel Household

Staple Apparel Carpet


13

An example of this is air-jet texturing. Sometimes it is desirable to combine air-jet with false-twist texturing. Air-jet texturing gives a product that is nearer to a staple yarn than is a false-twist textured yarn. It has much of the hand and appearance of the staple product. False-twist machines with built-in air-jets are now becoming common.

1.4.4 Tow and man-made staple fiber Man-made staple fibers are made from tow, which is extruded in the same basic way as with filament yarns; however, the number of filaments involved is vastly larger. The linear density of the filaments in the tow depends on the end use. A large extruder supplies a number of spinnerets and several extruders are ganged together to produce a thick rope of filaments. These filaments must be fully drawn before they are cut into staple or shipped to the spinning mill. The major production of most tow makers is cut within the organization and the product is sold as staple fiber. However, some tow is sold to those mills which elect to stretch-break or cut their own staple. Usually, such mills make long-staple yarns. Where the fiber maker cuts the material, great care is needed to blend very large volumes of material to ensure uniformity of the product over long periods of time. Care also is taken when the fiber makers’ processes are altered in any way because the slightest change can cause tremendous difficulties in the mills. The fibers are batched in so-called ‘merges’ so that changes can be equalized and controlled. This is not to say that there are never variations in fiber properties, but rather that they are sensitive to change and that great technical expertise is needed to control them. For long-staple processes, where the mill chooses to convert the tow within the mill, different standards apply. There is no longer an opportunity to merge batches in gigantic blending operations. Each tow supplied to a mill must stand on its own merit and the tolerances on the filaments have to be even more strict than with general tow production because there is little opportunity for doubling; thus the cost is higher. Another factor is that even the modern stretch-breaking machine used for converting tow to staple is limited in the linear density of tow that it can handle. The tows supplied to mills are often much lighter than those used internally by the fiber maker. A simple calculation illustrates the point. If the strength2 (tenacity) is, say, 35 cN/tex and the tow has a linear density of 106 tex, the breaking strength of the tow is 0.35 × 106 Newtons (over 30 tons). Thus, the loads needed to break the fibers in a filament tow are very high and the stretch-breaking machine has to be very robust. Such machines are expensive. Even the lightest tows used today might have a quarter of a million filaments in the cross-section.

1.4.5 Staple yarn production Staple fibers usually arrive at the mill in compacted bales containing about 500 lb of fiber. The bales are stored according to fiber classification; this makes blend component selection easier. Fiber supplies might come from brokers who deal in natural fibers, from synthetic fiber makers, or from both. In the latter case the product is often referred to as ‘blended yarn’ although, in reality, all staple yarn is blended. An example of a flowchart for cotton and melt spun staple fibers is sketched in Fig. 1.7

2 A tex is a measure of ‘thickness’ or linear density equivalent to 1 g / km

14


Tow production Grow Harvest

Draw Wind

Gin Bale Grade

Cut, oil & crimp

Staple fibers

Bale Warehouse Open Clean Card Draw Draw Spin Wind Staple yarn

Fig. 1.7

Staple fiber processing

and any combination may be used according to the market being served. Somewhat similar routes apply to wool/man-made fiber blends but in this case there has to be a wet process stage somewhere to remove the wool grease. This is increasingly being done in the agricultural sector.

1.4.6 Short-staple yarn production Short-staple yarns are produced from bales of short staples, which are delivered to the mill by the fiber suppliers. The yarn maker ‘opens’ the bales to produce a flow of more or less discrete fibers, which are then combined into a rope-like strand called ‘sliver’. Yarn making requires that the fibers be well oriented and therefore must use processes which will straighten and parallelize the fibers. An important one of these is drawing, which is a process of elongating the strand to make the fibers slide over one another and hence help in orientation. Drawn sliver has to be reduced in size and then twisted to make yarn. There are various processes of drawing (sometimes called drafting) and twisting. For some specialized uses, yarns can be twisted together to form ply yarns.


15

1.4.7 Long-staple yarn production There are three major types of long-staple yarn production systems; these are called (a) worsted, (b) woolen, and (c) stretch-breaking. In practice, some worsted and woolen manufacturing plants also make their own staple by cutting or stretch-breaking tow. There is much more variation in plant layout for long-staple yarns than there is for short-staple ones. The worsted system was devised for twisted yarns made from wool but it has been adapted for man-made fibers such as acrylic and its blends. In principle, it is similar to the short-staple systems just described except that a wet scouring operation precedes the mechanical processes, different types of card are used, and the machine elements are generally larger than for short staple. It is an important process. Woolen processing, despite its name, can also involve the processing of a variety of long fibers ranging from wool to man-made fibers. The process (one of the original short-process lines) consists of a card set comprising several roller-top cards assembled in series on a single frame as well as spinning frames; and winders. The yarns are softer and weaker than worsted yarns. Stretch-break systems use only man-made filament tows and usually start with a stretch-breaking machine, omitting the carding and preceding processes. It is a relatively short process line. In carpet yarn production, the system is in competition with bulked filament production, which has limited its growth as a system. Very occasionally, the stretch-broken material is fed to a card when there is a desire to blend it with a natural fiber. Process schedules for different products are given in Table 1.1.

1.4.8 Man-made carpet yarns Carpet yarns are considerably heavier than apparel yarns and the poundage produced is very large. Carpets produced with man-made yarns have displaced those made of natural fibers in many markets. Nevertheless, the total market size has increased and traditional carpets still hold a significant portion of it. This is of sufficient importance to be mentioned at an early stage. Several alternative manufacturing routes may be used, two of which are indicated in Fig. 1.8. On the left is the traditional route in which tow is cut, oiled, and crimped by the fiber maker before shipping bales of staple fiber to the yarn manufacturer (the crimp is intended to make the fibers mutually cohere to make further processing easier). On the right is a system in which the tow is shipped to the yarn maker and the cutting or stretch-breaking is carried out in the mill to convert the material to staple fibers in sliver form. The sliver is then bulked so that the yarn made from it has a soft hand with good cover. Alternatively, the fiber maker might bulk the yarn before shipping it directly to the carpet maker. The productivities of the equipment in manufacturing bulked filament yarns are very high and the economies of size favor the fiber maker over the small installations in the mills in this particular field.

1.4.9 Composite yarns A variety of systems are now emerging that combine filament and staple processing. Some of these involve the wrapping of filaments round staple fiber bundles or low twist staple yarns; some include a filament core for strength and others use binders. These are referred to generally as composite yarns. Where low twist is used, the yarns

16


Tow production Draw Wind

Cut, oil & crimp Bale

Yarn bulking

Yarn mfr Bulking Winding

Winding

Bulked yarn to end user

Fig. 1.8

Carpet yarn production

become softer in hand, but there is a danger that the filament will ‘grin through’3 the staple fiber covering. This might result in an unacceptable appearance of the fabric. Adequate cover of the filaments is generally required and this imposes limitations. Other changes in yarn structure may also alter the aesthetics of fabrics and hence much market research is needed for such new products. The category of composite yarns also includes industrial yarns where the most important attribute is strength. Some man-made filaments are exceedingly strong and the technology of composite yarns becomes very important where there is a need for a sheath of fibers with different characteristics that surround a strong core. A common example is that of sewing thread, where a non-meltable sheath is desired.4 Some non-textile composite materials are made in which strong yarns are embedded in a matrix to reinforce it. For example, concrete can be reinforced by high tenacity yarns or fibers.

1.4.10 Review of processes Since the cost of fibers is a large proportion of the final cost of yarn, it is important that the spinner understands something of the processes and products of the fiber3 Grin through is a term used for exposure of the filament surface in the fabric, which may cause changes in light reflection and areas of differing dye pick-up. 4 ln sewing, a thread passing through a needle at very high sewing speeds might be caused to melt in places by the high temperatures in the needle eye. For example, nylon thread might melt on the surface; but if it has a cotton sheath it will be protected because cotton does not melt.


17

producing industries. Not only do the average attributes of the fibers influence the efficiency of mill processing and the quality of the yarn, but the defect levels and variability within the fiber supply are also highly relevant. Thus quality control is important. Consequently, this book may be regarded as containing three sections. Chapter 1, which is a general overview, may be regarded as the first section. Chapter 2 is written in three segments to emphasize the materials employed and may be regarded as the second section. The subsequent chapters deal in detail with the wide range of processes used to convert fiber to yarn; these chapters may be regarded as the third section of the book. The subject matter covers a wide array of processes with many interlinked ideas. There is an interplay of topics in the complex web of production processes and there are some common principles that cut across the boundaries of the various process sequences. For example, two important ones are drawing (or drafting) and twisting. Drawing occurs in both filament and staple yarn processes. So does twisting. Consequently, the next section begins with a discussion of these common principles, which should lay a basis for the chapters that follow.

References 1. 2. 3. 4. 5. 6. 7. 8. 9.

Looney, F and Lord, P R. The Future is Not Just an Extension of the Past: Globalization – Technological, Economic, and Environmental Imperatives, Textile Inst Ann Conf, Sept 1994. Greenwood, F A. The Textile Mill of the Future: the Effect of Modern Technologies, Textile Inst Ann Conf, 1973. Cook, J C. Handbook of Textile Fibres, Merrow Publishing, Watford, UK, 1964. The Wool Assoc of the NY Cotton Exchange, Inc. Wool and the Wool Trade, 2nd edn, Riverside Press, Cambridge, USA 1955. Morris D and Stogdon A. World Markets for Wool, The Economist Intelligence Unit, New York, 1996. Brunnschweiler, D and Hearle, J W S. Tomorrow’s Ideas & Profits: Polyester, 50 years of Achievement, The Textile Institute, Manchester, UK, 1993. Livingstone, I. Cotton Inc, New York, US Cotton Textiles in a Global Environment, 5th EFS Conf, 1992. Livingstone, I. Cotton Inc, New York, Textile Competition in a Global Market, 7th EFS Conf, 1994. Thomas, P R, Banfi, O, Brusadelli, E, Derencin, L, Gresteau, J P, Hansen, G, Hoffmeister, P, Kampl, R and Leitner, J. World Markets for Spin Yarns: Forecasts to 2000, CIRFS Special Report No 2637, The Economist Intelligence Unit, Dartford, May 1994.

2 Textile products and fiber production

Section A

2.1

Textile materials (fabrics, fibers, and filaments)

2.1.1 The nature of fabrics Textile fabrics usually have the attributes of being soft and pliable with a capability of being molded or draped over non-flat surfaces. The ‘hand’ of a fabric (which describes its tactile characteristics) is very important in determining its acceptability for many applications. For example, to obtain the desirable characteristics required of apparel, the fabric must be made from fine yarns; also there must be some degree of freedom for them to move within the fabric structure. The sensation obtained when there is contact between human skin and fabric is determined to some extent by the stiffness of the hairs or fiber loops projecting from the surface of the fabric. The finer these outstanding hairs or fibers, the softer the fabric feels to the light touch. For this reason, too, many fabrics are made from fine filaments or fibers.

2.1.2 Fibers and filaments Let a filament be defined as a continuous fine strand whose length is so very long that it can be considered infinitely long. Staple fibers, examples of which are cotton and wool, exist in relatively short fiber lengths. We will refer to ‘continuous filaments’ and ‘staple fibers’ to help differentiate between filaments and fibers. Most natural fibers are staple fibers although silk is an exception. (Silk is sometimes chopped to make staple fibers.) Man-made fibers can be used as staple or continuous filament. The very fine fibers used in textile fabrics make it necessary to use special units to express the idea of fineness or ‘diameter’. Unfortunately, the many sectors of industry have each invented their own systems over the years. Appendix 1 gives some of the measuring systems used.

Textile products and fiber production 19

Another desired attribute of most textile fabrics is that they should have a good appearance. This usually implies that the fabric must look even and have no blotchiness, cloudiness, barré (see Fig. 2.1 and note that the bars are components of the condition known as barré), or streakiness. This, in turn, implies that the yarns should have uniform fineness, hairiness, and color along their whole lengths. With such materials, faults such as thick and thin spots in the yarn and neps should be avoided because of their disturbing effect on the appearance of the fabric. (Neps are tiny balls of fiber, which degrade the appearance of fabrics). Also, light reflects differently from various surfaces and a change in the structure of a yarn (or fabric) can cause unwanted change in the appearance of the fabric. For example, variation in yarn twist, hairiness, or fiber fineness can cause such undesirable changes. One must pay careful attention to preserving quality in these respects; for this, and other reasons, quality control is an important topic. Appearance is extremely important in the marketplace. With many of the yarns, great efforts are made to ensure yarn uniformity in all details. However, there is a special class of fabrics, which uses color patterns, random disturbances (such as nubs), and thick and thin spots in the yarns to produce effects which give interesting textures; such yarns are termed ‘fancy yarns’ or ‘effect yarns’. Nubs are random thick spots induced into the yarn to produce visual effects in the fabric. Each sort of yarn has its set of physical and mechanical properties which influence the performance of the yarn. To best select yarn for a particular purpose, one must understand the properties of the materials. Since there is an interaction between the technology and the properties of the yarn produced, it is very important to study all aspects of these topics.

2.1.3 Classification of fibers The principal division is between natural and man-made fibers. The ‘natural’ classification is subdivided into fibers of vegetable, mineral, and animal origins. The vegetable subclassification is shown partially in Fig. 2.2. Dotted lines indicate that there are other members of this particular subdivision that need not be considered here because of their small market share. Cotton fibers are the most important in this category, and they will be considered. Stem fibers are also known as bast fibers. Details of the fibers in the two bottom rows will be described later. Full classifications can be found in the literature [1–4]. About the only mineral fibers are asbestos and glass. Asbestos, in modern times, has been associated with asbestosis and has been banned in many parts of the world. Glass has a substantial market in industrial insulation, non-wovens and, of late, optical fibers for communication. The glass is

Bar 1

Bar 2

Fig. 2.1 Knitted fabric with barré

20

Handbook of yarn production Natural fibers

Vegetable

Stem

Leaf

Mineral

Seed

Cotton

Flax

Fig. 2.2

Jute

Hemp

Animal

Hair

Filament

Wool

Silk

Natural fibers – partial classification

melted and extruded and the techniques of production have much in common with manmade fibers. Wool is the most important animal fiber and large quantities of fiber have been used for carpets and apparel. Like the vegetable ones, they are natural and variable. 2.1.4 Polymeric materials Fibers are made of polymers; some are natural polymers and some are man-made. A class residing in between these two consists of regenerated fibers, made, for example, from regenerated cellulose from trees, waste cotton fibers or others of the many natural sources of cellulose, and modified natural fibers, made by reacting natural polymers with chemicals to alter their properties. It is a misconception to view only synthetic fibers as polymers. Natural polymers are cultivated in the agricultural sector whereas man-made fibers are produced in the industrial sector. The synthetic polymer is first produced in chip or similar form, or is supplied directly to extruders in a liquid state. If it is produced in chip form it is later melted or otherwise liquefied and extruded. A textile polymer is made up of long-chain molecules. A long-chain molecule may be regarded as a long string of atoms; these ‘molecular strings’ are flexible (if they are not cross-linked) and give to the fibers many of their desirable characteristics. The analogy between the behavior of a long flexible fiber and a long flexible molecule is no accident. However, the analogy must not be carried too far because there can be strong bonds between the molecular chains that make up the fibers, which are not simulated by the fibers in a yarn. The polymer has to withstand the end use conditions. For example, it would be of little use making a fabric that would melt or soften in hot water. It also has to be strong enough to fulfill its purpose. Other properties have similarly to be taken into account. Many polymers can be set by raising their temperature above the so-called glass transition temperature (Tg ), deformed and then cooled. Tg is the temperature at which the polymer softens. Some fibers (such as cotton) cannot be permanently heat set, but easy care properties can be induced into fabrics by a chemical treatment called cross-


linking. This treatment joins groups of molecular chains together and reduces their ability to move with respect to one another. The linking reduces the loss of energy of deformation and makes it more likely that the retained energy is available to return the fabric to its original shape. The cross-linking also makes the fiber structure stiffer. Many cross-linked fabrics, especially for apparel, have easy care properties. Some fabrics have creases or shaping set into them; thus, even after laundering or cleaning, they retain the desired shape or crease. One class of yarns is made by setting the filaments themselves into certain shapes and this is called texturing. When heat or mechanical stress causes a variation in the molecular structure, it can alter the way dye is taken up at various places along the length of the yarn. It can cause a fabric to look streaky. A classification diagram for some man-made fibers is given in Fig. 2.3; again, some members of the various subdivisions are omitted. Polyesters’ long-chain synthetic molecules are composed of esters of aromatic dicarboxylic acids and glycols. There is a family of polyesters but one of the most common is polyethylene glycol terepthalate (PET), which is widely used in staple form. It is often blended with cotton for apparel. The blends give some of the benefits of each component. The moisture absorption and feel of cotton are perceived to add comfort to apparel made of the fiber. Polyester has durability and recovery properties that add to the easy care attributes of any fabric made from it. Polyamides are long-chain synthetic polymers made from diamines and dicarboxylic acids. The most widely used are collectively known as nylons and the various chemical types are indicated by adding numbers which indicate the monomers from which they are formed, e.g. nylon 6, nylon 6.6, and nylon 11. They are widely used in both staple and filament forms for carpets. Growth of this market has been at the expense of wool. Acrylic fibers are another class of synthetic polymer, composed of at least 85% by weight of acrylonitrile units. Where less than 85% and more than 35% of the polymer comprises acrylonitrile units, the fibers are termed ‘modacrylics’. Textured Man-made fibers

Regenerated

Synthetic

Rayon

Polyamide

Acrylic Polyester

Fig. 2.3

Polyolefine

Man-made fibers – partial classification

22


acrylic fibers have become popular for garments such as sweaters and have displaced wool for some portions of the market Polyolefines, such as polyethylene and polypropylene, are made by polymerizing olefins such as ethylene (ethene) and propylene (propene). Polyolefines have come into widespread use for wrappings and have displaced jute from a large segment of that market. Wrapping fabrics are not necessarily made from conventional yarns; they may be non-wovens, or made from tapes rather than the more or less cylindrical yarns. These alternatives will not be further discussed. There are a number of special polymers used to make high performance fibers and filaments for industrial applications. For example, various aramid fibers have high tenacity and high temperature resistance. They are a form of polyamide. There are also the polyurethanes, some of which are endowed with enormous elongational capabilities.

2.1.5 Staple versus filament As previously mentioned, most natural fibers exist in discrete lengths whereas manmade fibers are produced as extremely long filaments, which can be processed as continuous filament yarn or may be converted into staple fiber. Natural fibers are agricultural products subject to changes in properties due to exigencies arising from variations in growing conditions. Man-made fibers are usually controlled more closely but variations still occur; many of the factors that do vary are rather subtle in their effects. There exists an extremely important demand from the non-technical end user in favor of the natural fibers and blends between natural and man-made staple fibers. Aspects of hand and appearance in consumer products are very important in this market and some of the expectations of better quality from manmade fibers arising from closer control have not been realized. Users of technical products such as ropes, belting, and other industrial materials are usually more concerned with strength rather than appearance and technical factors assume a greater importance. The point is that there are market divisions with widely differing requirements. The production of the two classes of fiber, natural and synthetic, is radically different and has to be discussed separately. Since natural fibers are those with the longest history, they will be discussed first. The sharp contrast between the methods involved in the two cases will be noted. But it will be appreciated that often a spinner has to deal with both man-made and natural fibers and there is a need to know about the sources and idiosyncrasies of both.

Section B 2.2

Natural fibers (types and production)

2.2.1 Cotton Cotton fibers Cotton is a vegetable fiber that grows from the surface of the seed. Each fiber is essentially a long thin tube of cellulose with a central feed channel, called a lumen,


which runs almost the whole length of the fiber. In modern production, cotton is cultivated as an annual plant rather than letting it grow into a tree. Harvesting is easier working in this way and fiber properties are better controlled; also, cotton plants left in the ground after harvesting are subject to attack by pests. The use of this so-called ‘stump cotton’ has been banned in the USA. The length to diameter ratio of the fiber is in the order of several thousand; this makes it mechanically flexible and suitable for textiles. Wild and cultivated species of cotton [5] have been placed in the genus of Gossypium and in the order of Malvales. Five species have been cultivated, Gossypium herbaceum, Gossypium arboreum, Gossypium hirsutum, Gossypium barbadense, and Gossypium peruvian. The first two of these are sometimes known as Asiatic species, the third is commonly known as American upland cotton, and the last is known as long-staple cotton (Sea Island, Egyptian, Peruvian tanguis, Pima and others). Asiatic species have historically been known for having short fiber lengths, whereas the G. barbadense is known for longer fiber length than the average. Much breeding work has been done to change the characteristics of the short fibers, particularly if they were coarse (i.e. of high linear density). Cotton fibers are elongated single plant cells, varying in length, the average length of which changes according to species and conditions of growth. Fibers develop as elongations of the outer layer of cells of the cotton seed and each fiber consists of layers. The outer and inner layers are called the primary and secondary walls, respectively. The wall has a structure of fibrils as sketched in Fig. 2.4(a). A growing fiber exists as a tube with a wall thickness defined by the primary and secondary walls. The wall thickness and length alter as the fiber grows but there is little change in the outer diameter of the fiber. A cross-section of the undried fiber reveals the existence of daily growth rings, in more or less concentric circles surrounding the lumen. At temperatures less than 72°F or more than 90°F, the plant becomes dormant but within this range, growth occurs and the diurnal changes in temperature produce the growth rings. An erstwhile fiber tube flattens when it matures and dries. The deformed tubular fiber gives a variable cross section that is sometimes as sketched in Fig. 2.4(c) and sometimes in other configurations of a flattened tube as shown in the micrograph in Fig. 2.4(d). According to Scott Tagart [6], the wall thickness is not constant, and when the lumen dissipates, an irregular collapse is caused by irregularities in the wall. It is not possible to show the length of the fiber in proportion to its cross-section and only a part is shown in Fig. 2.4(b). The convolutions or spirals of the twisted ribbon of the dried fiber make it easily spinnable, which is an important consideration. It has been stated that there can be as many as 250 twists along a single fiber but the direction of twist does not remain constant; there are a number of reversals along the length. Wall thickness is also important because immature fibers with thin walls tend to collapse into neps during processing. These neps are a great nuisance during processing and can seriously degrade the value of the final product. Also wall thickness and structure can affect dye affinity which, in turn, affects the color of the finished product. Fiber length is also of great importance and a premium is paid for long fibers. Extremes in length vary from less than 0.5 to over 2 inches. The former is of little use in yarn manufacture and the latter is expensive and somewhat rare. Also various species of cotton have differing average diameters when growing and, of course, the

24

Handbook of yarn production Fibrils Wall

Lumen (a)

Mature dried fiber (b)

Cross-section (c)

(d)

Fig. 2.4

Cotton fiber

diameter varies with the growing conditions. The concept of diameter is of little value because of the changes caused by drying as just mentioned. Rather, it is preferred to talk of fiber fineness or linear density.1 Color of the fiber is also important; some species are white and others have various depths of yellowness, which affect the final product. There are some cotton fibers which have been bred to have a range of natural colors but these only occupy a small proportion of the market at the time of writing. Variations in fiber characteristics make it very important for a mill to choose the fiber carefully and to blend it well and consistently. After the cotton flower has gone, a ‘boll’ is formed, which is a fairly small, fat, pear-shaped capsule. It bursts open as it ripens exposing the mass of fluffy white fibers. Seed fuzz begins to develop from epidermal cells at about one week after the opening of the flowers, whereas the fibers useful in spinning begin to form on the day of flowering. The short fuzz is known as ‘linters’ and is sometimes used for paper making, wadding or as a source of cellulose. Fibers useful for spinning are known collectively as ‘lint’; these useful cotton fibers are obtained from the output of the gin.

1 Mass/unit length of fiber. Also, the term ‘micronaire’ is used which is really a standardized test of air permeability of a given mass of fiber in a certain restraining volume.


Fibers are attached to the seed at one end of the fiber, and they develop as the seed ripens. The fiber, at the attachment point, is hooked, and this portion of the fiber is susceptible to nep production. The aggressiveness of the ginning process (separating of fibers from unwanted material) determines how much of the hooked portion passes into the lint used by the spinner and thus the nep potential of the fiber. There is continuous activity by breeders who strive to improve the fiber length, strength, and other fiber properties. For example, over the period from 1990 to 1994, the average fiber strength of US cottons improved from 26.3 to 28.4 gf/tex. The strongest specimens yielded values as high as nearly 40 gf/tex. Fiber fineness is also judged to be very important. In addition, the breeders strive to introduce varieties which are resistant to disease and give high yields. In recent times, genetic splicing [7] is augmenting the practice of culling the best from a large number of varietal developments using traditional techniques. Cotton growing Cotton is a tropical plant requiring moisture and sunshine. Cultivation requires a climate that avoids frost damage to the growing plant. If the climate supplies insufficient rainfall, then it is necessary to use irrigation. The requirement for sunshine means it is normally grown between the latitudes 47°N and 40°S, which includes the Americas, Africa, Australasia, and Asia. Little is grown in Europe. More than 500 species of insect attack cotton plants and many of them are very destructive. Lush foliage, large flowers with their nectaries, and the extended fruiting period make cotton a host for many insects. Consequently, care has to be taken to control boll weevil, bollworm, aphids, nematodes, and other pests, but the use of modern pesticides has reduced the problem. The boll weevil and such insects destroy the boll by consuming leaves, bulbs, and the bark of the plant. Damage caused by aphids, whitefly, and the like is more subtle. Secretions deposited after the boll is opened convert to sugar [8], which makes the fibers sticky and difficult to handle in processing. Deposits on metal surfaces in ginning and mill equipment can cause substantial losses in production. Control in the field is not simple. Early season pesticide application can disrupt the natural suppression of other pests and accelerate resistance to evolution. Mid-season aphid populations cause yield reductions and late season populations produce sticky cotton. Some aphids are highly reproductive under the optimal environmental conditions. The presence of other aphid hosts such as sweet potato nearby can increase the damage because of the increased populations surrounding the cotton fields. For example, cotton leaf hair is an important factor in infestation of cotton [9] by the Bemesi tabaci whiteflies from the sweet potato. The infestation produces sticky cotton. Sugar deposits can come from other sources, such as honeydew (a secretion from aphids). Irrespective of source, they pose a problem that still causes anxiety. Pathogenic fungi, bacteria, and viruses also attack cotton. The point of this discussion is to show some of the difficulties and underline the need for mills to watch for these deposits and infestations because of the difficulties they can create in yarn manufacture. Various methods of measuring cotton stickiness [10, 11] are available. Yield, disease resistance, fiber length, strength, fineness, and maturity are primary factors used to select cultivars [12].2 As time goes by, new strains of cotton are

2 A cultivar is a plant of a kind which originates and persists under cultivation.

26


produced which improve the performance of ginning and textile machinery, as well as enhance the value and quality of the product. The growing of cotton needs skill and attention but space precludes any great discussion of matters other than those which affect the spinner. Suffice it to say that the rows of plants have to be spaced to allow agricultural machinery to move down the rows. Modern practice uses picking, spraying, and other machines that straddle two or more rows. Because of this, the machines are characterized by their tallness. There is a necessity to control weeds, especially those which produce oily seeds, act as hosts for the pests just described, or otherwise inhibit growth of the fiber. Before harvesting, the plants are defoliated by chemical means; much of the embrittled foliage falls off in the field and some is removed in handling. This reduces the trash in the fiber before it is harvested. In advanced countries, bolls of fiber are machine-picked and transported to a gin in a module (which contains about 6000 lb of cotton) or trailer (which contains, on average, about 4000 lb). Two types of mechanical harvesting involve the use of a spindle picker and a cotton stripper. The processes are often referred to as ‘machine picking’ and ‘machine stripping’ respectively. The spindle picker uses tapered, barbed, rotating spindles to remove seed cotton only from well-opened bolls. ‘Seed cotton’ refers to material from which the seed has not yet been removed. The cotton stripper is non-selective; it includes cracked and unopened bolls along with burrs and other foreign matter in its output. Fouled spindle pickers produce spindle twists that are twisted masses of fiber difficult to handle in any process. Stick content and grade reduction due to bark is controlled by the aggressiveness of the roll settings on the stripper. Under conditions of improper setting and wear, particles of rubber from the stripper become embedded in the lint and cause ‘black spotting’ in the yarns ultimately produced. Thus, the product reaching the gin is affected not only by the seed and the growing conditions, but also by the setting and maintenance of the harvesting machinery. Unopened (‘green’) bolls are not only regarded as non-lint but they also carry considerable moisture. When the green bolls burst, the moisture migrates through the surrounding seed cotton. Changes in retained moisture in the supply can upset the moisture content of the cotton already being processed; defects may be created in the product supplied to the mills. Moisture content is frequently expressed as ‘moisture regain’, the amount of moisture in a sample compared with that in an oven-dried sample. A fairly typical distribution of unwanted material in lint is given in Table 2.1. As the growing plants pass their harvest time, they are prone to become gray because of weathering. The decision when to pick can provide a problem if the weather is uncertain. Harvested fibers left out in the open can become discolored and stained by exposure to the weather; the severity of this staining is thought to be Table 2.1 Non-lint content of some cottons. Values expressed as mass percentage, i.e. {(non-lint/useful lint) × 100%}

Burrs Sticks Fine trash Motes Total non-lint

Machine picked

Machine stripped

7% 2% 5% 6% 20%

90% 23% 22% 5% 140%


influenced by the amount of wax produced by the plant. Thus, differences between varieties and growing sites can play a part in this respect. However, moisture content is a most important factor and length of storage is obviously a factor too. Consequently, care is taken to keep the newly harvested cotton under proper storage conditions. Suitable outside storage sites [13] should be well drained and free of gravel, stalks, long grass, and debris. They should have a smooth, firm, and flat accessible storage surface. Obviously staining or weathering can degrade the part of the crop affected. The variability of the products in the various stages of yarn manufacture will be discussed in later chapters. However, the stage is set by the variabilities within the cotton itself. For example, it has been variously reported that the fiber fineness within a single plant can have coefficients of variation of the order of 15%. This can be compared to coefficients within a single field of cotton that are of the order of 10%. These variations become partly homogenized by the blending that occurs in the various process stages but it is important to realize the magnitude importance of these sources of variance. Cotton ginning An important step in the production sequence is ginning. In the ginning operation, sticks and coarse trash are removed from the input material. Also, since the input to a normal gin is from a variety of growing areas and a gin processes very large quantities of fiber, the process acts as a first blending of massive quantities of fiber. The term ‘gin’ is sometimes used to describe a whole establishment that provides the service of ginning, but sometimes it is used to describe the machines. The part of the machine line that separates fiber from seeds is often referred to as a gin stand. The Churka gin, which, like all gins, had the purpose of separating the seed from the lint, was developed in some unknown period BC; Eli Whitney is credited with inventing the cotton saw gin in 1794. The basic idea was, and still is, to grip the lint protruding through a set of ribs by a moving surface and wrench the fiber from the seeds, which are unable to pass through the ribs. Rotary saw blades are commonly used as the moving surface just mentioned, but a specially prepared roller surface is sometimes substituted. A perfect operation would result in damage to neither seed nor fiber, and the undesirable small portions of fiber at the attachment points to the seed would be excluded. Unfortunately, this state of perfection is not possible. The aim is therefore to minimize the damage and thereby maintain the salability of both products. Apart from this, ginning has developed into a process stage that involves more than just removal of the seed from the lint. Before going on to discuss the elements in ginning, let us consider the seed. Seed is an important by-product of cotton manufacture and most of the seed is crushed for oil or used for animal feed. The percentage of the US crop that was crushed for oil declined from over 85% in 1970 to about 50% in 1994. On the other hand, the percentage used for feed changed from about 10% to over 35% over the same time. To the spinner, the seed represents a hazard because, if seed-coat fragments are excessive, mill processing is made more difficult. The economics of ginning are affected by the sale of the by-products, and thus they have some effect on the cost of cotton to the mill. Ginning, warehousing, and merchandising [14] accounted for 90% of the cost of cotton lint in 1977, of which over half was taken up in warehousing and merchandising. The costs generated between the ginning and the mill processes are significant. It becomes all the more important to ensure that, not only is the cost controlled, but also that the quality of the product is maintained within specified

28


limits. The cost of cotton to the mill represents roughly half that of yarn and the spinner must therefore have a strong interest in the basic fiber production. A modern ginning establishment [15] contains some or all of the following process items: 1

2 3

4

5 6 7 8 9 10

One or more green boll traps, used to remove green bolls, and heavy foreign matter such as rocks. These green boll traps are often little more than a Ushaped diversion to the flow of air and seed cotton. A counter-weighted trap door releases the green bolls when they have collected in sufficient quantity to counterbalance the forces holding the trapdoor shut. Automatic feed controls to provide an evenly dispersed fiber flow without wet clumps. Dryers such as is shown in Fig. 2.5 – two dryers are normally used which expose the flowing fibers for 10 to 15 seconds to hot air and raise the fiber temperature to no more than 350°F. Two or more cylinder cleaners – these consist of spiked cylinders rotating at 400 to 500 r/min over grid bars. Interaction between the moving and fixed elements breaks up large wads of fibers to permit more even distribution of moisture and temperature among the fibers. This, in turn, induces removal of fine foreign materials such as leaves, trash, and dirt. ‘Stick’ machines to remove burrs and sticks. A conveyor-distributor to convey cotton to the gin stands, where the separation of fibers from seeds takes place. A feeder to control the flow rates to the gin stands. The gin stand – this is the central item in the system, about which more is written in the following paragraphs. Lint cleaners to remove immature seed fragments and other trash. Bale presses to compress the lint into bales to facilitate transport and storage.

The need to clean the fiber at ginning is driven by the urge to elevate the grade to get the best possible financial return for the farmer. However, a high grading obtained by excessive cleaning always causes disappointment in the mill due to fiber degradation and the fiber may, therefore, not fetch the high price envisaged. Hot air + cotton

Dried cotton

Fig. 2.5

Drying fiber at the gin


To make removal of trash easier, the cotton is heated to get the moisture content down to about 7%. Hot air is used for the drying, and the time to dry the cotton depends on the starting moisture content and the air temperature and flow rate. (The moistures content of a fiber in hot air declines approximately exponentially with time.) There is a temptation to use high air temperatures in order to speed up the process. However, this, or the slowing of the fiber flow, or changes in initial moisture content can lead to overheating. Associated ills are damage to the fibers by scorching, electrostatic charging of the fibers, and driving of the more volatile components of the wax that coats the fibers. Wax on the fibers is a valuable lubricant without which later processing would be very difficult. Electrostatic charges impede the separation of the fibers and lead to fiber damage. For those reasons, too low a moisture content arising from overheating the fiber is counterproductive. A modern saw gin stand [16] has between 100 and 200 disc-like saws separated by ribs about 0.5 inch wide. Saws up to 18 inches in diameter are used. A simplified sketch of a typical saw gin stand arrangement is given in Fig. 2.6(a), but the sizes of the saw-teeth and brushes have been exaggerated for clarity. The cutting edges of the teeth are angled (line CD, which is parallel to the tangent of the rib AB) as shown in Fig. 2.6(b). The reason for this is to prevent the seeds from sliding to the base of the teeth and accumulating there. The gin stand comprises a feed system and spiked Seed cotton

Brushes

Saw Lint

Gin rib Huller rib

(a) Debris

Rib

(b)

Fig. 2.6

Elements of a saw gin stand

30


roller cleaners, as well as the gin components shown. Seed cotton is introduced to the saws by a feed roller. The teeth of the saws pull the lint through the rib gaps into a moting chamber (not shown) where seed particles are removed by a mote bar. From there, the lint is carried to the brush doffer, which removes the lint and transfers it to the air transport system which, in turn, carries the fiber to the lint cleaner. Most cleaners consist of saws and mote bars that flail the lint against the bars. Brushes move lint from the saws. An alternative to the saw gin is the modern roller gin (Fig. 2.7) which was developed in the 1950s and is used mostly in Arizona, California, Texas, and New Mexico. An important feature of this gin is the covering of the roller, which has to be resilient, yet durable, and have a surface that will grip the fiber. The productivity is only about 20% that of a saw gin. Since ginning accounts for about 38% of the cost of cotton, roller ginning is used only when there is a particular need to preserve fiber quality. Pima cotton (Gossypium barbadense) and similar long-staple cottons are usually roller ginned. Woody and other unwanted materials are unavoidably collected in the fiber when mechanical harvesting has been used. All unwanted matter is collectively referred to as trash. Seeds and non-lint materials are removed in cleaning but some good fiber is also lost. Too rigorous a cleaning can damage fibers and degrade the product. In seed removal, it is inevitable that some fiber damage will occur; seed particles are removed by the violent action of separation. Moisture is very important in this respect. For example, the strength of upland cottons is about 1.8 times the force needed to separate the fiber from the seed at 7% moisture content. The short fiber content of the lint is increased by about 1% for each percentage reduction in moisture content [17] This is partly because the increased fiber electrification in the ginning process makes fiber separation more difficult. Some unwanted matter is taken out by the lint cleaners, which operate in series with the gin. It is a matter of debate how much cleaning should be done at the gin and how much in the mill. If dirty cotton is shipped to the mill, the yield of usable cotton per bale drops and the effective cost of the fiber rises, but the quality of the fiber is better. Similarly, the mechanical removal of fiber, which accompanies any cleaning operation, reduces the ‘out-turn’ of the gin. (Out-turn is defined as the mass output as a percentage of the corresponding input.) This tends to reduce the financial return to the ginner and farmer, unless there is a compensating

Seed cotton Moisture spray Rotating knife Stationary knife

Ginning roller

Debris Lint

Fig. 2.7

Elements of a roller gin


increase in premium due to improved cleanliness or grade. The use of a larger number of lint cleaners leads to a reduction in trash content but only at the expense of a decrease in fiber quality. The addition of a third lint cleaner in ginning causes a greater loss in fiber quality than that caused by the second cleaner. Since fiber damage is irreversible, there is some question as to the wisdom of using a third lint cleaner in ginning. However, the question is one that is usually settled by the market. After the completion of ginning, the fibers are compressed into bales to facilitate transport to the mills. A bale usually weighs about 500 lb and is wrapped in a fabric to protect it from damage, and strapped with wires or metal tapes to maintain the compression during transport and storage. These ties are strained up to 2000 lb force and extreme care has to be taken in bale breaking (i.e. removing them). The pressure used to compress a bale is also of importance. If several bales decompress to differing heights when introduced into the bale laydown in the mill, the bale plucking machinery will not sample the bales correctly until the top of the laydown has been leveled. Improper sampling of this sort could lead to barré (see Fig. 2.1) in the final product. Bales come in a variety of sizes and, even within the USA, there are several standards such as flat, modified flat, and universal. A bale is box-shaped (i.e. a rectangular prism) whose dimensions are X, Y, and Z. With US cotton, X ≈ 55 inches, Y ≈ 20, 21, 25, or 28 inches, and Z depends on the degree of compacting. For a 500 lb bale, densities vary between 20 and 30 lb/ft3. In earlier times, bast fibers were used to make the bale wrapping, but now they are commonly made from polypropylene tape. The wrappings assume significance because failure to remove all vestiges of the wrapping leaves ‘foreign fibers’ in the product stream, which cause blemishes in the final product.

2.2.2 Wool Wool fibers Wool is normally defined as the fleece of sheep, the fleeces from other animals can also be used as textile fibers. Camel and goat hair (some goat fiber is known as mohair) are highly prized although they have only a small market. For this reason, they will not be described here. Wool is a protein called keratin, which has a main polypeptide chain with amino acid side chains. It is an outgrowth of the epidermis (skin) of the sheep and the surface of the fiber has minute overlapping scales extending lengthwise and pointing to the end remote from the root or cuticle. The root is embedded in the epidermis. Wool grows in tufts, in or near the follicles in the skin of the animal; however, the useful, outermost portions of the fibers on the animal are no longer growing. Growth occurs by multiplication of the soft cells of the papilla, which exist at the base of the follicle. The papilla is a vascular arrangement of connective tissue extending into and nourishing the root of a hair. The useful part of the fiber is displaced from the cuticle as new cells are added and the fiber gets longer. A scaly surface is produced, as shown in Fig. 2.8. Thus, the fiber grows in length even though the outer part is no longer living. Lack of nutrition, or disease, affects the development of the fiber; if there are periods when the animal is adversely affected, portions of fiber become weak. Wool with such weak spots is referred to as ‘tender’. The central part of the fiber near the skin contains the medulla (the inner pithy part of the structure) and the cortex (a layer of tissue connected to the papillae). Certain moth and beetle larvae such as Tinea bisselliella, Tinea pellionella, and

32


Fig. 2.8

Portion of wool fiber showing scales

Anthrenus verbasci attack wool and it is necessary to protect the fiber. Mothproofing does not always protect against beetle larvae because the eggs may have been laid before the treatment. Some insecticides do not protect against all pests that attack the fiber and insects build up resistance [18]. Thus, it is desirable to have a broad range of protection. Common agents used are organochlorine compounds; microbial pathogens, parasites, and biological control also offer possibilities of control. Wool is a source of anthrax contamination in humans but heating during the drying cycle is sufficient to be lethal to the micro-organisms concerned. Any danger comes from handling untreated wool from geographic areas having problems with the disease. Coarse wools are stiffer than finer wools and this property is especially important at the tips of the fiber. Garnsworthy et al. [19] deduced that, if the fiber ends protruding from an apparel fabric are capable of supporting loads of over about 1 mN, the wearer will suffer mechanical irritation of the skin. These stiff fiber ends irritate the human skin by mechanical disturbance of the surface of the epidermis. The effect can be lessened through surface treatment of the fabric, but the main point here is that fineness of the fiber is a significant property. In some applications, such as in carpeting, stiffer fibers are preferred, but in most, more flexible fibers are preferred to avoid the problems of ‘prickle’. Follicles are deep pits in the skin of the animal where the fibers are nourished and where wax and sweat glands are situated. (Sweat glands are known as ‘suint glands’.) Thus, wool removed from the sheep is coated with wax and suint, as well as vegetable matter picked up by the animal during foraging. Raw wool in this state is referred to as ‘greasy wool’ or ‘grease wool’. The foreign materials, which can represent over half of the weight of the unprocessed fleece, have to be removed before the normal mechanical textile operations can be performed. Such cleaning operations used to be carried out in the mill but, increasingly, they are being performed nearer the shearing operation. Cleaning is divided into wet and dry cleaning; the former operation is referred to as ‘scouring’. Further discussion of cleaning is given in Chapter 8. Wool fibers are naturally crimped. The fiber crimp levels range from 6 per inch (in cross-bred wools) to about 24 per inch (in Merinos). The fiber cross-section is slightly elliptical and the balance of forces in the growing fiber cause it to curl in growth. The fiber crimps are related to this curling. Sheep bred for fine wool usually produce almost white wool and the presence of any color can downgrade it. The color can vary from white through yellow and fawn to brown; also shades of gray may be present. Tenacity varies in the approximate range of 100 to 150 mN/tex when dry but only 80% to 90% of that when wet. Elongation varies between about 25% and 35%. The average diameter of the fibers varies from 8 to 60 microns (approx 0.0003 to 0.0024 inches) and the useful length can be anywhere between a fraction of an inch to 40


inches. Obviously, the length depends on the growing time, the breed, and other considerations. Some fibers consist of only cuticle (outer skin) and cortex whereas others are medulated; some fibers contain parts that are medulated and parts that are not. The term ‘medulated’ refers to a pithy core sometimes found in a fiber. Besides normal wool, the animal grows ‘kempts’, which consist of coarse straight fibers with tapered tips that are often shiny. They show up as undesirable inclusions in the final product and therefore should be avoided. Discriminating between the various sorts of wool is the job of the ‘classer’ (classifier) and it will be realized that classing is a highly skilled job; much of the skill is based on experience. No single, universal classification system exists, but one commonly used grades the fiber according to the finest yarn that can be spun with it. An alternative system is the ‘blood’ system, which refers to the closeness to the Merino breed of sheep. Highly graded fibers are classified near to 100% blood and, as the percentage blood reduces, so the quality is lowered. Another system grades from ‘super’ down through AAA, AA, A, and 1st, to 2nd. As we have seen elsewhere, fiber length, fineness, strength, elongation, uniformity, color, and luster are important and are all parts of the value judgment. Fiber lengths greater than about 2.5 inches are regarded as suitable for combing and for processing into worsted yarns. Fibers less than about 1.25 inches are used in the woolen system. The yarn count is usually quoted in the woolen or worsted systems, where the count is measured in hanks/lb. The lengths of yarn in these types of hank are 1600 yd and 560 yd, respectively. Wool production Wool is obtained by shearing it from living sheep or by pulling it from the skins of slaughtered sheep. Shorn or sheared wool is the most common. Pulled wool may alternatively be described as ‘skin wool’, or ‘slipes’. Fiber taken from a dead sheep is called ‘dead’ or ‘murrain’ wool [20]. Australia and New Zealand produce a large percentage of the world’s wool. Much of New Zealand wool [21] comes from the NZ Romney sheep and its crosses; Coopworth and Perendale breeds are also significant. In Australia, the finer Merino wools predominate. In these countries, very large areas are often involved in sheep raising. Sheep can exist in rough mountainous regions under arid conditions, where forage is variable and herbage sparse. With large flocks, careful organization is required to harvest the wool because it is necessary to collect and confine the animals in close quarters to permit marshaling to allow the clip to proceed efficiently. Fine wool Merino sheep are able to survive under such conditions. Naturally sheep can also do well with lusher vegetation, as evidenced in England and New Zealand. However, some of the wools produced are relatively coarse and more suitable for carpets than apparel. Each sheep is periodically sheared to remove the fleece. The natural mutual cohesion of the fibers enables them to cling together even after shearing. A skilled shearer clips close to the body to keep the fleece essentially in one piece, and produces over 100 fleeces per day. Removal of heavily soiled wool around the crotch (crutch) of the animal is called ‘crutching’ and it is usually carried out before lambing and before the normal shearing; this reduces the amount of soiled wool to be removed later. After shearing, the fleece is sorted and classed. Sometimes the fleece is skirted locally (i.e. inferior pieces such as the short belly fiber are removed) and these skirtings are packed separately. Dirty or stained wool and rough, coarse ends are removed. Sweaty locks and parts containing a high percentage of burrs are also removed. All sorts of

34


vegetable matter picked up by the animal in its wanderings are usually included in the term ‘burrs’. The remaining fleece is divided and rolled to bring the rib and shoulder portions to view. The fleeces are then classed and shipped to warehouses for storage and eventual distribution. Discolored wool, ‘bellies’, and some other portions are sometimes sold separately from the main fleeces. Also wools containing faults are separated; these include cotted, stained, and muddied wool, as well as double fleeces. The term ‘cotted’ refers to heavily felted and matted material; double fleeces refers to the material removed from sheep missed in the previous shearing. Annual shearing is the normal practice and the yield thus normally depends on the growth rate of the wool. If left longer than a year, the fiber length may be more than that obtained with present practice; however, any increase in value with fiber length has to be set against the reduction in average annual yield. The jobs described are seasonal and are highly labor intensive; this drives up the cost of production. Some consideration has been given to dosing the animal with a drug to weaken the fiber near the follicle [22] so that the wool could be pulled from the skin surface rather than be sheared; it will be interesting to hear of the outcome. As mentioned previously, some movement has been made to clean the fleeces before sending them to the mills. Scoured wool and ‘tops’ command higher prices of course; the cost of shipping is lowered and environmental problems for the buyer are greatly reduced. ‘Tops’ are thick ropes of fiber, otherwise called sliver. Some 65% of New Zealand wool is now scoured before shipping. In contrast, as of 1988, 80% of the total Australian clip was shipped as greasy wool. However, it should be noted that wools from these countries go to different markets because of the differences in fineness of the wool and the differing preferences of their buyers. Arriving fleeces are closely examined by the buyer, despite the prior classing. Different qualities, known as matchings, are placed in separate lots to maintain quality control. The necessity to re-class underlines the variability of the wool from any one sheep, let alone the variations between the sheep. Up to the 1970s, most of the judgment regarding quality was made by appearance and touch. However, since then there have been developments in instrumentation that have been accepted and applied. According to Whitely [23], 90% of the variation in clean prices is accounted for in fiber diameter and vegetable-fault level. Style categorization is an indicator of staple length, average fiber diameter, and vegetable fault content. Scanning systems are beginning to be used to record color, crimp, and staple length of the greasy wool as it passes on a conveyor. This gives a glimpse of the advances which are being made, not without opposition. Similarly difficult transitions have been met with other fibers and in other geographic areas. However, the drop in wool prices has reduced the resources available to promote the process of adopting instrumented measurements. The value of wool depends, amongst other things, on its fineness. Heavier wools are discounted [24] and fiber fineness is an important factor in setting the price (which is not always directly related to value). Wool prices languished through the last decade or so, with the result that research and development was slowed. The lack of significant growth has also been a disincentive for machinery makers to invest in solving the difficult technological task of improving productivity. There have been fewer developments than in the short-staple arena; there seems to be some recovery but, as is so often the case with textiles, it is difficult to predict the outcome.


2.2.3 Bast fibers Flax Flax (Linum usitatissimum) is an annual, herbaceous plant grown in temperate and subtropical areas. After flowering, the bolls or capsules contain up to ten seeds. The fibers occur in the bark of the stem and it is the long stemmed varieties that are used for linen. Bast stems contain bundles of fibers that act as hawsers in the fibrous layers lying beneath the bark of dicotyledenous plants. (A dicotyledon is a plant having two seed leaves.) They help hold the plant erect. The Soviet Union was the largest producer before the collapse of communism but is no longer. Some satellite countries of the former USSR, such as Slovakia, produced large quantities of flax and linen yarns, some of which were directed to the manufacture of tarpaulins and other industrial uses. Linen yarns have remarkable resistance to sub-zero temperatures, which cause deterioration of properties in many synthetic fibers. Any conception that linen is solely an apparel fiber is misguided. In fact, in common with other bast fibers, it is beginning to find a use as a reinforcement for composite materials in automobile manufacturing because of its strength and biodegradability. Other major producing countries in recent times have been Poland, Germany, France, Ireland, Rumania, Belgium, and Holland. Strands of commercial flax fiber may consist of many individual fiber cells. The cells vary from about 0.25 to 2.5 inches in length. They exist as thick walled, cylindrical tubes with a diameter of about 0.0008 inch and the central lumen (the central canal in the cell) tapers to a point towards the end of the fiber. The fibers do not have the convolutions typical of cotton and the width of the fiber may vary several times along its length. It is stronger than cotton but it is an inextensible fiber and the elongation at break is only about 2%. It is 20% stronger wet than dry. Flax is still the main vegetable fiber grown in northern Europe [25]. The plants are subject to attack by pathogenic fungi (wilt) and viruses (curly top). Wilt-resistant varieties have been developed. Reasonable control can be exercised by chemical treatment of the seed and the use of fungicides. Flax is harvested when about half the seeds are ripe (yellow or brown, shiny, and flattened) and the leaves have fallen from the lower two-thirds of the stem. Modern practice is to use pulling machines that remove the plants bodily from the soil and bind them into bundles which are set into ‘stooks’ or ‘shocks’ in the field for drying or curing. The stooks are assemblies of bundles of stems arranged like an elongated cone that promotes natural airflow through the bundles. When they have dried, the stalks are de-seeded by threshing, combing, or beating, and the product at this stage is referred to as ‘straw’. Before the fiber can be used for textile products, it has to be removed from the stems. The dried straw is ‘retted’ to break down the gums that bind the fibers together in the bark. ‘Retting’ is a controlled rotting process which is brought about by exposure to the weather, or soaking in ponds, sluggish streams, or vats. Bacterial action and the physical effects of weathering or soaking cause the decomposition of the gums. Retting is complete when the bark becomes loose so that it can be easily removed from the woody portions of the stems. The process takes one to three weeks according to the weather or the temperature of the water. The retted straw bundles are set up in open shocks or ‘wigwams’ to dry. The fiber is separated from the woody material by ‘scutching’. This involves the use of fluted rolls and beating blades which break the brittle woody parts into ‘shives’ but leave the fibers largely intact. The scutched fiber is baled and sent to the mill.

36


Jute Jute fibers are obtained from two species of Corchorus, namely C capsularis and C. olitorius. There are also a number of jute substitutes such as Bimli (from Hibiscus cannabinus) and China jute (from Abutilon theophrasti). Jute fabrics formed the ‘sackcloth’ of Biblical times and are now used for wrappings, bindings, etc. Commercial jute fiber consists of overlapping cells which average 0.08 inches long by 0.0008 inches equivalent diameter (cells are not round; the equivalent diameter has the same cross-sectional area as the cell). The color varies from yellow to brown with various degrees of grayness and tends towards brown when exposed to sunlight. Like flax, the fibrous material surrounds the woody core and is embedded in the nonfibrous material under the bark. The strands nearest the bark run the full length of the stem and other strands further from the bark become progressively shorter. The cells are about 0.1 inches long and, although retting destroys the tissue that holds the fiber bundles together in the natural state, it usually does not separate the cells in a given fiber. Cultivation requires well-drained, fertile soil and a hot, moist climate. The crop is ready for harvesting when the flowers begin to fade. If cut too early, the fiber is weak, and if cut too late, it is strong but coarse and lacking in luster. Like flax, the stalks are retted to free the fibers from the natural gums that bind them. If the stems are removed from the retting basins too soon, the fiber is difficult to remove and suffers mechanical damage. If they are allowed to stay immersed too long, the fiber is degraded and is weakened. The separation of the fiber is termed stripping. The material is graded and baled before shipping to storage. Hemp The botanical name for hemp is Cannabis sativa. Sisal and manila hemps are hemp substitutes. As mentioned earlier, C. sativa produces fiber, seed, and narcotics. Cultivation is not unlike that of other bast fibers and, again, the time for harvesting has to be judged carefully. The fibers are soft and fine if they are harvested as the pollen begins to shed, but they are weaker than those obtained from later harvesting. Hemp made its mark because of the strength of the fiber. The cells vary from about 0.5 to 1 inch long, and, like flax, they are thick-walled tubes, although the lumen has blunt ends. The fibers may be up to 6 ft long and are roughly cylindrical with cracks, swellings, and other irregularities. The process of retting is similar to that already described for other bast materials. The devices for separating the fiber and ligneous material are called ‘brakes’ and the process is called breaking, but essentially the process is similar to that already described. Ramie Ramie comes from plants with the botanical name Boehmeria niva or B. tenacissema. Fibers are removed by decortication, which is a process whereby the fibers are removed from soaked stalks by scraping or beating. Gums are then removed by soaking in caustic soda followed by neutralization in an acid bath. The fiber is then washed and oiled. The thick-walled cells often reach 18 inches long. Normally the fiber is rather stiff but mercerized ramie has some qualities that allow it to approach the performance of cotton.


2.2.4 Silk Silk fibers Silk is different from the natural fibers previously discussed because it occurs as a filament, and the highest quality silk is worked as filament rather than as staple fiber. That is not to say that it cannot be chopped and mixed with other staple fibers. The visual and tactile characteristics of silk make it very attractive in both forms. As with all textile fibers, silk has long-chain molecules as its backbone, but attached to these are various sorts of side chains. Crease resistance and yellowing are two problems that have been addressed by epoxide treatment [26]. Some of the treatments are intended to cause the fiber to behave in a more nearly elastic manner. Silk is extruded by the silkworm into a cocoon and the silk has to be reeled from that cocoon before it can be used. Silkworms are of the Lepidoptera family, and of the Bombyx species, which feed only on mulberry leaves. Cultivated species are often B. mori, but there are also other species such as B. textor and B. sinensis. Indian Tasar silkworms are Antherea proylei and A. mylitta, which feed on leaves other than mulberry. A. assamensis (Mugar) silkworms and others are also used for fiber harvesting. The silk glands of the larvae produce fibroin. The freshly made fibroin is transferred to two holding cavities for the fluid to ripen. When the caterpillar reaches maturity, fibroin is extruded through a spinneret in common with a second secretion called sericin. Fibroin is an amphoteric colloid protein and sericin is a natural gum. This sericin solidifies straight away and two entering filaments of fluid are converted to a single emerging strand that is used to cover the insect in an oviform envelope. The filament varies from white to yellow in color. It has a high tenacity and it is capable of 20% elastic (i.e. fully reversible) elongation, which is remarkable. After the seracin gum has been removed, raw mulberry silk strand consists of two smooth rod-like fibroin filaments and has a white lustrous appearance. The crosssection of each filament is roughly triangular. Obviously, a supply of larvae is needed, but this will not be addressed here. Also, adequate supplies of appropriate leaves are required to feed the larvae; thus the first step is to cultivate the mulberry trees or other plants. A uniformly hot climate is needed to hatch silk ‘seeds’ or eggs, which are usually set out in trays. Incubation is timed to coincide with the leafing of the feed plants. After being taken from the incubator, the trays containing the newly hatched eggs are spread with gauze on to which chopped leaves are spread. The feeding period continues for about 40 days. The caterpillars, to spin their cocoons, inhabit a structure of cells. This structure is rotated whilst the cocoon is being spun. The cocoon takes about 60 hours to complete. It is essential to keep the cocoons separate during spinning because, if they stick together, it is almost impossible to reel the silk from the cocoon. The chrysalises are then killed, often by steam, otherwise the pupae would damage the cocoon in emerging therefrom. In some areas, reeling is carried out in a portion of a silk processing establishment called a filature. The reelable cocoons are boiled in water for about 10 minutes to soften the sericin. The ends are then sought for each of several cocoons and the group is reeled to make a skein or small bundle (called a ‘book’). Sericin has poor solubility in the presence of tannins and this makes it difficult to degum nonmulberry silks containing tannins. Modern developments include automatic reeling machines; also enzymatic and other forms of decomposition have been used with some success. The books are baled for further processing elsewhere. The count of raw silk is expressed as denier. The inverse of 4 464 531 yd/lb is equivalent to 1 denier. The

38


discarded non-reelable cocoons are mechanically converted to staple and are used to make a spun silk described as ‘schappe’. The materials from breaks in reeling are also used for spun silk yarns. Staple silk spinning is, in some countries, a cottage industry with spinning wheels and mules still in use. These cottage industries are often promoted for social reasons. Throstle spinning and ring spinning are used in other areas. Matsumoto et al. [27] describe an improved throstle in which a rolled sheet of silk fiber is inserted into an intermittently rotating stuffer tube.

Section C

2.3

Man-made fibers (polymer extrusion and yarn production)

2.3.1 Outline of processes to produce man-made fibers It is possible to emulate the silkworm by liquefying the polymer before forcing it through a ‘spinneret’ to produce a number of fine streams. The streams of liquid are then solidified to make filaments. These filaments usually have to be ‘drawn’ at a later stage to further orient the molecular structure to give the desired physical properties. The method of liquefying the polymer depends upon the type of polymer. In some cases a solvent is used, and in others the polymer is melted. Polymer solutions are solidified by removing the solvent by evaporation (dry spinning), or by coagulating them in a liquid bath (wet spinning). Polymer ‘melts’ are solidified by cooling them below their melting points (melt spinning). Some fiber cannot be melt spun because the material starts to decompose before it melts, or because the melting temperature is not within an acceptable range. For example, with some acrylic polymers the high temperatures required for melt spinning can cause the fibers to discolor. Therefore, either a wet spinning method is used, or a melt spinning operation is operated using a blanket of inert gas to prevent oxidation (oxidation causes the yellowing, as well as some other undesirable changes to the polymer). Melt spinning is the most important of the two processes. In all cases, the liquefied polymer has to pass through some form of pump to produce the necessary pressure to force the material through the very fine holes in the spinneret. In commercial production, the pressures are high. The flowing polymer has also to be filtered to prevent lumps, such as gels and foreign bodies, from clogging the holes in the spinneret. Dry spinning Dry spinning is used to produce cellulose acetate fibers from an acetone solution; also several vinyl fibers and polyacrylonitrile fibers can be produced from solution in other organic solvents. The first step is to produce polymer solution, which is then filtered and pumped through the spinneret as indicated in Fig. 2.9. Choice of solvent is governed by considerations of solvent power, boiling point, heat of evaporation, stability, toxicity, hygroscopicity, ease of recovery, and cost. Low boiling point solvents with high heats of evaporation may cause polymer condensation on the surface of the filament and produce an undesirable surface.


It takes a finite time and considerable energy to remove the solvent from the filaments in dry spinning. The process of solvent removal reduces productivity and increases costs. This is because the mass transfer and heat transfer are far from instantaneous. The spinning apparatus has to incorporate a long ‘chimney’ to remove solvents (see Fig. 2.9). The volatile solvents needed for the process are nearly all toxic and/or flammable. The vapors cannot be released into the air and they have to be recovered. Also, solvents are expensive and it is an economic and ecological necessity to recover them. Increases in delivery speed sometimes require disproportional capital costs; there is also an upper limit to the production speed. Normal spinning speeds lie in the range 800–1000 m/min. There is a limit to the length of undrawn and unsupported filament that can be handled. Flow through the spinneret is controlled by several factors. These include the pressure and viscosity of the liquid. A typical solution runs at viscosities in the range 500–1000 poise; this viscosity is mainly determined by the concentration of solvent and the temperature of the mixture. The yarn take-up speed depends on numerous factors that include shrinkage associated with solidification. The types of polymer and solvent affect the cross-sectional shapes of the filaments. Rarely are the fibers round in cross-section, and changes in cross-sectional shapes can be important in determining luster as well as other physical properties of the fiber. When the material is chopped into staple fiber, the cross-sectional shape can Input: Filtered polymer solution Pump Solvent + gas to recovery system

Spinneret

Filaments Chimney

Output: Yarn Air or inert gas

Godet

Ring & traveler Bobbin

Fig. 2.9

Dry spinning

40


affect cohesion between the fibers which will, in turn, affect the processability as well as the properties of the final staple yarn. Wet spinning Wet spinning is a chemical precipitation process. Coagulation of the filaments involves two-way mass transfer with the coagulating agent (e.g. acid) diffusing inwards into the filaments and the chemical products of coagulation (e.g. salts, H2S) diffusing outwards. It is sometimes necessary to use an intermediate process to produce a solution. For example, in viscose rayon production, a soluble derivative (cellulose xanthate in this case) is produced and this is dissolved in dilute dolium hydroxide to produce the liquid suitable for extrusion. Solvent is leached out by the liquid in the bath; the latter must be miscible with the solvent but must be a non-solvent for the polymer. Thus, in a generalized diagram of such a process, it would be necessary to specify the extrudate as filtered ‘polymer derivative’ or ‘polymer solution’ according to whether an intermediate step is necessary or not, but in the case cited above, the extrudate is a polymer derivative (Fig. 2.10). During coagulation, several simultaneous processes occur, in different ways for different polymer/solvent systems. Their coagulation is slow; up to 3× drawing is possible. The more rapid the coagulation, the more inhomogeneous is the crosssection. The heat- and mass-transfers between the extrudate and the liquid of the coagulation bath affect the temperature and solvent distributions within the fiber. Any maldistributions make it difficult to obtain uniform properties throughout the cross-section of the strand. The outside surface of the filaments hardens and this tends to inhibit the mass transfer required. Furthermore, the migration of the solvent through this hardened ‘skin’ reduces the volume of the material enclosed with the result that the skin wrinkles. Thus, wet spun filaments usually have a convoluted cross-section. Wet spinning is commonly used to produce viscose rayon, and polyacrylonitrile (PAN) fibers. (It will be noted the acrylic fibers can also be manufactured by dry spinning; see above.) The polymer derivative usually has to be ripened because as it ages, it changes Input: Filtered polymer derivative solution

Spinneret Output: Filaments Coagulation bath

Pump Input: Filaments

Washing Airflow

Chemical treatment Output: Filament yarn

Drying section

Not to scale

Fig. 2.10

Ring & traveler Bobbin

Wet spinning


viscosity and character. Often it is necessary for there to be a storage system between the preparation of the polymer derivative and the final extrusion in order to accommodate the ageing process. The viscosity of the polymer solution is an important variable. Generally, the higher the concentration of polymer (desirable for economic reasons), the higher the viscosity. High viscosity solutions spin well because the desirable cohesive effects of high viscosity outweigh the undesirable effects of unavoidable surface tension which tend to cause the liquid to degenerate into droplets. However, a high viscosity liquid is difficult to filter and pump, and there must be a compromise. Frequently, the solution is heated to reduce the viscosity during filtering. Cellulose fibers may be spun at about 50°C (122°F), whereas polyacrylonitrile fibers are frequently spun at 170°C to 180°C (338–356 °F). A typical spinning speed is several hundred meters per minute. Following the actual spinning operation, it is usually necessary to have a chemical treatment such as neutralization of the acid from the coagulating bath, etc., followed by washing and drying. The application of spin finish and the winding of the filament yarn follows this operation. Wet spinning plants have environmental problems and the counter-measures push up the costs of production. Melt spinning In melt spinning, the material supplied to the extruder is sometimes in a solid granular or ‘chip’ form, especially for small operations. In this case, the chip is conveyed from the storage silo to the hoppers of the extruders by a pneumatic transport system. From each hopper, the polymer passes through the extruder, conveyed by an ‘auger’ or ‘screw’ (Fig. 2.11). The polymer is then melted by the heated barrel and by friction Polymer chip input Heating jacket Insulation

Barrel Auger or screw rotates

Filter + pump

Quench air

Filament output

Fig. 2.11

Simple fiber extrusion

42


from the screw. Heat flow is the major factor in determining the viscosity of the liquid polymer in the working extruder. The viscosity affects the pressure generated by the screw forcing the liquid polymer through the spinneret. Additional backpressure is generated by a filter pack. The design of the screw is a very important feature of a modern extruder. In other cases, the polymer can be supplied in a continuous molten form (rather than in the intermediate chip form) direct from the polymerization reactor or an intermediate heated storage tank. By supplying the polymer through heated pipes, it can be maintained in the liquid form and air can be easily excluded to prevent oxidation with its deleterious effects. The liquid polymer may be pumped through the filter pack and spinneret by several sorts of pumps including the extruder screw. Back-leakage, polymer overheating, and degradation by excessive working are factors that have to be taken into account in choosing an appropriate pumping system. It should be noted that these entire polymerization/spinning systems are very large and the total polymer synthesis and extrusion equipment requires considerable floor space and headroom. Usually a multistory building is required. The capital cost is very high. As mentioned, the rate of heat transfer from the extruder barrel into the polymer is very important in determining the viscosity of the molten material approaching the spinneret. This, in turn, helps to determine the flow rates and the ultimate yarn properties. The rate of heat flow away from the extruded filaments leaving the spinneret helps to determine the morphological structure of the yarn. Morphology relates to the degree of crystallinity and orientation. At high speeds, the shear rate in the extrusion zone (which is a function of the filament velocity) also affects the morphological structure. The amount of subsequent drawing of these filaments yet further affects the properties of the yarn. Such drawing might be carried out near the extrusion operation, or during texturing, or both. The properties of the extruded filaments and fibers are important but it is beyond the scope of this book; the reader is referred to reviews by Mukhopadhyay [28] and Brunnschweiler and Hearle [29]. The mechanical process appears to be inherently simple. However, as indicated above, there are a number of less obvious complexities involved which become very important at the high speeds now in use (of the order of 5000 m/min). Since the drawing speed is limited by the mechanical nature of the process, the extruder delivery speed would become virtually fixed at a relatively low level if the filaments were fully drawn. If part of the drawing procedure is deferred until the material is in the texturing machine, the yarn leaving the extrusion frame is only partially oriented. The orientation is completed by drawing in the texturing process and the use of this strategy results in economic advantage. Draw ratio changes affect the strength of the partially oriented yarn (POY). The strength of the POY produced at low draw ratios is insufficient for high speed texturing and it becomes desirable to draw at the texturing stage to increase the filament strength. Some drawing is necessary at the extrusion stage to give sufficient strength and stability to the POY. Thus, when the POY is being produced for draw-texturing, the texturing speeds in effect become linked to the extrusion speeds. Draw-texturing relates to the process where drawing is carried out at texturing. Since it is economically advantageous to do as much drawing as possible at the texturing stage, the choice of draw ratio at spinning becomes fairly critical. Commercial filament extrusion is more complex than indicated in Fig. 2.11. The filters are larger and the molten polymer is distributed to groups of spinpacks fed from a main spin distributor and pump system. The whole system is carefully crafted


to avoid stagnant flow zones, to conserve heat, and to preserve the temperature of the melt with heat transfer fluids that are sometimes of two-phase type, which hold the temperature at the boiling point of the fluid. An example of a two-phase system is given in Appendix 3, but the example is not intended to imply that steam is always used. It will be realized that it is very important to control the temperature and viscosity of the melt because the uniformity of the yarn depends on it. For example, with some polyesters it is necessary to hold the temperature between the limits 300 ± 1°C. Mechanical working of the melt also affects the viscosity; therefore the design of the extrusion and distribution systems is critical. The extrusion systems also have to be made to permit the changing of filters and spinning heads with minimal interruption to production. It has to be realized that cessation of flow causes problems and if the polymer is allowed to solidify, this is a disaster! The extruder The screw and barrel of an extruder fulfill a number of functions. First, the screw acts as a propulsion unit that transports the feed material to the spinneret. Second, it acts as a pump in which the feed is compacted or compressed and later (after the polymer has melted) forced through the various obstacles ahead. Third, it acts to work the melt and to make it more homogeneous. The barrel acts as part of a heat-exchanger to maintain the temperature of, or to melt, the moving polymer. The molten material has to be filtered (perhaps with the generation of extra pressure) before it passes to the spinneret. The first part of the process is part of the extruder head and comprises the phases of propulsion, compression, heating, working, filtration, metering, and extrusion of the polymer through the spinneret. Filtration and metering will be discussed later. The second part follows and comprises quenching, drawing, and winding the filaments. Clearance between the screw and the barrel (Fig. 2.12) is of some importance. If the clearance is too large, there is appreciable pressure loss and the molten polymer leaks backwards down the screw. If the clearance is too small the screw may seize up. As the barrel is heated, the bore diameter increases due to metal expansion, and as it cools the bore becomes smaller due to contraction. Because of thermal inertia, the barrel can cool down faster than the screw. Care has to be taken to ensure that the contracting barrel will not grip the screw and cause a seizure. The procedures for starting and stopping have to be carefully executed to avoid such mechanical seizures. D X B X

Screw

Clearance

A

Barrel Polymer

Fig. 2.12

Cross-section at X–X

Polymer flow in the extruder barrel

44


Of course, the extruder should be empty or the polymer in the barrel should be heated to liquefy it before any attempt is made to start. If the polymer were to become crosslinked due to oxidation and it were no longer possible to melt it, the material would have to be chipped out mechanically. It is therefore normal to operate extruders continuously for 24 hrs/day, 7 days/week, to avoid these problems. The screw rotates and the movement of the surface of the helical portion in direction D causes the polymer to move in the direction C (Fig. 2.12). Polymer flows along the groove in the screw, and the mass flow, Q, at any cross section such as (X-X) is given by: Q = ρAV

[2.1]

where Q is the mass flow ρ is the density of the polymer (defined as 1/specific volume) A is the cross-sectional area V is the mean velocity component in direction of flow. As the chip is compacted, liquefied, and then pressurized, ρ changes and it is necessary for AV to change accordingly. Consequently, the core of the metal screw is tapered with the thick end towards the outlet. The forced flow pressurizes the fluid polymer ready for delivery to the pump/filter system that precedes the spinneret. The spinneret should produce one filament per hole; thus for a normal yarn, it must contain many fine holes. The shape of the holes determines the cross-sectional shape of the filament; however, the cross-sectional areas of the filaments vary from those of the spinneret holes for the reasons discussed later. Hot fluids usually circulate through channels in the barrel to provide the heat rather than using direct heating. Heat flow into the polymer is equal to a proportion of the heat flow from the heating medium plus local frictional heating. The absorbed energy is carried away from the system by the polymer as a change in state from solid to liquid and/or as a change in temperature. Heat transfer properties are also affected by the changes in state and temperature. The frictional heating component and the heat transferred from the heater are directly affected by the changes in the polymer and there is a very complex interactive situation. Local pressures, specific volumes, coefficients of friction, and viscosities of the melt vary according to the temperature of the polymer. Since the actual extrusion through the spinnerets is highly dependent on the viscosity of the melt, careful control is required of the variables. The complexity of the operating conditions, plus the ambiguity in flow caused by the multitude of parallel flow streams, leads to the possibility of uneven distribution of the polymer flow. This phenomenon, known as ‘channeling’, can cause more polymer to flow through some spinneret holes than others. The result is that filaments have differences in linear density. Furthermore, the flow pattern can change continually during operation, particularly if the polymer viscosity is incorrect. Operating under such faulty conditions leads to quality control problems concerning ‘denier’ variations. It is important to protect the polymer at high temperature from oxidation. Any such oxidation causes changes in viscosity, cross-linking, and deterioration in the final product. Consequently, antioxidants are usually included in the original polymer chip or molten polymer supply Also, hydrolytic degradation is limited by drying the polymer just before extrusion. Filtering and metering The material leaving the screw may not be perfectly homogeneous. It is particularly important to remove any hard elements or highly viscous concentrations (i.e. gels)


from the fluid polymer stream lest they block the very fine holes in the spinneret. The metal block in which the holes are drilled is called a die. Such blockages not only interrupt the individual fluid streams from the affected holes but, even if the impediment removes itself, it is unlikely that a filament will be re-established. Instead, one is likely to get a drip (which is unoriented). Such unwanted polymer drips can coalesce with adjacent filaments and the result is a fault that can seriously affect subsequent operations in staple or textured yarn manufacture. For these reasons, it is normal to filter the molten material before it reaches the spinneret. Metal webs, fabric, or carefully graded sand is often used for this purpose, but in the latter case the body of the filter has to be carefully constructed to preclude particles of sand from clogging the spinneret holes. The filter introduces a high shear stress in the polymer, which affects its viscosity and further complicates matters. In some cases, the filter assembly is made in two parts, one of which is waiting to be used while the other one is in use. At an appropriate time, when the pressure drop has risen or a given time of use has expired, the second filter is substituted. This enables the first one to be cleaned at leisure. If the linear density of the filaments is to be maintained, it is necessary to accurately control both the liquid flow rate and the yarn or tow take-up rate. To accurately control the flow rate of the liquid polymer, it is necessary to use a metering device (which is usually in the form of a pump), so that changes in viscosity and viscosity distribution shall have little effect on the mass flow rate. The pressure upstream of the metering pump has to be controlled so that it is not adversely affected by the metering device. Leakages in the metering pump can also adversely affect the denier of the filaments. Such variations are not easy to see at the extrusion stage and therefore very careful observation and testing are required to give a high quality product. Any variations permitted to go unchecked are likely to show up in the final product as faults in dyeability and bulk which will create customer complaints. Diameter D1 (Fig. 2.13), and polymer density (ρ) determine the linear density of the filament. This, in turn, is determined by the mass flow (Q) and the velocity of take-up (V). The mass flow is the same at all cross-sections. Hence, Equation [2.1] may be applied. It will be seen that the size of the hole in the spinneret, D1, plays no direct part in determining the linear density of the filament. However, the shape and size of the hole determines the flow lines in the polymer as it begins to solidify, and the shape of the hole does affect the cross-sectional shape of the filament. Also the shape of the hole, viscoelastic variables of the polymer, and the speed of take-up affect the ratio D1/D3. If the polymer solidifies quickly after leaving the spinneret, these factors can materially affect the morphological character of the filaments produced. This is

Portion of spinneret nozzle

D1

Polymer flow

Bulge

Hot pin

Heat

Fig. 2.13

Polymer bulge

D2 D3

46


because the crystal nucleation is affected by variations in shear stress, temperature, and viscosity in the regions near to the spinneret holes. Also, under certain conditions, there can be periodic changes in D2, due to vibrations within the polymer stream, and these vibrations can lead to variations in the denier of the fibers. Filtering does not reduce the debris generated at the exit of the extruder die and it is possible that such debris might cause trouble in ensuing processes. Good housekeeping at the extrusion stage is an essential ingredient of quality control. Quenching Liquid emerging from the spinneret has to be converted to a solid filament at a reasonable distance from the spinneret face (i.e. the corresponding temperature has to fall below Tm). Unless this occurs, it is very difficult to control and draw the filaments at that stage. With slow crystallization, it may be extremely difficult to handle the filaments once they are produced because of the lack of orientation. Hence, it is normal to quench the newly emerging material with a low speed flow of dry air or inert gas, usually blown perpendicular to the polymer stream. It is important to restrict the velocity of the gas flow to prevent one molten (or semi-molten) filament blowing into the path of an adjacent one. When such filaments touch, they will usually cohere and produce ‘married fibers’, which can be a great nuisance. Materials with significant numbers of married fibers or polymer drips cannot be used and are scrapped. Equal distribution of the quench medium is also important because of the necessity for equal cooling rates throughout the whole filament bundle. Unequal cooling rates not only vary the morphology of filaments across the bundle but also make some filaments more likely to break than others. This, in turn, affects the rate of creation of undesired drips. In any case, the ill effects would show up in the final product as changes in dye affinity. At very high production rates, the speed of the filaments affects the quenching rates significantly. Where it is intended to orientate the polymer during extension, the filaments must be cooled quickly before the effects of stream orientation are dissipated. Elongational forces acting on the viscous fluid passing into the draw-down zone, where the semi-molten polymer changes to a solid, tend to align the molecules. If cooled quickly, such orientation can be frozen to give a material which can be handled and which might be, if research results can be transferred to commercial application, suitable for use in draw-texturing. The relative velocity of the quench air affects the Reynolds Number of the air ‘skin’ surrounding the polymer stream and this in turn affects the heat transfer or cooling rate in the quenching phase. (The Reynolds Number is the ratio of viscous and inertia forces; it is a dimensionless parameter useful in normalizing the mathematical units.) The cooling rate helps determine the morphology of the POY. Perhaps the most difficult problem concerning quenching occurs in tow production because of the number and size of the spinnerets as well as the density in which the filaments are packed in the extrusion zones. Also, with the large number of ends, the chances of a break are much greater. Filament take-off and drawing Solidified filaments are gathered and carried from the spinning zone by devices that grip them without squashing them. The filaments are often wrapped around rotating cylinders or ‘godets’ and the capstan friction generated applies enough driving force to withdraw them, draw them and transport them to the take-up system. The speed of


take-up is very high but it is very rare for the filaments to be drawn significantly at this stage. After the draw stage, the filaments are at least partially oriented and the delivery speed is even higher than that of the take-up. However, it is necessary to draw the freshly extruded filaments at some stage, to orient the molecular structure and give the desired mechanical properties. It might also be mentioned that it is extremely important that the drawing be uniform from filament to filament and along the length of each filament. Thus, any mechanical inaccuracies in the draw rolls produce periodic variations in the draw. A common cause of this sort of error is due to the uneven build-up of finish on the rolls (see next section). Unfortunately, this not only causes variations in linear density, but also variations in dye affinity; such variations lead to streaking and barré. The drawing of a yarn can produce filament to filament variations in draw ratio and this can produce similar undesirable effects. During normal drawing, a neck (Fig. 2.13) is formed, and the position of this neck usually has to be stabilized by a hot pin or plate. Undrawn polymers change their characteristics fairly rapidly; this is referred to as ageing. The more the polymers have been drawn, the slower the ageing takes place, and fully drawn filaments have very long shelf-lives. In drawing filaments, ageing of the spun, undrawn yarn has to be controlled because it affects the natural draw ratio, the drawing tension, and the physical characteristics of the material. The phases of extrusion and final drawing occur at different locations when working with POY, and the material is stored during the interim. Consequently good inventory control is needed to keep the product within acceptable time limits between extrusion and final drawing. To start up a high speed drawing operation, it is necessary to use an aspirator. Such a device sucks the yarn from the spinneret (or other source) as fast as it is produced (it being realized that the source cannot be shut off in many cases). The ends are then ‘painted’ round the threadline and are wrapped around the take-up godet or rolls before cutting free the material inside the aspirator – a simple operation that needs skill to execute. Fiber finish and treatments Fiber finishes are necessary to lubricate the fibers or filaments and to reduce static electrification during subsequent operations; these finishes are mostly applied by the fiber maker. Without such ‘spin finishes’, the increase of drag due to the high coefficients of friction might cause end-breaks or other processing difficulties. Static electricity causes fibers to attract or repel one another, and causes some fibers to adhere to other surfaces (such as machine parts). In either case, a high degree of static charging causes considerable difficulty in processing. The fiber finishes can, in some cases, be used to provide a degree of cohesion between the filaments by acting as a sort of size, similar to that used in weaving. They also protect machine surfaces from wear and can prevent local fusion of fibers (especially at points where the fibers or filaments rub guides and other machine parts during high speed winding). Fiber finishes usually comprise a base lubricant, an antistatic agent, an emulsifier or solubilizing agent, and various special additives. The special additives include bactericides, antioxidants, and friction modifiers. The base lubricants are usually alkyl esters of fatty acids, hydrocarbon oils, waxes, vegetable oils, or mixtures thereof. These finishes have to be formulated with a regard to their sorption, moisture uptake, and surface tension characteristics, as well as to their effect on the dielectric and flow properties of the finish. Care also has to be taken to control the volatility, smoke potential, and flash

48


point of the finish so that problems in subsequent processing are minimized. These factors are particularly important in texturing, where surface temperatures of the fiber can reach high levels. During processing, particles of finish and fiber become detached and deposited on various machine surfaces. Two examples of the problems caused by this may be cited. In texturing, they can form deposits on the heaters and other working parts. In rotor spinning, deposits in the rotor can be troublesome as discussed in Chapter 7. It is very important that the amount of finish applied to the fiber be strictly controlled and that the nature of the debris should be such as to minimize difficulties in the ensuing processes. Also, the finish should have no detrimental effect on the package including its shelf-life. Sometimes the polymers include substances such as titanium dioxide (TiO2) as brighteners or other modifiers. Brighteners hide any tendency to yellowness in fabrics made from the fibers and makes colors more brilliant, but sometimes the additives are abrasive. For example, fibers containing TiO2 tend to wear guides and it becomes necessary to use ceramics at the wear points and to avoid frictional contact as much as possible in the design of the yarn-handling portions of the equipment.

2.3.2 Man-made staple fiber production Tow Fiber produced for the manufacture of man-made staple yarn is first produced as tow. The word ‘tow’ has many meanings but in the present context it means a thick bundle of continuous filaments. Tow has to be cut, broken, or abraded to convert it into staple fiber. The abrasion technique is restricted to light tows and for a very few specialty purposes; it will not be further discussed here. Fiber makers cut tow and blend it before baling to ensure uniformity of the product. A very large volume of fiber is cut in this way for short-staple spinners. Some longstaple is also dealt with in a similar way but some is supplied to the mill in tow form. This tow is cut or stretch-broken in the mill. Tow intended for stretch-breaking (see below) in the mill usually has a linear density of about half a million denier. (The linear density of tows for use by the fiber makers for cutting into staple is many times greater.) It is difficult to find common ground in the early processes because the needs vary so widely, as can be seen by examining Table 2.2. Stretch-breaking tow Stretch-breaking is a form of drawing in which the draw ratio exceeds the breaking elongation of the filaments, with the consequence that they break as they pass through Table 2.2

Fiber to sliver conversion

Process fiber Filament tow Bast fiber Wool Cotton Man-made SS Baled MM LS

Clean mech

Clean chem

X X X

X X **

Cut or break in a mill X * * **

Open

Card

X X X X X

X X X X X

Notes: X = always, * = in some cases only, ** = rarely, SS = short staple, MM = man-made, LS = long staple.


Roll stand

Roll stand

the draw zone and create staple fibers. The most popular use of stretch-breaking is to produce long-staple sliver from which high bulk staple yarns are made. There are several phases in the process which will be described separately but, in fact, all the phases are often incorporated into a single machine. The phases are: (1) heat the unbroken filaments and cool under tension, (2) break the fibers by applying elongation stress, perhaps accompanied by a beating action, (3) relax the fibers by heating to create bulk in the product. There may be one or more repeat stages of phase (2) (called re-breaking) before stage (3). In its simplest form, the machine produces a variable fiber length and the mean length is determined by the ratch setting (the distance between consecutive roll pairs in a roller drawing system). To break a large bundle of strong filaments requires a very robust set of drawing elements with strong gripping power. For this reason, the total fiber denier must be limited simply because the load cannot exceed the gripping power of the rolls. Also damage to the rolls has to be avoided. If load-sharing between filaments in a disorganized bundle is poor, uneven breaking will occur. Thus, it is desirable to have a sheet of parallel fibers entering the break zone but this is not practicable. In the early stretch-breaking machines it was not possible to process tows heavier than about 100 000 filaments of 1.5 dpf (denier per filament). More modern machines can process tows of up to 500 000 filaments and the acceptable fiber fineness has increased also. The exact specification of machine capability depends on the fiber because the fiber properties obviously play a large part in determining acceptable loads. The loads on the rolls are measured in tons and the machines have to be very robust. The machines are mainly used as tow-to-top machines that produce sliver. A ‘top’ is the name for a sliver as used in the wool processing mills. It is usual to heat the filaments above Tg (glass transition temperature) whilst they are under tension and allow them to cool before the tension is released (phase 1). The heat-stretch phase of the process (Fig. 2.14(a)) reduces the breaking elongation in the stretch-break zones and makes this part of the operation easier. After the heat-stretch

Input tow

Heattreated tow

Heat flow

Sheet of tow under tension Cold air (a) Heat-stretch Breaker bars

Heattreated tow input Roll stand (b)

Fig. 2.14

Stretchbroken tow output

Roll stand Stretch-break

Stretch-breaking tow

50


zone come the stretch-break zones where the cooled, heat-stretched tow is broken into staple fibers (phase 2). The locked-in extension is released when the fibers are reheated above Tg and this process causes shrinkage (phase 3). This is a valuable way of inducing bulk in the material. Older machines used intersecting breaker bars to control the staple length (Fig. 2.14(b)). This practice is declining and stretch-break/re-break systems are taking their place. The re-break stage is merely the stretch-breaking of sliver that has already been stretch-broken once; the second stage selectively breaks the longer fibers and reduces the variation in length. The intersecting breaker bars have an onerous duty and wear rates are something of a problem. Modern machines are very robust and are designed for very high speeds. The capital cost is high but they can be cost-effective where 100% manmade fibers are to be processed and the system can be integrated into the operation without undue disruption. Stretch-breaking not only changes the linear density of the bundle by drawing but it also changes the linear density of each of the filaments. The filaments are stretched to their breaking point and this involves an elongation of the fibers. Elongation is accompanied by a reduction in linear density of the fiber; the change in dpf (denier/ filament) can be significant. The fact that the fibers are stretched whilst heated causes flats to form on their surfaces [30] and this gives the resulting yarns a greater crispness in hand than otherwise would be the case. Fiber cohesion is low in freshly broken tow and, to be able to manipulate the material, it is necessary to improve it by fiber crimping. The usual crimper is a stuffer box in which the sliver is fed to a heated stuffer chamber at a speed faster than the offtake in a manner similar to that described in Chapter 4. Fibers buckle under the compressive load and become crimped (16 to 20 crimps/inch is normal). Crimped fibers cohere well and a sliver made of such fibers can be handled and carded properly. Where breaker bars are used, significant amounts of fly (airborne fiber and debris) can be generated and this fly must be taken away from the breaker zone otherwise the product becomes contaminated, to the detriment of following knitting operations. One great advantage of the stretch-breaking process is that it produces high bulk yarns [31]. Bulk is generated by the differential shrinkage of the fibers, the stage being set for this in the heat-stretch zone. Not all fibers suffer the same tensions or reach the same temperature with the consequence that not all of them shrink equally. The fibers that shrink the most cause the others to become compressed along their length; the compressed fibers buckle and the buckled fibers take up more space. Thus, a stretch-broken sliver is naturally bulky, but the effect can be heightened by mixing non-heat-stretched sliver with heat-stretched sliver at the drawframe and then autoclaving (heating with steam under pressure) to produce the shrinkage required. These bulky stretch-broken yarns are a close approximation to wool yarns and they produce soft ‘woolly’ fabrics. The yarns are often referred to as high bulk staple yarns. Whilst the traditional way of developing the bulk was to use an autoclave, some modern machines have a continuous heating system attached to them, which fulfills the same purpose. The operating temperature is about 240°F (115°C) and steam is often used as the heating medium (see Appendix 3). Within limits, an increase in heater temperature or draw ratio generally increases the tenacity of the fiber, but too high a temperature leads to degradation of the polymer, which, in turn, leads to a loss in strength. Too high a draw ratio or too low a heater temperature leads to end-breakages (i.e. stoppages) during processing and causes increased amounts of


fly. Stretch-breaking is technically possible for tow-to-yarn systems (direct spinning) as well as tow-to-top systems (tow-to-sliver) but the high cost of tows of suitable quality renders the system uneconomical for direct spinning. Also there is no chance of blending between the outputs of different machines to reduce the risk of barré. The object lesson here is that production efficiency cannot always be reconciled with quality of product. Cutting tow for long staple In cutting tow to produce long-staple fibers, a spiral cutter is commonly used, which meshes with a smooth, hardened anvil roller as shown in Fig. 2.15(a). The tow is spread out into a sheet of uniform thickness before passing through the cutter. The staple length is controlled by the pitch of the cutting edges and, to a lesser extent, by the angle at which the fibers pass through the system. Also, involuntary variations in fiber attitude cause a spread in staple length, Fig. 2.15(c). It is possible, by altering the angle at which the tow passes through the cutter, to slightly change the staple length as shown in Fig. 2.15(d). Only minor changes can be made by altering this angle and any major change requires that a different cutter be used. Any damage to a cutting edge is likely to allow double-length fibers to be discharged and these can cause difficulties in the following drawing and drafting processes. For this reason, the cutting edges are not razor-like but are rectangular, and function by locally crushing the filaments at the point of contact between the cutter and anvil roller. There is difficulty in handling fine denier fibers if the cutter is not precisely set and in perfect condition. Maintenance of the cutter is a vital part of the operation. Fibers tend to be bonded along the cuts by the pressure exerted by the cutter and this is undesirable. Therefore, the ribbon is caused to flex to create shear, which debonds the fibers as shown in Fig. 2.15(b). Also, tow leaving the cutter has welldefined lines of weakness along each cut since there can be little fiber entanglement. If the emerging ribbon of freshly cut tow was simply condensed, the resultant sliver would be extremely weak. To overcome this, the sheet is sheared by a process called ‘shuffling’ as shown in Fig. 2.15(e). In the case shown, an apron is used as the bottom element to provide a reaction to the two top rolls. The position of the cut end in the top of the sheet is now displaced from those below as shown in Fig. 2.15(g). The distribution of cut ends disperses the zones of weakness. Finally the sheet of cut fibers is rolled to make a sliver as shown in Fig. 2.15(f). The elements shown in these diagrams are often parts of a single machine so that the input is filament tow and the output is staple fiber in sliver form. Fiber finish and subsequently added dressings are often added in the mill to aid the tow cutting process [32] but such additives can adversely affect the performance of the sliver in the yarn making operation. A dressing that makes the cutting operation easier may cause fibers to cohere in a non-uniform manner. This, in turn, may cause unevenness in the yarns produced. The Pacific Converter type of cutting equipment, which is the type just described, is often used to produce a sliver or top, whereas the cutters used to produce short-staple fibers are quite different. Cutting tow for short staple Long-staple processing is more tolerant of multi-length fibers than is short-staple processing. Short-staple or mid-range systems are less tolerant of fibers greater than the ratch setting of the drafting system because they bridge the drafting zones and either break or slip at the drafting rolls. A ratch setting is the distance between

52

Handbook of yarn production Cutter pitch Uncut input tow

Cutting edges Cut tow Cutting roll Pressure bonding at each cut

Anvil roll

Debonded tow Flexing breaks Bonds along cut lines (b) Debonding

Cut tow output

Cut lines

E D

B A (c)

Cut lines

Tow cutting

Cut lines

(a)

C

F

AB, CD and EF are fibers CD > AB > EF

Effects of varying fiber angles

L2

L1

L1 ≠ L2

(d)

Effects of varying cutter angles

Input sheet of debonded staple fiber

Output sheet of shuffled, debonded staple fiber Input sheet of shuffled, debonded staple fiber Roller Sliver

Vb V t Sheet rolled into sliver (e)

Shuffling

(f)

Rolling

Vt

After

Before (g)

Vb

Vf

Vt > Vf > Vb

Shearing action caused by the shuffling process

Fig. 2.15

Tow-to-top conversion by cutting

adjacent sets of rolls in a drafting system (see Chapter 3). These events disturb the flow of normal fibers. This is detrimental to the efficiency of the process and the quality of the product. One solution is to wrap the tow around a cutter of the type shown in Fig. 2.16, first to create pressure between the filaments and the cutting edges and second to apply internal suction. Few over-length fibers pass into the

Textile products and fiber production 53 Air

Air

Air

Cutter

Tow input

Fig. 2.16

Fiber + air

Tow cutter for short staple

output and the system is suitable for producing short or mid-range staple fiber. The fibers are baled for transmission to the mill. To permit carding, it is necessary for the fibers to be crimped so that there is a degree of mutual cohesion, as was discussed earlier. Tow size and quality is important; the larger the tow, the more difficult it becomes to maintain uniform tension in processing. Lack of uniformity in thickness across the tow sheet can cause problems and, in particular, the tendency for the sheet to fold at the edges can lead to problems. Tow knotting is also an operational problem because the knots have to be removed before cutting. The knot removal operation can leave gaps in the ensuing webs which result in excessive waste. Fiber crimping and finish As discussed, it is normal to crimp the fibers. A ribbon of fibers can be deformed under heat by overfeeding it into a stuffer box or by passing it through the mesh of fine toothed gears (gear crimp) as shown in Fig. 2.17. Alternatively, tow can be stretched hot and then cooled to lock in the extension in the manner previously Feed rolls

Heat

Sliver Vo output

Vi Input

Vi > V o

(a) Stuffer box crimping Heat Cut tow input

Crimping gears

Crimped fiber output (b) Gear crimping

Fig. 2.17

Fiber crimping

54


described. The fibers must be properly lubricated to prevent damage in the opening and carding processes. Also, it is essential that the finish applied to the fiber should minimize any tendency for the fibers to charge electrically due to friction suffered in processing. Electrostatic charges interfere badly with normal processing and application of a suitable finish in appropriate quantities is very important. As pointed out earlier, any finishes applied to aid the conversion to sliver must not cause it to perform badly in the yarn manufacturing operation. General comments Conversion of tow to sliver is a short mechanical process that can be described in relatively few words. Nevertheless, adequate quality control involves not only the mechanical processes but also the chemical and morphological characteristics of the polymer and fiber finish. Brevity of explanation should not be taken to mean that any one of the processes is unimportant.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

Cook, J C. Handbook of Textile Fibers, Merrow Publishing, Watford, UK, 1964. Von Bergen, W and Krauss, W. Textile Fiber Atlas, Textile Book Publishers, New York, USA, 1949. Terms and Definitions, 8, The Textile Institute, Manchester, UK, 1986. Textile World, McGraw Hill, New York, annually. Encyclopedia Brittanica, Wm Benton, London, 1964. Tagart, W S. Cotton Spinning, Vol 1, pp 20–6, Macmillan, London, 1913. Trolinder, N L. How to Genetically Engineer Cotton, and other papers by various authors, Proc Beltwide Cotton Conferences, pp 165–77 National Cotton Council, USA, 1995. Lalor, W F. Sticky Cotton Action Team Activities, 7th Annual EFS Conf, Cotton Inc, 1994. Norman, J W, Sparks, A N and Riley, D G. Impact of Cotton Leaf Hairs and Whitefly Populations on Yields in the Lower Rio Grande Valley, Proc Beltwide Cotton Conf, pp 102– 3, National Cotton Council, USA, 1995. Hendrix, D L. The Relationship between Whitefly Populations, Honeydew Deposition, and Stickiness in Cotton Lint, Proc Beltwide Cotton Conf, p 104, National Cotton Council, USA, 1995. Sasser, P E. Automation and Validation of the Sticky Cotton Thermodetector, Proc Beltwide Cotton Conf, p 105, National Cotton Council, USA, 1995. Websters New Collegiate Dictionary, p 202, G & C Merriam Co, Springfield, Mass, USA, 1963. Lalor, W F, Willcut, M H and Curley, R G. Cotton Ginners Handbook, USDA, Agricultural Handbook No 503, p 21, Washington, DC, 20250, 1994. Shaw, D L, Cleveland, O E and Ghetti, J L. Economic Models for Cotton Ginning, Texas Tech University, College of Agriculture, Scientific Publication No T-1-158, Texas, USA, 1977. Anthony, W S. Cotton Ginners Handbook, USDA, Agricultural Handbook No 503, pp 43–6, Washington, DC, 20250, 1994. Sutton, R M. 198 Gin Stand and Dual Roller Lint Cleaner, Proc Beltwide Cotton Conf, National Cotton Council, USA, 1995, pp 50–1. Mayfield, W D, Anthony, W S, Baker, R V and Hughs, S U. Cotton Ginners Handbook, USDA, Agricultural Handbook No 503, p 237, Washington, DC, 20250, 1994. McPhee, J R. The Mothproofing of Wool, Merrow Monographs, Merrow Publishing, Watford, UK, 1971. Garnsworthy, R, Mayfield, R, Gully, R, Westerman, R and Kenins, T. Proc Int Wool Text Res Cong, III–190, Tokyo, Japan, 1985. Wool and the Wool Trade, 2nd edn, Riverside Press, Cambridge, USA, 1955. Carnaby, G A, Stanley-Boden, I P, Maddever, D C and Ford, A M. Mathematical Concepts and Methods in the Industrial Utilization of the New Zealand Wool Clip, J Text Inst, 79, 1, 1988.

Textile products and fiber production 55 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32.

Ryder, M L. The Production and Properties of Wool and other Animal Fibers, Text Prog, 7, 3, 1975. Whitely, K J. Quality Control in the Processing of Wool and the Performance of Wool Textiles, J Text Inst, 79, 3, 1988. Anderson S L. Textile Fibres: Testing and Quality Control, Manual of Textile Technology, Textile Institute, Manchester, UK, 1983. Anson, R. Natural Alternatives, Text Horiz, ITBD, London, Dec 1995. Tsukada, M, Shiozaki, H and Urashima, H. Changes in the Mechanical Properties of Silk Fabrics Modified with Epoxide and their Relation to the Fabric Construction, J Text Inst, 80, 4, 1989. Matsumoto, Y, Harakawa, K, Toriyumi, K and Tsuchia, I. A Study of Throstle-spun Silk/ Raw-silk Core-spun Yarn, J Text Inst, 82, 4, 479, 1991. Mukhopadhyay, S K. The Structure and Properties of Typical Melt-spun Fibres, Text Prog, 18, 4, 1989. Brunnschweiler, D and Hearle, J W S. Tomorrow’s Ideas and Profits: Polyester, 50 years of Achievement, The Textile Institute, Manchester, UK, 1993. Wray, G R. New Trends in Yarn Production, Modern Yarn Production, (Ed G R Wray), Columbine Press, Buxton, 1969. Reid, J J. Tow-to-sliver Stretch Breaking, Modern Yarn Production, (Ed G R Wray), Columbine Press, Buxton, 1969. Kirk, E. Tow-to sliver Cutting Methods, Modern Yarn Production, (Ed G R Wray), Columbine Press, Buxton, 1969.

3 Common principles

3.1

Introduction

Many principles relating to yarn production apply across the range of processes. For example, eccentric or deformed machine elements can produce periodic errors in filament yarns just as much as in staple yarns. An understanding of the common principles is vital to the comprehension of the technologies involved. The spectrum covers many technologies and yarn production systems. It includes the mechanics of drawing, doubling, and twisting. For reasons of economy, it is useful to discuss some of these common principles before dealing with the details of each process. Readers unfamiliar with the industry will find reference to Appendices 1 and 2 helpful.

3.2

Twist in strands

3.2.1 Purpose of twist In the present discussion, the words ‘twisted strand’ encompass any yarn or intermediate product that is twisted. Staple yarns or rovings are twisted to induce lateral forces. Friction created by these forces acts to control fiber slippage in a strand under tension. A simple experiment can demonstrate this. Take a length of sliver and it will be found that fibers can be withdrawn from the ends with ease. If the sliver is now twisted tightly, it will be observed that the diameter decreases as the twist is added. Lateral forces act to compress the strand and bring the fibers closer together. As the fibers are pressed together, it is harder for the fibers to slip and it will be found that it is difficult to break the twisted sliver. If the sliver is then untwisted, it becomes weak again; failure is caused by fiber slippage. A yarn is false twisted when a torque is applied to a running yarn, and the consequences are highly significant. Examples include texturing, rotor spinning, ring spinning, roving production, and yarn splicing. Distinctions between real and false twist will be drawn in the following text and the importance of each will be discussed.

Common principles 57

3.2.2 Twist direction It is necessary to define the direction of twist before continuing. Referring to Fig. 3.1, the direction of twist can be determined by matching the visible surface fibers to the center portion of the letter S or Z, whichever is appropriate. The convention is to refer to S twist or Z twist according to the direction. It is also conventional to spin single yarn in the Z direction but to ply in the S direction.

Fig. 3.1

Twist direction

3.2.3 Twist and flow Twist multiple and linear density are the most important parameters in determining the character of a twisted yarn made from a given fiber. However, in setting up a spinning machine, the linear velocity of the strand and the rotational speed of the twister must be set in their correct proportions to produce the required yarn. The relative speeds are controlled by a transmission system consisting of gear trains and belt drives. In practice, a single gear (called a twist gear) is changed to alter the velocity ratio, which gives the required rate of advancement of the strand for the given spindle speed. The rest of the transmission is typified by a twist constant for the machine. Twist density is often measured in twists/inch (tpi), which is calculated from the ratio of the twist constant and the number of teeth in the twist gear. Twist density can also be measured in the metric system. Some simple examples of such calculations are given in Appendix 2.

3.2.4 Effect of twist on a staple yarn Consider a simplified model of a staple yarn where a number of fibers at a given radius exist in a roughly helical configuration. Each fiber is under tension and a series of resultant forces acts towards the center of the fiber bundle (see Appendix 5). Taking all the fibers at a given radius, there is a sort of tube of fibers, all of which press inwards on the bundle of other fibers inside the tube. If a sufficiently high pressure is maintained on the inner fibers, there can be little fiber slippage and the whole structure becomes capable of bearing load. This requires that at least the outer shell of fibers should be kept under tension. One way to do this might be to tuck in the ends of each of the outer fibers rather in the manner of cord ends tucked in when whipping the end of a fishing rod. Although this is impracticable for yarn production,

58


Yarn tenacity

it serves to show a principle. In fact, the actual process of spinning causes portions of the outer fibers to be entrapped in the inner structure. Fiber migration is the name for this phenomenon and it causes the whole of the structure to interlock. In an ideal yarn, if the fibers were totally unbreakable, the strength of the yarn would increase with twist in the manner indicated by the cohesion curve in Fig. 3.2. Yarn failure occurs because the fibers slip over one another. On the other hand, in a yarn where the fibers cannot slip but must break, the strength would decline with twist. This is because of the reduced components of fiber tension resisting breakage as the twist angle increases1; it is shown as the obliquity curve in Fig. 3.2. ‘Strength’ is usually quoted in normalized units; it is then referred to as ‘tenacity’. Tenacity is the quotient of force and linear density. The linear density is often quoted in tex and the units commonly used for tenacity are mN/tex. It has the virtue that the value is similar for all staple yarns made from a given fiber and twisted to the optimum degree, irrespective of linear density. It is, therefore, a useful comparative measure of the strength of the yarn. As will be discussed later, twist is usually quoted for this purpose as twist multiple (TM). The manner of calculating TM is dealt with in Appendix 1. This is also a normalized value and the yarn characteristics vary little for a given fiber length. Thus, diagrams similar to Fig. 3.2 serve as models for all staple yarns made from fibers of a given length and strength. Real yarns are made from fibers that can be broken and which do slip. The strength of a twisted bundle varies, as shown in the lower curve of Fig. 3.2. It will be seen that twist weakens a fiber bundle by making the fibers oblique with respect to the yarn axis; too much twist seriously weakens the yarn. This eventually overwhelms the increased cohesion at twist levels above the optimum. The so-called obliquity curve refers to fibers oriented at an oblique angle. The yarn strength has an optimum value that is less than the sum of strengths of all the fibers in a cross-section. Above the optimum twist, the yarn fails by fiber breakage and a distinctive snap can be heard when the yarn breaks. If there is appreciable fiber slippage during the breakage because the twist is well below optimum, no snap can be heard. Sometimes, where strength is unimportant, yarns are produced at less than the optimum twist for economic

Obliquity curve

Cohesion curve

Actual result

Optimum twist

Twist multiple

Fig. 3.2 Effect of twist on yarn strength

1 Simple trigonometry shows that the component of tension contributing to strength = T cos β, which indicates that the helix angle of the fiber (β) is very important in determining the strength of the yarn. When β is very small, as in the case of some filament yarns, an anomaly arises because of maldistribution in loads between fibers. The yarn tenacity at zero twist may be slightly less than that achieved when the yarn has producer twist.


reasons. Sometimes the twist used is below optimum to give a soft hand to the product. The cohesion curve can be changed by altering the staple length, l, of the fiber or by altering the effective coefficient of friction, µ. The latter is altered by varying the fiber lubricant (i.e. fiber finish) or the crimp level of the fiber. If l or µ is increased, the cohesion curve moves from curve D to curve C along path x in Fig. 3.3. The actual tenacity curve also alters to reflect these changes. Providing the obliquity curve remains the same, the optimum moves from point B towards point A. It will be seen that the optimum twist level is reduced and the maximum tenacity is improved by increasing the staple length or the interfiber friction. This explains why a premium is placed on the longer staple fibers and why short fibers are often removed from the material to be spun. As will be discussed in Chapter 8, there is a limit to how long the fibers can be before there are processing difficulties. However, there is another limit; fibers beyond a certain length add very little to the resistance to fiber slippage. For indirect count systems twist density = TM √N and in the direct systems twist density = TM/√n both measured in twist/unit length.2 The symbols ‘N’and ‘n’ stand for indirect yarn count and the direct yarn count (or linear density) respectively as defined in Appendix 1. Some people use alpha to describe the metric version of direct twist multiplier. Since TM describes the nature of the yarn, it does not vary greatly within a units system (see Table 3.1). It will be realized that the twist density required (tpi) is strongly dependent on yarn count. Thus, to set up a spinning machine to make a certain class of staple yarn, it is necessary to know the specified TM and count of the yarn before the twist density can be calculated. The need for twist rises with count; this is why fine yarns are more expensive than coarse ones (the adage ‘twist costs money’ comes to mind).

O

Yarn tenacity

C

A D B

X

Twist multiple NB O = Obliquity curve C = Cohesion curve 1 A = Actual curve 1 X = Path of optimum tenacity

Fig. 3.3

D = Cohesion curve 2 B = Actual curve 2

Optimum twist

2 ASTM D861 uses twist density in t /cm rather than t/m.

60


Table 3.1

Typical twist multiples

System

Length

Use

Ne cotton count

Nm metric count

Nw worsted count

Cotton

Short

Warp Filling Hosiery

4.0–5.0 3.2–3.8 –

120–150 110–115 –

– – –

Cotton

Long


3.4–3.8 2.5–3.0 2.2–2.6

100–115 75–90 65–80

– – –

Wool

Long


– – –

65–75 55–65 45–55

1.8–2.0 1.5–1.8 1.4–1.5

3.2.5 Ply twist Where durable and pliable yarns are required, it is the practice to twist several yarns together and this is called plying. A singles yarn might, when relaxed, tend to take up a shape sketched in Fig. 3.4(a). It is then termed twist lively. It is usual to ply in the direction opposite to that in which the component strands were originally twisted so that the resulting plied yarn is no longer twist lively. Fibers on the outside of the ply normally appear to be roughly parallel to the axis of the yarn. Such a yarn produces a result similar to that in Fig. 3.4(b). If the amount of ply twist used is just sufficient to remove any residual torque (i.e. the plied yarn is non-twist lively, or ‘dead’), the yarn is said to be balanced. Such balanced plied yarns are useful in reducing difficulties in handling the yarn during any post-spinning processes, as well as in lessening distortion of knit fabrics. Twist structure is normally described in shorthand fashion. A singles yarn of count Ne = 20 is usually written as 20/1 (or 20s). When two such yarns are twisted together to make a plied yarn, the equivalent count3 is 10equ. The ply yarn is described as a 20/2 (but in some areas it is described as 10/2, which is meant to indicate 10equ/2). If four 20/1 yarns are plied, the result is a 20/4 yarn that has an equivalent count of 5equ. The designation ‘equ’ stands for equivalent and it is usually omitted, which is confusing. It is useful to check the context before working with yarn numbers for plied or cabled yarns. With worsted yarns, the order of the numbers is usually reversed; if two yarns

(a) Twist lively

Fig. 3.4

(b) Non-twist lively

Twist liveliness

3 See Appendix 1 for calculations. There are alternative designations to indicate that the number refers to equivalent count.


of Nw = 40 are plied together the result is designated 2/20equ or 2/40. The ply twist multiple is calculated on the equivalent count. When plied yarns are twisted together to produce a complex structure, this is referred to as a cable. Such cabling has a structure that is much more flexible than a simpler one. An example will illustrate how the twist structure is designated: if six 20/2 yarns are twisted together, the result is a cable, which is sometimes described as 20/2/6; the equivalent count is 10/6 = 1.66s. In some areas, the numbers are written in a different order. With cables, there could be an ambiguity and care should be taken to check the context.

3.2.6 Twisted filament yarns It is unnecessary to twist continuous filament yarns to impart strength; nevertheless, some small amount of twist is inserted merely to control the fibers. An untwisted bundle of filaments is difficult to handle because odd filaments and loops project from the surface of the bundle. These tend to catch up in guides, tangle with adjacent yarns, or otherwise cause difficulty. Some man-made fibers tend to balloon out quite severely because they accumulate electrical charge. Filaments or loops protruding from the yarn are often called wild filaments. Even a low level of twist in the yarns helps to reduce the number of these wild filaments; twist inserted for this purpose is called producer twist. Filament yarns are sometimes twisted to a fairly high level to break up the luster of the yarn or to impart some other attribute to the yarn for effect purposes. However, high twist levels reduce the tenacity of the yarn and make the yarn leaner (i.e. have a smaller diameter). Another use of twist in filament yarns is to create texture. A false twisted yarn will coil or snarl if it is subject to the correct sequence of twist, set, and untwist. If properly relaxed, these textured yarns become bulky and have many desirable features. A major advance was made when it was realized that the process of false twisting provided the opportunity to carry out such a sequence in a continuous manner. To understand how that works, it is necessary to be knowledgeable about false twist.

3.3

Twist insertion

3.3.1 Real and false twist So many practical cases involve false twist that it is thought desirable to discuss it in its own right. It is necessary first to discriminate between real and false twist. First, let the word ‘strand’ be used to widen discussion. It is used here to include not only yarns, but also rovings and possibly other forms of intermediate strands. We return now to the subject of false and real twist. Real twist is created when a ‘crankarm’ of a strand is rotated about an axis to insert twist and the material delivered to the takeup package retains all the twist, as shown in Fig. 3.5(a). The theoretical twist remains constant from the point A until the yarn is wound onto the package at a level τ = U/V, where τ = strand twist in tpi, V = linear speed in inches/min, and U = rotational speed in r/min.4 Some of the practicable means of achieving this are described later. 4 Fig. 3.5(a) does not show a balloon or yarn package so that the diagram may also cover two-forone twisting.

62


A

A

Vτ r /m

Flow velocity along the strand axis = V False twister

B

Twist flow

U r/m C C Exit strand is twisted

Exit strand is not twisted

Torque produces real twist

False twist level = τ tpi (b)

(a)

Fig. 3.5

Real and false twist

A typical example of this mode is in ring spinning, where the point C is in the upper reaches of a yarn balloon. The yarn package (not shown) rotates at a speed similar to that of the yarn in the balloon. In consequence, real twist extends over all the length AC. False twist is created when a strand flows through the torque producing means. No twist exists in the strand delivered and the false twist is locked in the system above the twister while the strand continues to run. Systems involving both forms of twist are possible, in which case there is twist in the strand delivered but it is different from the value upstream.

3.3.2 Mechanics of false twist In the simplest form of false twisting, neither the supply nor take-up packages are rotated to insert twist. Thus, the net twist in the delivered strand is zero and twist is trapped upstream. To explain how false twisting works, consider Fig. 3.5(b). A is just below the nip of the feed rollers, B is the point at which twist is applied, and C is the nip of the take-up. Twist carried downstream from B by the yarn is –Vτ twist/min, the amount projected by the twister is +U twist/min, and the twist delivered is the sum of them. The difference in sign is because the false twister generates S twist on one side and Z twist on the other. Twist flow input to zone AB is U twist/min and the loss is Vτ twist/min.


Thus the total twist in zone AB of length L is τL + (U – Vτ)t, where t = time. (U – Vτ) must be zero, otherwise the twist level would change and the system would not be stable. The input to zone BC is –(U – Vτ) twist/min, which results in no twist in zone BC. Thus, the running false twisted yarn behaves as if twist was inserted at A and removed at B.

3.3.3 False twist insertion Only examples of false twisting may be discussed at this stage. Some are by design and some are involuntary. Some uses are aimed at texturing the yarn and some are aimed at temporarily strengthening the strand during processing. For brevity, only one example will be given of each. In texturing, twist is deliberately inserted by stacks of discs. The shape of the filaments and the structure of the twisted yarn are frozen and then the twist is removed, leaving the fibers in a stressed condition. The fibers are separated and the stress is relaxed, which causes the filaments to texture themselves to create a bulky or stretchy structure. In roving, rubber grommets are used at the top of the flyer to provide a combination of false and real twist in the weak strand leaving the drafting system and entering the twister. The rubber surface grips the roving, which is in contact with the inner top surface of the rotating grommet and they move at different speeds. The shear from this pumps twist into the section between the drafting system and the grommet; this is in addition to the twist generated by the rotation of the flyer (discussed in Chapter 6). In this case, the outgoing twist is similar to that determined by the rotational and linear speeds; however, this does not imply that the structure of the strand is unaffected by the false twist. Often of major importance is that the twist in a vulnerable zone is enhanced and the strand is temporarily strengthened. The effect reduces end-breakages with beneficial economic consequences.

3.3.4 Real twist insertion Ancient systems of twist insertion were discontinuous; the yarn did not flow through the machine in the steady and continuous manner employed by modern systems. However interesting these ancient systems might be, space limitations preclude any discussion of them. Conventional systems have endured for some 200 years and they are very well developed as commercial processes. They appear in several forms. Commercial twisting is always carried out as the textile material passes from the supply to some sort of twisting. Where twisting is the sole objective of the operation, either up-twisting or down-twisting may be used. It is usual to use down-twisting when other operational phases are involved in the process (such as drafting). Two of the most common forms of down-twisting are flyer spinning and ring spinning. As the first example (Fig. 3.6), consider the production of singles staple yarn. A stream of fibers is supplied from a drafting system, then twisted, and the resulting yarn is wound onto a bobbin situated inside the yarn balloon. The second example is of up-twisting, where yarn is withdrawn from a package before further processing (Fig. 3.7). Alternatives capable of economic exploitation have been sought and some of these newer developments are discussed later. There may or may not be another process involved. One of these involves two-for-one twisting as shown in Fig, 3.8 (discussed

64

Handbook of yarn production Yarn

Balloon

Bobbin

NB Yarn winds on

Fig. 3.6

Down-twist (down-twist is used to wind yarn on to a package)

Yarn Pigtail guide

Yarn rotates at ωy

Lightweight flyer rotates at ω f

Bobbin rotates at ωb Wind-off speed = ω f – ω b rb

Fig. 3.7

Up-twist (up-twist is used to wind yarn from a package)


in Section 3.3.5); one yarn to be twisted is taken from a package situated inside the two concentric yarn balloons and is wound onto another one after twist has been inserted. An extension to this is found in plying (doubling), where the input consists of two or more yarns and the output is a single composite strand in which the input yarns are twisted together. The twisted yarn is usually wound onto a cheese or cone and this gives an economic advantage. Now let us consider the twisting as an abstract idea somewhat remote from the supply and take-up systems. With conventional technology it is necessary to rotate a package about the axis of the yarn to insert the twist. Depending on whether the upstream or downstream package is rotated, we have either up-twisting or downtwisting. In such cases, it is normal to have a common axis for both the rotating package and the yarn balloon, as shown in Figures 3.6 and 3.7. By having a common axis, it is possible to wind yarn onto (or to unwind yarn from) the rotating package with the same motion used to put in the twist. This makes a very neat design of machine but it requires that the yarn package inside the balloon be small in diameter and limited in height. Also, the yarn has to be laid in organized layers as nearly parallel as possible to its neighbors; this is important if the process is to run smoothly at high speed. Down-twisting is the most commonly used of the choices enumerated. The restrictions imposed by the balloon are substantial. (The details appear in Appendix 9 because the analysis is rather complicated.) To make the system work, there must be not only a twisting system but also a system to control the build of the package. Control is often applied by the motion of the ring-rail as discussed in Chapter 7. The element that is changed to alter the build is called a lay gear. Up-twisting combines twisting with winding and results in a change of package shape. In up-twisting, the package from which the yarn is drawn is rotated to insert the twist. The receiving package is rotated only to wind the strand; thus there is a beneficial separation of winding and twisting. Control of the yarn leaving the package is necessary, otherwise the rate of unwinding might vary with the consequence that the yarn tension would vary. Ideally, a constant tension is required so that a stable and uniform yarn cheese or yarn cone is built. Physical control of the yarn balloon can be achieved by use of a ring and traveler or by a tiny wire flyer such as that sketched in Fig. 3.7. The flyer or traveler rotates at a speed slightly different from that of the bobbin. The wind-off speed = ± (ωf – ωb) rb inches/min, where ωf = flyer speed in rads/min, ωb = bobbin speed in rads/min and rb = (1/2) × diameter of bobbin in inches. The idea is somewhat similar to that used in down-twisting.

3.3.5 Two-for-one twisting The foregoing cases relied on at least one package being rotated to put in twist. However, there is a possibility that requires no package rotation to insert twist. Consideration of the case sketched in Fig. 3.8 will show how this may be done. If the yarn is doubled back on itself to make a loop, which is rotated, then one turn of A inserts one turn of twist in each of legs B and C. Furthermore, the direction of twist is the same in each with the result that the twists add together. With this arrangement there is no need to rotate either package to twist the strand; one revolution of the spindle puts in two turns of twist. Such machines are known as ‘two-for-one twisters’. The problem is that the large package has to be held inside the yarn balloon. The concept first started to be used for tire cords in the 1930s, but it was a further

66

Handbook of yarn production Yarn rotates about XX to form a balloon enclosing the supply package C

X

Yarn from supply package

Yarn to take-up package B A

X

Fig. 3.8

Two-for-one twisting

20 years before it came into commercial use for staple yarns. Penetration of the shortstaple market took another 20 years. These machines are now used for carpet, industrial, and other yarns, as well as for sewing threads. The attraction is that, with a two-forone twister it is not necessary to rotate a large, heavy package to insert twist; consequently high twisting speeds can be used. However, the problem of suspending a non-rotating yarn supply package inside the yarn balloon leads to some mechanical design difficulties. It also leads to a certain awkwardness in piecing up because of the relatively complex threadline path. It is normal to use a compressed air system to blow the new end through the fairly complicated passageway. A tension control disk, coaxial with the package(s), gives stability to the large balloon (see Chapter 9). It could be inferred that high speed yarn balloons of large diameter are needed. However, high yarn tension results from various combinations of large balloon diameter and high speed. The result is that two-for-one twisters are normally used for twisting relatively strong strands. Some machines are used for plying, in which case two yarn cheeses may be mounted coaxially inside the balloon. The reader is referred to a review by Lorenz [1]

3.3.6 Wrap spinning The direct cabling machine has one sort of wrapping spindle arrangement; other sorts are based on the ring frame in which a hollow spindle is used. In wrap spinning, one or more small strands are wrapped around a core yarn. Basically, one or more yarns are wrapped around a core yarn so that the fibers in the outer sheath differ from those in the core. The wrapping may be a strong filament or yarn to enhance the yarn strength or the wrapping might create a texturing effect. The core often has inferior properties and the system offers financial incentives as well as possibilities of enhancement of the technical or visual properties of the yarn. One


sort of wrapping machine has an arrangement based on the two-for-one principle. Another is based on a ring frame in which a hollow spindle is used.

3.4

Confined and non-confined systems

The means of twisting so far considered have required that a package be confined within a yarn balloon. There are several systems that are not so restricted and these will be discussed next. 3.4.1 Open-end spinning If the number of fibers in the flow cross-section is sufficiently reduced as they flow from one package to the other, it is possible to create a so-called open-end. Such an open-end may be rotated about the axis of the yarn to put real twist in the yarn without great interference from the incoming fiber. It is no longer necessary to rotate either package to twist the yarn. This is known as open-end (OE) spinning. The process involves the separation of fibers by a severe drafting action, followed by re-condensation. This is discussed further in Chapter 7. In OE spinning, the staple fiber flow is separated by drafting so that individual fibers (or small clumps of fibers) are added to the ‘open-end’ of the forming yarn (shown diagrammatically in Fig. 3.9). A rotor is normally used to collect these fibers and support the open-end. This is discussed in Chapter 7 and Appendix 10. Yarn can be spun providing there is a steady flow of clean fibers into the moving rotor and the yarn is continuously removed. There is no significant yarn balloon; there is neither ring nor traveler. The result is that the package size is limited only by the ability to wind the package. Also, the speed is not limited by a traveler. Consequently, an OE spinning machine is capable of high production rates. In fact, an OE ‘spindle’ is capable of producing up to ten times as much yarn per hour as a ring spindle and, as a result, this process has become very important. 3.4.2 Alternating twist systems It is possible to insert twist into one or more parallel strands by using a pair of plates Wind

Open-end Twist

B

A

Draft

Fiber transport Feed Discrete fibers are detached from the feed at A by the drafting system, transported and then added to the open-end of the yarn being made at B.

Fig. 3.9

Open-end spinning

68


pressed into contact with a strand, as sketched in Fig. 3.10. Alternatively, a pair of rolls can be made to move parallel to their axes to produce a similar effect. If the process is to be continuous, the plates or rolls have to oscillate in the direction of the arrows. In the case of woolen yarns, the strand is called a roping. The twist cancels after passing through the reciprocating rolls but sufficient cohesion between the fibers is generated by the process to give the strand enough strength to carry it to the next stage of processing. In the case of self-twist yarns, the rolls are called ‘shuffling rolls’. They rotate to deliver yarn and at the same time they oscillate parallel to their axes to produce two (or more) strands, each of which now contains alternating twist. The component yarns are in close proximity to one another. A length of newly twisted yarn has an unbalanced torque (i.e. it is twist lively). If two such strands of the same twist direction are brought together in close contact along their length and are given freedom to rotate about their common axis, they will ply themselves in the opposite direction to relieve the unbalanced torques. The resultant ply tends to be balanced. The shuffling roll in the self-twist machine puts in twist that alternates from Z through zero to S, back through zero to Z, and so on. Two such strands placed together so that the Z twist is opposite Z twist and S twist is opposite S twist, ply themselves to give S ply through zero ply to Z ply and so on. The result is a ply yarn in which the direction of the ply alternates. Again, twisting and winding are separated with the result that large packages of unbroken yarn can be made. A much higher processing speed can be attained than with other devices, because there is no conventional spindle or rotor.

3.5

Twist evenness

3.5.1 Torsional stiffness An uneven yarn has a varying torsional stiffness; if a torque is applied to a length of such yarn, some portions will become more twisted than others. Torsional stiffness of a yarn is dependent on the yarn diameter, the disposition of the fibers in the crosssection, and the torsional stiffness of the fibers. (Torsional stiffness of the fibers depends on their cross-sectional shape and their modulus of elasticity.) The torque might remain constant along the length but there can be rotation of one segment with respect to a neighboring one, which is controlled by the torsional stiffness; the result is shown in Fig. 3.11. This phenomenon is known as twist migration; the perception Yarn

Plates oscillate in opposition to one another to produce alternating twist in the moving yarn.

Fig. 3.10

Alternating twist


Fig. 3.11

Twist distribution in an uneven yarn

is of twist running to the thin spots (which is largely true, but changes in torsional stiffness can produce similar effects).

3.5.2 Variation in behavior of twisted strand Clearly, an uneven twisted strand, such as roving, has varying diameters and hardness; consequently it has variable behavior as it goes through a drafting system, which results in variable twist densities. This is a particular problem with roving. where highly twisted compact portions of the strand (called ‘hard ends’) enter the drafting system of a ring spinning machine and cause slubs and end-breaks because of the conditions just described. Two means of controlling this problem in ring spinning are (a) to obtain as even an input strand as possible, and (b) to use as low a roving twist as possible. Too low a twist will not run on the spinning system concerned. Also, in texturing, migration of twist can disrupt the structure and create faults.

3.6

Tension control

3.6.1 Axial movement Moving strands are nearly always under tension and the tension needs to be controlled. If the strand is flowing along its axis, there are two main simple alternatives for the creation of extra tension for control purposes. One is to use an additive tensioner (Fig. 3.12(a)), and the other to use a capstan tensioner (Fig. 3.12(b)). A device can use both methods. In the additive system, the drag from the tensioner is simply added to the existing upstream tension. In the capstan system, the wrapping of the yarn over F To

Ti

To = Ti + µF F (a) Additive tensioner Ti

p θ

To ≈ Ti eµθ To

θ is measured in radians

(b) Capstan tensioner

Fig. 3.12

Tension controllers

70


the segment of subtended angle, θ, produces a transverse pressure, p, and creates a frictional restraint to flow. The appropriate equations are given in the diagram. Note: T = tension, µ = coefficient of friction, θ is the angle subtended by the yarn, and e = 2.718 (the base of naperian logarithms), and the subscripts i and o refer to the input and output respectively. It must be pointed out that the coefficient of friction, µ, is not a fixed value and varies according to the velocity of sliding. Various factors have an effect and these include: (a) temperature of the surfaces, (b) moisture content of the fibers, (c) additives applied to the fibers, (d) hairiness of the yarns, (e) condition of machine surfaces, and (f) presence of contamination.

3.6.2 Orbital movement If a yarn orbits an axis it is subjected to centrifugal force that makes it ‘balloon’. This is a complex subject that is addressed in Appendix 9. The tensions created are lessened by coaxial control rings, which confine the balloon. In some applications, such as rotor spinning, the rotating element supports some of the fibrous assembly.

3.7

Drawing

3.7.1 Terminology Historically, the term ‘drawing’ was used in connection with the drawframe in staple spinning. ‘Drafting’ was used regarding roller drafting systems in roving and ring spinning. Upon the appearance of man-made fibers, the term ‘drawing’ was also used to describe the elongational process to improve the molecular orientation of the filaments. Custom still insists on the use of the historically founded words but in essence there is little fundamental difference between drafting and drawing. Linear density is defined as mass per unit length of a strand or along the flow path of a stream of fibers.

3.7.2 Purposes of drafting or drawing Drafting occurs when a stream of fibers passes through an acceleration zone5. The place where the acceleration occurs is called a ‘draft zone’ and it is necessary to control the fiber flowing through it. The solutions to the problem of fiber control are diverse and only a few examples can be given to illustrate the importance of mass flow control by passive devices. There are two major reasons for drafting or drawing, which are (a) to better orient the molecules or fibers in the strand, and (b) to change the cross-sectional area of the strand6. In the drawing of polymers, one very important objective is to orient the long-chain molecules to give the filament better properties. In staple processing, an important objective is to orient the fibers within the strand by causing them to slide over one another to give the strand better properties. It should be noted that improved orientation can only be achieved by drafting the strand to give a smaller output crosssection. 5 Conversely, when a stream of fibers passes through a deceleration zone, condensation occurs. 6 In staple spinning, drawing is sometimes considered to include doubling.


There are cases that are not always regarded as drawing but which really are. For example, in extrusion, the linear density of the molten polymer approaching the spinneret is higher than the sum of the linear densities of the output filaments even before conventional drawing. The speed of the output material is faster than that of the input. While an extruder is not regarded as a drawing machine, it always is.

3.7.3 Control of flowing material Both polymer and staple drawing and drafting have instabilities in flow. Control is exercised by imposing restraints on the systems. With polymer in the solid state, control is exercised by hot pins or the like. Heat flow from the control surface permits control of the local visco-elastic constants of the polymer in such a way as to promote stability. In the case of staple processing, the variable frictional forces between the flowing fibers are a strong factor in producing the instability, which reduces their value in both yarn and fabric forms. These instabilities produce quasi-random errors in the product. The addition of an external retarding force to the flowing fiber reduces the instability.

3.7.4 Principle of drafting or drawing Consider a sample of the input material before and after discontinuous drafting or drawing. If there were no losses in the process, the mass of the input sample would be the same as it is after drawing. Let ρ be the packing density (not to be confused with linear density), a the cross-sectional area, l the sample length, ρi a i l i be the mass in the input sample, and ρoaolo be the mass after drafting. It follows that: ρiaili ≈ ρoaolo and if the packing density is constant, aili ≈ aolo

[3.1]

For the purely theoretical case, the change in cross-sectional area is inversely proportional to the change in length. This is discontinuous drafting. However, in production, the process of elongation takes place continuously with the input and output mass flows nominally constant. Thus, the formula of Equation [3.1] can be restated to say that the cross-sectional area is inversely proportional to the speed ratio. In practice, this is modified by changes in the packing density and small losses have to be taken into account, but it forms the basis of all drafting and drawing.

3.7.5 Drawing in staple fiber processing In staple spinning, the material flows through the drafting or drawing zones of the equipment. (The term ‘drawing’ is often used to describe the particular overall process but it is common to refer to the components that carry it out with the adjective ‘drafting’. Thus we speak of drafting rolls and draft in a drawframe which seems odd, but that is the common usage.) Fibers are accelerated as they pass through each zone. Also fibers can, and do, migrate with respect to one another along the direction of flow. Conventional theory has been mainly restricted to roller drafting, in which there are fiber acceleration zones within the spaces between two consecutive sets of rollers. (A similar idea

72


applies to filament drawing but godets are used rather than rollers. Godets are cylinders about which a yarn is wrapped to grip the yarn for the purpose of elongating it.) However, fundamentals merely require that the exit material moves at a greater velocity than the entry material. The theory in Appendix 8 seeks to include the case where fibers are drafted by toothed rolls.

3.7.6 Cumulative draft It is not possible to achieve sufficient drafting or drawing in one step; consequently most systems use multiple, consecutive draft or draw zones (Fig. 3.13). As shown in Appendix 1: ∆ = ∆1 × ∆2 where

[3.2]

∆ = total draft ratio ∆1 = draft ratio in stage 1 ∆2 = draft ratio in stage 2.

NB The term draft ratio is technically correct but it is frequently shortened to ‘draft’. In staple spinning, there are usually two zones. The first (or break-draft zone) has the function of breaking frictional bonds which form in roving (or other strands) due to (a) setting, (b) fiber migration, (c) fiber crimp, or any combination thereof. Newly drafted material is easier to draft immediately after such an operation even if the break draft is small because the crimp gets set over time, and the fibers no longer slide over one another as smoothly as freshly drafted material. The break draft varies according to the type of fiber and the linear density of the strand; it usually varies between 1.1 and 1.4. Overall draft is the product of the break and main drafts and it varies from about 6 to 30 according to the machine concerned. In polymer drawing, there is often more than one stage of drawing (perhaps using different machines) to complete the total process and the mathematical treatment is the same as for drafting in a staple process. However, one would use the term drawing rather than drafting. Nevertheless, for simplicity the explanation will be expressed in terms of draft. Normally, it is arranged that there is little change in fiber characteristics, to prevent the need to change the draft program and hence unnecessarily escalate costs. For more than one stage, all the drafts are multiplied together to give the overall draft. In staple spinning, the process starts with a bale laydown that might be regarded as an extremely thick strand (a linear density of perhaps a billion (109) tex). The yarn leaving the mill may have a linear density of less than 102 tex. (1 tex = 1 g/km or 1 mg/m as discussed in Appendix 1.) The mill can be regarded as a gigantic complex Input

Drafting rolls

Output

Thick Slow

Thin Fast

Break draft = ∆i Main draft = ∆o

Fig. 3.13

Draft distribution


drafting system and it is clear that a drastic amount of drafting is needed over all the various machines in the production line. Although the foregoing has been explained for staple spinning with roller drafting, much of it is equally applicable to toothed drafting (as in an opening line). Some machines, like cards, have draft ratios of roughly 100, whereas machines such as drawframes, roving frames, and ring frames usually have overall drafts of the order of 10. A large number of stages of drafting are required including those that precede the card.

3.7.7 Effects of roller errors It is essential that the operating surfaces of all rolls, gears, and other cylindrical elements should be perfectly round and concentric if periodic errors are to be avoided. It might be noted that the operating surface of a gear is at its pitch-circle diameter. An eccentric element produces a sinusoidal error. If a drafting system is left standing with the pressure acting on the soft cushion rolls, deformations might be developed in the rubber. Such deformations cause periodic errors in the textile product, which contains fundamental and harmonic components. Even though an elliptical roll is a rarity, it is useful to demonstrate the effects. Therefore consider an elliptical roll in a simple four-roll staple system such as is shown in Fig. 3.14. (Other deformed rolls will produce somewhat similar effects, irrespective of the type of system.) The bottom front (delivery) roll has been drawn as excessively elliptical for the purposes of illustration. All the other rolls are perfectly round and concentric; the back rolls deliver material at V inches/s. The elliptical bottom front roll rotates at ω radians/s and the surface velocity is V1 = ω r1 inches/s, where r1 is radius of the roll at the point of contact. The middle diagram refers to the bottom front roll after it has turned through 90°. The active radius is now r2 and the velocity is V2 = ω r2 inches/s. Meanwhile, the back roll speed, V, remains unchanged. Consequently, the draft changes from V1/V to V2/V as the front roll moves through 90°. As the elliptical roll rotates, there is a periodic change in draft, which in turn causes a periodic change in linear density of the output strand. In this case, the periodic wavelength is half the circumference of the deformed roll. A similar effect would have occurred if the roll had been round but off-center (i.e. eccentric). In this case, however, the error wavelength would have been the whole circumference of the deformed roll. Any deformity of the roll produces an error and, as mentioned earlier, a common cause of such errors is deformation of the top rolls (which are normally rubber covered). The rubber is used to improve the grip on the fibers but it is visco-elastic and will deform if the load is left on while the machine is stationary. It might be added that the rubber coverings harden unevenly with time and use. The result is that the deformation of the rubber also becomes uneven. Even if no geometric error is present, an uneven strand is produced because the rubber deforms in a cyclic fashion. These problems are controlled by using special tools to measure roundness, concentricity, and rubber hardness on a regular basis. There is a further complication. The nip-to-nip distance changes, as shown in Fig. 3.14(c), when an elliptical or any other non-round roll meshes with another. At the given angle of the bottom front roll, the setting has changed by δL. In effect, there is a cyclic variation in setting that not only produces a cyclic error of its own but actually magnifies it. Consequently a great deal of trouble is taken to keep the rolls, and other elements, round and concentric. The spectrogram is useful in this regard because out-of-true rolls generate a spike at a wavelength λo, which can be used to

74


V

V1 r1

(a)

V2

V r2

(b)

V

V0

δL

L (c)

Fig. 3.14

Deformed rolls

diagnose the source of the error. Further, any error produced upstream is elongated by the drafting to be ∆ times as long, where ∆ is the overall draft. Consequently, the spectrograph can show multiple sources of error. (An actual example is given later, in Fig. 3.18.) In symbols: λo = λ1 × (∆/k) where λo λl ∆ k

[3.3]

= error wavelength in strand measured = circumference of bad roll = draft between bad roll and point of offtake of the material measured = a factor which is an integer that takes into account how many lobes are on the bad roll.

λo and λ1 must have the same units of measurement.


3.7.8 Drawing a filament Filaments are made to grip the surface of the drawing elements (godets) by the simple expedient of wrapping the filaments several times round the godet as shown in Fig. 3.15. The pins, P, lie at an angle; this merely serves to separate the turns on the godet. The wrap friction effect is the same as is used in a capstan winch; indeed it is sometimes referred to as capstan friction. Yarn is wrapped round two godets rotating at different surface velocities, and the draw ratio is calculated from the velocity ratio. It is important that the surfaces of the godets are concentric with the axis of rotation, and round, otherwise errors similar to those described earlier will occur. A common reason for problems arises from irregular deposits of finish and debris on the operating surfaces. Vi Inclined pins

P

Heat Filament flow

P

Vo

Fig. 3.15

Filament drawing

3.7.9 Drawing a sliver (staple processing) In the drawing or drafting of staple fibers, pairs of rollers are caused to grip the strand as shown in Fig. 3.16. Weighting by deadweights, springs, or pneumatic systems is used to press the rollers together and prevent slippage between the fiber and the rolls. Normally, one roll is made of metal and is fluted; the covering of the other is usually made of synthetic, elastic material (i.e. it is a cushion roll or ‘cot’). As previously indicated, the cushion rolls should not be left under pressure, otherwise the rubber becomes deformed and produces mechanical errors in drafting. Fiber condensers are necessary to gather the fibers and introduce enough fiber migration to give the sliver cohesion. Drawframes are made to facilitate easy access to the elements, for example,

76

Handbook of yarn production Multiple sliver input Weighting

Weighting

Rubber-covered top rolls

Rubber-covered top rolls Fluted bottom rolls

Reaction

Reaction

Single sliver output

Fig. 3.16 Staple fiber drafting

easy removal of parts liable to fairly rapid wear (such as the cots). They are also designed to give a direct fiber flow path to minimize chokes. A sliver is an untwisted rope-like strand of loosely aggregated fibers that are held together solely by interfiber entanglement. To make good yarn, it is desirable that the fibers be aligned as well as possible, and this is one of the purposes of drawing. However, alignment or orientation of the fibers lowers the strength of the sliver. Sliver becomes weak if it is drawn too much or has too low a linear density. Thus, there is a limit to how much a sliver can be drawn and there is a limit to how fine it can be drawn before it is too weak to handle. The minimum linear density is affected by the degree of fiber orientation and crimp. Therefore, it is normal to set the mechanical draft to be about the same as the number of slivers fed. This limits the draft for one ‘passage of drawing’. The term ‘passage’ refers to a sliver passing through a drawframe a single time.

3.8

Consequences of roller errors on the textile product

3.8.1 Periodic errors Roller or godet defects such as those previously described translate into periodic errors in yarn, roving, sliver or tow, which are sharply defined. Not only does the linear density of the material vary in consequence but so also does the structure of the material strand.

3.8.2 Random errors Textile strands also contain random errors with a very wide spectrum of errors.


3.8.3 Cumulative effects of drafting Where there is a number of drafting stages, the results are cumulative and the range of error wavelengths can be very large. Yarns show not only an extremely large range of error but these errors translate into faults in the fabric. The end result of these irregularities is that the fabrics made from the yarns show undesirable patterning known as moiré or barré, which reduces their value.

3.9

Control of irregular flow in drawing or drafting

3.9.1 Irregular polymer flow in drawing An experiment with an undrawn nylon monofilament, or a strip of undrawn or partly drawn nylon sheet, will show that the draw does not always proceed as expected. The strand or strip tends to neck as indicated at Fig. 3.17(a) but the process of necking is not always stable. The thin portion consists of oriented strong material, whereas the thick portion is largely amorphous and capable of plastic flow. As the draw continues, material flows from the thick to the thin portion in regions and becomes oriented as it does so; the flow causes local heating, which tends to localize the flow. A partially drawn material may have ‘lumps’ in it if several necks form during the draw, as shown in Fig. 3.17(b); clearly this is undesirable. Polyester, nylon, acrylic and some others fibers are drawn during normal processing to improve their molecular orientation, but various materials, such as the cellulosics have a limited potential for improved molecular orientation by drawing. In the following discussion, the narrow class of textile materials capable of benefiting from drawing will be called ‘polymers’ for simplicity even though the term ‘polymer’ really covers a very much wider range. The flow of polymer in the drawing operation absorbs energy and the temperature of the strand rises as it is drawn at high speed. A change in temperature changes the characteristics of the polymer. To control the mechanical flow, it is necessary to control the heat flow; hence the use of the hot pin mentioned earlier (see, Fig. 2.13, p 45). The heat flow and mechanical drag caused by the pin are intended to keep the neck in its proper position. A polymer has a natural draw ratio, which is a function of the degree of molecular orientation and the draw becomes unstable if the machine draw ratio differs from this. The position of the neck will advance or retreat according to whether the machine draw ratio is lower or higher than the natural draw ratio. If the machine draw ratio is too high, the tensions rise to the point where the filament breaks. If it is too low, the system is unstable and the product consists of a mixture of drawn and undrawn lengths. In a continuous flow process, the position of the neck has to be stabilized; without such stabilization, the neck is likely to move in one direction or the other in respect to the godets. (There is an exception when the Vout

Vin (a) Bulge

Vin

Vout

(b)

Fig. 3.17 Unstable polymer flow in ‘neck’

78


mechanical draw ratio is the same, numerically, as the natural draw ratio.) The neck retreats or advances until it reaches a godet, where the strand will then break. Any oscillation of the position of the neck tends to give uneven filaments and therefore great care has to be taken in the design and operation of the drawing system. This is especially true if the filaments have to be dyed at a later stage; variations in draw cause corresponding variations in polymer morphology, which give rise to barré in fabrics.

Amplitude

3.9.2 Drafting waves in staple systems The problem of uneven polymer flow in drawing a polymer has its counterpart in staple spinning. In drafting staple fibers, the effective length of the fiber is an important factor. Fibers supplied to a drafting system in normal practice vary in length, fineness, crimp, and finish, natural fibers being more variable than man-made ones. Each of the variables mentioned alters the force that can be transmitted by a fiber under these circumstances; the force may be taken as a measure of the effective length of the fibers. A perfectly smooth fiber behaves as if it were much shorter than it really is. A crimped fiber is physically shorter than its fully extended length but it engages neighboring fibers because of the crimps. Thus, the question of fiber length is far from an easy one. For simplicity, the following discussion will refer solely to the effects of length variation. Irregular flow of the fibers changes the position of fibers relative to their expected positions after drafting and creates unwanted variations in linear density. With natural fibers, length is quite variable and the error is distributed. The variation produces a ‘hill’ typical of this type of error, as shown in the actual distribution in Fig. 3.18(a) and it is easily distinguished from a mechanical error (Fig. 3.18(b)), which shows up as spikes like those at A and B (also there are less easily distinguished peaks such as shown at C). The first of these types is often called a ‘drafting wave’ and it is a fiberborne error. Fig. 3.18(a) shows two hills, which indicates that two different drafting waves were created, one from a rearward drafting zone and a large wave from a forward one. Since a difference between the roll setting and effective fiber length is an important factor, variations in fiber length can produce undesirable results, which show up as blotchiness or streakiness in the fabrics. Actual distribution shown by bars Theoretical distribution in gray

Amplitude

0.1 1.0 10 Error wavelength, log scale (yards) (a) Drafting wave A

B C

2 4 8 16 32 Error wavelength, log scale (inches) (b) Mechanical error

Fig. 3.18

Actual and theoretical variance distributions in a drawframe sliver


Not only is control of fiber length an important matter, but so is the maintenance of correct roll settings. There are thousands of drafting zones in a normal mill, and the aim is to set all of them to standard values appropriate to the material being processed. Changing the ratch setting (i.e. the distance between the nip lines of successive pairs of rolls in a drafting system) throughout a mill is a major operation. Settings of all the drafting systems must be maintained within close tolerances to the values standard in the particular mill. Also the fiber purchasing agent must seek to acquire fibers within a standard range of fiber length distributions. These are management and maintenance problems; changes are not made lightly. Once set up, the ratch settings are usually maintained until the next maintenance period. Thus, it is useful to constrain the variability in the fiber population by blending and careful stock control.

3.9.3 Control of fibers by mechanical restraint The evenness of the final product, which is usually yarn, can be gravely affected if these errors just discussed are not kept under control. This is true even when the faulty drafting is in an earlier process. On the other hand, if the fiber movements could be constrained to minimize drafting waves, the distribution would approach the theoretical value as shown in Fig. 3.18(a). In the example, to the left of the picture there is a significant hill and in the center there is a minor one. The large one came from the main draft zone and the small one from the break draft (back) zone or an earlier process. Obviously there must be concern about the irregular flow through the front draft zone where the draft ratio is the largest. Fortunately, there are some design features in modern machines that help to restrain the unwanted relative fiber movement. These features work by adding frictional forces, which tend to keep floating fibers at a speed at or near that of the back rolls. These floating fibers within the draft zone are not gripped by either nip. In drawframe design it is normal to incorporate a pressure bar (Fig. 3.19(a)) or some other device to restrain these floating fibers. In a ring frame or roving frame, aprons are commonly used to fulfill a similar function (Fig. 3.19(b)). Aprons are pairs of relatively wide flexible bands. They are pressed together sufficiently to restrain most fibers so that they move at the apron speed until the leading ends of the fibers are trapped by the delivery rolls. The linear speed is usually close to the surface speed of the back roll. The use of aprons has helped staple spinners achieve remarkable improvements in yarn quality in this century, by greatly reducing the unstable fiber flow through the drafting system. The aprons are pressed together by pressure P (Fig. 3.19), merely to restrain the floating fibers and to allow them to slip without gripping them sufficiently to cause fiber breakage. This is in contrast to the rollers, which are pressed together by forces F to eliminate, as far as possible, fiber-to-roll slippage. The aprons press on the floating fibers and add their influence to the competing effects of the fibers. The competing forces arise from the frictional contact between the fibers and the front and back rolls. The concentration of fibers is greatest at the nip of the back rolls and the surface speed is greatest at the front rolls. As mentioned before, the aprons retard any premature accelerations of the floating fibers but they must be maintained in good condition to work properly. If the draft is too high or the linear density of the strand is too large, the wear rate of the aprons increases markedly. Aprons are not normally used in sliver drawing because of the high wear rates. For roving frames it is normal to use long bottom

80

Handbook of yarn production Pressure bar

F

F F V

F

F F

(a) Apron

P F

F

F

F

F V F P Apron

(b)

θ

(c)

Fig. 3.19

Fiber control in a drafting system

aprons, such as those shown, to prolong their life, whereas simple short aprons might suffice in ring spinning. However, many modern ring frames also use long aprons for the reasons cited. The setting of the forces F and P is critical to good performance. If F is too small, defects are created in the output strand, whereas if it is too high, the rubber rolls quickly become damaged. Setting P is part of the art of spinning. There is a choice of roll layouts but it is usual for the first drafting zone met by the sliver to be the break draft zone. The second operational zone is the place where the main draft occurs. Aprons in the break draft zone have been found to wear quickly and have never gained a foothold in practice. The roll layouts are referred to as ‘3 over 3’ or ‘4 over 4’ according to the number of rolls involved. A 3 over 3 system implies a system similar to that sketched in Fig. 3.19. A 4 over 4 system has four top and four bottom rolls but there are still only two drafting zones. The reason cited for this design by the makers is that it is beneficial to have a rest zone between the two draft zones, but additionally, the extra spacing insulates one drafting zone from the worst effects of fiber slippage from the other. In a conventional ring frame, which has roller drafting, the draft might well be up to 30, and in certain cases much higher drafts have been used. This applies especially to air-jet spinning, where the width of the ribbon of fibers presented to the twisting device is wider than normal. Also, some sliver-to-yarn systems are capable of handling remarkably high drafts (up to 80). Properly designed high draftframes yield higher yarn strengths, the yarn is more even, and the productivity is higher than with conventional frames. Increased precision in setting the high draft machines is required because high drafts are often associated with increased short-term error.


Condensers are also used to help control the flowing fibers by forcing them through an orifice (or other constraint) with a narrow throat, which compresses them. Compression of the fibers approaching the draft zone keeps the fibers together and prevents so-called ‘cracking’ of the fiber sheet. It is possible thereby to obtain a smoother and more even drafting action.

3.9.4 Prevention of fiber slippage over driving surfaces If the input strands are unequal in size, some may be gripped by the rolls more firmly than others and the loosely gripped portions may be pulled forward from the feed prematurely, causing a fault. If the input strands are cored, the outside sheath of fibers may be improperly gripped by the roller pair. (Cored slivers have a hard central core. See Section 5.10.4) In such cases, slippage between the rolls and the fiber occurs, which leads to irregularities in linear density of the output product. A pair of rolls clamping a strand does not have a simple line contact because the fiber assembly squashes as shown in Fig. 3.20(a). There is a distribution of pressure in the nip zone, but a leading fiber end is free from the direct influence of the nip before it arrives at the compression zone, and the trailing end comes free after it has left. The size of the compression zone varies according to how thick and compressible the strand is. The effective ratch setting is thus different from the theoretical value, and the difference depends on the material being processed. The setting differential just mentioned is a function of the linear density of the strand being processed and the type of fiber. For example, the ratch setting in a drawframe should have a greater differential than that in a roving frame. Another factor also intervenes. The effective length of the fiber increases during the drafting processes because hooks and fiber-crimps are pulled out and the fiber is generally straightened. It is therefore not surprising to find, in practice, that the best ratch settings are often determined by trial and error. If there is fiber loss or slippage between the fibers and the rolls, the actual draft becomes slightly reduced. Perhaps more important is what happens when the amount of slippage varies with time or position. Obviously, if the slippage varies with time, there are corresponding variations in linear density of the output strand that constitute a degradation of yarn quality. Such time-dependent variations can be caused by variations in agglomeration of the input material. For example, in ring spinning, twist in the roving input tends to concentrate in the thin spots and this makes so-called ‘tight spots’ more difficult to draft. The fiber tensions in the draft zone rise and this, in turn, causes slippage between the fibers and the back rolls. In drawing, the use of slivers of different linear densities in the creel causes the ‘thinnest’ ones to slip, as demonstrated in Fig. 3.20(b). Strands 1, 3, and 4 are compressed by the forces F and the resistance to slippage in each of those cases is µF where µ is the coefficient of friction. Strand 2 is too small to be gripped and slips under any applied tension. Even if Strand 2 is gripped, but to a lesser extent than the others, errors still arise. Highly irregular input slivers produce similar effects. The same type of condition can often be found in a comber lap machine in which many parallel slivers are combined side by side and are drafted to form a comber lap. Extra control can be obtained by wrapping the fibers partially around any rotating surfaces with the intention of restraining fibers movement (Fig. 3.19(c)). A partial wrap design improves the grip between the back rolls and the flowing material. The wrap idea is also sometimes applied at the front rolls and permits some control of the

82

Handbook of yarn production Strand

Roll

Compression zone

(a)

F

F

F

Roll 1

F

2

Roll

3

4

F

F

(b)

Fig. 3.20

Unequal compression of strands in a nip

fibers passing through the twist triangle on the output side. Wrapping a strand around a roll to get a better grip is common to both staple and filament processing. In filament processing, the use of godets to grip the filaments is very common. A pair of rolls squeezed together on an incompletely solidified filament would cause flats on the surface, which would be undesirable under most circumstances. The use of a number of wraps around a large diameter godet roll produces much less surface stress for a given gripping power. There is a similarity in concept between this and the partial wraps just mentioned. However, when multiple wraps are used, it is necessary to keep the coils separated. This is achieved in a very elegant fashion by the inclined pins shown in Fig. 3.15. Also in the godet system, the resistance to slippage is a function of the cumulative wrap and the coefficient of friction (µ) between the fiber and the metal. In these cases it is important to ensure that an adequate number of wraps is used and that the surface of the godet roll is free from contaminants, which would affect µ. In the production of filament yarn, each spinning head produces only one yarn. However, when tow is being produced, there is more ambiguity in the load distribution between the component feeds and consequently there is a greater chance of error. The filament length is virtually infinite and problems associated with longitudinal fiber migration do not arise.


3.10

Doubling

3.10.1 The principle of doubling If m similar streams of fiber converge, the theoretical variance of the total combined stream is theoretically reduced to 1/m of that of the individual input streams. The theoretical coefficient of variation (CV) is reduced by a factor of √(1/m), where CV = St dev/mean and St dev = √Variance. The same applies if m similar strands of sliver (or other strands) are laid in parallel in the feed to a machine. This combining of multiple fiber streams is known as doubling. An example is where slivers are placed in the creel of a drawframe; they are combined into one during and after the drawing phase of the operation. (A ‘creel’ is that portion of a machine where a multiplicity of input strands are removed from their packages and are delivered to the strand processing device. An example is shown later in Fig. 6.1.) Other cases exist of placing streams of fiber or strands in parallel and combining them. Many errors are created in processing; consequently, even if the material is delivered to the process evenly, the output contains variation. Sometimes doubling is used to offset the errors and the theory works reasonably well providing the errors are random. (Strictly speaking, the mean values and variances of each strand should be similar for the theory to apply.) The actual CVs are higher than the theoretical values. When doubling and drawing are combined, the input materials are doubled to reduce the long-term errors; however, new errors of shorter wavelengths are added as a result of the process of elongation. There is an exchange of relatively long-term for short-term error.

3.10.2 Combinations of drawing or drafting with condensation Doubling occurs in some processes that are not widely regarded as a form of doubling. For example, in filament tow production, parallel streams of filaments are combined before they are chopped or otherwise separated into staple fiber. The multitude of parallel streams reduces the total error in the output material. The collection of the cut material also creates a further doubling effect because of the mixing that takes place as the stream of cut fibers is condensed into the bales. In condensation, deceleration causes the incoming product stream to fold and produces multiple doubling of adjacent elements. The effect is a reduction in variance which, although it is somewhat less than the theoretical value, is still substantial. Drafting in the early stages of staple fiber processing is always followed by a condensation stage because of the need to control the flow of several machines in series. Doubling occurs in every condenser, chute feed, blending machine or any other place where the fiber flow rate is slowed down and the fibers accumulate. In fact, if it were not for this doubling, the drafting problems in the opening and carding would be noticed more. As it is, the increases in variance caused by the toothed drafting in the opening line are mostly offset by the large doubling factors that also prevail. This is not to say that the extra variance did not exist.

3.10.3 Homogenizing multiple streams of a nominally similar product Another valuable aspect of doubling is that it can be used to offset variabilities from one set of equipment to another. Each set of equipment tends to produce a product

84


with different characteristics according to its state of maintenance, setting, and design. If a downstream machine were always fed from the same source, there could be differences in product that would show up in the fabric, in the form of barré. This is called channeling. The effects are greatly reduced if the product is properly distributed to all the downstream machines in a systematic way that avoids patterning. According to Uster [2,3], the CVs of linear densities of sliver from the card, 1st drawframe, and 2nd drawframe sliver for the top 10% of yarn makers were 3.4%, 3.5%, and 3.6%, respectively in 1982. For the bottom 10%, the respective CVs were 5.7%, 7.5%, and 7.2%. This means that the gains due to doubling at the drawframe are just about offset by the losses in regularity caused by drawing. One purpose of doubling is to blend product streams. The implication is that it is not just the linear density of the product stream that is important, but so are all the other aspects of homogeneity of the stream. In the case of filament processing, this might be represented by differences in polymer morphology. In staple processing, it might be represented by differences in fiber attributes other than linear density. Some practical results for cotton processing are shown in Table 3.2. In this case, the CVs of short fiber content after carding rose slightly when compared with the average value in the appropriate bale slice, whereas the CV of fiber fineness dropped considerably.

3.11

Effects of shear

3.11.1 Definition of shear Shear may be defined as trapezoidal deformation or relative movement of elements in a structure, which causes the elements to slide over one another. When a viscoelastic body is stressed, some dislocations in the structure remain after the external stress is removed. There is often a residual stress pattern too. Such dislocations and residual stresses form the basis of a number of phenomena that are exploited in yarn manufacturing. They can be considered at the molecular and at the fiber levels, the former relating mostly to texturing and the latter mostly to staple yarn processing. At the molecular level, the elements are segments of long-chain molecules; the movements are measured in microscopic units and it seems suitable to describe these movements as dislocations. In many solid materials, crystalline areas are separated by amorphous areas and the ‘crystals’ can, and do, move with respect to one another under stress. In staple fiber processing, fibers migrate and the composition at any cross-section of the material undergoing drawing changes because of that processing. The phenomenon can still be regarded as visco-elastic but obviously the elastic forces are proportionately less than those in polymer molecular dislocations. The relative movements of fibers Table 3.2

Bale slice Sliver

Fiber attributes Linear density %CV

Fiber fineness micronaire %CV

Short fiber content %CV

– 3.1

4.2 2.2

15.5 17.3


can be quite large and, historically, the term migration has been applied; therefore it is proposed to extend that practice here in respect of other fiber movements.

3.11.2 Molecular dislocations The visco-elastic properties of polymers change when heated, especially at the transition temperatures, i.e. softening point (Tg) and melting point (Tm). Above Tg, many of the bonds between molecules are broken and relative movement of segments of the molecules occurs. When the temperature drops below Tg, new bonds are made with the molecules in their new shapes and relative positions. This makes possible the heat setting of filaments into desired yarn textures suitable for commercial use. When two polymers are involved, as in bicomponent yarns, differential stress patterns caused by the cooling polymer cause filaments to curl, coil, or loop. Also, the visco-elastic constants of a layer along the length of yarn can be altered to produce similar effects. The edge-crimp method is one such example, where the filament is dragged over an edge to produce a disoriented layer and that is sufficient to make the fiber deform significantly (see Chapter 4). Similar behavior can be found in staple yarn processing, where the result is not at all desirable. Fibers are dragged over sharp teeth in a number of machines and they stand a chance of becoming edge-crimped. Fine fibers that are stressed by this kind of action can form into tiny tight balls called neps (which are a cause of loss of quality).

3.11.3 Lateral fiber migration At the fiber level, most of the phenomena discussed relate to relative movement of one fiber with respect to its neighbors. For example, in ring spinning, segments of some fibers are subject to higher stresses than others in the twist triangle, and they move radially within the structure of the yarn. Highly tensioned fibers tend to move to the core and slack fibers move to the outer perimeter. In consequence, fibers thread their way between what would otherwise be concentric layers and stabilize the structure to make it self-locking. This is called fiber migration but it really should be called lateral fiber migration or some such term. In air-jet texturing, segments of filaments are forced across the yarn structure so that they, too, form an interlocked stable structure. Interfiber friction is the medium by which the structures become locked. A useful analogy to these effects is the common knot.

3.11.4 Longitudinal fiber migration As explained in Appendix 8, fibers flowing through a draft zone do so irregularly; there is shear between the fibers and some have a higher mean velocity through the drafting system than others. Generally, shorter fibers accelerate from the nip of the back rolls in a drafting zone earlier than longer ones that enter at the same time. The relative motion between two fibers delivered by the device is called ‘longitudinal fiber migration’. Maximum longitudinal fiber migration = (∆ – 1) × (L – S)

[3.4]

where ∆ is the draft, L is the length of the long fiber, and S is the length of the short fiber.

86


The ratch setting is usually set a little larger than L and the longitudinal migration of fibers varies between zero and the amount given in Equation (3.4). The fiber population of a given cross-section of the entering material differs from the population of the corresponding zone in the emerging material; it becomes difficult to predict the performance of the fibers in drafting because of these changes in fiber population.

3.11.5 Effect of migration on evenness The longitudinal fiber migration just described has an important effect on the evenness of the strand. To demonstrate the mechanism by which the evenness is degraded, consider the following example. The strand shown in Fig. 3.21(a) has two components and the composite strand has been leveled by some mechanism such that the evenness of the composite is perfect and the linear density of it is constant at nav units. However, component B has a thick spot, which is compensated by a conforming thin spot in component A. A normal autoleveling device cannot discriminate between the relative natures of the two components and can only make its adjustments based on the total linear density. Two conjugate intervals are marked by hollow headed arrows. The component A is now moved to the right by some means. Figure 3.21(b) shows how the linear density changes near the blend anomaly. The marked intervals do not change their heights but they become separated. Bearing in mind that the height of the diagram represents linear density, it will be observed that the strand is now uneven. The linear density now ranges within the limits nav ± n1. Variations in fiber length along the strand are converted into variations in linear density. If the concentration of short fibers varies (as was shown earlier), then the longitudinal fiber migration varies and the result is that a perfectly leveled strand can become uneven after passing through the process stage. Thus, for example, a roller drafting system converts variations in fiber content into variation in linear density. It can be seen that irregular blending produces some undesirable side effects.

3.12

Integration of sub-processes

3.12.1 Historical examples of process integration As will be detailed more exactly in Chapter 5, the development of cotton card sliver production in the twentieth century provides an example of the integration of a variety of machines arranged in serial fashion in a process line. This is the first case.

A nav

B (a)

n1

A B

–n1

(b)

Fig. 3.21

Longitudinal fiber migration and strand evenness


In the 1950s, each machine was free standing. The fiber transfers, to and from the machine, were executed by manual labor and were also controlled manually. By the close of the century, the transfers were automatic and so was the control. Very little labor is involved in the operation of the series of machines that open, clean, and blend the fibers taken from the bale. No appreciable labor is needed until the sliver emerges from the card. In contrast, the subsequent stages (in which the card sliver is converted into yarn) still involve considerable manual work. This is the second case. Various schemes of automation are being applied, but not universally. The first case shows a mature system and the second one shows a system developing in a somewhat similar direction. Integration and automation of process lines are a theme common to all phases of yarn production.

3.12.2 The driving force behind process integration Competitive pressure mandates economical means of production. Since labor costs have been much of the total cost of yarn, there has been a powerful motive to reduce the amount of labor needed. This pressure is offset by the costs of capital needed to implement technical solutions. Consequently, not every solution is adopted by industry and those that are adopted are put in place with considerable caution. In the first case just cited, the solutions adopted involved relatively modest amounts of capital and yet, over the years, have yielded significant reductions in the labor costs involved. The linking and automation of the next series of processes require investment and the savings are relatively modest; thus progress is relatively slow. Another aspect of reducing labor costs in spinning is the need to deal with end-breakages. There is a contrast between the initial and final processes that helps explain the dilemma. In the blow room there are normally only two or three production streams, whereas in spinning there are many, many parallel streams. Consequently, the acceptable capital cost per stream is relatively low in the last case and this hinders the development of process integration. Nevertheless, there is movement in this direction and future generations will see it mature. Filament yarn production seems to be less affected because of the shortness of the process line, but it is not immune to the pressures. For example, extrusion, texturing, and drawing were originally separate operations in series, but now draw-texturing is well established with little or no increase in capital cost. The relatively small labor cost was reduced. The development of the use of partially oriented yarn (POY) was responsible for that commercial advance, rather than some mechanical solution. Thus it can be seen that there is no single route to increased process integration, but the economic pressures ensure that progress in that direction will continue.

References 1. 2. 3.

Lorenz, R R C. Yarn Twisting, Text Prog, 16, 1/2, 1987. Anon. Uster Bulletin, Zellweger Uster Inc, Uster, Switzerland, No 31 Dec 1982. Furter. R. Uster Bulletin No 36, Zellweger Uster Inc, Uster, Switzerland, Oct 1989.

4 Filament yarn production

4.1

Introduction

Although most filament yarns used today are synthetic fibers that need texturing, there are some that need no modification in this way. Industrial filaments made from synthetic polymers constitute one case and natural filaments, such as silk, another. This chapter will concentrate mostly on textured yarns but a brief discussion of silk throwing will be included for the sake of completeness, at the end of the chapter. Industrial filaments are so diverse that little discussion will be given. Suffice it to say that the majority of the successful processes exploit the exceptional strength that can be obtained with some drawn polymers. During the period since 1975, manufacturing facilities have sprung up in countries such as China, Taiwan, Korea, Mexico, and Brazil. These countries operate to fill some of the demand of new markets. They also serve the established ones in the USA, Japan, Europe, and other developed areas. Such changes affect the price and distribution of the materials. The total consumption of textured yarn in the USA, Japan, and Europe has declined but there has been steady growth in industrial and carpet yarns. According to Wilson and Kollu [1], 51% of the textured yarn produced in 1983–4 was false twisted polyester filament, 22% was false twisted nylon, 18% was bulked continuous filament (nylon and polypropylene), and the remainder was made up of air-jet and other forms of textured yarns. Obviously, false twisting is very important in this field. However, the market has forced many filament yarn makers to move to products nearer to staple yarns in character and consequently the use of air-jet texturing has risen. Atkinson and Wheeler [2] state that air-jet textured yarns have maintained about 5% of the market for false twist textured yarns and most of that goes into automotive upholstery. Polyester has largely displaced nylon in that particular market.

Filament yarn production 89

4.2

Texturing filament yarns

4.2.1 Purposes of texturing The prime purpose of texturing filament yarn is to create a bulky structure that is desirable for the following reasons: 1 2 3 4 5

The voids in the structure cause the material to have good insulation properties. The voids in the structure change the density of the material (which makes possible a lightweight yarn with good covering properties). The disorganized (or less organized) surface of the yarn gives dispersed light reflections, which, in turn, give a desirable matte appearance. The sponge-like structure feels softer than a lean twisted ‘flat’ yarn. The crimped or coiled filament structure gives a lower effective modulus of elasticity to the structure when compared with that of a flat yarn.

From this it will be realized that, in order to make yarns to these specifications, it is necessary to deform the individual filaments and set, or otherwise hold, them in the desired deformed condition. When deformed in this way, the filaments in the whole bundle are unable to lie side by side in close contact and the required voids are produced. Furthermore, the non-straight, separated filaments are much more easily deformed than are those in a flat yarn, and one obtains a softer hand and greater ‘stretch’. There are two general classes of textured yarns that relate respectively to thermoplastic yarns only and to those which can be more widely used. In general, the first classification involves the stages of deforming, heating, cooling, and relaxing the filaments. The process is known as heat setting despite the fact that it is the cooling that does the setting. Theoretical filament structures are shown diagrammatically in Fig. 4.1. In the second case, the texturing of non-thermoplastic materials, filaments are deformed and are held in their deformed state by frictional contact with the neighboring filaments. An example of the latter is the air-jet method that will be described later in this chapter. Meanwhile, we will continue with heat set yarns.

4.2.2 Physical basis of texturing Before considering the methods of false twisting, let us review the mechanics involved. It will be recalled that the process phases in false twist texturing consist of:

(a)

Fig. 4.1

(b)

Theoretical yarn structures

90


1 2

Deforming the filaments. Applying heat to raise the filament temperature above the glass transition temperature, Tg. Cooling the filaments to below Tg. Rearranging the filaments under suitable tension. Winding the textured yarn.

3 4 5

Theoretically, phases (1) and (2) can be interchanged or be coincident, provided the deformation persists until the filaments are cooled below Tg and the polymer becomes set. However, time is a factor in determining the degree of set achieved and, in high speed machinery, it is usual to apply heat as soon as possible in the process. If temperatures of some polymers are raised too high, they tend to yellow and this gives trouble with the end products, particularly those of light color shades. The deformation can be of any kind, but in false or real twisting, the primary modes of deformation are torsion and bending. Since the real twist process is simple, it will be used for explanation although it is no longer commercially important.

4.3

Real twist texturing

Explanations are a little easier if we consider the early types of discontinuous processes. Various forms of twister were used to induce the initial deformation. A batch of packages of yarn was then taken from the twister and placed in an autoclave.1 The temperature of the yarn was raised above Tg (but below Tm), and then allowed to cool. The product taken from the autoclave was non-twist lively or ‘dead’ (see Fig. 3.4), but the fiber deformations were set into their newly twisted shapes. To develop the bulk, it was necessary to untwist the yarns until the filaments were approximately parallel and separated, and then relax them. It will be noted that filament separation in the phase (4) was necessary for the bulk to form without undue interference between neighboring filaments. In untwisting yarn from the set condition, a torque is applied to each filament. The sum of the individual torques is the total applied to the yarn. The torque places it in a state of stress, which is retained until the fibers are relaxed. Untwisting and relaxing the yarn allow the newly imposed stresses to be relieved by changes in the shape of the filaments as they move within the structure during the process of relaxation. This form of texturing is shown diagrammatically in Fig. 4.2. When relaxed, each filament seeks a minimum energy state, two of which are depicted in Fig. 4.1. If the structure is open enough, most of the filaments will achieve one of the minimum energy shapes, but a tight structure prevents full relaxation. In the latter case, not all the potential bulk is developed. A normal yarn structure will consist of shapes similar to those shown, or combinations of them if yarn is untwisted and the filaments are separated before release. Some methods of texturing produce alternating directions of coiling. The result is that the yarn produced has little or no twist liveliness because torques from the opposing filament coils cancel. This form of texturing is shown diagrammatically in Fig. 4.2. Consider extreme cases. The adjacent helical coils in Fig. 4.1(a) take up a great 1 A vessel that uses high pressure steam to obtain the necessary temperatures. For the characteristics of steam, see Appendix 3.


Twist Heat

Cool

Untextured filaments

Untwist

Separate and relax Textured filaments

Fig. 4.2

Principle of twist texturing

deal of space and we have a so-called ‘bulky’ yarn. The other model, Fig. 4.1(b), consumes relatively little space and we have a low bulk, high stretch yarn. As the yarn is extended, the intermittently snarled filaments are progressively converted to straight parallel filaments. There is a great deal of yarn stored in the snarls, and, consequently, there is a surprisingly large extension of the yarn before the snarls are fully converted to straight parallel filaments. Furthermore, the tension needed to pull out the snarls is relatively low, and thus the yarn behaves as a low modulus material (until all the snarls are removed). Of course, as the filaments change from the snarled to the straight condition, they are subjected to torsional and bending stresses, and energy is stored in the extended yarn. Once the tension is removed, the yarn attempts to return to a minimum energy state and contracts. Thus, the stretch yarn behaves rather like a rubber band and its principal characteristic is the enormous and almost elastic extension that becomes possible. A practical yarn is intermediate between the extremes. There are varying proportions of each kind of minimum energy shape according to the method and conditions of texturing. Also, there are modifying factors. Helical portions tend to intermesh, parallel portions tend to migrate (and become non-parallel), many filaments fail to reach their minimum energy state, and many filaments interfere with one another. Consequently, there is a wide range of combinations of bulk and stretch that can be achieved, but generally the higher the stretch capability, the lower the bulk. Of course, even the adjacent coil model provides a yarn with a moderate degree of stretch because the helices act as coil springs. In practice, the breaking elongation might vary from 10% for a bulked yarn to 500% for a stretch yarn.

92


4.4

False twist texturing

4.4.1 General comment One of the most important types of yarn modification is false twist texturing. As mentioned in the last chapter, a running yarn twisted as shown in Fig. 4.3 causes false twist to be trapped between the feed system and the twister. The feed yarn has little or no twist, the yarn between A and B has false twist, and the yarn leaving B has the same twist as the input. If heat is applied in the zone AX and the yarn is cooled in zone XB, then the yarn approaching B will be heat set in the twisted condition. Overfeeding (not shown) and untwisting slackened filaments at B facilitates the necessary fiber rearrangement and separation. (An overfeed is where the input speed is slightly more than the output speed.) When the filaments relax, the uneven contraction of the filaments causes them to rearrange themselves laterally. If heat is applied in zone CD, the latent crimp can be developed to produce a bulked, set yarn in one continuous process.2 In the particular case shown, a godet is used to grip and feed the input yarn; however, no twister is shown for reasons of clarity. All the phases mentioned in the previous section are embodied in this continuous process. The integration reduces costs of machinery and material transportation. The savings have been so large that false twist texturing has become a major system for yarn production. The Untextured yarn input

Godet

A False twisted yarn

Heat X B

Cool

Twist Zero twist filament output

Develop texture

Fig. 4.3

C

The temperature in the zone AX is raised above Tg

D

False twist texturing

2 Notice that care was taken to avoid saying that the output yarn had no twist.


means of twisting has changed and the systems will now be reviewed in a more or less historical sequence.

4.4.2 Pin twister type of false twist texturing machines To heat set the twisted filaments and relax them afterwards to produce bulk, it is necessary to heat the running filaments at two places and so we have two-heater machines to produce the developed yarns. To produce yarns in which the filaments have not been relaxed only one heater is required. Examples of a two-heater machine are shown to a small scale in Fig. 4.4. It is necessary to use high twist levels to produce adequately textured yarns; for example, with a 70 denier yarn, one might well use some 80 tpi. (This would give a TM of about 10 on the cotton system.) To get high production, it is necessary to use very high twisting speeds, of around 500 000 r/min. This calls for special designs of twisting unit in which the mass and size of the rotating element are as small as practical (or the element is eliminated). It also calls for special bearings, or suspension systems. In the pin twister shown in

Tensioners Godets

Heaters in false twist zone Twisters

Twister pin (enlarged) Secondary heaters

Godets

Winders

Take-up packages (a)

Fig. 4.4

(b)

Manufacture of false twist yarns

94


Fig. 4.4, the spindle is frequently less than 0.25 inch diameter × 1.5 inch long (approximately 6.4 mm diameter × 38 mm) and it is held against drive rollers by a magnetic field; this obviates the need for a direct bearing. The bearings of the drive rollers have to rotate at only a fraction of the speed of the spindle (typically 12–15%). It should be noted, however, that the spindle gets very hot because of air drag and magnetically induced eddy currents within the metal. Also, the false twist pin (shown inset) is usually made of ceramic or sapphire to withstand the abrasion caused by the yarn passing over it. A given element of polymer must reside in the hot environment for a sufficient period to reach Tg because it takes time to soften the polymer. If, for example, the time is 0.5 second, the spindle speed is 500 000 r/min and the twist is 80 tpi, the heater length has to be at least 52 inches. Thus it can be seen that the heaters must be long. It also takes a significant time for the yarn to cool sufficiently to freeze it into the twisted configuration. Thus, a certain distance is needed between the heater and the false twist pin. The needed heating and cooling lengths increase with spindle speed and this leads to increases in the threadline length. Not only do high production machines become very tall, but there is also increasing difficulty in handling the long, heated filaments. Frictional drag of the yarn over the heater plate is a significant factor. The frictional coefficient is modified by the fact that the yarn rotates at high speed about its axis as it passes over the heater plate. At very high speeds, the design of the heater becomes extremely important and it sometimes becomes necessary to use forced cooling of the yarn leaving the heater. Where two heaters are used (to produce a set yarn), the threadline length is almost doubled, as shown in Fig. 4.5. If the threadline is vertical and the two heaters are immediately above one another, a two-story building becomes necessary for high Feed roll Oiler Feed roll Second heater Winder Feed roll

False twister

Tensioner First heater Floor Feed roll

Fig. 4.5 Two-heater false twist machine


speed machines. Alternatively, a more complex threadline may be used; for example, the heaters might be inclined to the vertical. In all cases, the modern machines need a great deal of headroom. Threading up (or ‘stringing up’) needs skill because of difficulties in handling the hot, high speed yarns. It might be added that the use of air to piece and to thread godets, and other high speed elements, is very common in the filament industry. To reiterate, the temperature of the polymer has to be raised to a level between Tg and Tm. Within these limits, the higher the temperature, the better the set, but as the temperature approaches Tm, the yarn strength deteriorates and excessive differences in dye affinity are likely to be created. Atmospheric conditions should be controlled because moisture affects the setting process and can lead to degradation of the polymer. Generally, an air temperature of 75 ± 5°F (24 ± 3°C) and an rh of 65 ± 2% are used, but the conditions might vary according to the yarn being textured. Excessive humidity causes yarn to drag over contact surfaces, which leads to erratic tensions in the yarn. This, in turn, leads to variations in the bulk developed. Insufficient humidity leads to the production of static electricity and, on all of these accounts, control is very important. Tension in the yarn within the heater is controlled by the feed uptake rates. The feed rolls have to be adjusted to give an overfeed of 2 or 3% to take into account twist contraction and shrinkage. Insufficient overfeed leads to high tension, which causes unacceptably high end-breakage levels and low bulk. Too much overfeed leads to low tension, which results in the formation of tight spots (sometimes called ‘voids’), poor set, and, again, deterioration in the end-breakage or filamentation rates. The tight spots are seen as apparently untextured (or lightly textured) segments in the yarn that show up as defects in the fabric. These tight spots are caused by twist slipping over the false twist pin in an erratic manner. Segments of yarn leave the twist pin containing real twist; a twisted segment of yarn is unable fully to develop bulk. Over-twisting the yarn can produce a similar result. The twist level determines the hand and appearance of the material; a high twist gives the fabric a soft, fine texture, whereas a low twist yields a rough, pebbly look. High twist gives a relatively high crimp contraction and therefore more stretch potential. It also causes more tight spots and weakens the yarn (up to 20–30% strength loss for nylon, but very little for polyester or acetate). Fiber producers apply a finish to the surface of the filaments immediately after extrusion to help drawing and subsequent operations. The finish is intended to reduce static electrification and friction, but when it is heated in the texturing operation, any volatile fractions of the finish are driven off, giving rise to unwanted fumes. Heavier fractions can oxidize or otherwise deteriorate and cause problems with the deposit of solids in the heater zones. This is especially so if high heater temperatures are used (say 400°F, about 200°C). Loss of the fiber finish can also create a problem and it is often desirable to apply a lubricant after texturing. These so-called ‘coning oils’ replace the losses and facilitate winding and fabric manufacture. However, any such oil should be stable and capable of being scoured away without detriment to the color or performance of the yarn. A sufficiency of fiber finish or additive is important but excessive amounts of finish are to be avoided. Also, variations in the add-on levels of finish should be kept to a minimum. Some fibers are dulled by the addition of titanium dioxide (TiO2); this additive affects the wear rate of guides and pins. Such wear can adversely affect the quality of yarn being produced as well as the efficiency of the operation. With a single-heater machine, it is necessary to soft-wind the yarn packages to

96


permit satisfactory subsequent autoclaving to produce set yarns. With two-heater machines, it is necessary to overfeed the yarn into the second heater to allow the crimp to develop. This overfeed level is normally about 4 to 5%. The single-heater machine used in conjunction with an autoclave is less efficient than a two-heater machine. With the batch process of autoclave setting, variations between batches are more likely and thus there is an increased risk of producing barré in the fabrics. This is because of the changes in bulk and dye affinity arising from non-constant heat treatment conditions. Whatever system is used, great effort has to be taken to strictly control all temperatures, tensions, and twist levels so that they are similar from spindle to spindle, from time to time, and from batch to batch. The consequence of a failure to control, in all these respects, is that streaks and barré will be produced in the dyed fabric. Modern machines are equipped with control devices; in addition, strict quality control is exercised by means of proper sampling and testing. However, the potential flaws are rarely visible in the yarn coming from the machines. Therefore, it is necessary to carry out tests on dyed yarn at a very early stage before large inventories are accumulated.

4.4.3 Limitations of the pin twister machine The size of the false twist spindle dictates the maximum rotational speed that can be used. Remembering that the power absorbed by a spindle due to air drag alone is roughly proportional to D4U3 (where D is the diameter and U is the rotational speed), it will be readily realized that the spindle has to be kept as small as possible (see Fig. 4.6). However, there is a practical limit to smallness. It must be possible for a knot to pass through the spindle and this means that the diameter of the central hole in the spindle must be several times that of the yarn diameter. Thus, with 150 denier (167 dtex) yarn, the central hole must be of at least 1 mm (≈ 0.04 inch) diameter; for heavier yarns, the hole must be larger. Requirements for the false twist pin and the need for sufficient space to permit the threading operation control the minimum size of the largest diameter of the spindle. Centrifugal forces acting on the yarn, spindle and drive system can be very high. In the case of the spindle, it is necessary to ensure that it is dynamically balanced; otherwise, at high speeds, it will tend to ‘tramp’ like an unbalanced wheel on a car, and the drive tires might suffer considerable damage as a consequence. As well as encountering considerable centrifugal force, these tires are also subjected to high temperatures (due to frictional heating). The combination of the two can cause polymer creep, with a result that the tires sometimes grow in diameter during service. A change in diameter alters the forces acting on the surfaces. Growth usually signals impending failure of the tires. The surface of the tires can also suffer damage due to high shear stresses caused by the localized loading, and the damage shows up as a pitting of the surface. If the spindle is unbalanced, the loads are greatly increased and failure of the tire surface is hastened. There is usually a finite life for these tires and the units have to be replaced from time to time. Damage and imbalance cause an increase in noise level and faulty machines are difficult (if not impossible) to operate within the legal noise level limits of some countries. The yarn is pressed against the wall of the axial hole inside the spindle by the centrifugal forces. This causes the yarn to drag, which can cause filament breaks, and since the drag is related to ω2d (where ω is the spindle speed and d is the hole diameter), it is obvious that a large central hole in a very high speed spindle is

Filament yarn production 97 Section X–X Yarn lifts off pin

Y

Y

Yarn presses against wall

Hard pin (a) Access hole

Section Y–Y

X

X

(b)

Fig. 4.6

A pin twisting element

undesirable. This is especially important when producing fine yarns. Eccentricity can induce quite strong yarn ballooning in the heater zone. As will be realized, the variations in distance between the yarn and the heater surface can greatly affect the local heat transfer rate. Under certain circumstances, this can affect the set of yarn in a periodic fashion and produce patterning or barré in the final fabric. Additionally, centrifugal force acts on the yarn wrapped around the pin inside the spindle. A portion of the yarn wrap sometimes moves away from the pin as shown in the enlarged sketch in Fig. 4.6(a). Eccentricity of the wrap causes it to pull away even more and the eventual restraint is from the walls of the access hole. The grip on the yarn by the pin is then reduced and twist slips over the pin. Intermittent slippage of this sort generates undesirable tight spots in the yarn. Twist is associated with tension and this is an unstable relationship, which can lead to surges that give operational problems as well as the undesirable periodic tight spots. At the high linear speeds of yarn take-up associated with high speed operation, there is frictional heating of some of the outer filaments of the yarn. Such heating occurs (a) at the twist pin, (b) in the central hole of the spindle, (c) at various guides, and perhaps (d) at the heater surface (if the yarn is not properly controlled). At these ‘hot’ spots, there is likely to be filament damage or breakage. The undesirability of

98


breakage has already been mentioned. Apart from the problems of wild filaments (uncontrolled filaments not bound into the body of the yarn) and reduced yarn strength, the local overheating might cause segments of some filaments to fuse together. Furthermore, it might result in changed local yarn extension, or it might change dyeability at the local spots. Whichever combination of such faults is generated, it impairs both the efficiency of the operation and the quality of the product. In all these cases, the higher the speed, the worse the problems become. Consequently, there must be practical upper limits to speed and this, in turn, means that there are practical upper limits to the productivity of pin twisters. Improvements in the technology continue to raise the limits, but it becomes increasingly more difficult and costly to do so. In fact, the rise of friction twisting caused further machinery developments of pin twisters to show unsatisfactory returns on investment. Whether pin twisters will find a market in the future is uncertain.

4.4.4 Friction twisters In the search for ever higher productivity, the false twist element has, over the years, become ever smaller. The ultimate stage was that the diameter of the high speed rotating element was reduced to that of the yarn itself. After that we had friction twisting with its enormous potential for increased speeds. An example of friction twisting is shown in Fig. 4.4(b) and two embodiments of the principle are shown in Fig. 4.7. In Fig. 4.7(a), friction between the bore of the rotating tube (bush) and the yarn causes twist to be inserted into the yarn. In Fig. 4.7(b), it is the friction between the outside surface of the disk and the yarn that gives the effect. In both cases, there is slippage and therefore it is not possible to calculate the twist insertion rate from the ratio of diameters (i.e. rotating element diameter/yarn diameter). It is better to consider the torque generated. From Fig. 4.7(a), it may be seen that the reaction F must balance components of yarn tensions Tin and Tout resolved in a direction perpendicular to the axis of the bush. For the present purpose we may ignore the components F3 and F4. In other words: F = F1 + F 2

[4.1]

where F1 = Tin cos γ F2 = Tout cos α Since torque is (force) × (radius of action), and the relevant radius is that of the yarn under operating conditions, we may write: Torque = µkFd/2

[4.2]

where d is the diameter of the yarn in the free state, and k is a factor that takes into account the local compression at the contact zone between it and the twister, as well as the end effects at the edges of the twister. The factor k < 1 and µ is the coefficient of friction. In the simple case shown in Fig. 4.7(a): Torque generated by the twister = µ (kd/2)(Tincos γ + Toutcos α)

[4.3]

If n is the linear density of the yarn, the effective yarn radius is K√n, where the factor K includes k / 2 used in equation (4.3) as well as the factor relating diameter to linear density:

Filament yarn production 99 Tout F3

F2

Vout α

U r/m γ

F1

Vin

F F4

Tin

(a) Rubber end caps

β

F

Vout α

Tout γ

Tin Vin

(b) Rubber tire

(c) A selection of disk profiles

Fig. 4.7 Friction twister elements

Torque generated by the twister = µK√n(Tincos γ + Toutcos α)

[4.4]

In other words, the torque is influenced by the linear density of the yarn and its compressibility. It is also influenced by the coefficient of friction, the tensions applied as well as the angles taken up by the entering and departing yarns. Similar logic can be applied to the disk twister, but in this case, K is further affected by the attitude of the yarn on the surface of the disk (the angle β shown in Fig. 4.7(b)), which is discussed in the following paragraphs. The disk type of machine is more widely used, therefore we shall restrict most further discussion of false twist machines in this chapter to that form. There is a degree of self-adjustment in the angle β. However, under unstable conditions, there is surging and the angle fluctuates. At high speeds, torque and tension surges lead to difficulties and impose a limit on the speeds that can be achieved. A feedback mechanism involving the phase relationships between the tension and the rotational speed of the yarn leads to the surging. Equation (4.4) shows that the degree of texturing is strongly affected by the coefficient of friction, the linear density of the yarn being textured, the applied yarn tension, and the yarn angles. The angles α and γ may not be the same, but for the purposes of explanation let them be typified by a single value, θ. The twist level is also a function

100


of the stiffness of the yarn, as well as the torque. For a given yarn, it is important to use high values of µ and θ. To give high values of µ, bushes or disk tires made of urethane or some other high friction material are used. It is difficult to get a high value of θ with a single disk ( (Cross-sectional area) × √πE/F

[4.5]

This suggests that the crimp is dependent on three major factors, namely: the buckling force F, the modulus of elasticity E, and the geometry of the cross-section. The force F is principally determined by the degree of overfeed. The polymer and its heat treatment determine the modulus. The geometry of the cross-section is established during extrusion and is a function of the linear density of the filament. Thus the texture is seen to depend partly on the feed rates and the temperature within the stuffer box.

F

y

F

l

A

l/2 l

Fig. 4.10

Fiber collapse in a stuffer box

Filament yarn production

105

4.6.2 Stuffer box Some modern systems depend on a controlled overfeed and a fiber transport system within the stuffer box such as is shown in Fig. 4.11. An overfeed is a condition where the input speed is greater than the speed further along the process flow line. The transport system is intended to improve the uniformity of the process at high speeds. Without it there can be a tendency to intermittently choke. Even partial chokes affect F and thus the crimp level. Hence, a smooth flow of fiber through the stuffer box is essential. Another difficulty at very high speeds lies in ensuring that each filament is heated to the same temperature. Not only is it necessary to raise the filaments above Tg, but all filaments should have identical temperature histories so that conditions are the same for all. Failure to provide such conditions leads to a variation in crimp level from filament to filament. Although it is not feasible, in practice, to transfer the heat equally to all filaments, at least the variation should be kept to a minimum. Some fine stuffer box textured yarns are plied to give the material a resistance to snagging and filament breakage in the fabric during normal use. However, plying is expensive and there is a loss of bulk in the yarn (which was the purpose of texturing in the first place). Sometimes the bulk is not fully developed until the fabric has been finished and this means that some potential faults are not discovered until the fabric finishing process is completed. Omission of a heat setting stage, or the use of improper Flat yarn input

Vin

Vin > Vout

Stuffer box feed rolls

B

Tractor feed transports fibers through the stuffer box

Heat

VS Cool A Controller

Vout

Winder

Fig. 4.11 Stuffer box texturing

106


processing temperatures, cause changes in bulk and dye affinity, both of which can lead to barré in the fabric. Thus it is necessary to test the product [3] for shrinkage and dye affinity at the yarn processing stage to avoid expensive claims from customers because of improper quality. It has become possible to process yarns at up to 1200 m/min. This may be compared with the speeds obtainable for friction twisting. Unfortunately, the crimp stability and the uniformity of stuffer box yarns is not so good as with false twist textured yarns. Nevertheless, the system is capable of handling relatively heavy yarns so it has become quite important in carpet yarn manufacture [4]. A stuffer box takes up little space and can easily be placed in line with another process. Because of the high speed capability, it is often used for crimping tow. It would be very difficult to do this with other methods because of the large number of filaments involved. Hot fluid texturing is a variant of stuffer box texturing, where the solid filaments in the stuffer box are replaced by jets of hot fluid polymer. As the material enters the nozzle in a plastic condition, the strands are looped or otherwise disturbed before they impinge on the plug of filaments in the stuffer chamber. The outgoing yarn is wrapped around a cooling drum to set the crimp. This is a form of bulked continuous filament (BCF) production, which spins and texturizes the filaments in one operation; it is used mostly to produce nylon and polypropylene yarns for floor coverings [1].

4.7

Air-jet texturing

4.7.1 Simple air-jet devices All the foregoing methods of texturing require that the yarns be thermoplastic so that they can be heat set. This precludes the use of non-thermoplastic yarns like rayon. Air-jet texturing provides a means of creating texture in such materials. Further, it is a useful means of producing a yarn structure near to that associated with staple yarns. This is an important concession to the tastes of the ultimate consumer. False twist and air-jet texturing can be combined. The major principle involved is the tangling effect given by highly turbulent airflow acting on filament feed yarns. Entanglements within the yarn structure are made, and are interlocked by inter-filament friction to form a stable yarn. In some ways, these air-textured yarns resemble staple yarns made by traditional spinning methods. To get the needed air turbulence, high pressure air is supplied to a nozzle and this produces supersonic airflow at the exit. Also, an obstruction or asymmetry is introduced in the airstream to cause a series of violent eddies; this is known as a von Karman vortex stream. The obstruction can be in the form of a hollow needle through which the feedstock is fed. Because the emerging airstream contains shock waves (like those seen in jet engine exhausts), there are some severe pressure gradients in the air discharge. A diagram of the divergent portion of a nozzle with a filament injection needle is shown in Fig. 4.12(a) where the swirling airflow (gray arrows) passes over an obstruction such as needle, creating turbulence downstream (shown in black). The attitude of the needle, and its rotational position about its own axis, are adjusted to maximize the quality of the textured yarn. Because the needle is hollow, it acts as an injector since the static air pressure in the throat of the nozzle is less than atmospheric pressure. Thus, a filament feed yarn can easily be inserted into the exit airstream (Fig. 4.12(b)). Separated filaments follow different flow paths and when the filaments are recombined at an integration point, there are lengthwise displacements of one


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Textured yarn output

Filaments separate from each other

Needle

Yarn input Airflow (a)

Fig. 4.12

Airflow (b)

Air-jet texturing

filament to another; some filaments are overfed and the result is that a structure with loops and bows is formed, as shown in Fig. 4.13. A bow in this context means a curved portion of filament that does not make a complete loop. The needle causes the airstream, which is passing over it at high speed3 to break up into eddies. These eddies can be superimposed on a general vortex motion tending to untwist the feed yarn. The untwisting allows separation. However, separated filaments possess torque because of the untwisting and, if overfed, the filaments tend to curl or snarl and occupy more space. Since the filaments are separated, different filaments are caught by the progression of eddies and there is a tangling effect as the snarls and loops become caught up in each other. Filament separation is an essential part of the texturing operation. The subsequent tension applied to the filaments after they recombine at the integration point causes the loops and tangles to interlock to give a moderately bulky yarn. The yarn has characteristics similar to staple yarn. Longitudinal migrations of portions of the filaments, caused by differing path lengths taken by the filaments between separation and integration, enhance the texture because some filaments are temporarily overfed with respect to their neighbors (in Fig. 4.14, filament a has been overfed with respect to b and c.) The excess lengths produce loops and bows. Compared to false twist textured yarns, air jet yarns are considerably less extensible. In some operations, the entering filaments are moistened, which enhances the texturing operation because of better separation of filaments within the nozzle; control of the flowing filaments is also improved. This is referred to as the wetting process, where one or more yarns pass through a water bath before entering the air-jet. Care has to be taken to remove the debris or finish particles that accumulate, so that the jet nozzles do not become blocked. Alternatively, water applicators are used, which allow finer control of the water applied. A baffle is sometimes used to divert the flow, to create extra turbulence and to 3 The Reynolds Number must be above the critical value.

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Fig. 4.13

Air-jet texturing yarn

Integration point Bows

Filament migration

c

Separation of filaments

b

a

Nozzle

Fig. 4.14

Filament separation


109

lessen air consumption. Baffles can be used to limit the filament bow size and control the loopiness of the yarn. Bearing in mind that stability of the yarn depends on interfilament friction, it might be realized that a drawing stage following the texturing can stabilize the structure by pulling the closely looped portions tighter. The drawing process in this case is like tightening a knot. A thermal process may follow the texturing [5] to achieve a reduction in loop size and to reduce shrinkage in boiling water.

4.7.2 Effect yarns As a class, effect yarns are a speciality of interest to fabric designers looking for special effects in their products. Yarns with nubs, bouclé yarns with loops on the surface, and many more, are members of the class. It is beyond the range of this book to deal with them all, but a few processes will be mentioned in passing to give a flavor of a few possibilities. There are special mechanical attachments that can be fitted to normal spinning machines to produce effects such as aperiodic nubs or loops. Some of these are based on a random speed varying device that affects the draft in staple spinning. However, these are not very useful when drawing a filament yarn because of the variation caused in the molecular structure. More likely one will find devices that raise loops or break them to produce the desired effects. There are also some treatments based on unequal shrinkage of components within the yarn structure to produce bulk, perhaps in a randomly induced fashion. Air-jet texturing is sometimes used in series with the basic yarn process. Some spin staple fibers to form a sheath around a core of filaments; these (together with those described later in this section) are called core yarns. Such core yarns are sometimes regarded as ‘effect yarns’ when they produce special effects rather than act as replacements for traditional yarns. If the components within the combination of fibers or filaments can be induced to shrink differentially with respect to one another, then extra bulk can be produced, sometimes evenly and sometimes not. If some fibers are capable of being set and others are not, then a further set of possibilities arise. Slitting or fibrillating thin polymer sheets may make flat filaments, like miniature ribbons, which can then be made into yarns. Fibrillation may be carried out by drawing a sheet of certain polymers such as polypropylene and concurrently applying lateral stress to produce a yarn of flat filaments without the need for slitting. These so-called flat filaments may be mixed with some of those already discussed to produce interesting visual effects arising from their differing optical properties. Combinations of various of the yarns described in the various sections bring the possibility of a wide range of effects. The idea is extended by extruding different polymers through the same spinneret and combining them as a ‘co-extruded yarn’ (see Section 4.8.6). Alternatively different spinnerets are used for each polymer and the filaments are mingled together before taking-up prior to winding to produce a ‘co-mingled yarn’. For example, it is possible to use a component to give strength in the core and a more aesthetically pleasing fiber as the sheath. The component delivered to the nozzles at the highest delivery speed is the ‘effect’ component, which goes mainly to the sheath, and the component fed at the lower speed becomes the core. (The more slowly moving filaments approaching a mingling point are under more tension than the faster ones, which produces a

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migration similar to that described in Section 3.9.3.) It is possible to use POY as one of the components and to include a drawing stage in the process.

4.7.3 Modified false twist texturing Air-jet texturing is now being used in conjunction with false twist texturing to produce filament yarns with staple-like characteristics [1]. Modifications to the structure involve surface loop control and/or the production of free fiber ends in the surface to simulate staple fiber yarns. Feeding two or more sets of filaments into the yarn at different rates can form loops, and also modifying the polymers can change yarn properties. The conditions in melt spinning can also be varied to alter the structure. The ability to extrude very fine filaments has also increased the range of possibilities. The great number of alternatives not only makes the modern machines much more complex than formerly but the technology draws on a much wider base. The result is a wide range of product possibilities. Control of fiber speeds, tensions, and temperatures at all positions is an essential prerequisite for consistent and acceptable yarn quality. To get high productivity and adequate bulk, it is necessary to use expensive high pressure air. Also, to control bulk, it is essential to maintain the settings, which uses expensive labor. On the other hand, the air texturing produces no appreciable morphological changes in the polymer and at least one source of barré is removed. Productivity is very high.

4.8

Other texturing techniques

4.8.1 Bi-component yarns The basic idea of a bi-component yarn is to use filaments that consist of two parallel components, each having different physical attributes (which affect their shrinking or swelling characteristics). A composite structure has the potential to curl if a filament consists of polymers A and B disposed side by side as shown in Fig. 4.15(a). The filament curls when polymer A is caused to shrink relative to polymer B. This is because of the forces generated by the shear due to shrinkage. If the differential in shrinkage is sufficient, and the ends of the filament are restrained, the curl develops into the reversing-coil helix sketched in Fig. 4.15(b). As with other textured yarns, this improves the bulk and lowers the effective modulus of the yarn. However, the result is obtained without mechanical texturing and therefore is not restricted in the same way. There is potential for very high speed production, but the method is often applicable only to very fine yarns. One method of producing such a structure is to extrude compatible but different polymers through the same spinneret. It is important that the components mutually adhere. This rules out using polyester at the present. Usually two forms of nylon are

(a) (b)

Fig. 4.15

Bi-component yarn


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used. Another method is to combine two dissimilar strands from adjacent spinnerets in such a way that they adhere to produce a bi-component yarn. Again, it is very important to make certain that there is adequate bonding between the components. A considerable volume of such bi-component yarn is used for ladies’ hosiery.

4.8.2 Edge-crimping A product related to bi-component yarn, but not always regarded as such, is edgecrimped yarn. If a yarn under tension is run over an edge (Fig. 4.16), a lengthwise layer of polymer is disoriented and possesses different shrinkage characteristics from the rest of the yarn. The effect can be demonstrated by running a human hair over a finger nail and watching it curl. One of the problems with an edge-crimping process is the maintenance of the edge over which the yarn slides. Variations in conditions at the edge lead to variations in crimp and thus to quality control problems. A further related idea is that of asymmetric quenching of the yarns at extrusion (or elsewhere). The rate of cooling affects the crystallinity and is associated with variations in density. In other words, asymmetric quenching can also produce a texturing effect. It is believed that similar effects could be produced chemically. In any of these cases, the bulk can be developed by heating, which can cause further differential shrinkage (or swelling) to augment the effect.

4.8.3 Twisting and folding of filament yarns It should be explained that ‘folding’ in this context is jargon used in the filament trade; it has a similar meaning to the ‘doubling’ discussed earlier, inasmuch as strands are laid more or less side by side before they are integrated into the final yarn. The process is often a two-step operation with a forming twist being first applied to single ends and then cable twisting the composite to achieve the desired end result. The final product has a low or zero filament twist, but the ply twist is sufficient to control the surface of the yarn. Often two-for-one twisting or a variant of it is used for these operations. There is little or no need for the improvement in evenness that such doubling brings. Reasons for this operation include [6, 7]: (a) entrapment of wild fibers or broken filaments, (b) torque balancing of false twisted yarns, (c) improvement of load sharing between the filaments, (d) changing the load elongation characteristic of the yarn, and (e) changing the optical and tactile character of the yarn.

Edge

Filament yarn input Oiler

Textured yarn delivered

Fig. 4.16

Edge-crimp texturing

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4.8.4 Knit-de-knit texturing The fundamental idea of knit-de-knit texturing is simple. If a fabric is knitted, heated, and cooled and thermoplastic yarn is unraveled from the fabric structure, then the yarn is found to have a texture set into it. The newly unraveled yarn has repeating deformations, but these can be manipulated to redistribute the zigs and zags of individual filaments and create a textured yarn. It is used for certain specialty yarns. For example, where low bulk, lustrous fabrics are required using a fiber such as Quiana® (a high cost nylon used as a high fashion silk substitute), then the knit-deknit process might be appropriate. In such specialty markets, it is aesthetic results that are more important than high productivity and low price. 4.8.5 Elastomeric yarns Elastomeric fibers are characterized by very high elongations at break (up to 100%) and have a composition of at least 85% segmented polyurethane [8]. They owe their extensibility to the soft, elastic material used. Polyethers or polyesters are used as segments of block co-polymer chains, which are joined together by urethane groups but which are not cross-linked. The result is a polymeric structure capable of high ‘power’ yet which can be heat set into desired shapes. In this context, ‘power’ refers to the ability of the material to recover from elongation or other deformation. A large proportion of this material is used in foundation garments, swimwear, and hosiery. Sometimes an elastomeric core is sheathed with another type of fiber to give good aesthetic properties. Care has to be taken that the elastomeric core does not ‘grin’ through to give unsightly changes in color or reflection due to different dye behaviors. 4.8.6 Texturing by co-extrusion Co-extrusion is where two or more polymer components are extruded through the same nozzle to produce a filament with stripes of different polymers (Fig. 4.17). It is difficult to manage more than two components; thus two component systems are likely to be most significant commercially. There are two distinct possibilities. The first is to have the stripes firmly bonded to each other in such a fashion that treatment will cause it to curl or otherwise texture in the manner of a bi-component yarn. The second is to make the stripes have little or no bonding, in which case the filament can be decomposed into a series of finer ones. Ultra fine filaments can be separated from the main body to make silky yarns and a variety of surface effects are possible by altering the cross-sections of the separated fibrils. Multi-lobed cross-sections diffuse

(a)

(b)

(c)

Fig. 4.17 Co-extruded filament yarn and components


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reflected and refracted light to give a dull effect whereas flat cross-sections give a sparkle such as that associated with silk. The author has no details of the production of these materials.

4.9

Industrial filaments

Polypropylene (an olefin) is sometimes used for some non-apparel yarns but care has to be taken to protect the yarns from sunlight, which degrades them. The moisture absorbency is less than 1%, which is a serious disadvantage for apparel and some home uses. However, it does have good dimensional stability if the temperature is kept below about 120°C (≈ 250°F). The main use is in industrial fabrics. For that reasons there is little need to consider texturing the yarns. High tensile man-made filaments, such as those made from aramid polymers, are also used for many industrial applications, such as ropes and cables, because of their very high tenacities. Other common industrial filaments are those of polypropylene and similar polymers, which are used for carpet backings, bale wrappings, etc. Space precludes discussion of the technical aspects of ropes and cordage but the reader is referred to the work of Backer [9]. Other fibers are used because of their modest cost and/or their high strength. Glass and high modulus, high strength fibers, such as carbon, are increasingly used for reinforcement of composites but discussion of this important sector must be curtailed because it carries us beyond the production of yarn. When sheets of certain polymers are stretched, they split in the direction of stretch with a result that the sheet is transformed to a web of interconnected filaments. This process is called fibrillation and it was discussed briefly in Section 4.7.2. The use of chopped fibrillated material falls outside the range of our discussions although some fibrillated materials do end up as yarn, even if only in tape form. Often these fibrillated filaments have a rectangular cross-section. Sometimes the position of the slits is precipitated by ridged roll surfaces, or the sheets are slit. According to Schuur and Gouw [10], it is a pity that water bath quenching is less suitable for making thin films because of draw resonance, which gives unacceptable thickness variations. In other words, it seems that it is not yet possible to make fine fibrillated filaments. The stretching of the film is carried out in ovens with forced-fed hot air. A stretching force of 1 to 2 g/den (i.e. 9 to 18 g/tex) is normally used. Sometimes bi-component structures are created by using laminated sheets of different polymers, e.g. polypropylene and polyethylene. This gives a structure that is easily textured to give bulk. If the sheets are slit into narrow strips, the result is a textured yarn. Untextured strips of polypropylene are used directly as yarns where more robust use is contemplated, as in the manufacture of sacking, bale coverings, carpet backings, and the like.

4.10 Silk filaments and staple yarns Silk filaments are converted into yarn by a process known as throwing.4 The filaments from the skeins arriving from reeling in the filature have to be plied. This requires a 4 From the Anglo-Saxon ‘thrawan’, to twist.

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twist of perhaps 4 or 5 tpi (0.1 t/m) to be added during the plying process. The plied yarns are then twisted to the level required for the end use. Twisting is sometimes carried out by ring frames similar to that shown in Fig. 7.3, but sometimes there is a twister included in the reeling equipment that produces hanks of silk yarn. In many of the silk producing areas of the world, silk goods are an encouraged cottage industry. In those areas, there is still a considerable amount of manual manipulation of silk filaments in the production of yarn. Staple yarns are often thrown using spinning wheels and mule spinning frames. The plied silk yarn usually has considerable amounts of gum left on it, and it is quite normal to produce a warp yarn that needs no sizing for weaving. Most other staple yarns and some filament yarns need to be sized by the addition of a softened adhesive to withstand the rigors of weaving.

4.11

Morphology and dyeing

Dyes are color producing substances that can be permanently attached to or incorporated into the fiber. The affinity between the dye and the fiber depends on the physical and chemical properties of both. As has already been mentioned, the physical characteristics of the fiber depend upon its mechanical and thermal history. The morphology of a polymer changes as it is heated and cooled. It also changes as the fiber is drawn. The dye affinity of the material changes accordingly. Thus, the texturing operation can affect the dyeing operation materially. If there are periodic variations in polymer morphology arising from any of the manufacturing stages preceding the dyeing operation, there will be periodic changes in the color of the yarn along its length. If the wavelength of the error is small, the fault appears in the fabric as a moiré effect. If the wavelength of the error is large, the fault appears as barré. Such periodic errors could be caused by finish deposits on a feed or take-up roll in the texturing, or by faulty winding, or some other mechanical error. Many yarns are dyed in the form of relatively low density cones or cheeses and the winder on the texturing machine has to be configured accordingly. Staple yarns are sometimes dyed in hank form. Thus, if there is uneven dye penetration into the package, a range of error wavelengths may be found from this cause also. It is possible, and desirable, to determine these wavelengths by dyeing a knitted test sleeve, or by other means, to find the source of the problem. In addition, there can be more random types of variation arising from a variety of causes, such as spindle-to-spindle variations in the texturing conditions, mechanical or thermal instabilities in the texturing machines, faulty winding, etc. These variations tend to show up in the fabric as shading or streakiness.

References 1. 2. 3.

Wilson, D K and Kollu, T. The Production of Textured Yarns by Methods other than the Falsetwist Technique, Text Prog, 16, 3, 1987. Atkinson, C and Wheeler, M J. New Developments in Air-jet Textured Yarns for Upholstery, Int Text Bull, 1, 1996. Du, G W and Hearle, J W S. Threadline Instability in the False-twist Texturing Process, J Text Inst. 81, 1, 36–47, 1990.

Filament yarn production 4. 5. 6. 7. 8. 9. 10.

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McCormick, W H. Bulked Yarns Produced by a Stuffer Box Method, Modern Yarn Production, (Ed G R Wray), Columbine Press, Buxton, 1969. Bock, G and Lünenschloss, J. An Analysis of the Mechanisms of Air-jet Texturing, Textile Machinery: Investing for the Future, Textile Inst Ann Conf, 1982. Fischer, K E and Wilson, D K. Air-jet Texturing – An Alternative to Spun Yarn Production, Textile Machinery: Investing for the Future, Textile Inst Ann Conf, 1982. Lorenz, R R C. Yarn Twisting, Text Prog, 16, 1/2, 1987. Craig, R A and Ibrahim, S H. Elastomeric Fibers, 4th Shirley Int Seminar, The Hague, Netherlands, 1971. Backer, S. The Mechanics of Bent Yarns, Text Res J, pp 668–81 and a number of later papers, 1952. Schuur, G and Gouw, L H, Future Prospects for Fibrillated Polypropylene Film Processes and Products, 4th Shirley Int Seminar, The Hague, Netherlands, 1971.

5 Carding and prior processes for short-staple fibers

5.1

Introduction

Short-staple fibers are nearly always processed dry using mechanical means. The first stages in a short-staple spinning mill comprise a number of machines, usually arranged in series, which are connected by fiber transport systems. The most common of these transport systems is where air is pumped through large ducts and carries the fibers in the airstream. The line of machines described is called an ‘opening line’ and it supplies a set of cards in which the fiber flows are usually arranged in parallel. There are usually two or more opening lines in an establishment and the space occupied by opening lines is sometimes known as the ‘blow room’. One function of the blow room is to blend the fibers into a homogeneous product. The term ‘blend’ applies to the mixing of nominally similar fibers or to the mixing of unlike fibers similar to polyester staple fibers and cotton. In the latter case, the blending may be carried out in the blow room or in a process following carding. However, the case being considered in this chapter is that of blending nominally similar fibers. It is possible to blend dissimilar fibers by using the same process as described in this chapter although many operators prefer to do it in processes following carding as will be described in Chapter 6. Fiber attributes vary from bale to bale and within a bale. Even man-made fibers, in which the fiber length and fineness are strictly controlled, have variations in fiber crimp and finish. Variations of crimp and finish alter the mutual cohesion of the fibers within a strand or clump and these can strongly affect the ease of processing. Natural fibers vary in all of their attributes. In both cases it is very desirable to blend as early as possible in the blow room, and to use every succeeding opportunity to carry on the process of blending. The primary stage of blending is carried out by removing clumps of fibers from a succession of bales and mixing them in the machines that follow (see Section 5.4). The secondary stage is carried out in a blending machine with the intention of homogenizing the material in transit. Mixing also occurs in every machine in the opening line as well as during transit. Further blending occurs in processes following

Carding and prior processes for short-staple fibers

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carding as will be seen later in Section 6.4 in Chapter 6. All contribute to the degree of fiber homogenization in the total spinning process; however, for now we must concentrate on the blow room. Raw material is supplied to a mill in highly compressed bales of fiber. One important function of the blow room is to disintegrate these bales into a flow of very small clumps of fiber, which are sufficiently small in size to be digested by the cards. The cards then further divide the clumps into single fibers (or very small groups of them) and assemble them into rope-like strands called ‘slivers’. The function of breaking up the bales into clumps, and the subsequent reduction into single fibers, or very small groups of them, is referred to as ‘opening’. (Also the term is used as an alternative to ‘blow room’ but the context usually makes it clear which meaning is intended.) A third function is needed for natural fibers, the most important of which in shortstaple spinning is cotton. As was discussed in Section 2.2.1 in Chapter 2, cotton ginning is imperfect as far as removal of the trash is concerned. Ideally, unwanted matter must be removed so that it neither interferes with operations nor causes significant deterioration in the quality of the product. In practice the ideal is not reached but modern technology allows a close approximation to it. The mechanical cleaning function is not required when spinning 100% synthetic fibers, but many mills have this broad capability irrespective of the fiber actually being spun; this gives them operational flexibility. Recombination of fibers into a larger mass occurs at various stages along the opening line and I will call this phase ‘condensation’. Such condensation is necessary to accommodate the control of the fiber flow in a continuous line and to aid the processes of accumulating fibers to make the feed systems workable. Feed systems often use moving lattice aprons to collect fibers deposited from streams of air or gravity feeds. A lattice apron is an endless permeable belt of slats, each of which is positioned perpendicular to the line of the belt movement, but parallel to one another. Air is often sucked through the gaps between the slats. The slats contain metal spikes to retain the fibers. A rotating condenser is a perforated cylinder, to which suction is applied so as to collect fibers from an airstream. The process of condensation is really a form of doubling (see Section 3.10.1), which improves evenness along the fiber stream. The word ‘stream’ is meant to include airborne fibers flowing in ducts, thick blankets of fiber (called batts or fleeces) being carried by mechanical transport mechanisms (such as lattice aprons), and sliver being delivered from the cards. It is impossible to blend the fibers into an intimate blend without opening them first. A perfect intimate blend would have a single fiber of one sort in very close proximity to single fibers from each of the other sorts. Imagine trying to blend clumps of fibers of, say, 1 cu ft in size into a homogenous product. It would be rather lumpy and the blend would hardly be characterized by the word ‘intimate’! Also, adequate cleaning of natural fibers is not possible without opening the clumps first. In the case of cotton fibers, it is relatively easy to remove the trash and dust from the outside of a clump but it is much more difficult to remove spiky trash or even dust from inside without damaging the fiber. With so-called cleaning machines, there is, of necessity, a great deal of opening and a certain degree of blending. Dust and trash has to be removed from the bale plucking machines. Dust and trash is ejected from blending machines, the main job of which is to accumulate fibers in reservoirs to facilitate blending, as discussed in Section 5.4. Dust and trash is removed from socalled opening machines as the clumps are divided. Consequently, each machine in the opening line performs functions of blending, opening, and cleaning in varying

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proportions. Thus the processes called opening, cleaning, and blending should be regarded as various phases within the operation of each machine and this is distinct from the labels applied to the machines. However, a major point of this paragraph is to emphasize that the labels applied to individual machines in a blow room describe their major function; all the machines perform all three functions but to varying degrees. Cards are included in this analysis because they share many of the functions of the preceding machines, and they are physically part of the modern linked system for performing the processes described. The whole system described in this chapter is integrated. Bales of fiber form the input and sliver is the output. In most modern systems, the fiber is untouched by human hand from the time that the bale is placed into position in the bale laydown until the sliver emerges from the card. It is now a continuous operation and there are few demands on labor. Such a system is known as a chute feed system because the fiber is fed to the card by way of a ‘chute’. (A chute is really a temporary storage chamber that contains automatic flow control devices to maintain the linear density.) Any failure to control at this point would eventually result in exceptionally long-term errors in the yarn. Errors in the yarn arising from this cause are so long that they extend over the mass of yarn on many consecutive bobbins. Once the bobbins become mixed with others, the error appears as a random count variation. Consequently, flow and control will also be discussed later in Section 5.9. This chapter will be written using imperial units common in the USA and many other English-speaking areas. However, metric conversions will be given but in a paragraph with several such conversions, they will gathered at the end of the paragraph to minimize distraction to the reader.

5.2

Opening line

5.2.1 The elements of the chute feed system The elements of a system are shown in Fig. 5.1. The diagram is deliberately incomplete because the space in the diagram was at a premium and the intention was to give an Opening and cleaning machines Bale plucker Bale laydown

Mixer

Cards Card slivers

Fig. 5.1

Elements of an opening and cleaning line


119

impression rather than a prescription. The actual machines installed are determined as a matter of operational need and preference. For example, the blending machine is shown as being the last in line before the cards on the basis that the best blending is achieved when the fiber clump size is very small. However, some prefer to install it earlier on the basis that good blending aids the processes of opening and cleaning. Some use more than one blending machine per opening line but the extra cost of the machines is sometimes hard to justify. Only four cards are shown in the diagram but in an actual plant there are several times this number; the actual number is determined by the relative productivities of the cards and the bale plucker (the device that removes clumps of fibers from the lines of bales). There are usually two or more opening lines, because this permits a shutdown of one for maintenance, adjustment, or other purpose without closing the whole mill. In similar vein, the ductwork for the machines has not been joined up to emphasize (1) that a variety of ductwork transition pieces are needed to complete the fiber flow circuits and (2) that bypasses are often fitted which require flap valves and forked ductwork. Safety is a special concern in the work zones about to be discussed; consequently a special Section (5.12) about such matters has been added to the chapter.

5.2.2 The historical perspective Szaloki [1] points out that there were few changes in the design of opening and cleaning equipment in the first half of the twentieth century. There was then a surge in development spurred to some extent by the increased need to clean the cotton as it increasingly became picked by mechanical means. He gives a review of opening and cleaning equipment as of 1976. Remarkable progress was made during the last century in developing means of connecting discrete machines into continuous production systems. A good example is the blow room just described. At the beginning of the century, it required many workers to control and transfer material from machine to machine in the series.

5.2.3 Conservation of flow Opening and cleaning machines have to be connected in such a way that matches the productivities of the various components. Since the machines in an opening line are all connected, mass flow has to be conserved. The conservation includes not only the fibers flowing into and out of any element, but also the trash and dust removed. In other words, what goes in should come out! Appropriate fiber transport systems have to be provided so that a continuity of fiber flow and control can be maintained. Also there has to be a distribution system that connects a series of cards in parallel to the supply system. The change from a series path to a set of parallel paths is needed because the equipment in the series path has a much larger production capability than the individual cards in the parallel paths.

5.3

Bale preparation

5.3.1 Selection of bales from the warehouse An intimate blend starts with the selection of an appropriate number of bales from large lots in the warehouse. Lots are usually segregated to provide compatible content,

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and a bale withdrawn from a lot should have, in theory, similar attributes to the rest of the bales in the lot. In fact, there are large variations and part of the art of blending is to arrange the assignment of bales to the various lots in a way that minimizes the variance within the lot. The bales withdrawn are arranged in sets to make a laydown as described in Section 5.4.

5.3.2 Opening (the process of removing straps and bale covers) Bales arrive at a mill in a compressed state and they are protected by a covering and stored in a warehouse. The bales are moved to the work area a day or so before they are needed, the straps and the coverings are then carefully removed, and the bales are allowed to condition. The bales are said to be ‘opened’. (It will be noted that the term ‘opened’ takes on a different meaning from that defined earlier and care has to be taken to make sure of the context of the word.) Removal of the straps can be dangerous if not carried out with proper equipment because when the straps are cut, they relax violently and injury could result if due care is not taken. Failure to completely remove the bale coverings can lead to the inclusions of ‘foreign fibers’ that produce faults in the yarns and fabrics. (Removal of the last vestiges of the covering from the underside of a 500 lb (≈ 227 kg) bale is not easy.)

5.3.3 Bale conditioning The conditioning just mentioned allows the moisture content and temperature of the fiber to approach stability. The bales, freed from the restraints of the straps and bindings, expand and they are said to ‘bloom’. Bale blooming is a natural process in which the bale grows in size as the stresses in the fiber, arising from compression, release themselves. Conditioning not only allows the moisture content of the bales to approach equilibrium but, in cold weather, eliminates condensation of moisture on the cold fibers. Damp fibers are difficult to process. At the other extreme, excessively dry fibers are subject to being damaged. The storage environment should be at about 70°F (21°C) and the rh at roughly 45%. Care should be taken to avoid storage of cotton in freezing conditions otherwise the strength of the fibers will be reduced permanently. A conditioning curve for a typical bale is shown in Fig. 5.2, which shows how the density of a typical bale might take several days to reach a stable state after the straps have been released.

Bale density * (arbitrary scale)

3 2 1 0 0

5 10 Days after release

15

N.B. * Bale density was measured by penetrometer

Fig. 5.2

Bale density changes upon releasing the straps


5.4

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The first stage of blending and opening

5.4.1 The bale laydown Two or more parallel rows of conditioned bales are laid on the floor in proximity and this is called a ‘bale laydown’ as was sketched in Fig. 5.1. The overall purpose of opening is to break down the supply material into an open mass of very small clumps of fiber that can be handled at carding. The word ‘opening’ is used here in a different sense from that described in Section 5.3.2. The first step is to remove clumps of fiber from the bales and this is referred to as ‘bale milling’ or ‘bale plucking’. A bale is a tightly packed mass of staple fiber usually weighing about 500 lb (≈ 227 kg). Several packing densities are used to yield so-called flat, standard, and high density bales. Mills within the USA use standard bales but high density bales are used for transoceanic transport to conserve space and cost. Flat bales are of low density and are used by mills close to a gin. It is desirable to allow the bales to bloom sufficiently and to choose bales of the same relaxed size. Otherwise the first cuts from the bale laydown will not be according to plan. Bales are supplied from the warehouse in carefully selected sets designed to minimize variations in the fiber attributes. To these bales, others containing recycled fiber are often added. However, the number of these should be strictly limited if quality is to be preserved. Each bale should be inspected for fragments of wrappers or wire before they are assembled into a laydown. Bales should be assembled so that they are in close proximity to their neighbors in the laydown and similarly oriented to create a compact mass of fiber suitable for the bale plucker operation. Care should be taken to keep the height of the bales similar.

5.4.2 The bale plucker The first mechanical processing stage commonly used in the mill today is a patrolling ‘bale plucker’ or ‘bale milling machine.’ Sets of rotating spikes or teeth are used, which cut into the operational surfaces of the laydown in a manner similar to that shown in Fig. 5.3(a). A typical cutter and associated press rolls are shown in Fig. 5.3(b). The press rolls are to keep the bale surface firm at the time of cutting. Most (b)

(a) Fiber

Cutting head

Bales

Fig. 5.3 Milling a bale laydown

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machines use at least one pair of cutters. Often the other one in a pair is a mirror image and rotates in the opposite direction. Some machines cause one of the cutters to lift in order to balance the offtake of fiber between the cutters. The changes occur at each reversal of the bale cutter head (at the ends of the bale laydown). Some makers stop the trailing cutter because the teeth are facing the wrong way. The depth of cut is an important parameter in determining productivity and degree of fiber separation. A setting more than, say, 0.2 inches (5 mm) might cause the top surface of the laydown to become roughened with fiber tags. These tags are torn off in the next pass to form unacceptably large clumps of fiber in the offtake. The theoretical minimum number of bales in a laydown is determined by the adequacy of the blending equipment and the diversity of the fiber characteristics from bale to bale. Currently the maximum set by machine design is about 50 bales but improvements in the technology of bale milling and blending will increase that value. In practice, the number of bales is set to give a work schedule that is suitable for management of the personnel and minimizes costs. The greater the number of bales in the laydown, the fewer the number of laydown changes and the lower the cost of operation. Also, the use of bale pluckers that deliver fibers in small clumps contributes greatly to the solution of the problem of opening. The clumps at this stage should be the largest in the system and the spikes or teeth that temporarily grip the clumps have to be proportionate in size. It is desirable that bales in each laydown should be of similar size and density. If they are not of the same height, the first cuts will differ in composition from later ones. If they are of different density, the blend make-up in the output will not always be as predicted from the initial bale data.

5.5

The process of disintegration of fiber clumps

5.5.1 Opening (the process of division of fiber clumps) In the stages of the opening following the bale plucker, machines with an opening function have the task of separating clumps of fiber into smaller ones. The sizes of the clumps, and of the teeth that deal with them, are progressively reduced. In general terms, grasping clumps of fibers with sets of teeth and dragging the clumps across another set of teeth or grids perform the opening function. The engagement of a clump with two sets of teeth in which there is relative movement applies a shearing action that pulls the clump apart. Since this process alone cannot be seen in any machine, and the design of the machine is affected by the other processes it has to perform, further discussion of the machine design will be deferred. Most machines have feed rolls and toothed elements that are significant drafting systems. Fibers are caused to slide over one another as fiber clumps are divided and the resulting daughter clumps or single fibers are removed. The divided fiber clumps, fibers, and non-lint material are carried to the next machine by the airflow and the material is discharged into a receiver that might be a chute feed, condenser, or the like. The receiver has a doubling function as will be discussed in the next section.

5.5.2 Specific volume of the fiber stream The specific volume of a bale varies. However, the changes are dependent on the fiber characteristics, the original bale density and the atmospheric conditions prevailing


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at the time the bale is blooming. The specific volume of the bales at the time the bale plucker is removing clumps affects the size of the fiber clumps and the performance of the downstream machines. Thus bales should be opened at least a day before they are put in a laydown. Of course, the specific volume of the fiber is much higher in transit through the pipes connecting the machines because the fiber clumps are dispersed in air. It also decreases during condensation at the collection points at each mechanical feed system. Every time the material is further opened, the specific volume increases and these changes have to be taken into account in calculating pipe sizes, feeder speeds, and so on. The degree of fiber openness also affects what sort of cleaning is effective.

5.5.3 Maintenance of the machine elements The working elements of the various machines such as beaters, grids, etc., must be kept in good condition and should be properly set. If the elements in contact with the fiber become bent, nicked, or otherwise damaged, the fiber is likely to be damaged. Fibers may collect and clog the machine, and neps may be produced. (Neps are tiny balls of fiber that degrade the appearance of yarn and show up strongly on the surface of fabric. Unfortunately neps often dye to a different shade; this emphasizes their presence and reduces the value of the product.) As was noted earlier, the size of the machine elements gets smaller as the fiber passes downstream in the process line. This makes them more vulnerable to damage and increases the likelihood of producing fiber damage especially if they are not properly maintained and set. Furthermore, where the relative velocities are high, the abrasion of the metal surfaces increases. A major purpose of the opening line and card is to reduce clumps of fiber to single entities. Sufficient working is needed to do this but excess working can only damage the fiber, produce nep, and remove useful fiber mass. Careful assessment is required to make sure that there is only just sufficient opening and cleaning.

5.6

Condensation

5.6.1 Feed arrangements of the various machines in the opening line The fiber stream is carried through some machines purely by the flow of air (e.g. the axiflow machine in Fig. 5.4(a)) but in others, a mechanical feed is used. The fibers and the air have to be separated to allow a mass of fibers to be advanced by mechanical means into the working area of the machine concerned. The first sort needs no explanation as far as the feed is concerned. The latter does need some explanation.

5.6.2 Accumulation of fibers at the feed As mentioned earlier, the process of separating the fibers from the air and the accumulation of fibers on a surface is called condensation. An example is the weighpan feeder (Fig. 5.5), which uses lattice apron feeds. The lattice apron is a permeable ‘belt’ on to which fibers are collected and the air passes through the belt. Another example is a rotating ‘condenser’, which collects fibers on the inside of a porous drum. Dust from the fibers is pulled through the perforations to be carried away by suction. A fiber take-off system is applied to the outside surface to keep the quantity of fiber on the surface at a given level. The openings in the lattice

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Handbook of yarn production Beaters rotate

Fibers out

Adjustable louvres Fiber in Grid bars Trash (a) Beaters

Fiber output Fiber input

Adjustable grids

Trash (b) Fiber + trash input Condenser

Cleaned fiber output Dust + Air

Gridbars Trash (c) NB Drawings not to the same scale.

Fig. 5.4

Fiber cleaners

apron are fairly large and the size of fiber clump that can be captured is also fairly large. On the other hand, the openings in the condenser surface are small so that small clumps and single fibers can be collected on the outer surface. As mentioned earlier, the process of accumulating many layers of the incoming fiber causes an appreciable degree of doubling. It will be recalled from Chapter 3 that the term ‘doubling’ implies a considerable diminution of the unevenness in the material


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Doffer

Level control

Fiber flow Weighpan

Fiber feed

Fiber out

Conveyor belt Lattice apron Lattice apron

Fig. 5.5

Return flow

Weighpan feeder

collected on the collection surface. Thus, there is an improvement in the short-term evenness of the product at that point. (However, after the material passes through the subsequent machines, the error assumes an ever longer wavelength, and in the yarn it will be seen as a very long-term error.) The doubling just mentioned tends to hide the unevenness produced by the opening processes. Fibers and fiber clumps are attracted to the outer surface of the condenser just described. Perhaps what is more important, fibers are attracted preferentially to the thin spots in the fiber mat lying on the screen because more air flows in these zones. Conversely, there is less airflow to the thick spots and the rate of fiber flow to these thick spots is reduced. Thus, there is an automatic regulation effect.

5.6.3 Mechanical feeds The previous paragraph dealt with lattice aprons as condensers, but they also fulfill another function. They serve as transport systems that move the fiber from place to place. They often serve as a means to project fiber clumps into a stream of air. (The airstream is, of course, another means of transport.) A second mechanical system is to use a pair of rolls to grip the incoming fiber and feed it forward. The rolls may be pinned or fluted and they have to be set at a distance apart that will entrap the incoming fiber and not jam up or choke. A third mechanical feed system is to use fluted or pinned rolls. The rolls engage a batt of fiber and induce it to slide over a smooth surface. Figures 5.8 and 5.10 relating to chute feeds often used to supply fiber to a card will be shown later.

5.7

The process of cleaning

5.7.1 Philosophy of cleaning Natural fibers, in the state that they reach the mill, have mineral and vegetable particles lodged between them. With cotton, there are often seed coat fragments attached to them. It is difficult to remove some of the extraneous matter without vigorous mechanical action and without adequate opening. Every time a clump of

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fibers is divided, a new surface is exposed from which it is relatively easy to remove the loose unwanted matter (trash) but trapped or bonded material is a different matter. The machines in the opening and cleaning line are intended to remove waste but the amount of waste in the raw material is quite variable. At one extreme (man-made fiber) the waste is very low. At the other extreme, with some natural fibers, there might be as much as 10% (or even more) non-lint material. There might also be some unwanted nep-prone fiber. As was discussed earlier, cleaning nearly always accompanies an opening function. As the material is opened, the specific volume of the fiber mass changes considerably. To maintain an approximately constant mass flow of fiber through that process phase, the mean velocity has to increase. Imposing such acceleration on the moving fibers is really saying that the flowing material has been drafted. Although it seems obvious, the material removed during cleaning must not be allowed to re-enter the fiber stream. This is because, not only would it be inefficient, the material removed would contain fibers damaged by the cleaning operation, and which might cause extra problems in subsequent processing.

5.7.2 Various means of cleaning There are several ways in which fiber can be separated from trash. Newly removed fiber is removed from the bale plucker by an airflow running at perhaps 100 ft/sec (30 m/s). A substantial stream of air carries the fiber to the next machine. It is usual to have a magnet in the air duct that can remove ferrous materials from the flow and thus reduce damage to the following machines from these foreign objects. The magnets are sometimes called ‘humps’ because of their unusual geometric arrangement. Also some operators use pneumatic separators that throw out nonferrous foreign objects, as sketched in Fig. 5.6. (The sketch is based on sales literature of Trützschler Gmbh & Co, Germany, to whom acknowledgments are made.) Heavy particles are thrown out because air is forced to flow in a circular direction; air and fiber are sucked back into the airstream but heavy particles such as wood and stone are ejected. It is surprising what is sometimes found in bales of fiber! In Figures 5.4(a) and (b) it will be seen how the exposed surface of a clump of fibers is gripped by pins or teeth and dragged over one or more edges formed by grids to remove trash. The trash drops through the slots and the fibers go on their way. Grid bars or screens are used in several places in the opening line and in the card. It must Input = Air + Fiber + Heavy particles Output = Air + Fiber

Output

Input Rod grid

Returned fibers

Heavy particles

Fig. 5.6

Heavy particle removal


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be appreciated that with cotton, the trash particles are often attached to the fibers and it is not easy to remove all of them. As the fiber is worked, trash tends to become separated from the fiber and it is necessary to have a fairly large number of cleaning points to be effective. As was explained earlier, the fibers are usually gripped in these machines by using saw-like teeth (Fig. 5.4(c)) or pins. The pins can be very large such as those used in the cleaners shown in Fig. 5.4(a) and (b) or they can be very small as in the case of pinned rollers used for improved fiber separation. These latter pins are commonly conical in shape with sharp points and they are more susceptible to damage than the others. Also the more aggressive action makes fiber breakage more likely and it is important the fiber clump size be reduced as much as possible before entering the machines equipped with fine pins. The saw-like teeth are similarly used where the fiber clump size has been reduced. The saw teeth are used extensively in carding. Some machines include one or more perforated screens to which suction is applied for the purpose of removing some of the dust and fine fiber particles. In some other machines, a rotating condenser collects fibers from an airstream on its outside surface and dust from the fibers is pulled through the perforations to be carried away by suction. Another way in which fiber can be separated from trash is to use the differences in mass and air drag between a fiber and a trash particle. This is rather like the method used by the primitive farmer who winnows the chaff from the corn by allowing the wind to carry away the chaff. The fibers and the trash particles have different trajectories that permit separation. A batt of fibers may be beaten and/or vibrated to cause unwanted particles to filter down through the mass so that the unwanted material can be removed. However, hard, spiky, or attached particles do not respond to this treatment unless they have been crushed and/or abraded in a prior operation so that they become detached or less spiky than they were. The thicker the batt or the more dense the clump of fibers, the more difficult it is to remove the trash. Effective cleaning in this way cannot be carried out until the fiber is well opened. Many machines are effective in removing the dirt on the surface of a tuft; they are less so in removing dirt from the center of the clumps.

5.7.3 Some examples of cleaners Pre-cleaning machines are sometimes inserted first into the line to divide the clumps and removing the worst of the trash. In the case of the ‘axiflow’ machine shown in Fig. 5.4(a), there is only that overall drafting which is caused by any acceleration of the fiber stream. However, the tearing apart of the clumps provides drastic local episodes of drafting. The main emphasis is that of removing trash from the outside of the fiber clumps. An adjustable louver, or some other air control device, is provided because it is important that the air pressure should be balanced across the grids. Lack of proper pressure balance can cause newly released trash to be reintroduced into the main fiber stream, or more usable fiber is taken out with the trash than is necessary. If there are local zones of low air pressure, trash can be sucked back into the main flow. To this end, it is important to make sure that access doors and hatches are properly closed during operation. The inclined cleaner shown in Fig. 5.4(b) provides multiple stages of cleaning with the first stage usually being the most effective. The type of cleaner mentioned

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in this paragraph is used at the beginning of the opening line where the size of fiber clump is relatively large and there is a need to guard against too violent an action because it would damage the fiber. Each case has to be judged on its merits depending on the quality level being sought. The type of cleaner shown in Fig. 5.4(c) is used in later stages where the clump size has been reduced and the more aggressive parts of the working elements (i.e. the teeth) are less likely to cause fiber damage because of the openness of the material being worked. The degree of cleaning has to be balanced against the cost of the fiber removed, remembering that fiber costs are about half the total cost of the yarn.

5.7.4 Maintenance of cleaning machines Cleaning machines work in a hostile environment. When working with man-made fibers, abrasion can be caused by the fiber finish or even by the fibers themselves. With cotton, much of the dust is silica (or other minerals) from the soil in which the plants were grown and abrasion of the components of the cleaner is increased by its presence. Similar remarks apply to other vegetable fibers. In the rare cases where short-staple animal fibers are used, accumulations of grease or other material tend to clog the machine. In the various cleaning and opening machines, distances between co-operating surfaces have to be set to control the material. As already mentioned, it is desirable to get as small a tuft size as possible without impairing productivity or damaging the fiber. As elsewhere, good maintenance is required to make sure that beaters and grids do not become nicked or worn because such damage can damage the fiber and cause increases in nep level, both of which reduce the value of the yarn.

5.7.5 Fiber handling Fiber handling is a term that encompasses not only transport of the fibers, but also the condensation, cleaning, and control of the fiber flow. Transport of the fiber is usually by a pneumatic system that carries fiber from the bale plucker or weighpan feeder to the following machines, between adjacent machines in the line, and eventually to the chute feed that condenses the fiber before carding. This involves the use of high volume fans that generate the pressure difference to create the flow of air. Sometimes the fiber passes through the fan blades, in which case there is danger of fiber damage and nep creation. Sometimes the fan is at the receiving end and a condenser is used to collect the fiber and allow the filtered air to pass to the fan. The design and installation of the air ducts have to be carried out with care because sharp bends and joints that create turbulence can cause nep. Also jagged metalwork, protruding screws or the like can create fiber strings that are very difficult to separate in later processes. A ‘string’ is created by a clump of fiber being caught up by some projection in the ductwork, twisting in the wind, and trapping more and more fiber as it lengthens until it looks like a piece of thick string. Fiber strings then break off intermittently to produce material that is difficult to divide without fiber damage and is liable to produce chokes. Machines along the process line often have a sort of chute feed that not only condenses the fiber but also regulates the flow by mechanical means. A discussion relating to flow and regulation is given later. In general, the chute contains some device to measure the height of fiber in the


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chute and some means of controlling the packing density of the fibers in the column. Vibrating front plates, airflows, and careful geometric design are elements often employed to maintain the packing density. Once that is maintained, a simple volumetric control of the fiber feed suffices. It might be realized that machines with beaters and saw-toothed coverings are extremely dangerous if left unguarded. In many countries it is a legal requirement to provide proper interlocked guards that will cause the machine to stop if removed. Also remember that most of the machines are heavy and the inertias involved do not permit them to be shut down quickly. Not only are the safety requirements legal in nature but there must also be a strong commitment by employed and employers alike in the matter of safety if accidents are to be avoided.

5.8

Intimate blending

5.8.1 The consequence of poor blending Fibers may be blended in a mill by either mixing them together before carding or by running several slivers of each sort in a creel of a drawframe or other sliver processing machine. The former process is referred to as ‘intimate blending’ and the latter as ‘creel blending’. Creel blending might be carried out at the drawframe or the comber but we must defer that discussion until the next chapter. For the purposes of initial explanation, it is useful to consider an intimate blend of two fibers, say polyester/ cotton. If the fiber tuft size is too large, the card web will contain streaks of 100% polyester or 100% cotton, and these streaks appear in the sliver. Even with nominally similar fibers, streakinees will exist. Certain fiber characteristics will exist as streaks in the card web just in the same way that the streaks of polyester or cotton had appeared. Thus it may be seen that clump size in the feed to the card is an important factor in following stages of production. Next consider bales of all the same nominal type of fiber. Despite being of the same type, there are differences in fiber attributes from one bale to another. During opening, fiber is taken from sets of bales. These sets are set in sequence along the laydown. Concentrations of fiber from a given bale set might not be completely dispersed among the rest by the blending. Variations arising from the bale-to-bale changes in fiber will then appear in the sliver produced. There has to be a substantial element of mixing in the opening line to make sure that the fibers from the original bale laydown are properly homogenized. Failure to do this produces results similar to those just discussed, except that the size and distribution of the streaks are on a larger scale. Difficulties do not appear until the yarn is made up into fabric, at which time so-called ‘dye streaks’ and ‘barré’ occur. Streaks of fiber that do not match the neighboring areas of a fabric can produce an effect very disturbing to the user. If the streaks are long enough, they appear as bands. Also if cones of yarn have different properties from others in a lot, this too will produce bars in the fabric. This is called barré. Such faults in the fabric are common causes of customer complaint with the responsibility being laid on the yarn maker; the settlement of such claims can be very costly. 5.8.2 Coefficient of variation (CV) as a measure of blending efficacy (Efficacy is used in the subheading rather than efficiency because the latter is difficult to define, as will be realized from the following text.)

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Before launching into explanations concerning variation, it might be useful to define some textile measurement terms. The fiber attributes quoted in Tables 5.1 and 5.2 are commonly used in the cotton industry and they are listed below. MIC =

Micronaire (a measure of fiber fineness) This is an old measure of fiber fineness analogous to linear density, which is still widely used in the cotton industry. The values used to be quoted in the unlikely units of mg/inch but they are now regarded as just indexes. It is really a measure of permeability of a wad of fiber in a specified enclosure as described in ASTM standard D1448. UHM = Upper half mean length (inches) The population of fibers in a sample may be divided into those longer than average and those shorter. The short fibers contribute little to the strength of a yarn. The long ones make more than a proportionate contribution to strength. The mean of these long fibers yields a single figure of merit that gives an idea of what useful length is available in the lot of fiber that was sampled. STR = Fiber tenacity (gf/tex) This is an old standard definition of normalized fiber ‘strength’ or ‘tenacity’, which is still in use. The new standard is in terms of mN/tex. A tenacity in gf/tex is multiplied by 9.81 to get it into mN/tex. ELO = Fiber elongation at break, % Fiber reflectance determined by a Nickerson-Hunter colorimeter according Rd = to ASTM standard D2253 SFC = Short fiber content (%). The length of these fibers < 0.5 in (12.7 mm) +b = A measurement of fiber yellowness determined by a Nickerson-Hunter colorimeter according to ASTM standard D2253 CGRD = color grade, which determines the grayness of the fiber Area = Percentage of a test surface covered with trash removed from a sample of cotton under standard conditions. If a blend were perfectly homogeneous, there would be no variation in the fiber attributes over any number of samples. Clearly the blend cannot be homogeneous if any, or all, the various fiber attributes such as micronaire, length, etc., vary over the set of samples. Variations in fiber attributes tend to be independent; the value of CV of one attribute is not necessarily reflected in the others. Often, color grade and short fiber content of the sample have very much higher CVs than the rest of the attributes. The blend is usually significantly worse concerning short fibers than the upper mean length of the fibers (UHM). Most notable is the tendency for most CVs to decrease in the opening line and then increase again in subsequent processing. In the opening line there are large drafts but there is always a compensating amount of doubling in the mixer, chute feeds, and the like. There is also removal of some undesirable matter. In the case cited in Table 5.1, it will be noted that the trash and dust levels fell markedly in the opening line as, of course, they should. There is often a slight rise in CVs between the bale and card sliver. In post-carding stages of drawing, the draw and doubling ratios are usually roughly equal and there is little removal of material even though the CV might be relatively high. Nevertheless drawing still tends to reduce the trash and dust content but it often causes slight increases in the CVs for the other fiber attributes. Such a result was found by El Mogahzy [2] in an industrial case study. He also found that variations in

Carding and prior processes for short-staple fibers Table 5.1

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CVs of fiber attributes at various stages

Bales Chute feed Card sliver First drawing Second drawing

MIC

UHM

STR

ELO

Rd

Trash/g

14.4 2.4 3.1 2.1 2.9

5.0 1.7 1.3 1.4 2.1

8.3 5.5 3.8 3.3 3.5

12.1 6.3 5.0 5.5 6.2

6.3 1.1 1.2 1.6 1.5

87 16 28 24 24

Dust 73 51 28 27 8

color grade were highly significant in the particular case (see Table 5.2). Space precludes inclusion of all the data available. It can also be seen that the within- and between-bale CVs were comparable and that implies that control of the bale laydown quality is imperfect if no account is taken of the within-bale variance. Normally bale selection is based on two or three samples per bale and this may be insufficient in some cases.

5.8.3 Intimate blending As was mentioned earlier, the first step is to assemble a bale laydown, and the number of bales in a laydown is usually determined by the operational need to run without replenishment for a round period (commonly 24 hours). The physical arrangement is that which is best suited to enable the bale plucker to remove fibers layer by layer. Once prepared, the bale plucker is set in motion and starts a flow of fiber into the opening line. The cutting head usually moves to and fro across the top of the laydown and the height of the head above the ground is progressively decreased after every traverse. The depth of cut is thus determined. The depth of cut and speed of traverse of the bale plucker decide the degree to which the material is separated into tufts. Both of these parameters affect the productivity of the machine and the size of the fiber clumps generated. There are other schemes for taking fibers from the bale supply but space precludes further discussion. Once a stream of fiber clumps is established, there are several ways to homogenize the flowing material. Apart from the mixing created as the fiber stream passes through each of the machines already described, there is normally a blending machine, whose main function is to homogenize the flowing fiber. A typical machine of this sort is a sandwich blender in which fibers are laid in layers on surface AA as in Fig. 5.7(a). Table 5.2

A selection of CVs of fiber attributes

Bale no.

2

3

4

6

7

8

9

MIC UHM STR ELO Rd +b CGRD Area SFC

2.4 1.3 5.0 6.3 4.2 4.1 38.2 106 19.4

2.3 1.4 4.2 6.6 4.9 6.0 43.0 110 15.4

2.1 1.3 4.2 7.8 4.4 4.9 35.8 99 14.9

2.2 1.5 4.7 5.3 4.4 5.0 38.4 104 14.4

1.8 1.2 5.1 6.0 4.3 5.1 38.6 131 14.6

2.1 1.2 3.9 5.3 4.2 4.3 42.1 132 13.2

2.0 1.2 4.4 7.4 4.6 4.5 46.6 133 12.8

CV (%) between bales 3.3 0.7 in 3.1 g/tex 4.7% 4.6 4.8 – –% 11.1%

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Handbook of yarn production Output fiber flow B

A

Input fiber flow A

(a)

Cell number

1

2

3 4 Fiber input

Cells

Feed rolls Beaters Fiber output Conveyor belt Trash (b)

Fig. 5.7

Blending machine

The fibers flow to B where they are removed perpendicularly to the flow line across the many layers accumulated. This provides a means of ensuring reasonable homogeneity in the fiber blend over a certain mass of fiber within the output. If there are variations, which are very long, they overwhelm the blender and it can only ‘smear’ the boundaries of the changes. Such a situation certainly can occur when introducing a new type or merge of fiber. Some blending machines do not work as effectively as the one just described. In a common type, a series of cells is filled from the top to produce roughly horizontal strata that are often about 0.5 inch (≈ 12.7 mm) thick. Pairs of feed rolls empty the cells of stock and deliver the material to a conveyor belt below. The belt discharges the fiber clumps into an airstream that carries them to the next machine. If the strata remain roughly horizontal across all the cells and all the feed rolls were turned off and on at the same time, there would be little blending. In fact, the cells are not emptied uniformly and each cell should deliver fiber at different times. In Fig. 5.7(b), cell 2 is being emptied causing the strata of fibers within the cell to move relative to the others (NB the diagram shows a four-cell machine but larger numbers of cells are available). This causes blending by taking vertical slices as described earlier. The positions of the strata marked Z show the effect that is obtained.


133

The volume in a cell is limited and the amount of mixing is spread over a limited volume. Again, very long-term changes in blend may not be smoothed. The question of recycling waste arises. Klein [3] quotes maximum percentages of recycled waste fibers as 5% for ring spinning; also between 5% and 20% for rotor spinning according to count. Hard waste (e.g. roving waste) is often returned to lower grade products after some disintegration process where the recycled intermediate product is reduced to more or less separate fibers. Some spinners recycle their waste through weighpan feeders to give an even flow of waste rather than baling the waste and reintroducing it into the bale laydown. This is effective in stabilizing the waste percentage in the fiber stream.

5.9

Fiber flow

5.9.1 Fiber flow control The crudest of controls is a level switch regulating an earlier but neighboring machine in the line. When the fiber in the chute rises above the set point, the delivery from the prior machine is cut off until there is a demand for more fiber. More sophisticated versions might involve two-speed delivery rates to give a smoother control, but these are more expensive. Of course, there is always a need to cut off the inflow where choking is imminent. Continued supply with inadequate removal causes material to become packed in the ducts and/or machines. Such a disruptive event is a financial burden to clear because of the human effort needed and the idle time of the machine. Control of the fiber flow becomes an important matter if the yarn produced is to fall within proper tolerances of fiber composition and linear density. One form of control uses an electric eye, mechanical feeler or some other device to measure the height of fiber within a confined space. When the height exceeds the set value it restricts, stops or diverts the fiber supply until the offtake has reduced the fiber to a slightly lower level as shown in Fig. 5.8(a). Machine 2 is controlling Machine 1, because Machine 1 is supplying fiber to it. Failure to stop or slow the flow when the hopper or chute of Machine 1 is too full would lead to a blockage in its entry port or the ductwork. Since several machines are used in series, similar controls can act to stop or slow the feed of fiber from the machines elsewhere in the chain. Machines 1 and 2 are drawn as cleaning machines but they could have been some other devices in the line. Some machines incorporate an overflow system. If there is an excess of fiber, the excess is diverted and recirculated to the feed. As was mentioned earlier, this is a rough form of evenness control but it also acts to prevent choking. This latter is important because if the machine has to be stopped to remove the choke in the feed, much of the production is lost during that time. Usually, there are only a few opening lines and therefore the economic repercussions of a shutdown can be quite severe. Modern systems use chute feed systems to supply fiber to a card. The chute contains some device to measure the height of fiber in the chute and a means of controlling the packing density of the fibers in the column. Vibrating front plates, the flow of air and careful geometric design are elements often employed to maintain the packing density. Once that is maintained, a simple volumetric control of the fiber feed suffices. It becomes necessary to insert automatic control to regulate the final yarn count on a bobbin to bobbin basis. It will be remembered that errors in the sliver produce long-term errors at spinning. Full bobbins are often randomly mixed with the

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Handbook of yarn production Machine 2

Machine 1 Fiber input

Switch

Signal Sensor

Feed rolls

Control line Fiber output Level switch controls feed roll of Machine 1 (a) Material flow

Fiber input

O S F Fiber output Signal

Off–slow–fast controller (b) Input sensor

Drafting rolls

Output sensor

Reference signal Comparator

CPU C = Speed measurement and control NB Signal flow shown in black and material flow in gray. (c)

Fig. 5.8

Control systems and chute feed

result that material from bobbins of ‘thin’ yarn might appear next to material from bobbins of ‘thick’ yarns in the fabric. Devices to control the level and densities of the fiber tufts are necessary to give good carding. The fiber level in the feed and fiber packing density of the fiber in both the reserve box and the main chute must be controlled within strict limits for successful operation. There are many forms of chute feed but space only permits the inclusion of one example (Fig. 5.8). This example has been chosen because it shows the need for careful control to ensure uniformity of mass flow in the supply of a fleece of uniform linear density and openness to the card.


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5.9.2 Control and autoleveling Simple controllers are found in the opening line, which are little more than on/off switches. They stop the feed when the textile material in a receiver in the flow line (such as a chute) reaches the set point. The set point is the desired level; the user normally adjusts it for the given conditions. If the material level reaches the off position in the receiver, it shuts off the supply and when it drops to the on position it starts up the supply again. It has to be remembered that each control has a lag time; shut off and restart does not cause immediate cessation or restart of the material flow. These lags produce error and if several such control systems are daisy chained, the cumulative lag can become quite large. The lags can lead to instability in the system unless it is properly designed and set. A slightly more sophisticated version has two supply speeds (S and F) as well as an emergency stop O (Fig. 5.8(b)). The slow feed is at a rate slightly below the normal feed rate and the fast feed is slightly above. The supply rate thus oscillates only within narrow limits around the mean level and the system can be more accurate and stable than the on/off version. At the next level of sophistication, it is possible to measure the linear density of a strand delivered to a machine. An error signal from this measurement can then be used to offset errors in the supply system. Devices like these are sometimes fitted in cards and drawframes; they are called autolevelers. The signal is an electrical voltage, air pressure or some other means of conveying information. If the measurement is made on the input and the output speed is changed, the device is a feed-forward device. The drawback to this method is that it takes no account of the changes it makes or of any changes in conditions. It has to work by dead reckoning. Thus if the calibration changes, or some other factor intervenes, an error is created. A typical means of controlling the linear density is to measure the output but control the input; this is called a feed-back system. A feed-back system has the consequence that the results of any change are carried by the material to the measuring device after a delay, but this can lead to instability. An example is where a transducer measures linear density of a card sliver as it emerges from the card, and the measurement is used to determine the feed roll speed. The change in draft alters the linear density of the sliver. If the variation is perfectly periodic, the system can be tuned to give excellent performance because the variations can be predicted on the basis of the history of measurement. In practice, random variations in linear density are present and an important component of the signal from the transducer is unpredictable. Some idea of how the problems arise can be obtained by considering a single thick spot in an otherwise flawless portion of sliver passing the transducer. The one-time increase in signal strength causes an increased draft for the duration of the passage of the thick spot through the measurement device. If the thick spot passes before the system can react, a thin spot is created in the following material. This thin spot later causes a negative signal and creates a thick spot and so on. Thus not only is there the original error but also echoes of it in following portions of the strand. Since an actual signal is a mixture of repetitive and random errors, a feed-back system can correct some components but not others. If the estimate of the time lag or the amplitude of correction is wrong, the strand will contain not just the random errors but some of the harmonic ones as well. It will contain the echoes just discussed. Thus feed-back systems have to be used with care. A step toward further sophistication is to use both feed-back and feed-forward devices together in a combined system as shown for a roller drafting system in Fig. 5.8(c). The diagram shows only an input sensor but there are alternatives, which

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cannot be further discussed. Roller drafting is sometimes used on sliver leaving a card before it enters a can and the draft is varied as part of the control system. A device compares the actual measurement of linear density with the desired one and passes an amplified value of the difference (i.e. the error signal) to the central processing unit (CPU). The computer unit uses algorithms designed to reduce the instabilities of the feed-back portion of the system. It is normal to change the back roll speed in a drafting system in a card or drawframe. If the front roll speed is controlled, the speed of the can has to change also. However, the mechanical inertia of the coiler mechanism and the associated can full of sliver, resists sudden speed changes that might be called for by the control system. Such sudden changes produce heavy mechanical loadings on the coiler drive system. Consequently, the rate of change of speed of the output system with its high inertia has to be limited. This is done by smoothing the demand for change by limiting the rate of change of coiler speed. An alternative is to work with a temporary sliver storage system operating with a low draft roller system in series with the sliver take-off from the card.

5.9.3 Weighpan feeders Some mechanical feeders use oscillating combs that remove excess fiber from the belt or lattice apron and give a measure of volumetric control of the fiber flow rate. This is a crude leveling device. More sophisticated feeders of this type are fitted with weighpan controllers that dump specified masses of fiber on to a conveyor belt as was sketched in Fig. 5.5. A number of such feeders are used to give close control of blend proportions; even though the technology is old, many are still in use today. Modern design favors continuous control rather than the intermittent supply inherent in the weighpan feeders.

5.10

Carding

5.10.1 The function of carding Carding is where the last major stages of opening and cleaning occur. It is also where separated fibers are converted into the rope-like sliver form. The functions involved, like the other machines described, embrace opening (the division of fiber clumps), cleaning (even though this function is little used when making sliver from man-made fibers), blending, and condensation. There is, however, another function involved. This is fiber orientation. The card is the first stage where the fibers start to be straightened and get some orientation in a common direction. Thus the two following subsections will deal with fiber separation and the carding action. This latter action deals with fiber straightening, orientation, and a degree of condensation as well as the other functions. Following this, doffing (the removal of fiber from the cylinder) will be considered. Doffing entails a considerable condensation process as well as the conversion of the fibers from a sheet-like form to the rope-like one called a sliver. Cleaning will also be given a special subsection. Apart from removing short fiber and trash, the card also has the task of removing more nep than it creates. Other aspects of carding will also be considered. In reading the following it should be appreciated that the speeds are high for such a large cylinder (therefore care has to be taken to keep the cylinder true and balanced).


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Also, the surface speed is high and the teeth tend to pump a considerable amount of air and it is important to keep the moving surface covered, where possible, to prevent disruptive turbulent air currents from forming. Further, the surface has to be kept covered for safety reasons. The design of the card developed in the nineteenth century. According to Gunter [4] there was little basic change over the next century except for the introduction of ‘revolving flats’ that move slowly over the surface of a rotating cylinder. The word ‘revolving’ does not mean that the flats revolve about their own axes but merely that they move around a specified path. The teeth on the active elements have to be very fine because they have to be capable of handling single fibers. The order of magnitude of a typical dimension is 0.1inch (≈ 2.5 mm). It also means that they are vulnerable to damage from foreign objects. This sets the tone for a discussion of cards. A sketch of a short-staple card is given in Fig. 5.9.

5.10.2 Fiber separation The main feed roll advances a batt of cohering fibers and a thick fringe of these fibers is combed by the teeth of the licker-in as shown in Figures 5.9 and 5.10. This results Moving flats

Flat cleaning

Fiber input

Trash Screen Doffer

Feed plate Licker-in Cylinder

Crush rolls

Sliver output

(a) Fiber to flats

Licker-in

Feed roll Cylinder Fiber feed A Feed plate Trash

Trash Mote knife (b)

Fig. 5.9

Short-staple card

138

Handbook of yarn production Fiber feed from overhead duct (a)

Feed rolls

Beater

Fiber level control Fiber to card cylinder

Chute

Feed roll for batt Main feed roll

Fiber batt Feed roll Licker-in

X

Y

Feed plate Enlargement of drafting zone (b)

Fig. 5.10

Card feed

in a major separation and acceleration of the fibers, which implies a major overall drafting stage. Numerous further localized drafting episodes take place between the cylinder and the flats, which drafts clumps or tufts of fiber and reduces many of them to individual fibers before the process is complete. (Many dictionaries define a tuft as being anchored at one end but some textile authorities regard a tuft as a small aggregation of fibers. In this context, the aggregation of fibers is anchored temporarily in the card clothing and it might be felt that it is a more appropriate word to use in carding than ‘clump’.) As mentioned, the processes of drafting in a card cause some fiber orientation. Some of this orientation is retained because of the restraints provided by the proximity of the elements carrying out the process. The orientation is far from perfect but fibers within the tufts are no longer disposed in random fashion. Considering this opening function, assume that the clump entering carding machines is 0.1 lb in size (approximately 107 fibers). Let these clumps each be first divided into 0.05 lb portions. Next, let the 0.05 lb portions be divided into 0.025 lb tufts and so on. There would have to be 24 stages of division (224 = 16.8 million) before the tuft


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would be reduced to single fibers. This is a theoretical case and it is much more likely that there would be large and small portions at each division. The total number of stages of division would then be dependent on reducing the large portions. Assume that, on average, each tuft is divided into 4/5 and 1/5. It would now take more than 73 stages of division ((5/4)73 = 11.8 million) to reduce it all to single fibers. The point of these very approximate calculations is to show the necessity of many stages of division and redivision. (Note: 0.1 lb ≈ 45 g, 0.05 lb ≈ 22 g, 0.025 lb ≈ 11 g.) Attention is drawn to the multitude of flats on the top of the machine. As mentioned, the licker-in removes relatively small tufts of fiber from the ingoing fiber batt, partially cleans them (if necessary), and delivers them to the main cylinder which carries them to the first flat. Many of the clumps or tufts are caught by the wire on the flat and a shearing action causes tufts to be torn apart. (The term ‘wire’ is used to describe the teeth on cards and some opening devices.) Most of the fibers initially caught by the flat are returned to the cylinder. Many of the divided tufts and the remaining undivided tufts are then temporarily caught by a second flat and are torn into smaller tufts. The process continues in this fashion until the fibers are almost completely separated and lie as a web on the surface of the main cylinder as it leaves contact with the last flat. Some 40 flats are needed in the carding zone to obtain the desired fiber separation and comparison may be made to the earlier calculations. Non-moving carding segments may be interposed between the licker-in, flats, and doffer to give a greater carding action. The segments carry fixed wire and are placed close to the moving cylinder wire; an action occurs there that is similar to that between the flats and the cylinder.

5.10.3 The carding action Two alternative arrangements of the carding elements exist; in one, moving flats cooperate with the cylinder and in the other, fixed plates or segments are used. In the first case, some 40 flats are linked together and move slowly over an arc of the rotating cylinder. The surface speed of the cylinder is usually in the range of 500 yd/ min (≈ 457 m/min). There is a small clearance between the teeth, the setting of which can be varied from 0.008 to 0.02 inches (≈ 0.2 to 0.5 mm) according to the fiber being processed. Thus the shear rate is very high and a tuft of fibers caught by one set of teeth is wrenched apart by the opposing set. There is very little time for the fragments of the tuft to relax until the next division is applied to them. Consequently the fibers within the tuftlets retain some orientation in the direction of shear, i.e. in the direction of movement of the surface of the cylinder. This permits a carding action. Portions of such systems of flats are shown in Fig. 5.11(a). It is possible to replace the flats and their cleaning apparatus by a simple curved plate with fixed teeth (or even with a roughened surface) when carding clean fibers of relatively even length. Some designs exist in which trash can be evacuated from between the segments by interposing small wedge shaped plates that deflect the flow of air (Fig. 5.11(b)). The distances between the tips of the teeth on the fixed tops and tips of the teeth on the cylinder (i.e. settings) have to be carefully adjusted. Also care has to be taken with trash evacuation systems to ensure that they do not choke. Such chokes might not be detected immediately but cause deterioration in quality that might not be diagnosed in the early stages. Worn teeth (Figures 5.12 and 5.13) give trouble and it is customary to test the card output for nep on a regular basis to provide a control. When the nep levels exceed a level determined by experience, grinding or rewiring become necessary. Excessive

140

Handbook of yarn production Flats move slowly

Cylinder (a) Flat-top card Air + Trash

Air

Cylinder Tooth sizes exaggerated for clarity (b) Fixed-top card

Some card flat arrangements

Fig. 5.11

Flat wire

X

α Sharp teeth

W

Cylinder wire

X β

Setting (a) Worn teeth

A′

A

B′

B

Clearance between teeth increases from AB to A′B′ as teeth wear Nep rolls due to action of blunt teeth (b) Cylinder Wire

Section X–X (c)

Fig. 5.12 Card wire and wear therein


(a) Correctly ground

(b) Worn

141

(c) Over–ground

Fig. 5.13 Microscopic views of card wire

wear as shown in Fig. 5.12(b) would require regrinding to remove the metal between AA’ and BB’ before an adequately sharp edge could be attained. (Of course, the clearance would be restored to AB but the tips become wider.) The teeth are case hardened and consequently there is only a limited number of regrinds that can be carried out under normal conditions before rewiring becomes necessary. Case hardening means that the body metal has a thin skin of harder metal. For the extreme case portrayed, it would then be likely that the case hardening had been ground away, in which case there would be very rapid wear when the wire was put in to service again. Also it would be questionable whether a sufficient degree of fiber penetration could be achieved with the wide tooth tips. In regrinding, too heavy a cut with the in situ apparatus used to grind the tips of the wire causes burrs to form (see Fig. 5.13(c) for a view as seen with a pocket microscope). This condition might give good nep performance at the start but the performance deteriorates rapidly thereafter. If problems persist, it might be time to investigate other designs of card wire. The fibers leaving the flats on the surface of the cylinder are sometimes exposed to another carding and/or cleaning process. Carding segments somewhat similar to, but larger than, the flats carry out the carding at this stage and further cleaning may be carried out by installing a knife edge with proper air pressure control and waste removal facilities. A cleaning edge is an effective way of removing pepper trash but care has to be taken to monitor the condition of the knife edge. Hard particles and abrasive material tend to nick and wear the vulnerable edge that then creates nep and causes operational problems. Merényi [5] reported the sensitivity of the plate-to-cylinder and flat-to-cylinder settings. With a 0.008 inch flat setting the mass of flat strip removed increased by 150% as the plate setting was changed from 0.017 to 0.019 inch. The work was probably carried out with wire1 clothing but it still has some relevance. (Note: 0.008 in ≈ 0.2 mm, 0.017 in ≈ 0.43 mm, 0.019 in ≈ 0.46 mm.) The reason can be imagined when it is realized that the ingoing nip of two large cylinders rotating in proximity creates a considerable pressure especially along the line A–A in Fig. 5.14. Unless the pressure is controlled and contained, it tends to blow out in the direction of the gray arrows and carry dust and lint with it. There is low pressure under the cylinder/doffer nip and the flow of air from the high to the low pressure zone affects the fiber orientation in the fiber transfer zone. The air pressure gradient in this zone affects air leakage as well as the fiber transfer between the cylinder and doffer. 1 Modern use of the term ‘wire’ refers to saw-like teeth but originates from the use of wire embedded in a material fixed on the surface of the cards used in the nineteenth and the first half of the twentieth centuries. In this particular case, wire refers to the original meaning.

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Handbook of yarn production Cylinder

A

A Air

Air

Doffer Detaching roll* Sliver formation

Vc

Vd

Trumpet

VC > Vd

Sliver

Enlargement of fiber transfer zone *The toothed detaching roll shown is one of several alternative means of removing fiber from the doffer.

Fig. 5.14

Cylinder doffing

Not all fibers are removed by the doffer and many of them recirculate. Grosberg and Iype [6] carried out an experiment in which the doffer speed was varied sinusoidally and it was found that the amount of recirculated fiber varied similarly provided that the frequency of oscillation was not greater than about 1 per second. This was not a recipe for a practical operation but a means to determine how much the fiber recirculates and how much is taken off by the doffer. The work showed that there is a mechanism of fiber storage on the cylinder that might have importance in blending and doubling within the system. In other words there is condensation on the cylinder caused by the numerous layers of fiber being collected there.

5.10.4 Doffing Modern cards remove the web from the doffer by a so-called wire-covered detaching roll of small diameter followed by a pair of smooth control rolls. Sometimes crush rolls are used to crush seed coat fragments so that they can drop out of the fiber stream. The web is then gathered together and passes through a trumpet or condenser which converts the web of fibers from the doffer into a rope-like sliver. (Note: this sort of condenser is quite different from the ones that collect fibers and extract dust.) In modern machines, the fiber flow is assisted in the transfer by aprons that condense the web to an intermediate condition before passing through the trumpet and calender rolls. A sketch of the major parts of a doffer system, but with the belts removed for clarity, is shown in Fig. 5.14. Should the fiber take-off system be improperly adjusted, it is possible to produced cored slivers, which are dense or entangled in the central core. Such cored slivers are difficult to draft in ensuing operations. Calender rolls press the fibers together to give the sliver added cohesion and then the sliver sometimes passes to a drafting system, which adjusts the linear density. (Automatic control of the linear density of the sliver output is common today.) The


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sliver is coiled as it is put into a storage can and the device that does this is called a ‘coiler’. The can is used to transport the sliver to the next operational stage. Returning to the main cylinder and doffer, the teeth are so angled and the distances so adjusted that the main cylinder gives up the fiber easily and the doffer collects some of it. The condensation occurs in the fiber transfer zone (shown enlarged in the diagram). It might be noted that the faster cylinder speed tends to ‘brush’ the fibers on the doffer with the consequence that the sliver emerges with a predominance of trailing hooks. In older cards, the web between the control rolls and the trumpet is in a free triangular shape with the fiber being withdrawn directly from the surface of the cylinder. The open web is helpful in judging the quality of the web. The web is removed from the doffer by a vibrating comb. Sometimes the web passes between a pair of crush rolls to break up seed and trash particles and the crush rolls might replace the detaching rolls. The crush rolls are set with their axes not quite parallel so that the inevitable deflection in the center of the crush rolls will cause the nip line to have almost constant pressure along their length. Crush rolls should not be used with pressure sufficient to damage the fiber and they should be avoided when processing sticky cotton. Newer cards use different means of removing the fiber from the doffer. Sometimes belts are used to carry the sliver to one side of the cylinder; sometimes the sliver is ‘rolled’ and sometimes other means are used, but the principles discussed here remain the same. For a reasonable output per card, say 100 lb/hr, and a thin web of fibers on the main cylinder, it is necessary to have a high surface speed. A cylinder speed might be, say, 142 r/min or roughly 500 yd/min at the surface. If the sliver were delivered at 65 grains/yd, the sliver delivery speed would be 7.5 yd/min. The difference in speed illustrates how there is a condensation effect at the doffer. Even though the overall draft in a card is, say, 100, the draft at the licker-in might be 500 with the speed of the fiber stream varying accordingly. The data are given merely to give an idea of scale. (Note: 100 lb/hr ≈ 45 kg/hr, 37 yd/min ≈ 33 m/min, 500 yd/min ≈ 457 m/min, 65 grains/yd ≈ 4.6 g/m or 4.6 ktex.) The cylinder surface speed is higher than the corresponding speed of the doffer. This speed difference means that the web is condensed on the surface of the doffer and the web is usually several times as thick as it was on the main cylinder. Also, it has to be converted from a thin sheet to the required rope-like configuration. The normal way to do this is to remove the web from the main cylinder by a doffer whose surface speed is less than that of the cylinder (see Fig. 5.14).

5.10.5 Fiber cleaning in carding Since man-made fibers need little cleaning and the major short-staple fiber is cotton, this section is devoted to the cleaning of cotton in carding. Although the cotton gin is designed to remove seed hulls, some debris from broken hulls is inevitably caught in the fibers entering the mill. The seed particles are woody and have seed hairs attached. Sometimes there are waxy, oily or sticky materials present. In seasons of insect infestations, cotton can contain sugar and insect excreta, which are sticky. Such stickiness makes carding difficult and it certainly adversely affects doffing. Very bad infestations can shut down a mill if there is not a sufficient diversity of sources of cotton in the mix supplied in the laydown. The only reliable solution to this problem is to avoid the sticky cotton.

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The fibers are not completely separated from the woody material in the cleaning machines, nor are they in the carding process [7] as can be seen in the left-hand picture in Fig. 5.15. (For that matter, even drawing does not completely remove the trash particles as shown in the right-hand picture). Further, fibers still attached to a seed particle at this point are more liable to nep once the particle is removed. It can perhaps be realized how difficult it is to clean cotton in the process; it is not just a matter of shaking out the dirt. Cleaning functions occur in three major zones in the card when carding cotton or other natural fibers. The first is in the region of the licker-in, the second is in the region of contact between the flats and the cylinder, and the last is under the card. We take these in turn. Any fibers or trash combed out from the main fiber supply are carried under the licker-in over grids or other trash separating devices to the fiber transfer zone. This zone is at the nip of the licker-in and main cylinder (not shown). According to Gunter [4], the distance X in the enlargement of the drafting zone in Fig. 5.10(b) should be no less than 0.5 inch. This is because if it were much larger, the trash and tufts of fiber would become too deeply embedded in the wire. He also says that if Y is too large, plucking will occur and fiber clumps will be fed to the cylinder. Presumably this is really a reference to the nip to nip distance, which would be analogous to the ratch setting in a roller drafting system. Certainly the result is the same. After leaving the licker-in, the fibers are carried by the cylinder wire to the zone of the flats. Varga [8] quotes research from the Shirley Institute that established that the best performing diameters of the main elements of a card were 10 in, 40 in, and 20 in for the lickerin, cylinder, and doffer respectively. (Note: 0.5 in ≈ 13 mm, 10 in ≈ 0.25 m, 40 in ≈ 1 m, 20 in ≈ 0.5 m.) The flats that have just been removed from the carding zone are brushed clean and are later returned to service at the front (or back) of the card. Material removed from the flats is called ‘flat strip’. Forward moving flats are the most common; in this case the cylinder motion helps drive the flats and the removal of waste is easier. Where rearward moving flats are used, they meet the fiber in a clean condition at the front of the card but they accumulate short fiber and dirt as they continue to the back for cleaning. The dirty flats are not brought into contact with the cleanest fibers. The wire on the flats retains sufficient amounts of short fiber to clog them (i.e. load) fairly quickly unless the fibrous material is removed. Thus it is common to brush the flats continuously to clean them. Not all the fiber on the cylinder is removed by the doffer; a fairly thick film of fiber is carried under the card back to the licker-in. It is normal to place screens, grids, and other cleaning devices under the card between the doffer and the licker-in to help control the airflow around the cylinder and contribute to the overall cleaning From card web

From drawframe

Fig. 5.15 Trash in card web and drawframe sliver


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of the fiber. This cleaning would not occur unless some fibers recirculated on the cylinder. Care has to be taken to avoid the droppings from being sucked up back on to the cylinder surface. For that reason the doors to the under-card space must be kept closed. The trash that drops out here is often intermittently and automatically purged by a suction system. A poorly designed or defective purging system might agitate the trash under the card to the extent that every time it activates, it causes trash to be sucked back on the cylinder surface. This can cause periodic episodes of trashy sliver to be produced. Consequently it is useful to inspect the underside of the cards during a purge to see if there is any malfunction.

5.10.6 Card wire So-called ‘card clothing’ is a continuous length of ‘wire’ containing teeth that is wound under tension on a plain cylinder and the ends are secured. A cross-section of the wire is L shaped with the upstanding portion containing the teeth. The base forms a foundation in contact with the cylinder (Fig. 5.12(c)) and sets the distance apart of the teeth across the width of the cylinder. The bottom of the L shape of the wire rests on the periphery of the cylinder. Careful heat treatment is needed during manufacture of the wire because the tips of the teeth have to be hard enough to withstand wear (≈ 1000 Vickers). However, such heat treatment makes the metal brittle and the main body of the tooth must retain its toughness; therefore the body of the tooth is tempered (≈ 200 Vickers). Skilled use of a flame is required to give the right distribution of temperature during the hardening process. The hardened tip of the tooth is only about 1 mm thick even when new, so there is only the possibility of a limited number of regrinds. The shape and size of the teeth are altered for different fibers and frequently different pitches of wire are used on the cylinder and flats. The wire for the licker-in is always of a coarser pitch than either of them. The wire of the flats is easier to access and clean and therefore finer wire pitches are often used. Too fine a pitch causes the wire to ‘load’ (i.e. become jammed with fiber). Usually, the spacing of the wire is described in points per sq inch or sq cm. An aggressive wire is illustrated in Fig. 5.12(a), which would penetrate the masses of fiber and grip them well, but it might cause fiber breakage with some fibers. Even more aggressive designs use serrated wire and large values of α or β. Point populations vary from 250/inch2 (≈ 39/cm2) for coarse, long synthetic fibers to 1000/inch2 (≈ 155/cm2) for long fine cottons. Less aggressive designs have smaller attack angles α or β and non-serrated wire; these are gentler but not so effective in opening the fiber masses. The general comments apply to the wire on all surfaces, but the values differ. There is a whole range of designs according to the fiber being processed and the degree of initial preparation. To make informed choices, the mill manager should consult the wire makers. As stated before, the clearance between co-operating sets of wire (i.e. the setting) is also important, as is the point population. A lower point population gives better penetration of the fiber mass and better opening, but such wire is incapable of separating the fiber as well as wire with higher point populations. When the wire is reground, the tips of the teeth are removed to get to a new sharp edge. In so doing the width W of the tooth ends increases as shown in Fig. 5.12(a); after several regrinds, the tip cannot achieve good fiber penetration and the card has to be rewired. New wire has a tip that is almost completely pointed. Allowing heavy wear is undesirable; reasonably frequent regrinding should be scheduled. The actual period depends on the fiber and the

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requirements for freedom from nep. Maintenance of the wire condition is of great importance to the performance of the whole mill; it is not a matter to be taken lightly. The choice of wire is an art and experience is very important. Mechanical loading of the teeth is caused by the carding action and this creates a problem with highly angled wire. The forces generated tend to bend the tips outwards and to reduce the distances between the tips of the cylinder wire and those of the flats. A reduction in clearance of this sort increases the loading and intensifies the problem. The tips have been known to touch and destroy the whole clothing at a cost of $1000s per machine affected. Wire makers are very careful to avoid these problems but there are constraints in the design of the wire. Users should always remember that the danger of accidents of this sort increases as the settings decrease. Close settings are sometimes used to decrease the incidence of nep. However, the use of close settings should be closely monitored to check on their stability. The settings between teeth (typical values are given in Table 5.3) are measured by feeler gages and sometimes the flats are set to be slightly tilted so that the entering cylinder teeth meet the widest setting. Consequently care has to be taken to measure the settings in the correct places.

5.10.7 Airflow within the card An aspect of airflow relates to the involuntary flow found within machines. When two parallel cylinders rotate in proximity, pressure differences are generated on both sides of the nip. If the rotational directions are opposite, as with a doffer and cylinder, an increased pressure is generated at the ingoing nip and a decreased pressure at the outgoing nip. Pressure differences of this sort create an airflow particularly when the surface velocities are high and the surfaces are rough. An example can be cited with a card. Lauber and Wulfhorst [9] showed that an upward airflow existed which was directed upwards towards the ‘nip’ between the cylinder and doffer. The magnitude of the flow was about the same as the surface speed of the doffer. Other tests confirmed this and established, at least for the cases tested, that the airflow swept droppings from the doffer towards the nip. The use of proper covers and enclosures permits control of the flows of air that otherwise would create defects in the product. The size of the apertures formed by the teeth in the nip and the degree to which they are loaded with fiber affects the airflow between the high and low pressure zones. In the case just discussed, the flow is vertically downward through the gaps between rows of wire. It is suggested that this airflow is a significant factor in the transfer of fiber from cylinder to doffer. On the entry side of the nip there is a tendency to expel fibers parallel to the axis instead of letting them continue unhindered in the tangential direction. Some fibers remain on the cylinder after passing the doffer. Considerations of conservation of mass flow dictate the mass of fiber to be transferred but do not control the fiber population on the wire. If the population is high, it takes significant time for conditions to equilibrate after starting. Table 5.3 Position

Some typical settings for a cotton card Licker-in– cylinder

Flat–cylinder back

Flat–cylinder Flat–cylinder middle front

Cylinder–doffer

0.007 0.010

0.022 0.024

0.008 0.009

0.005 0.010

0.017 0.034

inches inches


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5.10.8 Nep control All fine fibers reaching the card have some percentage of nep, and, of course, this has to be minimized. The settings of the card are very important in this respect. Too wide a setting between the cylinder and flats will fail to remove as much nep as normal and will favor the creation of nep. Too close a setting (say 0.008 in) can lead to the problem of wire damage. Also wear on the wire widens the settings. Problems with bearings or ‘bends’ can increase the setting needed to avoid the wires touching. (Bends are guides on which the moving flats slide.) Normal settings are made by inserting a feeler gage of the right thickness into the gap and feeling the drag between the metal of the feeler and the wire tips. If the drag is heavy, the gap has to be widened and if it is too light, the gap has to be closed. Setting is a skilled art and even professionals vary in the actual settings they produce. Checks are often not performed until trouble is experienced. One machinery maker offers an electronic device for testing the setting while the machine is running. Several transducers are fitted on a rigid strip that replaces one moving flat. Electrical eddy currents give a signal that indicates the magnitude of the setting distance and this permits a frequent check of the settings without shutting down the card. Other settings are made with feeler gages and doubtless technologies will become available for continuously monitoring these too. As productivities rise and the use of fine fibers increases, the demand for proper control will rise. New technologies will be required to make it feasible to work in a region where very close settings indeed are required. The closer the setting that becomes practical and the more automatic the adjustment, the better will be the nep performance of the card. The condition of the bearings and deflections of the machine parts govern the minimum setting. The fiber diameter and the loading determine the maximum setting. If the setting is much larger than one fiber diameter, there is a tendency for the fiber(s) to roll into a nep as shown earlier in Fig. 5.12(b). This is especially true if there is a nucleus already there or if the teeth are blunt. Nep is a cause of much concern, especially when spinning fine fibers. One prime site for nep creation is the zone between the flats and the cylinder. Also, damaged teeth in any of the card elements can create neps. Several major items have to be controlled; the wire must be sharp, without damage, and of the correct design for the job. A control chart of nep count should be maintained and the limits suitable for the market served should be established. A typical chart of the nep from a card is shown in Fig. 5.16 [10]. It will be seen how the nep count trend slowly rises (a regression equation is given for this portion of the curve). Further, it can be seen how the nep count drops after the card is reground and reclothed. Periodic examination of the condition of the wire with a pocket microscope is advised; this helps to catch any mechanical damage that has occurred. The wire life is taken by some manufacturers of cards to expire after about 800 000 lb of fiber have been processed by the cylinder or doffer. The life of the licker-in wire is less and is quoted by the manufacturers to be about 200 000 lb. At 100 lb/hr, this is equivalent to running for 333 days for the cylinder and doffer or 83 days for the licker-in. Changes in fiber or mineral dust content alter these figures considerably; also the linear density and type of fiber are factors. Continuous monitoring of nep count is a necessity in a modern mill. The card has to be taken out of service periodically and the time for this to occur is often judged on the deterioration of the nep count. The regression equation given in Fig. 5.16 applies to the period before reclothing. It implies a cylinder wire life of about 200

148


Nep count/sq inch

200

y = 0.3x + 111

150

100

Before reclothing

50

After reclothing = Regrinding 0 0

50

100 Days

150

Fig. 5.16 Card wire life

days’ continuous use (the graph is no more than a single sample of the normal deterioration of the wire; other cases can differ greatly). Reclothing refers to the mounting of new wire on the cylinder and other toothed elements. A wise operator does not rely solely on the machine maker’s estimate but needs to check the performance of the particular machines directly. (Note: 800 000 lb ≈ 363 000 kg, 200 000 lb ≈ 91 000 kg, 100 lb/hr ≈ 45 kg/hr.) 5.10.9 Air ducting When a flow of fiber is distributed to a series of parallel branches, the distribution system requires approximately equal air velocities at all points in the ductwork. After an offtake from the system the volume of air flowing onward is reduced; the crosssectional area of the duct has to be correspondingly reduced to maintain constant velocity. Changes in air speed create a danger of the heavier fractions of fiber being deposited in the early offtakes. Turbulence becomes more probable when an overlarge duct is used and while this is good for blending it can cause unwanted agglomeration of fiber clumps. As was mentioned earlier, rough edges in the ductwork cause strings of fiber to become entangled and, when they break free, they suffer fiber damage as they pass through subsequent machines. It is also associated with the generation of some nep in the system. Most systems work at pressures slightly below atmospheric to prevent the egress of dust and fiber. Leaks in the system waste energy because extra, unwanted volumes of air have to be pumped. Also a serious leak can reduce the flow in the upstream ducts and adversely affect the performance of the machines served by the starved ducts.

5.10.10 Sliver storage Sliver is a soft, weak, rope-like strand that is easily damaged by stretching, crushing, or perturbation of the fiber order. Normal short-staple sliver is rarely wound externally on a package such as used for stronger strands (because of the danger of involuntary and uncontrolled drafting by stretching). Rather it is coiled into a sliver can in the fashion shown in Fig. 5.17 and this also facilitates easy withdrawal. The can is a large (usually) cylindrical vessel into which the sliver is fed for storage. The sliver is either:


149

Central hole

Can

Piston Spring

Fig. 5.17

Plan view showing coiling pattern

Sliver can

(a) delivered through a rotating coiler head and is laid in a slowly rotating can or (b) fed through an epicyclic device to generate a sliver pattern without a rotating can. There is an increasing tendency to use ever larger cans because this reduces handling costs. If it costs 10¢ to handle a can, then it costs 1¢/lb to handle a can containing 10 lb whereas it would only cost 0.2¢/lb if a can holding 50 lb were used. The reason for using large cans is self-evident but it also leads to space problems especially in the creel of a following machine. The action of the coiler leaves a cylindrical hole down the center of the sliver in the can. Too large a hole reduces the storage capacity of the can. Too small a hole causes the sections of sliver around the periphery of the hole to be tightly packed, which produces false coiler patterns in the spectrograms of any such sliver tested. The coils of sliver must be laid with precision to optimize the mass stored and to prevent damage. To do this, it is necessary for the top layer of the material in the can to be near the coiler. This is achieved by using a spring-loaded false bottom to the can. The weight of the sliver depresses the so-called piston and the spring constant is so arranged that the level of the top layer remains at about the same level. A can with a bad spring or piston, or an overfilled can, causes the sliver to become crushed and the crushed portions are more difficult to draft than the rest. The cross-overs of the coil are regular and tend to be crushed most, so frequently a crushed sliver produces a periodic error in the material produced. The removal of a coil from a can puts in one turn of twist. The effect is scarcely noticeable with large cans but it does produce a slight effect with the very small cans sometimes used for open-end spinning.

5.11

Waste control

5.11.1 Waste generation Operating machines produce waste products. These waste products may be classified as reworkable or non-reworkable. The latter are divided into subcategories of (a) nonlint materials removed during processing, (b) fiber unacceptable for the intended process, and (c) fly removed from the air conditioning or machines. The non-lint materials include trash, dust, and extraneous objects found in the fiber supply. The materials in categories (a) and (c) are usually disposed of as discussed later.

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Klein [3] quotes the waste percentages for various fibers. The blow room losses range from 6% for 1 inch cotton, to 3% for 1.5 inch cotton (compared with approximately 15% and 10% respectively of total losses in a carded yarn plant). In buying cotton, the price includes the non-lint material and has to be considered in valuing the material. For a plant producing, say, 5000 tons/year, the losses due to unusable waste in the blow room can be as high as 300 tons/year. The quantity of recycled waste is higher than this. The value of the waste from a single mill might be measured in $100 000s/year and the waste that has to be disposed of is measured in $10 000s/year. Understandably, mills do not clean fibers more than necessary. At one time, it was thought that the use of an increased number of cleaning machines would improve the cleanliness of the cotton supplied to the card. It has since been found that, beyond a certain point, repetition of the same process was ineffective [1] and only induced unwanted fiber breakage. However, in the working range, the decision about the amount of cleaning must be the result of a compromise between costs, quality, and sales. (Note: 1 in ≈ 25 mm, 1.5 in ≈ 38 mm, 1 short ton as commonly used in the USA ≈ 907 kg, 1 metric ton = 1000 kg.) 5.11.2 Waste separation Cleaning machines are unable to remove the non-lint material without removing some usable fiber; at times the waste may contain up to 50% usable fiber. Neither are they able to remove all the non-lint material. Indeed, the removal rates vary from 40% to 70%, depending on the type of waste, the type of machine, and the running conditions.

5.11.3 Disposal of non-reworkable waste Arrangements have to be made to deal with the waste produced. Returns from the spinning rooms such as pneumafil waste, remnants of sliver and roving, etc., are usually worked into the flow stream in such a manner to distribute it evenly in the fiber flow (Fig. 5.18). Pneumafil waste is fiber recovered from the drafting systems in the ring frames; the fibers are of good length but have been overworked and should be recycled sparingly. It is rare to exceed about 5% reworkable waste if good quality spinning is desired and some prefer to keep it down to 3%. Non-reworkable waste is sometimes sold for uses other than yarn manufacture; more often incineration, controlled dumping or some other form of authorized disposal is used. Klein [3] points out that the capital cost of the blow room is less than 10% of the total and a more serious financial concern is the cost of the waste. All the material that goes to waste has been paid for at the going price of the fiber concerned. To that must be added a portion of the running costs of the plant, the costs of baling or otherwise condensing the waste for transport, the transport itself, and the disposal costs. The waste costs are the sum of these costs less the resale income, if any. He points out that waste costs can amount to tens of thousands of dollars a year. The air discharge from the fans contains dust, fiber debris, and particles that must be removed before the air can be returned to the atmosphere. Often a two-stage process is used in which cyclone filters separate the bulk of the waste and cloth filters carry out fine filtration. The latter are large and are usually installed in a ‘dust house.’ Sometimes parallel systems are used for processing different workroom air discharges. The size of filtration plant is of the order of 300 tons/year and the associated energy


151

Cleaners

Trash

Non-reworkable waste

Slivers to processing rooms

Cards

Bale plucker

Reworkable waste Line from spinning room

Fig. 5.18

Waste system

costs are significant. Since the air is expensively conditioned, it is normal to return the clean air to the operating rooms concerned, but an air wash is needed to remove remaining dust and rehumidify the air. The waste material from the dust house is compressed into bales or briquettes to facilitate handling. It may then be burnt. Briquettes are compacted to about 80 lb/cu ft (≈ 1300 kg/m3) and this is about the same density as a common house-building brick. Modern day work regulations in many countries apply the rigor of law to ensure compliance. Thus waste is transferred and collected pneumatically; acceptable designs of fiber separation, waste baling, and dust house are required. Some fiber may be reworkable but not useful in the particular mill, in which case the fiber has to be separated from trash and perhaps de-dusted before resale. The waste is often baled for disposal; in which case, bale presses are needed. It is helpful to have a bale press for each type of waste, e.g. comber waste, licker-in droppings, flat waste etc., depending on the market or use for a given sort of waste.

5.11.4 Blending reworkable waste Examples of reworkable waste are: 1 2 3

Short fiber (called noil) removed in the process of combing. Spoiled product from the particular machine. Waste from which usable fiber can be recovered.

Noil is clean and a saleable item (Section 6.3). Alternatively, it can be blended into laydowns to supply cards that make sliver for rotor spinning machines. There are limitations to option (1). Yarns are difficult to recycle, roving less so and sliver is relatively easy to deal with. Roving has to be stripped from the bobbins before it can be recovered. Sliver of poor quality and the stripped roving just referred to may be reworked by making a bale of it, and then placing it in a subsequent bale laydown. However, care has to be taken to avoid dirty, oily or overprocessed material and only one or two bales of it should be inserted into any one bale laydown. Overprocessed fiber behaves poorly during processing and this is why the percentage of recycled material has to be limited.

152


Option (3) includes the reworkable waste from post-carding operations but excludes those mentioned under (2). All the reworkable waste has to be recycled with care. High percentages also give problems. If waste bales are near to one another in the bale laydown, a cyclic variation in waste percentage in the fiber stream is produced that might cause intermittent problems. When the fiber mix is too rich in waste fibers, production difficulties in subsequent processes are to be expected. By keeping an even flow of waste, the benefits of recycling can usually be garnered without undue trouble. Failure to control the waste flow can give most undesirable concentrations of poor fiber that can gravely affect the performance of the whole mill. Reworked fibers behave like short fiber because they have been overworked. Fiber crimp levels have been reduced, lubricants have been removed, and the surface of the fiber has been damaged, or perhaps broken. Such reworked material in excessive proportions in the main fiber stream produces drafting errors at every stage and causes increases in ends down. The result is that running efficiency drops, waste levels rise, and product quality further deteriorates. Also a portion of the recycled waste drops out during processing and this is associated with the amount of fly generated (which is often associated with badly performing mills). However, there is an economic benefit to recycling a small amount of fiber because the fiber costs are such a large percentage of the total for the yarn.

5.11.5 Card waste Waste cannot be ignored in product flow calculations. For the moment, let us assume that the production efficiency without waste is 100%. If the system produces x% waste and the throughput without waste is P, the actual output is P(1 – {x/100}). If y% of the waste fiber is recycled, the regain is {xy/104} and the feed of fiber is P(1 – {x/100}) + {xy/104}). In other words, the loss in production is {x /102 –xy/104}P. Naturally, the production P falls in proportion to the efficiency and the actual loss depends on how the plant reacts to the changes in fibers used. A flat top card produces flat strips that are removed from the flats as they leave the proximity of the cylinder. These strips are accumulations of short, damaged, and usually unwanted fibers mixed with some good ones. Settings of the clearances between the cylinder and the co-operating surfaces are important. As the settings are reduced below the normal levels, the short fiber content of the fiber carried away by the flats increases. These wastes are often treated as non-reworkable. Fixed top cards produce no flat strips but do produce waste. Mote knives, gids, and/or screens under the cylinder allow waste to drop through, and there is also waste discharged from the licker-in. This non-reworkable waste probably contains cotton dust harmful to the worker; therefore it is now the practice to remove the material pneumatically by automatic means. Fine trash is referred to as pepper trash. Many cards are now equipped with crush rolls which calender the emerging card web and crush particles of seed. Some of these particles are likely to be spiky or attached to fibers before the crushing operation. After crushing, a great deal of this unwanted material is then able to fall out instead of being carried forward by the product. It might be noted that there are powerful air currents generated by the card cylinder, doffer, and licker-in; these air currents can sometimes recirculate small trash particles and trap them in the material being delivered.


153

5.11.6 Effects of varying the opening and cleaning If some of the opening machines are bypassed, the card licker-in waste increases and so does the flat strip. For example, private data showed that bypassing a cleaner in a production line gave 36% and 4.8% increases respectively; the yarn irregularity was increased by 1.7% CV and the strength decreased by about 7%. On the other hand, adding a machine can cause problems too. In another example, an added cleaning machine caused the licker-in waste to increase by 13.6% and flat waste by 2.5%. This was because of the fiber damage caused by excessive working. The yarn deteriorated too, with CV increasing by 1.6% and the strength decreasing by about 5%. Excessive opening and cleaning can do as much harm as having none at all. A careful balance is required.

5.12

Safety

5.12.1 General concepts The machinery used in opening and carding can be very dangerous. Nearly all the machines use rotating beaters or surfaces with teeth or pins rotating at high speed. Great care has to be taken by the operators to avoid the dangers of being caught by the machinery or by the ingoing textile material. Neglect in this area can result in serious physical injury. By law, machinery has to be provided with suitable guards to prevent the operator coming into contact with the dangerous parts of the machines while they are operating. Interlock switches are nearly always mandated by law and these are designed to stop the machine if a guard is removed. The working environment is affected by discharges of particulate matter and noxious substances. The blow room is particularly vulnerable to discharges of dust into the atmosphere of the workplace and is, in most countries, subject to regulation. 5.12.2 Safety in the blow room Historically, the blow room and the carding areas were dungeons of unimaginable filth. The machinery had many dangerous units with rotating beaters and sharp teeth moving in unguarded enclosures, leather belts flapped waiting to entrap the careless, large volumes of dust hung in the air with the result there were many accidents and many workers became sick. Things have improved very markedly since then but this is no reason for complacency. Modern machinery has eliminated much of the risk to the operators (it would be unwise to say that it has been eliminated). However, there is still much old machinery working in various parts of the world and it is useful to look at some of the out-of-the-ordinary risks. A few of the dangers are: 1

2

3

Bales are handled by fork-lift trucks and rules applying to the control of vehicular traffic in restricted spaces have to be instituted and enforced if accidents are to be avoided. The release of the straps from the bales has to be carried out under controlled conditions to avoid injury from the violent release of the straps when the coverings are removed. The bale plucker is a ponderous machine and cannot be stopped in an instant; consequently control of personnel in the operating range of these robots is important.

154

4


Many of the machines are very tall and means have to be provided for safe access to the higher elevations of the machines and ductwork.

As was just mentioned, dust could be a problem; it certainly used to be in cotton mills and thousands of workers in cotton mills became ill with byssinosis. Machinery today is required by law to be fitted with means of suppressing dust emissions and the space in which they operate has to be adequately filtered. Fortunately modern machinery is enclosed and a variety of safety locks and protective devices are installed so that the risk to the operators is greatly reduced. Older machinery requires more scrutiny, adjustment, and strict management to approach the safety levels required.

5.12.3 Safety in the card room Most of the warnings relating to the blow room apply here as well. However, there are more machines that are often closely packed and certain additional warnings are in order. The very high inertia of cards poses a particular hazard. It takes some minutes for the cylinder of a card to stop after the motor is switched off, even if a brake is applied. The normal interlocks are to little avail if a guard is removed and the operator then carries out a dangerous act while the cylinder is still rotating. All too many workers have lost fingers, hands, or even arms by disregarding the rules of safety. The most dangerous areas are the zones around the feed roll supplying the licker-in and the doffer. In the first case, never attempt to adjust the batt just entering the feed rolls because it is all too easy to get caught in the material as it enters the pinch of the feed roll. In another case, it has been the practice to scoop up the web emerging from the doffer take-off system and feed it into the sliver take-up device or merely to take a sample. The danger is in touching the doffer surface with its sharp teeth moving at a considerable speed.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Szaloki, Z S. Opening, Cleaning and Picking, Inst Text Tech, Charlottesville, VA, USA, 1976. El Mogahzy, Y E. On-line Monitoring of Fiber Quality: Merits and Challenges, Private communication, Dec 1994. Klein, W. A Practical Guide to Opening and Carding, Manual of Textile Technology, 2, Textile Institute, 1987. Gunter, J K. Carding Innovations, Josef K Gunter, Durham, USA, 1994. Merényi, G. The Effect of Reduced Flat Speed on Cotton Carding, Tech University, Budapest, 1957. Grosberg, P and Iype, C. The Effect of Dynamic Changes in Doffer Speed on Cylinder Loading. J Text Inst, 82, 4, p 457, 1991. Criado, J J. TX590 Project Report, N C State Univ, Raleigh, 1977. Varga, J M J. Technical Innovations in Carding Machines, Tomorrows Yarns, (Ed Hearle, J W S) UMIST Symposium, June 1984. Lauber, M and Wulfhorst, B. Non-contact Gauging of the Fiber Flow During Carding and Drafting Cotton by using LDA, RWTH, Germany, 1995 Beltwide Cotton Conferences, National Cotton Council, Jan 1995. Propst A. AFIS In-plant Quality Control, 7th Annual EFS Conference, 1994.

6 Sliver preparation

6.1

Introduction

As previously mentioned, the modern opening line is almost completely automated and very little labor is required in that department. Between carding, and drawing, there is less linkage. It is common to use can-changing devices that defer some of the manual work to more convenient times without impinging on the productivity of the machine. Such machines are kept in 24 hr/day production between maintenance cycles except for rare event stoppages, which can only be dealt with by a human operator. There is increasing use of automatic can movers and bobbin transfer systems but these are not yet in widespread use worldwide. Linking is always possible, but it is still early in the machinery development cycle for such devices to obtain universal adoption. There are several classes of spun yarn. One class is that of blended yarns that contain dissimilar fibers, and another is one where the fibers are nominally the same. An example of the first class is a polyester/cotton yarn. In the second class mentioned, it is necessary to blend the components because, in fact, fibers from different lots are not similar (although they are of the same type) but they are never described as blended yarns. With cotton yarns there are two subclasses. These are so-called carded and combed sliver. Carded yarns predominate but the more expensive combed yarns have a good market. Since combing is used for cotton processing and very rarely for other fibers, the topic has been separated from the other processing and it is discussed in Section 6.3.

6.2

Drawframe

6.2.1 The concept of drawing Sliver drawing improves fiber orientation, intimacy of blend, and sliver evenness, as has already been described. Each drawing head is supplied with a number of slivers contained in cans. Each sliver comes from a different can, and the combination of

156


these slivers constitutes the doubling part of the operation. The sliver then is passed to the drafting zone by a power creel, which transports the fragile slivers without uncontrolled stretching or other damage (see the left-hand part of Fig. 6.1). The drafting elements (which perform the drawing operation) then elongate the whole group of slivers to produce a single output sliver as shown in Fig. 6.1. The sliver leaving the drafting zone passes through a condenser to a large can where it is stored in readiness for transport to the next stage of the operation. (This sort of condenser merely consolidates the passing sliver but produces no doubling effect.) The drafting system usually consists of three or four pairs of steel rolls and the top rolls have thick rubber sleeves called cots. (Drafting and drawing are discussed in more detail in Chapter 3 and Appendix 8.) A linear speed of 500 yd/min is common and higher speeds, currently up to 800 yd/min, are possible. This has the result that the productivity of the machines is in the order of 500 lb/hr for each head. For this reason, very few drawframes are needed and the cost/lb of drawing is low. Fiber wastage is also low at between 0.5 and 1%. A creel containing from four to eight feed slivers is used. The linear density of the delivery sliver ranges from 40 to 80 grains/yard. A sliver is often passed through two drawframes; the first passage is called ‘breaker drawing’ and the second, ‘finisher drawing’. (Note: 500 yd/min = 457 m/min, 800 yd/min = 730 m/min, 40 grain/yd = 2.8 ktex, 80 grain/yd = 5.7 ktex, 500 lb/hr = 227 kg/hr.)

6.2.2 The draw zone Loading on a drawframe roll is high because of the mass of fiber being processed. For this reason it is not practicable to use aprons and special care has to be taken to use the correct bottom roll fluting, hardness of rubber on the top rolls, and settings. Typical types of fluting are spiral or axial. Another factor is the dust emission, which tends to increase with speed. Machines are now designed to removed dust and fly. The accretion of fly and contaminants, as well as the progress of wear on the elements, has to be monitored carefully. Errors can have a profound effect on later processes. One faulty component can produce a large amount of substandard material even in a short interval. The top rolls usually have rubber hardnesses that vary between 70° and 90° Shore, to control wear and performance. Buffing of the cots has to be carried out with care to avoid roughening the rubber, or else lap-ups will become a problem. When the rubber layer is reduced by buffing to an unacceptable level, the rolls have insufficient elasticity to control the fibers and the evenness of the sliver delivered Sliver condenser

Power creel

Coiler

Drafting rolls

Input sliver cans in the creel

Fig. 6.1

Output sliver can

Drawframe layout

Sliver preparation

157

deteriorates. Sometimes the rubber surfaces are coated with lacquer, treated with acid, or hardened by UV irradiation to increase performance, but some treatments lack durability. Changes to the rubber also produce error; for example, accidental irradiation by prolonged exposure of one portion of the rubber to sunlight causes variations in hardness that produce results similar to a mechanically damaged roll.

6.2.3 Sliver condensation Sliver leaving the drafting rolls passes through a condenser containing a sharp contraction designed to produce lateral fiber migration and enhance sliver cohesion (Fig. 6.2). It then passes through a trumpet, which further condenses it. One or more devices to measure linear density are usually mounted here. The trumpet should be changed for differing sliver weights. A rule of thumb is that the throat diameter (in mm) of the trumpet should be between 1.6√n and 1.9√n (n is the linear density in ktex) depending on the weight of the sliver. Take-up rolls discharge the sliver into a sliver passage, which rotates about XX. Sliver is deposited into a can, which rotates at a different speed and about a different axis, to make the coiled pattern described earlier. There are also alternative, planetary systems. The sliver trumpet and the underface of the coiler head get very hot. A hot spot that is particularly hot and wears more than elsewhere is the exit of the sliver passage marked Z in the diagram. This must be smooth and properly shaped when spinning polyester or other man-made fibers that produce oligomers and other materials that can sublime at high temperature. (Sublimation is the process of a change in state from a solid to a gas.) Any emission of gas at these hot spots condenses on cooler surfaces to form a hard, crystalline deposit. These deposits alter the local coefficient of friction and provide sites for fiber damage.

Exploded view From drafting system

X Condenser Trumpet

Take-up rolls Sliver passage Coiler head

Z X To sliver can

Fig. 6.2

The machine, of which this component is part, is known as a drawframe. The system of rolls is usually referred to as the drafting system.

Sliver delivery from a drawframe

158


6.2.4 Autoleveling One of the purposes of drawing is to improve the evenness of the sliver and the process of doubling is not enough. It is now common to fit an autoleveler on the first drawframe and the principle of such an autoleveler has already been mentioned in Section 5.9.2. The autoleveler measures the linear density of the sliver and compares it to a standard. A signal, proportional to the deviation, is used to cause a change in draft ratio sufficient to reduce the error signal and correct the error as far as possible. At present, little more than linear density is used to control the sliver preparation. Increasingly, this is measured using online measurements from tongue-and-groove, capacitive, or optical transducers. Often, these transducers are sited at the input and output of each of the machines between carding and final drawing. Measurements are also made in the laboratory on samples of sliver.

6.2.5 Fiber hooks Carding produces hooked fibers, which cause errors in drafting, reduce the strength of yarn, increase the end-breakage rate, and lead to a general deterioration in performance. The hooks are pulled out to some extent in the two passages of drafting, but sufficient are left to make it worthwhile to present survivors to the ring frame as trailing hooks. Since there is a reversal in hook direction at every transfer (Fig. 6.3), and since the card produces a predominance of trailing hooks, it is necessary to have an even number of transfers. This implies that there should be an even number of passages of drawing. Within reason, the more passages of drawing, the less the hairiness of the yarn produced. However, four or more passages overwork the fiber and the normal custom is to use two passages unless a combing process is used. Each transfer adds a small cost to the product.

6.2.6 Monitoring An interesting development is the coupling of a computer to the transducers of all the drawframes, the signals being used as a means of monitoring the performance of the frames and personnel. The signal is monitored automatically at whatever interval is selected within the capacity of the system and exceptions to the normal are reported. The program can be made to print out spectrograms, which can indicate, at a very Input sliver with leading fiber hooks

Filling can

Fig. 6.3

Output sliver with trailing fiber hooks

Emptying can

Fiber hook reversal

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159

early stage, any mechanical trouble that develops. This is a valuable feature since normal quality audit procedures might not find the trouble for many hours, in which time great volumes of faulty sliver are produced. The drawframe is a favorite place to site an autoleveler because there are fewer drawframes than cards; there are arguments for using either of these positions and many operators use both. Initial expense alone is not always a sufficient reason for omitting such control systems. The levelers have to be set to correspond with the correct interpretation of the error signal. For example, a capacitive type transducer produces a slightly different signal for polyester than for cotton. Also, different fiber stiffnesses can cause a pneumatic trumpet to give a different result. Consequently, careful calibration is essential for best performance. Furthermore, variations in the sliver being processed can cause blend inhomogeneities and it is useful to level the components at as many stages as is feasible. However, autoleveling is no substitute for good preparation; the effects of variable preparation may be disguised by autoleveling, only to appear again at a later stage. Drawing is a very important process stage, and it is used in all forms of staple yarn production. As has already been mentioned, it serves not only the functions of fiber alignment, blending, and long-term error reduction; it also serves to smooth out inevitable differences in card sliver, especially if the routing of sliver cans is carefully controlled. Drawing is a sort of central clearing house because there are so few frames needed and a sizable proportion of the total production passes through each drawframe.

6.3

Combing

6.3.1 An outline of the combing operation For high quality yarn, an extra process is introduced called combing. The purposes of combing are to (a) remove short fiber, and (b) improve fiber orientation. Combed sliver has a ‘silkier’ appearance than card sliver because of the enhanced fiber alignment. The first stage in this series of processes is lap winding which follows a drawing stage (Section 6.2.1). One or more passages of drawing are used before combing to straighten and orient the fiber hooks for best combing performance (Fig. 6.4(a)). A batch of cans is moved from the transient storage area following drawing to the creeling area for a ‘lap winder’. Comber lap is then prepared from drawn slivers. The cans are assembled in a lap winder creel, which transports the slivers to the running lap winder (Fig. 6.4(b)). Many slivers (often 12) are combined to make a closely spaced sheet of slivers, which are wound as a continuous layer on to a cylindrical center (Fig. 6.4(c)). The resulting ‘lap’ might be as large as 20 inches (≈ 0.5 m) in diameter by 12 inches (≈ 0.3 m) wide and weigh about 45 lb (≈ 20 kg). The lap is transported to the combing machines where combed slivers are produced (Fig. 6.4(d)). The sheets of slivers are combed and thereby drafted down to fiber webs. These webs pass over curved guide plates to the table of the comber where the fiber webs are layered to form a sandwich. They then pass to a drafting system that restores the linear density of the strand and converts it to a combed sliver. Combed sliver is coiled in a can and passes to a transient storage area. The last stage in this series of processes is the ‘finisher’ drawing using conventional drawframes as shown in Fig. 6.4(e). For space reasons, the combing machine is shown as having four combing heads but actual machines have more. The process involves a great deal of doubling, thus one

160

Handbook of yarn production (a)

Carded sliver

Drawframe creel

Drawframe

Can movement

(b)

Slivers

(c)

Lap winder creel Center tube Lap winder Lap movement Webs of fiber pass over curved guide plates

(d)

Webs from each combing head are layered on top of the ones from the preceding head(s)

Drafting Comber Can of combed sliver

Can movement Drawframe creel Drawframe (e)

Can of combed sliver

The drawings are symbolic and the components are not necessarily in scale nor are all components included.

Fig. 6.4

Drawframe to lapper sequence

Sliver preparation

161

expects the slivers to be more even than usual; further the combing process introduces a desirable orientation to the fibers that provides the yarns with a silky desirable appearance. Extraction of short fiber coupled with the good fiber orientation increases the strength of the yarn made from combed sliver. In normal practice, only fine cotton yarns are made using the combing process; it will be realized that the yarns are expensive.

6.3.2 Lap quality The output speed of a lap winder is up to 120 yd/min (≈ 110 m/min) and it has a draft of only 2 to 4; consequently there is a considerable doubling effect and very little drafting error. Within the limits imposed by the capacity of the cans in the creel, the short-term evenness should be improved. Theoretically, the CV should be 1/√m of the average value in the input slivers, where m is the number of slivers in the lap ribbon. However, the variance between the slivers has to be taken into account. Variation between the slivers can cause some to be gripped weakly by the drafting rolls and extra errors are created in drafting; it is important to use even and similar slivers. It is also important that the ribbon of fiber is wrapped on the lap at the correct tension. Too low a tension produces a soft lap that is prone to damage in transport and handling. It also increases storage space needed. Too high a tension makes it difficult to unwind the lap at combing, especially the last few layers. The ribbon does not part cleanly; there are hairy connections between the departing ribbon and the remaining cylindrical part. These are known as ‘split laps’. Klein [1] reports a web doubling process in which the pre-combing drawframe stage is replaced by a ribbon lap machine that follows a sliver lapper. Web doubling has many features to commend it and, in the future, it may well appear in other processes, perhaps even in carding. Many laps are mounted on a combing machine to yield the same number of comber webs, which are combined by layering and the layered ribbon is drafted and then condensed to form combed sliver.

6.3.3 The combing process Figure 6.5 shows the main elements of the combing process. Such machines are complex and the sketches are meant to extract only the essence of the process. A common feature is the combing roll, which is shown in different positions in diagrams (a) and (b). Diagrams (a) and (b) show the left and right parts of the machine. In some machines this is called a half roll. The combing roll contains one or more segments, known as combing segments, which have toothed wire or needles to penetrate the fringes and remove short fibers. Diagram (a) shows a ribbon of slivers that has been delivered from the lap by the feed rolls A. The ribbon is then nipped by the elements X and Y, with a fringe of fibers protruding to the right. The diagram shows the fringe being combed by the combing segment at B. When the combing segment has passed, and the primary combing portion of the cycle is complete, the nippers are moved towards the detaching rolls and are then opened. Meanwhile, the detaching rolls F have reversed and carried back a portion of the fiber web processed during the last cycles. The newly combed web carried forward by the nippers is now laid on the returned web to make a piecing D. At this point, the comb E in diagram (b) penetrates the two layers as the detaching rolls resume their forward motion. The movement through the comb provides a secondary combing that removes some fibers that escaped

162

Handbook of yarn production Feed rolls A Lap

Nipper X B

Combing segment Combing position

Combing roll

Cushion plate Y (a) Comb E

Brush F C

G

D H

Combing Piecing zone roll (b)

Detaching rolls

Combing segment

Fig. 6.5 Stages in combing

earlier removal. The comb is then withdrawn and the nippers are returned to their original position to start the next cycle. As the combing segment passes underneath, it meets a brush that removes the noil, which is then removed by a suction system. Noil is a by-product of the comber; it comprises good and reasonably clean fiber but it is often used as a component of the fiber supply for rotor spun yarns and some other products. The rectilinear motions have to be balanced because modern combers work at up to 300 nips/min. There are multiple combing positions in a single machine and the output from each position is combined with that of the others. The doubled web is then condensed through a trumpet and drawn to form a sliver. Adjusting the draw ratio permits adjustment of the output linear density. The doubling and drawing continue at constant speed. Since the detaching roll has an oscillating motion superimposed on the steady speed, there is a need for a loop G in the individual webs to accommodate the differences in velocity. In some machines the web is condensed into sliver before doubling, and in others the webs are laid together sandwich fashion. If the teeth of the combing elements become bent or damaged, nep and other defects can be produced. The outermost teeth seem most prone to damage and inspection of the selvages of the webs sometimes provides an early indication of a need for maintenance work. Settings can be adjusted to remove the desired amount of noil. Removal of too much is an expensive proposition because of the cost of fiber, but removal of too little reduces the quality of the product. Economics usually decide the issue; the sales appeal of a combed yarn often resides in the label ‘combed’ and the decision is not wholly a technical one. Fractionation of the fiber is imperfect and some long fibers are removed with the shorter ones; also some longer ones are broken. Thus, the comber is an instrument that usually improves the short fiber content but it does not bring it to anywhere near zero.

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163

6.3.4 Comber noil Parallelization of the fibers in the input, and the linear density thereof, are important in combing. With insufficient drawing or too heavy a lap, the comber tends to pluck clumps of fibers from the stock rather than handle single fibers. Uneven slivers tend to give uneven gripping of the web which allows more plucking to occur than would be found with good slivers. To avoid overloading the combs, the sliver weight should be limited, otherwise quality suffers. Many people regard the percentage of noil removed as a litmus test of quality, whereas it is often the case that more can be gained by paying attention to the quality of the lap than by increasing the amount of noil removed. Short fiber is removed to improve the characteristics of the yarn. The average fiber length in the noil can be varied. Within limits, the more short fibers that are taken out, the stronger the yarn, and the less hairy and more expensive it is [2]. Figure 6.6(a) shows various regions of a typical fiber diagram before combing. The section colored black represents the proportion that is almost completely removed as comber noil and the detachment setting controls the proportion. The section shown as light gray is almost completely retained and the section shown as dark gray is only partly retained, the remainder going to comber noil. Several factors influence this mid-zone including fiber type, CV, and orientation, as well as machine condition and speed. Normal levels of noil removal vary between 6 and 14%. A fiber diagram of the noil removed is shown in Fig. 6.6(b) and it will be seen that removal of some longer fibers occurs; this is unavoidable.

6.3.5 Web layering There is a doubling arising from the layering of the webs from the individual combing heads on the comber table. If there are x heads, the theoretical CV is 1/√x of the mean input value but to get the linear density of the output sliver back to a value compatible with the following drawframe, there has to be a drafting stage. As will be realized, the drafting will introduce some error and the improvement in CV is less than might be expected. The extent of the change in evenness in the combing process depends on the fiber and the setting of the machine. Nip of detachment roll Partially retained Detachment setting

Fiber length

Feed/Cycle Retained

Removed Number of fibers Bite of nippers

Fiber length

(a) Feedstock

Number of fibers (b) Noil

Fig. 6.6

Fiber diagrams relating to combing

164


6.4

Creel blending

6.4.1 Basics of drawframe blending Drawframe blending is used to improve the uniformity of the blend and this applies no matter whether the slivers being ingested are similar or are completely different. Slivers of each single component are blended at the drawframe; this avoids difficulties in carding fibers of widely different characteristics. The idea is to combine a number of slivers as they enter the drawframe and this very process of doubling blends the fibers. In the first passage of drawing, each ribbon could be of a specific type of fiber. In this case, we use a deviation in blend rather than a deviation in linear density for our calculations. Suppose a spot in one of the slivers has only 40% of fiber A instead of 50% and the difference in mass is made up by fiber B. This is 80% of the expected value for fiber A, which makes the blend ratio 40/60 in the bad spot and 50/ 50 elsewhere. Assume that the linear density of each input sliver is 4 g/m and there is normally 2 g/m of both fibers A and B. At the bad spot the linear densities of fibers A and B in the one bad sliver are 1.6 and 2.4 g/m. The theoretical blended eight-sliver totals are now (7 × 2) + 1.6 and (7 × 2) + 2.4 g/m giving the proportions 15.6/16.4. This represents 97.5% of the perfect value for fiber A instead of 80% in the bad spot. Thus, not only does the doubling reduce the error in linear density, it also improves the blend evenness. However, as mentioned elsewhere, drawing introduces error that offsets these gains. It will be noted that in Table 5.1, drawing did not always decrease the CV and in the second passage the CV usually went up a little.

6.4.2 Fractionation A card and opening line can separate blend components, especially if the fibers differ in attributes. For example, the flats in a flat card tend to remove the coarse and short fibers and the delivery might be depleted of these components irregularly. The fractionation of the fiber changes the population of fibers and the consequential variations in populations of fibers affect the blend from place to place in the material flowing through the system. The blend for one particular fiber attribute differs from that for another attribute within the same population. This is because of the many permutations of fiber properties in the stream of material passing through. A typical industrial performance is shown in Fig. 6.7 and it will be noted that, not only does the linear density (or ‘sliver weight’) have a CV but so do all the fiber properties. The CV of the component level is a measure of the perfection of the blend. Obviously, a 0% value would mean that the blend is perfectly even. The ‘Uster 25%’ refers to the Uster statistics, which show the range of worldwide mill performances in respect to evenness of linear density of the product. A 25% rating means that 25% of all spinners are better than the subject one. The other symbols are explained in Table 5.1 but special mention is made of the CV of the short fiber content (SFC) which is usually much higher than the other values discussed here.

6.4.3 Longitudinal fiber migration The process of drafting causes fibers of differing characteristics to move relative to one another during processing. This migration of fibers mixes them and is an unintended form of blending and it applies whatever input fiber components are involved.

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165

12

Sliver CV%

Carded sliver Drawn sliver 8

4

0 Uster Sliver 25% weight

MIC UHM STR ELO SFC

Fig. 6.7 Variability in sliver

Assume that the input material to a machine that drafts the material enters as a series of sections labeled A, B, C, and D, as shown in Fig. 6.8. The number of fibers in each cross-section is assumed to be the same. After passing through the machine, drafting causes longitudinal migration of the fibers and a cross-section of the output contains fibers with mixed labels as shown. The labels refer to strata parallel to the material flow. The fiber order in the input is not preserved and there is a mixing of the input segments. Not all components migrate at the same rate. Mixing will be biased according to some fiber attribute or attributes. The exact mechanisms are still unclear at the time of writing. Each output cross-section contains a sampling of several length-segments entering the machine.

6.5

An industrial case study

A case of a particularly bad laydown in a mill not equipped with a mixer is now discussed to emphasize the importance of adequate blending [3]. This is not a normal circumstance and should be considered as a worst case scenario. The bale laydown consisted of 40 bales and the bale plucker took about 5 minutes for a round trip. The length of card sliver delivered in the average time for the bale A B A B B D B

D D A B A C D

C

B

Material flow

Machine Output

B

B Input

C A C A C B

A

D

Cross-sections

Fig. 6.8

Effect of processing on a blend

C

D

166


CV of fiber fineness

plucker to make one round trip was about 700 yd (≈ 640 m). Tests were made at 10 yd (≈ 9 m) intervals along the sliver and at corresponding times from the bales. FFTs of the data series were made to show the periodic components of the various fiber attributes. (FFTs are Fast Fourier Transforms, which convert data using the time along the X axis to a frequency or wavelength basis. A typical use of the wavelength basis is the radio spectrum.) To simulate the blending occurring in the opening line, the data were smoothed over a moving interval of 20 bales. (A ‘moving average’ is a succession of the averages over successive batches of data and the width of each batch is termed an ‘interval’. If all the intervals are made the same we refer to a ‘moving interval’.) The CVs of fiber fineness (micronaire) from these tests are shown in Fig. 6.9(a). The result was primarily affected by the bale-to-bale difference in the laydown. The laydown contained many bales, the round trip time of the bale plucker cycle was long, and the mixing volume of the blending system was small. There was an approximately 700 yd (≈ 640 m) error wavelength in micronaire due almost entirely to this cause. Tests at other establishments with adequate blending only showed weak tendencies towards this sort of error. The importance of the particular variability shown arises because of differences in dye uptake with cottons of varying fiber fineness. There are factors other than fiber fineness affecting the dye shade but they showed little or no effect on the result in this case. It is interesting to look at yarn made directly from card sliver because this avoids the distortions from yet another production stage. It was expected that different dye shades would show similar patterns in the fabric.1 One yard (≈ 0.9 m) of each 10 yd (≈ 9 m) sample of sliver was converted to Fiber source Bales Sliver

Fiber

Color variance

(a) Color wavelength 400 nm 600 nm

Fabric

100

700 1000 (b) Error wavelength in equivalent card sliver, log scale (yards) NB All samples were taken from a single laydown and no blender was used.

Fig. 6.9

Fiber fineness and dyeability

1 Color is also measured as wavelength, but the unit is nm, that is 10–9 meters, rather than the thousands of yarns used as the abscissas in the graphs here.

Sliver preparation

167

yarn on a sliver-to-yarn ring spinning machine, and a portion of each sample of yarn was knitted into fabric. Each product could be directly related to the corresponding length of card sliver. Figure 6.9(b) shows that the fabric had variations in dye uptake consistent with movements in the bale plucking head across the bale laydown. Extremely long-wavelength errors such as those generated in very long bale laydowns, or large variations throughout the height of the laydown, cannot be completely extinguished by normal blending devices in the opening line. However, mixers of adequate capacity can usually smooth the results to an acceptable level.

References 1. 2. 3.

Klein, W. A Practical Guide to Opening and Carding, Manual of Textile Technology, 2, Textile Institute, 1987. Pillay, K P R. Text Res J, 34, 663, 1964. Lord, P R and Rust, J P. Blending as a Systemic Problem, Proc. Beltwide Cotton Conferences. Nat Cotton Council, Vol. 3, p 1631, Jan 1994.

7 Short-staple spinning

7.1

Ring spinning

7.1.1 Ring spinning and associated processes A ring-spinning machine is an uncomplicated, flexible, low cost device that is well established with a wide range of applications. In the past, differences in fiber length between cotton and wool determined whether the system was regarded as being in the short- or long-staple category; that view persists even though there are now many more types of fiber and machines. However, in this book we shall persist with the original demarcation and, for the present purpose, we may define ‘short staple’ as covering the range of fiber lengths up to, say, 2 inches (≈ 50 mm). Short-staple spinning machines may process a variety of fibers, the most important of which are cotton, polyester, and blends thereof. Although it was initially developed in the nineteenth century, ring spinning still is attractive for a wide range of services and is likely to endure for many more years. The systems described in this section include roving production, ring spinning, and winding. Roving is an intermediate product made from sliver and it is normally used as a precursor for yarn. A problem that requires attention is end-breakage in spinning, roughly half of which arise from faulty roving preparation. This is mentioned to underline the need to consider the whole production line; concentrating on individual machines is not sufficient. Since sliver production has already been discussed, we now continue with roving production. Essential parts of a roving frame are: 1 2 3 4

A creel, which contains cans from which sliver is drawn to feed the drafting systems (see Section 6.2.1), Drafting systems to reduce the linear density of each sliver to that of rovings (see Section 3.7), Flyers to twist the emerging rovings. Individual winders to take up the twisted rovings onto bobbins.

Items (3) and (4) are usually combined as will be seen in the next section.

Short-staple spinning

169

7.1.2 Flyer twisting and winding of roving The flyer (Fig. 7.1(a)) is ideal for twisting low strength strands such as roving, but it has a limitation in speed because of the sheer size needed and the associated mechanical stresses. However, the productivity is reasonable for roving because both the count and twist are low. One revolution of the flyer inserts one turn of real twist in the roving emerging from the drafting system. One turn of the bottom feed roll delivers D inches of yarn, where D is the diameter of the roll in inches. If the flyer rotates kπD times during a single rotation of the front drafting roll, the twist is k tpi. The factor k is controlled by a twist gear, which is part of the gear train connecting the bottom front roll and the flyer. Since the feed roll speed is fixed, it is also necessary to fix the flyer speed to maintain an unchanging twist level. The roving is supported inside the flyer arm or in a slot, and this reduces the forces acting on it due to centrifugal force. Winding tension is, in part, controlled by wrapping the roving around the presser arm two or three times. The more wraps, the higher the tension and the more dense the roving package. In addition to real twist there is also false twist involved. The rubber grommet provides a surface on which the whirling yarn rolls to produce false twist between the grommet and the feed roll. This false twist strengthens the weak twist triangle and reduces end-breakages. It is not enough to merely twist the strand; it has to be wound on the bobbin and this requires that the bobbin speed be different from the flyer speed. In some machines, the bobbin speed is greater than the flyer (one can tell by the direction of the foot) and sometimes vice versa. The one is referred to as a ‘bobbin-lead’ machine and the other as a ‘flyer-lead’ machine. In both cases, a so-called ‘warp wind’ is used, and this means that cylindrical layers of roving are laid onto the bobbins (Fig. 7.1(b)). This has certain consequences. After the first layer of roving has been laid on the bare surface of the cylindrical bobbin, it is necessary not only to change the direction of lay but also to adjust the bobbin speed, because the diameter has just become larger by two roving thicknesses. After each complete layer is wound onto the bobbin (i.e. after each lay), the direction of lay is changed and so is the bobbin speed. This is usually accomplished by means of a pair of opposed cone pulleys and differential gearing. The cone pulleys are set parallel but in opposite directions so that a belt connecting them may be moved parallel to the cone axes without being stretched or going slack. Movement of the belt produces different speed ratios. The differential gearing is merely a mechanical means of combining two separate input speeds to give an output speed proportional to the difference of them. In a conventional roving frame, one input is calculated for the case when the flyer and bobbin speeds are equal; the other is arranged to give the appropriate wind-on speed. The ‘wind-on’ speed is changed at the end of each traverse by altering the position of the belt on the pair of cone pulleys just described. In some modern machines, the speed change is controlled electronically. If the speeds are improperly adjusted, the winding tension is changed after each layer of roving is placed on the bobbin. If the roving is larger in diameter than allowed for in the calculation, the bobbin becomes very compact and difficult to unwind at the next stage of processing. If the strand is wound too slackly, the package becomes unstable and prone to damage. (If the changes are too great, there is danger of causing a machine stop due to a broken end.) Thus, selection of the correct lay gear is important. The precise choice depends on the fiber being used, as well as the count and twist of the roving. A production unit usually has data, based on experience, for the best value to use. The drive train controlling this speed change

170

Handbook of yarn production Grommet to provide false twist Feed α

Threading slot

Bobbin rotation (ωb)

Db

Flyer rotation (ωb)

t L

Traverse Warp wind Several wraps of roving around the presser arm to control tension

r

D Each layer contains m coils/inch (b)

(a)

α A B

Diagrammatic representations Not to scale (c)

Fig. 7.1

Roving spindle and bobbin


171

contains the lay gear. If an end breaks and the machine is not stopped, these speed adjustments get out of kilter; further operation is not possible until the machine is reset. To give good stability to the roving package, it is normal to progressively change the length of traverse of the flyer foot with respect to the bobbin. Each successive layer involves a progressively shorter traverse with the result that the completely filled bobbin has conical shoulders of about 80° to 100° inclusive angle (which reduces damage from handling). The setting of the traverse mechanism determines the slope of the shoulders of the package. The strand is fed to the twister at a steady pace and therefore the flyer speed has to be maintained to keep the twist level constant. However, the winding-on speed has to be varied to match the feed of the roving. The ratio of bobbin and flyer speed1 is: Ub /Uf = 1 ± 1/τc

[7.1]

The sign in the equation changes according to whether it is a flyer-lead or a bobbinlead design. The twist is adjusted by changing the twist gear (part of the gear train connecting the front and back rolls). There was discussion of twist and draft in Chapter 3.

7.1.3 The roving machine Good descriptions of roving machines are given by Klein [1]. The machines comprise between 60 and 120 spindles, each containing a drafting system and a flyer twister. Rotation of the flyers twists the strands and, since the strand is supported within one of the flyer arms, centrifugal force does not cause it to be tensioned due to ballooning. It is not possible to rotate such a flyer at very high speeds because of mechanical design difficulties; speeds up to 1600 r/min are obtained in practice. Bearing in mind that the productivity of a spindle is a function of the linear density of the strand, it will be realized that a flyer frame finds its best use in processing relatively thick, weak rovings that vary between 0.2 and 1.2 ktex. Input to a roving frame is ‘drawn’ sliver, taken from a can filled in the last drawing stage. Sliver is normally drafted by the roving frame to roughly 1 hank/lb and it is then twisted just sufficiently to permit handling before being wound onto a large bobbin. A roller drafting system is used as discussed in Chapter 3 and in Appendix 8. The top rolls are similar to those used in ring spinning (Fig. 7.2(b)). The spindles and flyers each share common drives and the bottom rolls of the drafting system are formed from long steel bars, which are articulated along the length of the machine. These arrangements enable the gearing to be concentrated in a headstock at one end of the machine; they also enable the gearing to be enclosed for safety and cleanliness. The top rolls are similar to those used in ring spinning. Note: 0.5 hank/lb ≈ 1.2 ktex, 1 hank/lb ≈ 0.6 ktex, 3 hank/lb ≈ 0.2 ktex. The building motion is controlled by the steady upward and downward movements of the rail containing the bobbins and spindles. The bobbins and spindles are coaxial; the flyers all operate at the same speed and direction (by custom, Z twist). The bobbins rotate at a common speed. The best time to set the tension is when the build 1 Relative winding speed = material supply speed, thus (Uf ± Ub)πD = Uf πD/τc which leads to Equation (7.1). τc is the twist/unit length, Ub is the bobbin speed, and Uf is the flyer speed. It might be noted that Uf and τc are virtually fixed by the machine design and product, respectively.

172

Handbook of yarn production Break draft

Rubber-coated top rolls

Break draft Main draft

Main draft

X X

X

Fluted metal bottom rolls

X

Twist triangle Yarn

Short apron Twist triangle

Long apron

Yarn or roving (b)

(a)

Wear point Apron Front top roll

X X

Bottom roll

Yarn

Nose piece or bridge

Setting

Apron (c) The aprons shown are thin reinforced rubber bands, about 1 inch wide, which move at about the mean speed of the forming yarn. Where they protrude into the nip of the front rolls, they slide over nose pieces X.

Fig. 7.2

Apron drafting

of the bobbin just begins because, if an end breaks and the machines are not stopped quickly, these speed adjustments get out of balance and further operation is not possible until the whole machine is reset. The twister has already been described. Because the roving packages are large they are usually arranged in two rows (Fig. 7.1(c)). In some machines there is a difference in the angle α made by the roving entering the grommet; this leads to differing false twist and tension between the front and back rows. The result shows up as differences in yarn coming from one roving bobbin and another; the differences can produce barré in fabrics. The solution is to use extensions, which raise some grommets from B to A to give equal amounts of false twist. Worn grommets can cause similar problems but they are occasional rather than prevalent.

7.1.4 The roving process Roving usually has a count of about 1 or 2 hank/lb irrespective of whether it is in cotton or worsted processing. The count of roving is often quoted as ‘hank roving’.2 2 See Appendices 1 and 2 for definitions and calculations relating to Ne or Nw.


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Since this roving has then to be drafted to make yarn, and this drafting involves the sliding of fibers over each other, it is not practical to insert a high level of twist in the roving. Twist multiples (TM) of less than 1.0 are typical, but precise values depend on the staple length, fineness (or denier), and type of fiber. Data given by Klein [2] suggest that the TM for cotton is given by the regression TM = 1.785 – (0.46 × fiber length in inches). Cottons need a higher twist than synthetics; coarse fibers need more twist than finer ones. In addition, lightweight rovings need more twist than heavier ones. More roving twist and the use of lightweight rovings lead to less yarn hairiness, but too high a roving twist impairs the drafting operation in ring spinning. High roving twist is liable to cause defects in the yarn. There has to be a compromise between having a high enough twist in the roving to give it sufficient strength to withstand processing and having a low enough twist to permit proper drafting in ring spinning. Imperfections in drafting at this stage are likely to lead to thick and thin spots in yarns produced in ring spinning. A thick spot (or ‘slub’) is more difficult to twist than a thin one, and when a strand containing thick and thin spots is twisted, the twist therefore concentrates in the thin spots. Irregular roving is weak at places and overtwisted at others; it is difficult to process. Such poor material can cause considerable problems at the ring frame because of the so-called hard ends or tight spots. It becomes more likely that an undrafted portion of the roving is drawn through the drafting system of the ring frame creating a thick spot followed by a thin spot in the yarn. The weak spots give so-called creel breaks. A rule of thumb is that the roving should be just strong enough to remove it from the bobbin, manually, in a direction parallel to the axis of the bobbin. Differences will be found in this attribute if the roving is stored for any considerable time. The twist becomes ‘set’ and the layers more firmly bedded, with the result that it is sometimes difficult to process such aged material in the ring frame. High humidity in the workplace can produce a similar effect. A machine is incapable of shutting down immediately and there is always some discharge of fiber when an end breaks. In consequence, the whole frame is normally shut down automatically when an end does break. When an end breaks in roving, a free end of the weak roving lashes the adjacent structure creating a veritable snowstorm of fibers until the machine finally stops. In addition, there is always fly (airborne fiber) from other machines. Fortunately, this is not normally a frequent occurrence. Nevertheless, it still requires vigilance by the operator and the ability to shut down quickly. Uneven or faulty sliver should be avoided. It goes without question that the machine must be correctly set. The solution of increasing the roving twist to reduce end-breakages of the roving is rarely acceptable. There are a few machine designs emerging which break out the sliver supply when an end breaks. Fly contains dirty fibers and aggregates into masses, which are often picked up by the sliver or roving to give an undesirable thick spot. These can cause flaws in the materials or end-breaks in subsequent processes. A snowstorm of fiber also causes considerable masses of unwanted fiber to be deposited on adjacent rovings and serious yarn faults can also result from this. To reduce this danger, it is now standard practice to install traveling cleaners, which patrol a series of machines to blow fibers from sensitive areas and pick up the material disturbed by the blower. The cleaners consist of long, vertical, pendant tubes with air nozzles protruding at appropriate heights and a suction nozzle, which sucks the displaced fibers from the floor. These systems of tubes are moved along elevated tracks that pass along the fronts of several

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machines. The fibers swept up are filtered from the air and might even be reworked. Substantial lint collection systems with adequate suction are also needed. The lint collection systems have to be maintained, which involves regularly cleaning (or replacing) the filters, and removing the reusable fiber. The air for blowing is usually supplied by a blower on the traveling cleaner itself.

7.1.5 Apron drafting in roving and ring spinning Both roving and ring spinning usually have apron drafting. It is usual to angle the drafting systems with respect to the vertical (Fig. 7.2) to improve access and to help control the fibers in the twist triangle. The top rolls hinge upwards, out of the way, when the system needs rethreading, cleaning, or maintenance. Roving is removed from the drafting system at a fairly shallow angle to the horizontal and there is a little wrap around the front roll. Fully formed yarn is nearly always taken from the drafting system with a partial wrap of the fibers on the front roll, which causes moderate pressure to be applied to at least a part of the twist triangle. This gives a measure of control of the weak ribbon of fibers emerging from the front nip. Many end-breaks occur in this zone and control is important. There is a choice between short and long aprons (Fig. 7.2(a) and (b)). Long aprons are less likely to choke and are easier to fit than short ones but the long ones have a larger initial cost. Short aprons have better fiber control. To some extent the choice is governed by the rate of fly production and the class of yarns being made. Correct aprons and settings are needed to control the unevenness of the yarn since the greatest added variance to the product is created at the ring frame. Setting assumes a different meaning from that applied to a drawframe, and an example of it is given in Fig. 7.2(c). The extent of the variance is related to the high drafts used. Also, the use of correct hardness of the rubber cots on the top rolls is important. Referring to the whole drafting system, the back rolls have to control more fibers than the front ones. Consequently, heavier pressures have to be applied to control the fibers, and harder rubber is used there. Typical hardnesses of the rubber are 80° to 85° Shore at the back and 65° to 70° Shore at the front rolls. Rolls tend to harden in use and vigilance has to be maintained to spot any loss of fiber control due to the hardening of the rubber. The softer rubbers wear more rapidly than the harder ones and the hardest rubbers consistent with fiber control should be used. Also, soft rubber cots tend to lap more easily. When fibers first lap a roller, a surface is created that tends to collect more fibers, and very quickly a dense wrapping of fibrous material forms. This build-up can be so powerful that it forces the rolls of a drafting system apart and can even bend the shafts. The condition is created when the fibers are longer than a certain percentage of the roll circumference, or when the fibers or the surfaces are sticky. When rubbercoated rolls are used, a build up of static electricity can also start the undesirable process of roll lapping. Maintenance of correct rh in the vicinity of the rolls minimizes this problem. A discussion of the phenomenon of lapping is given in Section 8.2.1. Experience has to be used to make the final choice, depending on local conditions. Related to this, it might be seen that maintenance is important; a normal cycle includes buffing the rolls at intervals varying between 150 and 200 production days. This interval depends on the fibers being processed, roll weighting, draft, and rubber hardness. In ring spinning, the possibility of a lap-up is increased when an end breaks and


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the fiber stream is diverted into the pneumafil waste suction system. (A pneumafil waste system collects fibers emerging from the front rolls of a ring frame at those times when an end is broken; it is uneconomic to stop the whole frame, or even a section of it, for a single break.) Schiffler [3] postulates that the wrap frequency varies inversely with time, logarithmically with respect to the number of fiber ends entering the waste suction, and inversely with the apron clearance. Schiffler calls a lap-up a ‘wrap’ and he goes on to say that the wrap frequency (the number of wraps/ unit time), is normally a very small number. The apron clearance is defined as the distance apart of the aprons at the nose. The state of the rolls and the extent of ageing of the rolls affect the issue, as does any change in fiber, roll size, weightings, etc. Periodically, farmers suffer infestations of aphids and other insects, which eventually produce contaminants such as sugars on cotton; the fibers become sticky and difficult to handle in processing. Fiber lapping then becomes a problem. The effects of sugar reduce gradually as microbial action breaks down the substance, but insect excreta take longer to break down and sidelining the affected bales is then no longer a reasonable option. In man-made fiber production, the application of too high a fiber finish level can produce a similar result. Carelessness in tending the machines can sometimes result in deposits of oil or grease on the rolls; this too can produce these effects.

7.1.6 The ring spinning machine The ring spinning machine took about two human generations to replace the mule but, by 1982, some 150 million spindles were installed throughout the world, of which 80% were used for short-staple spinning [4]. The ring frame consists of a large number of spindles. One traveler and spindle co-operate with a bobbin, to twist and wind the yarn from a drafting system as shown in Fig. 7.3(a). This sub-system is replicated several hundred-fold in a ring frame because of the low productivity of a spindle. As with the roving frame, the bottom rolls are sometimes long cylinders, extending over many spindles, articulated at intervals along the frame and connected to gearing in the headstock. There are now some very long frames of about 1000 spindles/machine, and articulation is necessary to prevent trouble from changes in floor height that might distort the whole frame and cause bearing problems. The spindles are driven by one or more tapes (thin flat belts), which engage the whorls (pulleys) that project from the bottom of the spindle. Slippage of the tapes can lead to twist losses, which vary from spindle to spindle and which, in turn, give barré and streaking problems when the yarn is assembled into finished fabric. Consequently, it is desirable to carry out periodic checks with a stroboscope to find spindles that are out of tolerance regarding synchronism. This is not a small task because the spinning area is very large and sometimes covers acres of floor space. The ring frame is normally fed with roving from a large bobbin, and delivers yarn to a smaller one. Because of this, the roving bobbins in the creel have to be renewed less frequently than the yarn bobbins (ring tubes). When spinning a coarse count, the ring bobbins have to be doffed every few hours. This used to consume considerable amounts of labor. These days, the doffing is carried out automatically or semiautomatically and this process is referred to as autodoffing. At the time of writing, there is also some use of automatic or semi-automatic creeling in which the roving bobbins are transported to the ring frame by a rail system and the empty bobbins are automatically replaced by full ones as necessary. This further reduces the labor needed.

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Roving bobbin

Pigtail guide forms node for yarn balloon

Drafting unit

Aprons Ring

Components of the system are not in scale with one another to enhance illustration

Separator plates

Ring rail

Traveler (a)

Ring tube Yarn

Ring bobbins

X

Enlarged view of pigtail guide (b)

Ring

Traveler Ring flange Enlarged view of ring and traveler (c)

Fig. 7.3 A typical ring frame position

Typical spindle speeds for new machines in 1995 were in a range around 18 000 r/min, but new materials have been introduced, which inhibit ring and traveler wear and it now possible to raise the speeds to about 25 000 r/min. However, when spinning yarns with abrasive fibers or fibers with poorly formulated finishes, or fine yarns, speeds have to be reduced. Occasionally, two rovings are fed to each spindle (‘double creeling’) to even out errors by doubling, but the draft ratio is thereby increased, which also increases the errors generated by the drafting process. The result is a trade-off between improvement in long-term errors and deterioration in short-term errors. The practice is avoided by many, purely for economic reasons. High levels of error are often obtained with high draft. However, with attention to design detail and proper settings, short- and medium-length errors can be reduced to very acceptable levels. Nevertheless, high draft also tends to increase long-term errors and to produce yarn hairiness, unless condensers are used in the main draft zone [5]. Unfortunately, the lengths of the long-term errors fall outside the ranges


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normally measured, and often these defects are not detected until the yarns are assembled into fabric. Twist seeks the thin spots because of the low stiffness there; this phenomenon can give exaggerated defects in the fabric. With a good, even yarn there is little problem, but with an uneven one there may be complaints.

7.1.7 The twisting phase in ring spinning In ring spinning, the energy to drive the twisting mechanism is derived from the bobbin, but the level of twist is controlled by the traveler. The traveler is a C-shaped piece, which slides around a flange on a ring that is set into a ring rail as shown in Fig. 7.3(a). The rotating yarn balloons out, and it is necessary to use separator plates to prevent the clash of yarns from neighboring spindles. (If the balloon gets large enough to impinge on the separator plates, the yarn becomes more hairy and the spinning efficiency is impaired.) The mass of the traveler has to be balanced against the yarn linear density, and the so-called ‘traveler weight’ is an important factor in determining the yarn tension. The yarn tension, in turn, is an important factor in determining balloon size as well as the end-breakage rate. Each revolution of the traveler inserts one turn of twist into the yarn. There is a twist gradient across the pigtail guide (Fig. 7.3(b)), and some false twist caused by the yarn rolling on the internal surface of the pigtail guide. Consequently, the twist above the pigtail guide is a little less than might be expected. The bobbin rotates faster than the traveler and the trailing yarn drags the traveler behind it (Fig. 7.3(c)). The difference in speed causes the yarn to wind onto the constant speed bobbin. The yarn winds at different diameters during the build of the package and this causes slight variations in the twist insertion rate, but the differences even out over the length of the yarn. Productivity of a ring spindle is very low. For example, a spindle producing a 4 TM, 36s yarn at 18 000 r/min and 95% efficiency delivers only 0.039 lb/hr. Consequently the machine has to contain many spindles to achieve economy of tending and workspace. These are reasons for the ring rails, which hold many rings and serve many spindles. As an aside, it might be mentioned that it is also a reason for the bottom rolls of the drafting system to cover many spindles. It is an economical arrangement that reduces the capital cost of spinning.

7.1.8 Package build To build a yarn package, it is necessary to move the ring rail, which extends the length of the machine and carries the rings. This rail oscillates over about 2 inches (≈ 5 cm), in an asymmetrical pattern, during which the spindle rotates several hundred revolutions on, say, the upstroke and a much smaller number on the downstroke or vice versa according to the design of the machine. This short oscillation is called a chase; the term serves to distinguish it from the creeping build motion (Fig. 7.4(a)). The asymmetric building motion gives a wind structure that is stabilized by the longer yarn spirals generated on the downstroke. Each cycle of the chase creates an interlocked double conical layer of yarn on the top of the package (Fig. 7.4(b)) and this is called a weft wind. The actual number of rotations of the spindle can be set by changing the appropriate gearing. Superimposed on this motion is a slow lift, which changes the rail height sufficiently to accommodate the conical layer of yarn just

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Handbook of yarn production Creeping motion

Chase

Bobbin rotates

Bottom rail Belt or tape

Weft wind (b)

Top rail oscillates and moves (a) slowly upwards during the build

Movement of element

Pigtail guide

Balloon control ring

Ring & traveler

(c)

Fig. 7.4

Time

Package building

laid. Many machines not only move the ring rail but also control the position of the balloon control ring and pigtail guide (Fig. 7.4(c)). This helps to keep the yarn tension variations within a closer band than otherwise would be the case. The tube height is about 5 ring diameters plus about 0.2 inch (5 mm) and the total lift of the ring rail is about 0.8 inch (≈ 20 mm) shorter than this. The balloon height is about 6 ring diameters and this is important because an excessively tall balloon induces a high spinning tension and increases the chance that the top of the tube will interfere with the yarn in the balloon. These circumstances are undesirable because of increased end-breakage in one case and variable yarn hairiness in the other. Some travelers create more yarn hairiness than others.

7.1.9 Ring and traveler Travelers are shaped to accommodate the ring flanges and flowing yarn; they are normally made from small lengths of wire of a variety of cross-sections. There have


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been developments over the years in the profile and size of the ring flange, as well as in the corresponding traveler.3 These developments have been aimed at increasing productivity or reducing end-breakage rates. Also, the traveler has been changed to lower the point of contact of the yarn to lessen the tilting action that leads to jamming. A sampling of typical flanges and traveler profiles is given in Fig. 7.5. Diagram (a) shows a typical shape and (b) shows a design of ring that has two flanges to permit reversal so as to provide a new wear surface when the first flange is worn; this extends the life of the ring. The SU ring shown in (c) has a conical flange that distributes the load from the traveler to lessen wear and give greater stability, but it has a tendency to be difficult to piece. Piecing refers to the repair of an end-break, which is discussed in Section 7.1.11. Special techniques have to be developed to deal with the piecing problem. All rings should be uniformly smooth and properly centered, otherwise once-per-revolution variations in yarn tension occur and the end-breakage rate is increased. Diagram (d) shows a selection of traveler wire cross-sections. Round cross-sections are used for wool and long-staple spinning, whereas flat and half-round cross-sections are used for short staple. Flat cross-sections are often used for cotton because such travelers help clean the yarn; they shave off projecting hairs, which help lubricate the ring, but produce fiber build-ups on the traveler. The halfround cross-sections are frequently used in elliptical travelers. Elliptical shapes give a lower center of gravity to the travelers, which reduce the tendency for them to tilt in operation. Some travelers are made from special wire, which more readily transfers heat from the sliding surfaces and permits high speed operation. Both rings and travelers should be run in at low speed for a period before they are used in high speed operation. In practice, it is rather onerous to run in travelers and many do not do it. Normally, no oils can be used for lubrication of the traveler, otherwise there is a risk of oil stains on the product. Crushed fiber debris is usually sufficient for lubrication; sometimes non-liquid anti-friction surfaces are used. When lubrication fails, shortlived micro-welds form, which disrupt the smooth movement of the traveler. The choice of traveler is conditioned by the type of ring flange used, as well as by the intended product and the production speed. If the machine elements are improperly Yarn

Yarn

Yarn

F F F

(a)

Ring cross-sections Ring flanges at F (b) (c)

Traveler crosssections

(d)

Fig. 7.5 Ring and traveler cross-sections

3 Rotating rings have been tried in attempts to overcome the sliding problem just discussed, but although productivity increases of up to 40% have been cited [6], the capital cost of the ring is increased, and the lack of simplicity has prevented the system gaining significant market penetration.

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placed, or the sliding surface on the ring becomes damaged by rust or micro-welds, the defect is reflected in a periodic variation in yarn tension. An eccentric spindle also produces variations. Since the probability of an endbreak is greatest at the peaks of yarn tension, it is the maximum tensions that matter rather than the average. Thus, reductions in the deviations from the normal are important in minimizing the end-breakage rates. The efficient running of a mill depends, in part, on ensuring that the rings are well maintained and that the correct travelers are used. The foregoing implies that the traveler must be changed when the yarn count is altered and sometimes when the fiber is changed. If the traveler is too heavy, or the spindle too fast, the load between the tiny traveler and the ring flange becomes sufficiently high to cause excessive wear or burns. Travelers wear and have to be changed frequently. Consequently, not only must the correct traveler be chosen for the job but also the traveler changes have to be scheduled on a regular basis. Burned and worn travelers can fly off and eye protection is advisable in the ring room. Attempts to run the traveler too fast not only cause the contact area of the traveler to burn, but also cause the yarn to be damaged. Speed is limited by the traveler, which, in short-staple spinning, is rarely lubricated with oils. Even if the travelers are nickel plated or otherwise treated, they can only slide up to between 100 and 150 ft/sec (30–46 m/sec) during their short working lives. The precise speed depends on the ring diameter, smoothness of the ring surface, yarn tension, and the traveler weight and design. The ring surface must not be polished since a suitable micro-structure of the surface is needed. The life of a traveler is very short and is measured in days of running time. The combination of the various forces acting on a running traveler causes it to tilt and this affects performance. According to Klein [2], normal running pressure between the traveler and ring is 35 N/mm2; consequently the ring has to have a hard, smooth (but not polished) surface and the traveler has to have a less hard running surface so that there is sacrificial wear on the cheaper traveler. As mentioned, it is good practice to ‘run in’ new rings to produce a viable surface. This entails running at reduced loads and speeds for some hours before resuming normal operational conditions. It can be calculated that the speed of winding on the ring frame is only a very small percentage of the spindle speed, which is fortunate because the spindle speed is fixed and it is the traveler that controls the twist insertion rate.4 Some calculations in this regard are given in Appendices 1 and 2. As the bobbin diameter changes, so a very small variation in twist occurs. When the bobbin diameter builds from (say) 1 inch (≈ 25 mm) to 1.75 inch (≈ 44 mm), the traveler speed might vary from 15 950 to 15 900 r/min, a difference of about 0.3%, which is negligibly small as far as twist is concerned. The outside diameter of the yarn on the package must be less than the ring size. The diameter of the empty tube onto which the yarn will be wound has a minimum size of at least 45% of the ring size, otherwise excessive yarn tensions would be generated. The density of the yarn is approximately fixed. As mentioned earlier, the length of the package is limited to about 5 ring diameters. This limits the changes in yarn tension caused by increases in balloon height. Since practically the whole bobbin has to fit inside the balloon at some time or other, the length of the package has to be limited and therefore the mass of yarn that can be stored on a ring tube is often limited to just a few ounces (1 oz ≈ 28 g). 4 It is not correct to say that the traveler puts in the twist; the energy derives from the spindle.


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Forces acting on the yarn passing through the traveler, and forces acting on the traveler, have to be in equilibrium. There is an important balancing mechanism provided by the traveler. The forces acting can be classified as (a) those coming from the yarn, (b) centrifugal force acting on the traveler, (c) frictional force acting on the traveler, and (d) the reaction component between traveler and ring, acting normal to the ring. The resultant of force components (a), (b), and (c) is balanced by the reaction force (d). Part of the centrifugal force acting on the traveler is absorbed by the ring at the point of the sliding contact, and part applies tension to the yarn entering and leaving the traveler. The very important balance between these two parts is affected by the angles that the yarn and traveler adopt under the given running conditions. The balance changes with the ring rail position and so do the forces (and tensions) involved. The tension averaged over, say, one second, changes with the geometry of the system. There is a variation that more or less follows the movement of the ring rail. During the chase, however, the tension is not in exact synchronization with the rail position because the winding point on the bobbin lags behind the recent movement of the rail. In the main, the yarn tensions are at a maximum when the rail is near the top of its chase but near the beginning of the build. Some machines control the spindle speed according to the position of the ring rail, to restrain the yarn tension [7]. If the yarn tension is too high, the probability of an end-break increases. The practical way of adjusting the yarn tension in mill practice is to alter the traveler weight and type according to the yarn being spun. Changes in traveler weight and design affect yarn hairiness, but there is some dispute as to the extent [8]. The traveler weight also affects the yarn package density. Too light a traveler can produce an undesirably soft yarn package with poor yarn storage capability, but what is more important, it can shift the operational condition to a zone of instability when a bobbin is in the early stages of build. The balloon collapse associated with the instability leads to end-breaks. The complex subject of ballooning is considered in Appendix 9. The traveler weight is selected on the basis of the yarn count and traveler type. Travelers are described commercially by numbers in both the direct and indirect systems of counting; for example, they are often quoted in grains per 10 travelers but they might be quoted in the number of travelers per unit mass. However, it is helpful to use a traveler mass unit related to the linear density of the yarn instead. In that case, recommended traveler weights range from 2.6 mg/tex for high yarn counts to 3 mg/tex for counts in the range of 20 to 30 tex. The values given are subject to adjustment for the type of traveler in use and the spindle speed.

7.1.10 Spindle eccentricity Lünenschloss et al. [9] showed that eccentric spindles can produce increases in hairiness of the yarn as well as influence its strength and elongation. This was especially true at high eccentricities. With a 20 tex, 65/35 polyester/cotton yarn, the hairiness changed from about 1000/m to 1700/m at a spindle eccentricity of 2.5 mm. With combed cotton, the change at the same eccentricity was from about 500/m to 700/m. Apart from its effect on the yarn, the life of an eccentric spindle is shorter than that of one that runs true; also, the noise level is worse. An eccentric spindle, or a displaced guide or ring, can increase the end-breakage markedly because of the once-perrevolution cycle of tensions produced; this has important economic repercussions. (Note: 407/yd ≈ 500/m, 640/yd ≈ 700/m, 914/yd ≈ 1000/m, 1550/yd ≈ 1700/m, 0.1 inch ≈ 2.5 mm, 20 tex ≈ 30 cotton hank/lb (Ne.)

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7.1.11 Dealing with end-breaks End-breaks cause a loss in production because the spindle produces no yarn after an end-break until it is repaired. An operator may serve up to several thousand spindles in modern plants and it is unavoidable that, on occasions, an end-break will cause a delay of perhaps twenty minutes or even longer before a repair is made. If an endbreak were to occur every hour on every spindle, the production efficiency would be terrible; even if it occurred once a day, there would still be a significant production loss. An end-break has to be a rare event if reasonable production efficiencies are to be maintained. When an end breaks, normally the textile fiber keeps flowing and is sucked away by a pneumafil or waste fiber suction system. To manually repair the breakage, the operator has to retrieve the end from the bobbin, thread it through the traveler, balloon control ring (if one is fitted), and the pigtail guide before inserting it into the nip of the front drafting roll. With an experienced operator, this takes only a second or so, but the time spent in patrolling to find the end-break is quite another matter. Machines, or attachments to the ring spinning machine, can simulate the action, but the capital cost is high. It is also possible to interrupt the roving supply (known as a ‘roving stop’ system) to prevent wastage and choking. The complexity of the roving stop results in an initial cost that amounts to a significant sum because of the thousands of spindles involved, but it does reduce the fiber loss significantly and it improves yarn quality as a consequence. Because of the capital costs involved with the roving stop system, most users prefer the conventional method but it involves the expense of dealing with about a 2% fiber loss. Waste fiber can be recycled, but only with care because it does not spin well. It has to be mixed and diluted with virgin fiber. In spinning, there is always some fiber loss from the twist triangle zone and suction is always required to remove the waste. When an end breaks, the amount of waste increases. Over all the spinning frames, there is a level of waste that is dependent on the mean end-breakage rate and beyond a certain level it becomes difficult to absorb the wastage without deterioration of the mill performance or the quality of the product.

7.1.12 Ring frame limitations As already mentioned, the ring size is limited on the large side because of traveler burns at normal production speeds, but, on the other hand, the ring cannot be too small, otherwise the bobbins would hold so little yarn that the cost of changing bobbins would be prohibitive. One of the limitations of the ring frame is the traveler speed. With non-rotating steel rings and steel travelers, the linear speed is limited to about 100 ft/sec (≈ 30 m/sec). The limit arises because the poorly lubricated traveler makes micro-welds with the surface of the ring, which are immediately broken as the traveler goes on its way. This creates incremental damage to the surface of the ring, which still endures over the life of many, many travelers. However, the roughening of the ring surface also progressively shortens the life of travelers. One solution is to use ceramic materials for the rings to minimize the damage. For a frame to run at a higher speed, the ring diameter has to be smaller and clearly there is a limit to how small the ring can be. The volume of yarn on the package is less than ρ( π /4)( D12 – D22 ) L, where D1 is the outside diameter of the package which must be less then Dr, the ring inside diameter, D2 is the diameter of the bobbin, and


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L is the length of the bobbin covered with yarn. D1 must come as close to Dr as practicable and the corresponding mass needs to be as large as the limitations in ring geometry will permit. Currently the ring sizes for short-staple spinning vary from 1.4 to 2.1 inches (≈ 36 to 53 mm) inside diameter and they use several sizes of flange varying from 0.125 to 0.16 inches (≈ 3.2 to 4.1 mm). Smaller rings mean not only that the mass of yarn that can be stored is reduced but that the frames have to be doffed more frequently. With automatic doffing and good splicing this is of less importance than formerly. However, doffing still takes time from production and the smaller the ring, the lower the spinning efficiency (but only incrementally so). An offsetting factor can be that smaller rings mounted on a machine that is designed to take them, reduces the capital cost. Another limit is reached when spinning medium to fine counts of highly twisted cotton and blend yarns, in which case the speed has to be reduced to suit the prevailing conditions. The heat dissipation at the traveler is approximately proportional to the third power of spindle speed and the traveler has to be able to dissipate the heat generated by conduction or radiation. When the lubrication breaks down, the safe temperature limits of either the yarn or traveler might be exceeded. In the one case there are molecular changes to the polymer structure, or even surface melts of the fiber. In the other case, the metal structure changes; the metal changes color and eventually breaks. One solution is to run with a lighter traveler but if this is carried too far, the balloon collapses and there are ends down as a result. Of course the speed can always be reduced but that begs the question because we look to higher speeds to improve economic performance. Yarn tensions are also responsible for a good portion of the excess end-breaks in spinning and this is why attention is focused by some on the ballooning mechanics (see Appendix 9). Figure 7.6 gives an example of tension variations in which the black line is the running average over a 0.2 second interval and the light gray points are the variations from this running mean [10]. The running average shows the variation caused by movements of the ring rail, with some superimposed variations due to changes in linear density. When the tension variations exceed a level indicated by xx, the tensions become high enough to pose a risk of breakage. The probability of an end-break is determined by the statistical distributions of yarn strength and tension. The height of the line xx is influenced by the strength of the weakest links in the yarn being spun. The points at risk are shown as small black crosses and the number of these should be rare if there is to be reasonable spinning efficiency. Klein [2] reports that the majority of end-breaks occur as the ring rail approaches its topmost position in the chase, rather than on the downstroke.

7.1.13 Mill balance Returning to the question of productivity, let us establish a range. Compare the productivities per spindle when making 81s and 9s yarn, both with a TM of 4.0. The first is a fine yarn and the second is a coarse one. Assuming the spindle speed to be 18 000 r/min and the efficiency to be 95%, the productivities are 0.012 and 0.314 lb/hr respectively (using the formulae in Appendix A1.5.1). Considering that a mill might produce, say, 20 tons/day (1867 lb/hr), at least 155 000 and 6 000 spindles, respectively, would be required. The way in which productivities alter with average count leads to an operational difficulty. Suppose the mill had been set up to produce 36s. The number of spindles required would have

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Handbook of yarn production When the specific tension is lower than the breaking stress (xx), then the chance of an end-break is low. When the specific tension is higher than the breaking stress (xx), then the chance of an end-break is high.

40

x

x Specific tension (g/tex)

30

20

Running average of specific tension

10

0 0

10 Time (seconds)

Fig. 7.6

20

Yarn tension in spinning

been about 48 000. If a heavier yarn such as the 9s were substituted, there would be 42 000 excess spindles, whereas, if the very fine 81s were substituted, there would be a shortfall of 107 000. In this exaggerated first case, over 87% of the ring frames would be shut down with an accompanying loss of employment for the operators. In the second case, also exaggerated, the mill would be capable of supplying only 30% of the requirement from production. The figures are exaggerated to make the point; normally the average count is kept reasonably near the design value. It is very expensive to change the so-called balance of the mill, once it has been set up.

7.1.14 Automation in ring spinning At this stage, automation from sliver through to yarn will be discussed. Sliver handling is automated to different degrees according to circumstances. Nearly all modern cards and drawframes have automatic doffing. Some mills use automated guided vehicles (AGVs) to marshal the cans in the creels of the following machines. In rotor spinning, this segment of the costs plays a larger proportionate role than it does in ring spinning. With roving, the usual solution is to use an overhead rail system with the bobbins suspended from carriers. In many of the systems, the transport system carries the bobbins from station to station, passing occupied positions and making exchanges for an empty bobbin where a full one is needed. There is a risk with this system that a few bobbins might circulate for a long time before they find a home. Periodic checks for such ‘joy riders’ prevent difficulties. With roving, it is necessary to control the loose


185

ends during transport and this is normally accomplished by making a wrap of the end in a secure place before doffing. The use of AGVs is also possible. Mechanical piecers have been introduced from time to time. They work well but they have not become established. There is a feeling that there is still a need for patrolling operators to clean, check on performance, and perform other duties. Automatic ring frame doffing has been widely accepted and the most common system involves rails that reach from end to end of the frame. These rails are designed to carry the full bobbins during the doff, and the empty ones during the replenishment phase. The doffer rail carries apertures for each spindle and each spindle is equipped with a grasping device. The grasping device is often an inflatable cuff which fits over the bobbin and grasps it. The purpose is to lift the full bobbin from the spindle without damaging the yarn. Two series of pegs are mounted on a belt running the length of the machine. One series of pegs carries empty bobbins which have been mounted before the start of the doffing sequence. The ring frame is stopped automatically when the bobbins are full, then: (a) the ring rail is lifted clear after the ends of yarn have been trapped at the base of the spindle; (b) the doffing rail is dropped over the full bobbins; (c) the grasping devices are activated and the rail is used to lift the full bobbins from the spindles; (d) the full bobbins are deposited on the vacant pegs on the belt just mentioned; (e) the doffing rail then picks up the empty bobbins from the belt; and (f) the rail deposits these empty bobbins on the empty spindles. On start-up, the yarns should still be threaded through the travelers and the rotating bobbins should catch the yarn and start spinning automatically. In practice, a few ends fail to catch and have to be pieced manually. Thereafter, the belt moves towards the end of the spinning machine and the full bobbins are either removed or continue on to the winder. When the bobbins are transported directly from the autodoffer to the winder without human intervention, it is known as ‘linked spinning’. Clearly, some sort of quality control is needed because deformed or improperly filled bobbins are unlikely to unwind properly and it is best to discard them. The defects would be difficult to trace if not caught before winding. Consequently, the bobbins are gauged at the exit from the spinning machine and faulty ones are rejected. Looking to the future, monitors could be fitted to keep track of the number of endbreakages and other performance data. The cost of these monitors is high and few mills are willing to invest in them until there is a more assured way of translating the large volumes of data they provide into an effective control system, capable of yielding an economic gain. So far, the complex factors and interactions in the process are not well enough understood to permit accurate prediction of the outcomes.

7.2

Open-end spinning

7.2.1 Basic principles The basis of open-end (OE) spinning is that fibers are added to an ‘open-end’ of a yarn, as indicated in Section 3.4.1. Twist applied to the newly added fibers converts them into yarn and the new elements of yarn are continuously removed from the twisting zone. The theoretical advantages of such a system are: (a) it is easier to rotate the small open-end of the yarn than it is to rotate a whole yarn package as in the case of ring spinning; and (b) the twisting and winding can be separated. The first point implies a tremendous potential for increased productivity and the second point means

186


that the package size is limited only by the design of the winder, which leads to greatly reduced handling costs. To produce an open-end, it is necessary to use a very high draft so that the fiber flow is reduced to just a few fibers in the cross-section. This prevents twist from running back into the fiber supply to produce false twist, which would defeat the object of the exercise. The technology has developed to the point where OE machines are fed with sliver and this eliminates the roving frame. A sliver might have (say) 20 000 fibers in the cross-section, and, if the fiber flux just before the open-end is as low as (say) 2, the initial draft would then be 10 000:1. It is not possible to use roller drafting alone to produce this sort of draft at the speeds required. Rather, it is more normal to use a toothed roller, which acts in much the same way as a licker-in in a card. The emerging yarn might have several hundred fibers in the cross-section and thus there has to be a condensation of fibers leaving the open-end. It follows that the essential phases in the spinning operations are as shown in bold font: 1 2 3 4 5 6 7 8

Drafting. Fiber transport. Fiber alignment etc. Cleaning (if necessary). Fiber condensation. Twisting. Yarn removal. Winding.

In practice there have to be some intermediate phases as well and these are also listed.

7.2.2 Open-end systems There are many embodiments of the basic idea of OE spinning and, although only one (rotor spinning) has taken a large market share, it is worthwhile to mention briefly a number of the other contenders. The early mechanical systems of the nineteenth and twentieth centuries were too cumbersome to work at high speeds. Generally, it was the invention of the mechanical/ pneumatic systems that led to workable prototypes, which had a chance of commercial development. An early variety of OE spinning used an air vortex device. Another variety was friction spinning, but although this reached industrial production, the fine yarn version did not develop fully due to lack of yarn strength and other problems. Friction spinning for coarse yarns and core spinning did become established for a segment of the market. One successful form of friction spinning was the DREF machine in which fibers are ‘rolled’ into yarn by a pair of condenser rollers as sketched in Fig. 7.7. Rotor spinning itself is now well established. In the early days, a rotor was easy to piece up at the, then, low speeds, but it produced a yarn that was some 20% weaker than ring yarn. Because the yarn was acceptable for some uses, and because of the economy of operation, rotor spinning established a foothold despite these difficulties. A further disincentive was the high capital cost component of yarn produced; it could only be made to pay its way if low count yarns were made. Thus, there was pressure to increase the count range spinnable, by increasing the operating speed. The first commercial machines operated at 30 000 r/min, but, by the mid-1990s, speeds of 120 000 r/min were possible. Productivity raced ahead of capital costs to the point


187

Input fiber

Airflow

Feed roll Combing roll

Output yarn

Perforated suction rolls

Fig. 7.7

Friction spinning

that the break-even count, discussed so much in the 1970s, was no longer an issue. That is not to say that the capital cost did not go up. For example, it was found that manual piecing was impractical at the high speeds now established; automatic piecers had to be designed and manufactured. Such further developments have a considerable cost that has to be passed on to the machine buyer in the form of higher absolute capital cost. Nevertheless, the capital cost of the machine/lb of yarn produced was reduced.

7.2.3 Rotor spinning Rotor-type OE spinning was first used commercially in 1969 [11], but by 1995 it was a major part of the US yarn spinning capacity and had found widespread use elsewhere. At the beginning of the development, rotor spinning was offered for both short- and long-staple spinning but long-staple rotor spinning did not become established. Further discussion will be limited to short-staple rotor-type OE spinning.

7.2.4 Drafting in rotor spinning It is possible to combine a rotor with a combing roll drafting system to make a spinning machine, so let us consider the drafting system before explaining the functions of the rotor. The combing roll drafts fibers in much the same way as does a licker-in of a card. Fibers are detached from a beard presented by a feed roll, as shown in Fig. 7.8(a); the detached fibers pass into the rotor at much higher speeds than the advance of the beard and thus there is a drafting action. To sustain the high exit fiber speeds, an adequate airstream is needed to carry the fiber forward in a sufficiently opened state to prevent twist from running back to the drafting system. If one required the ideal exit fiber flux of unity, the draft ratio in the combing roll would be equal to the number of fibers in the cross-section of the sliver feed and that could number tens of thousands. In practical terms, the fiber flux is usually greater than unity, but the value cannot be very high or the system would not work; draft ratios at the combing roll are usually measured in thousands. The fiber flow is later condensed inside the rotor as

188

Handbook of yarn production Rotor Fiber to rotor Toothed combing roll Feed roll

Cleaning edge

Pressure Combing roll Sliver input

Trash out

Sliver

Feed roll

Combing edge Feed plate

Bearing

Fiber beard Fiber to rotor

Tape

Reaction (b)

(a)

Fig. 7.8

Rotor spinning drafting system

a precursor to yarn. The overall draft ratio is that calculated from the ratio of linear densities of input sliver and output yarn, measured in compatible units; it is the mathematical product of the draft at the combing roll and the fractional draft (i.e. condensation) in the rotor. Various feed systems are possible, but the one that has become established is the roll and table type shown in Fig. 7.8(b). Sliver is gripped between the slowly moving feed roll and a stationary plate; the passageway reduces in size to increase the pressure at the point where the fiber beard is formed. The action of the teeth of the combing roll on the fringe removes fibers; these fibers are then discharged into the rotor where the precursor to the yarn is formed. The combing roll can be clothed with needles or saw-toothed wire but the latter has established itself firmly in the market. Hunter [12] notes in his survey that many find the pins superior to wire for opening capability and wear; he cites up to 60 000 hours’ life when spinning cotton. However, hardened and ground card wire with a surface of diamond dust embedded in nickel has been developed since then [13]. Other sharp edges prone to damage are similarly treated with wear-resistant materials. Siersch [14] found that there were advantages to spirally wound saw-toothed clothing because the helix angle of the wire causes successive teeth to penetrate the beard in positions that move across the beard as the roll rotates, and this gives a good distribution of fiber separation. He found that saw-toothed wire gave lower CVs in the yarn than did comparable needle arrangements. Wire-wound clothing is recommended for cotton and cotton blends running at high throughputs, whereas pinned combing rolls are recommended for fragile fibers such as acrylics and rayon running at moderate output rates. The force acting on the fiber beard increases ever more rapidly as fiber length is increased; beyond a certain level, fiber breakage then becomes a problem. Thus, the device is best suited for short-staple fiber. The fibers can disengage the teeth soon after leaving the combing zone and travel along the inside periphery of the combing roll housing at a velocity lower than the surface speed of the combing roll [15] (the housing is shaped to allow for the fiber flow). Friction between the housing


189

and the fiber slows the fiber until it reaches the duct that carries it to the rotor. An airstream is generated by the suction in the rotor, which accelerates the fibers and straightens them somewhat. Lünenschloss [16] states that the fiber velocity at the exit of the transfer duct is determined primarily by the negative air pressure inside the rotor (i.e. suction). Without an adequate airstream, the fibers relax, become disorientated, and tend to clump together. It is desirable for the feed material being presented to the combing roll to be as free from hooks and tangles as possible, in order to reduce fiber damage. However, it is difficult to keep the entering fibers aligned as they are manipulated by the combing roll. Stalder [17] has shown photographs illustrating how a single fiber can lie across several rows of combing roll teeth before it is carried away into the rotor. Lawrence and Chen [15] showed that short fibers were removed, and that longer fibers abraded the edge of the feed channel. Combing roll speed affects the yarn hairiness [18, 19] and can affect nep production. Too high an overall draft ratio can give high end-breakage rates but good trash removal; values in the order of magnitude of 100 are usually satisfactory. For cotton, the sliver weight depends on the sliver preparation and the yarn count, but common sliver weights vary between 50 and 70 grains/yd (≈ 3.5 to 5 g/m); for man-made fibers, the slivers should be about 25% lighter than with these. Fiber orientation in the sliver feed channel is important in terms of yarn tenacity and evenness. For coarse yarns in which yarn strength is not very important, it is sometimes possible to use card sliver as the input to the open-end spinning machine. For such carded yarn, performance is enhanced if the sliver is drawn in the carding machine. Otherwise it is normal to use drawn sliver. Occasionally, combed sliver is used. Another way to lessen fiber damage is to reduce the number of fibers in the fringe being combed by the combing roll, and this is achieved by using a lighter sliver, which adds to the cost of production. This can be offset against the advantages of rotor spinning. Fine combed sliver is difficult to handle.

7.2.5 Combing roll performance The cleanliness of cotton feedstock depends upon the degree of cleaning at the gin, opening, and carding. Cotton fiber ends tend to be damaged by the combing roll and at higher combing roll speeds there is fiber breakage. The combing roll speed is an important factor in this respect and this speed also affects the cleaning capability of the combing roll system (Fig. 7.9). A cleaning edge separates most of the trash before it can enter the rotor but it is difficult to remove all the dust (but often, when spinning man-made fibers, no cleaning edge is used). Separated trash passes to a ‘dirt box’, which is emptied periodically. Even when spinning man-made fibers, fiber breakage

Non-lint (% of sample)

30

Rotor

20 10

Dirt box

0 4000

6000 8000 Comber roll speed (r/min)

Fig. 7.9

Trash extraction

190


and the deposition of debris in the rotor is not eliminated. Polyester fiber finish, fiber debris, and oligomers are stripped from the fiber in passing through the combing roll and are then deposited in the rotor. Naturally, weak fibers are liable to breakage, but also damaged fibers have a tendency to pill. An optimum comber roll speed needs to be established for each product to give the best compromise between productivity and product quality. The clearance between the comber roll flanks and the casing should be strictly controlled [20], otherwise airflow can cause fibers to become trapped and jam the roll. Unfortunately, dust and particles tend to be deposited unevenly, particularly if large particles enter the rotor. Uneven deposits cause periodic unevenness in the yarn and produce unacceptable moiré patterning in the fabric made therefrom. As trash builds up, the quality of the yarn suffers and drifts lower as time elapses; when the rotor is cleaned there is a sharp improvement. Changes in evenness, appearance, and strength are caused by these cycles of rotor fouling and cleaning. They pose significant quality problems unless controlled. This circumstance has sparked the development of automatic rotor cleaning, which, in turn, requires automatic piecing to be effective. Cotton fibers can be worked at high speeds but man-made fibers are usually worked at lower speeds. The shape of the comber roll teeth is important because an aggressively forward-raked tooth, such as is used with cotton, can be seriously eroded by wear. This is especially true if it is used with dusty cotton or certain man-made fibers. Dusty cotton is sometimes found after particularly dry growing seasons. Improper or damaged wire can also produce neps, which can give serious quality control problems. Also, abrasive materials from fiber finishes, fiber debris, and silica dust from dusty cotton can cause excessive wear. Sharp edges (such as those at A and B in the line of fiber flow, Fig. 7.10(a)) and combing roll teeth (Fig. 7.10(b)) are subject to damage. It is desirable to avoid such finishes and fibers but, where this is not possible, the combing roll speeds, the sliver weight, and the rotor speed should be kept low. Yarn

Rotor Navel Wear on tips of teeth

Doffing tube

F

Fiber supply channel

Cleaning edge B

A

F Nicks on sharp edges

Cleaning aperture Comber roll housing (a)

Fig. 7.10

Comber roll housing Relative fiber motion shown by arrows marked F (b)

Comber roll wear


191

Lawrence and Chen [15] describe how short fibers tend to be thrown out of the trash escape aperture while longer fibers are retained for a little more time on the combing roll wire. The suction applied to the rotor removes the fibers from the combing roll teeth but the fibers drag over edge B as they pass into the rotor. This causes wear; both edges A and B have to be reinforced with especially hard material. The fibers become damaged at these points also, with the result that dust and nep are sucked into the rotor. Not surprisingly, the extent of trash separated by the combing roll varies with the state of cleanliness of the sliver supplied as well as with the speed and design of the roll. Rotor machines do not necessarily have such cleaning devices. Machines without trash removal are more suitable for spinning man-made fibers, but there is still a build-up of debris in the rotor due to the accumulations of fiber finish and fiber debris. In all cases, the debris in the rotor causes a deterioration in the yarn characteristics so it is important that the rotors be cleaned periodically. The length of the period between cleaning depends on the type of fiber being spun as well as the speed and type of spinning machine concerned. The combing roll speed is particularly important in this respect. A 3 inch (≈ 76 mm) combing roll normally runs between 5000 and 8000 r/min; the higher the speed, the more fiber damage ensues and the more debris is deposited in the rotor. However, the higher combing roll speeds tend to give better trash separation. Trash build-up causes a gradual deterioration in yarn quality. In particular, yarn hairiness, nep, unevenness, and other fault rates increase. Barella [19] and many others found that regular cleaning of the rotor is required to preserve yarn quality. Choking of the feed mechanism must be avoided. If too heavy a sliver is fed, or if the end inserted into the feed roll nip is doubled, it might overload the combing roll, causing it to jam. During running, badly stored sliver in the feed can cause a loop of sliver to be lifted from the can and to be fed into the drafting system. This produces a similar unfortunate effect. If such choking is allowed to persist and the machine continues to run for a long time, the whorl becomes overheated and so does the surface of the drive belt. The result is that the contact surface of the belt becomes glazed, causing slippage in all the combing rolls in the set. Such slippage might be uneven in time and from combing roll to combing roll. This has an adverse effect on yarn quality. Over long periods of time, overheating the combing roll assembly can cause the grease in the bearings to harden and add to the difficulties by causing some combing rolls to slip. Therefore, associated with choking is the possibility of bearing damage. Combing roll bearings with race tracks indented by the balls have been detected. Such damage increases the power requirements and increases the noise level in an operating machine. An increase in power demanded by a comber roll assembly increases the risk of slippage and of deterioration in yarn quality. Most combing rolls are driven from a single tape or belt. Auxiliary pulleys, spaced along the belt, control the path taken by it and apply the necessary force between it and the whorls at the bottom of each drive shaft. These arrangements are discussed in Section 7.2.8. Increases in combing roll speed are often associated with increased output. However, high combing roll speed and point populations can increase fiber breakage and lead to increased end-breakages in spinning, as well as a reduction in yarn strength. Some experimentation by the user is called for, to find the best compromise between machine productivity and quality for the particular product. An exceedingly high machine productivity does not necessarily produce the best financial return. Some trend curves for combing roll performance when spinning cotton are shown in Fig. 7.11.


Thin Thick Nep

Ends down/h

Yarn faults

Yarn evenness

Yarn tenacity

192

Tooth angle

Fig. 7.11

Comber roll speed

Trend curves for a comber roll performance

7.2.6 Fiber flow into the rotor Most machines will spin cotton or short-staple synthetic fibers. The trash in cotton is deposited in the rotor at a fairly rapid pace unless it is removed before entry into the rotor and it is difficult to remove all this trash by conventional means. Consequently some rotor machines have a cleaning edge or cleaning aperture built into the combing roll housing, as shown in Fig. 7.10. However, it is still of very great importance to clean the fibers well in the opening and carding operations, otherwise the deposition of dust in the rotor becomes a very severe problem with important economic consequences. Fibers flow from the combing roll through a fiber transport channel and are assembled in the rotor, where yarn is formed. Fibers must be completely removed from the combing roll and be transported to the rotor without being crumpled, disoriented, or clumped together. This means that an airflow velocity exceeding that of the surface of the combing roll must be used. To get such an airflow, it is necessary to run the inside of the rotor at a vacuum of several inches of water.5 Today, the practice is to use an external fan. The inside of the rotor gets hot and the fan fulfills the purpose not only of inhaling the fibers into the rotor but also of removing hot dusty air from it. The high temperature reduces the rh of the air inside the rotor and levels as low as 20% have been recorded. It is necessary to properly condition the input sliver. The ratio of air speed to that of the surface speed of the combing roll should be 1.5 to 2. (Some authorities suggest 1.5–4.) This is to ensure that the fibers are doffed properly; too high an airflow can increase fiber waste. 5 An inch of water denotes a difference of air pressure equal to about 1/400 of an atmosphere.


193

The disposition of the transfer channel or duct with respect to the combing roll is important. Lawrence and Chen [15] showed that short fibers are thrown from the combing roll at the transfer duct opening, and travel along the tube with what was the leading end still leading. Longer fibers are dragged by the combing roll teeth over the edge between the combing roll and the transfer duct, and then they are aspirated into the duct with their erstwhile trailing end now leading. The hooked fibers entering the transfer duct have a shorter fiber extent than if they were straight and, if such fibers remain hooked or convoluted in the yarn produced, they reduce the strength of the yarn. Thus it is desirable that some fiber straightening mechanism be employed. For this reason it is common practice to use a tapered duct that accelerates the flow of air and fiber as it approaches the duct exit. This tends to straighten the fibers; however, observation of photographs shows that the acceleration is insufficient to remove all the hooks and other fiber deformations in the transit stage. Fortunately, provided the suction is not too strong, the surface of the rotor that first contacts the fibers emerging from the transfer duct will be moving faster than the fibers. This sliding contact tends to straighten the fibers [21] although they are rarely, if ever, completely straightened and parallel at this point. The speed of a fiber as it enters the rotor should be about 80% of that of the metal surface on which it lands; the transitional draft at that point should be between 1.25 and 1.4. Fibers entering the rotor are deposited on the internal sliding wall (Fig. 7.12(a)) and move on this surface to the rotor groove, where they collect to make the fiber ring. Figure 7.12(b) shows the sliding path,6 which is approximately fixed in space with the rotor moving relative to it. Except in zones where the fibers sliding up the inside of the conical portion of the rotor interfere with the outgoing yarn, fibers are usually laid in the rotor groove in an amazingly parallel, straightened fashion. The sliding wall is part of the transit system; surfaces must be well designed and they must be kept clean. Fortunately, the movement of the fiber cleans the surface. It must be realized that the yarn rotates at high speed with respect to the sides of the rotor groove; centrifugal force presses the rotating yarn into the groove. Furthermore, the wedge action of the acute vee of the groove causes the centrifugal force to be magnified. Any abrasive particles that might be present then heighten the ‘lapping’ or abrasive action of the rotating yarn. For this reason, the inside surfaces of the rotor are treated to resist wear. Steel alloy rotors have been developed and it is now standard practice to diamond coat the surfaces. The condition of the inside of the rotors is of great importance. Yarn is removed through a doffing tube.

7.2.7 Twist in rotor spinning Real twist is applied to the yarn by the motion of the rotor acting on the yarn arm that passes from the rotor groove to the yarn withdrawal point inside the rotor. Each revolution of the rotor causes about one turn of twist to be inserted into the yarn, and 1/τ inches of yarn are removed (τ is the twist in tpi). There can be movement of fibers with respect to the metal of the rotor during twisting. This is because the fibers are not firmly held by any discrete nip at this point. Twist usually runs back along the rotor groove and some fibers are laid onto an already twisted core of fibers. This 6 For economy of line in a complicated diagram, the picture shows fibers aligned along the sliding path, but this is not necessarily true as they can move crabwise along the path.

194


Yarn

Yarn withdrawal tube

Rotor groove Fibers Rotor Yarn

Fibers

Navel Fiber sliding path Fibers slide on wall of rotor during entry (a)

(b) Yarn in rotor groove

N.B. Some twist runs into the rotor groove Yarn arm rotates False twist + real twist in yarn arm

Real twist in departing yarn Navel Enlargement of navel (c)

Fig. 7.12 Fiber and yarn in the rotor

affects the yarn structure. The center of the end of the yarn withdrawal tube is fitted with a non-rotating navel7 through which the departing yarn flows, as shown in Fig. 7.12(c). Sometimes a separator plate is introduced to prevent the premature capture of the incoming fiber by the outgoing yarn. This makes a less desirable transport system because of the complexity of the passageway, but in separating the incoming fibers and outgoing yarn it fulfills a useful function. The yarn entering the navel rolls on the inside surface. This rolling action produces a false twist in the section of yarn inside the rotor. The false twist is in addition to any real twist created by the movement of the rotor. Twist is trapped in the running yarn between the point of twist application and the nearest upstream twist trap. In the present case, the point of twist application is at the navel and the twist trap is on the collecting surface of the rotor. The flare radius of the navel affects the false twist generated, as shown in Table 7.1. Spinning performance and yarn character depend on the yarn twist inside the rotor (which is false twist dependent) rather than just on the apparent twist in the yarn delivered. Often, navels are grooved, as shown, to increase the false twist, but as speeds rise there is less need for this. Also grooved navels tend to make the yarn weaker as well as more bulky, neppy, and hairy, particularly 7 Experiments have been made with rotating navels, but they have not gained acceptance in the market.

Short-staple spinning Table 7.1

195

Effect of navel radius

Yarn linear density

59 tex fiber

Navel radius (inches) Yarn tenacity (gf/tex) Yarn elongation (%) CV of linear density (%)

0.06 12 18.0 6.2

25 tex polyester fiber 0.2 21 17.6 9.1

0.06 22 13.9 7.1

0.2 11 11.0 8.1

at high rotor speeds. The grooves cause the yarn to bounce off the surface of the navel for very brief periods of time. Yarn tensions measured inside the rotor are very close to the theoretical figure given by the formula ω 2 rr2 n but there are pulses due to the yarn riding over the grooves, if there are not too many of them. Unpublished work at NC State University showed that the number of pulses rose with the number of grooves until four grooves were cut. Increasing the number from four to eight gave only four pulses and this was interpreted to mean that the yarn jumped over alternate grooves. False twist is also affected by how close the front surface of the navel is set towards the flat inner surface of the rotor. Local shear in the air is produced by the rotor wall moving close to the stationary navel. This shear can produce a small amount of false twist in the yarn. Enlarged portions of yarn can interact with this space if the gap is set too narrowly. Gages are used to set the distance accurately. There will be differences in the coefficients of friction of the navel surface and the yarn; also the navels wear. It has become common to use ceramic navels because of their long lives but there can be unacceptable differences in the surfaces. As the navel varies, it causes the nature of the yarn, and the efficiency of the operation, to vary. Thus, it becomes important to make sure that all navels used in a given lot of yarn are similar, or operational and barré problems will result. The range of usable twist multiples is much affected by these considerations; a typical set of twist multiple curves is given in Fig. 7.13. Generally the twists are higher than for ring yarn and the combination of the higher twist and the more disorganized yarn surface creates a rougher hand. At one time this was of major concern, but fabric finishing techniques have improved and the market has adjusted for the differences; the lower cost outweighs the tactile disadvantages. As mentioned earlier, end-breakage rates are, amongst other things, a function of rotor diameter and

Yarn tenacity (mn/tex)

Polyester/cotton 150 Acrylic 100 Cotton

50 0

3

4 5 TM (machine value)

Fig. 7.13 Yarn tenacity curves for rotor spinning as a function of twist

196


speed. Whereas the larger rotors used in the 1980s gave minimum end-breakage rates/lb at about 70 000 r/min, the newest small rotors (down to 28 mm are reported) have minimum breakage rates at over 100 000 r/min. The minimum twist level achievable is of interest because low twist yarns have a good hand; also, the spinning machines have higher productivities at low twists. Generally, the minimum twist diminishes with rotor speeds up to about 70 000 r/min and then levels off; under some circumstances it rises at higher speeds. The lowest value of twist is a function of the radius at the base of the rotor groove and the type of navel in use. The navels might be grooved or non-grooved; they might be of steel or ceramic. Generally, the higher the rotor speed, the less need there is for grooved navels. As previously mentioned, ceramic materials are used to increase the life of the navels. The combination required for a given product is often initially determined by a manufacturer’s recommendation, which is followed by trials to find that best suited for the job.

7.2.8 Rotor bearing system A rotor spinning machine has multiple rotors driven by a single tape and, because of the very high speeds involved, special bearing arrangements are necessary. A common design is to support each rotor on an assembly of pulleys, which rotate at speeds lower than that of the rotor. Normal ball races are unable to survive at the highest rotor speeds now used. Air bearings have been tried, with various degrees of success, but the type of rotor support system most common now is similar to that sketched in Fig. 7.14. The supporting rubber-tired pulleys are mounted on ball races and these disks can safely rotate at the lower speeds.

Belt loading provided by jockey pulleys (not shown) Rotor shaft

F

Rotor

The bearings of the pulleys with tires (not shown) provide a reaction to the transverse belt load (F).

V

Fig. 7.14 Rotor support system

Belt


197

7.2.9 Winding on a rotor spinning machine The winding function of a rotor spinning machine is separate from the rest. All that is required is that the yarn be taken up at a constant rate. The rate of yarn removal is determined by the surface speed of a pair of take-up rollers. The gearing between the fluted sliver supply roller and the yarn take-up roller determines the machine draft ratio. The yarn can be stretched involutarily, in which case there is a difference between machine and actual counts. The speed ratio between the rotor and yarn takeup roller determines the twist level. Unlike a ring frame, the winding and twisting functions are divorced and this permits the building of large yarn packages whose size is limited only by the capability of the winder. As previously mentioned, the yarn is then usually wound on a large cross-wound cheese that might (when fully built) weigh some 10 lb (≈ 4.5 kg). Some rotor machines are capable of producing yarn cones of about the same size. Use of these large packages reduces the yarn handling costs. Rewinding (comparable to the winding from bobbin to cheese or cone in ring spinning) is usually unnecessary in rotor spinning because the number of yarn faults per package is usually low. However, it is difficult to anticipate which rotor will develop a fault, and when a faulty condition arises a great deal of yarn can be made before the fault is discovered. A cheese or cone running on the drive roller for long periods without yarn being laid becomes damaged by the drive surface. Therefore, some rotor yarn is occasionally rewound, but care is needed in rewinding because the yarn can be overstrained in the process. A better alternative is to monitor the rotor to detect a fault or an end-break immediately after occurrence and thus prevent the building of a bad package. Commercial devices cause the package to become disengaged with the drive roller when triggered by a fault; this is to prevent damage due to abrasion of the unchanging surface of the package during a non-productive period. Winding on an open-end spinning machine differs from winding on a separate frame. In the former case, the yarn feed rate is set by the take-up roll at a constant value, whereas in the latter case the yarn is supplied on demand. Compensation for the change in length of yarn between the guide and the lay-on point on the package is needed in open-end spinning. A simple scheme is to use a bow such as is described in Section 9.1.6. The yarn diverted by the bow approximately compensates for the change in length just mentioned and preserves a reasonably uniform yarn tension. When a cone is being wound, a yarn storage system is required which is capable of compensating for the differing wind-on speeds as the yarn traverses between the small and large diameters of the cone. The winder should control the yarn tension and maintain a uniform package density. In addition, a pattern breaker is required to remove unwanted variations in package structure, which arise as the diameter reaches certain critical measurements. Furthermore, cradle pressure control is required to compensate for changes in package weight as the package grows in size. A cradle lifter is often used to relieve pressure when an end breaks (this prevents scuffing of the surface of the package). Sometimes the yarn user requires a waxing attachment to apply paraffin (wax) to the yarn; this is particularly true where the yarn is for a knitting application. Further discussion of winding is given in Chapter 9.

7.2.10 Automation Rotor machines are currently available that are capable of running at 130 000 r/min. The maximum depends on the fiber being spun. Cotton can be spun at the highest

198


speed, acrylic at 20% less, and polyester and polyester/cotton blends at about 35% less (90 000 r/min). It is impossible to use manual piecing at such high speeds and automation becomes an operational necessity. The rotor machine lends itself to automation, and patrolling robots that piece, clean, and doff are commonplace. The robots follow a track round the machine. One sort of robot patrols, opens up rotors, cleans them, and re-pieces according to program or need. Another sort patrols and doffs when required. Frequently, machines have automatic start-up programs and built-in monitoring systems that will read out the machine performance over any reasonable period. Automatic doffing requires that a supply of empty tubes have a starter yarn bunch wound on each of them for start-up with the automatic piecer. Thus, there has to be a supply of empty tubes, a bunch winder, and a transport and loading system, as well as a system capable of removing the full bobbins in a safe and effective manner. The latter has importance because a damaged cone or cheese is rarely recoverable and it represents a considerable waste of effort and money. Damage is not restricted to a violation of the surface but also includes the loss of effective transfer yarn tails, which are so important to yarn users who tie packages nose to tail to reduce their package handling costs. Each package should have a starter and finisher tail disposed in a standard manner so that other machinery can find them. 7.2.11 Piecing in rotor spinning Before an acceptable piecing can be made, the rotor has to be cleaned. Manual cleaning involves stopping the rotor, opening the front cover, cleaning, shutting the cover, and restarting; a time-consuming endeavor. Automatic methods of fulfilling these functions are now standard to all new machines. The dirt in the rotor is usually loosened by one or more blasts of compressed air through ducts in the rotor cover, or the application of a scraper, and suction carries the debris away [13]. If a scraper is used, the blade is made of a soft material to prevent damage to the rotor and the sacrificial wear of the blade means that it has to be replaced periodically. Automatic piecing requires careful control of the opened fiber entering the rotor at start-up and the introduction of an end from the winder. After the inside of the rotor has been cleaned, new fiber is introduced and a ring begins to build up. To explain this, consider the series of pictures in Fig. 7.15. The end of the piecing yarn is shown as square cut. The steps are exaggerated for the purpose of explanation. It is assumed that the rotor has just been cleaned and that the thickness of the lines represents the number of fibers in the cross-section. In diagram (a), a starter yarn (or piecing yarn) is shown approaching the rotor groove; the fiber supply has just been started and a thin ring of fibers has been laid in the rotor groove. The yarn is sucked in by the vacuum and the air inside the rotor exists as a vortex. Thus, the yarn rotates about the rotor axis at a lesser speed than the rotor and, because of the twisting actions already described, the yarn end also rotates about its own axis. As the yarn is fed still further into the rotor (diagram (b)) the end is laid in the rotor groove and it tries to rotate and entangle the fibers already in the groove. The entangled end breaks into the fiber ring (diagram (c)). At an appropriate time, the yarn is withdrawn from the rotor at an appropriate speed. As the yarn is further withdrawn, the fibers continue to be supplied to the rotor groove and extra layers of fiber are laid over the break (diagram (d)). This process continues until equilibrium is reached. Meanwhile, the piecing contains a thick spot followed by a thin one and this is not acceptable. This is why the end is conditioned.


199

Thickness of fiber ring at time t

Piecing yarn Full groove (b)

(a)

(c)

Groove fills during the piecing operation

(d)

Fig. 7.15 Stages in piecing a rotor

For the purposes of explanation, let the circumference of the fiber ring be unzipped to permit linear drawings (Fig. 7.16). The timing of the piecing is important, especially at high rotor speeds. Thin spots break under the high tensions and, unless accurate timing is used, it is impossible to get spinning going. Because a human operator cannot reliably synchronize the stages in piecing at very high rotor speeds, automatic systems become a necessity. If a square cut piecing yarn is introduced into the rotor too soon and the rotor ring is thin, a very thin spot is generated in the piecing as shown in Fig. 7.16(a). If the yarn is introduced late and the rotor groove is nearly full to its normal operating level, a very thick spot is created. If the yarn withdrawal is started too soon, there is a thin spot, and if it is started too late, there is a thick one. Such a piecing is never perfect but it can be improved upon by tapering the end of the yarn introduced into the rotor, because this reduces the sudden changes in linear density, as shown in Fig. 7.16(b). Consequently, yarn end tapering is automatically carried out by the machine, often using a pneumatic stripping device. At first sight, some of the profiles shown do not seem to differ much. However, if the ordinates are plotted to include all three components at any point, the result is quite surprising; diagram (c) shows two such plots of the data for conditioned ends. The bad case shown is for early yarn introduction, which resulted in a distinct thin spot; the other case is a good piecing. There is another good reason for conditioning the sliver end before piecing. Endbreaks occur randomly and the time between the break and the piecing varies. While the end remains unrepaired, the combing roll is working the sliver end about to be fed to the rotor. Fiber alignment, as well as linear density of the fringe, is affected. To achieve standard conditions, the overworked sliver end should be discarded. The easiest way to do this is to feed sufficient fibers into the rotor in the normal way and then clean it before piecing. The machines are now designed to do this automatically.

200

Handbook of yarn production Early yarn introduction

Fiber in groove at break-in

Piecing yarn

Added fiber Late yarn introduction

Added fiber Early yarn withdrawal Added fiber Late yarn withdrawal

Added fiber (a) Square-cut yarn ends Early yarn introduction

Fiber in groove at break-in

Added fiber Late yarn introduction

A good piecing

Added fiber

Added fiber Early yarn withdrawal Added fiber Late yarn withdrawal Added fiber

(b) Conditioned yarn ends A good piecing

n n = linear density

Early yarn introduction

(c)

Fig. 7.16

Piecing diagrams

Some modern start-up devices reduce the machine speed for piecing. Other devices assign the piecer to control the initial yarn withdrawal and synchronize it with the accelerating machine before handing over control to the winder.

7.2.12 Fiber requirements Bridging fibers can cause deterioration in yarn performance. The probability of a bridging fiber is the ratio of the fiber length and the circumference of the rotor; thus long fibers used in small rotors produce poor yarn. Conversely, within limits, short fibers can produce reasonable yarn. (Comber noils can be spun successfully in rotor spinners, which fact indicates that fiber length is not of paramount importance.)


201

Some experimenters have stated that removal of noil from the feed material by combing has little beneficial effect on yarn quality whilst others say that combing improves yarn quality sufficiently to justify its use. There is an optimum fiber length beyond which no further increase is beneficial; the precise value is determined by the rotor size and geometry as well as the nature of the fiber. In making medium and coarse yarns, some variability in length may be acceptable but it is worthwhile controlling the short fiber content when spinning fine yarns. Deussen [13] points out that the effect of fiber length should not be underestimated. Man-made fibers are cut at 1.25 inches for coarse and medium counts whereas 1.5 inches is preferred for fine counts. (For ring spinning, the standard fiber length for man-made fibers at all counts is 1.5 inches.) Poor length uniformity can degrade the yarn quality but not so much as with ring spinning. Highly crimped fibers tend to aggregate and behave as longer ones; generally, high crimp is undesirable. Fiber finish also plays a part but, although low friction finishes are desirable up to a point for man-made fibers, too slick a finish can lead to yarn unevenness. Distinction must be made between fiber-to-metal friction and fiber-to-fiber friction. The former should be at a minimum but the latter should be high enough to prevent fibers slipping within the yarn structure when it is stressed. Generally, rotor spinning is now confined to short-staple spinning with relatively fine fibers. Fiber fineness plays a part not only in yarn strength, evenness, etc., but it also affects the hand of the product. For this reason, fine cottons have become quite popular with some rotor spinners. Experience has taught that a minimum of about 100 fibers is required in the yarn cross-section and the normal spin limit8 for cotton varies between 24s and 50s cotton count (Ne). The spin limit is related to yarn strength; higher strength fibers tend to reduce the end-breakage rate and increase the spin limit (this is because it is the minimum strength of the strand that determines whether or not a yarn breaks rather than the minimum linear density). End-breakage rates can vary from 15 to 150 ends down per thousand rotor hours. The practice in cotton marketing is to use micronaire as a measure of fineness, but this clouds the issue of spin limit because micronaire is affected by fiber maturity as well as fineness. The range of fiber finenesses now available for the man-made fiber components makes closer matching possible with natural fibers to make good blends. This is particularly true of polyester/cotton blends, which form a significant part of the market. Man-made fiber development has led to the production of finer fibers and the fineness has gone from the standard 1.5 denier to a range that includes values below 1 denier. The fine fibers are prone to nep but they spin reasonably well in rotor spinners except at the highest speeds. There is a tendency to break the finer fibers during spinning, which was demonstrated in a study by Looney [22]. Reducing the fineness of polyester fibers from 3 to 1.5 denier increased fiber breakage from 5% to 15%. Fiber strength is an important parameter in rotor spun yarn because, in part, of the comparisons made to ring yarns. A graph of the relationship between a particular set of cottons and the yarns made from them is given in Fig. 7.17 as an example. (Some people use the term ‘C × S’, which means yarn count multiplied by skein strength, and it has the dimensions of tenacity.) At twist multiples higher or lower than the optimum, the yarn strength declines. The cottons shown varied in length from 11/32 to

8 Spin limit is the count, or linear density, of the finest yarn which can be spun under the prevailing conditions. There are other definitions.

Handbook of yarn production Yarn skein strength (C × S)

202

Fig. 7.17

4000

NB C × S is known as the countstrength product and the units have the dimensions of tenacity.

3000

2000 25

30 35 40 Cotton fiber tenacity (gf/tex)

Optimum rotor yarn tenacity as a function of fiber tenacity

13/8 inch (≈ 26 to 35 mm), the TM varied from 3.9 to 5, and the shortest fibers gave the lowest yarn strength. The most important attributes for natural fibers used in rotor spinning are fiber tenacity, fineness, and cleanliness, in that order. Fiber tenacity is reflected in the yarn strength, and rotor yarns tend to be weak. Fine fibers work better in rotor spinning than coarser ones. Dirty fibers create a problem in keeping the rotors clean. Polyester, especially when an optical brightener such as titanium dioxide is used, is somewhat aggressive and creates more wear than cotton. For man-made fibers, cleanliness includes freedom from excessive fiber finish, debris, and oligomer. With man-made fibers and blends of these with cotton, the factors can be ranked in the order: fiber finish, tenacity, fineness, and length. An area of interest in rotor spinning is the spinning of acrylic fibers, which, with some machines, can be spun into yarn with a surprisingly low twist multiple. The yarns produced are soft and of interest to knitters who want even, knotless yarns with a soft hand and where strength is not of great importance. For this market, it is possible to make the yarns economically because the low twist permits high production. Economic success has been reported at counts as high as 30s cotton (45s worsted or 20 tex). Another area of interest is the spinning of waste fibers. For example, cleaned comber noils provide a cheap source of fiber and, in certain markets, the strength is acceptable, but the dust must be removed before spinning because combing does not remove dust from the noils. In some markets, noil is added to virgin fiber and produces acceptable yarns; in some cases 100% noil can be used. The use of noil in this way is of particular interest to makers of combed ring yarns because of the ready availability of noil. Blend yarns do not derive proportionate strength from the tenacities of the fiber components because fiber elongation also plays a role. Each component may be assumed to be extended by the same amount when a yarn is elongated. The load in each fiber is a function of both the elongation and the modulus of the fiber. A yarn made of a blend of stiff, weak fibers and extensible, strong fibers will usually fail when the weak fibers reach their breaking elongation. At that point, a strong fiber will only have contributed part of its strength to the composite at the time of failure. Thus, for example, a polyester/cotton blend produces an effect such as is shown in Fig. 7.18 [23]. However, if the percentage of weak fibers is small, the strong fibers might be able to bear the entire load when all the weak fibers have failed. This is not the usual case.


Tenacity (g/tex)

30

203

Ring

20 OE 10

Polyester (%) 0 0

20

100

80

40

60

80

100

60 40 Cotton (%)

20

0

Fig. 7.18 Blend yarn tenacities

7.2.13 Maintenance High speed precision machinery needs scheduled and thorough maintenance. It has been suggested that 2 to 3% of the initial cost of the machine should be spent on maintenance each year. Rotors and combing rolls are supposed to have operating lives of up to five years but lack of proper maintenance, or abuse, can significantly shorten useful life. The type of fiber being spun also has an influence. If the fiber is cotton, then freedom from silica contamination is very important; if it is polyester, the use of TiO2 or any other abrasive additives is a factor.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

Klein, W. A. Practical Guide to Opening and Carding, Manual of Textile Technology, 2, Textile Institute, Manchester, UK, 1987. Klein, W. A. Practical Guides to Combing & Drawing and Spinning, Manuals of Textile Technology, 3 & 4, Textile Institute, Manchester, UK, 1987. Schiffler, D. A. Roll Wraps in Ring Spinning: Part II, Effect of Fiber and Spinning Frame Variables, Text Res J, 1993. Shaw, J. Short-staple Ring-spinning, Text Prog, 12, 2, 1982. Lünenschloss J. Textil Praxis, 22, 689, 760, 1967. Stalder, H. Possibilities for Increasing the Productivity of the Ring Spinning Frame, Textile Machinery: Investing for the Future, Textile Inst Ann Conf, 1982. Herdtle, M. Fadenbrucherfassung, Minimierung und Behebung, Melliand Textilber, 6, 1984. Barella, A. Yarn Hairiness, Text Prog, 13, 1, 1983. Lünenschloss, J, Külter, H and Hoffman, B. Der Einfluss der Spindlexzentrizität auf das Fadenbruchverhalten und die Garneigenschaften, Forschungsberichte des Landes NordrheinWestfalen, Nr 2417, Westdeutscher Verlag, 1974. El Mogahzi, Y. Private communication, 1994. Rohlena, V. Open-end Spinning, Elsevier Scientific Publishing Company, Oxford, 1975. Hunter, L. The Production and Properties of Staple-fibre Yarns Made by Recent Developed Techniques, Text Prog, 10, 1/2, 1978. Deussen, H. Rotor Spinning Technology, Schlafhorst Inc, Charlotte, N.C., USA, 1993. Siersch, E. Ein Beitrag zum Mechanismus der Fasertrennung und des Fasetransportes beim OE-Rotorspinnen, Fortschritt Berichte der VDI Zeitschriften, 3, 56, 1980.

204


15.

Lawrence, C A and Chen, K Z. A Study of the Fibre-transfer-channel Design in Rotor Spinning, J Text Inst, 79, 3, pp 367–408, 1988. Lünenschloss, J, Coll-Tortosa, L and Phoa, T T. Die Untersuchen der Fäserströmung Im Faseleitkanal einer OE-Rotorspinnmaschinen, 24, 355–8, 478–85, Chemiefasern TextileIndustrie, May June 1974. Stalder, H. Faserauflösung und Faserführung beim Rotorspinnen, Vorträge anlässlich der 3. gemeinsamen Tagung der Aachener Textilforschunginstitute, 1977. Vila, F, Pey, A and Barella, A. A Contribution to the Study of the Hairiness of Cotton Openend Spun Yarns, Part 1, J Text Inst, 2, 55, 1982. Vila, F, Pey, A and Barella, A. A Contribution to the Study of the Hairiness of Cotton Openend Spun Yarns, Part 2, J Text Inst, 73, 124, 1982. Barella, A. Yarn Hairiness, Text Prog, 13, 1, 1983. Landwehrkampf, H and Schreyer, F. USP 3 524 312, 1970. Looney, F S. Mechanics of Open end Spinning, Private Communication, c 1975. Lord, P R and Grady, P L. Proc 15th Canadian Textile Seminar, p 33, Kingston, Canada, 1976.

16. 17. 18. 19. 20. 21. 22. 23.

8 Long-staple spinning

8.1

Introduction: Effects of lengthening the staple

Long-staple spinning was originally designed to work with wool and other long fibers, whereas short-staple spinning had been designed for cotton. Not only is wool much longer than cotton, it is much more variable in length. Wool fibers normally vary between 3 and 18 inches (≈ 76 to 457 mm) in length and between 8 and 60 microns in diameter. Therefore, the systems for wool must be able to cope with a rather wide range of conditions. It is also interesting that wool is naturally crimped at between 6 and 24 crimps/inch (≈ 0.2 and 1.0 crimps/mm); this is comparable to the levels introduced artificially in cut or stretch-broken tow. This means that one would expect more diversity in the machinery and processes used for long-staple opening and carding than found with the corresponding short-staple processing discussed in Chapter 5. Man-made fibers are now used in blends but much of the appeal of wool lies in its special character. Many consumers are willing to pay high prices for 100% wool yarns. This applies in both apparel and carpet markets, and consequently there remains a healthy market in fabrics made from pure wool. This is in addition to the market for blends of wool with man-made fibers. We can divide the field into two without losing too much of the total breadth. The two fields, which will be the subjects of major discussion, are the worsted and woolen systems. The worsted process as used for wool has similarities with the total short-staple process involved in ring spinning as discussed in Chapter 5 except that fiber cleaning has to be totally different because of the greasy nature of wool. The mechanical part of the worsted process applied to man-made fibers, like its short-staple counterpart, involves relatively little, if any, fiber cleaning. The worsted yarns produced by this system are twisted and have a much higher strength than the woolen yarns just about to be discussed. (Note: ‘woolen’ refers to the process and not the wool fiber.) The woolen system is a short process designed to make relatively inexpensive yarns. Stages of drawing, combing, and roving are dispensed with and the yarns are spun directly from a card cylinder. The loss of the multiple doublings coming from drawing and combing is countered by paying great attention to fiber blending in the

206


early stages of the process. Use of the woolen system has declined over recent years but a description is included because the technology contains the roots of many devices used in other processing.

8.2

Wool fibers and their preparation

8.2.1 Further effects of lengthening the staple It is possible to improve fiber cohesion in yarn by increasing the fiber length; this allows a lower twist multiple to be used. The lower twist, in turn, permits higher productivity and yields a softer hand. Thus, there are a number of advantages to working with long-staple fibers as compared to the short-staple ones we have already discussed in Chapter 5, but there are limitations. Long fibers are difficult to manipulate and the chance of fiber damage increases with fiber length. There are interactions with the design of the machine; under certain circumstances, fibers that should flow past, will adhere to the surface of a roller and be carried round that surface, as shown in Fig. 8.1. If the fiber length is larger than the circumference of the roller, a roll lap is likely to be produced. The fiber may be trapped on the roller surface as shown in Fig. 8.1(b) instead of being carried away by the linear fiber flow. If this happens, other fibers tend to be caught by the trapped fiber and a ribbon of fibers accumulates around the surface of the roll. The ribbon is tightly packed and continuously builds up, often causing damage to the rolls; these are called lap-ups. Such lap-ups usually have to be removed by cutting them from the roll; this, too, can cause damage. The ratio of fiber length and roll circumference is related to the tendency to lap. In cases where fibers are incompletely separated, it is the tuft length that controls the situation. Consequently, if the length of a tuft is longer than the circumference of the roll, lapups can occur. Fiber finish and crimp can also strongly affect performance in this respect and these parameters must be closely controlled. Generally, when the roller circumference is about twice the fiber length and normal fiber finish and crimp are used, an acceptably low incidence of lapping is obtained. Thus, long-staple systems usually have large diameter rollers in their drafting systems. In addition the ratch settings have to match the fiber length. It follows that long-staple drafting systems are larger in all dimensions than their short-staple counterparts. Long-staple yarns are usually heavier than short-staple ones; this makes it desirable to use larger bobbins to reduce doffing costs and thus the twisting portion of the machine is also larger. An increase in size is nearly always associated with a decrease in speed. Therefore, it

x″

x′

Fibers adhere to rolls at x′ (a)

Fig. 8.1

Fibers trapped under new layer of fiber at x″ (b)

Roller lapping

Long-staple spinning

207

should be no surprise to find that long-staple machinery is to a larger geometric scale, and is slower, than short-staple machinery. Long fibers tangle more easily than short ones (a man with a crew cut has much less difficulty in combing his hair than does a person with long hair). Proper lubrication of the fiber eases the problem. Crimped fibers tangle more easily than smooth ones. The carding process aims to disentangle the fibers and separate them, but it is found that the flat-top card damages long fibers in attempting to disentangle them. Consider the carding action between two sets of teeth moving at different speeds in about the same plane. One fiber end is caught in one set of teeth and the other end is caught by the second set. Figure 8.2(a) shows a typical fiber snaking between the teeth on a surface from A at one end to B at the other. If there is already a tension acting at A, then the tension at B is given approximately by applying Amonton’s Law.1 Appropriate angles as well as the input and output tensions are as shown in diagram (b). It will be seen that the more sinuous the shape of a given fiber, and the closer the pins are set, the greater will be the accumulation of tension. In practice, the tension at A is generated by interfiber friction in the fiber supply, or an interaction between fibers and pins in a preceding section, or both. That at B is X times as much. A factor Z = Tout/Tin is due to the fiber reactions against the pins just described, but another factor Y is affected by the fiber crimp and stiffness. (Hence the approximately equal symbol in the equation in the footnote.) The precise relationships between X, Y, and Z are not known. The reactions are caused by the in-built tendency for the fibers to take up a zigzag shape. Pressure from the teeth de-crimps the fibers as they are forced into the interstices, and this produces a friction force that contributes to the tension at A. In φ2

φ1

Motion of the fiber relative to the pins

T1 T2

B

T2

A

φ3

T1

φ4 (b)

(a) Fiber

Feed roll

T

C

D (c)

(d)

Fig. 8.2

Fiber tensions created by combing

1 Tout ≈ Tin eµθ where the angle θ = φ1 + φ2 + φ3 + φ4 and µ = coefficient of friction.

208


any event, the output tension at B is heavily affected by the input value at A. The angle of wrap, the fiber crimp level, the pitch of the teeth, and the coefficient of friction also affect it. The lubrication and crimp are of increasing importance as the staple length increases beyond 4 inches. The cumulative angle of wrap is affected by how many zigs and zags there are along the length of the fiber. Beyond a certain critical length, fiber breakage will occur. Using a pinned feed as shown in diagrams (c) and (d) can double the critical fiber length; such systems are commonly found in long-staple machines.

8.2.2 Wool fiber cleaning in worsted mills Wool is usually supplied to the mills as shorn or pulled fiber. Shorn wool is taken from living animals and pulled wool is from carcasses. These untreated materials are known as greasy wool because of the grease, suint (from old French ‘suer’, the verb ‘to sweat’), and other animal excretions that coat the fibers. Together with vegetable and mineral particles, there can be up to 70% foreign matter in greasy wool. The wool grease (or so-called ‘brown grease’) coating the fiber contains a waxy material called lanolin, which is a valuable by-product useful in making ointments and cosmetics. Suint is dried perspiration that contains valuable potassium salts. Greasy wool will not yield to purely mechanical means of cleaning and therefore it is necessary to scour the fibers before further mechanical treatment. There were relatively few developments in the first half of the twentieth century; however, the problem then started to receive more attention, especially in the grower countries. The traditional method of scouring is to divide the greasy wool and feed it to a fork frame (Fig. 8.3(a)). A procession of forks carries fiber through a succession of liquor troughs (or bowls) in which the fibers are washed and rinsed. The frame may be some 60 ft (≈ 18 m) long. The scouring liquor is traditionally an alkali (soda) soap solution, since it readily permits the recovery of lanolin, but this requires soft water, otherwise there are troublesome insoluble lime deposits. There is a diversity of ways of running the liquor through the frame and we can only describe one. Care has to be taken to control the temperature and the pH level of the liquor, otherwise the fibers will be damaged and the dyeability of the fibers affected. The degree of alkalinity or acidity is recorded by pH readings. Generally, the temperatures and pHs of the first stages can be kept high because the grease tends to protect the fiber. As the protective grease is removed from the fiber in its passage downstream, the temperature in successive bowls is reduced and the pH level is moved nearer neutrality. The final bath is merely a rinse in plain water, after which the wool is passed through squeeze rollers to remove most of the moisture. An average time for the wool to pass through the fork frame is 8 minutes. Scouring tends to make the fibers brittle unless they are oiled and the fibers are kept below about 120°F (≈ 50°C) during scouring. The wool grease has to be separated from the scouring liquor to allow a reasonable run of the equipment. Once again there is a diversity of systems and it is only possible to describe an example in this book (see Fig. 8.3(b) for a simplified diagram). The liquor still degrades, and the scouring liquor has to be completely changed every few days. In addition, there is a need for liquor make-up and fresh water to rinse, and thus the process is a great consumer of water. Solids, such as sand and other heavy particles, are allowed to settle in tanks in the scouring liquor circuit and these solids may be intermittently pumped out or otherwise removed. Fiber loss into the scouring liquor causes a problem because fibers tend to bind the contaminants, and when they


209

Greasy wool input Direction of harrow forks

Scouring liquor out Scouring bowl Scouring liquor in Filter

Settling tanks

Filter

Centrifuge

Squeeze-roll system

Rinse water out Rinse bowl Rinse water in Replenishment Hot air + moisture out liquor (a) Fork frame

Cool air in Recirculated and Heavy sludge cleansed liquor removal Waste Clarify wool grease

Crude lanolin

Fiber transfer

Scoured wool output

Heaters (not shown)

(c) Dryer

(b) Scouring liquor treatment

Sketch not in proportion, so as to illustrate the principles more clearly. The drawings have been simplified. For example, only one scouring bowl is shown.

Fig. 8.3

Fork frame

decay, they become malodorous. Fatty substances contaminated with fiber and other light substances tend to float to the surface in the settling tanks, and sludge centrifuges may be used to remove these lighter solids continuously. (Centrifuges are machines for separating solids from liquid suspensions.) Disposal of the liquor not processed by the recirculation system is discussed in Section 8.2.5. The wool grease is separated from the contaminated fatty solids and clarified (see Fig. 8.3(b)), to be later refined by the purchaser. If a market exists for potash at a price sufficient to cover the costs of separation, the salts are converted to whatever intermediate the buyer will accept. Some attention was given to solvent scouring but no significant market penetration was achieved. The use of detergents was once claimed to give whiter, loftier, and stronger fiber; detractors’ claims and counter-claims were not so significant as the impact on costs and the convenience of being able to scour to the desired pH level. In recent times, the development of effective detergents caused costs to become competitive with alkali solutions. Whichever method is used for scouring, the main aim is to remove the wool grease at minimum cost with as little damage to the fiber as possible. Cost considerations are a large factor in the choice of method. An important past problem was the felting of the wool during scouring, but suitable chemical treatments are now available to reduce the ensuing shrinkage [1]. Felting occurs because scales on the wool fiber act as ratchets, which favor movement of the

210


fiber in one direction rather than the other. The result is that wool structures tend to become densely packed as the mass shrinks due to the relative movement of the fibers. Felting makes the disentangling of fibers difficult. Some machines were developed in the mid-twentieth century to improve performance by increasing productivity and reducing the tendency to felt; one of these is the suction-scour machine. In this machine, wool passes round the lower surface of a number of horizontal, perforated drums rotating in the scouring bowl. A stream of scouring liquor flows from the outside to the inside of the drums and the wool floats near the surface of the liquor in a stream flowing from one drum to the next. The wool leaving the scouring plant should have a moisture content of less than 40%. This means that, where aqueous scouring systems are used, the wool has to be passed through at least one pair of squeeze rollers (mangles) to reduce the excess moisture. The damp wool is then passed through a dryer in which hot air is circulated through the blanket-like layer of wool carried on a lattice conveyer. Cool air is admitted at the delivery end and is progressively heated as it passes through the fiber as shown in Fig. 8.3(c). This enables the dried wool to be delivered in a cool state. After chemical treatment, it is essential to oil the fibers before proceeding with further mechanical treatment. Scoured fiber is usually hot air dried and oiled to a level of 0.75%2 to facilitate further working. It is interesting to note the need to oil the fiber after the surface has been rather drastically cleaned. Use of the wrong oil or the wrong quantity of oil can result in processing difficulties. See Section 8.2.4 for further discussion. Where the vegetable matter among the scoured and oiled fibers is present only in moderate quantity it can be dealt with at the card. Dirtier fibers need some precleaning. A partial alternative to the mechanical removal of vegetable matter is to destroy it by chemical action. Sulfuric, hydrochloric, or other acids (such as those produced by salts such as aluminum chloride when heated) may be used. These treatments reduce the unwanted matter to carbon and the process is known as carbonizing. Usually, the dirty scoured wool is steeped in acid, dried, and baked. Drying is accomplished by draining, mangling, and heating. The particles are then crushed and beaten out before diluting the acid and then neutralizing by soda washes. The dilution is necessary to prevent overheating the fibers due to the exothermic reaction during neutralization. Rinsing and drying follows it. Such carbonization gives some loss of fiber amounting to 5 or 6%. Also, there can be some loss in fiber strength, particularly if high drying temperatures are used. At 250°F (≈ 120°C), the loss in strength can be as much as 30%, but at 100°F (≈ 38°C) the loss is negligible. Again, it must be pointed out that there is a necessity to oil the fibers after chemical treatment to prevent fiber breakage and electrification. It will be seen that the wet cleaning process is complex and it can be added that it is expensive to both install and run. This alone tends to make wool products expensive; consequently a great deal of care is usually taken to preserve quality. The first mechanical cleaner in line is called a burr picker. A second cleaning is achieved by blowing air through the burr picker to remove light dust and impurities. One of the last opportunities mechanically to remove all but the last remnants of vegetable matter is in the carding operation. These mechanical operations are described in Section 8.3.2. Any residual vegetable matter left in worsted slivers is usually

2 A higher level might be used occasionally, especially if obsolete Noble combers are in use.


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removed in the combing process. An extensive discussion of the variety of processes is given in the Wool Handbook [2].

8.2.3 Fiber cleaning for woolen mills Many traditional woolen mills use recycled fibers and then there is no need to use methods described in the previous section, but there is a need to clean and sterilize the fibers. A shortened cleaning system is then used. However, if the mill wants to use virgin wool fibers, then a cleaning system such as that just described is required for at least part of the intake of fibers.

8.2.4 Fiber lubrication The importance of proper fiber lubrication has already been stressed but it should be realized that considerable care has to be taken in the choice of oil and the method of application. There are many oils used (and even more claims, many of which are unsubstantiated) and such materials as olive oil plasticize, lubricate, and reduce electrification. Over the years, many competing oiling formulations have been derived and it is beyond the scope of this book to discuss them further. The oil is usually applied to a layer of fiber, another layer of fiber is added, then more oil is applied, and so on. Too much oil, or one that is too viscous, or one that becomes sticky over time, causes lap-ups and chokes with the result that the fiber becomes unworkable. Also insufficient lubrication or lack of plasticizer can cause an undue amount of fiber breakage in the mechanical operations. Such fiber breakage impairs operational efficiency and reduces the quality of the product. Not only does the oil change the coefficient of friction, but it also reduces the tendency for the fiber to charge electrically. Insulated surfaces sliding over one another create electrical charges. All fibers are subjected to sliding contact with other surfaces during processing and thus the ‘oil’ has to possess some ability to allow an electrical current to flow to minimize the charge. If not, since like charges attract each other and unlike ones repel, the results are that (a) the fibers cling together and they are difficult to separate and (b) some fibers adhere to machine parts and cause uncontrolled fiber flow and breakage. Such fiber behavior can make processing exceedingly difficult and increase the probability of lapping and tangling. (It might be remarked that these situations provided the early makers of man-made fibers with some valuable insights as to what they had to do in the way of fiber finish to make their fibers capable of being worked.) The presence of a sufficiency of proper oil can reduce the need for a high atmospheric humidity. Humidity also reduces the tendency towards electrification of fibers. For a woolen system with only small amounts of oil, the rh has to be at least 65% and a humidifying system becomes a necessity in many climates. Air at 65% rh and 70°F (≈ 21°C) holds about 5.2 grains/cu ft (≈ 12 g/m3). The quantity of water that has to be sprayed into the atmosphere can be quite large and the cost significant. Table 8.1 gives some typical rh values at various processing stages in the worsted system. It will be noted that the rh values needed for carding and spinning in the worsted system are considerably lower than the 65% just quoted. Wool fibers in the worsted system are oiled. When different sorts of fiber are to be blended, it is desirable that each sort of fiber be lubricated separately. Many man-made fibers already have fiber finish applied by

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Handbook of yarn production Table 8.1

Typical % rh in processing of wool

Mixing and blending Carding Combing Drawing (French) Spinning

% Min

% Max

65 60 65 65 50

70 70 75 75 55

the fiber maker, whereas wool is scoured before mechanical processing. Most manmade fibers process reasonably well at 55% rh and thus the further importance of oiling the wool can be seen. Use of appropriate moisture contents and oiling help to keep the wool fibers pliable and resistant to rubbing. It has been explained that the lubricant has to perform some important functions but there are other requirements. The oil must be capable of being easily removed after it has fulfilled its purpose; it should not degrade, stain, or otherwise mark the fiber; it should not damage the machine in any way. It should be compatible with the finish on any man-made fiber used with it; both should be chemically stable under a variety of conditions during prolonged storage; and it should not produce any fire or health hazards.

8.2.5 Disposal of wastes from scouring Disposal of the wastes from scouring is still a problem and traditional methods are no longer acceptable. Waste water from scouring is a particularly difficult effluent to deal with because it contains both organic and inorganic components. Inert sediments and vegetable matter produce little problem in this respect, but the wool grease and soluble organic salts do have highly significant effects. The polluting effect of such aqueous wastes is indicated by its biochemical oxygen demand (BOD) to achieve the decomposition of organic waste by aerobic bacterial action. The rate of discharge of these waste materials can thus be measured in pounds of BOD per day (or by the equivalent of that generated by a number of inhabitants of a community). The BOD rate depends upon the waste contents of the fibers, the level of production, and the efficiency of the waste treatment systems used. Several methods of treating wastes exist. Biological methods are low in cost and are capable of dealing with both soluble and insoluble impurities. However, they are sensitive to variations in ambient temperature, they can be adversely affected by poisoning, and they produce large volumes of sludge. Gravity settling of the sludge in lagoons is used to produce the biological action. If the sludge is spread on the land as a way of disposal, a large acreage is required. Flocculation of the liquors has not proved to be satisfactory on the small scale that most mills require. Evaporation of the water from the liquor followed by condensation to recover the water is another concept that has been tested and used at various places because it has the attraction of a reduction in water usage and a high yield of valuable wool grease. The cost of water is a severe burden in some areas of the world. An acid-cracking method is possible in which the suint liquors are evaporated and the residues of salt are calcined to produce saleable potash. However, the evaporation process can be expensive.


8.3

213

Worsted systems

8.3.1 Long-staple processing within the worsted family of systems Within the worsted family of processing systems, there are many variations. These systems range from the traditional processes used for making high quality yarns from wool, to short processes that use man-made fibers solely and require no carding or combing. These systems use some of the same equipment at certain stages of their manufacture and it is convenient to use the adjective ‘long-staple’ when the process stage is useful over a wider range than might be inferred by the adjective ‘worsted’.

8.3.2 Worsted system carding Worsted yarns are made from long, lustrous varieties of wool and they are usually combed to improve the luster, smoothness, and strength of the yarn. Wool fleeces are sorted into pieces of reasonably uniform quality; these sorts of greasy wool are blended and, if necessary, deburred before being scoured. The scoured, dried wool is conveyed by an airstream (or by a lattice conveyor) to a temporary storage bin, or area, from which a lattice card feed draws fiber at a controlled rate. An automatic weighing system is usually used to control the thickness of the fiber fleece fed to a roller-top card (Fig. 8.4). Breast works, diagram (a), are used to feed fibers to the first main cylinder of a card of the sort shown to small scale in diagram (c). It was once thought that at least five licker-ins were needed to card wool, but the introduction of ‘metallic’ wire has reduced the necessity for so many. The so-called metallic wire refers to saw-toothed wire of the type illustrated in Fig. 5.12. Special metallic wire (Fig. 8.4(b)) is used to keep burrs and other extraneous matter on the surface of the cylinders and yet allow the fibers to be pressed between the rows of teeth. This arrangement permits the removal of the unwanted vegetable material by bladed burr beaters. Special wire-covered Morel cylinders work with burr beaters and are often substituted for the redundant licker-ins, to provide extra deburring stages. Morel beaters are equipped with special flat-topped wire to keep the fibers on the surface and so enhance the cleaning function. The main sections of these cards give little other opportunity for cleaning. Burr beaters are in the range of 5 to 6 inches (say, 130–150 mm) diameter and run up to 2000 r/min. Atkinson and Saunders [3] showed that eight-bladed burr beaters, run at about 1000 r/min, were nearly as effective at removing seed/shrive and vegetable matter as similar beaters running at 2000 r/min. Eight-bladed beaters were preferred to two-bladed ones. Various blade profiles were used and high relief angles were used. Burrs were removed at up to 80% efficiency and seed/shrive at up to 60%. Morel clearance and surface fiber density are of importance. The wool in the waste varies between 10% and 20%, the bulk being mostly vegetable matter. Efficiency of removal of unwanted matter depends largely on the speed of the beater, up to a limiting value of about 1800 r/min. Clearance between the burr beater and the Morel has an effect on efficiency; opening up the clearance by 2.5 mm drops the efficiency by about 15% [4]. On single swift cards, two Morel/burr beater pairs are placed in tandem between the forepart, or breast works, and the swift [5]. Deburred and roughly opened fiber is transferred from the last licker-in or Morel beater to the first main cylinder (or ‘swift’). The main cylinder carries the fiber to the first worker/stripper fiber combination (Figures 8.4(b) and 8.5(c)). The surface speed of the worker (which is the larger of

214

Handbook of yarn production Scoured fiber input

Output fiber web on first main cylinder Divider

Divider Stripper

Brush

Enlargement

Burr beater with saw-tooth wire

Takers-in

Divider Tightener

Worker/stripper sets (b)

Divider Tightener

Burr beaters shown with black fill (a)

Swift Doffer Breast cylinder (c)

Fig. 8.4

Roller-top card

the two rollers) is less than that of the cylinder, and the teeth are angled to pick up fibers rather like a doffer. As the fibers picked up by the worker move away from the cylinder, the rapidly moving cylinder teeth comb out the trailing ends of the retreating fibers. The stripper runs at a higher surface velocity than the worker does, and this causes fibers to be stripped from the worker and to be drawn and combed again before being returned to the cylinder. Fiber can take various paths as indicated in Fig. 8.5(b). The surface speed of the stripper is less than that of the swift, so it acts rather like a licker-in, and the teeth are angled to facilitate the fiber transfer. Typical relative velocities of the various components, taken in the order that most fibers meet them, are shown in Fig. 8.5(c). Within the normal range of settings between the wire surfaces, there is surprisingly little effect on the production of noil obtained in later combing [5]. However, the speed of the set is important in determining the noil percentage which implies that it has an effect on fiber breakage. Robinson [6] suggests that fine worsted-style wools can be carded at higher rates than normal, provided an antistatic lubricant is used. It will be noted that worker/stripper sets replace the flats used in short-staple processing. Worker/strippers not only fulfill a similar function of dividing tufts, but also give greater longitudinal blending than can be achieved with flats.


215

6 7 Stripper

Fancy

Worker

5

2 3

To doffer

4

Cylinder

1 Cylinder

Wire size exaggerated for clarity

Alternative fiber paths: 1, 4, 8 1, 2, 3, 4, 8 1, 4, 5, 6, 7, 3, 1, 4, 8 etc. (b)

The fancy roll raises fiber to the surface of the wire to facilitate doffing (a)

Velocity, log scale (ft/min)

104 Cylinder

8

Fancy

103 Strippers 102

Licker-in Doffer

10 1

Workers Feed Stage

(c)

Fig. 8.5

Carding elements

The layers of fibers on the swift may be considered to consist of a component from the feed and a component of recycled fibers, due to the inefficiency of the doffers. The swift-to-doffer setting, and the speed ratio between them, affects the efficiency of fiber transfer to the doffer. The fact that there is a choice of fiber path at each worker/stripper means that there are longitudinal displacements of successive proportions of fiber and this gives a useful blending action. Errors tend to be smoothed out by these random lengthwise relocations of the fiber population. There are several stages of working and stripping on each main cylinder. Normally, several main cylinders are used in series in a normal carding set; consequently, there can be up to about a dozen worker/stripper stages in the total process. Incomplete fiber separation and entanglement of fibers in the material delivered by the card affects fiber breakage in later processes. Web is transferred from one main cylinder to the next by means of an intermediate doffer cylinder. Often crush rolls are used at this point, to reduce the size of any small burrs left in the stock after the deburring phase of the process. Many of the crushed burrs fall out from the fiber because the crushing tends to break up the spikes and sharp edges of the burrs. Also, a Morel burr cylinder is often fitted at the transfer point. Fibers leaving the last worker/stripper tend to be deeply embedded in the cylinder wire, and it is necessary to raise the fiber to the surface to facilitate doffing. A ‘fancy roll’ (Fig. 8.5(a)) carries out this function and the member has long, flexible wire teeth. The surface speed of the fancy is higher than that of the cylinder, with the result that the fibers are raised and brushed forward to be better caught by the doffer. The doffer is of conventional design and operates much as was described earlier, except for the condensing stage.

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8.3.3 Man-made fibers in worsted spinning The production of man-made staple fibers has been addressed in Chapter 2 but some mention should be made of their use in worsted spinning. If the fiber is to be carded, the man-made fiber is supplied in bales. The unopened bales of fiber should be conditioned for two or three days before cutting the bands and removing the bale wrappings. An rh of between 55% and 70% is usually needed for this purpose. Since the fibers are clean, it is not necessary to risk nep creation by over-carding and ICI recommended low carding rates [7]. The sliver should not be ‘backwashed’, otherwise the risk of nep production in later processes is increased. Backwashing is a washing procedure which will remove fiber finish from the man-made fibers. Changes in frictional characteristics as compared to wool might require alterations in the back draft in combing. It is considered inadvisable to add oil or other lubricant to the sliver after combing. However, if the sliver is dyed or printed to produce special effects, the fiber finish is removed and a suitable dressing becomes needed to control static generation in subsequent processes. If the sliver becomes matted or compacted, a preliminary opening in a gill box is suggested. Where the man-made fiber is to be blended with wool, it is advisable to do so in the preparatory gilling stages (Section 8.3.4) before re-combing. This implies that the slivers of wool and man-made fiber have been produced separately. When intimate mixtures of man-made fibers and wool are combed, mixed noil is produced, of variable fiber proportions, and this is of lower value than unmixed noil. Color matching might become a problem, but comb mixing is recommended for the production of high quality tops (i.e. sliver). Differences in coefficient of fiber friction also affect the slubbing (or roving) twist needed; lower values than those normally used for wool might be employed. This has economic advantages. In spinning, different ratch settings might be required from those used with wool. The spinning and folding twist levels required for a given end use are affected by the fibers used. Differing levels are necessary for the various man-made fibers, wool, and blends. Strength is not always the main concern; hand and appearance are often more important.

8.3.4 Long-staple drawing A traditional drawing operation (‘gilling’) is carried out on pin drafters. Faller bars with pins in a comb-like configuration move similarly to the simplified fashion shown in Fig. 8.6(a). Two sets of intersecting faller bars are usual. One set enters from the top of the fiber stream, and the other enters from the bottom. There up to 100 such faller bars in a machine and about 30 of them engage the sliver at one time. Studies of pinning density and fiber loading have shown that increased pinning densities can lead to irregularity, probably because of the increased drafting forces produced. However, if the pin density is not increased to extreme levels, the pins act to control unstable fiber movement in the drafting zones. At each drawing stage, a number of slivers are creeled to form the input and this produces a useful doubling that improves long-term regularity and blend. There are usually four stages of drawing before spinning [4]: three stages of intersecting gills and a final stage on a rubbing finisher or speed frame (i.e. roving frame). The gill frames have a restricted productivity of about 2000 faller drops/min and there is an incentive to replace them with rotary gills, or caterpillar drafting or the like. These newer systems obviate the noisy, rectilinear motion of the faller bars. Also, caterpillar and chain gills are capable of a greater length of contact in the draft zones as shown in Fig. 8.6(b). They operate at up to four


217

Path of faller bars Caterpillar

Sliver

Brush

Faller bars Delivery rolls

Feed rolls

Back rolls

Brush

Front rolls

Caterpillar (b)

(a)

Fig. 8.6

Gilling

times the productivity rates of the traditional gilling frames. Automatic doffing (autodoffing) of the sliver cans is becoming well established with can sizes up to 60 inches (≈ 1.5 m) diameter. The front roller system in a classical machine is a three-roll set. The drafting, feed, and sensor rolls are often pneumatically weighted and sometimes pneumatic sliver transport systems are used to carry the sliver to the front roll assembly. Sliver mass sensor systems are fitted to new machines and the electrical signals are used as computer inputs; the algorithms in the computer control the short- and long-term linear density variations. Stepless drive motors allow tension controls, as well as the differential speeds necessary to control drafts. Autoleveling systems are frequently used, especially on the first stage of drawing.

8.3.5 Top making It is more common to make long-staple sliver ready for spinning or drawing elsewhere than it is in short-staple spinning. The combed wool sliver is a high value product and it is worth the extra cost of transport if the material produced by a specialist is of superior quality. Another reason is that the long-staple sliver is more durable because of the use of long fibers. It is common to add as much value as possible before shipping and the material shipped is known as a ‘top’. These tops are frequently stored as a ball, in which the sliver is cross-wound onto an external package as shown in Fig. 8.7. This is made possible by the high fiber cohesion in these long-staple slivers. 8.3.6 Long-staple combing Long-staple slivers are often combed to improve fiber orientation and the appearance of the final product. Blends of natural and man-made fibers may be made by combining ends of sliver in the drawing and combing processes. Two main combing systems exist but the major one uses the Heilman or French comb, which has a rectilinear, intermittent action somewhat analogous to the cotton comb. The machines are larger than their short-staple counterparts and operate at up to 240 nips/min. Alternatively, tops may be made from tow by stretch-breaking as was described in Chapter 2. At the machine level, new arrangements of drive cams

218


Fig. 8.7

Sliver ball

combined with the use of lightweight materials have permitted increases in speed without substantially increasing the noise level or adversely affecting the performance of the machines. In modern machines, draw-off aprons control the emerging web, draw-off cylinders are equipped with lap detectors, combs are fitted with cleaners, and computers are used to control automation.

8.3.7 Long-staple roving and spinning Roving and yarn may be made on equipment that is fundamentally similar to that described for making short-staple yarns. Draft ratios are higher as is the fiber cohesion. Fiber oiling has to be considered. As mentioned earlier, the increased fiber length in worsted systems means that the size of the rolls and the distance between them are correspondingly larger. Yarn counts are heavier and the yarn packages have to be correspondingly larger to prevent undue loss in production due to doffing. This means that larger flyer and ring sizes and lower speeds are used than in short-staple spinning. In drafting, variable fiber length is often quoted as the most important factor in producing irregularity; however, increasing fiber crimp has also been shown to have a detrimental effect on the ends-down rate in spinning. There is an inference that high crimp causes irregularities. High drafting forces lead to the need for higher roll nip pressures in the drafting system, otherwise slippage between fiber and roll leads to irregularity. High pressures between rolls and soft surfaces in a drafting system tend to damage wool. Apart from the dangers of fiber lapping, it is desirable to use as large a diameter drafting roll as possible because small rolls produce high pressures. The diameter is limited, of course, by the roll setting and that, in turn, is related to the fiber length. Historically, various twisting systems were used that included ring, mule, and cap spinning; but the use of the latter two has declined and we need only concentrate on the ring frame. It is interesting to note that better results are obtained with increasing mean fiber length, but that variability in length seems to have little effect. Fiber diameter and percentage of short fibers correlate with the frequency of thick spots in the yarn [8]. 8.3.8 Worsted spinning In principle, long-staple ring spinning is similar to that described for short staple. Consequently, the descriptions will not be repeated. Yarn tension in spinning is a


219

function of the yarn count, spindle speed, ring size, and some other factors. With the heavier counts in long-staple spinning, it is desirable to increase the ring size with the heavier yarns, not only to reduce doffing costs but also to reduce the incidence of knots or splices. To maintain an acceptable spinning tension, it is necessary to reduce the spindle speed and/or reduce the balloon size. It is here that collapsed balloon spinning finds its place (see Appendix 9). Differences in count and running conditions lead to means of controlling fiber flow that differ from those already described. Because of the wide range of products, there is a wider band of technology in longstaple spinning than in short staple. As examples: rings are sometimes lubricated; traveler designs are more varied; and travelers can be made of polymeric, composite, and steel materials. The large packages typical of worsted spinning require that the bobbin length be large. This implies a significant variation in spinning tension during spinning unless the bobbin is moved up and down during the wind, to keep the balloon length about the same. Some machines do this and some oscillate both the bobbin and ring rail for this purpose. The mechanical complication of this is repaid by a reduction of the variation in tension, which, in turn, leads to a more consistent product and a reduction in end-breakages. Automatic winding has become established as has the use of electronic clearing, but the joining of end-breaks and cuts during winding is rather more difficult than with short-staple yarns. For medium length staple, splicing is feasible, as explained in Chapter 9. Splicing can give an almost faultless join but knotting is still often used for the heavier yarns. Many worsted yarns are plied and several forms of twisting are used. Traditionally, worsted warp yarns have had to be plied to withstand the rigors of weaving. Plied and quasi-plied yarns are made by ring uptwisting, two-for-one twisting, Sirospun processes (see Chapter 10), novelty twisting systems, and others; the products include not only simple plied but also certain fancy yarns. These fancy yarns have spiral, loop, nub, bouclé, ratine, flake, chenille, or other special effects, but they are too various and complicated to be explained here. In such a wide range of yarns, the requirements for fault clearing in winding vary enormously. It is obvious that the market expectations of a particular yarn must determine the levels at which faults should be removed, and any system must permit variation according to need.

8.3.9 Semi-worsted and related systems There are two main divisions in long-staple processing of natural fibers and these are known as the worsted and woolen systems, both of which are based on systems originally designed for wool. As new man-made fibers have been developed, the tendency has been to blend them with wool, or to displace wool altogether (wool has become a relatively expensive fiber). The wholly man-made, long-staple yarns have become popular for carpets. Attempts have been made to shorten process lines to gain some economic advantage. This philosophy has been applied to the worsted system, and although the solutions are somewhat diverse, it is possible to categorize them under the heading of ‘semiworsted systems’. The most popular use is in carpet yarn manufacture and the count range is between 3s and 12s worsted. A typical system uses a one- or two-cylinder card set with appropriate deburring stages, and there may be six or seven worker/ stripper combinations per card. Sometimes two doffers are arranged to give a split web and the left and right portions are converted to sliver. It is possible to use multiple doffers, each producing split webs. Care has to be taken when producing

220


multiple slivers to balance the outputs to keep uniform sliver weights. Such multiple doffer split web cards are made at up to 100 inch width with productivities of up to 450 lb/hr. Often, two or three passages of drawing are used, according to the yarn count (above 10s worsted, say, three passages would be used). In the post-carding processes, the first drawframe normally has an autoleveling attachment to even out the errors from carding. The first frame can have up to 12 slivers per head in the creel, so there is a considerable amount of doubling to improve the long-term regularity. The intermediate and finisher drawframes might take three slivers per head with a draft of about 8. Therefore, there is a progressive reduction in sliver weight from about 300 grains/yd at the card to about 50 grains/yd at the finisher drawframe delivery. The productivity of the first two frames is about 200 lb/hr but the finisher frame produces only about 60% of this because of the reduction in sliver weight. Tow-to-top sliver making systems have replaced the preparation up to and including the card in some cases but these modified systems are capable of dealing solely with man-made fibers. The yarn is usually spun from sliver. (Note: 100 in ≈ 2.5 m, 450 lb/hr ≈ 200 kg/h, 200 lb/hr ≈ 91 kg/h, 10s worsted ≈ 90 tex, 300 grain/yd ≈ 21 ktex, 50 grain/yd ≈ 3.6 ktex.) 8.3.10 Twisting and doubling Yarn ‘folding’ is almost universal for worsted and woolen yarns or blends with wool [9]. Folding is otherwise known as twisting or doubling. (See also Chapter 3.) Longstaple fibers, when well drafted, display low fiber migration, and the peripheral fibers can easily be peeled off; folding stabilizes these fibers and improves the yarn characteristics. Weaving folded yarns is accomplished with fewer end-breaks and the fabrics contain reduced numbers of faults. Two-for-one twisting enables much longer lengths of twisted yarn to be produced without a knot or joint. This, too, improves weaving performance and reduces twisting costs; there is now even less burling and mending of the fabrics and this has helped to assure the penetration of two-for-one twisting in the industry. (Burling is the removal of yarn faults from the fabric.)

8.4

The woolen system

8.4.1 General comment The woolen system often uses blends of fibers (sometimes with waste fibers) to make a relatively inexpensive, soft, hairy, and full yarn. Fibers used are usually short by wool standards and so are likely to felt. Fabrics made from woolen yarns are often deliberately milled or felted to make them dense with a napped appearance on the surface. Frequently, the yarn strength is low. Layout of the processes is quite variable in this industry because of the diversity of raw materials. Even if all the components are wool, the fiber properties might vary widely and so might their state of cleanliness. To achieve consistency, it is necessary to blend thoroughly before carding and this might involve several stages. For example, a diversity of fibers might require that some fibers should be cleaned differently from others and each lot might be blended within itself. Even after scouring, color differences might be recognized that require further blending to even out the variations. The intermediate product resulting from the preparatory and blending process is known as a ‘willeyed blend’ and this product is the feed for the carding process. The card


221

produces a condensed slubbing that goes direct to spinning. Woolen spinning has a short and efficient process line that is less complex mechanically than some of the other systems described. Since the process after the card feed is such a short one and the feed materials are so diverse, there is little further chance of rectifying unevenness (unless the material is reworked). Every opportunity is taken to blend the components because any reworking degrades the material and adds unnecessary expense.

8.4.2 Woolen opening According to Richards and Sykes [10], wool can vary greatly in its state. The conditions range from clean to greasy and different amounts of dirt are sometimes embedded in the grease. The dirt may be only loosely associated with the fibers. The size of fiber clump can vary from locks to relatively large pieces. Thus, the amount of cleaning and/or opening needed for the different categories varies widely. Excess opening can damage or entangle the fibers and therefore it is sometimes necessary to process different lots of fiber by different equipment running under different conditions before any final blending. The process of opening uses equipment not dissimilar to the types described for ginning and opening in short-staple spinning or in worsted spinning. Compared to the short-staple devices, the teeth or spikes used are larger but that would be expected in view of the increase in fiber length. Suffice it to discuss only the differences from types already described. The teazer (otherwise known as a wool willow, wool opener, or devil) has many similarities to the opening machines already described. A significant difference is that material is fed to the machine and is left for a predetermined time (the ‘draw’) to undergo opening within the machine before being removed. This is in contrast to the continuous flow systems described earlier. Another feature, not met with elsewhere, is the possibility of grease build-ups in the equipment. The picker opener is similar to machines already described and will not be further discussed. The Fearnought opener is akin to a coarse-toothed card because it has a cylinder, workers, and strippers. The teeth or pins are much larger than those found on cards. It is a thorough opening device and promotes blending but it is never used to disintegrate the wool pieces to single fibers or even to small tufts because this would interfere with carding. If recycled material is used, yarn or fabric has to be decomposed into fibers. Briefly, the rags are shredded in a rag picker and then reduced further in a garnet to separate the fibers sufficiently for them to be carded. Naturally this process is rather severe and fiber breakage causes a considerable loss in fiber strength and may cause damage that will show up later. Thus, although it is acceptable for the class of product involved, there are hazards that are not usually found elsewhere. Yarns that have not been completely decomposed into fibers cause trouble in carding and subsequent processes. The so-called threads degrade the product. Some idea of the damage that can be done to recycled fibers is illustrated by Fig. 8.8 where it will be seen that close settings can severely shorten the threads, and by deduction, also the fibers. Recycled material is no longer widely used because of governmental regulations regarding the labeling of reused fiber: the introduction of many different blends and types of fiber has made the identification of the fibers in the rags difficult and this makes compliance with regulations difficult. Furthermore a great deal of the economic motivation has disappeared, and therefore the traditional woolen system is becoming rare. In modern times, large volumes of synthetic fibers are used in the blends. The opening process

222

Handbook of yarn production 300 in/min

Thread length (inches)

3 64s wool

150 in/min 2 300 in/min 1

50s wool

150 in/min

0 0.015 0.020 0.025 Breaker setting (inches)

Fig. 8.8 Thread length

has been replaced by more conventional fiber opening and cleaning, using virgin or man-made fiber. Where virgin wools are used, layers of fiber are oiled to plasticize and lubricate them. The man-made fibers need little opening and cleaning; consequently they are introduced later along the process line.

8.4.3 Woolen blending Hopper feeders are still used to blend fibers. The opportunity is often taken to stockdye fibers. In such a case, the material then has to pass through squeeze rolls, a dryer, and an oiling section before further processing. After opening and cleaning the fibers to bring them to a compatible state, the main process of blending occurs. Sometimes this is manual but, increasingly, mechanical systems are being used. In blending, good stock records and good housekeeping are prerequisites to satisfactorily uniform blends. Mistakes at this point usually involve reworking the material, which (a) is an unnecessary expense, (b) increases fiber damage, and (c) adversely affects carding. When reworking becomes unavoidable, it is preferable to take the material from the line before it reaches the Fearnought process, to avoid as much fiber damage as possible. With manual blending, human variability gives rise to additional error possibilities. A reasoned procedure, which takes into account the varying properties of the feed material and the needs of the product to be made, assumes an even greater importance. In manual blending, the components are spread as layers, in an order determined by the specified plan, to take into account mass and composition of the various feed lots. The thickness of each layer has to be as small as is consistent with an even coverage of the area of the laydown. Sometimes the feed lots are pre-blended by various means. Obviously this is a labor intensive and skilled operation. In a climate of high wages, mechanical devices will eventually replace the traditional methods. Modern methods favor pneumatic handling of the fibers and mechanical means of layering or mixing. One form of mechanical blending is the rotary spreader, which deposits a circular layer of fiber in a suitable bin or receptacle. Several feeds can supply the spreader at the same time. An alternative is to pre-blend intermediate lots into the final blend. If Z sections of intermediate lots are each built from the same feed lots and in the same proportions, then a blend with good longterm evenness will result. No matter what the magnitude of Z is, long-term differences are thus avoided. Various opening type machines can be used to improve the local blend, providing they do not break down the tufts too much or damage the fibers. It


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is possible to supply the willeyed blend directly to a Fearnought or other blending machine if the production rates are properly synchronized. Such a line can be integrated with an oiling system. A site to add the processing agent to the wool is needed. Scouring removes the natural lubricant.

8.4.4 Willeyed blends A customary final blending is to pass the material through a Fearnought willey that blends what should be small wool pieces or tufts. Despite the care in preparation, willeyed blends still contain differences. Cleanliness can vary, especially if scoured wool is bought and added to the blends. The wool can contain varying amounts of coloration, processing lubricant, burrs, and other vegetable matter. Where man-made fibers are to be blended, no cleaning is needed and the materials are blended at the latest possible stage. Such blending is almost an irreversible process where separation of the fibers becomes quite impractical and the possibilities of reworking become correspondingly smaller. The question of what lubrication is needed for such blends is heavily dependent on the fiber finish applied by the man-made fiber maker. Inhomogeneity arises if the wool pieces are large or the building of lots has been irregular. Sampling and testing inhomogeneous feed lots and the resultant blends are important parts of the technology, the variability; and characters of the feed lots determine sampling rates. The material is often baled and stored until required for use in further processing. Alternatively, it is stored in large bins, in which case pneumatic fiber transfer systems can be used. The material within each of the bales or storage bins should be as nearly homogeneous as possible in tuft size, fiber composition, and cleanliness. Of course, if stock-dyeing has been used, the variety of dye shades increases the difficulty of stock and quality control.

8.4.5 Initial woolen carding Conventional woolen cards are fed by card hoppers that usually have a weighpan device to control the mass flow of fibers. Fiber is carried from the main storage bins by lattice feeds to the hopper. The action of the hoppers removes much of the variability that comes from manual feeding. A hopper feeder has two or more bins as part of its structure, together with the necessary moving lattices. Fiber from the large bin is carried on an inclined lattice, which supplies fiber to the smaller weigh-bin. Rakes are used to roughly level the fiber sheet on this lattice and the fibers, which are removed tumble back into the bin. The tumbling action gives a degree of blending and the action of the pins opens the fiber masses somewhat. However, the tumbling can cause aggregation of fiber clumps to unacceptably large new ones. In such cases, the danger of choking becomes greater. The solution is to use appropriate lattice speeds so that the excess removed is not too large. Controllers are used for the lattice drivers and this improves the accuracy of control, especially at high production rates. The contents of the weigh-bins are dumped periodically (several dumps/minute) on another moving lattice and the flow control is based on the mass of fiber in each weigh-bin. Often the dumping rate is partly determined by the lattice speed. There can be several hoppers, all of which dump their discharges on a common lattice feed, which goes to the card to give another stage of blending: in this case, the dumping times of the various hoppers are co-ordinated. Devices using strain gages and/or

224


other sensors are displacing mechanical weighing systems, and the electrical signal outputs facilitate the use of computer control. Such computer control is capable of leveling several consecutive weighings to deliver a moving average of the input fiber mass and this reduces long-term irregularity in the slubbings. There is a trend to the use of chute feed systems, often in co-operation with a type of hopper feed; this reduces the need for labor and might provide a site for control. Since the card feed is one of the few practical points for mass flow control, further measurements of the feed sheet thickness are made and there are a number of methods available to do this. The use of a dancing roller, with sensors to detect changes in height, is one way that gives an average across the width of the sheet. Measurement of the penetration by ultrasonic or gamma beams through the sheet provides an alternative source of control signal. In these cases, it is necessary to either (a) choose measurement sites that sample the sheet accurately across the width, or (b) use a scanning device which moves to and fro across the width of the sheet. One reason for the extra control stage is that changes within the hopper can produce errors, especially if there is a tendency for the feed system to choke. Evenness also is needed in fiber proportions and state. The action of pinned rolls can not only fractionate the material delivered into different fiber types and clump sizes, but also cause agglomeration of smaller clumps into larger ones [10]. Varying quantities of dirt, oils, residual grease, and other materials can influence performance and it is quite likely that the measurements used to control the fiber flow will not properly reflect these causes of error. Moisture and some processing agents evaporate over time and this is another cause of variation, especially if water sprays are used in blending. Careful human monitoring is needed to ensure that the blend is within the limits of uniformity in all respects.

8.4.6 First stage in woolen carding Woolen cards (Fig. 8.9), like worsted ones, nearly always have roller tops because they are less likely to become clogged than non-rotating elements. Locks of fiber can be handled with relative ease by the worker/stripper in combination with a swift (i.e. cylinder). It may be recalled that the workers and strippers perform a blending function because fiber is not immediately removed from the worker/stripper sub-system. The lag in fiber transfer acts to mix early arriving fibers with later ones, as well as to perform the fiber opening and orientation actions of the sub-system. To give some idea of the dimensions of these cards, a typical swift is about 5 ft (≈ 1.5 m) diameter, the workers and strippers about 8 inches (≈ 0.2 m) and 4 inches (≈ 0.1 m), respectively, and the whole card set might be 20 yd (≈ 18 m) long. The width can be several yards (meters) and the swifts might rotate at up to 200 r/min. These card sets are enormous pieces of machinery and they can process material at speeds up to 500 lb/hr (≈ 227 kg/h), although the range of speeds and sizes varies considerably. Carding forms a large part of the processing set-up in woolen spinning. This means that if a card set becomes non-operational, a large fraction of the production is lost. Woolen carding is noted for the wide range of equipment groupings and the best that can be done in limited space is to describe a typical set-up. Such a set-up might include a feed system, a breast with perhaps three workers and strippers, a scribbler with perhaps four sets of workers and strippers, a cross lapper, a two-swift carder (i.e. two-cylinder card) with a total of about eight workers and strippers, crush rolls, and a condenser that produces the slubbings. Let us start the description at the feed end. A silhouette of a typical first unit of a set is given in Fig. 8.9.

Long-staple spinning Worker

225

Stripper

Fancy

Swift Breast

Delivery

Feed Transfer roll

Fig. 8.9 Typical first carding unit of a woolen system

Weighpan hoppers (not shown) drop fiber onto a moving lattice that forms the feed to the card set. The feed rolls are often pinned to grip the fibers without undue compression. The items shown in dark gray may have fairly coarse garnet clothing, whereas later sections may have flexible wire. Garnet clothing is a metallic wire similar to that used in short-staple processing, but with larger teeth. The delivery from this unit may pass to a scribbler (breaker card), which has a similar swift/ worker/stripper arrangement, although the swifts are usually larger. The delivery from the last swift of the first carding unit is doffed and passes through a crush roll set. At this point, embrittled vegetable matter is reduced to powder and drops out of the web as it passes through to the carder by way of a cross-lapper. 8.4.7 Cross-lapping The woolen system differs from the worsted in several different ways. One of the most important differences is that the output often consists of a number of slubbings, each drawn from a narrow band of fiber from the card web. A slubbing might be taken from a region ranging from one selvage to the other. It is highly undesirable for the material to vary over that range otherwise the slubbings would be dissimilar. It is therefore essential that the web be even across its width. One way of improving the across-width evenness is to cross-lap, using a Scotch feed or similar device, somewhere between two of the main cylinders (usually between the scribbler and the carder). The principle is illustrated in Fig. 8.10. Web enters at W on a feed lattice oriented perpendicular to the paper and oscillating in the direction X. The web is laid zigzag fashion on another moving lattice, which moves in direction Z. The direction of oscillation X is usually roughly perpendicular to the direction Z. In many practical set-ups there are two such cross-lapping sections. Web-feed lattices are arranged to complete the changes in orientation of the body of the sheet, so that the feed to the next machine is again in line with the original flow. 8.4.8 The carder The cross-lapped web then passes to the carder (finisher card), a diagrammatic arrangement being shown in Fig. 8.11. As before, each cylinder has its quota of workers and strippers and the theme of blending continues. It has to be mentioned that many stages of division of fiber tufts are required. Interactions between the swifts, strippers, and workers throughout the system cause tufts to be broken down into smaller ones and thence (mostly) to single fibers. Fortunately, there is incomplete

226

Handbook of yarn production W

X

S

Y

Z

Fig. 8.10

Cross-lapping

Workers and strippers Fancy

Fancy

Swift

Swift

Doffer

To tape condenser

Transfer rolls From X lapper

Fig. 8.11

Carder

doffing at these various interaction points and a typical fiber lock recirculates many times; the fiber receives several hundred stages of opening and blending before it leaves the system. The carder delivers web to the tape condenser and all these major units are set in line to make a continuous system. Dirt gathered under the card has to be removed and safety regulations require the machine to be enclosed. Manual cleaning requires a shutdown if a risky cleaning operation is to be avoided; consequently pneumatic systems are likely to become more prominent. Management of the carding system takes skill and experience. Flexible wire has not been discussed elsewhere because it is obsolete in shortstaple spinning, but it still serves an important function in woolen spinning. In working wool, the fine wires are effective in teasing fibers apart without undue damage. The


227

wire population density varies according to use but it ranges from 100 to 800 points/ inch2 (≈ 0.15 to 1.24 points/mm2). Wire, usually of round cross-section, is embedded in a base material that is mounted on the surface of the element concerned (e.g. swift, worker, doffer). The cross-sectional area of the wire is small compared to its length and this is what gives it its flexibility. The shape is cranked as shown in Fig. 8.12(a) because it is necessary to have an adequate angle of attack, α, and the angle, β, is chosen so that the height does not change under load. The clearance between cooperating wire surfaces might be as little as 0.008 inches (≈ 0.2 mm) and there is little tolerance for reduction. Changes in the height of the wire alter the setting, and if the setting is reduced, very expensive damage can occur. Properly selected and maintained wire is important. Another problem associated with flexible clothing is its tendency to load. Fiber becomes embedded deep into the wire and, after a time, this can impair the efficacy of the card. For this reason it is necessary to use a ‘fancy’ to bring fibers to the surface. The fancy is made of long, flexible wire, and the wires penetrate those of the swift. Despite the fancy, the wire will still load in time. Periodic stripping or fettling of the wire is necessary, which means that fiber trapped by the card wire is removed. Depending on the fiber being processed, it is possible to card between 1000 and 8000 pounds (≈ 450 to 3600 kg) of fiber between fettlings. A card behaves abnormally after fettling until a sufficiency of fiber has been deposited in the wire to give equilibrium conditions. Thus, maintenance involves not only the grinding of the wire, but also the cleaning of the wire interstices (i.e. fettling) and re-establishment of equilibrium conditions. An overloaded surface will not collect fibers and the process begins to break down. As the loading increases beyond a certain limit, the weight of the web diminishes and so does the eventual yarn count. Loading of the cylinders also affects the opening power of the card and it has to be controlled to produce a good yarn without slubs or defects. The cylinder, worker, and stripper speeds have to be adjusted to suit the fibers being processed. Excessive worker and stripper speeds damage the fiber because of the increased rates at which the fiber tufts are drafted. Workers on the finisher swift (cylinder) should be limited in speed to prevent irregularities, but those on earlier

α

103

(i)

64s wool

(ii)

Nep/g

102 (i) 300 in/min (ii) 150 in/min 10

50s wool

(i) (ii)

β 1 0.015

0.020 Setting (inches) (b)

(a)

Fig. 8.12

Flexible wire and nep

0.025

228


cylinders might be speeded up to increase the opening effect. Doffer speed and feed rates affect the loading, as do the settings, and these affect performance. Production rates depend on the speed of the swift. Settings and speeds become critical. The mechanism of transferring fibers calls for comment. The surface speed of the swift is much faster than that of the worker or doffer, and the fibers on the web surfaces are layered like overlapping tiles, with straightened ends projecting forward. Portions of the fibers on the doffing surfaces are brushed by the others on the swift to produce fiber hooks, which affect subsequent processing. The size of the doffer has little effect on the fiber transfer rate, providing the surface speed is kept constant. Moreover, the direction of doffer rotation does not affect it either [6]. As with short-staple spinning, the condition of the wire is important, as are the settings. Worn wire leads to high nep production, which has just as deleterious an effect on quality as in the other systems. The settings are somewhat more difficult to maintain because the wire is not always as rigid as with the systems previously discussed. A definite trend for increased nep was shown as the settings were increased in the experiment portrayed in Fig. 8.12(b). The designation of the wool as an Xs means that the wool is suitable for spinning to an X count of yarn. Thus a 64s wool should be capable of being spun to a 64s yarn. It is a designation of the expected spin limit. The web leaving the doffer presents another of the easily accessed control points. The web thickness can be measured by various means and the doffer speed can be controlled using the signal from the transducer. Photoelectric web thickness measuring equipment used to adjust the doffer speed can be used to improve the productivity; this is called the autocount system. There is a considerable inertia effect and no shortterm control is possible. Limits on the loadings on the swift mean that the average speed of the doffer has to be coupled with the input to the system. Very long-term errors cannot be controlled from signals obtained solely from signals derived from the web; long-term control is better done at an earlier stage. To obtain more accurate control, signals can be obtained from intermediate positions by measuring the fiber on the swifts or doffers using scanning, flat laser beams, or other devices. There is a band of error wavelengths that can be controlled from signals generated by transducers on the card, providing that the long-term average mass flow is also controlled. The web is still a major source of error in woolen yarns. Apart from linear density, longterm variations in the amount of oiling or opening or blending adversely affect the operation; fiber loss or degradation varies accordingly. The blending length of the card set, although very long, is still limited, and the card set cannot remove all these errors. A blending length is defined as the length of web over which variations will be attenuated to a prescribed level. A short blending length implies that the integration process in the card merely ‘smears’ the variation over a short length and even longerterm variabilities pass through almost unsmoothed. The woolen card set is deliberately designed to have a very long blending length. This is because all irregularities in composition and thickness of the web are passed directly to the slubbing and may show up as variations in linear density, color or some other attribute. Thus, every opportunity has to be taken to improve evenness. One of the several purposes of the card set is to remove unwanted material and the machine is quite efficient at doing that. If, however, the percentage of unwanted matter varies widely within the blend, its removal produces a complementary irregularity in the fibers delivered to the tape condenser (see following section). If the feedstock is properly prepared, this is not too much of a problem, but unless experienced, the


229

workers in preparation might not realize the importance of their work. In addition, the card set can still contribute its errors to the products. Errors produced in, or near, the tape condenser show up as relatively short-term errors in the slubbings. Consequently, there is a very wide spectrum of error and there is no drawframe doubling to even them out. Thus, it has to be again emphasized that there needs to be a great deal of attention to evenness at all stages.

8.4.9 The tape condenser A prominent difference between the conventional woolen and worsted systems lies in the card delivery. Traditionally, a woolen card set delivers slubbings rather than sliver, and the slubbings are converted to yarn in a spinning machine. However, some woolen systems produce sliver, much of which is used for sliver knitting. The system is also used for non-wovens. These last two uses do not involve yarn and will not be further discussed. The slubbings or ropings are like roving but have no recognizable twist. A woolen card delivers up to 200 such slubbings, which are created by splitting the web into a number of similar ribbons of web that are rubbed to give the needed fiber cohesion. Consolidation of the fibers at this stage helps retain the shorter ones and avoid undue fiber loss; the number of active tapes is related to the yarn count required and the web thickness. The system of separation of the sheet into tapes is designed to limit fiber damage, but inevitably there is some short-term irregularity caused by the process. Card web is laid onto a series of tapes, which separate the ribbons (Fig. 8.13). There are a variety of tape arrangements possible but the figure of eight pattern is probably the most popular. The tapes have to be durable and retain their surface characteristics over long periods of time. Slick or greasy spots on a tape can cause local irregularities in the web being divided and so can wear at the edges. Variation in tape tension is another factor, particularly between banks of condensers. The web selvages are usually too uneven or of the wrong weight and are usually discarded or reworked. The useful ribbons are about 1 inch (25 mm) wide and they pass to pairs of rubbing aprons that roll them into ropings. The whole assembly is called a tape condenser. Each slubbing is taken up on a spool and these go direct to spinning. The

Slubbing output to winding section

Reciprocating rubbing aprons Web division Twisted tapes (b)

Card web input

Tapes (a)

Fig. 8.13

Woolen condenser with rub aprons

230


material at this stage is fragile and needs careful handling. Out-of-true carding elements, periodic differences in loading, wire sharpness, etc., produce periodic error in the slubbings that can be measured in the laboratory. Truly periodic errors can be recognized and the source of error can usually be determined. However, there is often a larger and more distributed random component for which diagnosis is difficult. Experience and trial-and-error tests are needed to find the sources of these types of errors. Of course, with such a wide range of fiber properties, high levels of random errors are to be expected. A card set stores a large volume of fibers during operation because of the size of the elements involved and the degree of loading. This has advantages in averaging medium-term errors but it causes significant inertia in the system, which is particularly noticeable on start-up. As the fiber delivery rate alters, so does the wire loading; it moves exponentially to some steady value. Stability is not achieved for several minutes. Care has to be taken in disposing of the slubbings produced during start-up or shutdown procedures. For a five-part card, the transient can be as high as 8 or 9 minutes.

8.4.10 Woolen spinning Slubbings are usually formed into groups of cheeses on a mandrel and the assembly is referred to as a bobbin. These are then transported directly to the spinning machine, which is often in the form of a ring spinning machine. (Mule spinning still survives in some areas but space precludes a discussion of this.) Bobbins often have flanges to protect the material, because the cheeses are soft and are vulnerable to damage. Denser packages are preferable, because of the more efficient use of space, and it then becomes possible to dispense with the flanges on the bobbins. However, the denseness of the cheeses is quite dependent on the tension that the slubbing can bear, as well as on the wind structure. Undue tension can cause uncontrolled drafting and end-breaks; this represents a limit to the possibilities. Winding lag has to be avoided to get the improved package density that arises from a precise lay. One solution is to use a grooved tension plate. The objective is to lay the material directly on the surface of the cheese, near the nip, between the spool and a drive roll. Emerging practice is to transport a series of bobbins by endless chains, and creel them in the spinning frame automatically. Arrangements have to be made to ensure that a given lot of slubbing arrives at the proper spinning frame and obviously computer control has a part to play in this. Direct spinning from the slubber has not found favor over the years, probably because of the lack of flexibility inherent in such a system. In spinning, a strand’s weakness makes necessary special precautions in order to locally strengthen the yarn at the weak points. In drafting, false twist is introduced to bring the drafting point closer to the nip of the delivery rolls than otherwise would be possible with large rolls. The false twist runs to the twister surface. Consequently, the effective ratch setting is between the twist transition point and the nip of the delivery rolls. A typical system is shown in Fig. 8.14. The false twist spindle runs at anywhere from 20% to 60% of the main spindle speed. Higher percentages apply mostly to the higher speed spindles [10]. The lower part of the drafting section usually comprises a series of rolls that are fairly conventional. Fiber control is also needed in drafting. Fig. 8.14(b) shows a typical set of rolls. Rolls A and B are the ones shown in diagram (a). D is a fiber control device that fulfills much the same function as an apron in short-staple spinning. The distance between D and E is a function of the fiber length. The output is twisted and wound by a ring spinning machine similar to those


231

Back rolls

False twist draft zone

Roping

From the false twist draft zone False twister A

B

Front rolls D Main draft zone

C

E Yarn To ring bobbin (a)

(b)

Fig. 8.14

Control of fibers in woolen drafting

already discussed. The tensions caused by spinning are reduced by using collapsed balloons or balloon control rings. Woolen yarns are used as both singles and ply yarns. In this trade, plying is known as doubling. Since woolen yarns have the characteristic of being soft, with good insulation properties, singles yarn has a market for hand knitting yarns and garments where those qualities are prized.

8.5

Bast fiber spinning processes

8.5.1 Conversion of stems to sliver The process of converting flax, jute, and hemp consists of hackling, preparation, and spinning. The first step (i.e. hackling) is to (a) split and separate the fibers that are gummed together at the start of the process, (b) disentangle them, and (c) parallelize them as far as possible. The remaining broken, shorter, raveled fibers form a tow, which is a byproduct of the process and regarded as inferior. Yarns are also made from this tow. Traditional hackling was performed by hand, using spiked boards, but more modern practice uses machinery. The first stage of hackling is known as roughing. A principal component of a roughing machine is a moving band carrying spiked bars containing hackling stocks, which work on the ‘stricks’ of rough flax and carry out the first rough separations. Successive stages have ever finer teeth, more closely packed to finish the process, the finest pitch being about 60 pins/inch (≈ 2.4/mm). Fiber bundles are moved from one hackling band to a neighboring finer-toothed one for further processing. This process continues until the bundle reaches the finest hackling band. The root ends are hackled first, the ‘combed’ end is clamped, and the other end is hackled in a manner similar to that just described. The tow made during these processes is stripped from the teeth of the bands by brushes whose surface speed exceeds that of the pins. The stricks of hackled fiber are then sorted into different qualities; smoothness, luster, hand, and cleanliness are factors that determine the quality.

232


Fiber bundles of similar quality are fed to spreading frames which transport sheets of fibers to a drawfame for a gilling operation similar to that already described. The lengths of flax on the spread sheet entering the drawframe must overlap to give cohesion and evenness; also, several parallel sheets are used to give a doubling that reduces unevenness. The output of the drawframe, just as in the other similar processes described elsewhere, is referred to as sliver and is stored in cans. In all, it is practice to use at least four drawings and to creel between four and twelve slivers to give a large amount of doubling. It is common to use a cumulative doubling ratio approaching 1500:1. (It will be remembered that the doubling ratios for each stage are multiplied together to calculate the cumulative doubling ratio.) Manufacture of yarn from tow follows a different process. First, the tow contains shives (woody material) and other impurities, which have to be removed. Opening and cleaning equipment is somewhat similar to that used in short-staple spinning. It is then carded using one- or two-card sets, depending on the quality and cleanliness of the input material. The card sliver is then drawn and spun in similar fashion to that used to produce other bast fiber yarns.

8.5.2 Spinning bast fibers Spinning can be done using one of two systems. The dry spinning system is used for coarse yarns; a roving stage similar to those already described is used to produce an intermediate product. Sometimes another flyer spinning system is used to produce the final yarn and sometimes a ring frame is used. Wet spinning is used for finer yarns. The rove (roving) is passed through a trough containing hot water and the rest of the spinning is carried out wet. The water dissolves the gummy substances and provides freedom for the fiber to slide in a controlled fashion in drafting, with the result that evenness is much improved. Twistless spinning of cotton using wet drafting showed the same effects [11]. Spun yarn is then usually wound into hanks containing 300 yd. These hanks are referred to as leas or cuts. The grist (count) is calculated by the number of leas/lb.3 There are other count systems, which will not be enumerated. The hanks are dried and then worked by twisting and untwisting them to dispel the wiry feel of the yarn; this breaks down the gummy adhesions, which give the wiry hand. Fine linen yarns are often bleached before passing to the lace maker or weaver. Linen thread involves the plying of very fine linen yarns.

References 1. 2. 3. 4. 5. 6. 7.

Wood, G F. Wool Scouring, Text Prog, 12, 1, 1982. Von Berger, W. Wool Handbook, Interscience, New York, 1982. Atkinson, K R and Saunders, R J. Burr Beater Design and Operation, Part 1, J Text Inst, 82, 4, 1991. Plate, D E A. Advances in Early Stage Processing of Wool, Textile Inst Conf Proc, Sydney, Australia, 1988. Atkinson, K R. An Analysis and Theory of Burr-beater Operation, J Text Inst, 80, 2, 1989. Robinson, G A. High-speed Carding of Wool, J Text Inst, 80, 1, 1989. Anon. Processing of Staple Fiber on Woollen and Worsted Systems, Technical Booklet 12, ICI, Harrogate, UK 1970.

3 This is yet another yarn count system to add to those described in Appendix 1.

Long-staple spinning 8. 9. 10. 11.

233

Henshaw, D E. Worsted Spinning, Text Prog, 11, 2, 1981. Lorenz, R R C. Yarn Twisting, Text Prog, 16, 1/2, 1987. Richards, R T D and Sykes, A B. Woollen Yarn Manufacture, Manual of Textile Technology, Textile Institute, Manchester, UK, 1994. Selling, H J. Twistless Yarns, Merrow Monograph, Watford, UK, 1971.

9 Post-spinning processes

9.1

Winding

9.1.1 Introduction Bobbins from a ring frame contain too little yarn to be useful in modern fabric making equipment and it is necessary to rewind the yarn onto larger packages. Special, high speed winding machines are used for this purpose. It is very important that the yarn which is to be sold or used in a subsequent process should be ‘put up’ into the correct sort of package – usually cones, cheeses, or occasionally, hanks. The transfer of yarn from the ring tube to the cheese or cone provides an opportunity to remove yarn faults. Looking towards the consumer’s needs, it has to be realized that the density and structure of the package delivered are important. For transport and storage, the package should be as dense as possible. For ease in unwinding, the package should have a regular structure without over-dense portions, which might impede the unwinding process. Poor unwinding properties cause difficulties for the user and increase the costs. For dyeing, a low but regular package density is required so that the dye liquor can penetrate the package easily and evenly. Irregular dye penetration yields streaks and barré in the final product. Variations in winding tension produce similar effects. The needs of the customer or user therefore dictate the type of package and the density of winding. If the yarn is returned as a complaint, the spinner’s costs are increased. Dye packages are usually wound on sprigs (porous package centers) and are shipped mounted on pegs, which form part of a transport frame. Sprigs permit the flow of dye liquor in dyeing and the peg-frames prevent the packages rubbing together and becoming damaged. As previously stated, ring frames produce low volume bobbins that contain blemished yarn; also, these bobbins have a combination wind, which is unsuitable for the next process. The final yarn packages are usually cross-wound and contain several pounds of yarn. This means that there must be many joins in the yarn on each package because there must be at least one join for every ring bobbin used. To these must be added another one for every blemish removed in the clearing operation. Yarn faults

Post-spinning processes

235

outside the prescribed limits are removed as the yarn is transferred from the spinning bobbins to the cones or cheeses. Faulty portions of yarn are cut out and the ends are spliced together to make, as nearly as possible, a perfect join; all of this is done automatically. This latter process is called ‘clearing’ and, while it is very effective, it is better to have as few a number of original yarn faults as possible, rather than rely on the clearing capabilities of the winding machine. Each intervention by the winder adds to the cost and slightly degrades the quality of the yarn. Winding and clearing of staple yarns are normally carried out on the same machine. In filament production, and in certain advanced staple spinning systems (such as rotor spinning), the primary process produces a large, cross-wound package and there is no need to rewind to change the package size, although sometimes there may be a need to rewind to a low density package for dyeing and sometimes there may be a need to clear defects. However, every effort is made to avoid such costly rewinding. The technology of winding has developed to the extent that automation is the rule. It is therefore important to consider this aspect fully. Economics and quality control become very important factors in the seemingly simple task of rewinding yarn (or ‘winding’ as it is normally called). Winding is carried out for the following purposes: 1 2 3 4

To To To To

change the type of wind. change the package density. remove yarn faults. create a package which is not susceptible to damage.

9.1.2 Machine principles Machines are required to produce acceptable yarn packages as just outlined; this often involves the manufacture of a cross-wound package using a reciprocating guide, as shown in Fig. 9.1. Consequently this type of machine will be used to open the discussion. The yarn is laid on the surface of the rotating package by this reciprocating guide. The idea is that overlapping sinusoidal wraps of yarn interlock and provide a stable package. The relative rates of traverse of the guide and yarn package determine the type of build. As the package grows in size, its tangential speed increases unless it is controlled in some way. This implies that, for a given traverse oscillation rate, the geometry of each layer of yarn gradually changes as the package grows. As will be seen later, this has some important consequences.

9.1.3 Package build The package build most used is a cross-wind in which the yarn is traversed across the face of the cheese (or cone) several times during one rotation of the package. For convenience, a cone or a cheese will be referred to as a package because many of the following remarks apply to both. Yarn on the surface of the package is roughly sinusoidal and, ideally, out of phase with the coils lying beneath. Yarns cross one another and friction holds most of the yarn in an × formation (Fig. 9.2(b)) which is highly stable; this makes packages very durable under normal conditions if abuse in handling is avoided. The winding tension, angle of wind, and cradle pressure affect the structure and package density. The socalled ‘cradle pressure’ refers to the force acting between the package and the drive roll. The density of a regular package should be controlled because it might later be

236

Handbook of yarn production Package

Yarn

Reciprocating guides (a)

Package

Yarn

(b)

Fig. 9.1 Cross-wound package

unwound at high speed and too hard a package then gives trouble.Winding tensions should be limited due to the danger of overstraining the yarn. Changes in cradle pressure cause changes in winding tension and adjustment of the cradle pressure is a means of exerting control on tension. As the diameter of the package builds up, the number of traverses per rotation of the package changes and this changes the structure of the package. When the package is the same diameter as the drive roll, the coils on the surface of the package lie exactly on top of the ones just laid. A number of coils laid on top of each other as the diameter builds through the critical zone produces so-called ribbons that are very troublesome (see Section 9.1.4). In the case of a dye package, the dye penetrates the ribbon at a different rate from the rest of the package; the result is a periodic difference in dye shade in the yarn. In unwinding, the yarn is reluctant to leave the surface of the dense parts and this gives rise to tension pulses which also can cause difficulty. Ribbon breakers are normally fitted to the machines to prevent the problem. Some machines cause the package to lift from the drive roll momentarily, to allow some slippage to disperse the ribbons at the crucial times. Other machines move the package sideways to achieve the desired dispersion. Some modify the drive roll speeds. The yarn in the shoulders of the package plays a considerable role in the durability of the package. There should be sufficient traverses per revolution of the package to prevent loose portions of yarn from lying parallel to the shoulders. The shoulders should feel firm and stable, but not hard. As has been explained earlier, yarn is sometimes parallel wound. In such a case, either the package must be wound onto a bobbin with flanges, or the package must be small and have sloping shoulders. Hank or skein winding can also be used. Sometimes skein dyeing, and occasionally skein mercerization, is used; skeins are often sold in the home craft market and they frequently consist of heavy yarns.


237

9.1.4 Cross-wound packages For simplicity of explanation we will confine ourselves at first to a cheese. The reason for stability can be seen in Figures 9.2 and 9.3, where it may be noticed that the yarns on the surface of the package interlace at an angle. Each layer of yarn imposes a restraint on the sinuous ‘coils’ beneath. The best angle is between 12° and 20°. Resolving the tensions in a given layer of yarn, the radial components acting inwards at every intersection have a magnitude of F; these depend on the winding tensions, T, and the angle of wind, θ. The number of intersections depends on the wind, and θ again enters the picture. The radial component and the number of intersections determine the density of the cheese, and the package density is a function of T and θ as shown by the example in Fig. 9.3(b) and (c). The force needed to make one coil slide over the others depends on the coefficient of friction µ, as well as F, and the number of intersections, m. A force greater than µmF, acting along the length of the yarn, could cause whole coils of yarn to slip. A cross-wound cheese has a large number of intersections and it is, therefore, inherently stable. It is quite possible to build a stable cross-wound cheese containing over 10 lb (4.5 kg) of yarn. However, it still has to be remembered that the winding tension is an important factor in determining both the package density (Fig. 9.3(c)) and stability. As previously mentioned, too high a tension can damage the yarn; the aim is to maximize the stability without exceeding the tension limit. The limit depends on the type of yarn being wound. Referring back to Fig. 9.2, a phase change, φ, occurs as each layer is added and: φ = πD – mλ

[9.1]

where m is an integer, λ is fixed and D is variable. As the cheese builds up, φ changes periodically with the consequence that the package structure also changes periodically. πD λ

φ C

A′

A

C′

B′

B (a)

(b)

1 2

(c)

Fig. 9.2

Build-up of package surface

238


T1

θ

F

D T2

(a)

Package density (g/cm3)

0.5

0 0

30 60 Angle of wind (θ degrees) (b)

90

Cradle pressure = 2 lb (0.9 kg) 0.5 Package diameter = 6 in (0.15 m) Ne = 20

0 0

Fig. 9.3

10 Winding tension (gf) (c)

20

Cross-wound packages

At the times that φ is reduced to zero, yarn from one ‘layer’ is laid exactly upon the one below and the yarn piles up in a dense sinuous ribbon on the surface of the cheese until the diameter grows sufficiently to give φ a significant value again. As m reaches successively larger integers, new ribbons are created; this causes considerable problems in dyeing and unwinding. The flanks of the package also show signs of the ribboning. At the critical diameters, the effective yarn traverse is increased and the reversal points protrude; these are called overthrows (Fig. 9.4(c)). Not only do patterns occur when φ passes through zero but they also occur when φ is a fraction of π D (i.e. when φ/π D = 1/2, 1/3, 1/4, 3/2, 5/2, etc.). A solution to this problem is to vary the lateral position of the yarn lay, as shown in Fig. 9.2(c). There is an inherent variation in package density in a cross-wound package. Consider two slices, A and B, of equal width, δl, taken perpendicularly to the axis of the cheese, as shown in Fig. 9.4(a) and (b), where just one coil of yarn in the geometrically developed surface is shown. Of course, each slice contains many yarns. A typical length of yarn in slice A is δlA and it lies parallel to the shoulder. In fact, all the yarns in the slice are more or less parallel. The lengths in slice B, such as δlB, criss-cross


239

(a)

B

A δl

δ lA

A

B δl

δ lB

(b)

(c) Overthrows shown by arrows

Fig. 9.4 Hard shoulders and overlaps

within it. Thus, the shoulders of the package at slice A are denser than those within slice B or any other intermediate slice. Therefore there is a need for several traverse motions per revolution of the cheese to keep the percentage of yarns lying more or less parallel to the shoulder. Ribboning and dense shoulders also occur with cones and although the mathematics are more complicated, the ideas are essentially the same. Ribbons occur periodically with the same unwanted effects, and the solutions are similar to those already described. A minimum of several traverses per revolution of the cone is also required.

9.1.5 Cones Although a cheese is easier to wind than a cone, there are advantages that favor the cone. In knitting, yarn is withdrawn slowly and there is insufficient speed to cause the yarn to balloon away from the surface of the cone. Yarn drags over the surface of a cheese and disturbs the lay of the other yarns; the drag generates more and variable yarn tension, as well as making the yarn more hairy. With a cone, the taper causes a progressive release of the yarn from the surface, providing it is withdrawn in the direction of the apex of the cone. There are fewer surface entanglements and the yarn flows more evenly. Even at the higher speeds used in warping and high speed weft insertion, the cleanliness with which the yarn is withdrawn from a cone is a considerable attraction. If the cone and the traverse are driven at constant speeds, the yarn speed required to lay the yarn on the small-diameter end (B in Fig. 9.5) is different from that needed

240

Handbook of yarn production A A B

B

Fig. 9.5

Cone surface

at the base (A). To make such a system work, there can either be intermittent yarn storage or the traverse rate can be made to vary across the traverse. Ideally, the yarn cone should be driven by another cone, to avoid scuffing of the surface, because two cones mesh together perfectly. However, some winders use a wide cylinder to drive the yarn cone which is kinematically incorrect. The cone tends to run with its mean surface speed in synchronization with the drive cylinder; the base runs faster and the tip runs slower, the differential movement causing scuffing. Often, a narrow cylindrical rim is used to drive the cone. Although this avoids most of the scuffing, it causes some local compression on the surface of the cone.

9.1.6 Traverse mechanisms One common type of winder has a reciprocating traverse. As mentioned, by varying the relative speeds of traverse and package, or their relative positions, one may obtain a pattern breaking effect. A displacement of lay 1 with respect to lay 2 can produce a pattern breaking effect as indicated in Fig. 9.2(c). Care has to be taken to keep the final guide close to the surface of the package otherwise the yarn lag can be troublesome and hard shoulders will be produced. The yarn lag is due to the uncontrolled length AB in Fig. 9.6. When the guide is moving leftwards, the yarn lies at an angle such as is shown at EF, but it lies at an angle such as GH when traveling in the other direction. The lag tends to concentrate yarns at the reversal points and give hard shoulders. The simplest and a widely used form of yarn traverse is the grooved roller (Fig. 9.7). The grooved roller not only drives the package but it also lays the yarn on the surface at approximately constant spacing. However, since the coil spacing is fixed, there is ribboning every time the package reaches a multiple of the effective diameter A FG B H

E

Yarn at time 1 Yarn at time 2

Fig. 9.6

Yarn lag in winding


241

Yarn package or cheese

Yarn

Grooved roll

Fig. 9.7

Grooved roll traverse

of the grooved roll. There is also a fractional relationship as already explained. One solution to the problem is to use a large diameter grooved roll, and another is to use a pattern breaker. Some pattern breakers oscillate the package or the grooved roll, sideways, randomly. Local linear speeds at which yarn is laid onto the surface of a package, which rotates at constant speed, vary with the angle of lay. If the surface speed is Vs and the yarn speed is Vy, then at special positions, Vs = Vy, but elsewhere this is not so because the yarn lies at an angle. One way to overcome the problem is to place a bow piece in the plane of the yarn offtake (Fig. 9.8). The length of yarn between the supply and the laying-on points varies in such a way as to take up the slack caused by the variations in wind-on position and angle. In building a cone, more yarn has to be laid on the base of the cone than on the tip. The angle of lay has to be biased away from Yarn reciprocates over the bow Yarn Bow

Cheese

Fig. 9.8

The use of a bow to control yarn tensions

242


the sinusoidal and length compensation has to be introduced to prevent periodic high tensions or overfeeds occurring. A special grooved roll or reciprocating traverse may also be used.

9.1.7 Precision winding The machines discussed so far have had a constant traverse rate and such machines produce ribbons of denser structure at periodic intervals as the package grows in diameter. There is a kinematic solution in which the traverse rate is varied. If we differentiate Equation [9.1] with respect to time, we may restate the result as: dφ/dt = K(πDU – mλV )

[9.2]

where dφ/dt = the rate of phase change of yarn layers, K is a constant, U = rotational speed of the cheese in rev/s, V = the speed of the traverse in oscillations/s, and the remaining symbols are as in Equation (9.1). If mV and DU are kept in synchronism, dφ/dt = k, where k is another constant. If k is chosen appropriately, the ribbons can be eliminated and a denser package can be made. Separate drives are needed for the package and traverse and this makes the apparatus expensive. Some machines use a precision wind in which the diameter is sensed and the traverse speed is adjusted to give a constant value of the coil advance.

9.1.8 Direct winding Where the yarn is wound directly on the cones or cheeses at the yarn manufacturing stage (for example, in rotor spinning), great care is needed in setting the bow piece and traverse mechanism. Unlike the case where the yarn is being withdrawn at will from another package, the supply velocity is fixed by the process. As was seen earlier, the velocity at which the yarn should be laid on the surface of the rotating yarn package is not constant. If one requires a well-built cone with a considerable cone angle, it is necessary to have a more sophisticated yarn storage and tension control. An improperly set bow, or the operation of a bow piece system beyond its limits, gives a poor package structure and a high frequency of end-breaks during operation.

9.1.9 Winding tensions Since yarns are visco-elastic in nature, any tension applied to them alters their characteristics. Yarns need to be wound at controlled tensions because over-tension damages the yarn; under-tension gives an unstable package of low density. Damaged yarn performs badly as can be seen from the example in Fig. 9.9. Progressively increasing the winding tension increases the tenacity at first and then it drops again. At first sight, the increase in tenacity would seem to be an advantage. However, careful examination will show that there is a progressive decrease in the breaking elongation, which implies no improvement in the energy to break. Also, the weak spots are strained more than normal, which is a bad feature. Another example, a cotton OE yarn, showed that application of stresses up to 20% of the breaking value increased the tenacity of the yarn between 1.0 and 1.5 gf/tex, but at the expense of the breaking elongation. In weaving, any lack of elongation gives problems with warp


243

Elongation Tenacity 10

Tenacity

Elongation (%)

Tenacity (g/tex)

10

Elongation

5

5 0

Fig. 9.9

1 Winding tensions (g/tex)

2

Effects on yarn performance of high winding tensions

breaks.1 The value of such strength increases is therefore dubious and, without doubt, over-stressing a yarn damages it. The effects on ring yarns are less because of the more organized structure. Fortunately, OE yarns are rarely rewound.

9.1.10 Unwinding The amount of yarn on a ring bobbin is insufficient for commercial use and many ring bobbins contain faulty yarn, which has to be removed before final packaging. The yarn is transferred to cheeses or cones and this involves the unwinding of the ring bobbins as part of the transfer. A frequent wind found among the supply packages is the filling or weft wind (which typifies the ring package as sketched in Fig. 7.4(b)). The wind-on tension varies throughout the wind because of variations in (a) tension during spinning, and (b) the balloon during unwinding. High winding tensions are frequently met when a ring bobbin is just started because of the smallness of the winding diameter, and this is frequently a time when one sees an increase in the fault rate. Thus, a cheese or cone has variations in yarn structure. Tensions change as yarn is taken from the beginning or end of the ring tube; this changes the package structure. The appearance of thin and thick spots further complicates the picture. It is not uncommon to find a thick spot followed by a length of thin, over-twisted yarn. Even after piecings are cut out, one finds that knots or splices are followed by thin, weak yarn with a different appearance. There are a number of differences that are likely to show up in the final product; often the problems are made worse by high winding tensions. Bobbins from the ring frame have to be unwound at very high speeds as yarn is transferred from the ring bobbins to the cones or cheeses. In the ring spinning process, bobbins had rotated at high speed and new layers of yarn had been laid on to a plushlike surface created by the hairs being held out by centrifugal force. When the yarn is removed during the winding process, the yarn tends to twist as it is removed; this action removes fibers from adjacent coils on the bobbin. The consequence is a microunevenness that is not easily detectable on a spectrograph but is objectionable when the yarn is assembled into fabric. The problem becomes worse as the ring frame speeds increase. Also, it is a good reason to control yarn hairiness in spinning. Yarn appearance is among the largest categories of customer complaint; it has been seen 1 Energy to break, i.e. tenacity × elongation, is the best criterion.

244


to rise at times as high as 40%. Even if a yarn has adequate strength, evenness, and the correct twist, it is not always satisfactory. Rust and Peykamian [1] have shown that the hairiness of yarns increases due to the winding process. During the winding process, fibers are transferred from yarns on a supply package to the yarn just being removed. Large transfers of fiber yield yarn faults; the normal transfer rate is less than 10 fibers/m. The presence of yarn defects adversely affects unwinding performance; it may also reduce winding efficiency, not only because of the increased clearing needed but also because of the end-breakages in the process. High speed unwinding of a package, whether it be a ring bobbin, cheese or cone, gives a balloon that is chaotic, and peak yarn tensions can be very high. The difficulties arise because the radius at which yarn is taken from the package varies rapidly because of the build. Without any form of balloon control, these peak yarn tensions severely limit the speed at which the yarn can be removed; this, in turn, limits the winding speed of the whole winder. Balloon breakers are used to reduce the tension variations and one of the latest types is shown in Fig. 9.10 [2]. All balloon breakers increase the yarn hairiness and the mean winding tension. The take-off point of yarn from the bobbin moves up and down the chase as yarn is pulled from the package; this causes variation in yarn tension. The balloon breaker, which might also traverse up and down, limits the tensions. The yarn winding tension as it goes on to the new package is partly determined by the unwinding tension from the supply package. It is further determined by the additions and/or multiplication of tension (see next page) produced by the tension controllers and guides. Some tension increase is inherent in the design of the system; some is used as a means of control. The latter is dealt with later; for the moment we shall concentrate on the involuntary portion. A simple balloon breaker can double the Yarn

Before winding Control tube @ 1500 m/min Conventional @ 1000 m/min

Yarn balloon 0

Height, arbitrary units

Balloon control tube

Bobbin

100 200 Hairiness, arbitrary units

Fig. 9.10

Balloon control and yarn hairiness


245

hairiness of the original yarn; it also increases the winding end-breakages unless the winding speed is limited. Improved designs, such as those with a control tube, as shown in Fig. 9.10, limit the hairiness increase [2]. Turning now to the voluntary tension controllers, there are two basic forms of these, namely, addition and multiplication types. The former gives an output tension equal to the input tension plus a frictional component; the latter gives a result, which follows Amonton’s Law. Many tension systems incorporate elements of both and, in general: Tout = K1 µ F + Tin (1 + K2 eµθ )

[9.3]

where K1 and K2 are constants, µ is the coefficient of friction, F is the normal force acting on the element in the tensioner, Tin is the input tension, Tout is the output tension, θ is the angle of wrap in radians, and e = 2.718. Amonton’s Law states Tout = Tin (1 + K2 eµθ ).

9.2

Yarn joining

9.2.1 Defect removal Clearing of yarn to remove faults is a crucial role for most staple yarns. The market rarely accepts yarn with numerous and large defects; to maintain price and reputation, it is essential to remove all unacceptable slubs, thick spots, thin spots, etc. The level at which they become unacceptable is a matter of agreement between the buyer and seller of the yarn. Yarn is sometimes wound at high tension so that weak spots are found during winding rather than at some later stage. An automatic knotter or splicer is used to join the ends created by the removal of the defect. To remove weak spots consistently, the winding tension has to be uniform. In many cases it is anything but uniform and, under such circumstances, some weak spots are missed; the efficiency of clearing is thus reduced. Furthermore, some weak spots are overstrained and this might cause problems later. Thick spots are often detected by passing the yarn through nub plates that contain slots that will pass normal yarn but not the thick spots. The rise in tension when a thick spot is caught in the nub plate causes the yarn to break; the ends are then knotted or spliced to give continuity. Again there is a periodic over-stressing of the yarn. Defects, and the consequential damage, are often concentrated in yarn taken from the base of the ring tubes (because end-breaks in spinning are more frequent there). A satisfactory alternative is to use a sensor to detect the defects, and to use the signals from it to actuate a cutter and a knotter or splicer. Optical or capacitive sensors are frequently used. Accurate defect removal requires measurement of the linear density (or ‘yarn diameter’) of the running yarn. When a bad spot which is outside the prescribed limits passes through, the winding head should stop and the section of bad yarn should be removed. Any newly cut or broken ends are then joined before the winder recommences winding. A patrolling piecer assembly finds the ends on the supply and uptake packages, splices the ends and then restarts the winding head. A machine might contain tens of such piecers serving, perhaps, a hundred winding heads. The procedure is automatic and needs no human intervention unless (a) the winding head is improperly set, (b) the section of yarn is particularly bad, or

246


(c) there is a mechanical failure. The limits can be set independently to include the thickest and thinnest allowable linear densities at various fault lengths. Capacitive sensors are most commonly used to detect the defective portions of yarns. Sometimes, however, optical devices are used, in which case one has to be careful with the use of external lights around the machine unless the sensors work in the infra red part of the spectrum. Optical sensors have difficulty in discriminating between thick and fluffy spots (they are not sensitive to yarn mass). On the other hand, capacitive sensors are moisture sensitive and they cannot discriminate between changes in linear density, fiber composition, or moisture content. Both of the sensors mentioned can perform satisfactorily providing that the moisture content is properly controlled by appropriate air conditioning and that there is reasonable quality control in spinning. Their use permits the setting of the winding tension to appropriate levels. Supply packages should be conditioned for an adequate time because moisture penetrates at a relatively slow rate; it is prudent to condition them for at least 24 hours before winding. Since water is cheaper than yarn, it is necessary to define the moisture content of the yarn in the contract between buyer and seller. It might also be pointed out that many fibers swell when they imbibe water; this swelling and the subsequent shrinkage can disturb the lay of the yarn on the package. Conditioning of finally wound packages can lead to the production of hard shoulders. This leads to difficulty in unwinding the packages and yarn damage. It is preferable to condition the yarn before winding and then maintain the moisture content by appropriate control and packaging of the final cones and cheeses. The winder is used not only to join the component yarns as just discussed, but it provides an opportunity to test the yarn for defects, cut them out, and join the new ends. The consequential reduction in defects is of the greatest importance for quality and for profitability of the mill. Clearly, if the clearers are set too close, too many minor defects are removed and replaced by splices. There can be a deterioration in quality because even the splices (as good as they may be) are still minor defects. Equally clearly, failure to remove serious defects can have serious undesirable effects on the trading relationships involved. The presence of unwanted defects may be the subject of a complaint by the customer that may involve payment of a financial settlement and a loss of confidence.

9.2.2 Splicing Until the last decade, it was common practice to knot yarns together, but the knots were a source of weakness and were defects in their own right. Nowadays, yarns are spliced together using mechanical or air-jet splicers, which produce a joint that is usually at least 70% of the strength, and generally less than 130% of the thickness of the parent yarn. Splice efficiency is used as a measure of the strength of the spliced portion of the yarn, expressed as a percentage of that of the parent yarn. The adoption of splicing has greatly reduced problems in weaving, knitting, and dyeing. There are two means of splicing in common use. One is in the form of a hand-held device and the other is part of a winding machine. A single winding machine might have, perhaps, ten splicers serving something in the order of 100 winding heads. A typical winder/splicer makes between 10 and 30 pieces per package and ejects from 5 to 10 mg of fiber per package into the atmosphere. The splicers are complex, expensive devices and there is a need to conserve capital by letting a single splicer serve several winding heads. Also, that way, the splicer is kept in fuller employment


247

than otherwise would be the case. Most devices involve air splicing but there are some mechanical types. Both sorts can produce good splices providing they are properly set. A review in the early 1990s of the industrial performance of a series of splicers gave the results shown in Table 9.1 [3]. An old air splicer performed poorly compared to more modern ones; this points to the improvement in piecer design over a decade. Some spaces have figures in parentheses and these reflect the diminished performance when a single machine (of the few needed) was poorly set. This is a matter of personnel training rather than machine design. In a different survey of several mills, the CVs of the settings of a given type of splicer were found to be extraordinarily high. This is a bad sign as far as fixer training is concerned. To make a satisfactory splice, the two yarn ends have first to be prepared to make them properly tapered. Also, the fibers must be adequately separated and paralleled so that they are capable of intermingling when the splice is made. Consider a typical machine as shown in Fig. 9.11. Remember that the winders work at high speed, and have to interrupt the winding to cut out the faulty portions of yarn before the new ends are spliced together. It will be noted that scissors are provided to cut the unwanted yarn ends after the two yarns have been laid in place. At this juncture, the ends of the yarn are parallel and face opposite directions as shown in Fig. 9.11(a). Automatically actuated clamps grasp the yarn at the appropriate places before the main splicing

Table 9.1

Performance of splicers

Splicer type

No of spinning plants

% strength efficiency

% bad splices

% CV of splice strength

Mechanical Air (Maker 1) Air (Maker 2) Air (Maker 3) Old air type

2 4 3 3 2

98 98 (94) 93 91 (81) 85

0 0 (8) 3 0 (27) 10

10 16 (19) 12 12 (21) 16

Yarn B

Yarn B

(a)

Ends being spliced (d)

Yarn B

(b)

(c)

Scissors clamp X

Spliced yarn (e)

Air Splicing nozzle X

Air

Splice

Clamp Scissors Yarn A Time 1 Lay and cut

Yarn A Time 2 Conditioning ends

Yarn A Time 3 Form loops to retract ends

Time 4 Splice ends

X = End-conditioning nozzle

Fig. 9.11

Stages in splicing

Time 5 Remove spliced yarn

248


procedure begins. The free ends of the two yarns are sucked into end-conditioning nozzles and air blasts are provided to condition them before joining. To condition the ends, the yarns have to be gripped and fibers sucked from the exposed ends to taper them (Fig. 9.11(b)). A spectrum of fiber lengths is removed from the ends. The violent airflow causes some fiber entanglement in the remaining fibers at the yarn ends, which makes it easier for the fibers to intermingle during the splicing operation itself.2 Splicing is carried out after the two conditioned yarn ends are laid inside the splicing chamber so they are parallel, facing opposite directions and appropriately spaced without the tips of the conditioned ends protruding. One way to do this is to withdraw loops of yarn as shown in Fig. 9.11(c). The splicing chamber of an air splicer is sometimes made in two parts which open to allow easy insertion of the yarn ends and then permit closure for the splicing phase of the operation. The two ends are spliced together by a rapidly rotating body of turbulent air inside the splicing chamber. The turbulence is induced by air that enters the cylindrical chamber tangentially. The air blast first intermingles the fibers and then causes the newly made joint to rotate to produce false twist. The yarn is then removed from the splicer (Fig. 9.11(e)) and winding is recommenced. If properly restrained, the false twist at stage (d) accumulates on one side of the splice until the air ceases to flow, at which time the false twist flows into the joint. Twist distribution during the splicing operation is shown in Fig. 9.12(a), the yarn size having been exaggerated to show the twist directions and level. This latter is important because the twist in the splice gives the joint an appearance similar to that of the parent yarn (and strengthens the joint too). Consider the case of a splicing chamber designed for use with normal Z twist yarns. A temporarily high Z twist exists on one side and an S twist (or very low Z twist) exists on the other while the air blast operates. There is about 15 to 20 mm of zero twist between them then. After the air blast stops, the twist is redistributed to give what appears to be reasonably uniform Z twist throughout the splice. There is a distinct chance of damage to, or failure of, the portion of the yarn with low twist during splicing. Also, if the tensions within the zone are kept low, there is a probability of snarls forming in the high twist portion during splicing. Consider the nature of a splice. To avoid a thick splice it is necessary to taper the ends to be spliced so that the joint is not obvious. These ends have to be in the proper relative positions when the splice occurs. In the case shown in Fig. 9.12(b), the tapered ends are misplaced to give a thin spot. This is also usually a weak spot, which is undesirable on that account. The yarns could have been overlapped too much, in which case there would be a thick spot and two undesirable splice-tails (Fig. 9.12(c)). These tails catch up in knitting and weaving and are often the subject of customer complaints. The splicer should be set to avoid these tails, even at the expense of a slight loss in splice strength. Figure 9.13 shows two such bad splices, the top one of which has a wrapper on the right-hand side and the bottom one a quite undesirable tail on the left-hand side. For these reasons, and to uphold quality standards, it is very important to maintain the correct timings, settings, and tensions. The various motions described are performed by a series of cams and levers, and it is essential that the components be well maintained. 2 If there are 50 splices per cone, the count is 20s (≈ 30 tex), and each splice converts about 1 inch of yarn to fly, then 0.083 lb (≈ 38 g) of fiber is ejected per cone, which is a significant source of pollution.


249

Yarn 2

Yarn 1

Clamp

Clamp

Low twist

High twist

False twist (a)

Splicing chamber Thin spot

(b)

Thick spot Unwanted tail

Unwanted tail (c)

Fig. 9.12

Fig. 9.13

Splice structure

Imperfect splices of a dark yarn with a light one

Accumulations of lint, loss of lubricant, and wear on machine parts can alter the performance of the machine and degrade the quality of the product. The alternative method to air-jet splicing involves a mechanical device which untwists the yarn end locally and pulls the tail away, causing the remaining fibers to become parallelized. The distribution of fibers in the remaining tail gives a tapered end as required. A shear field produced by two counter-moving disks is used to produce the twisting action needed to splice the ends together. This mechanical false twister has rubber elements, which apply pressure to the assembly during the splicing operation and, again, careful maintenance is required to keep the system in good working order.

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Splices are commonly tested for strength, in the field, with a hand-held dynamometer. The parent yarn on either side of the splice is normally tested also and so-called splice ‘strengths’ (more properly, ‘splice efficiencies’) are usually expressed as (100% × splice strength ÷ average of the parent yarn strengths). With a good splice, it is not easy to distinguish the splice from the rest by eye. Nevertheless, if the yarn is looped in the region of the splice, it will be seen that the splice is stiffer and does not conform to the same radius as the rest. This is helpful in finding the exact position of a splice that is about to be tested.

9.2.3 Winding machines Most modern winding machines used in staple spinning incorporate not only the function of winding but also that of removal of faults. They involve unwinding the supply package (usually ring bobbins), splicing the yarns after cutting out faults, and winding the package that goes to the customer or to some intermediate process. On the other hand, winders for filament yarns are not concerned with the removal of faults and splicing. All winding machines are designed to produce stable packages of undamaged yarn at maximum speed. A typical modern cone may well weigh more than 10 lb (≈ 4.5 kg) and can be composed of yarn from up to 50 ring bobbins. The winding machines work at speeds up to 1500 m/min (≈ 1370 yd/min) and the process can impose considerable strain on the yarn if the machine is not properly set. The surfaces touched by the yarn are usually coated or treated to reduce friction and abrasion. Increasingly, ceramic inserts are being used at critical places to reduce the wear on the machine. The minimization of the wear and tear on the yarn is also important; however, it is not always easy to detect the damage until it enters a subsequent process. Wear can produce hairiness, nep, and other appearance problems. Consequently, maintenance is essential. Another function of the winder can be to meter the amount of yarn on a package. This can be important to a customer, because it is desirable that all yarn packages in a creel of a warper or knitting machine should run out at the same time. In this way, there are few or no remnants of yarn left on the package centers to become waste. Winding is so near to the customer interface, that it should be taken very seriously. The author’s experience in the 1990s was that roughly 25% of claims made on complaints arose from winding problems. This figure is not fixed; it is amenable to change through improvements in technique and equipment.

9.3

Ply yarns

9.3.1 Plying For sewing threads, as well as certain speciality and industrial yarns, it is necessary to ply (i.e. to double or fold) the yarns to give them a smoother and less hairy character. Doubling improves the evenness; plying balances torque if carried out correctly and binds some of the hairs on the component yarns. The traditional methods include assembly winding (Fig. 9.14) to place the single yarns parallel to each other as a closely spaced pair (or group) of yarns on an intermediate package. The new package is then used as a feed for a twisting machine and the output is a plied yarn. However, the cost of assembly winding approaches 25% of the total winding costs and the system is prone to problems. If one of the ends breaks in the process, then


Fig. 9.14

251

Assembly winding

only a single yarn is wound until the machine is stopped. Another problem is that an end from an adjacent package can be caught up to form a three-fold (or four-fold), instead of the desired two-fold, yarn. Such faults, if not spotted and removed, are a sure cause for complaint by the customer. The lengths of yarn on every package are best matched to avoid wastage. Such practice also eliminates the need for the existence of partly consumed feed packages on the winding frames and is desirable since the presence of surplus packages increases the risk of the three- and four-fold yarns just discussed. Because of these problems, it is modern practice to wind and clear the ring yarn so that standard cones may be used for the feeds. This may not be the most efficient way of converting the yarn, but the field is not large enough for there to be promise of much machine development.

9.3.2 Singeing Where smooth cotton yarns are required, as in sewing thread, they are sometimes singed (or ‘gassed’) to remove the hairs. Such yarns are often two-ply; long-staple cottons are frequently used to give maximum strength and resistance to abrasion. A micrograph of a typical ring yarn before and after singeing is shown in Fig. 9.15. To singe the yarn, it is passed through a flame at a steady speed; the rates of fuel gas and air are carefully adjusted so that sufficient hair is removed without damaging the core.

252


Fig. 9.15 Ring yarns before and after singeing

The process normally produces CO2 and soot. However, if the ratio of fuel-gas to air is incorrect, the process will produce toxic carbon monoxide rather than CO2. Therefore, care has to be taken to set the equipment properly. Careful venting of the workspace is vital because cotton dust is highly flammable. The singeing is carried out in a walled-off space otherwise there could be explosions in the fly-laden atmospheres found in other fiber processing rooms. Other points to consider: soot deposits can ruin otherwise perfectly good yarn packages, heat of combustion needs to be removed, and the health of the workers must be preserved. Singeing of yarns containing meltable fibers, or ones that char (like wool), is not recommended.

9.3.3 Sewing threads Although the manufacture of sewing threads does not necessarily involve new technology, it does have special requirements for the yarn structure. Sewing threads are often plied to give strength and uniformity to the product; they are often made from cotton, but special threads of linen and silk are also produced. Yarn hairiness has to be controlled to reduce the fly build-ups around the sewing needle area. To give the desired strength and smoothness, cotton threads are often singed and then mercerized. The latter is a process that swells the cotton fibers with caustic soda and stretches them to improve molecular orientation. This, together with the high twists used, makes them rather expensive. One of the reasons for using cotton is that it does not melt. With the ever-increasing speeds of sewing machines, needle melts can be a problem with many of the man-made yarns, especially with certain stitch constructions. Thus, a source of high tensile threads is denied to some garment makers unless special needle cooling arrangements are installed. There are some special man-made fibers, such as Nomex, a heat resistant aramid fiber, but they are costly and this is not a universal solution to the problem. Other solutions include the use of a man-made filament core, sheathed with natural fibers to protect it and to give the desired aesthetics. In all cases, waxing of the yarns is essential. The wax must be sufficient to give lubrication, even at the elevated local temperatures in the needle eye. However, the quantity must not be so much as to cause clogging of the guides and needle eye. Problems can also come from differences between the yarn and fabric in the matters of elongation, tension differences, and dyeability. If the thread contracts more


253

than the fabric being joined, the seams pucker. If the dye affinity of the thread differs from the fabric, it becomes visible.

9.3.4 Other plied yarns Plied yarns are used for purposes other than sewing threads. Plied yarns are not only more even and of enhanced strength, they are durable, flexible, and low twists may be used to give a soft hand. Acrylic and wool yarns are often plied for the hand knitting market; this is because of the soft hand that can be created. Aramid yarns are plied in various complex structures to give strength to ropes and load-bearing strands. Thus, plying is widely used within certain specialty markets, but for run of the mill products the process is too expensive. The improvements in quality of singles yarns over the last half-century have undermined the broad plied yarn market.

9.4

Automation

9.4.1 Patrol theory Consider the case of manual winding. To optimize the use of a winder, a relationship has to be established between cost and the number of winder spindles to be assigned to a worker. If the assignment is too low, the machine efficiency is high, but the operator is not fully occupied and the operator efficiency is low. If the assignment is too high, spindles stand idle waiting for the operator and the machine efficiency is low. It is necessary to balance the two forms of efficiency so that the overall cost is at a minimum. For simplicity, ignore the cost of doffing the large output package and any waste. Assume that the operator progresses steadily in one direction, and only repairs end-breaks or replaces empty input ring bobbins by full ones as he/she comes to them (the interventions are called ‘events’). Spindles that have been passed have to wait until the next circuit. The theory is similar to that which applies in spinning (see Equation [12.1] in Chapter 12), except that the number of events/hr, E, replaces the breaks/hr, B, and some terms are neglected. Suffice it to say that the estimated optimum winding assignment, aw, is given approximately by: aw = √{CLE/(VtCfw)} where E V CL Cfw t

[9.4]

= number of events/yd = velocity of yarn in yd/hr = labor cost/hr = Fixed cost/hr for winding machine = average time in hrs to complete one patrol during which time all spindles in the assignment are inspected once (including workbreaks).

The assignment is strongly affected by the pre-existing number of faults in the yarn. The spindle speed Vw, the patrol time t, and the cost ratio CL /Cfw play a significant part. The performance of the winding department is strongly affected by the preceding operations. This estimate can be used where the winder transfers yarn from one large bobbin to another but with ring yarns, manual winding is onerous and expensive. It is normal to then use automatic winders.

254


9.4.2 Automatic winders With automatic winders, the knotting or splicing function is taken over by a traveling automatic device which searches for the broken ends, makes a join, and then sets the spindle in production again. Operator attention is only required when the robot has failed to make the join after several attempts; a signal light draws attention to such a condition. The robot treats the exhaustion of a feed package in the same way as an end-break, except that, in this case, a new feed package is presented automatically. Various functions are carried out by separate systems that form part of the winding machine. The traveling splicer operates independently of the other functions of the winding machines. It patrols to and fro along a fixed path searching for end-breaks or exhausted supply packages. When there is an exhausted ring tube, another mechanism replenishes the magazine and then leaves it to the knotter or splicer to make the join. There is also a mechanism that detects when the output package needs doffing, substitutes an empty core for the full package, and restarts winding on the new core. Consider the supply system. Several bobbins of yarn should be available to each spindle. When there is a need for a fresh supply, the machine will discharge the empty bobbin and bring a fresh bobbin into place. It will then find the end and join it to the end from the delivery package before continuing to wind. It will do this without assistance from the operator except when the machine malfunctions. This enables the operator to distribute his or her work amongst several machines; it is only necessary to ensure that the turret or magazine has a sufficient supply of bobbins. In modern machines, this too has been automated. A hopper of ring bobbins supplies them to an automatic device, which aligns them, rejects bad ones, and carries the aligned bobbins to the individual spindle magazines. Suction arms move to the surface of the supply and/or uptake packages to find the ends lying on the surfaces of the packages when the operation has to be restarted. A restart is needed when a new supply package is introduced, an output package is doffed, or an end-break is detected. These ends are inhaled into the suction arms for the limited duration of the transfer. During this time, yarn is consumed, but it is under a controlled tension that makes the laying of the yarn into the piecer manageable. The suction arms move to position the yarn as is shown in Fig. 9.11(a), so that the splicing can continue. After the splice is made, the machine restarts automatically. If there is a failure to splice, the machine attempts to splice again. However, if the number of retries is beyond the pre-set limit, a warning light appears to notify the operators that the particular winding head requires attention. The frequency of appearance of warning lights is often used as a crude measure of the quality of the yarn leaving the ring spinning machine. The operator has a trouble-shooting function rather than that of a server; the operator assignment is different from that for manual winding. Similar factors operate in determining the assignments but the coefficients differ. The capital costs of the knotter or splicer relative to the rest of the machine should, in theory, determine the assignment of the robot. The mathematics of optimum operator assignments applies (see Chapter 12); the theory indicates the importance of keeping the yarn joining frequency within the economic limits of the machines. This implies that the CV of the yarn and the defect levels should be carefully controlled so that they do not exceed the design level (which is relatively low). The defect level of the input yarn directly affects the number of joins per package, as well as the value of the yarn. The number of joins should be at a minimum. Generally, a poor performance in winding is a sign to expect poor spinning and future problems in fabric manufacture. Many problems arise in the early processes, and clearing acts as a sort of filter to


255

buffer the impact of these early problems, but does so at a price. Clearing does not completely remove all effects of the faults produced in the early stages; the process of clearing is carried out at the expense of winding efficiency. Whilst it might be better to clear yarn and accept the low winder efficiencies than vice versa, it is better still to spin good yarn in the first place.

9.4.3 Linking Recent practice was for spinning bobbins to be gathered in tubs as they came from the autodoffer. Tubs full of bobbins were placed randomly in the hopper of the winding machine, which sorted the acceptable bobbins from the rest. The accepted ones were then passed to the winding heads for the winding process. In the process, the bobbins suffered considerable jostling. There is a trend nowadays to connect a dedicated winder to a ring spinning machine. This has advantages and disadvantages. The bobbins pass smoothly from the autodoffer to the winding head without being subjected to jostling and possible damage. The transfer is automatic and saves labor costs. It is also possible to keep track of the source of faulty bobbins, which is of great value in quality control if appropriate records are kept and used. However, the productivity of the ring frame is highly dependent on count and the winder is less so; thus, there is a balance point between ring frame and winder at which the productivities match. Departure from that balance gives an excess of winding heads or a lack of them. The latter cannot be contemplated; the result is that a mill usually has excess winding heads. Some machines are made so that the heads can be moved from one linked system to another to ease this problem.

9.5

Two-for-one twisting

To make ply yarns, it is necessary to twist two or more singles yarns together. Following the assembly winding stage, paired yarns are often twisted using two-forone twisters. (There are still ring twisters in use, but they are expensive to run.) The principle is shown in Fig. 9.16(a). Some yarn makers use two-for-one twisting without assembly winding, where two packages are mounted inside the balloon (Fig. 9.16(c)). However, the process is not as efficient as assembly winding because more space is used up inside the balloon envelope and the winding continues less smoothly. Figures 9.16(a) and (b) show two methods of controlling the yarn balloon, which surrounds the yarn package(s). In one case, a circular balloon rail is used and in the other, a cylindrical pot. This is analogous to the conventional balloon control ring in a ring frame. It may be compared to the control pot shown earlier in Fig. 9.10. The yarn is protected by smooth metal pots in order to facilitate start-up, to separate the yarns, and to protect the feed yarn from the ever present fly. Plastics tend to become damaged in use and stainless steel has been found to be satisfactory. To conserve valuable space inside the balloon, it is preferable to use precision wound packages because the package density can be raised by about 25% when compared with normal crosswound packages. However, this is not always feasible, especially if the cheaper rotor spun yarn is used as a feed. Rotor spun yarn is frequently used because the packages from the spinning machine need no winding before two-for-one twisting. Sometimes, for very fine yarns, up-twisting (see Section 3.3.4) can be used instead, because the

256


Pigtail guide

Yarn passes down through the spindle and then up outside the balloon rail Rotating flyer

Neither balloon rail nor package rotates

Rotating whorl Tape (a)

Central tube Yarn Package Pot (Stationary)

Storage disc Outer pot omitted for clarity (b)

Fig. 9.16

T

S

Yarns shown grossly oversize (c)

Several forms of two-for-one twisting

damage to the surface layers of the yarn is less. The increase in cost is a reasonably small percentage of an already expensive yarn. The package(s) inside the balloon in a two-for-one twister is/are usually held in place with magnets, which act on armatures in the yarn package center. This enables a vertical spindle arrangement to be used which conserves space. Any rotating member in the magnetic field has to be non-conducting, otherwise there is an energy loss and some local heating. Yarn tension is controlled by a choice of various spring-loaded and ball-type devices, mounted in the central hollow shaft of the winder through which the yarn travels. The use of various pots not only protects and separates the yarns, but provides some protection against balloon collapse. Tensions in this type of twisting do not vary greatly when everything is properly adjusted. All surfaces in contact with the yarn need to be polished and to have an immunity to wear and ill use.


257

It is normal to have a storage disk S under the rotating throw-off plate and a yarn tensioner T as shown in Fig. 9.16(c). The angle of wrap on the storage surface plays an important role in determining the tension. If the tension leaving the tensioner in the hollow spindle becomes too low, the balloon becomes unstable. In that case, the rate of wrapping the storage disk (Fig. 9.16(b) and (c)) increases, with the result that the tension in the balloon rises due to the increased capstan effect. Conversely, too high a tension has the opposite effect. The maximum yarn tension is at the balloon node because the energy for yarn movement along its axis comes from the winder. (This differs from the balloon in a ring machine where the energy for yarn translational movement comes from the bottom of the balloon.) The winding portion of the machine must be controlled (a) to preserve constancy of the twist level, (b) to be independent of variations in tension arising from the twisting unit, and (c) to produce a package structure suitable for the end use. In other words, it must be possible to unwind the yarn at speed and give a product as nearly uniform as possible in all respects. This requires a good tension controller and lay mechanism to produce the necessary structure. Twisting machines are often used for waxing and this is usually done to meet the needs of the knitter, although the wax can help in the winding process also. Passage over guides and machine surfaces tears out fibers from the surface, especially when frictional forces are high. Waxing and plying are both methods of limiting that increase in hairiness and in wild fibers. Damage created in twisting also results in the generation of fly and dust, which brings other quality control problems. The use of correct speeds and judicious amounts of lubricant limit the problem.

9.6

Customer concerns

9.6.1 Yarn contraction When a strand such as a yarn is twisted, it tries to become shorter. If the yarn were allowed to move freely, no tension would be generated and the yarn would become shorter due to the twist; this is known as ‘twist contraction’. A typical value for twist contraction for a typical 30s cotton ring spun yarn is about 4%. Contraction of twisted yarn after it is wound on a package generates tension, which causes the package to become more tightly packed. Absorption of moisture causes contraction in hydrophilic yarns because the fibers swell and need more room. Thus, changes in moisture content of such packages cause variations in package density, and sometimes make it difficult for the customer to unwind the package as was discussed in Section 9.2.1. The effect is greater with highly twisted yarns. Yarn is sometimes conditioned by steaming to adjust the package weight to standard conditions if it is too dry to meet the agreed specifications. Over-conditioning can make the yarns absorb too much moisture. Not only does over-conditioning affect the weight of the yarn packages, it affects the tightness of the package to the point where it creates the difficulties for the customer in unwinding as already discussed. 9.6.2 Winding and yarn dyeing Dyeing of yarn is often carried out in an autoclave similar to that described elsewhere. Dye is forced through the package under pressure and the whole autoclave is held at a sufficiently high pressure to gain the necessary dyeing temperature. An autoclave is an expensive piece of equipment in which the yarn packages have to be closely

258


packed to fill the working volume of the steam chamber. (An autoclave is briefly described in footnote 1 in Chapter 4) Usually a series of cheeses or cones is mounted on a mandrel and several of these loaded mandrels are inserted into the autoclave, with their axes parallel to one another. The mandrels are usually part of the autoclave itself because the dye solution travels through the perforated hollow centers on its way to or from the dye packages. Frequently the direction in which the dye is pumped is alternated; inside to outside, outside to inside, and so on. As might be deduced, the dye pressure depends on the steam pressure in the autoclave, as well as on the density of the packages. Low density packages tend to have larger passageways through them than more dense ones. Thus, the package structure affects the dye pressure needed. The yarn package transport system for these products should be designed with the capacity and arrangement of the autoclave in mind.

9.6.3 Preservation of yarn tails on packages Cones and cheeses are usually made with yarn tails from the inner and outer layers of the package secured at some agreed upon position on the package centers. The purpose of this is to enable the head of one package to be tied to the tail of another. Thus, when the first is exhausted, yarn is automatically taken from the next, and so on. Packages of this sort are frequently used in creels of machines in which many short chains of packages supply whatever equipment the customer is operating. Within these chains, the packages are tied head to tail. This permits the labor of tying the ends to be deferred by the operator because the packages actually supplying the machine act as reservoirs; it makes possible the continuous operation of the yarn using machine. It is commercially important that the tails always are present in the form specified by the user. Winding machines need careful adjustment to produce the desired tails reliably. Even if the packages are used in-house, the receiving department should be treated as a customer.

9.6.4 Shipping It might seem so obvious that it needs no statement, and yet mills lose thousands of dollars by errors in shipping. In business, there is a time and efficiency factor in the goodwill generated. If the goods do not arrive on time, no matter that they are perfectly satisfactory and reasonably priced, the shipments might be returned. This brings in no revenue to the spinner and only produces extra costs that have to be subtracted from any profit. If the goods are shipped to the wrong place, or the wrong items are shipped, the effects are equally disastrous. It is imperative that the shipping operation be efficient and effective. The relatively new idea of just-in-time (JIT) shipping is one where incremental orders are transmitted electronically, from user to supplier, as required; the supplier ships goods in timely quantities to keep the user’s operation going without large inventories. Inventories would otherwise consume working capital and JIT reduces interest charges and fees. However, there is a downside to the scheme. A sales system has inertia; some of it is due to the inventory stored in various parts of the supply line. JIT seeks to reduce this cushion of stock; but in times of sudden demand, the lack of a cushion can make severe demands on the primary producer, both in the shipping and the production departments. Rather than lose business, many primary producers keep an inventory to meet the need, but it involves risk and cost. The cost tends to be


259

passed down the line because, if one business fails for these reasons, competition is reduced and prices tend to rise.

References 1. 2. 3.

Rust, J P and Peykamian, S. Yarn Hairiness and the Process of Winding, Text Res J, 62, 11, 685–9, 1992. Muratec Advertisement, Int Text Bull, 1, 21, 1996. Lord, P R. Unpublished data from the author’s private records

10 Staple systems and modified yarn structures

10.1

Yarns of complex structure

Traditional ring spun yarns consist of fiber spirals and parts of each fiber approximate to helices of the same pitch. Yarns of this sort can be untwisted to give roughly parallel fibers. The qualifiers inherent in the above definition arise because of lateral fiber migration, fiber hooks, and convolutions created in processing. Despite such distortions, a fully untwisted yarn possesses very low strength. Processes such as open-end spinning, air-jet spinning, and other specialist systems produce a structure that contains varying pitches of the helical fiber segments. As a result, they can never be untwisted to a point where the fibers are all roughly parallel. The yarns thus possess significant strength when untwisted to give a minimum value of strength. The variety of structures is wide. Structures for rotor spun and air-jet yarns are described in Chapter 7 and Appendix 10. The process of rotor spinning also is considered in Chapter 7.

10.1.1 Composite yarns A series of developments related to the various specialist systems have been made in which staple yarns and filaments are combined to give composite yarns. Usually, the filaments are used as the core of the yarns, and staple fibers make up the surrounding sheath. In this way, the filament yarns are placed where their strength is of the greatest advantage and the staple fibers are placed where they can have the greatest aesthetic value. Unfortunately, filaments are relatively expensive and so increase the cost/lb of the yarn. Also they are often shiny, so that if the sheath does not cover the core properly, the shiny filaments ‘grin’ through the cover to give streaky effects in the final fabric. A description of one technology to produce composites by wrapping is given later is Section 10.6.

Staple systems and modified yarn structures 261

10.2 Processes using modified twist 10.2.1 Processes using false twist The advantage of using false twist is that there is no need to rotate a yarn package to put in the twist needed, but the twist created is transient. The following sections sketch two examples of how difficulty can be circumvented. The first example is the air-jet spinner, which encourages a structural change in the yarn before the false twist is released (Section 10.4). The second example is of self-twist yarns in which one yarn with an alternating twist is plied with another to make a stable ply yarn (Section 10.7).

10.2.2 Processes using another form of modified twist Streams of untwisted fiber can be encouraged to merge in such a way as to produce a structure that looks like a ply yarn (one might call it a mock ply). It does this by exploiting the conservation of torque. Torque applied to the outgoing ply can be made to transfer to the ingoing component strands to create a ply twist (Fig. 10.1). (Reactions R occur at the front drafting rolls.) The result is similar to a regular plied yarn and it is made without the mechanical complexity of the traditional plied yarn operation. The yarn structure is either an S-on-S or Z-on-Z ply rather than the conventional balanced torque type of S-on-Z or Z-on-S. The yarn and process is described in Section 10.7.

10.3

Compact spinning

10.3.1 Fibers in the twist triangle in ring spinning The twist triangle in ring spinning determines much of the character of a ring yarn. As discussed in Section A5.2.1, each fiber leaving the nip of the front rolls of a conventional drafting system has a tension that depends on its lateral position with respect to the rolls. There a wide distribution of fiber tensions and fibers traversing at least one of the two selvages of the triangle exist at a high tension. Fibers in the central zone often go slack. Cameras using short duration exposures have produced images which have shown so-called wild fibers emerging from the front roll nip of the drafting system and these are also slack. Parts of these wild fibers exist in space remote from the twist triangle itself and contribute to the hairiness of the yarn. Fibers R1 T1 Strands from drafting system

Tout

Yarn

T2 R2

Tout = T1 + T2 + Losses T1 + T2 = R 1 + R 2 T = Torque, R = Reaction

Fig. 10.1

Torque balance in mock ply

262


under high tension migrate to the center of the yarn as they enter the twisted transition structure at the output of the triangular zone. Slack fibers migrate to the outside of the yarn and form part of the loose hairy surface. The structure of fibers within the twist triangle changes continuously (although for simplicity, descriptive models often assume that the average tension distributions are symmetrical about the center-line of fiber flow but this is not always true). Consequently, there is good reason to try to control the distribution and variability of the fiber tensions within the twist triangle.

10.3.2 Processes of fiber control If the fiber tensions can be controlled externally by a system of restraints, it becomes possible to reduce the migration of what have been slack fibers and which might otherwise have formed a loose structure at the surface of the yarn. Such a constraint would be expected to produce leaner yarns with well-organized surfaces, in which the outer fibers can bear a larger percentage of the load applied to the yarns. Consider a system containing a perforated surface with fibers flowing on one side of the perforations and suction applied to the other side. Friction forces will be generated between the fibers and the perforated surface, which increase the fiber tensions by a small amount. If the flowing fibers are those in the twist triangle, and sufficient suction is applied, there should be few, if any, slack fibers entering the twist transition zone and the tendency for what would have been slack fibers to migrate to the surface will be lessened. Furthermore, if the suction apertures are controlled to fit the shape of the twist triangle, the number of wild fibers can be controlled, which again tends to reduce the loose structures mentioned and to reduce hairiness. There are several possible arrangements that could fit this specification. A few of them are: (a) a perforated hollow front roll with suction applied to the inside, (b) a ‘back-to-front’ perforated apron projecting from the front roll under (or over) the twist triangle with suction acting through the apron thickness, (c) an option similar to (b) but with a slot or aperture in a cover plate to control the position of application of the suction. Several such designs were shown in the 1999 ITMA machinery exhibition. Possible drawbacks to such designs are: 1

2 3 4

When an end breaks, it is more difficult to apply the conventional pneumafil devices because the two suctions compete with the result that the dangers of a lap-up are greatly increased. Some machines have to be specially engineered, which increases the cost. Accumulations of fiber debris from fibers entrapped in the perforation are likely to increase maintenance problems. Ineffectiveness for heavy yarn counts.

Relating to (1) above, the use of a roving-stop system to solve the lap-up problem increases the cost of the machine, but the roving-stop mechanism does have the advantage of eliminating pneumafil waste and all of the problems involved in recycling it. Advantages include the possibility of producing smooth strong yarns, which would be of interest to weavers because of the reduction in hairiness, coupled with a modest increase in strength. This would make beaming, slashing, and weaving easier and more efficient. Another area that could benefit would be in thread production, where it might be possible to dispense with singeing. This suggests that, if such devices become commercially viable, the first users are likely to be producers of yarn for weavers or thread makers.


10.4

Air-jet spinning

10.4.1 The principle Air-vortex and air-jet developments led to air-jet machines, which are not truly openend spinning machines but are related. In OE spinning there is an open-end, which can be rotated, whereas in some of the yarns about to be discussed, continuity in flow is given by a core. Fibers outside that core can be rearranged and trapped in the structure to give different yarn characteristics. Götzfried [1], and later Pacholski [2], showed that air-jets entering tangentially with respect to the bore of a nozzle cause a vortex within it, and the high speed rotation of the air can be used to twist yarn passing coaxially through the vortex. The pure air-vortex spinners did not succeed commercially but they laid the groundwork for the modern air-jet spinning system. They also laid the groundwork for some of the textured and composite yarns. If the jet in the nozzle is inclined in the direction of yarn flow, it can help transport the yarn. This is by virtue of the extra increment of yarn tension generated due to the axial component of the air drag between the air and the yarn. The rotational speeds of the vortex can approach a million r/min but the yarn rotation is likely to be limited to a region around 200 000 r/min [3]; there is a large potential for high speed yarn production. An important development was that of the ‘fasciated’ (wrapped) yarn principle [4]. The original idea was based on the addition of fibers to a flowing, false twisted structure followed by the removal of torque at the exit of the false twister, which causes the added fiber to be reverse twisted as shown in Fig. 10.2. Closely related to this was the idea that the hairs on an incompletely formed yarn could be wrapped about its core. Such hairs, entrapped in the structure, give enhanced cohesion to the strand, even after untwisting. In patents by various authors [5–8], several sets of apparatus and processes were disclosed which produced similar effects. The most important of these will now be discussed. In some examples, which are attributable to Murata [5, 6, 8], two twisting devices were used, one to produce, say, S twist at the exit of the twist triangle, followed by a device producing the opposite twist. The second device removes the false twist from

False twisted yarn

False twist is removed as the yarn leaves the twister

Added fiber

Fiber locked on the yarn structure before untwisting Resulting fasciated yarn

Fig. 10.2

Fasciated yarn

264


the core and completes the wrapping of the outer fibers about the core. The twisting devices shown are air-jets, but other devices can be used. Directions of twist can be reversed to produce a yarn that simulates the opposite yarn twist but this requires different nozzles or settings. The common use of such machines is for the longerstaple fibers in the short-staple range. For example 1.5 inch (38 mm) polyester or cotton fibers spin well. It is normal for the wrapper fibers to be in the Z direction. In air-jet spinning, the idea is to use the twist triangle leaving a roller drafting system to produce a very hairy intermediate. The portion of the fiber flow that makes the core of the yarn, and the extent of the yarn hair, are unlike the corresponding values with conventional yarn. This is because the feed ribbons are much wider and the fibers in the triangular zones (marked H in Fig. 10.3) are a greater proportion of the total than normal, and there is a greater degree of drafting there. Zones marked T contain fibers under tension derived from the pull of the air-jet. The zone marked M contains fibers that go slack due to their shorter path length (when compared with the others) between the nip of the drafting rolls and the vertex of the triangle. Slack fibers migrate laterally in the core structure to interlock it, give it fiber cohesion, and create fiber loops and hairs (see also Appendix 5). Compared with a conventional yarn, there is a wider range of fiber tensions distributed over a greater width, and this promotes migration and hairiness. Hairs on the outside of the core at A in Fig. 10.3 are more or less autonomous and make only a loose and easily disturbed sheath structure. Figure 10.4 shows fibers emerging from a roll nip that have then been twisted by an air-jet; the fiber flow coalesces into yarn (shown at Y) due to the twist and the yarn is taken off in the direction of the black arrows. The separation of the surfaces of the rolls creates a depression along Drafting system

Twister 2 Z twist

Yarn

Sliver

Air jet 1 S twist

Twist triangle H Hairy yarn A

T X

Tension

M Motion T H

Torque H = Selvage zone T = Fiber tension zone M = Fiber migration zone

Enlargement of twist triangle zone

Fig. 10.3

Air-jet spinning

Staple systems and modified yarn structures 265 X Air

Air

Roll nip Y

Y

Air

Air Elevation

Fig. 10.4

View in direction X

Air and fiber flow in a roll nip

the nip zone that induces airflow (shown with gray arrows) into the nip and this airflow aggravates the hairy condition. The diagram shows two views of this airflow, and the limited view in direction X also shows how disorderly the fiber mass can be in this region. The major part of the twist triangle cannot be seen because it is masked by the top roll. Similar, but much clearer, pictures were obtained by Jones [9]. An extension of the principle just explained is to feed two adjacent yarns being made on an air-jet machine to a single take-up mechanism to create an assembly package (Section 9.3.1). The yarn can be twisted later to make a ply yarn. This idea has been used successfully on long-staple yarns and an example of it is given by the Suessen Plyfil system, which can spin up to about 8 inch (220 mm) wool (and similar fibers) from sliver in the range from 160 to 380 yd/min (150–350 m/min).

10.4.2 Machine design aspects of air-jet spinning As mentioned, the hairs are important because they are laid on the core of false twisted yarn leaving the twist triangle; the false twist is removed with the hairs in place. The spinning action wraps the hairs around the core and there is enough lateral fiber migration to lock the structure. The final product has little or no twist in the core, but has a twisted sheath, which gives the structure integrity. Leaving aside the single nozzle versions of air-jet machines, the false twist and rearrangement of the sheath fibers are usually carried out by two air-jet nozzles set in line, close to the drafting system. However, it is possible to replace the second nozzle by a mechanical twister. The entry of the air-jet orifices has one component angle tangential to the cylindrical main channel through which the yarn moves, and another component angled relative to the axis of yarn flow. The latter is an important parameter because it helps define the relationship between the twisting and linear translation speeds. Oxenham and Basu [10] showed that if the jet orifice was inclined more than 60° to the axis of the yarn there was difficulty in spinning, but at 45° spinning went on well. The diameter of the vena contracta of this channel was usually 1.6 mm and the orifice was 0.5 mm. In their experiments, the frictional characteristics of the chambers of a first nozzle were altered by coating the surfaces with PTFE to give a low coefficient of friction. Another nozzle was made from a ceramic material to provide a higher coefficient. The PTFE coated nozzle produced the best yarn tenacity and the ceramic

266


one produced the greatest CV of tenacity. Air pressures of up to 3 kg/cm2 were used in the first nozzle and 4 kg/cm2 in the second. Air-jet spinning machines with more sophisticated drafting, fiber reconsolidation, and twisting systems than those shown have now become established. Table 10.1 shows some data relating to different fiber finishes and it will be seen that finishes with high coefficients of friction give poor results. New machines with mechanical twisters following the first fiber consolidation nozzle have appeared. The mechanical twister, which replaces the second air-jet false twister, is formed by pairs of rolls with their axes crossed (Fig. 10.5). This layout of rolls gives torsional as well as transport components of motion to the yarn as it leaves the consolidation nozzle. The greater force acting on the hairs to press them towards the center of the yarn increases the chance of integration into the yarn structure. One certain result is that this arrangement produces less hairy yarn and possibly increases the range of fiber length that can be spun successfully. Air-jet machines often feature a five-roll drafting system with two pairs of double aprons capable of drafts up to 400. The drafted strands are entangled or fasciated by air-jets, as just described. The use of a wide ribbon passing through the drafting zone helps enormously in this respect. In this way, a sliver-to-yarn system is possible, which avoids the use of a relatively expensive intermediate roving. The machines can produce cheeses or cones and thus separate winding costs are eliminated. Traditional assembly winding is also avoided (assembly winding is discussed in Section 9.3.1). Adjacent pairs of air-jet yarns can be laid side by side before winding to create an assembly wound package and then pairs of yarn can be twisted subsequently, usually by a two-for-one twister, to make plied yarns. Although variants exist, the basic idea Table 10.1

Characteristics of air-jet yarn

Fiber length (mm)

Fiber fineness (d tex)

Yarn tenacity (cN/tex)

Imp per km

CV (%)

Stops per hr

38 38 38 a 38 b 32

1.7 2.2 1.4 1.4 1.7

18.0 15.3 15.5 15.0 19.5

280 – 48 20 40

15 16 14 14 14

18 38 85 20 40

Notes a = high friction fiber finish, Imp = Imperfection, b = low friction fiber finish, CV = CV of linear density.

Fig. 10.5

Crossed-roll twister


is to avoid the roving and winding operations and to produce the equivalent of traditional plied or singles yarns.

10.4.3 Air-jet performance Figure 10.6 shows a micrograph of an air-jet yarn made from 50/50 polyester/cotton staple fibers, the polyester fibers having a length of 1.5 inches (≈ 38 mm) and the cotton an upper half mean length of 1.05 inches (≈ 27mm). The wrappers that give the structure cohesion are denoted by W in the micrographs. Polyester dominates the wrappers and cotton is more prevalent in the core. This illustrates the importance of fiber length. Long fibers produce longer hairs approaching the final twister and they have a greater chance of becoming entangled with the core. Thus they generate tension within themselves as they wrap around the structure in helical form. The greater these tensions, the more radial forces are produced and the more cohesion within the structure is produced. Short cotton fibers give problems with this sort of spinning and long ones are relatively expensive. Nevertheless, a good reason for persisting with cotton fibers is that consumers seek cotton yarns. A successful air-jet process to make 100% cotton yarns is a worthwhile target. A 100% cotton yarn is hairy and this suggests a deficiency in integrating the hairs entering the twister. However, in general, it is claimed that the yarn defect level is lower than with comparable ring yarns. Looney [11] showed that the fault rate for 100% polyester was reduced to about 40% of that of the blend. He concluded that the result was, in part, due to the relatively short, high micronaire cotton fiber used (0.93 inch (24 mm) and 4.24 micronaire). It is interesting to note that some of the traditional remedies intended to improve quality sometimes do not produce the intended result. For example, increasing the number of drawings from two to three caused a decrease of about 20% in the fault rate as expected but it decreased the sliver cohesion by about 75%, which made the handling of the sliver difficult. Mishandled sliver caused stops and faults. Intimate blending can produce a reduction of up to 20% in yarn fault rates over drawframe blending. Delivery rates up to nearly ten times that of a ring frame were reported. Some idea of the performance traits of an air-jet machine when spinning polyester yarns is shown in Fig. 10.7, based on data given by Looney [12] in 1984. The point being made was that the yarn CV increases with the linear density of the fiber and the yarn tenacity decreases. The system seems best suited for fine fibers. In contrast, the work of Oxenham and Basu [10] showed that, when spinning 32 mm (1.26 inch) cotton of 26 g/tex fiber strength, yarn tenacities did not exceed 5 cN/tex (5.1 g/tex) and the yarn elongations at break were less than 8%. Thus, despite the use of long cottons, there can be some problems in running 100% cotton, although polyester/cotton blends are usually satisfactory. W

Fig. 10.6 Micrograph of an air-jet yarn

W

Handbook of yarn production Tenacity Yarn CV

20

Yarn CV (%)

18

22

12 tex

X

20

20 tex

16 12 tex

X

18

Tenacity (cN/tex)

268

20 tex

14

16 10

Fig. 10.7

10.5

15 Fiber fineness (d/tex)

Air-jet spinning performance, 1984

Sirospun yarns and process

10.5.1 Sirospun yarns As mentioned previously, worsted warp yarns are often doubled (i.e. plied). Plying has a number of benefits: (a) plied yarns have a better evenness because of doubling, (b) they weave more easily and this reduces costs in weaving [13], and (c) fabric made from them is more durable and less likely to pill. Surface fibers on a singles worsted yarn are sometimes loosely wrapped around the body; plying avoids difficulties due to such wild fibers [13]. The Sirospun process makes S-on-S or Z-on-Z ply yarns with somewhat similar characteristics to a normal S-on-Z ply yarn except that the unidirectional structures can never be completely balanced.

10.5.2 The Sirospun process A structure that is similar to the plied yarn can be produced on an ingeniously adapted ring frame (Fig. 10.8). The yarn is called Sirospun. Two rovings (A and B) are fed to a ring frame, with separators to ensure that each roving is drafted individually. The two strands emerging from the drafting system converge into a single yarn, at J, before they reach the lappet guide (otherwise known as a pigtail guide). The variations in linear density and tension cause a random fluctuating twist to be generated in the emerging yarn structure which gives it some of the characteristics of a plied yarn. If a Sirospun yarn is untwisted, the individual strands have a low fluctuating twist, which defines the strands such that, when they are twisted, they give the character of a ply. Quite low levels of random twist generate enough surface fiber trapping to improve weavability significantly; it is not necessary for the random twist to be unidirectional for this purpose. A typical strand twist is of the order of 1 turn/inch and this is sufficient to bind the wild fibers in place. The low twists result in potentially high productivities. However, the system is restricted to long-staple fibers. A disadvantage of the system is that if one strand breaks, long lengths of singles yarn will be interspersed with the quasi-plied yarn. Such faults are known as ‘spinners singles’. The difficulty is overcome by having a break-out device (shown at B in Fig. 10.8), which stops production when either of the two component strands is missing. A suction then removes the fiber from the emerging strands. There is a cost to this

Staple systems and modified yarn structures 269 A

B

J B X

Fig. 10.8

Sirospinning

solution inasmuch as each spinning position has to have an extra piece of equipment compared with regular spinning. Remembering that the productivity per spinning position is still low, any extra cost of this sort is not trivial. On the other hand, the cost of the detector is considerably less than that of a traditional plying operation set-up on a comparable basis. It has the advantage that existing frames can easily be modified.

270


According to Lorenz [14], the Sirospun system has gained a respectable share of the worsted market and it remains to be seen to what extent it can penetrate other sectors. In short-staple spinning, the existing alternatives to ring spinning offer much higher productivity and lower costs and penetration of that market will be difficult. Lamb and Junghani [15] compared wool yarns made by the Sirospun system with similar ones and found that the index of irregularity of the Sirospun yarns was between those of conventional two-fold and Plyfil (air-jet) yarns. Three twist multiples were tested (α = 73, 96, and 122) with similar results. The two-fold yarns were ring twisted. They noted an increase in short-term unevenness in Plyfil yarns, which they attributed to the high draft. The question of whether the twist levels could be reduced was not resolved and it was pointed out that it is not the tenacity of the yarn that is important but rather the weavability. Both the Plyfil and Sirospun yarns were torque unbalanced, which was a disadvantage for knitters.

10.6

Hollow spindle spinning

10.6.1 Wrap spun yarns The basic idea of a wrap spun yarn is for the machine to insert little or no twist in the core and, at the same time, wrap another yarn or filament around the core at high speed to make a composite yarn. The wrapper yarn or filament provides the forces to compact the yarn structure. Frequently the wrapper yarn is a filament, which being strong, can be wrapped at high speed without suffering the number of end-breaks that would be encountered by a staple yarn.

10.6.2 Wrap spinning by the hollow spindle process This technology provides a means of wrapping filaments about core yarns to enhance the performance of the composite. Figure 10.9 is a diagram of a hollow spindle system in which filament is taken from a bobbin mounted coaxially with the yarn Y. A hollow spindle with a hook rotates about the same axis. The hook engages the yarn and creates false twist above the hook, but the staple strand below the hook should have little or no twist. The filament yarn, F, passes through the hollow spindle and should have sufficient twist induced above the hook for the filament and staple components to be brought into firm contact. The hook acts like an untwister similar to a false twist spindle in texturing. Most of the false twist in the staple component is removed as it passes through the hook. The twisting action causes the filament to follow the surface of the staple component and the filament becomes tightly wrapped about the very low twist staple core that emerges from the hook (yarn Y′). Because of the high tenacity of the filaments, high production speeds are possible (up 35 000 r/min, which is about twice that of ring spinning). Sometimes, sliver-to-yarn systems are used and the filament is wrapped around the drafted sliver. Occasionally, both a filament and a staple strand (roving or sliver) are passed through the drafting system to produce a bouclé or other effect. The system can handle short or long staple but it is predominantly used for long-staple wrapped yarns. Xie et al. [16] created a theoretical model and tested 64s wool yarns to find that yarn tenacities of up to 12 g/ tex were possible, with wrapper twists in the range 3 to 5 wraps/cm (≈ 1.2 to 2 tpi).

Staple systems and modified yarn structures 271 Staple strand

Drafting system

Y

False twisted yarn

Filament F

Rotating hollow spindle Bearing

Y′

Hook

Wrapped yarn Guide

Fig. 10.9

10.7

Hollow spindle spinning

Self-twist spinning

10.7.1 Self-twist principle If a pair of worsted rovings are drafted and the emerging strands are passed through a twisting system such as is shown in Fig. 10.10, a plied yarn is produced in which both the ply and strand twists alternate. The emerging strands are called self-twist (ST) yarns. The torque of the freshly emerging strands from the front rollers of the drafting system causes them to try to untwist. Hairs from one strand are caught by the other; the individual yarns twist about their own axes and consolidate the grasp of the hairs from the other yarn. Meanwhile, the pair of strands twist about their common axis to relieve the torque in the individual strands (Fig. 10.11(a)). There is a discernible zero twist zone at each changeover and this zone increases as the yarns wrap around each other in the separate twisted zones. The transfer of torque from the component strands is reduced as the local ply twist increases and the system comes to equilibrium. There is a resulting series of slightly extended zero twist zones, interleaved with ply twisted sections of yarn (Fig. 10.11(b)). 10.7.2 Self-twist yarns Let the length between changeovers be L (variable) and let the staple length of the fiber be fixed at S. The zero twist zone is a weak link in the chain because any fiber


Strand 1 input

Strand 2 input

Rolls oscillate axially in opposition to one another ST yarn output

Fig. 10.10

ST yarn process

τ = τ1 + τ2 τ1

(a)

L

(b)

Relative yarn tenacity

272

1.0

S = Fiber length 0

L/ S

1.0

(c) Relative yarn tenacity is a measure of yarn strength relative to that which might be expected from using a similar fiber in a ring spun yarn.

Fig. 10.11

ST yarns


that has an end in the zero twist zone contributes no strength. Assume that the fiber ends are randomly distributed and let m be the number of fibers in the whole crosssection. It is necessary for L<S to provide fibers to transmit load across the weak section. The number of ungripped fiber ends per cross-section in the zero twist zone relative to those in the twisted sections will then be L/S and the number of loadbearing fibers is: [1 – L/S]m

[10.1]

Ignoring fiber obliquity effects, the relative strength of the zero twist zone is: [1 – (L/S)] × 100%

[10.2]

Growth of the zero twist zone has to be limited. Unless the strands are prevented from rotating about their own axes, the torque in the strands tries to balance by removing more twist from the twist changeover zones. This was briefly alluded to earlier. Fortunately, long fibers catch on the other strand easily and the process of binding fibers from the co-operating strand joins them and forms a torque stop. Lengths of the zero twist zones are determined by this licking-in process. To get the necessary fiber wraps, twist flows between zones. As the strands untwist, the length of the zero twist zone increases. The strength of the zero twist zone varies with staple length in the manner shown in Fig. 10.11(c). It will be seen that it is necessary to use a staple length of several inches (unless the weak points are strengthened by some means) to get a reasonable strength. Of the various means to stabilize the weak zones, a few will be mentioned. Stomph [17] used intermittent air vortices created by switched nozzles to false twist the individual strands before assembly at what would have been the weak points. The system was complex and limited in speed to some 100 m/min; also piecing was not simple. Morgan [18] sized the yarns with a water-soluble adhesive before further processing, but this system did not achieve wide usage either. Air-jet texturing can create lateral fiber migration and this is useful in locking the structure. In practice, the ST process is confined to long-staple fibers.

10.7.3 The self-twist process The self-twist (ST) process was alluded to in Section 3.4.2 and the discussion is continued here. There is no conventional spindle or rotor and very much higher processing speeds than normal can be used for ST spinning. Twisting and winding are separated, with the result that large packages of unbroken yarn can be made. The shuffling/twisting rollers are capable of inserting twist at extremely high rates (>10 × ring frame productivity) and the system is capable of producing yarn cheeses containing up to 9 lb of yarn. To keep L small, high levels of alternating twists are required. The yarns tend to be weak but they can be produced at very high speeds at relatively low cost. Phasing the component strands so that the zero twist portions of each strand no longer coincide with the zero twist zone of the ply (Fig. 10.12) can alleviate the problem in patterning. In practice, this is easily accomplished by making one of the component strands take a longer path than the other, on its way from the twisting rollers to the guide, J (Fig. 10.13). When the strands unite, one strand is out of phase with the other. If the phasing is properly set, the patterning, as well as losses in strength, reduced but it is still necessary to use a long-staple system. All commercial ST yarns are phased.

274


Strand = zero twist

Ply = Zero twist

Fig. 10.12

Phased STT yarn

Strand 2 input Strand 1 input

Guides J

Rolls oscillate axially in opposition to one another

Phased ST yarn output

Fig. 10.13

Phased STT yarn process

The self-twist process is rather simple. A normal machine contains a number of production channels. Each channel comprises a roving supply package, a roller drafting section with an oscillating front roll (i.e. shuffling/twisting rollers), a strand combination system, and a take-up. The oscillating front roll provides the alternating twist as already described. The machines have an exceptional productivity, but even the phased product still causes some patterning in fabrics that makes them look streaky. The simple process has a restricted market and much of the yarn is twisted to make STT yarns as described in the next section.

10.8

Twisted self-twist yarns and processes

10.8.1 Twisted self-twist yarn As already discussed, alternation of twist in ST yarns creates a streaky effect in a fabric but it is possible to superimpose real twist sufficient to make the ply twist unidirectional to minimize this effect. Such twisted self-twist yarns are known as ‘STT’ yarns. Nevertheless, even if real twist is inserted by two-for-one twisters, the cost of twisting is relatively large compared to the cost of spinning and it is therefore less attractive economically than it first appeared. Shaw [19] estimated in the early 1970s that the costs relative to the ring frame varied between 80 and 88% for counts between 30s and 9s worsted respectively. When STT yarns are made, the real twist added has to be high to minimize the patterning. The result is that, with wool, the system produces a high twist, longstaple, plied yarn that has much of the character of a worsted yarn. A worsted yarn is a relatively high twist product and therefore the STT system is still quite attractive in this market.


10.8.2 Processes for twisted self-twist yarns Two processes in series are needed to make STT yarns. The first is the manufacture of the ST yarn and the second is the plying process. This involves transfer of yarn packages between the processes, which increases the cost. It does, however, give increased manufacturing flexibility which may be useful in a specialty market.

10.8.3 Processes using modifications of the ST process One development is to self-twist a staple strand with a filament and then self-twist this composite with another filament. This produces a staple core with filaments wrapped in opposite directions on the outside. Fine filaments are not very visible and the streaky effects can be minimized. The filaments add strength to the structure but they also tend to increase the cost/lb. The trade name for this type of composite yarn is ‘Selfil’. A second development reported by Miao et al [20, 21] concerns STT yarns modified by air-jet texturing. The air-jets interlace the yarns with the result that yarns tested somewhere between 20% and 60% stronger than with non-interlaced yarns. Patterning in the fabric was diminished as compared to normal ST yarns and fabrics.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

Götzfried, K. Process and Apparatus for Spinning Yarn, BP 825,776, 1959. Pacholski, J. The Pneumatic Spinning Machine PF1 – Characteristics and Application, Theory and Practice of the New Spinning Techniques, Instytut Wlokiennictwa, Poland, c 1980. Stalder, H. The Possibilities and Limitations of Various Short-staple Spinning Systems, UMIST Symposium on Tomorrow’s Yarns, Manchester, UK, 1984. The Random House Dictionary, Random House, New York, 1966. Morihashi, T and Murata, K K K. Japan, US Patent 4,183,202, 1980. Nakahara, T and Murata, K K K. Japan, US Patent 4,142,354, 1979. Hasegawa, J, Kawabata, S and Niimi, H. Toyoda, Japan, USP 4 434 611, 1984. Yamana, M and Kubta, N, Murata, K K K. Japan, USP 4 107 911, 1978. Jones, T. Application Note No 2, Oxford Lasers Ltd, Oxford, UK, 1990. Oxenham, W and Basu, A. Effect of Jet Design on the Properties of Air-jet Spun Yarns, Text Res J, 63, 11, 674–8, 1993. Looney, F S. Optimierung des Luftspinnens durch den Einsatz von Dacron-Polyesterfasern, Dornbirn, 1984. Looney, F S. Engineering of Polyester Fibers for Modern Spinning Systems, Tomorrow’s Yarns, (Ed Hearle J W S), UMIST Symposium, Manchester, UK, 1984. Plate, D E A and Lappage, J. An Alternative Approach to Two-fold Weaving Yarn, Part 1, J Text Inst, 73, 3, p 99, 1982. Lorenz, R R C. Yarn Twisting, Text Prog, 16, 1/2, 1987. Lamb, P R and Junghani, L. Drafting and Evenness of Wool Yarns Produced on the Plyfil, Sirospun and Two-fold Systems, J Text Inst, 82, 4, p 514, 1991. Xie, Y, Oxenham, W and Grosberg, P. A Study of the Strength of Wrapped Yarns, J Text. Inst, 77, 5, 1986. Stomph, J G. Spinning Short-staple Fibers on the CSIRO-system, 4th Shirley Int Seminar, The Hague, 1971. Morgan, W V. The CSIRO Self-twist Yarn Spinning System, 4th Shirley Int. Seminar, The Hague, Netherlands, Netherlands, 1971. Shaw, J. New Methods of Yarn Production, Private communication, c 1974. Miao, M and Lui, K K. Air-interlaced Self-twist Yarns, Text Res J, 188, 67, 3, 1997. Miao, M and Soong, M C C. Air Interlaced Yarn Structure and Properties, Text Res J, 65, 433–40, 1995.

11 Quality and quality control

11.1

Quality

11.1.1 Definition of quality Modern use of hyperbole has widened the meaning of quality to such an extent that it is desirable to narrow it for the present purpose. The whole textile enterprise is founded on bargaining between supplier and customer. Two of the most important factors in the contract (explicit or implicit) are quality and price. Other factors, such as delivery schedules, service, reputation, etc., also apply, but can be set aside for the present argument. In textile technology, quality is often defined in terms of various attributes of the fiber, yarn, or fabric, but this alone is insufficient. What forms the basis of an acceptable bargain for a given product for one particular end use may not be acceptable for another set of conditions. Consequently, quality may be defined as a set of attributes for a product that fulfills the needs of a customer or user. Interlinked with the physical characteristics of the product is the question of price. Securing superior physical characteristics of the product often involves higher costs for the supplier and this usually results in higher prices. Profit margins for the yarn producer have to be sought in the differences between cost and price. The consumer looks for the highest quality at the lowest price. There are several aspects of quality control, some aimed at preventing difficulties and some aimed at curing the cause of them. A routine of sampling, testing, and adjusting is the standard method of day-to-day control. Also customer complaints or difficulty reports often require test work. Consequently a quality and control department is usually equipped with fiber, yarn, and fabric testing equipment.

11.1.2 Quality factors Acceptable quality is determined by the user, and fabric makers are major users of yarn. The desirable attributes for fabric may be classified as those related to fabric appearance, fabric durability, and freedom from faults. Often, woven fabric durability and yarn strength are found to be linked. Similarly, the ease with which the yarn can

Quality and quality control

277

be manipulated in making fabric is often related to the yarn strength, fault rate, and hairiness. However, these relationships are not universal. For example, whilst warp yarns for weaving are required to be strong, knitted fabrics have no great need for a strong yarn. However, they both have a need for non-twist lively yarn with good evenness of linear density, hairiness, and dye affinity. The appearance category may be subdivided into evenness of linear density, hairiness, coloration, light reflectivity, and refraction. (A minor segment of the market specializing in novelty yarns may have different standards from those more generally practiced, but space precludes further discussion of these.) Evenness is often expressed in terms of variance, standard deviation, or CV of the attribute concerned; consequently there is considerable discussion of these factors in this chapter. Fabric durability is a matter of yarn strength, yarn structure, and fabric structure. Yarn strength is mostly a matter of yarn structure and fiber strength. These matters become more complex when blend yarns are used, especially those in which the fiber properties of the constituents differ greatly. Freedom from yarn faults involves not only minimizing the production of faults during spinning but also the removal of them in winding. Furthermore, removal of a fault requires the joining of cut ends and the join itself is sometimes an unacceptable fault. Yarn processability is important, not only to the fabric maker but also to the yarn maker himself. Obviously, a weak yarn is more difficult to process than a strong one. Other factors also apply, and these include twist liveliness, yarn hairiness, residual yarn fault level, and yarn package construction. Deterioration in any of these factors can cause difficulties in yarn manufacture and in quality of the product.

11.1.3 Analysis of customer complaints Since the standard of quality is set in the marketplace, and since the standard changes from time to time, it behoves the spinner to keep abreast of what technical properties the market is demanding and how the product mixes of his or her company meet that demand. Again, there is no absolute definition of what the demand is, and considerable skill is needed in interpretation of the data. The customer complaint level gives a good window on the technical requirements but it varies according to the state of the economy. When the economy is booming, complaints lessen and there is a danger of complacency among those responsible for the quality control system. When the economy declines, complaints mount, and unjustified complaints are mixed with the justifiable ones. Nevertheless, analysis of the customer complaints is a prime tool for keeping track of the quality levels. Technical analyses are also important because, not only do they permit the solving of problems, but they can provide an information channel to the marketing people. Thus, the testing facility is an important part of the business. A typical spectrum of complaints is shown in Fig. 11.1, but the spectra vary according to the nature of the businesses. Nevertheless, the sample can be used to make several points. Mundane affairs, such as shipping, can sometimes be even more important than a technical issue such as the CV of linear density. The sample quoted in Fig. 11.1 was for fine staple yarns and it might be noted that yarn appearance and winding ranked at the top of the list. Neither of these categories uses a single measurement as a criterion. Rather, the judgment is made using a complex list of factors. As a contrast, filament yarns have different criteria and complaints range more in the field of polymer morphology than in evenness of the product. Changes

278

Handbook of yarn production Customer complaints/year

20 Sample spectrum only

15 10 5

Fig. 11.1

Yarn weak points

Yarn count

Yarn strength

Yarn hairiness

CV of mass

Foreign fiber

Package build

Shipping

Winding

Yarn appearance

0

Spectrum of complaints

in mechanical and thermal stress history are very important in this latter field since stress history determines the dyeing performance of these types of yarns. Winding complaints are often derived from unsatisfactory conditions in the earlier processing. Thus, a high volume of complaints labeled under ‘winding’ might stem from bad spinning, which might, in turn, arise from poor spinning preparation. Yarn appearance usually includes yarn faults such as neps, thick and thin spots, etc. It also includes some factors that are difficult to quantify by laboratory measurements. The importance is illustrated by the fact that more than 16.5% of all fabric faults in shirting, in one set of market data, were from spinning faults. The item ‘foreign fiber’ usually comes from improper stripping of the bale coverings in the process of laying down the bales in preparation for the next offtake run. It is not easy to handle a bale of fiber after the straps have been removed. Consequently, the operators have to develop techniques of removing every last strand of bale wrapping from the underside of the bales. Regardless of whether the bale wrapping is jute, polypropylene or some other material different from the fiber within the bale, the foreign fibers show up after finishing the fabrics. Sometimes it is due to differences in dye affinity, sometimes to differences in fiber size and color.

11.2

Quality control

11.2.1 Fiber quality control Testing is a very important part of quality control and needs more space than can be allocated in this chapter. Most of the data presented in this section were gathered in the 1990s but they are subject to change as developments of equipment and techniques improve. Many of the various quality factors have improved by roughly 0.6% per year in the last half a century and the trend is likely to continue in the twenty-first century. The Uster Corporation periodically issues a very comprehensive statistical analysis of worldwide data under the title Uster Statistics [1] and the reader is referred to the current issue at the time of need for information. A well-founded quality control program should carry out tests in a controlled


279

atmosphere and all samples should be conditioned by immersion in that atmosphere for an adequate time. Further discussion is given in Appendix 4. Routine tests require well-founded sampling plans. The raw materials should be sampled regularly but the schedule depends on the fiber and the variability thereof. Generally speaking, there is less testing of man-made fibers in the mill as compared to that used for natural fibers. In this case, greater reliance is placed by the mill on the fiber maker for help in quality matters than is the case with natural fibers. Consequently, remarks in this regard will be mostly confined to natural fibers. Labeling and sampling varies according to the type of fiber. Modern practice with cotton is that every bale is labeled with the normal fiber parameters, but that does not necessarily mean that every bale has been tested. There is a growing practice of module averaging in which a fairly large number of samples is measured from a module and the averages are assigned to every bale taken from that module. Debate continues whether it is better than testing every bale with only two or three samples. Some yarn makers use HVI or other mass testing lines to measure every bale,1 to assist them in preparing optimum cotton blends. These testing lines carry out a battery of standard tests on fiber (usually cotton) in a continuous fashion. HVI stands for ‘high volume instrument’ used in testing cotton, and it is a proprietary name. With wool, the fibers may be in bulk, and sampling might require core-boring tools. The mass is sampled randomly; the samples are subdivided, doubled with other samples from the same mass, and then subdivided again before testing. Sometimes there are more than two subdivisions and doubling stages in the sampling process. The International Wool Testing Organization (IWTO) specifies that there should be at least 100 test zones. With bast fibers, at least 20 bunches are selected at random, and a strick is removed from each and tested. The strick is divided lengthwise, one portion is discarded, and the remainder is separated into tip and root portions. These are halved repeatedly as necessary, the tip and root samples being kept separate. Composite samples of both tip and root are tested. Even man-made fibers are tested by some with the dividing and doubling techniques. Of the fiber attributes, length is often considered the most important. Specific ways of testing are given in Appendix 4 but it is thought worthwhile to give an example of how processing and testing can influence the results. Normally, fist-sized samples of fibers from the bales are brought to the laboratory and are conditioned before testing. A clamp is used to secure and withdraw a sub-sample of fiber from each main sample submitted. The fibers protruding from the clamp are called a ‘beard’ and the fibers actually tested are in this beard. To measure length, it is necessary to straighten the fibers by some sort of combing action. The number of fibers in a cross-section of the beard is determined by light penetration through, or electrical capacity of, a small ‘slice’ of the beard running parallel to the clamp. The measuring head is traversed perpendicular to the clamp to a position where it measures 50% of the signal it had recorded at the clamp. The position of measuring head is then taken as a measure of fiber length. There are different ways of testing, which cannot be further discussed here. The combing removes loose fibers and changes the fiber distribution. Compare a 1 Choosing only one or two samples per bale provides little or no guidance about the within-bale variance of the fiber attributes measured. These variances are often of the same order of magnitude as the between-bale values measured. Consequently, one of the dimensions needed for total control is missing.

280

Handbook of yarn production Test zone

Fibers

(b)

(a) Clamp y

Fibers Clamp

x

x

Histograms (d)

(c)

Fig. 11.2

Fiber length sampling

theoretical result before combing with one after. Figure 11.2(a) shows a population of fibers, some of which are clamped and some are just free of the clamp. The latter are shown in gray whereas the clamped fibers are shown in black. A real sample would contain hundreds, if not thousands, of fibers but for clarity the diagram shows only a few. If all the fibers in the zone are counted, the histogram of length in the sample is as shown in diagram (b). If the unclamped fibers that were shown in gray are removed as in (c), the histogram changes as shown at (d). Even if the statistical frequencies are normalized to 100%, the histograms still differ. The latter is a lengthbiased sample, which differs from reality. A third form of sampling is when the fibers along the line xx are counted as a function of y. This is called a tuft curve. The results can be expressed as histograms or cumulative frequency curves. Thus, the method of testing and the history of the material can strongly affect the result. This is important when the results are the basis of decisions, especially if the methods used by the supplier and the supplied are not co-ordinated. In HVI testing, most of the loose fiber is removed from a beard and a 50% span length is measured. This is the value of y when the number of fibers along xx is 50% of the total in the clamp. The 50% span length is about 0.58 times the classers’ length.2 Detailed distributions are given in the Uster Statistics 1997 [1]. Mean shortfiber content is shown to be unaffected by processing but the amount of trash steadily decreases with processing. This is not to say that the CV of short-fiber content is unchanged; as will be shown later, there can be significant differences. Again, according to Uster, the short-fiber content by weight can vary from 14% down to 6% for the best 5% of production. A second important fiber attribute is fiber fineness. With wool, fiber fineness is measured by fiber diameter expressed in microns, whereas with cotton, the fineness is usually expressed by the micronaire index. Micronaire is a measure of the permeability of a fiber wad of a defined size when it is mounted in a defined chamber. While it is true that micronaire is related to fiber fineness, it is also related to the maturity of the cotton; nevertheless it is an industry standard. For many sorts of American cotton, there is little variation in the micronaire/fiber fineness ratio with fiber length. However, there are some high micronaire cottons that display up to 30% difference between short and long fibers. If long-staple cottons are excluded, the differences can usually 2 The classers’ length is one obtained by a manual method now largely obsolete.


281

be ignored for fibers between 3 and 4.5 micronaire; in other words the micronaire value is a reasonable measure of fiber fineness within the quoted range. A third fiber factor with cotton is yellowness, as measured by the coefficient ‘+b’ [1, 2]. The value varies from 7.9 to about 11 for short cottons and from 8 to about 13 for long cottons, the higher figures being more yellow than the low ones. The yellowness comes from natural dyes in the fiber and from yellow-tinged wax coatings. It is desirable that the yellowness should be low to reduce the scouring and bleaching of the fabric that might be necessary; above all, it is desirable that the figure should be uniform throughout the product. Preliminary measurements [3] suggest that there might be as much as 5% difference between adjacent lengths of sliver. Generally, few measurements have been made of this variability, but perhaps more attention should be paid to it. A fourth factor with cotton is the amount of trash present. A typical HVI measurement relates to the surface area of a bale sample occupied by dark colored trash. According to the Uster Statistics [1], the trash surface area reduces as the fiber length increases. For short cottons, the worst trash counts can change from a value of 2% to 1.5% as the fiber length changes from about 1.0 to 1.2 inches. The best case has a count of under 0.1%, irrespective of fiber length within the quoted range. The long cottons show a similar pattern, although the values are lower. Other important factors are nep and short-fiber content. With short cottons, the patterns of both neps and short-fiber content with respect to fiber length are similar. Long cottons show fewer neps and lower short-fiber contents than short cottons.

11.2.2 Quality control of intermediate products Intermediate products such as sliver, roving, etc. are tested on a routine basis; the items tested depend on the business. Commonly, linear density of the intermediate material, such as sliver or roving, is measured daily at each step; yarn strength, yarn hairiness, neps in the card web, and yarn fault levels are also checked daily. However, even strict periodic sampling may give erroneous results if there is a periodic variation from a prior process that is slightly different from the sampling frequency. The two frequencies are said to beat against each other and they produce a beat frequency that shows up in the result. Where exploratory tests are being used to diagnose a problem, a viable experimental plan is required and that involves a knowledge of the variables likely to cause the problem. When long-staple sliver samples have to be transported between plants or units, it is desirable that sliver be twisted to prevent disturbance of the structure and fiber distribution (for wool tops between 15 and 30 ktex, Anderson [4] recommends 20 turns/m). In assessing the results of testing, it has to be realized that the total variance3 of random errors measured is the sum of the variance between the bales, zones, or other large divisions and the variance within them. The between-zone variance is possibly due to fiber acquisition policy whereas the within-zone category is a micro-variation in the supply, often caused by processing. This distinction is sometimes helpful in seeking to reduce the variance in properties.4

3 Variance is the square of the standard deviation. 4 Standard deviations have to be weighted to take into account the number of samples taken and (Standard Error)2 = s2/ms + s 2 /mz.

282


11.2.3 General yarn defects Defects are usually regarded as single, random deviations from the normal parameters describing the yarn. For example, a single fairly long thick place (or ‘slub’) would be called a defect, whereas a systematic series of thick and thin places would usually be classified as irregularity or unevenness. Unevenness will be described later. Sometimes slubs appear periodically; they are recognizable by their torpedo shape. Defects can occur in either staple or filament yarns. In filament yarns, many defects arise from differences in morphology of the polymer rather than from differences in linear density. (Morphology is a term used to describe the molecular structure of polymers.) Changes in morphology result in alterations in dye affinity that have a powerful effect in the category of yarn and fabric appearance. However, married fibers, drips, and debris also cause problems. The dominant defects in staple yarns arise from processing the fibers and interactions between procesing and the fibers. The possibilities of producing unacceptable yarn for staple yarns are greater than for filament yarns because a large percentage of staple yarns contain natural fibers, with their inherently variable sets of characteristics. Also natural fibers are associated with non-fibrous materials, which have to be removed. This is not to say that filament yarns are without problems. Complaints arising from filament defects are a matter between the spinner and his or her fiber supplier whereas natural fibers are subjected to some uncontrollable influences such as weather. For these reasons, discussion will be centered on staple yarns. As mentioned, yarn defects can be caused by a variety of circumstances. Some machine errors tend to be organized, but others occur at random. The organized effects, such as those due to eccentricities and drafting waves, have already been discussed. Random errors, such as the production of slubs, piecings, corkscrews, crackers, etc., have not yet been discussed (see Fig. 11.3). Many of the fault types (a) through (d) are caused by drafting. Drafting under too dry conditions can cause static electricity to be generated by the sliding fibers, with the result shown in diagram (c). Balls of fiber on the yarn caused by accumulations of lint on the traveler (d) are often composed of short-fiber debris from drafting. Occasional long fibers bridge the nip lines in the drafting system and disrupt the process (e). Loose fly, captured by the Name (cause) (a)

Thin spot (drafting)

(b)

Slub (drafting)

(c)

Fishes (static)

(d)

Ball (traveler)

(e)

Cracker (long fiber)

(f)

Spun-in fly

(g)

Piecing

(h)

Nep (ginning & carding)

Fig. 11.3

Some fault types


283

Cumulative defects/unit length arbitrary units

Total defects/unit length arbitrary units

yarn between the front drafting rolls and the pigtail guide (f), is usually ‘spun in’ at only one end [5], whereas fly deposited before that is held more firmly. A piecing is the result of an end-break (g) and neps (h) have already been discussed. If the best mills now are compared to those described by Thomason in 1971 [6], one can see reasons for some of the improvements by merely walking around and noting the vastly improved state of cleanliness. Some results typical of the ring spinning industry are shown in Fig. 11.4(a) [2]. Following the discussion in Section 3.7.7, where the known increase in difficulty in drafting at high ratios is discussed, it should be no surprise to find that the fault rate is a function of the draft ratio. Uster Tester data [1] relevant to thin spots in the yarn show that an average carded cotton yarn spinner might experience ranges from 1/km to 150/km dependent on counts between 16s (38 tex) to 45s (13 tex) respectively. The corresponding thick spots range from 120/km to 800/km. Not only are the thick spots more prevalent but they can cause trouble in later processes unless removed. Defects have a range of thicknesses and lengths that are classified accordingly. The Uster Classimat [7], an apparatus for classifying yarn defects by length and thickness, has four length classifications, labeled A to D, which range from 1 mm to 40 mm. It also has 4 thickness classes, labeled 1 to 4, each centered at various levels between 100 and 400% of normal yarn linear density. Often there are relationships between these thicknesses and lengths (e.g. Fig. 11.4(b)). There are fewer large defects than smaller ones and there are fewer long defects than shorter ones, as might be expected. A profile of combinations of length and thickness is set to give limiting settings for defect removal in winding. This usually includes a protocol at various levels in the A3, B2, C1, and other categories. Multiple winding of yarn to remove defects can also damage it. Polyester/wool and acrylic fibers are especially vulnerable to such damage. In staple spinning, irregular faults can be produced by improper maintenance, machine settings, and raw material supply. For example, too close a roll setting in a drafting system can lead to fiber breakage, slub creation, and derivative faults such as traveler accumulations of fly. Wear of machine parts occurs after a period of use and this wear frequently leads to the production of defective yarn. An unclean or improperly conditioned atmosphere can cause problems. Failure to remove trash in opening and carding can lead to the production of yarn faults, especially for trash over a certain small size. One can often discriminate between failure of the cleaning machines in the opening line to remove trash and the failure of the card to remove it, by inspecting for trash particles embedded in the card flats. Poor cleaning in the

0

(a)

20 Draft

40

Short Long

0

100 200 300 Relative size of thick spot (b) [(n′ – n) × 100%]/n n ′ = linear density of thick part n = linear density of normal part

Fig. 11.4 Defects in yarns

284


opening line results in significant deposits of trash in the wire of the flats. The production of neps is frequently a significant quality control problem; controls at carding help in this respect.

11.2.4 Staple yarn defects arising from the fiber Defects inherent in the supply of natural fibers arise from the presence of non-fibrous material and defective fibers. These may be categorized as: 1 2 3

For cotton: immature fibers, neps, trash, etc. For wool: grease, suint, vegetable matter, etc. For man-made fibers: concentrations of finish, oligomers, undrawn or improperly drawn segments of fiber, etc.

Other fibers have foreign or unwanted matter in their supply too. These materials adversely affect processing, which, in turn, produces error. But here we consider the direct effects on such properties as dye uptake, fiber reflectance, and appearance of the final product. Differences in fiber color can also produce disturbing effects leading to complaints. The differences can be from batch to batch, there can be differences within a yarn lot, or there can be differences within a package of yarn. These differences are categories of yarn length (which range from a few yards to thousands, or even millions of yards) and the fabric faults they produce fall into the categories of barré and streaks. Figure 11.5 shows two examples of color variation in fiber yellowness. The top diagram shows the variation in card sliver that was made from a single bale of cotton. The inch-to-inch measurements5 show a variation range of about 5%, despite being taken from source that is often treated (wrongly) as invariable. The bottom diagram shows the variation in a sample of commercial combed sliver in which the number of sliver doublings involved was greater than 3000. Despite the doublings, a significant error is visible and the wavelength of the whole error cycle was probably in the hundreds of yards. Such a variation could have led to barré problems in greige fabric. Nep varies from processing stage to processing stage. According to Uster [1], the nep level in the bales (≈ 100 to 900 nep/g) is slightly lower than the level in the fiber approaching the card. This level is then reduced in carding to the range 25–300 nep/g. For the longer cottons used for combed yarns the nep counts in the bales are less and they can be reduced to the range 7–80 nep/g after combing. The neps in this form of measurement are of an absolute minimum size. The actual reductions are determined by the fiber as well as the design, settings, and maintenance of the machines. An average ring spinner produced carded yarn between 6s (100 tex) and 45s (13 tex) with nep counts of 150 and 50 nep/g respectively (AFIS Data [1]). In an Uster tester type of measurement, a nep is defined as a very short fault of more than 200% of the yarn diameter. Consequently, with this type of measurement, the size of the neps recorded, vary with the yarn count. Data from the Uster tester [1] suggest that average ring spinners produced nep levels in the range from 25 nep/km 5 It is not possible to measure such samples on an HVI instrument and therefore these measurements were made on a flat-bed scanner. Adobe ‘Photoshop’ software was used as a photometer applied to color images of sliver, the images were converted to the CMYK mode, and the yellow separation was used. The percentage of yellow represents the color depth and is related to the +b value normally used in textiles.


285

% Yellow

15

10 5 Card sliver from a single bale 0 0

5

10

% Yellow

15 10 5 Commercial combed sliver 0

0

20 40 60 Length along the sliver (yards)

Fig. 11.5

80

Variation in fiber color

(≈ 0.3 nep/g) at about 6s cotton count to 1300 nep/km (≈ 100 nep/g) at 45s cotton count. A 45s combed yarn gives an average nep count of about 12 nep/g (it would be rare indeed to make a 6s combed yarn). Thus it can be seen that care is needed in interpreting results. In rotor spinning, the corresponding AFIS figure is about 200 nep/g irrespective of count. Thus, rotor spinning produces a slightly inferior yarn as far as nep is concerned. This may be due to nep created in the combing roll of the rotor spinning machine. In cotton spinning, neps are often associated with other defects, for example as in Fig. 11.6(a). To emphasize the importance of the quality control regime in a mill, consider Fig. 11.6(b), which shows the results of tests in two separate mills spinning different counts of cotton yarn. A 50s yarn usually shows a different nep count from a 30s, but this is not the issue in this case. Rather, one mill Mill A: y = – 4.14x + 113 Mill B: y = 0.42x + 28

2

y = –0.006x + 1.446x + 10.33 150

r 2 = 0.898 r = correlation coefficient Nep/1000 yd

Nep/1000 yd

150

100

50

100 50s

50 30s

0

0 0

25

50 75 Thick spots/km (a)

100

0

Fig. 11.6

Nep control

10 Months (b)

20

286


had worked to reduce neps and the graph shows the result. The other mill had a reasonably good nep count for the end use and, unfortunately, saw little need to work hard at the problem. The example is intended to show the value of progressively plotting fault production. Some data relating to yarn faults in various types of yarns are shown in Fig. 11.7. The yarns are of average quality. All the faults shown decline in number as the linear density increases (i.e. the count decreases). Worsted yarns show up poorly in thin spots whereas 100% carded cotton yarns show up badly in thick spots. It is, perhaps, not surprising to find the number of thin spots for worsted yarns to be high when it is remembered that wool is more variable in length and fineness than cotton. Also, it is not surprising to find that combed cotton yarns perform well in this respect. It is interesting to see that the nep performance for wool is relatively good, whereas the performance of carded cotton is relatively poor. Presumably this is because cotton is so much finer than wool. It is a little disappointing to see the relatively poor nep performance of air-jet yarns; perhaps this is due to the extra drafting. Trash, dust, and visible foreign matter are progressively reduced by processing and the levels in combed yarn can be reduced to the order of 1% of the values pertaining to the bale material. The best and worst spinners of carded ring spun yarn produce 0.1 and over 10 trash particles/g, respectively, at 50s count; the figures for 6s yarn are approximately 8 and 30, respectively. Thus the quality of the processing is seen to play a very significant part in the quality of the product. The use of waste fiber affects quality. Before regulations restrained yarn makers by making truthful labeling mandatory, more mixed waste was used than now. Thus, we had shoddy in the wool trade, recycling of blend fibers of indeterminate blend ratios, filaments and fibers made from recycled polymers, and so on, but such practices are rarer today. The name ‘shoddy’ is now synonymous with poor quality. Comber noils are sometimes recycled within a mill but it is also a frequent practice to sell the noil to yarn makers who make lower grade products. For these latter people, the noil

Thick spots/km

K1 100

K2 A

10

A

1000

W

K3

K1 K2

100 10

W C2 C1

C1 & C2 1

1

20

20 50 100 200 Yarn linear density, log scale (tex)

K3 100

C1

A C2 K2

10 W 1

50

100

200

W = Worsted K1 = Carded, 50/50 P/C, rotor K2 = Carded, cotton, rotor A = Combed, 50/50 P/C, air-jet K3 = Carded cotton, ring C1 = Combed, 65/35 P/C, ring C2 = Combed cotton, ring

K1

20 50 100 200 Yarn linear density, log scale (tex)

Fig. 11.7

K3

Yarn linear density, log scale (tex)

1000

Nep/km

Thin spots/km

1000

Fault rates for average yarns


287

is a relatively cheap raw material. Undercard and flat waste are sometimes recycled, but much of it is disposed of. Pneumafil waste (see Section 5.11.3) is nearly always recycled within the staple mill. Intense competition and pressure on prices make prudent recycling a necessity. As in Fig. 11.8, the amount of pneumafil generated with cotton yarns is substantial and it rises with count. Recycling has to be carried out with care. In filament/staple processing there is less possibility of recycling the waste internally.

11.2.5 Staple yarn defects arising from processing The analysis in this section will be organized from winding back through the processes until we reach the bale laydown. Many yarn faults are removed during the clearing operation and are replaced by standard knots or splices. Not only does this make it possible for faults to be produced and then be removed without the manager being aware of the fact, but it calls into question whether the yarn should be tested before or after clearing. Normal practice is to test samples after piecing. Study of the piecings from a given operator or machine will quickly show a characteristic appearance. This sometimes enables the source of a problem to be traced. Every time an end breaks, the process has to be restarted by piecing, and these piecings are always imperfect, even if they are acceptable. If the purpose of testing is to detect the source and frequency of fault production, then there might be an incentive to test the yarn before clearing. It would then become necessary to sample the thousands of ring bobbins or equivalent packages entering the winding process. If the purpose of testing is to protect the customer, it is necessary to sample the wound packages destined for the customer. These latter packages contain the contents of the input bobbins minus the portions of yarn removed, but with the added knots or splices. It is not possible to regard these final packages as single specimens; in reality each one is a series of yarns from almost randomly chosen bobbins of yarn. However, with linked winding systems, the order of the component yarns on the cheese or cone is usually the same as that of the rovings on the ring frame, which can be turned to advantage. The amount of testing required to truly sample the material delivered is much higher than normally realized. This is especially so if there are large variances between individual bobbins. Remember that, as the CV reduces, the required sampling frequency is also reduced. The residual fault rate describes the yarn after clearing. It depends not only on the original processing during yarn manufacture but also on the setting of the clearing and splicing mechanisms. Thus, test results may be ambiguous. The levels at which the clearers are set determine the size of defect removed, and the setting of the splicer

Pneumafil (%)

6

y = 0.071x – 0.754

4 2 0 20

Fig. 11.8

40 60 Yarn count Ne

80

Pneumafil production rates

288


determines whether the splice is acceptable or not. The more defects removed, the greater is the chance of an unacceptable yarn. A frequent problem in this latter regard is the production of long fiber tails to the splice. Such tails interfere with the processes that follow and these faults should be avoided. Defects can be caused in roller drafting as well as the normal set of errors already described. Wrong ratch settings, worn guides, eccentric spindles, etc., might cause fiber breakages or other fiber disturbances. Nicked, fouled, or damaged pigtail guides, balloon control rings, travelers, rings, flyer grommets, or the like can also lead to defects in the product. The nicks become more prevalent after certain abrasive manmade fibers have been used on the particular equipment. Often these machine defects cause episodes of hairiness not easy to detect in the yarn but which become very troublesome in later processes. Apart from locally irregular drafting, fiber debris may be discharged into the air. Since the highest draft is at the spinning frame, this is a good place to look for the sources of defects. Concentrations of fly from this or other sources may then become wrapped around the yarn to produce faults. Slubs and fishes are usually created by electrification or by faulty settings of the drafting system. It might be noticed that the defect level is a function of draft, all other things being equal. A badly arranged or maintained traveling cleaner can blow concentrations of fiber onto the yarn, roving or other strand. Raw materials that contain an excess of short fiber might lead to an undesirable discharge of fly into the atmosphere. If the atmosphere is such as to encourage fiber electrification, this fly may concentrate into tiny clumps which deposit on the yarn during manufacture. Alternatively, the fly can be deposited on material being stored in the workplace. Undrawn or married fibers, particles of foreign material, irregular fibers and the like can temporarily interfere with the drafting process and produce slubs. Deposits of fibers on the traveler or elsewhere may cause defects if the deposits are suddenly licked into the product stream. Also they can cause the endsdown rate to increase. A sharp look-out for such accumulations is required. When spinning blends of natural and man-made fibers, there can be accumulations of fiber finish that become particularly troublesome on the balloon control rings, main rings, and travelers. The deposits are often white and powdery, which explains the common description of it as ‘snow’. They might vary in quantity from one fiber maker to another or from one fiber merge to another, and some are very easy to remove by washing, but economics do not allow for a washing operation of machine parts. There is an intermediate case where too light a roll pressure creates a string or a series of defects rather than a classical case of irregularity. The effects of too light a roll pressure can be magnified if the roving is fairly highly twisted and is irregular. The hard ends described earlier can cause outbreaks of such strings of defects. All these various examples have in common an irregularity of occurrence, but an experienced eye can usually recognize the probable source.

11.2.6 Staple yarn defects arising from air conditioning A cause of defects is the entrapment of accumulations of fly on the roving or yarn, either at the ring or roving frame. Spun-in fly can usually be associated with poor cleaning procedures or equipment. Every time an end breaks in a roving frame, the broken end lashes the neighboring machine parts and creates a ‘snow storm’ until the frame can stop. Inertia of the machine prevents an immediate cessation of motion.


289

Therefore, even if the machine is switched off immediately after the break, it continues to broadcast fiber until it stops. There is a ‘run-down’ time. Apart from this, there is a steady discharge of fibers from the roving and drawframes as they work normally. This discharge often results in the creation of very loose fluffy rolls of fiber on the floor of the mill; these often escape the patrolling cleaners, some parts of them becoming airborne again and migrating to other areas. Also, concentrations of fiber finish that accumulate on machine parts can cause irregular faults. If any of the accumulations fall into crucial operating zones of the machines, faults are created. In ring spinning, the traveler sometimes pushes trapped fly into ball-like shapes and this type of defect can sometimes be associated with a high incidence of traveler fouling. It is vitally important to keep the critical areas free from lint accumulations, which is no easy matter. For example, a small fraction of a percent of the fiber passing through a frame is liberated into the air to form fly. This may not sound much, but in fact it represents hundreds of pounds of fly being deposited every day. If the deposition rate is 0.02% of the fiber flow in a mill processing 2000 lb/hr, the deposition rate is over 60 lb of fly/week. If you do not believe it, fix a wire mesh in front of (say) one section of rings and you will find that, in an hour, several milligrams of dirty fiber will be collected. At the ring rail level, the fibers will be on the side of the mesh nearest the rings. At the level of the tops of the bobbins, the deposits will be on the outside. The rotating bobbins act as a pump that sucks air from the room into the balloon space. If the air is dirty (it usually is), fibers and contaminants are pulled over the freshly made yarn being wound onto the bobbin. Adequate cleaning is vital and this involves the use of traveling cleaners as described earlier. The main air conditioning should also have adequate cleaning capabilities. Management must also consider the sources of lint, because it is better to eliminate as many of the emissions as possible rather than clean them up afterwards. On the supervisor’s tour, a sharp look-out for accumulations of fly around light fittings, ceilings, and roof fittings is very helpful in this respect. A study of the airstreams within a mill can help to determine if fly is being transferred from a seemingly non-critical area.

11.2.7 Defects in fabric The ultimate product is usually in fabric form, and a judgment of quality is normally made on the number of defects per square yard. Frequently, demerit points are assigned according to the length, diameter, and type of defect. For that reason, many spinners knit samples of yarn to test for defects and barré. In weaving, there is a requirement for adequate yarn strength and a lack of any defects that would cause end-breaks in beaming or weaving. For example, consider a warp beam with 3000 ends of yarn that has a defect rate of ‘only’ 10 per 1000 yards. On average, there will be 3000 × 10/1000 = 30 faults per yard of warp. This is unacceptable. The faults can give problems in at least three ways: (a) the end-breakage rate in beaming and weaving is increased, (b) the fabric is degraded because of the yarn faults, and (c) the fabric is further degraded because of the weaving faults due to the increased breakage mentioned in (a). Figure 11.9 shows measurements of yarn strength made just after spinning and it will be seen that the strength is not constant. As mentioned elsewhere, end-breaks in spinning occur when the yarn tension exceeds the strength of a weak link in the yarn in the process line. A similar philosophy applies to weaving but now the weak spots concerned are in the wound yarn. Some faults have been removed but the yarn might have been strained by high tensions or damaged, and a new crop of weak spots


Yarn strength

290

x

x The line xx represents the applied load. The portions of the yarn strength curve shown in black represent potential yarn breakages. Time

Fig. 11.9 Variation in yarn strength

might now exist. Straining the yarn reduces the work needed for rupture and the ability to weave well. Thus, the picture given by yarn testing may well be too optimistic and the weaving performance might be worse than predicted. To demonstrate the impact of yarn faults, let us consider a square weave fabric made from the same yarn for both warp and filling. If the yarn diameter is proportional to (1/√Ne), where Ne is the yarn count, then the end or pick density is (k√Ne)/CF per unit length. The factor k includes not only the constant relating yarn diameter to (1/√Ne) but also takes into account the weaving crimp: CF is the cover factor, which is defined as (area covered by one or more yarns)/(area of the fabric). The length of yarn (L) in a square yard of fabric is: L ≈ [# ends/yd + # picks/yd) L ≈ (2k/√Ne)(CF)

[11.1]

If the defect frequency is f defects/yard, the average number of defects/square yard is Lf. If the fabric has a different construction (say, woven or knitted), we may replace 2k/CF by a different factor (K), which takes into account the difference in structure. Number of defects/sq yd = Kf √Ne

[11.2]

In ring spinning, the fault rate increases with draft, as shown earlier in Fig. 11.4; the major draft occurs in spinning and the largest number of defects arise there. The number of defects can, in practice, be related to the yarn count and the quality of the spinning operation. The cover factor does not vary greatly within a given class of fabric; therefore, fabric defect rates for a given class of fabric are affected mostly by changes in √Ne. 11.2.8 Testing for yarn defects Usually, yarn defects are classified by length, local linear density, and fault frequency. A typical distribution is shown in Fig. 11.4(b). It will be seen that short defects are more common than long ones. Generally, a series of combinations of length and linear density of the faults is used to determine whether the fault should be removed. The choice of these combinations depends on the market involved and the experience of the user. The amount of yarn that must be tested depends on the fault frequency. With a fixed length of yarn (say 100 000 yd), there are two opposite dangers. On the one hand there may be costly over-testing of a yarn, but on the other hand there may be a risk of having tested an unrepresentative sample. In consequence of the need for long samples, there is considerable incentive to use online monitoring.


291

11.2.9 Monitoring An example of the interplay between economics and technical advancement is the use of online monitoring at the ring frame. Because of the low output of a ring spindle, the amount of capital expense that can be justified is limited. Monitoring every spindle means that thousands of measurement points are involved and the provision of a transducer of any great complexity would be too expensive. A solution is to have a patrolling sensor that detects the presence or absence of the thread leaving the drafting system. One such device is carried by a patrolling cleaner and another uses a transport system mounted on the ring rail. The direct advantage is that those end-breaks are signaled immediately they occur and this permits closer management of the repairs. An indirect advantage is that this allows analysis of end-break patterns and becomes a means to study regional effects of the air conditioning, pneumafil settings, and machine maintenance. The system can have a beneficial impact on the processing costs and product quality but the capital cost is still high and the advantages have to be weighed carefully against the costs of installation. Another interesting development is the roving stop mechanism. The idea is to clamp the roving as it enters the drafting system when the end breaks. This reduces the amount of pneumafil greatly. When the end is repaired, the whole drafting system for the particular spindle has to be rethreaded. This can be automated. It has been seen to be very effective from a quality point of view and the cost can be moderate. Adaptations have been made to use the electrical signals that operate the clamps for spindle monitoring. Monitoring includes searching for defects. For example, it is common in rotor spinning and filament production systems to use online recording of the passage of defects in the flowing material. This is especially important because the product is rarely rewound before use in manufacturing the fabric. Such strategies move the testing from the laboratories to the production machines themselves.

11.3

Yarn evenness

11.3.1 General Yarn evenness is made up of several components. Repeating what has been said elsewhere, errors come from mechanical problems or from fiber flow problems. Either may produce short- or long-term errors. 11.3.2 Harmonic irregularity due to mechanical faults The class of mechanically caused errors consists of wholly repetitive variations such as those caused by roll defects. Such errors are harmonic and the repeat period may be expressed as error wavelength. A harmonic error produces a series of single ordinates on a spectrogram, and the highest of these lines is usually the fundamental wavelength. If a delivery roll is purely eccentric without any other error, the fundamental error wavelength in the output strand is the circumference of the front roll. A roll with a flat on it produces a fundamental wavelength and a number of harmonics (at 1/2, 1/3, 1/4, etc., of the fundamental wavelength). The defect is often caused by leaving the load on the rolls when the machine is not running. Occurrences of these harmonics in the test data should raise questions about the work practices in the mill. It is relatively simple to estimate the source of each spike. The spectrum of mechanical

292


Roll separation (arbitrary units)

peaks shown in Fig. 11.10(a) was made by recording the changing separation of the centers of the front rolls of a drawframe [8]. No textile material was passing through the frame at the time. The technique exposed the mechanical errors more precisely than testing the sliver; furthermore, the use of an encoder made possible exact correlations between the error and the result. The spikes at W, X, and Z were from the top front roll, W and X representing the second and third harmonics, respectively. (The spike Y came from another source and need not be discussed here.) On a spectrogram of sliver from a drawframe, the mechanical errors also produce spikes but it is unavoidable that there is some fiber-related error as well. A single spectrogram is often not very useful in determining mechanical errors because of the profusion of random spikes from non-mechanical sources. If spectrograms are taken at precise time intervals and they are assembled as a three-dimensional graph, then if the spikes persist throughout the time dimension of the graph then the cause is mechanical and they represent a truly harmonic variation. Random spikes can be separated from the harmonic ones in this way [2,9]. A faulty element in the drafting system causes organized ‘spikes’ to appear, as shown in Fig. 11.10(b). This example is an especially bad case, included merely to emphasize the difference between the two types of error.

Z

X

W

Y

1.66 2.5 5 Error wavelength (inches) (a) Mechanical errors Mechanical errors in spectrograms

Amplitude

Non-mechanical errors

m Ti

e

Error wavelength (b) 0.5 in

3 in

11 in

Fiber-related peaks A

B

C

1 yd Worsted Carded cotton Cut-staple MMF Continuous filament

(c) Fiber-related errors

Fig. 11.10

Three views of error spectra

10 yd


293

Fiber-related errors are more difficult to diagnose. The sources of error are many, and they are dispersed. They range from (a) variations in the fiber selected, through (b) the maintenance and setting of the machines, to (c) the environment in which the operation occurs. A starting point is the theoretical random error that is dependent on the number of fibers in the cross-section; consequently there are different values to be expected from sliver, roving, or yarn. The evenness also depends on the fiber length and its variability. Thus, there is a spectrum of values according to the strand involved, as indicated in Fig. 11.11(a). A yarn contains errors from spinning, roving, drawing, carding, and other processes. The error wavelength carried forward to the yarn gets ever longer as the initial generating point is positioned nearer the beginning of the process line. Also performance of one machine in a serial line of processes

Yarn 1

Short-term CV (%)

20

Roving

Sliver

2

10 4

3 2 4

5 1 2 3 4

= = = =

Worsted Carded cotton 1.5 den 1.5 in polyester combed cotton

2 4 Theoretical

1 10

100 Linear density (tex) (a)

CV of yarn mass (%)

25

95% USP 50% USP

20 15

1000

5% USP

Carded

10

Combed USP = Position in the Uster statistics percentile (USP) rankings = Results from a series of mills (combed yarns)

5

CV of yarn mass (%)

100 10 Linear density of ring yarns (tex) (b) 20 Actual 15 Theoretical 10 1940

1960

1980

2000

Year (c)

Fig. 11.11

Three views of strand variations (CV)

294


affects the following machines. A simple calculation can show that errors due to carding produce yarn errors in the order of a million yards. Errors from sources earlier in the process line produce even longer errors in the yarn. At the other end of the scale, the delivery rolls of a cotton ring spinning frame produce errors of only 4 or 5 inches. Fortunately, the errors produced from the early stages are smaller than those from the later ones. Mechanical errors from the ring frame produce short, highly organized errors that produce moiré. Errors from the early process stages produce barré in the fabric. The very long error wavelengths arising from the early processes produce bobbins that contain yarns that vary in average count. The spinning bobbins can become mixed between spinning, winding, and shipping. Consequently, the fabric making equipment often has to deal with random step changes in yarn count that show up as barré. Controlling the whole line is a technological art based on experience as much as on science and technology.

11.3.3 Irregularity in linear density due to staple fiber variations and their interactions with the machinery Figure 11.11(a) shows some values of CV for various yarns and intermediate products and it might be noted that an overall regression would not be parallel to the theoretical line. Diagram (b) shows some carded ring yarn evenness values as judged on a worldwide basis in the late 1990s. The values are for the error wavelength range 0.2 inch to 30 yd. Even the best yarns show CV values that range from 16% at 50s count to about 10% at low counts. Longer-term variations, represented by the bobbin-tobobbin values, range from about 1% CV for the best spinners to a value for the worst spinners that varies between about 3% and 5% at the counts mentioned above. These long-term variations arise in the preparation. Fiber-related errors in the last draft zone show up as distributions of variance on a spectrogram that produce so called ‘hills’. They encompass a range of wavelengths between one and ten fiber lengths, with the crest of the most common hill being located between two and three fiber lengths. Wavelengths shorter than about half a fiber length are unreliable and are not used. As with mechanical errors, the fiberrelated errors from previous drafting zones are elongated and the new crop of errors is added. This means that there is a hill from the front draft zone, and sometimes a less prominent one from the back zone. To get sufficient data to produce a spectrogram requires the testing of a sufficiently long length of the strand concerned. This strand then becomes waste fiber. Consequently it is not feasible to test for the errors from the earlier processes as well as those from the current one with the spectrograph. Materials from these earlier processes are tested immediately after the particular process concerned. Remember that the use of CVs does not discriminate between the sources of the errors concerned, and an alternative method is needed. One way to minimize the drafting errors is to (a) test the strand emerging from the subject machine for evenness, (b) adjust the roll setting by one increment, (c) retest the strand and see whether or not the amplitude of the crest of the hill has changed, (d) readjust the setting based on the difference in amplitude, (e) retest, and so on until the minimum value is found. By using the crests of the hills, one avoids the interference from other errors. The ‘hill’ discussed is not always very smooth and there are sometimes spikes that have little to do with mechanical errors. One way of resolving this is to take a sequence of spectrograms and array them. False spikes, unrelated to any mechanical error, will


295

stand out from the array, while the truly harmonic ones ‘march’ across the array in a steady straight line. Experience over many years has made possible the production of the Uster statistics, which give a good guide for judging performance. An example is given earlier, in Fig. 11.10(b), where the shaded lines represent the different percentiles of evenness from spinners throughout the world. Somewhat similar but more comprehensive diagrams published by Uster Corporation are very useful for normalizing data to compare plants on a meaningful basis. Rather than refer to a particular CV at a certain count, it is better to refer to the position in the Uster statistics percentile rankings (USP). For example, a mill spinning 30s then can be compared to another spinning 10s with reasonable results. Of course, differences in fiber, machine maintenance or operational difficulties intervene, but a normalized comparison is likely to expose difficulties that otherwise could be missed. A group of mills can accumulate their own data and pursue a similar strategy. There is a frequent practice of testing competitors’ products to get a basis of comparison but care has to be taken to preserve an ethical stance on such testing. Figure 11.11(b) shows some mill data of the early 1990s compared to the 1997 Uster statistics [1]. The particular individual mills belonged to a commercial group and their results then fell in the 50 to 60 percentile range (the lower the percentile, the better). However, mill data within any manufacturing group commonly vary less than with international experience because of differences in the range of skills and equipment used. Nevertheless, experience has shown it to be a powerful managerial tool because, once norms are established for the group, logical comparisons can be made despite differences in product and equipment. A similar procedure can be used for the Uster data regarding thick and thin spots as well as neps. Over the years, there has been steady improvement in the spinning equipment, the methods of testing, and the application of the technology. The data given in the foregoing are affected by this and some idea of the change can be obtained from Fig. 11.11(c). For example, the CV of mass for a 60s combed cotton yarn has declined over a half century. Thus care has to be taken to keep the standards up to date; the values steadily approach the minimum theoretical values. The same is true of other fibers and preparations. This is one of the reasons why defects have assumed such relative importance.

11.3.4 Irregularity of yarn hairiness Hairiness of yarn is a factor in the appearance of fabrics, and variations in hairiness can produce optical effects that show up as streaks or bars or other visual disturbances. The Uster hairiness value, H, represents the total length of all the hairs protruding from the yarn, in cm, with reference to a sensing length of 1 cm. It is measured using infra red light to avoid color problems [9]. In an average carded cotton ring yarn on the bobbin, H varies from about 10 for 6s (≈ 100 tex) yarn to about 4.5 for a 50s (≈ 12 tex) yarn [1]. Subsequent winding and unwinding changes these data dependent on the conditions prevailing at the two operations. The variation between the best and the worst yarns is about 30%. The values quoted are averaged over a substantial length of yarn and there are significant short-term variations. Uster report withinbobbin variations which are in a range that can be expressed as 18 to 28% CV. The between-bobbin values vary between 1.2 and 10%.

296


11.3.5 Irregularity of yarn strength Yarn strength for a staple yarn varies from about 10 cN/tex for the weakest yarns to 30 cN/tex for the best ones. A graph of the tenacities of various sorts of yarn is given in Fig. 11.12 and it will be seen that rotor yarn has lower values than ring yarn. The values for long-staple cottons for very fine yarns (not shown in Fig. 11.12) yield up to 30 cN/tex tenacities. At the bottom end of the scale, poor worsted yarns have tenacities around 7 cN/tex, poor rotor yarns produce about 10 cN/tex, and poor combed 65/35 P/C ring yarns produce about 19 cN/tex. Tenacities appear higher when tested at 400 m/min. Figure 11.13 shows the CV of tenacity for various yarns. The best cotton yarns vary from about 4% for 6s carded ring yarn to about 9% at 50s count (≈ 12 tex). The worst yarns have CVs of the order of 10%. Combed cotton yarns have CVs varying from 4.5% at 15s count (≈ 40 tex) to 12% at 120s count for the best yarns. The worst yarns vary from 8.5% CV to 15% CV for corresponding counts (≈ 5 tex) and worsted yarns have high CVs of tenacity. It should be noted that the CVs of tenacity measured at higher testing speeds tend to be a little higher than those quoted.

11.3.6 Faults in fabrics Coefficients of variation of the yarns do not tell the whole story; acceptability is conditioned by the wavelengths at which the errors occur. Spectrograms, such as Tenacity, log scale (cN/tex) CV of tenacity, log scale (%)

Combed cotton, ring Carded cotton, ring

20

0 P/C, air-jet Carded 50/5 10

20

50

Carded cotton, rotor

100

200

Yarn linear density, log scale (tex)

Comparison of yarn tenacities (best 5% of yarns tested at 5m /min) 20

Worsted

10

Carded cotton, rotor 50/50 P/C, rotor 50/50 P/C, air-jet

Carded cotton, ring

5 Combed cotton, ring 20

Fig. 11.13

Carded 50/50 P/C, rotor

Worsted 5

Fig. 11.12

Combed 65/35 P/C, ring

30

Combed 65/35 P/C, ring

50 100 Yarn linear density, log scale (tex)

200

Average CVs of tenacity of some staple yarns (tested at 5 m/min)


297

shown earlier in Fig. 11.10(c), show that different fibers produce the majority of their short-term errors at different wavelengths [9]. The area under the curves is a function of the variance in the yarn; the pattern of peaks is as important as the magnitude of the variation. The pattern affects what is seen in the fabric; the CVs of mass give only an overall value, with no wavelength component [10]. Another way of illustrating the same point is to observe the fabrics. Knitted fabrics are sensitive to yarn errors. A selection of photographs of knitted fabric appearances and their accompanying spectrograms is shown in Fig. 11.14. In the space available, only a taste of the subject can be given, but there is some value in discussing the types of error shown. Picture (a) is of fabric made with normal yarn and serves as a reference. Picture (b) shows the effect of poor fiber control in the front drafting zone of a ring frame – the mottled look will be noted – and picture (c) has errors produced in both front and rear drafting zones. The long waves from the rear zones produce the wood-grain effect. All yarns were 20s cotton. Experience has shown that improvement of short-term error without a corresponding improvement in long-term error leads to the production of fabrics of unacceptable visual character.

(a)

(b)

(c)

Fig. 11.14

Fabric faults arising from yarn errors

298


11.4

End-breaks and quality

Mean end-break duration (min)

40

20

10 10

15

Effect of traveler wear

10 5 0

60 40

90% 50%

20 10% 10 10 20 40 100 Yarn count, log scale (Ne)

30 50 100 Yarn count, log scale (Ne) (a)

200 400 600 Traveler use (hr) (c)

Fig. 11.15

800

(b)

Ends down/100 spindle hr

Ends down/100 spindle hr

End-breaks/1000 spindle hr

11.4.1 End-breaks in spinning The topic of end-breaks has already been mentioned. From an operational point of view, the end-breakage rate is a symptom of how well a plant is running. A high endbreakage rate points to a combination of machine, material, and human faults. If (a) the travelers need changing, machine maintenance is in arrears, or the machine is otherwise defective, or (b) the roving is bad, or there is a poor choice of fiber, or (c) the operators are not performing well, then a high end-breakage rate will result. If the training is poor, the assignment is too large, etc., then poor results should again be expected. The design of the machine is also a factor and so is the use of monitoring (human or otherwise). When all these factors are controlled, the author’s experience suggests that the end-breakage rate is then quite a strong function of yarn count (Fig. 11.15(a)). Some authorities have different experience and their rate is less variable with count because their range of expertise and equipment is much wider than in the case quoted. The mean duration between end-breaks is shown in diagram (b); three lines are shown and one might say that the lines represent the best, the average, and the worst spinners in the world. As has been mentioned elsewhere, the end-break duration is a matter of assignment and operator training. It also depends on the duties assigned to the operator other than piecing-up. It is fairly obvious that the end-break rate is a function of spindle speed but it is less obvious that it is a function of how long the traveler has been in service. The rate increases similarly to the case shown in diagram (c). Replacing travelers more frequently adds costs to production but it is offset by the reduction in costs from lower end-breakage rates. The ends down rate also is very strongly affected by the number of yarn defects/unit mass (usually compared on the basis of defects/bobbin) as shown in Fig. 11.15(d).

40

y = 10.8x + 11.33

30 20 10 0

1 2 Defect/bobbin (d)

End-breakage in ring spinning

3


299

One inexpensive and effective way of detecting the places where excessive endbreaks occur is for the supervisor to tour the ring room looking for deformed bobbins. When an end breaks, the building mechanism continues to work. The result is that, when an end is repaired after a delay (which is normal), the package looks like that shown in Fig. 11.16(b) instead of Fig. 11.16(a). The longer the delay, the greater is the deformity. Greatly deformed bobbins mean that (a) the operator assignment is too high, (b) the operator is poorly trained, or (c) the operator is inefficient. A high frequency of widely scattered deformed bobbins means (a) the machine maintenance is substandard, (b) the settings are wrong, (c) the sliver or roving supplied is substandard, or (d) the operator assignment, training and efficiencies have to be reviewed. Such tours should be frequent because it is only possible to make judgments on frames where the bobbins are built sufficiently to make the deformities evident. Thus, a single tour misses the frames where a doffing has occurred fairly recently. The tour can also be used to check the spindles. If viewed with a simple flashlight, the balloon can be seen easily. If a bobbin is out-of-plumb, it is quite possible that the upper tip of the bobbin will touch the yarn in the balloon when the rail is in a low position. Also, if a wrongly sized traveler is used, the same thing might happen. Such interference causes intermittent hairiness in the yarn which, in turn, leads to hairiness, moiré, and barré in the fabric. In addition, the use of too light a traveler can produce temporary balloon collapses which are likely to cause end-breaks. The collapse can be seen with the flashlight. The phenomena just described are seen most at the low rail position, soon after doffing. A localized pattern of end-breaks is often found near doorways,

(a)

Fig. 11.16

(b)

Normal and deformed ring bobbins

300


local heat sources, or in a group of machines supplied with substandard sliver or roving. An accurate way of accomplishing the objective just discussed is to use a monitoring system, but this is a somewhat expensive option. For ring spinning, one has to weigh the advantages and disadvantages of monitoring. For more highly productive machines, the advantages usually outweigh the disadvantages. Thus rotor spinning, air-jet spinning, and many processes associated with the production of filament yarns are likely to be fitted with a defect monitoring system.

11.4.2 Quality control and economics As an example of the rate of production of faults, if production of a 24s yarn is 1000 lb/hr and the spinning fault level is only 1 in 106 yards, then 24 × 840 × 1000/106 = 20.16 spinning faults are still produced per hour. (See Q1 Appendix 2 for the length of yarn in 1 pound.) If the fault rate is worse than this, the burden increases proportionately. High fault rates cause losses in efficiency of spinning, winding, twisting, beaming, and weaving. They also degrade the product. The degradation increases the complaint level, reduces the price that can be demanded, and reduces the volume of sales. As mentioned elsewhere, many of the spinning faults are caused by poor preparation. It is hoped that the foregoing illustrates that the decision of how and where the yarn or the preparation should be sampled is not a simple one. To be economic and effective, it is essential that the multitude of spindles in a ring spinning mill should be properly monitored. Also, it is of similar importance to monitor the earlier stages of production properly, particularly regarding those errors and faults that lead to later trouble. Frequently, it is necessary to backtrack to find the cause of error. This requires that the product be properly labeled. Testing is only effective when it is accompanied by good organization and interpretation. Similar thoughts apply to the production of textured and filament yarns. Many of the problems in the final product have their origins in the early stages of yarn production.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Färber, C and Furter, R. Uster Statistics No 40, Zellweger Uster, Uster, Switzerland, May 1997. ASTM D2253 Measurement of Cotton Color with the Nickerson-Hunter Colorimeter, Textiles, ASTM, Philadelphia, Annually. Lord, P R. Unpublished data from the author’s private records. Anderson, S L. Textile Fibres: Testing and Quality Control, Manual of Textile Technology, Textile Institute, Manchester, UK, 1983. Hattenschweiler, P and Bühler, M. Uster Bulletin No 21, Zellweger Uster, Uster, Switzerland, Nov 1973. Thomason, W A. A New Era in Quality Control – Yarn Fault Management, Uster News Bulletin No 18, Uster Corp, Charlotte, NC, Jan 1971. Douglas, K. The Uster Automatic Electronic Yarn Clearing Installation, Uster Bulletin No 22, Uster Corp, Charlotte, NC, USA, 1974. Lord, P R and Grover, G. Roller drafting, Text Prog, 23, 4, 1993. Douglas, K. Uster Bulletin No 35, Uster Corp, Charlotte, NC, USA, Oct 1988. Douglas, K. Uster Bulletin No 15, Uster Corp, Charlotte, NC, USA, Jan 1971.

12 Economics of staple yarn production

12.1

Yarn economics

12.1.1 Cost and price At the risk of stating the obvious, a distinction between cost and price must be drawn. Costs are incurred by the producer and the price is what is asked upon selling the product. Under non-monopoly conditions, price is determined by availability of the product already in the market and the quality of the product being offered for sale. The difference between price and cost is profit (or loss). The cost of a yarn may be divided into several categories. More than half is from the cost of fiber, part represents sales costs, and the rest comes from costs associated with the conversion of fiber (in the bale form) to yarn (wound on cheeses or cones). Buying fiber is a very important factor, not only in controlling costs, but also in determining mill performance. Buying involves specialized skills that require an ability to assess value in the raw material. A knowledge is required of what is needed to produce a yarn satisfactory to the customer and what is needed to give acceptable performance in the mill.

12.1.2 Conversion costs Conversion costs are a major component of the total. Evaluation of conversion costs involves a knowledge, not only of how the fibers interact with the machinery, but also of the commercial importance of yarn quality and cost. Major subdivisions of the conversion costs are (a) direct labor costs, (b) overhead costs, (c) capital costs of machines, (d) space costs, and (e) power costs. An assessment of cost proportions in 1995, calculated from ITMF data, indicated that, for a 30/1 combed cotton rotor yarn produced in the USA, 52% of the costs were attributable to the cost of fiber. Some conversion costs are shown in Table 12.1. The extra combing process (which is not universally used) affects the issue. Nevertheless, it is clear that labor costs in rotor spinning no longer dominate the conversion cost structure. This is different from previous decades when the labor cost dominated. Despite this, the 1990s saw a surge in ring spun cotton products for apparel coming from the Far East. In the USA, rotor

302

Handbook of yarn production Table 12.1

Conversion costs for 30/1 combed cotton rotor yarn in 1995

Labor Depreciation Interest Waste Energy Aux materials

10.4% 41.5% 18.5% 15.5% 8.3% 6.0%

spun cotton yarn has become common for denim products and ring spinning has become increasingly confined to underwear products; most other apparel products are imported and almost all of these are ring spun. A basis for estimating the current yarn market is provided by the sampling used by Uster Corporation for their statistics [1] and the result is shown in Table 12.2. Notable facts are that (a) the USA and Europe are prime users of rotor spinning, (b) the USA now has a diminished ring spinning capacity, (c) Asia is a strong supplier of ring spun yarns, and (d) Europe is the primary manufacturer of worsted yarns. Some private data are given in Table 12.3 and other private data show that the cost of fiber in some more recent operations in advanced regions of the world can reach 70% of the total. Operating expenses in more recent operations are generally closely guarded secrets and it would be improper to disclose them here. The relative reductions of labor costs have a strong influence on the conditions of international trade but this has been offset by the effect of international trade agreements. The above data may be compared to the figures presented by Thompson [2] in 1982. He showed that in North America the breakdown for a 30/1 yarn was 26.4% labor, 43.9% capital (i.e. depreciation), 10.6% for energy, and 18.8% for space. These may be compared to 9.3%, 60.7%, 19.1%, and 10.9%, respectively, for the Far East. The total cost for Far Estimated yarn supply percentages in 1997

Table 12.2

Cotton Cotton Cotton 65/35 P/C 50/50 P/C Polyester Worsted

Ring Ring Rotor Ring Rotor Ring Ring

Table 12.3

Labor Overhead Depreciation Energy Totals

Carded Combed Carded Combed Carded

W Europe North Asia America

South Africa, E Europe Total America Middle East

29 47 35 3 25 38 70

25 7 11 8 – 11 15

3 3 25 3 68 11 –

15 29 13 39 – 25 –

24 10 12 34 2 4 8

4 4 4 13 5 11 7

100 100 100 100 100 100 100

Percentage conversion costs for 36/1 combed cotton ring yarn

Prep

1978 Spin

Wind

Prep

1988 Spin

Wind

43.3 26.1 15.2 15.4 100

52.9 21.9 10.9 14.3 100

52.2 22.4 13.4 11.9 100

37.0 23.9 23.6 15.4 100

39.0 21.7 18.6 20.6 100

43.8 27.1 18.7 10.3 100

Economics of staple yarn production

303

Eastern countries was 72% of the North American cost. European total costs were said to be similar to the North American ones. The reason for the move of yarn manufacturing centers to the Far East is quite clear, as are the inequalities in capital investment, energy, and labor costs. It also helps explain the strong showing of rotor spinning in the USA, where the increased investment has led to substantial labor cost reductions, which have helped maintain viability. In ring spinning, labor costs are higher. A private analysis (relating to a different European area) yielded proportions similar to those given in Table 12.3. Accounting procedures, the count and the proportions of labor cost/hr, power costs, etc., differed in this case as compared to the data cited by Thompson. However, the higher labor costs, which still dominate in the ring spinning of medium to fine yarns, are quite apparent. In the decade illustrated, labor costs for the cases cited were reduced by about 10% by investing in new and more productive machinery. The corollary to this was the rise of more than 7% in depreciation costs which was associated with the increased investment. Energy consumption in some mills has almost doubled over the last ten years.

12.2

Productivity

12.2.1 Normalized productivity One measure of mill efficiency is the number of operator hours needed for a given task. This is a good measure because it enables comparison between mills running in various environments; it is not affected by the wage rate and it can be expressed in different ways. One way is to measure the number of operator hours to produce 100 lb of yarn. This form of normalization is represented by the acronym OHP. In the metric system, the unit of mass is 100 kg and the applicable acronym is HOK; the values for HOK are 2.2 times larger than for OHP. Basically, the units are expressed in operator hours/100 lb or kg.

12.2.2 Historical changes in operator productivity Technologies continue to change and enable mills to run with decreasing amounts of labor. However, as the labor costs are reduced by applying new technology, the competitive advantages between high and low labor cost regions diminish. Over the last half century, the OHP level has decreased to less than 10% of what it was. This is, in large part, because of the progressive introduction of various schemes of shortened process lines and automation. Consider some practical data, which make the point. Each year sees a development in technology that further reduces the values, as shown in Fig. 12.1. The exponent of the regression curve (–0.074) suggests that the improvement each year has averaged nearly 15%.1 The equation is exponential, which implies that the rate of improvement will steadily decline as the labor use factor approaches some steady value asymptotically. This estimate relates solely to one group and others may differ considerably, but the trend is common wherever considerable investment has been made. Suffice it to say that there is abundant evidence to show that labor productivity varies with time, as equipment designs evolve. 1 10 –0.074 = 0.843, thus the value of OHP is, on average, 0.843 that of the previous year.

304


6

30/1 cotton

OHP

4

2

y = 6.454 × 10–0.074x x = year – 1980 0 1980

1985

1990

1995

Year

Fig. 12.1 Historical decline in OHP

12.2.3 Division of operator productivity within ring spinning The values of OHP or HOK can be broken down by components. In rotor spinning, there are no separate winding costs as there are with ring spinning, and preparation costs are lower because of the omission of the roving stage; however, finer slivers are needed. Thus, labor costs are lower for rotor spinning than for ring spinning in the viable range of counts (Fig. 12.2(a) shows some old data). In short-staple ring spinning of fine yarns, preparation (all processes up to and including roving) takes up to 5% of the total, winding takes up to 4%, as do the combined overhead costs of management, maintenance, and general mill expenses. The rest is for spinning. For yarns towards the coarse end of the count spectrum, preparation costs are a much larger proportion of the total. Of that, a significant part is labor cost. When expressed in terms of OHP, the example shown in Fig. 12.2(b) shows the labor needed for spinning rises rapidly with count as compared to that needed for preparation and winding. Stryckman [3] and others have developed a series of HOK or OHP curves for various fibers and spinning systems. The types of machinery in use, the preparation, and the fiber lengths and types determine the equations. The data are good only for the time at which they were produced and for the equipment then in use. Changes are to be expected in future years. For fine counts of ring yarn, the OHP is almost directly proportional to count, as shown in Fig. 12.2(c) (each point represents a single mill). Regressions for the data are given. Data for other years have been plotted and, as expected, the results suggest that the values are time dependent. The linear relationship seems to hold up well for cotton counts above 36s. Linear regressions are given. In the five year interval shown in Fig. 12.2(c), the OHP for a 40s combed yarn appears to decrease from 2.27 to 1.66, which suggests a reduction of just over 5% per annum. On a worldwide basis, the OHP levels off for coarse carded counts, and it is much more variable from plant to plant. In the past, regions of lower wages produced less efficiently than more advanced regions and, on a worldwide basis, performance was represented as a fairly wide band rather than a single line. With wider markets and competition, the width of that band seems to be narrowing. The balance of technology has changed and many regions of lower wages now have modern machinery. Some idea of the relative magnitudes and the relationships to count is shown in

Economics of staple yarn production Preparation Ring spinning Winding

3

OHP

10

Ring

HOK, log scale

OHP, log scale

100

10

4

100

y = 3.646 × 100.010x Ring y = 1.817 × 100.017x Rotor

305

2

1

Rotor

1990

1983 0 10

20 30 40 Yarn count (Ne) (a) 5

50

10

20

30 40 50 Yarn count (Ne) (b)

60

70

1985 y = 0.080x – 0.934 1990 y = 0.077x – 1.425

OHP

4 3 2 1 0 10

Fig. 12.2

Ring spinning Each point represents 1 mill 20

30 40 50 60 70 Yarn count (Ne) (c)

80

Normalized operator productivity

Fig. 12.2(b). It will be noted that spinning increasingly becomes dominant as the count rises above 30s cotton. The balance between these is subject to changes as automation is progressively brought to bear on the labor costs in those technical areas. For coarse count yarns, materials handling costs, which account for much of the labor charges in preparation, assume great importance. It is also interesting that winding costs become progressively more important at the lower counts. At one end of the count spectrum, quality and piecing costs are of great concern, whereas at the other end, materials handling becomes a dominant problem.

12.2.4 Operator productivity in rotor spinning With modern industrial rotor spinning, the labor costs are about 50% of those for ring spinning. A rotor machine is often fed with sliver produced by an abbreviated preparation line; sometimes only one passage of drawing intervenes between the card and the rotor spinning machines. This reduces costs. With modern rotor spinning machines, automatic piecing is a necessity because human operators are not capable of carrying out the operation reliably at the rotor speeds now possible. As with ring spinning, automatic doffing is standard. Consequently, jobs that were once performed by human

306


operators are now done by machine. Also, the productivity of the rotor machines on a count-for-count basis is over five times that of a ring frame. Obviously rotor spinning falls into a different category, as do most of more advanced forms of spinning. Typical values of the ratios of OHP/count are 0.025 for rotor spinning, as compared to 0.05 for ring spinning.

12.2.5 Operator assignment Another way of judging mill efficiency is by the operator assignments. An operator of a ring spinning machine might tend many hundreds of spindles [4] and that would be his/her assignment. Assignments have increased appreciably over the years. There are some interesting consequences arising from the large number of items being controlled by a single person. Inevitably, some ends break whilst the operator is patrolling the spindle set. The cost of this operator has to be set against the savings he or she can make by repairing the broken ends. The cost of the operator depends on the wage rate but the assignment depends on the economic balance at which minimum cost, or acceptable quality, is achieved. The literature over the last half century was searched to find how the assignments and wages had changed and the results are plotted in Fig. 12.3(a); regression curves shows the differences between two common yarn types. Differences are to be expected because cotton is a natural fiber and more variable than polyester. Again, care has to be taken with comparisons, especially since the operator often has duties other than piecing.

12.3

Quality and economics

12.3.1 Yarn quality and operator assignment Figure 12.3(a) compares spindle assignments across the world. One can note that, in 1980, the higher the wage paid, the higher was the spindle assignment. Equation [12.1] on page 308 shows that this should be expected. However, the equation can take into account neither the range of technologies in use nor the influence of differing qualities of product from the various systems. Poor yarn quality not only degrades the price that the product will fetch, but it also imposes a cost penalty in spinning and winding. Logic suggests that operator assignment should also be influenced by the quality of the yarn produced [5]. The more weak places in the yarn, the greater is the end-breakage rate and the higher is the need for labor. Plotting data from a set of mills against their spindle assignments for like products (Fig. 12.3(b)) shows that the spindle assignment drops tremendously when the number of thin spots rises. The regression shows that the assignment for these cases was just about inversely proportionate to the number of thin spots per unit length. Some quality factors affect the immediate cost of production but the effects of others are not seen until a later process, possibly in the plant of a customer. Let these be called Category A and Category B problems, respectively. The latter are more difficult to deal with because the operators and their supervisors do not feel the effects at first hand. Nevertheless, they are very important. The major effect of Category B problems is to undermine the price of the product. Category A problems reduce the efficiency of operation but do not undermine customer confidence unless they are allowed to continue uncorrected.


Spinning assignment (spindles/operator)

Spinning assignment (spindles/operator)

10 000

40s Polyester/cotton 1000 20s Cotton 100 0.1

1 Wage rate (1980 $/hr) (a)

ye =

Breaks 1000 spindle hr

100

y = 25 777 x–0.96 x = thins/km

1 000

100 10

10

Break duration

100 Thins/km (b)

yd = 2.760x0.601

Break rate ye = 2.693x0.579 x = yarn count (Ne)

100

Duration Break rate

10 10

All scales are logarithmic

Yarn count (Ne) (c)

10 100

1000

yd = break duration (min)

y = 772x 0.92 y = 410x 0.56 x = wage rate ($/ hr)

10 000

307

Fig. 12.3 Factors in determining spinning assignment

12.3.2 End-breaks and operator assignment Operator assignment is strongly affected by the end-breakage rate in processing. Consequently, no discussion of the economics of spinning should avoid this aspect. A set of performance figures for a number of mills is shown in Fig. 12.3(c). As the number of ‘thins’ increases, so does the number of weak spots, and an operator can only serve a smaller set of spindles. In other words the assignment has to be lowered. Accurate data is difficult to acquire for several reasons, three of which are given. First, methods used by managers differ from one mill to another. Second, different fibers are in use. Third, there is a tendency for mill personnel to try to cast the performance of their mills in a favorable light. The correlations are poor but clearly both the end-break rate and the waiting time vary with yarn count. Other problems, such as the number of weak spots, are also associated with count (and other factors). Nevertheless, it is a reasonable expectation that fine yarns will be more difficult to spin than coarser ones. This is one of the reasons why a more careful choice of fibers should be made for fine yarns. The product of the end-breakage rate and the duration of the down time enables an estimate to be made of the lost production. Taking the data in Fig. 12.3(c) at face value, the loss in production during the time a break waits for repair is yd minutes (or {yd/60} hrs), the production between breaks is 1000/ye hrs and the percentage loss in production is approximately yeyd/600. The maximum probable value of loss due to end-breakage in those data was 2.37% and the loss was very roughly proportional to count. On this basis, spinning a 30s count might bring a loss of machine productivity of just less than 1%. Besides machine productivity losses, the cost of labor increases

308


due to the end-breakages. However, since the figures are very dependent on the fibers and practices used in a given mill, they can do little more than give an order of magnitude. Correlation between the loss in production and pneumafil waste collection is usually poor because pneumafil continues to be collected even when spinning continues. Examining thousands of spindles has shown that fiber is always being removed from the fiber stream entering the twist triangle during the spinning process. Furthermore, the amount removed is affected by the quality of the fibers. The waste figure increases if there is a large short-fiber content. Unfortunately, the short-fiber content is highly variable at this point. Of the 2% or 3% pneumafil waste levels common in ring spinning of cotton yarns, up to 0.6% is due to the continuous loss.

12.4

Cost minimization

12.4.1 Theoretical assignment For simplicity, let it be assumed that X spindles in a set are equally spaced around a circle and that end-breakages occur randomly. Further, assume that the operator deals with each end-break in sequence and never turns back. Under stable conditions, as the operator deals with one break and moves to the next, there is a probability that an end breaks somewhere else in the spindle set; that spindle then becomes unproductive. There will always be u spindles not producing, and, on average, they have to wait t hours before they are repaired. During the waiting time, the set fails to produce ptu out of the ptX lb/hr expected, where p is the productivity of one spindle in lb/hr. If the assignment is increased, there is a saving in labor costs, but the efficiency of the machine is reduced. There is a deterioration in the quality of the product because of the multiple piecings, which have to be removed in winding. The assignment has to be optimized and perhaps negotiated with the unions. It has been shown [4] that when piecing dominates the work load, the optimum assignment, a, is approximately given by: a = √ [Cl/{Bt (pCw + Cf)}] where

a B t p Cw Cl Cf Ch OHP

= = = = = = = = =

[12.1]

spinners assignment in spindles/operator end-breakage (rate/hr) average time (hr) for the operator to pass from one spindle to the next productivity for one spindle (lb/hr) cost of reprocessing the waste ($/lb) cost of labor ($/hr) fixed costs ($/hr) handling costs ($/hr) number of operator hours needed to spin 100 lb of yarn.

Bt is a function of yarn count, as is p. Thus the spindle assignment is affected not only by the economic factors, but also by the count and quality of the yarn being processed. Equation [12.1] indicates that the fixed cost is an important factor and the other major components are the end-breakage rates, the size of the operator sets, and the cost of labor. The cost of labor may be divided into three categories, namely (a) supervisory and maintenance staff, (b) mill personnel dealing with materials handling, and (c) personnel whose task it is to keep the machinery productive – this entails the


309

repair of end-breaks as soon as possible after they break to keep up the operational efficiency, as well as the regular maintenance. Category (a) is not normally considered part of the direct labor force because service usually continues over long periods. Consequently the costs from this source can be considered part of the fixed cost and, for the present purpose, can be lumped with costs of servicing the investment and direct maintenance costs. Category (b) is regarded as part of the handling costs, and category (c) is part of the variable cost. Spinning operator assignment can also be expressed in terms of OHP and machine productivity. a = 100/[OHP × p] spindles/operator

[12.2]

12.4.2 Capital and fixed costs Studies in 1990 covering many mills showed that the value of the machinery installed in a mill rises almost linearly with yarn count. Before 1990, the cost was estimated by various people to be the equivalent of up to $1 million per ton/day for each unit of count. In the twenty-first century, a new mill producing 30/1 yarn might be expected to cost perhaps $10 million for each ton/day capacity. In other words, the textile spinning industry has become a capital intensive industry. Despite the improvement in spinning technology, piecing a ring frame is still a major consumer of labor. Piecing costs also rise roughly linearly with count (with the factor standing at about 0.45¢/lb for each unit of count). Obviously the figures alter with time, as changes in the design of the machinery modify the costs and performance; also as currencies suffer inflation. Nevertheless, it is easy to see why the average count produced tends to have reduced over the last few decades. However, there is still room for the high count producer of specialty yarns, despite the cost. In ring spinning, the productivity per spindle is very low and since expenditures have to be paid for from the proceeds of sales, the amount that can be spent is also low. For example, consider a spindle producing 0.02 lb/hr for 7000 hr/year in an environment where the most extra charge that the market can bear is, say, 3¢/lb. The most that could be afforded is little over $4 per year per spindle. Even with a ten year payback, the most that could be spent is $40 per spindle! For a mill with 50 000 spindles, this latter figure is equivalent to $2 million, which is not a negligible sum! All of this has to be taken into account when considering monitoring and computer control. To the extent to which they can pay for themselves, they are fine, but there is little margin for cost overruns. Of course, these cost estimates are transitory and will change over the years. In the early days, the capital (or fixed) cost of the rotor spinning machines was so high that there was a break-even count above which the spinning system was uneconomic. It was not until rotor speeds could be raised to improve the output per dollar invested that the bar of a break-even count was overcome. With rotor speeds up to 130 000 r/min now possible, the cost of building in features such as automatic piecing can be absorbed into the original price of the machine without killing the market. Early fears of substandard yarn have given way to acceptance over a wide spectrum of products. Thus, we now have a category of high capital cost machines that need relatively small amounts of labor to operate. This is in contrast to ring spinning which has a relatively low capital cost but which needs more personnel.

310


12.4.3 Variable costs The cost of labor is mostly viewed as a variable cost. However, the charges for certain management and maintenance staff are regarded as administrative costs, which is part of the fixed cost category (their costs are not directly linked to production and therefore they are not viewed as direct labor costs). Of the variable costs, piecing of ring frames is the dominant portion for fine-count ring yarn production. As previously mentioned, piecing is automatic in rotor spinning, the end-breakage rate is less, and these are two of the reasons why rotor spinning needs less labor. Variable costs are important not only in the assignment equation but also as a measure of the mill’s performance. Automatic piecing for ring frames has been offered for sale by several machine manufacturers. They were technologically sound but were not commercially acceptable. The extra capital cost was an important factor in the lack of success. Nevertheless, a lower cost solution might still be offered and this would change the theoretical model that will be proposed later in this chapter. However, until an acceptable device becomes available, it is useful to analyze the operation as it now exists.

12.4.4 Handling costs In the first half of the twentieth century, the opening line consisted of manually operated feeders and a number of cleaning and opening machines not physically connected. Operators were used to transport material from one machine to another and were also needed to feed the material into the system. In a modern plant, no operator is required for material transfers in this section of a short-staple plant. The system is now automatic from the time the bales are laid into position until the sliver emerges from the card. The only personnel required in this department are supervisory, except for the operator who puts the bales in the laydowns about once per day. Between carding and spinning, operators are often required to move cans of sliver from one machine to another, and a few operators are needed to supervise the machinery. The introduction of automatic can changers and movers provides an alternative to the job of sliver can moving but, again, it is a case of capital expenditure being balanced against that of human operators. There is a similar choice in the zone between the processes of roving and spinning. Several degrees of automation are available for the transport and sorting of bobbins from the ring frame to the winder. On average, considerable labor is still used today in moving sliver cans, roving bobbins, spinning bobbins, and cones or cheeses of yarn. Bobbin transport systems and automated handling of the final packages have reduced the handling costs for those who can afford the capital outlay. By introducing automatic handling equipment, there is a transfer from the handling cost category to the fixed cost one. It is these sorts of transformations that increase the capital cost of the equipment and drive the industry to become ever more capital intensive.

12.4.5 Cost proportions Costs may be divided into categories such as are shown in Fig. 12.4. Some alter almost immediately according to demand; these are described as short-term variables. Costs in other categories change little or are not controllable by management and they take place over longer periods of time; these are designated long-term variables. Administrative costs vary according to the company structure and may include items


311

Total costs

Conversion costs

Short-term variability

Fig. 12.4

Profit

Overheads and shipping

Rent

Depreciation and interest

Fixed costs

Administrative costs

Power

Direct labor

Other materials needed in processing

Fibers and/or filaments

Variable costs

Taxes

Material costs

Long-term variability

Distribution of costs

not otherwise included in the foregoing. For the present purpose they will be taken as long-term items. Investment in new machinery nearly always incurs higher charges for depreciation and interest than the corresponding ones for the machines replaced. In fact, the machines replaced have often been written off. These charges are placed in the fixed cost category. Direct labor and power costs cover the variable costs in the mill; these have been further divided into materials handling (for short, ‘handling’) and other variable costs. If the total conversion cost is described by the three cost components just discussed, then: Total conversion cost = Cv + Ch + Cf

[12.3]

The variable cost, Cv, rises with spindle speed because of the increased end-breakage rates and power consumption. For a given set of equipment, this cost also rises with count because of (a) the diminished machine productivity at the higher count, (b) the increased cost of energy, and (c) the increased end-breakage rates at high counts. For simplicity, assume that the variable costs can be lumped together and described by Cv = KN m, where K is a constant, N is the count and m is fixed. Since a log graph of the lumped costs on the simplified basis is a straight line of slope m, it is useful to express the relationship as: Log Cv = Log K + m Log N The number of yarn packages produced is proportional to N – 3/2 and the handling costs, Ch, are proportional to those for a given set of machinery running at a specified reference speed. By arguments similar to those used for the variable costs, the log (handling costs) can be plotted as a straight line on a graph. Log Ch = Log K′ – (3/2) Log N

[12.4]

Fixed costs, Cf, are, by definition, fixed. The calculation assumes that all the equipment is in full production and there is no idleness due to malbalance of the mill. As will be discussed later, theoretical balance in a mill with a single product is achievable only in a plant that runs a single count. Where there is more than one product, it is difficult to even approach perfect balance. Most mills have a degree of malbalance. Figure 12.5 is a plot combining all three elements for each of two cases of differing variable costs. A curve of total cost is given in each case. It will be observed that the

312

Handbook of yarn production Total

Log (cost / hr)

C1

Cv 2

C2

Variable

C v1

Fixed

Handling

N1 N2

(a)

Log (cost / hr)

T1

Log N

T2

C2 Cf 2 C1 Cf 1 Ch1 Cv Ch 2 N2 N1

(b)

Fig. 12.5

Log N

Optimizing costs

addition of the three cost components, to give the total costs, results in curves with distinct minima. The symbol N is used for count and the appropriate subscript can be applied to whatever count system is in use. The graph shows two curves for variable costs Cv1 and Cv2, which could be caused, for example, by buying inferior fiber for one case. At the count for minimum cost, the use of poor fiber increases the labor cost from C2 to C1 and reduces the best count from N2 to N1. Thus the market becomes more restricted because of the lower count. The competitive position will be eroded if the additional cost shown to the right of N1 is not offset by the savings in fiber costs. For a given machine, the position of the minimum cost along the count axis is controlled by factors other than fixed costs (Fig. 12.5(a)). (As an aside, if one were to imagine the case of changing one rotor spinning machine for another, the fixed costs/lb are increased but that would not affect the optimum count). Increases in handling cost move the optimum count towards the higher counts and increase the value of the optimum cost. Increases in variable costs move the optimum towards the lower counts and increase the optimum costs. It is a matter of whether the handling or variable costs predominate that determines the result. Remember that variable costs comprise direct labor and power costs. Costs of labor and power can vary quite widely from area to area, and this plays a significant role in deciding whether new major investment is justified. For example, such an analysis might be used to see whether rotor spinning should be brought in to replace ring spinning.


12.5

313

Operational factors

12.5.1 Automation Perhaps the most important step in automation of modern times was the successful introduction of autodoffing. A similar process for the roving frame has been more recently introduced but is not universally accepted. Also, an automatic device for changing the travelers has been developed and this seems to work well. Each step of automation decreases the handling costs in return for an increase in capital or fixed costs. Figure 12.5(b) shows the effect of changes in handling costs from Ch1 to Ch2. An increase in fixed cost is inevitable and has to be taken into account as also shown in the same diagram. The value has been changed from Cf 1 to C f 2. An arbitrary ratio between the savings in labor cost and increase in fixed cost has been used for demonstration. The effects of upgrading the automatic handling equipment are to alter the best count (e.g. a shift from N1 to N2, as depicted in Fig. 12.5(b)) and the optimum cost. Whether or not the total cost is improved depends on the additional capital cost involved. The net result is that the total cost curve is flattened and the best count might be lowered. Automation tends to be more favorable for heavier counts and it is less sensitive to moves away from the best count. In a ring frame, automatic doffing has become standard and is now often included in the original cost of the machine. In rotor spinning and other forms of new technology, automation is almost universal. Automatic equipment leads to a substantial reduction in operator hours needed to produce yarn but, as was said earlier, there is an extra capital cost involved. All these schemes are technically feasible, but the problem is to finance them. The whole process of substitution of capital for labor requires access to capital. Furthermore, the final cost/lb must be no more than that of any competing systems, unless there is a special feature given to the textile product that enhances its value. This is a major reason for the slow adoption of the new technologies. On the other hand, if yarn manufacturers who have to pay high wage rates are to compete, the labor content has to be reduced and there will always be an interest in increased efficiency.

12.5.2 Mill balance Unless a mill is balanced, some machines will lie idle or work at a reduced throughput. Such a situation has an adverse effect on the economics of production. Ideally, each process stage will process approximately the same flow rate of material. The productivity of the spinning machine is a significant function of the count and is given by: P = Uη/(504 TM Ne √N e ) where P U η TM Ne

= = = = =

[12.5]

machine productivity (lb/spindle hour) spindle speed (r/min) efficiency per spinning unit twist multiple of yarn yarn count (hanks/lb).

For practical purposes, all factors other than count vary little; the spindle productivity is inversely proportional to N3/2 for a given spindle speed, since TM varies but little. The throughput of the opening lines is little affected by yarn count and intervening machines are weakly affected. Since mass flow must be conserved:

314


spinning output = output from prior stages of processing – waste generated [12.6] A change in spindle speed, twist multiple, or spinning efficiency affects the productivity mentioned in Equation [12.5] although the major factor is the count. Hence, if there are m spindles in the mill per opening line, the output can be written as PT ≈ mKN –3/2. If the net output of the opening line is Ko, then Ko ≈ mb KN – 3/2 at balance, where mb is the number of spindles needed for balance, and mb ≈ (Ko /K) N 3/2. However, Ko /K is a constant under the stated conditions and so mb is proportional to N3/2. The balance is heavily dependent on count. In practice, a mill is never completely in balance but with a stable order book, machinery is installed as necessary and a rough balance may be achieved.

50

0

5 10 CV of yarn strength (%) (a)

Statistical frequency

Ends down 1000 spindle hr

12.5.3 End-breakages and economics The foregoing makes clear that spinning contributes a major portion to the cost of yarn. In ring spinning, much of that cost is caused by end-breakage. In rotor spinning and other new developments, this may be less so. In any case, it is worth considering the aspect of end-breakage in yarn production. One factor in the end-breakage rate is the CV of strand strength. The word strand is taken to include the weak point in the twist triangle in ring spinning. An analysis of cotton yarns from a variety of mills showed a relationship between the ends-down rate and the CV of yarn strength (Fig. 12.6(a)). The correlation is imperfect because of the varying conditions in the various mills but there is a definite trend.2 As the yarn tension increases, the stress in the strand increases and so does the end-breakage rate. The probability of an end-break depends on the probability distributions of applied stress and tenacity of the strand (Fig. 12.6(b)). The area under the intersection of the two distribution curves gives the probability of a break. The applicable tenacity is that of the weak spots in the flowing material. In the case of ring spinning, this is often located in the twist triangle and is a variable with time. In rotor spinning, the weak spot is at the point where the yarn is removed from the rotor groove (except at very high speeds). Under high speed conditions, surges in false twist increase yarn tension temporarily at the navel and the yarn breaks occur there. There is an incentive to reduce the yarn tensions and CVs of yarn strength. It is assumed that the count and Applied stress Grayed area = probability of an end-break Yarn tenacity

Stress or tenacity in compatible units (b)

Fig. 12.6 CV of yarn strength and end-breaks

2

One obvious factor ignored in the above is the spindle speed, and another is traveler weight; in fact there was neither control nor measurement of the yarn tension which is a major reason for the poor correlation. Unfortunately, adequately controlled data of this sort is very difficult to gather in normal industrial practice.


315

twist are set by the market and are not a variable. As far as the economics are concerned, it is the number of piecings made and the number of bobbins containing piecings that are important. One has a large impact on the cost of spinning; the other impacts on the cost of winding and the quality of the product. There are both cost and price implications. To emphasize the last point, consider Fig. 12.7 in which the average number of ring bobbins spun per end-break was calculated for a range of mills spinning cotton yarns of various counts. Again, this was not a fully controlled experiment because it is almost impossible to get data on a wide enough scale under fully controlled conditions. Nevertheless, there is a clear trend and the correlation coefficient was better than might be expected. The technical difficulties in spinning high count are clearly seen. Quality control from a purely economic standpoint becomes ever more important as the count rises. The number of bobbins with an end-break leaving the ring frame is one important factor in determining the work load for the winder. If the end-breakage rate in spinning is high, there is probably a high rate of intolerable yarn faults. Not only will the winder have to remove most of the piecings but it will also have to remove the accompanying yarn faults. The chance of a piecing in a bobbin is low at fairly low yarn counts. On the other hand, the chance of an end-break within a bobbin of high count yarn is much higher. The number of breaks per cheese or cone is, of course, much higher still. There is a piecing for every ring bobbin used in making a cheese or cone. In addition, there is a piecing for nearly every objectionable fault, many of which are caused by end-breaks. A few faults escape through the aspiration device used to thread up the winder. For example, at 70s count, a break in every other bobbin might give 30 + 30 = 60 or so piecings per wound cone. At 30s, the figure might drop to, say, 30 + 2 = 32 piecings per wound cone. As an economic matter, clearly poor spinning begets a poor winder performance. Both undermine what otherwise might have been good economic results. Perhaps the operator costs for piecing should be based on the production they save by intervention rather than on the number of spindles they serve.

12.6

International competition

International competition is complex and in the space available it is only possible to outline some factors. First, let us make a comparison between a high cost, highly automated mill in one region, and a less sophisticated mill in a region of lower wage Bobbins spun/end-break

15

y = 0.012x 2 – 1.544x + 50.368 r 2 = 0.950 r = correlation coefficient

10

5

0 30

Fig. 12.7

40

50 60 Yarn count (Ne)

End-breakages per bobbin

70

316

Handbook of yarn production Area of advantage for Case A

Cost/hr, log scale

Area of advantage for Case B

Case B

Variable costs

Handling Fixed costs costs Case A

Yarn count, log scale (Ne)

Fig. 12.8

International competition

rates (Fig. 12.8). The scenario in this case is that a mill in an advanced region with high labor costs seeks to offset the disadvantage by using automatic equipment. This means that the fixed costs are high in this case. The mill in a region of lower wage costs feels less need to invest in expensive automation. There are trade-offs that might be in favor of one group or the other, depending on costs. Remember also, that relative conditions undergo continuous change and no fixed recipe can be recommended. In the particular case shown, the highly automated, high cost mill shows an advantage only over a certain low count range. The high fixed costs have the effect of leveling the total cost curve so that it is not so sensitive to count changes. The spin limit forms a natural boundary to the right of the diagram. As wage rates tend to equalize, these distinctions tend to disappear, but equalization of wages is a prospect for the distant future. A factor not included in the analysis is that of shipping costs. Transoceanic shipping decreases the margin of profitability. Since low labor cost producers are usually distant from the major markets in advanced countries, shipping costs offset their advantage of lower labor costs. This could be taken into account by adding shipping costs to the normal fixed costs. It might also be noted that interest rates available in some regions are less favorable than those granted elsewhere. This, too, influences the final cost. Further factors are those of quality, reliability, and promptness of delivery.

References 1. 2. 3. 4. 5.

Färber, C and Furter, R. Uster Statistics No 40, Zellweger Uster, Uster, Switzerland, May 1997. Thompson, A. Techno-economic Aspects of Textile-machinery Investment, Textile Machinery: Investing for the Future, Textile Inst Ann Conf, 1982. Stryckman, J. Une Méthode de Mesure de la Productivité du Travail et du Matériel en Filature de Coton, Centexbel, Belgium, May 1983. Lord, P R and Mohamad, S B. Economics, Technology and Development in Staple-yarn Manufacture, Managing Technological Change, 64th Ann Conf, Textile Institute, Oct 1980. Garde, E. Process Control in Cotton Spinning, ATIRA, Ahmedabad, India, 1974.

Appendix 1 Calculations I: Elementary theory

A1.1

Yarn and strand numbering systems

A1.1.1 The basic philosophies Textiles are often sold on a weight basis and consequently it is natural to express the fineness of a yarn in terms of mass (or weight). There are two basic ways in which this may be done. These are: (a) by specifying how much a given length of yarn weighs; or (b) by specifying what length of yarn there is in a given weight. These are known generally as the direct and indirect systems of yarn numbering. Direct number = mass/length

[A1.1]

Indirect number = length/mass

[A1.2]

It will be noted that one is the inverse of the other. In the first case, the number gets larger as the yarn gets heavier, and in the second case it gets smaller. The term mass has been used because this is technically correct, even though the popular term is weight. Mass is the amount of material in an object and weight is the force that acts on it when it is accelerated. Since we all live in a gravitational acceleration of about 32 ft/sec2, all objects are subject to a force acting towards the center of the earth; that force is what we call weight. The same mass on the moon would weigh a different amount. Each system has its advantages and disadvantages; each has found areas in which it has endured and has become established by custom. It so happens that, because the lengths are so very long for any reasonable mass, the yarn numbers would get impossibly small or large unless special counting systems are used. (Within the normal range of linear densities, one pound of yarn laid in a line would extend many miles. The numbers are unwieldy.) The following paragraphs explain a selection of the most important counting systems. A1.1.2 Direct systems The technical name for fineness is linear density1 and it is always expressed as 1 This should not be confused with the term density as used in physics.

318

Appendix 1

mass/unit length. In commerce, the technical name is used less than in the fiber industry or scientific community, such units of measurement as denier or tex being often used instead. Sometimes the term yarn number (explained shortly) is used, but this can be ambiguous. Two of the major subsections of the direct system will be cited. In one, the logically minded scientists have chosen the metric system and use g/km (the unit is called a tex). In the other, the technologists have chosen g/9 km (the unit is called a denier); this is based on an ancient measure of length but it still survives because it happens to be about the right size of unit to describe a typical fine fiber. The normal metric prefixes can be used in the tex system. For example, a decitex is one-tenth of a tex and a kilotex is 1000 times larger than a tex. One denier is equivalent to 1/9 tex or 10/ 9 decitex. The denier is a popular unit in the fiber industry and many fibers of 1.5 denier are supplied to blend with cotton; in the tex system, the commercial equivalent is 1.5 decitex. (The 10% difference is normally ignored.) Microfibers fall into the range of 1 denier or less. A carpet fiber often runs at 15 denier. In passing, it should be noted that a 450 denier yarn made up of 1.5 denier filaments would contain 450/1.5 = 300 filaments in the cross-section. There are also intermediate products, such as sliver, to which the direct system of measurements is applied. Sliver is a rope-like strand that is much heavier than yarn; a normal linear density is about 5 kilotex. In much of the industry, a system using grains and yards is used for sliver; typical values are in the range 30 to 100 grains/yd. There are 7000 grains per lb. In this book, the symbol n is used for measurements in the direct system of yarn numbering and the capital letter N is used for the indirect system. In all cases, the appropriate units of measurement should be placed after the quantity.

A1.1.3 Indirect systems Indirect systems utilize terms of length per unit mass. There is a large variety of systems, which is a legacy of the ancient crafts. Generally all the systems in this category are called yarn count or yarn number. The term yarn count is preferred. It is normal to specify the yarn count in hanks/lb where a hank contains a specified length of yarn; unfortunately each of the systems specifies a different length. Therefore, it is helpful to always specify the sort of hank being used when quoting a yarn count. Some specified hank lengths are listed in Table A1.1 With the indirect system, the number gets larger as the yarn gets finer. In the English cotton system, a 4s yarn is very coarse whereas a 50s yarn is fine. In cotton processing (and those technologies that have evolved from it), the units developed in England in the industrial revolution are still in use. A cotton hank is defined as 840 yd of yarn. (The number 840 is divisible by 1, 2, 3, 4, 5, 6, 7, and 8; one can imagine the value of that in early primitive societies.) Thus, if the count of a singles yarn is 20 cotton hanks/lb, there are 20 × 840 yd in a pound of yarn. It should be noted that the yarn count is usually written as 20s or 20/1. The symbol used in this book is Ne, where the subscript refers to ‘English’ and distinguishes it from Nm, which refers to the metric count (meters/gram). In the case of long-staple yarns, where the technology is derived from one of the processes for making yarn from wool, a worsted hank is defined as 560 yd of yarn. In this case the symbol Nw is used. Other systems of symbols are used by others. The American Society for Testing

Calculations I: Elementary theory Table A1.1

319

Strand numbering systems Direct

Fiber Cotton Wool Man-made Denier g/9 km Intermediate Card Fleece Kilotex Roving (cotton) Roving (wool) Roping Yarns Man-made All yarns Cotton type Worsted count Woolen count All yarns (European)

Indirect

Name

Units

Name

Units

micronaire fineness decitex –

Approx µg/incha mg/cm dg/km –

– – –

– – –

lap weight sliver weight g/m tex – –

oz/yd grains/yd – g/km – –

– – – hank roving roving weight roping weight

– –

denier tex – – – Metric count

g/9 km g/km – – – m/g

– – – – English cotton count ch/lbb Worsted count wr/lbc Woolen count wo/lbd Metric count m/g

ch/lbb wr/lbc wo/lbd

Notes: (a) This is an arbitrary index of fineness, (b) ch = cotton hank (840 yd), (c) wr = worsted hank (560 yd), (d) wo = woolen hank (1600 yd).

Materials uses Nec meaning English cotton count on some occasions, but ASTM Standard D2260 uses cc instead, and D1907 uses N (these are rarely used in mills throughout the world). The Textile Institute uses T for the direct system and N for the indirect system, and a variety of subscripts are used to distinguish between a number of subcategories.

A1.1.4 Conversion In normal practice, it is unnecessary to go through a calculation each time a conversion is required; generally a conversion factor can be used (Table A1.2). In the case of converting from one direct system to another, one merely multiplies the known linear density by the conversion factor. A similar procedure is used when converting from one indirect system to another. When converting from indirect to direct, or vice versa, then the factor must be divided by the known quantity. Referring to the use of Table A1.2, an example is to convert from cotton count to tex. In this case, 590.5 must be divided by the cotton count. These are known as cross-conversions. Another example: to convert from cotton count to worsted count, multiply the given cotton count by 1.5.

A1.1.5 Plied yarns and examples of calculation It is possible to make up a yarn by twisting together two or more finer yarns. The process is called plying and the yarns are called plied yarns. To show that a yarn is plied, it is normal to write both the yarn count and the number of plies separated by a slash. In some systems, the quoted yarn number is that of each component. A 100 den/3 yarn would mean that 3 plies of 100 denier yarns were twisted together to give

320

Appendix 1

Table A1.2

Conversion factors Direct

Indirect

To

tex

denier

cotton count

worsted count

woolen count

metric count

From tex denier cotton count worsted count woolen count metric count

– 0.111 590.5 885.8 310.2 1000

9 – 5315 7972 2791 9000

590.5 5315 – 0.667 1.905 0.591

885.8 7972 1.5 – 2.857 0.886

8.06 72.54 0.525 0.350 – 0.31

1000 9000 1.693 1.129 3.224 –

a yarn whose equivalent linear density would be approximately 300 denier. The twisting causes a slight effect but this can be ignored for now. With a direct system, one adds together the individual linear densities to arrive at the total. The problem is a little more complicated with an indirect system. Ignoring twist effects, imagine a number of cotton yarns lying side by side, each being of the same length L yards. If the masses of the strands are w′, w″, w″′ etc., then the total mass M = w′ + w″ + w″′ + etc. But

w′ = w″ = w″′, etc. = L/840   M = L  l + l + l + etc.  840  N ′ N ′′ N ′′′ 

[A1.3]

where N ′, N″, N″′, etc., are the counts of the individual component yarns. If the equivalent count of the ply is NT, then:

NT =

L 840 M

l = l + l + l + etc. NT N′ N ′′ N ′′′

[A1.4]

[A1.5]

In words, the reciprocal of the count of the ply is the sum of the reciprocals of the component strands. As noted in Chapter 3, the equivalent count is used by some in commerce, with no indication that the number quoted refers to the equivalent count. Thus, one finds yarn counts being written as, say, 10/2, meaning 10equ/2, whereas the practice elsewhere is to use the form discussed earlier (i.e. 20/2) for the same yarn. Dealing with short-staple yarns, the practice is to quote the ply as, say, 20/2, whereas with long-staple yarns the numbers are reversed (i.e. 2/20).

A1.1.6 Simple draft calculations The flow through a draft zone obeys the law of conservation of mass flow. In other words, Mass flow in = Mass flow out + losses of mass

[A1.6]

Calculations I: Elementary theory

321

In mass flow, the element of time is introduced and velocity is substituted for length. Thus Equation [A1.6] may be re-quoted in the form: Vini = Vono + losses

[A1.7]

where V = velocity of the fibers and n = linear density. Ignoring losses, ni/no ≈ Vo/Vi

[A1.8]

Often it is assumed that the speeds of the fibers and the rolls in contact with them are the same, but this is not always so, because of slippage. Also, it is sometimes assumed that the linear density of the final output product is the same as that of the material passing through the output rolls. This is not always true either, because there can be shrinkage immediately following the emergence of the strand from the front drafting elements. That is why the approximately equals sign appears in Equation [A1.8]. The ratio of linear densities of the output product and the input is called the actual draft ratio (often referred to as just draft). The ratio of the surface velocities of the media inducing flow (such as rollers) is known as the mechanical draft ratio or mechanical draft. When several stages of drafting are used, the overall draft across them is the algebraic product of the stage draft ratios. Taking two stages, designated by the subscripts 1 and 2, and realizing the output of stage 1 is the input of stage 2, then no1 ≈ ni2 and Vo1 ≈ Vi2. Let n ≈ no1 or ni2 and V ≈ Vo1 or Vi2. Thus, since: ni1/n ≈ Vo1/Vi1 and n/no2 ≈ Vo2/Vi2, (ni1/n) × (n/no2) ≈ ni1/no2 ≈ draft cross the two stages If no1 and ni2 are used, the drafts would give a close approximation to the actual drafts. As mentioned, these are defined by the ratio of linear densities of the strand at the input and output of each stage. However, in practice, spot measurements of linear density of material moving through the system are rarely completely representative and accurate. For the purposes of simple mill floor calculation it is normal to ignore the losses, slippages and contractions, which are quite small. In such cases, normal equals signs may be used as in Equation [A1.9]. The drafts so calculated are mechanical draft ratios. Thus, if ni1/n = ∆1 = draft in stage 1, and n/no2 = ∆2 = draft in stage 2 ni1/no2 = total draft = ∆ = ∆1 × ∆2

[A1.9]

This can be extended to total draft = ∆1 × ∆2 × ∆3 × ∆4 etc.

A1.2

Yarn diameter

A1.2.1 Diameter and cross-sectional shape of a yarn in service It might be thought that the obvious way to describe a yarn would be by its diameter, but there are difficulties with this approach. A textile yarn, by its very nature, has to be soft and can squash; therefore, although it is approximately round in cross-section when it is in the free state, it rarely remains round in fabric form. Different fibers are used in all sorts of combinations and it is a complex matter to calculate fabric weights

322

Appendix 1

because of the physical differences in the fibers and fabric structures. Nevertheless, it is helpful at times to have an idea of yarn diameter. For example, the diameter helps determine how closely the yarns can be packed to make a fabric, or how well a given yarn will cover in a given fabric.

A1.2.2 Theoretical diameter of a yarn in the free state Let the linear density of a yarn, ny, be equal to the product of the number of fibers, m, and the average linear density of the fibers, nf. Cover is the percentage area covered by one or more yarns as they lie in the fabric. n y = m × nf

[A1.10]

Assume that the fibers are evenly spread throughout the cross-section at a rate of b fibers per square inch. A round yarn, of diameter d inches, has a cross-sectional area of Ay, but Ay = πd 2/4 sq inch, and the yarn contains Ay b fibers. From Equation [A1.10] m = n y /nf, but also m = Ayb substituting for Ay, m = πd2b/4 substituting for m, n y /n f = πd2b/4 whence d2 =

4 ny π bn f

and d =

4 ny π bn f

[A1.11]

In a normal yarn, nf is relatively constant, but b is determined by the twist and structure of the yarn. With a given yarn with a fixed twist and structure, the only highly significant variable in the right-hand side of Equation [A1.11] is ny. Consequently, we may write the value of the equivalent yarn:

d≈K

n

[A1.12]

The approximation sign is meant to take care of the uncertainties due to changes in yarn shape; K is often treated as a constant factor, but care has to be taken in exercising this option.

A1.3

Twist multiple calculations (staple spinning)

A1.3.1 Derivation of twist multiple as a function of yarn count The helix angle at which the fibers lie is important in determining the properties of the yarn. Since there is a profusion of helix angles involved in a yarn structure, it is normal to define twist by the helix angle of the fibers in the outermost layer. This angle is very roughly 45° for a normal yarn.


323

A simple experiment will demonstrate the relationship between twist angle just discussed (i.e. helix angle) and diameter. Draw a line diagonally on a flexible transparent sheet and roll the sheet tightly. The line now appears as a helix consisting of numerous repeats along its length. Allow the roll to increase in diameter and it will be seen that the number of repeats decreases. Let the roll grow to such a diameter that there is only the single complete helix as shown in Fig. A1.1. If the diameter of the roll is now D, the length of a repeat (or wavelength) is λ, and the helix angle is β, then tan β = πD/λ. But tan β is proportional to twist multiple. Therefore TM = KπD/λ

[A1.13]

where K is a constant. A twisting or spinning machine has to be set to give a certain number of turns per inch (say τ) but, if the yarn does not change in length, this is really the number of helical repeats in one inch of yarn. If λ is measured in inches, 1/λ = τ (i.e. λ is the number of inches per turn). Yarn diameter is roughly proportional to the square root of the linear density (or to the inverse square root of the yarn count, according to the system used). Take an indirect system, where K1 is a constant and N is the count, D ≈ K1/√N. Substituting in Equation [A1.13] and transposing: Twist density = TM √N × constant, and the twist level is measured in turns/unit length. The constant is normally taken as unity and the twist is given by: Twist density = TM √N tpi

[A1.14]

In this case, the twist is measured in turns per inch and this is usually contracted to

πD

β

λ

D

λ

Fig. A1.l

Geometric development of the strand surface

324

Appendix 1

tpi. However, it must be emphasized that ‘tpi’ is not the name of the variable but merely describes the units of measurement. The variable is called ‘twist density’ and we use the symbol τ to denote it. The result of applying the formula in this case yields an answer in turns/inch. A subscript is added to N in Equation [A1.14] according to whether the measurement system is English cotton count, worsted count, metric count, or another indirect system.

A1.3.2 Twist multiple as a function of linear density In the direct count system, similar logic produces the relationship: τ = constant × TMdirect/N

[A1.15]

or twist density = α/n The factor α is frequently used in Europe in place of TM. An ASTM standard recommends using a twist density measured in turns/cm and assumes a constant of unity.

A1.4

Productivity of pre-spinning preparation machinery

A1.4.1 Opening line productivity A normal opening line is capable of producing a fiber stream of the order of 1000 lb/hr and there are usually at least two opening lines in operation in a mill. Thus, the minimum fiber stream found is of the order of 1 ton/hr. Let the minimum opening line productivity be Po. (It might be noted that 1 long ton (UK) = 2240 lb and this is approximately the same as 1000 kg (i.e. 2206 lb). The short ton (US) is 2000 lb.) The efficiency of the process, η, is sometimes measured per unit (pu) rather than as a percentage. Processing is carried out in a series of sequential stages. Machines following the opening line have much lower productivities and there have to be multiple parallel paths within a single stage of the later processes. The production should be approximately matched at all stages and it is necessary to calculate the number of each sort of machine required at each stage. The following calculations will be based on the assumption that all machines at a given stage have the same productivities and that the throughput is constant from stage to stage. This is not always true, but the calculation will lay out the basic rules. It is not a great step to modify the procedures to accommodate variations in the machinery mix.

A1.4.2 Sliver productivity Cards, drawframes, combers, all produce a stream of non-twisted sliver and their production rate is the mathematical product of the linear density and the delivery speed. The productivity, P, is usually measured in lb/hr or kg/hr, the velocity, V, in yd/min, or m/min and the yarn count, n, in grains/yd or g/m; the constant K has to be adjusted accordingly and the efficiency, η, is expressed in per unit terms. Let the machine productivity be P. P = KVnη

[A1.16]


325

A1.4.3 Card productivity The productivity of a card is usually measured by direct weighing and the result is expressed in lb/hr (or kg/hr). A typical figure, after losses, in the early 2000s may well be in excess of 300 lb/hr (say 150 kg/hr). Let this productivity be Pc is 100 lb/hr. If the cards run continually, there have to be at least Po /Pc cards operating in parallel, to match the prescribed Po output of the opening line. Prudence might suggest a spare to allow for maintenance but that would be expensive. Consequently for the figures suggested, we would require either [1 + (Po/Pc) = 1 + (2000/300)] or [2000/300] depending on the risk that could be tolerated. These results may be rounded up to 8 or 7 respectively (but we could just as well produce 2100 lb/hr with 7 or 6 cards) and speed up our opening line a bit. If production is stopped between can changes, efficiency drops and the number of cards required increases in inverse proportion to the per unit efficiency. If it is desired to reduce the throughput of the cards to improve on nep performance, or to prolong the time between maintenance stoppages, then a further increase in the number of cards is required.

A1.4.4 Drawframe productivity Drawframes can deliver sliver at up to 1000 yd/min, even if normal production rates are somewhat lower. The linear density of the sliver usually varies between 60 and 100 grains/yd. To establish an order of magnitude of drawframe productivity, let us assume the sliver weight is 70 grains/yd (which is equivalent to 100 yd/lb), the delivery speed is 800 yd/min, and an automatic can changer is in use (which implies a per unit efficiency close to 1.0): also assume that the efficiency is 100%. Let the machine productivity be P. 800 yd 70 grain lb × × × 60 min min yd 7000 grain hr = 480 lb/hr

P=

[A1.17]

Notice how the unit ‘yd’ cancels top and bottom, as do the units ‘grains’ and ‘minutes’, and we are left with lb/hr. The result shows that 4.17 breaker drawframes are needed to match the stated opening line output, and a similar number are required for subsequent stages of drawing. The resulting figures have to be rounded up; therefore the number needed would be at least 5 per passage. At least 10 drawframes would be required to match the throughput of the opening system if two passages of drawing were used.

A1.4.5 Roving productivity With roving, the product is twisted and the production is limited by the twister speed. Flyer speeds can range up to 1000 r/min, and the twist levels are typically 0.9 tpi; hence the linear speed is 1000/0.9 inches/min. (The value 0.9 is used rather than a round number to facilitate tracking the calculation.) The linear density is often measured in hank roving, where 1 cotton hank contains 840 yd of roving. A 1.2 cotton hank roving contains 840 × 1.2 = 1008 yd/lb. The productivity equation2 may be modified in form to give Pr = K′Vη/Ne. 2 K′ is a constant, Ne is the count, τ is the twist density, V is the linear velocity of the roving, and U is the rotational velocity of the flyer. Alternatively, Equation [A1.24] may be used by converting τ into TM.

326

Appendix 1

Also, V = U/τ and substituting for V we get:

Pr =

K ′Uη τNe

[A1.18]

Thus, if U = 1000 r/min, τ = 0.9 tpi and Ne = 1.2 cotton hanks/lb (these are common values for roving), a single spindle working without stop would produce:

yd lb Pr = 1000 × inches × × hank × min 0.9 1.2 hank 840 yd 36 inches × 60 min = 1.837 lb hr hr

[A1.19]

Again, the units cancel to leave the final units as lb/hr. If the efficiency were 0.9, then the net production would be 1.653 lb/hr and 1210 roving spindles would be needed per opening system capacity of 2000 lb/hr. The result has, of course, to be rounded up to find the number of machines needed. Each machine has a number of spindles specified by the machinery maker.

A1.4.6 Roving wind-on speed The winding-on speed is Uw = ± (Uf – Ub ) r/min, according to whether the flyer or bobbin leads. Some designs have the bobbin rotating faster than the flyer, in which case Uw = (Ub – Uf ). The general case may use the absolute value, which takes no account of sign, and it is written as |(Uf – Ub)| in the equation. When U is the speed in r/min and r is the radius of wind in inches (the subscripts refer to the flyer and bobbin), the linear velocity is: Vw = 2πr|(Uf – Ub)| inch/min

[A1.20]

The roving supply velocity is Vs. Vs = Uf /τ inch/min

[A1.21]

but Vs = Vw, therefore 2πr|(Uf – Ub)| = U f /τ

[A1.22]

But 2πrτ is the number of turns of twist put in a single coil of roving of radius r; let this number of turns of twist be τc. Substituting τc in Equation [A1.22] and rearranging, the ratio between the bobbin and flyer speeds is: Ub = 1± 1 Uf τc where Vw Uw Uf Ub r τ ||

= = = = = = =

[A1.23]

winding-on speed in inches/min winding-on speed in r/min flyer speed in r/min bobbin speed in r/min pitch radius of outer layer of roving in inches twist density in twist/inch means that + or – sign of the resultant within the ‘bars’ should be ignored.


A1.5

327

Ring frame performance

A1.5.1 Ring frame productivity The productivity equation for the production of a twisted strand shown in Equation [A1.18] can be modified further. In ring spinning, the twist multiple is the factor most likely to be kept fairly constant, irrespective of yarn count. If we insert the appropriate value of K′ in Equation [A1.18] we get:

P=

Uη Uη or P = 1.5 504 TM [ N e ] 504 TM N e N e

[A1.24]

With a typical count of 24 Ne and a TM of 3.5 being spun on a spindle rotating at 15 000 r/min, with an efficiency of 0.97, we get a productivity of 0.0701 lb/hr. We would require at least 28 531 ring spindles to match our hypothetical 2000 lb/hr opening line. If a machine contained 800 spindles, then we would require at least 35.66 machines. Rounding this up, a practical number would be 40 machines. The requirement for spindles also changes if the average count of the mill changes. An operator running with a bare minimum number of spindles would encounter a shortage of spinning capacity when the average count goes finer. If the average count goes coarser, there will be some surplus spindles. In practice, the number of spindles cannot easily be changed and the speeds are manipulated to balance the system as far as possible. It should be mentioned that the average count and twist are usually determined by the marketplace and the spinner has limited choice in the matter.

A1.5.2 Elements of mill balance The flow through the various production phases has to be balanced if the machines are to be fully utilized. The number of each sort of machine can be calculated from the quotient of total mass throughput divided by the productivity of the particular machine. Adjustments have to be made for fiber losses from each stage and the proportion of those losses recycled. An example of such a calculation is given in Q34 in Appendix 2. In addition, where dissimilar machines work in parallel, it is necessary to calculate the production of each of the parallel production streams and add the results to make a balance with the overall product flow.

A1.5.3 Bobbin flow A spinning bobbin might only contain 0.1 or 0.2 lb of yarn; thus, a large number of bobbins has to be handled. In our hypothetical case, the mill operator would have to handle at least 10 000 bobbins per hour. This highlights why attention has been given to automating the transfer between the ring frame and the winder.

A1.6

Winding performance

A1.6.1 An example of the reduction in winder productivity due to the need to splice A winding machine might run up to 1000 yd/min and, if it were not for the interventions needed to perform its clearing function, the productivity when winding a 6s yarn

328

Appendix 1

could be in the order of 12 lb/hr (0.2 lb/min). If, for example, a 5 lb cheese of yarn has 30 splices in it, and each splice needs 10 seconds to locate the fault and perform the splice, then 5 minutes of winder time is spent on splicing during the build of the package. If it had not been for the splices, our hypothetical winder would have taken 25 minute to wind the yarn. For simplicity, assume that the time to doff the full packages and replace them by empty ones is negligible and assume there is no loss of fiber through the various processes. The total winder head time consumed would have been 30 minutes and the average speed would have fallen to 10 lb/hr.

A1.6.2 An example of the effect on winder productivity caused by poor yarn quality If the yarn fault rate mentioned in Section A1.6.1 doubled and 10 minutes of winder time was on splicing, the average speed would drop to 5 lb/35 min = 0.143 lb/min, which is equivalent to 8.57 lb/hr. Thus, the number of winders required depends not only on the count of the yarn, but also on its quality. In the best of the two cases just discussed, we would have needed at least 14 winding spindles to deal with the production rates of our hypothetical mill. In the case of the higher fault rate, we would have needed at least 21 spindles.

A1.7

Rotor spinning machine performance

A1.7.1 General statement about productivity in rotor spinning Rotor spinning needs no roving or winding facilities and the preparation of sliver is much the same as that already discussed. The biggest difference is in the speeds that can be attained. Rotor speeds of up to 130 000 r/min are possible, and preparation has to be good enough to prevent rotor fouling and provide reasonable long-term evenness. Equation [A1.24] applies.

A1.7.2 An example of rotor productivity Appropriate data in this case might be: rotor speed = 100 000 r/min, spinning efficiency = 0.97, TM = 4.0, and yarn count = 24s. If we apply Equation [A1.24], the productivity is about 0.4 lb/hr and at least 5000 rotors would be needed for our hypothetical mill.

Appendix 2 Calculations II: Worked examples

A2.1

Yarn numbering

Q1. How many yards of yarn are there in 1 lb of 24s cotton yarn? Ans. In 1 lb of cotton yarn there are 840 × 24 = 20 160 yd. In other words, there are 840 Ne yd/lb in the cotton system of units. Q2. If there are 20160 yd in 1 lb of yarn, what is the worsted count? Ans. L/W = 560 × Nw where

L = length in yd and W = weight in lb Nw = L/(560 W) = 20 160/560 = 36 worsted hanks/lb or Nw = 36s From the foregoing, it will be realized that Nw is 50% larger than Ne for the same yarn. Q3. If Ne = 24s, what is the linear density in g/m? Ans. This is a conversion from an indirect system to a direct system; therefore, one is inversely proportional to the other. In other words n′ = constant × (1/Ne), consequently Ne should appear on the bottom line of the equation. There are 1.093 yards in 1 meter and 454 grams1 in a pound. A 24s yarn has a length of 24 × 840 yd/lb and, designating gram and meter by g and m, respectively: n ′′ =

1lb 454 g 1.093 yd × × (24 × 840) yd lb m

n″ = 0.0246 g/m Q4. What is the linear density of the yarn in Q3, expressed in tex? 1 The French spelling, gramme, is often used, to avoid confusion with grain, especially when hand written.

330

Appendix 2

Ans. The unit tex is the same as g/km. 1000 m n ′′ g × km m = 1000 × 0.0246 g/km = 24.6 tex

n=

Q5. If a 120 yd skein weighs 40 grains, what is the cotton count? Ans. We have to change the units. Remembering that there are 7000 grains in 1 lb, and 840 yd in 1 cotton hank,

Ne =

120 yd 7000 grain 1 cotton hank × × 40 grain lb 840 yd

Canceling out the units as well as the numbers, we get: Ne = 25 cotton hanks/lb

A2.2

Drafting

Q6. A roving of 1 hank roving (Ne = 1) is converted to a 24s yarn. If twist contraction is ignored, what is the actual draft ratio? Ans. Actual draft ratio = (output value of N)/(input value of N) in compatible units. Nei = 1.0 cotton hanks/lb and Neo = 24 cotton hanks/lb Actual draft ratio = 24. Q7. The linear velocity of a yarn leaving a drafting system is 100 ft/min, and the entering material has a velocity of 2 ft/min. What is the mechanical draft? Ans. Mechanical draft = Vo /Vi = 100/2 = 50. Q8. A roller drafting system consists of two pairs of drafting rollers; the front rollers are 1 inch diameter and the back rolls are 1.25 inch diameter. The front rollers rotate at 90 r/min and the back rollers at 3 r/min. The system is fed with 2 hank roving (Ne = 2 cotton hanks/lb). What is the yarn count if twist contraction is ignored? Ans.

Vo = πDoU = 90π inches/min Vi = πDiU = 3 × 1.25π inches/min Mechanical draft = VoVi = 90π/3.75π = 24 Output Neo = 2 × 24 = 48 cotton hank/lb.

Q9. If the yarn delivered in Q8 contracts by 3% before it is wound, what is the actual draft ratio? Ans. Without shrinkage (output Neo) = 48 × (input Nei). After shrinkage, the yarn is fatter and the output Neo is less, thus the actual draft ratio = 48 × (1.00 – 0.03) = 46.56 cotton hanks/lb. Q10. A sliver-to-yarn drafting system is fed with 50 grain/yd sliver and delivers a strand of Ne = 24 cotton hanks/lb. What is the actual draft? Ans. The input is expressed in a direct system and the output in an indirect one. Thus, the first step is to convert one value into the units of the other, because compatible units must be used.

Calculations II: Worked examples

Input N e =

331

1 yd 7000 grain 1 hank × × = 0.1666 cotton hanks/ lb 50 grain 1 lb 840 yd

The second step is to state in the input and output counts. Output Neo = 24 cotton hanks/lb The third step is to check the compatibility of the units and the fourth step is to calculate the ratio as follows: Actual draft = 24/0.1666 = 144 Q11. A drawn filament bundle is made up of filaments of 1.5 denier (i.e. 1.5 dpf). The draw ratio used to orient the molecular structure was 5. What was the denier of the original ‘spun’ filaments before drawing? Ans. Output linear density = no = 1.5 denier. Input linear density = no × draw ratio = 1.5 × 5 = 7.5 denier. Q12. A 150 denier yarn is made of 1.5 dpf fibers. How many fibers are there in the cross-section? Ans. No of fibers in cross-section = nyarn/nfil = 150 denier/1.5 denier = 100 filaments/yarn. Q13. A toothed drafting system takes in sliver at 53 grains/yd and converts it to a stream of fibers that average 5 fibers in the cross-section. The fibers have a linear density of 1.5 dtex. What is the draft ratio? Ans. Linear density of input = ni

ni =

53 grain 1 lb 454 g 1.09 yd 1000 m × × × × yd 7000 grain lb m km

= 3747 g/km Since 1 tex = 1 g/1000m, linear density of input = 3747 tex, and of output = 5 × 1.5/10 = 0.75 tex. The units of input and output are compatible, hence Draft ratio = 3747 tex = 4996 0.75 tex

Q 14. The foregoing is a very high draft ratio, typical of these devices. What is the draft when the thin stream of fibers is condensed into a 30 tex yarn? What is the overall draft? Ans. The new input linear density for the second stage is 0.75 tex and the output is 30 tex. Therefore the draft ratio is 0.75/30 = 1/40. In other words, the condensation stage gives a fractional draft. The overall draft is 3750 tex/30 tex = 125. Q15. Four 40s yarns are plied. What is the equivalent count of the plied yarn if twist effects are ignored? Ans. 1 = 1 + 1 + 1 + 1 = 4 40 40 40 40 40 NT

from which it follows that NT = 10 hanks/lb.

332

Appendix 2

Q16. A 40s yarn is plied with a 20s and a 10s yarn to make a fancy yarn. What is the equivalent count, if twist effects are ignored? Ans.

1 = 1 + 1 + 1 = 1 + 2 + 4 40 20 10 40 40 NT 40 = 0.025 + 0.05 + 0.10 = 0.175 (hank/lb)–1 This is the reciprocal of NT, hence NT = 1/0.175 = 5.71 hanks/lb This calculation is typical of all indirect systems. To apply it to a particular one, make sure to quote what sort of hank is involved. For example, if this had been wholly in the cotton system, the answer would have been quoted as 5.71 cotton hanks/lb. However, if it had been in the worsted system, the answer would have been 5.71 worsted hanks/lb. If the counting systems had been mixed, the answer could have been expressed in one of the systems but the units used in the calculation would have had to be consistent with the answer. Notice how the equivalent yarn count is smaller than that of any of the component yarns.

A2.3

Belt transmission

Q 17. Consider a belt or yarn being driven in the direction shown in Fig. A2.1, by a pair of rolls, one of which is 1.3 inch radius and it rotates at 110 r/min. The linear velocity of the yarn, V, equals ωr. What is the velocity? (Hint: care has to be taken with the units. If V is to be in ft/sec, then the rotational speed, ω, must be expressed in radians/sec and r in feet.)

V = 1.25 ft/sec

110 rpm

r = 1.3 inch

Fig. A2.1

Strand delivery


333

Ans. V = ωr ω = 2 π × 110 rev × min rad/ sec min 60 sec

ft 12 inch

r = 1.3 inch ×

ft V = 2 π × 110 rev × min × × 1.3 inch min 60 sec 12 inch = 1.25 ft/sec Q18. Determine the speed ratio of the pulleys shown in Fig. A2.2(a). The large pulley has a radius of rL inches the small pulley of rs inches and they rotate at UL and Us r/min, respectively. Ans. The belt speed can be determined by considering either the small pulley or the large one. The belt thus runs at: V = KULrL ft/sec, where K = 2π/(60 × 12) The small pulley radius is rs and it rotates at Us r/min, which gives: V = KUsrs ft/sec KULrL = KUsrs Thus ULrL = Usrs or

ULDL = Us Ds

[A2.1]

where D = diameter and the subscripts have the same meaning as already explained. It will be noticed that the constants cancel because we are dealing with ratios. Q19. An electric motor runs at 1800 r/min and drives a shaft by a pulley and belt US

UL

V

rs (a)

US

rL UL

V rS (b)

Fig. A2.2

rL

Belts and pulleys

334

Appendix 2

system. The pulley on the motor is 6 inches diameter and the pulley on the driven shaft is 18 inches diameter. What is the speed of the driven shaft? Ans.

Us = 1800 r/min, UL = ? r/min Ds = 6 inches, DL = 18 inches

From Equation [A2.1], UL = Us Ds/DL = 1800 × 6/18 = 600 r/min This answer is not completely accurate – see Q20 (b) and (c). Q20. (a) What would be the effect if the belt of Q19 is crossed? (b) What effects would slippage have? (c) What effect does belt thickness have? Ans. (a) If the belt were crossed as in Fig. A2.2(b), the direction of rotation of the driven member would be reversed and a minus sign can be introduced to take this into account. Thus the answer for the crossed belt case is minus 600 r/min. Ans. (b) There is always a slight amount of belt slippage, which slightly reduces the speed of the driven member. Ans. (c) The thickness of the belt cannot be ignored. It is usual to add one belt thickness to the actual pulley diameters in calculating the speeds. If a 1/8 inch thick belt were used in the foregoing example, and slip is ignored, the approximate speed would be: U1 ≈ 1800 × (6.0 + 1/8)/(18 + 1/8) ≈ 1800 × 6.125/18.125 ≈ 608 r/min

A2.4

Gearing

Q21. A motor runs at 720 r/min and drives a shaft by means of a sprocket and chain. The motor sprocket has 20 teeth and the driven sprocket has 80 teeth. What is the speed of the shaft? Ans. Let output speed = Uo

Output speed = 720 × 20 = 180 rpm 80 Q22. A compound gear system consists of a 20 tooth driver that meshes with an 80 tooth gear and the latter is locked concentrically with a 25 tooth gear that meshes with the output gear as shown in Fig. A2.3. The gear ratio is 16:1. How many teeth are there in the output gear? Ans. Let the output gear have m teeth. Gearing ratio = (–80/20) × (–m/25) = 16 whence m = 16 × 20 × 25/80 = 100 teeth.

A2.5

Machine speeds

Q23. To be able to get a reasonable output per card and yet only have a thin web of fibers on the main cylinder, it is necessary to have a high surface speed. Suppose


335

80 teeth

20 teeth Output gear 25 teeth Mesh Mesh

Fig. A2.3

Compound gears

there are 200 fibers/sq inch on the surface of a 40 inch wide card. There are 200 × 40 = 8000 fibers/inch of circumference on the card. If a single cotton fiber weighs 1.3 × 10–8 lb, there are roughly 8000 × 1.3 ×10–8 = 10.4 × 10–5 lb/inch of circumference. Assuming an output of 100 lb/hr, what is the surface speed? Ans.

1 inch vo = 100 lb × × 1 ft × 1 hr –5 hr 10.4 × 10 lb 12 inch 60 min = 1335 ft/min Q24. What is the rotational speed of the cylinder in Q23 if the diameter is 40 inches? Ans. U = V/πD r/min. The diameter concerned must be expressed in feet to be compatible with the velocity in ft/min. The diameter is 3.333 ft, V = 1335 ft/min, and U = 127 r/min. Q25. If the output is to be 65 grains/yd sliver, what is the sliver delivery speed in Q23? Ans. 1 yd 3 ft 7000 grain V d = 100 lb × × × × 1 hr hr 65 grain lb yd 60 min = 538 ft/min (or 179 yd/min)

A2.6

Twist calculations

Q26. What is the twist density, in tpi, of a 4 TM, 25/1 cotton yarn? Ans. From Equation [A1.14], twist density = TM √Ne = 4 √25 = 20 tpi. Q27. A 20 tex yarn has a TMdirect of 36 (α = 36); what is the twist level? Ans. Twist level = 36/√20 = 8.05 turns/cm. Q28. A yarn has 20 tpi and a count of 36s in the cotton system. What is the twist multiple? Ans. Twist multiple = TM = 20 / √36 = 3.33. No units need be quoted in this case.

336

Appendix 2

Q29. Plot a graph of twist level versus count, for a TM of 3.0. Ans. Set out a table of co-ordinates. Table A2.l

Co-ordinates of graph

Ne Ne τ tpi

4 2.0 6

9 3.0 9

16 4.0 12

25 5.0 15

36 6.0 18

The data are plotted in Fig. A2.4. 20

Twist/ inch

TM = 3

10

0 0

10

20 30 Yarn count (Ne)

Fig. A2.4

A2.7

40

Twist characteristics

Production

Q30. The front roll of a drafting system advances a strand into a twister that rotates at 10 000 r/min. The roll diameter is 1.2 inch. Calculate the front roll speed when a yarn of Ne = 25 hanks/lb and TM = 3.5 is being made. What is the speed ratio between the spindle and the front roll? Ans. Twist density = τ

τ = TM N e = 3.5 25 = 17.5 tpi Ut = twister speed in rev/min V = linear speed of yarn in inches/min = U t /τ V=

10 000 rev × inch min 17.5 turn

= 571 inch/min


337

But V = Ufr πD where Ufr = rotational speed of front roller and D = diameter of front roller. Substituting for V and D and rearranging:

rev U fr = 571 inch × = 151 rev/ min (i.e. r/ min) min 1.2 π inch Velocity ratio = 10 000/151 = 66. In other words, the spindle has to rotate 66 times as fast as the front roll of the drafting system. Q31. A roving frame running at 1000 r/min and producing a 1.1 hank roving (Ne = 1.1) at 0.9 TM will produce P lb/spindle hour. What is the value of P? Ans. Assuming the pu (per unit) efficiency is 1.0 P=

P=

Uη 504 × 0.9 × 1.1 ×

1.1

1000 504 × 0.9 × 1.1 ×

1.1

= 1.91 lb/spindle hr

[A2.2]

Q32. A traveler slides at 120 ft/sec on a 1.75 inch diameter ring. The twist density of the yarn being spun (τ) is 20 tpi and it is wound on to a 1.25 inch diameter bobbin. (Figure 7.3 shows a ring and traveler.) What is the percentage difference between the traveler and package speeds? What does this difference represent? Ans. Let ωt = rotational speed of the traveler, and since ωt = V/R: 2 ω t = 120 ft × × 12 inch = 1645.7 rad/ sec sec 1.75 inch ft Let Ut = rotational speed of traveler in traditional units

U t = 1645.7 rad × 60 sec = 15 718 r /min (i.e. rev/min) 2π sec min V = the linear speed of the yarn = U/τ Since τ = 20 tpi, V = 15 718/20 = 785.9 inches/min d = 1.25 inches and the wind-on speed = V/πd Uwind = 785.9/(1.25π) = 200 r/min This is 1.27% of the traveler rotational speed. Ring spindle speed = Ut + Uwind = 15 718 + 200 = 15918 r/min Note: As the bobbin diameter builds from (say) 1 inch to 1.6 inch, with a bobbin speed of 15 918 r/min, the wind-on speed varies from 785.9/π = 250 r/min to 785.9/(1.6π) = 156 r/min and the traveler speed varies from 15 918 – 250 = 15 668 r/min to 15 918 – 156 = 15762 r/min,

338

Appendix 2

a difference of about 0.6%. As the bobbin diameter changes, a small variation in twist occurs but the effect of this is neglected. Q33. A ring frame produces a yarn of average count of 25/1. The twist multiple is 3.5 and the spindle speed is 20 000 r/min with a spinning efficiency of 0.95. (a) What is the output for the given ring frame? (b) If the count were reduced to 36/1, what would be the output? Ans. (a) Equation [A1.24] contains the group Ne√Ne and it is easier to calculate this first. Ne√Ne = 25 × √25 = 125 Substituting this in Equation [A1.24] we get: 20 000 0.95 × × 1 = 0.0862 lb/ sp hr 504 3.5 125 (b) Calculating Ne√Ne as a preliminary step, Ne√Ne = 36 × √36 = 216 and inserting this in Equation [A1.24] we get: P=

20 000 0.95 × × 1 = 0.0499 lb/ sp hr 504 3.5 216 At least 12 spindles are needed in one case, and 20 in the other, to produce 1 lb/hr. P=

Q34. A mill has an output of 2500 lb/hr of yarn of 16/1 (Ne) at 3.8 TM spun on ring frames running at 15 000 r/min at an efficiency of 0.92 and a waste level of 1.8%. The ring frames are supplied with 1.1 hank roving (Ne), made on roving frames running at 1200 r/min and with a TM of 0.996. The efficiency of the roving frames is 93% and the fiber loss is 0.2%. (a) How many ring frame spindles, and (b) how many roving spindles are required? The mill has two passages of drawing and the drawframes run at 600 yd/min when producing 90 grain/yd sliver. (c) How many drawframe heads are needed if the operational efficiency is 95%, the sliver wastage is 1%, and each drawframe has two heads? It is intended to install cards, each with a productivity of 100 lb/hr. The waste fiber from carding and opening is 2% and the operational efficiency is 96%. (d) How many cards would be needed, (e) what input fiber flow would be required, and (f) what flow of new fiber would be needed if 50% of the waste from spinning, roving, and drawing is recycled? Ans. (a) Starting this question with the ring frames, the yarn flow required from them = 2500 lb/hr. Calculating the value of Ne√Ne = 16 × √16 = 64, the productivity of one ring spindle 15 000 × 0.92 = 0.113 lb/ sp hr 504 × 3.8 × 64 The number of ring spindles needed = 2500/0.113 = 22 124. If there were 800 spindles per machine, 27.66 machines would be needed; rounding this up gives us 28 machines. (This number would have to be increased to allow for maintenance shutdowns and repairs.) (b) Allowing for 0.008 pu fiber loss in spinning, the roving flow needed is: Prf =

2500 + (2500 × 0.008) = 2500 × 1.008 = 2540.16 lb/hr


339

TMroving = 0.996 and Ne √Ne = 1.1 × √1.1 = 1.154 1200 × 0.93 = 1.927 lb/ hr 504 × 0.996 × 1.154 Number of roving spindles needed = 2540/1.927 = 1318, say 1400. (c) Drawframe production/head for one passage, Pdf = Vsliver × nsliver Proving =

Pdf =

600 yd 90 grain lb × × × 60 min × 0.95 × 0.99 min yd 7000 grain hr

= 435.3 lb/ hr Allowing for 0.002 pu (per unit) fiber losses in roving, the throughput is: (the value in Answer (b) × (1 + 0.002)) = 2540 × 1.002 = 2545 lb/hr. Number of heads required = 2545/435.3 = 5.847 for one passage. For two passages the number required = 5.847 × 2 = 11.69 and rounding up, this would be taken as 12. With 2 heads/drawframe, 6 machines are required. (d) Allowing for 0.01 pu fiber losses in drawing, the card output is: (the value in Answer (c) × (1 + 0.01)) = 2545 × 1.01 = 2570 lb/hr. Taking the efficiency into account, the production rate/card is 100 × 0.96 = 96 lb/hr. The theoretical number of cards required would be = 26.77. However, one cannot have a fraction of a card so the number required is rounded up to 27. (e) After losing 2% of the fiber in carding and opening, the input rate is: 2570 × 1.02 = 2621 lb/hr (f) The specified wastes are: Spinning waste = 0.008 × 2500 = 20.00 lb/hr Roving waste = 0.002 × 2540.16 = 5.08 lb/hr Drawing waste = 0.01 × 2570 = 25.71 lb/hr ————— Total specified waste = 50.79 lb/hr Recycled waste = 50.79/2 = 25.4 lb/hr, which offsets the losses and the total fiber requirement drops by this amount. Thus, the net input fiber required in this case is 2622 – 25.4 ≈ 2597 lb/hr.

A2.8

Texturing

Q35. A texturing machine has a six-disk stack. The coefficient of friction, µ, is 0.2, the run-on and run-off angles are both 30°, and the inclination on the periphery of the disk is 0°. Assume that all disks are working disks and that they give no aid in moving the yarn through the stack. Calculate the output tension and the torque produced by disks 1 and 6. Ans. The angle of wrap for each disk = (90 – θ) × 2 = 120° (equivalent to 2π/3 radians). Let the input tension to disk 1 = T1 and, using Amonton’s Law (i.e. To = T1 eµθ), output tension from disk 1 = T1 e0.2×2π/3 = 1.52 T1. Since the passage past five disks accumulates an angle of wrap of five times that of the passage over a single one, and the angle appears as an exponent in the equation we may write:

340

Appendix 2

Input tension to disk 6 = 1.525 T1 = 8.11T1 Output tension from disk 6 = 1.526 T1 = 12.3T1 The torque generated by a disk = (T1 + T2) µ √n K cos θ where n is the linear density of the yarn, µ is the coefficient of friction, and K is a factor. Torque generated by disk 1 is ((1 + 1.52) × 0.2 × 0.866) KT1 √n = 0.436 KT1√n Torque generated by disk 6 is ((8.11 + 12.3) × 0.2 × 0.866) KT1√n = 3.54 KT1√n This torque for disk 6 is 8.1 times that generated by disk 1, but the maximum tension is 12.3T1; this is also 8.1 times the tension output of disk 1.

Appendix 3 Advanced topics I: Air conditioning and utilities

A3.1

Introduction

Water vapor is very important in yarn making in many ways. Frequently, steam is used as a heating medium. It is used because there is a unique relationship between pressures and temperatures of steam, which makes it relatively easy to control. There is often an advantage in applying moist heat to set polymers, especially when hydrogen bonding occurs within the polymer molecular structure. The air in which we live is really a mixture of air and steam. In a textile mill, where high humidities are needed, the proportion of steam is rather high and it is helpful to understand its characteristics.

A3.2

Units

Conventionally in the USA, units are expressed in pounds, feet, and seconds, whereas the SI system uses grams, meters, and seconds. The relationships between derived units may not be obvious and it may be of use to discuss them. In thermodynamics, the interest is in mass, temperature, and energy as well as some other parameters. Temperature is commonly measured on two scales (i.e. Fahrenheit and Celsius) with two important points being fixed by the freezing and boiling points of water under normal atmospheric pressure. On the Celsius scale, water freezes at 0°C and boils at 100°C. For many thermodynamics calculations it is necessary to work from absolute zero rather than from an arbitrary one fixed by the characteristics of a single substance. This absolute zero is –273°C on the Celsius scale, which is awkward; it is preferred to set absolute zero at the datum but use the same intervals as the Celsius scale. This is now called the Kelvin scale. Conversion is made by merely adding 273 degrees to the temperature in Celsius; the result is written as °K. The absolute zero measured in Fahrenheit is –460°F; when the intervals are the same as the Fahrenheit scale, the result is called the Rankine scale. The heat content of a material is the product of mass × specific heat × temperature, where the heat content and temperature are reckoned from some arbitrary levels. This

342

Appendix 3

assumes that there has been no change in state, such as from solid to liquid or from liquid to gas. The specific heat is a property of the material; it is proportional to the amount of heat that has to flow to or from a unit mass of the material to change its temperature by 1 degree. In the conventional system, the specific heat of water is 1.0 and this is used as a reference for other materials. Temperature is a measure of thermal ‘pressure’; heat will flow more rapidly through a material as the temperature increases. Care should be taken to discriminate between temperature and heat. A thermometer measures temperature but it is necessary to know the specific heat and mass of a substance before the quantity of heat can be determined. In the conventional US system, the unit of heat is the Btu and this is defined as that heat required to raise one pound of water through 1°F. Heat is a form of energy but there are other forms. For instance, electrical energy is measured in joules. Mechanical energy is measured in ft lb (that energy needed to raise 1 pound through 1 foot). These are mutually convertible; for example, 778 ft lb is equivalent to 1 joule. The Sl system expresses all forms of energy in joules. Power is the rate of using (or producing) energy. In engineering the common term is horsepower, which is defined as 550 ft lb/sec. In thermodynamics, the term is often expressed in Btu/hr, whereas in electrical engineering the unit is the watt. A watt can be variously defined as the energy flow in joules/sec or the product of voltage and amperage. One ampere is the flow of electrical charge in coulombs/sec; thus, since 1 coulomb may be defined as 1 ampere second, the two definitions come to the same thing. All forms of power are mutually convertible and the SI system uses the watt as the universal unit of power.

A3.3

Water vapor and steam

A3.3.1 Change of state When a material changes from solid to liquid, liquid to gas, solid to gas, or any of the reverse processes, we refer to a change of state. A change of state is nearly always associated with a taking in or a giving out of heat. This is an important property of most materials; water and thermoplastic polymers are no exception. Many of the ideas discussed relate to water, but they have their counterparts with other materials. Naturally, the values of the data can be widely different.

A3.3.2 The properties of steam Water absorbs energy as it is heated, with the result that the temperature rises until it reaches its boiling point. Beyond this boiling point, any further addition of energy causes water to be converted to steam but the temperature does not rise again until all the water has been boiled off. The boiling point rises with pressure. Table A3.1 shows how the latent heat changes. It can be seen that the temperature of wet steam can be controlled by the pressure in the steam vessel. With wet steam, some water remains in the steam and, as energy is added, the percentage of water drops. When all the water is converted, the steam is said to be ‘dry’. In practice, it is much easier to control pressure than temperature, and, for this reason, the use of steam as a heating medium is quite popular. Within limits, there is automatic regulation of temperature all the time that the steam is wet and at constant pressure. If an autoclave (or steam kettle) is used to set or dye yarn or fabric, it is usually necessary to adjust the kettle to some pressure higher than atmospheric. For example, if a temperature of 150°C is

Advanced topics I: Air conditioning and utilities

343

required, the autoclave should be run at 70 psi or 482 kilopascals, as shown by Table A3.1. The term ‘steam’ describes water vapor without any other gas or vapor present. Steam that is just dry (i.e. the last drop of water has just been converted to steam) is called saturated steam. A dryness fraction, q, is used to define the actual specific volume of wet steam in respect to the specific volume of saturated steam at the same pressure and temperature. The term ‘specific’ means that the quantity relates to unit mass; specific volume is the volume taken by a unit mass of the substance. The specific enthalpy or energy content of wet steam, H, is given by the simple formula: H = s + qL

[A3.1]

where: s = specific enthalpy of water (i.e. sensible heat), L = specific enthalpy associated with a complete change of state from water to dry steam (i.e. latent heat), and q is the dryness fraction.

A3.3.3 The properties of air/steam mixtures If two or more gases or vapors are mixed in a confined space and there is no chemical reaction, each component fills the whole volume and each exists at its own particular partial pressure. The sum of the partial pressures equals the total or applied pressure. This is known as Dalton’s law of partial pressures. With a perfect gas, the mathematical group P Vol/T remains constant for the particular gas and the constant is denoted by the symbol G, which relates to energy/unit mass, and stands for the universal gas constant. P is the pressure and Vol is the specific volume of the gas. The temperature must be expressed in absolute terms. With an imperfect gas or vapor, such as steam, the characteristic equation mentioned is inaccurate and one must determine the specific volumes from tables such as Table A3.1. In the case of steam/air mixtures, the absolute volume and temperature are common to both the steam and the air. The mass of each component is the quotient (absolute volume/specific volume). The absolute volume is the whole volume occupied Table A3.1

Properties of steam

Conventional units t

SI units

°F

P psiabs

H Btu/lb

Vol cu ft/lb

L Btu/lb

°C

t

P kPa

H J/g

32 40 50 60 70 80 90 100 150 212 250 293 320 358 417

0.0885 0.1217 0.1781 0.2563 0.3631 0.5069 0.6982 0.9492 3.718 14.696 30 60 90 150 300

1076 1079 1084 1088 1092 1097 1101 1105 1126 1150 1164 1178 1185 1194 1203

3306 2444 1703 1207 868 633 468 350 97.1 26.8 13.8 7.17 4.9 3.02 1.54

1076 1071 1066 1060 1054 1049 1043 1037 1008 970 945 915 895 864 809

0 4.4 10 15.5 21.1 26.7 32.2 37.8 65.6 100 121 145 160 181 214

0.61 0.84 1.23 1.77 2.5 3.49 4.81 6.54 25.6 101 207 413 620 1030 2070

2502 2491 2479 2465 2451 2440 2426 2412 2344 2256 2198 2128 2081 2010 1882

Vol m3/kg 206.3 152.5 106.3 75.3 54.2 39.5 29.2 21.8 6.06 1.67 0.861 0.447 0.306 0.188 0.096

Notes: t = temperature, P = absolute pressure, H = total enthalpy of dry steam, Vol = specific volume, L = latent heat

344

Appendix 3

by the mixture. The specific volume of steam must be found from tables, whereas for air, the characteristic gas equation can be used, i.e. Pa Vol = MG

[A3.2]

where Pa is the partial pressure of the air, and Vol and M are the volume and mass of air involved respectively. There are two cases in which steam/air mixtures assume importance. One relates to moist air, and the other to air leaks in autoclaves and boilers. In the second case, the effect of air leakage into a steam system is to reduce the partial pressure of the steam, which then causes a reduction in temperature. A normal gage can only measure the sum of all the partial pressures and it cannot detect the displacement effect of the intruding air. An inward air leak can cause a drop in temperature (which might be undetected), and this can cause difficulty in some dyeing and setting operations. The problem is compounded because water contains dissolved gases that are released on boiling; the released gases act in the same way as the air leak. Thus, boiler water should be de-aerated and steam traps should be used to permit removal of air and gas without loss of steam. The dissolved air can cause corrosion in boilers and equipment and it is prudent to remove it for this reason also. Air leaks are more likely where steam cools and the internal pressure drops below atmospheric.

A3.4

Humidity

A3.4.1 Humidity in the workspace Normal air is really a mixture of air and superheated steam. Superheated steam has been heated above its saturation temperature, but if the partial pressure is very low, the saturation temperature is also low. When the temperature of the humid air is reduced to its dew point, the steam starts to condense and droplets of water are precipitated to form a fog. In a workspace, water is deposited on cold surfaces and, since these are often of steel or iron, there can be a problem with rust. Thus, it is good practice to keep the temperature of the workspace above the dew point, always. As the temperature of the air is increased above the dew point, the specific volume of the air increases and the air is drier. The normal measure of wetness is relative humidity measured on a 0 to 100% scale. It is, in fact, the ratio of the amount of moisture that the air actually holds to the maximum that it could hold at the same temperature. It can also be defined in terms of the partial pressures. Steam tables such as Table A3.1 could be used to calculate such conditions, but it is more normal to use psychometric charts such as shown in Fig. A3.l. Two styles are given, one in SI (diagram (a)) and one in imperial units (diagram(b)). To use these charts, one needs to know the wet and dry bulb temperatures. These are measured by a pair of thermometers; one element is kept dry and the other is kept moist. The evaporating water from the wet bulb keeps the local temperature down to the dew point. It is possible to see how the moisture content increases as the air is heated at constant rh by tracking the line AE in diagram (b). It is also possible to see how the state point A is defined by (i) the wet and dry bulb temperatures or (ii) the dry bulb temperatures and rh. A3.4.2 Air conditioning A mill has to have a controlled climate if high quality yarns are to be made. In drafting, fibers can stick to the rolls and there are several possible causes for this

Air densities in m3/kg (shown with dashed line) A = 0.80 B = 0.85 0.03 C = 0.90 30°C Wet bulb

100 %r h 80% rh 60% rh 40% rh

345

30

0.02

20

0.01

10°C Wet bulb

Dew point (°C)

C

20°C Wet bulb

20 % rh

Mass ratio = mass of moisture/mass of air


B 10

A

10 20 30 40 Dry bulb temperature (°C)

%r h 80% rh 60% rh

(a)

E

70

ra

rh

pe tem B

60

20%

lb bu 50

60

40

Dew point (°F)

es

(° F

)

40%

rh

100

80

tur

100

W et

Moisture content (grain/lb)

150

40

A

D C

40

60 80 100 Dry bulb temperature (°F) (b)

Fig. A3.1

Psychometric chart

problem. One is stickiness, caused by too damp an atmosphere. Another is electrical charging of the fibers, which causes them to be attracted to a surface. The charge becomes a problem when the atmosphere is too dry. The best rh of the air depends on the fibers and the roll coverings, but a typical value is 55% rh.

346

Appendix 3

It is not a simple matter to get the rh to the correct value everywhere in the plant. For example, in a spinning room, it is not unusual to find zones that are too wet or too dry. Certainly, one must not rely on the wall-mounted hygrometers since they merely record at fixed locations. The use of a portable hygrometer will quickly reveal the bad zones. Hot areas, such as near a motor, give low values, and inappropriate values are often found near doors, especially when the outside conditions are far from ideal. To give an idea of the magnitude of the problem some mill experience will be quoted. In one mill where the ring frames were positioned very close to one another, the rh at the ring rail varied from 30% to 35%, even though the wall-mounted hygrometers read 55%. The mill performed badly. Another, with widely spaced ring frames and a welladjusted air conditioning system only showed ±1% rh. Most mills fall between these extremes. The setting of the air diffusers and the pneumafil suctions can greatly affect the uniformity of the rh throughout the room. A hint of poor distribution is sometimes given by accumulations of fibers on the ceiling and light fixtures. Old buildings with exposed beams and glassed areas are particularly difficult because of the heat transfer through the roof and the large volumes of air trapped there. Careful attention to air conditioning and distribution can save many later operational difficulties. The ducting in the air distribution system must be designed to give uniform distribution throughout the room. Also, the flows from the supply have to be balanced with the main return air systems, as well as with the suction systems removing waste fiber.

A3.5

Mill environment

A3.5.1 Energy balance in an enclosed workspace Considerable amounts of energy from electric motors and other devices are dissipated in the workspaces. Not only is there heat dissipated from the motors, but also from the machines themselves. The machines take mechanical energy from the motors and do work in overcoming the resistance to movement of the machine parts and this translates the energy into the heat form. Thus the machine parts get warm. For example, bearings and belts get hot. The movement of the parts disturbs the air and dissipates further energy; for example, the air leaving the rotors in OE spinning gets very hot. The temperature difference between the machine parts and the air also causes heat transfers to occur. Thus the original input electrical energy is translated into heat energy at every stage in the process. In cool winter climates, heat escapes from the entrances and exits as well as by conduction and radiation through the shell of the building. Balance is normally obtained by applying a heating system. In hot summer climates, the heat flows are reversed and air conditioning has to be applied to keep the temperature down. Consider the energy balance in an enclosed space: If Eelec = electrical energy input to the space, Ecomb = energy input derived from combustion of fuels Etherm = thermal energy passed through the shell of the space due to a difference in temperature between inside and outside. Etherm = Total mass of enclosure × (Toutside – Tinside) Tinside = is normally controlled to be constant and Etherm may have a positive or negative value. Eac = thermal energy pumped out of or into the space by the air conditioning


Emat

347

plant. Eac is negative in summer when energy is being pumped out by the refrigeration plant and positive in winter. = is the sum of the differences between input and output in mechanical strain, thermal and other energies resident in the textile material, which differences are normally insignificant.

Since there is usually no chemical or nuclear reaction involved, the energies described must be conserved and if similar units are used for all forms of energy: Eelec + Etherm + Ecomb + Eac ≈ 0

[A3.3]

Availability of thermal energy is determined by the temperature at which it exists. Every transfer degrades its availability. Eventually, it is all dissipated as low grade heat that cannot be recovered economically. It is not just a question of balancing Etherm + Ecomb and Eac but for every horsepower used within the space, there is a dissipation of 746 watts from the machines themselves in addition to losses from the electrical system. The energy from the lighting system is also dissipated as heat.

A3.5.2 Energy removal from the workspace With a fixed amount of moisture present in the atmosphere, temperature changes are accompanied by changes in rh. Consequently it is necessary to control both temperature and rh. The additional heat from all these sources has to be removed to maintain an even condition. In the past, air conditioning often has been given little priority, with small regard for the heat loading from the equipment within the building. However, as the equipment installed consumes ever more power in the quest for higher productivity, the importance of the heat loading becomes more apparent. In tropical or semi-tropical countries, each kW of power used must be pumped out again while the refrigerating air conditioning system is in use. This becomes particularly apparent with high speed rotor spinning. Schemes where the motors and hot parts of a machine are cooled separately from the main workspace have appeared and these are to be encouraged. This source of heat might be useful in a cold climate but in a hot one it adds to the air conditioning costs. Eckert [1] pointed out that a deciding feature of the air conditioning system for rotor spinning is the increased direct exhaust air capacity of the machine itself. The high temperature difference between the exhaust air and the ambient makes it easier to pump out the heat. He quotes values of 27°C difference. Since that article, speeds have risen and, because the energy consumed rises approximately to the 3rd power of speed, the temperature difference he quotes must be low by modern standards. The attractiveness of direct exhaust system cooling rises accordingly. Eckert also points out that the distributions of supply, exhaust, and return air are of utmost importance (as they are for any spinning operation). If the heat from the motor, head-, and tailstocks is directly exhausted, it is then only necessary to remove from the spinning room little more than the heat loading from the lighting and transmissions. This means that the amount removed from the room at normal ambient temperatures is less than one-third of the total. The remainder is removed at temperatures up to 40°C higher. A refrigeration process is sensitive to temperature differences across its cooling coils and this means that it is easier and cheaper to remove the heat from the hotter air. Also, heat removal by water washing of hot air reduces the temperature more quickly than it does with air at atmospheric temperature. Combinations of air-wash

348

Appendix 3

and refrigeration can be designed for optimum efficiency and the proportions for hot and normal temperature returns may well differ. Against the gains in operating cost have to be set the costs arising from the extra capital investment needed. Ducts to carry away the hot air (preferably underfloor) have to be installed. Separated air conditioning systems are also desirable to deal with the two classes of return air. Obviously it is an advantage to install such systems when the plant is first built.

A3.5.3 Filtration Not only does the air in a mill have to be maintained at an rh best suited to the particular task, but it has to be clean. In many regions of the world, air quality in mills is quite stringently monitored. There are regulations in many countries mandating maximum levels of particulate matter in the air in the opening and carding areas. Such filtration is especially important in cotton spinning because of the incidence of byssinosis (an allergic lung disease) in some workers. Even if automation is used to minimize the amount of human exposure, the regulations still apply. In other forms of processing, noxious chemicals can be given off and the climate has to be controlled there too. A further important reason for attention to cleanliness is the fire risk. Some airborne fibers and dust are flammable; fire and explosion risks are severe. Enclosed, spark-free motors have to be used and a number of fire hazards are outlawed. Special fire-fighting arrangements must be provided and most local authorities have a Fire Code, which requires compliance. Lint and harmful dust have to be removed from the return air. Where the concentrations of dust and fly are high, such as in the returns from the carding and opening areas, cyclone separators are used to extract the heavier fraction of waste. Most of the remaining dirt is removed by electrostatic precipitators, fabric filters or the like. The air is usually washed as well. One important concern is the maintenance of the atmosphere within the comfort zone of the operatives (Fig. A3.2), especially if maximum performance is expected.

Moisture content of air (grain/ lb)

A3.5.4 Fly Another concern arises about the level of fibers in the air (fly) because it can and does cause defects in the yarn produced. An accumulation of fly landing on the yarn being made in a ring frame obviously creates a blemish. Not so obvious is the result of fly on other products. For example, fly landing on roving during manufacture is 100

100% rh

70% rh 30% rh

Human comfort zone

50

40

60 80 100 Temperature, dry bulb (°F)

Fig. A3.2

Human comfort zone


349

often thrown off again during spinning, only to land on the yarn. A good spinner keeps an eye open for the sources of fly production and tries to eliminate them. Fiber and dust can carry electrical charges, and so can the surfaces of machinery. Friction between moving surfaces, and separation of those surfaces, also produce electrical charges. Much of the dust and fiber is highly flammable and if the electrical charges build up sufficiently to cause a spark, then a dangerous conflagration can occur. The remedies are to keep rh at a proper level and to ground all machinery by connecting it to earth with a conductive cable. In that way, electrical charge build-up is minimized and any that does form is leaked to earth before it can cause damage [2]. There are also other reasons for controlling the electrical charge. If the rh is too low, electrified fibers coagulate and interfere with processing. The optimum level varies from fiber to fiber and from process to process. A common symptom of incorrect humidification in spinning is when roll laps occur. A3.5.5 Lighting Most workspaces in a modern mill are lighted exclusively with artificial lighting. The electrical load can be limited to about 16 watt/square meter and still provide the necessary 550 lux level of illumination. To achieve this economy, efficient light sources, good reflectors, and clean, well-maintained, light-colored ceilings must be used. Windows not only allow passage of light, but they form an easy path for noise and heat transmission. Thus, natural lighting is avoided because of the increase in load for the air conditioning plant and the increase in noise radiation to the outside. Maintenance costs of the windows are also avoided. A3.5.6 Effects of chemical contamination Fibers or subsequent products treated with noxious chemicals can form a hazard. Chemical emissions from any product or machine in the mill must be strictly controlled and the necessary venting must be supplied. Such emissions are usually subject to regulation by the local authorities. Where singeing is used, not only must the products of combustion be properly vented, but the work area must be sealed off from the main work areas to minimize the fire risks. The particulate level in the air should also be monitored because soot inevitably escapes into the atmosphere. In filament texturing, where the filaments are raised to high temperatures, lighter fractions of fiber finish may boil off. It is important that the gases emitted are not toxic, harmful to the product, or harmful to the machine. Again, proper ventilation is required. Man-made fibers have fiber finish which sometimes becomes removed from the fibers and accumulates on certain important surfaces on various machines. Also some fibers can produce oligomers which deposit on the working surfaces. In places, the deposit forms a so-called ‘snow’ that is a sure sign of this sort of trouble. Cleanliness in this respect is a necessary condition for the production of high quality products.

References 1. 2.

Eckert, O. Up-to-date Engineering design and Planning in the Concept of Open-end Rotor Spinning Plants, Textile Machinery: Investing for the Future, Textile Inst Ann Conf, 1982. Anon, Static Electricity, Nat Fire Protection Assn, Boston, USA, 1947.

Appendix 4 Advanced topics II: Testing of textile materials

A4.1

Divisions in testing

A4.1.1 Introduction There is a gray area concerning quality control and testing. In one sense it is very clear that the technicians who carry out the testing should have some say in the sampling and interpretation of results. However, it is not always clear where the measurements and techniques stop, and where the use of the results as a control medium starts. The reason for testing is to acquire the data on which sound decisions can be made; the reasons for quality control are to ensure adequate quality of product and minimum trouble in processing. In a mill, testing is not an end in itself. Testing falls into two categories: laboratory (or offline) testing and online monitoring. In the first mode, shown in Fig. A4.1(a), samples of the product are taken to the laboratory for testing. Various sorts of test apparatus are used. The test laboratory must be air conditioned because the moisture content of textile fibers varies with ambient conditions and many tests are strongly influenced by moisture content. Normally, a laboratory is maintained at 70 ± 2°F (21 ± 1°C) and 65 ± 2% rh for 24 hours/day over the whole year. Samples brought into the laboratory have to be conditioned for sufficient time before testing to allow the moisture content of the textile material to reach equilibrium with the laboratory environment. Whilst it might only take a few minutes for a single fiber to reach equilibrium, a can full of sliver might take several days. As a guide, a tightly wound yarn package should be conditioned for 48 hours. In the second mode, shown in Fig. A4.1(b), sensors are fitted into the machine, which generate signals that describe some function of the material or machine performance; these signals are processed by a computer. The sensors should be insensitive to changes in the environment or should have corrections applied to neutralize any changes in anything other than the parameter being measured. The computer may be local or it may be a central unit. The output of the computer can be used either for control or for information or for both.

Advanced topics II: Testing of textile materials 351 Textile processing machine

Textile product

Sensor

Textile processing machine

Laboratory Control Computer Testing apparatus Data (a)

Data

Fig. A4.1

(b)

Testing modes

A4.1.2 Laboratory testing The priorities for testing filament yarns differ from those for staple yarns. In staple spinning, the most common measurements relate to variations in linear density, fault levels, fiber properties, yarn twist, and strength. Increasingly, laboratories are using high volume instruments (HVI) to measure many fiber properties in a semi-automatic measurement line. Other testing equipment is also becoming automated. Since the number and type of the individual measurements are likely to vary, no description of HVI lines will be described per se, but individual component tests will be described under the appropriate headings. In filament testing of yarn being used for household, apparel or industrial applications, bulk, fiber strength, elongation, and dyeability are among the most common tests made. For household applications, cover is a very important factor and consequently yarn bulk tops the list of priorities in these cases. In both staples and filaments, consistency of fabric appearance is important; consequently yarn defects, dye affinity, and variations in linear density, etc., are becoming of increasing importance. For many industrial uses, strength is the most important factor. A4.1.3 Online monitoring of production Many yarn and fiber attributes cannot be measured online because of the technical difficulties and costs involved. However, it is now established practice for the count of yarn, or any intermediate product, to be measured online. Also, yarn hairiness and defect levels are commonly monitored continuously. Sensors might be connected to devices that sound an alarm when the monitored attribute moves outside the control limits. Any of the signals generated by the sensors involved in monitoring might be used for control purposes, but in practice it is likely to be used only as a proxy for mass/unit length.

A4.2

Measurements on staple fibers

A4.2.1 Fiber length Drafting waves in roller drafting are caused when there is a significant difference between the ratch and effective fiber lengths. The ratch settings are set to standard values but variations in fiber length occur within any given sample, as well as from

352

Appendix 4

sample to sample. Thus, there are always problems with drafting waves and some control of effective fiber length is desirable. It will be recalled that ‘effective fiber length’ refers to the in situ behavior of the fiber; the effects of hooks, lack of fiber straightness, and fiber ends are all taken into account. Thus, it is necessary to consider not only the fiber length distribution but also how processing affects the geometry of the fibers. The traditional method of displaying the fiber length distribution is to progressively comb out fibers of descending length from a prepared sample. Fibers are then placed on a velvet board in the form of an array, as illustrated in Fig. A4.2(a) (the array shown is for American cotton). The actual procedure needs skill and is time-consuming. An alternative is to use a machine to create a fibrogram. The samples have to be prepared before being measured to ensure that the fibers are parallel and that unclamped short fibers are removed from the fringes to give a length-biased sample. This is done by using a comber roll. A fiber fringe is produced in which one end of the beard is clamped and the free ends extend away, with the fibers parallel to one another. A schematic sketch of such a fringe is given in Fig. A4.2(b). Fibers are viewed in a narrow aperture in an optical or capacitive system. The aperture is called a window in the diagram. The signal derived from the sensor elements provides an estimate of the number of fibers in the window. If the clamp is moved to change the dimension X, the number of fibers viewed in the window also changes. A plot of the number of fibers against X gives a fibrogram from which the distribution of fibers can be deduced. The fiber array is often typified by two span length readings. The span

(a) Fibers

Window

X Clamp (b) Reaction force measured by load cell a and b = hooked fibers Clamp Clamp

b a

Fibers

Force caused by downward movement of lower clamp (c)

Fig. A4.2

Fiber measurement

Advanced topics II: Testing of textile materials 353

length is the average length of those fibers that fall in the longest y% of the fiber population. Values of y commonly used are 2.5% and 50%, the long fibers being typified by the 2.5% span length and the short ones by the 50% span length. The span length changes and the hooks are pulled out as the material passes through the various draft zones. The added variance in drafting is greater for the 50% span length fibers than the 2.5% ones. It is normal to report the upper half mean length, uniformity index, and the short-fiber content. Uniformity index is defined as the ratio between the 2.5% and the 50% span lengths; it is often quoted as a percentage. The short-fiber content is taken as the percentage of fibers less than 0.5 inches. Care should be taken in comparing results from the various sorts of length sampling; different machines may produce results that are not truly compatible because of differing sampling procedures. Some testing equipment is able to report using differing sampling schemes, which is of considerable value if test results are to be compared in different departments or different mills. The clamping of hooked fibers can give ambiguous results. If hooked fibers are clamped at the base of the ‘U’ of the hook, then the fiber length, as seen by the testing machine, is shorter than the real length. If the fiber is clamped at the extremity of the longest leg, the measured length will be close to the real length. There are intermediate conditions, which give a variety of errors. This is despite the combing operation, which is part of the preparation procedure. It follows that, where there is a predominance of leading or trailing hooks, such as with sliver, the direction in which the material is mounted in the clamps becomes important. If there is doubt about the direction, the material should be tested in both and the higher of the two values should be used. Insufficient combing during fiber preparation produces error also. Some fiber breakage will occur during specimen preparation, but the error from this source will be small providing the fiber fringe is not too vigorously combed. A compromise between under- and over-combing has to be found for minimum error.

A4.2.2 Fiber strength Fiber strength is measured on a beard of fibers clamped as indicated in Fig. A4.2(c). One clamp moves relative to the other to extend the fibers and a load cell attached to the fixed clamp measures the load on them. If the number of fibers in the crosssection is determined, the fiber breaking load can be determined. Further if the average fiber linear density is known, the fiber breaking stress may be calculated. On an HVI line these measurements are made on the same samples, which helps in the matter of accuracy of result. The system is similar in principle to that later described for yarn. Again, hooked fibers can reduce the indicated fiber stress with respect to the actual one; furthermore, insufficient combing will reduce the indicated stress because of the lack of fiber orientation in the direction of loading.

A4.2.3 Fiber fineness Fiber fineness is measured by placing a given mass of fiber in a container of given volume and measuring the air permeability by passing air through the mass. Fiber fineness is related to this air permeability and the results are expressed as the micronaire index. If a fiber is loosely packed into a standard tube and air at standard conditions of

354

Appendix 4

temperature and humidity is passed through the packed tube, a pressure drop will be found. This air pressure drop/unit flow volume is proportional to: k (As)2ρM2/{(Kρ) – M}3

[A4.1]

where As is the surface area of the fiber sample, ρ is the packing density of the fiber, M is the mass; k and K are factors. For a standard sample mass of a given fiber, the pressure drop is a measure of the surface area of the fiber, from which the fiber diameter or linear density can be estimated. The importance of maintaining standard conditions is evident from Equation [A4.1]; in particular, the denominator is sensitive to error. Accuracy in sample mass is very important. Uniformity of fiber packing in the test volume is also an important factor and any tendency for the fibers to occlude will produce an error. Despite the care needed to get reliable results, the test is simple and rapid and it is widely used in mills. The result is described by the term ‘micronaire index’ (‘micronaire’ for short). Cotton fibers from a given geographical zone often have factors k and K which vary but little, and the test is useful within such zones. For example, the ‘micronaire’ test is widely used within the USA because of its reliability under normal circumstances for US grown upland cotton or other varieties. However, if cottons from many geographical zones or varieties are mixed, there could be difficulties. Other more sophisticated instruments are available, as described in Section A4.2.4. In the case of flax fibers, the fibers are gummed together at the beginning of the process and they have to be divided during processing. It is possible that some fibers are still gummed together at the end of the process and the variability in apparent fiber fineness might be more marked than desired. Also, since the product is a natural fiber, the fiber fineness is variable. Nevertheless, the fineness of the fiber is a parameter that should be controlled. The usual equipment for these measurements is a simple air permeability tester with a short test cylinder into which a U-shaped sample is inserted. Variations in the cross-sectional shape of the fibers affect the air permeability tests as just described. This is a disadvantage as far as measuring fiber linear density (micronaire) is concerned, but it brings with it the advantage of ease of use.

A4.2.4 Cotton immaturity Immature cotton has a different cross-section from mature cotton, and the permeability test helps identify this important parameter. Immature cotton fibers also have dyeing characteristics that differ from those of mature fibers. Thus it is often important to determine if the fiber being used is mature or not. The fiber fineness test described in Section A4.2.3 is incapable of discriminating between the effects of changes in fiber cross-sectional shape and fiber ‘diameter’ (i.e. fiber fineness). In trying to measure two parameters with one measurement, there is always an ambiguity. Sometimes, with a restricted source of supply, the immaturity and micronaire correlate but outside that sphere it is necessary to conduct a second test. One way is to carry out second permeability tests on fibers swollen by treatment with caustic soda and make comparisons with the first test on unswollen fibers. Alternatively, double compression tests can be used. A constant airflow device is used to measure the pressure drop of a standard sample at two different fiber-packing densities. A second alternative is to use image analyzers, but the expense of the equipment limits this option. Projection and other microscopes can be used to look at single fibers or small


groups of them, but the labor and equipment involved again makes this a fairly expensive alternative.

A4.2.5 Optical character Color and reflectance are measured by an optical system; yellowness (+b) and reflectance (Rd) are reported. Trash content is also measured by an optical system that relies on the fact that trash is darker than cotton. The percentage of the viewing area of the specimen that is dark is taken as a measure of trash content. The measurements of fiber length, strength, fineness, color, and trash content can all be made on a single, automated machine known as a high volume instrument (HVI). The use of such machines has virtually replaced manual cotton classing in the USA and they are widely used elsewhere.

A4.2.6 HVI calibration A problem that exists with the HVI relates to consistent calibration of the machines over time and between various laboratories. Since their use is mainly with cotton, emphasis is placed on the use of calibration cottons as a standard. The difficulty is that even the calibration cottons are variable; consequently, statistical control techniques have to be used to keep the machines within acceptable limits of calibration.

A4.3

Measurement of linear density of staple yarns

A4.3.1 Determination by weighing A commonly used manner of testing for yarn count is to weigh skeins and derive an average value. (A skein usually contains 120 yards of yarn.) However, the variations in count from skein to skein must be closely watched in order to prevent barré in fabrics. Commonly, this form of measurement is used both in the production facility and the laboratory. In a long specimen such as a skein, short wavelength errors escape detection; there can be no information gleaned about the inch-to-inch variation in linear density. Sliver is usually measured by a yard board (Fig. A4.3) which is simply a template

Cut 1 yd

Sliver

Cut

Hinged sliver board Sliver

Fig. A4.3

Yard board

356

Appendix 4

for cutting a set length of sliver. Other lengths can be measured using the same technique. It is a manual method often used in the mill, which is simple to use; it is a test that is reasonably accurate and the procedure is fast. Again, it is necessary to test a sufficiency of samples.

A4.3.2 Continuous measurement The discussion in the previous section centered on the mid and upper limits of length. now consider the lower limits. In continuous measurement, sensors are used to measure some set of attributes that reflect the parameter to be monitored. For example, linear density might be measured by using a pair of electrodes to measure the capacity of the electrode gap with the staple yarn, roving or sliver passing through that gap (Fig. A4.4). An example of this type is the Uster evenness tester [1]. A small, varying electrical voltage across the plate enables differences in the electrical capacity of the electrode system to be detected. As the mass of yarn in between the electrode changes, so does the capacity. The length of the electrode, L, is only a few millimeters and so the data stream reflects very short wavelength changes. Thus, the equipment is able to monitor continuously the changing linear density of a running yarn and the resulting electrical signals are converted to coefficients of variation, spectrograms, and strip charts. There is a relationship between the capacitance and the linear density under controlled conditions, which makes it possible to treat the output from the monitor as a measure of linear density [1]. As a second example, a beam of light can be used to project a shadow or image of the passing material and to translate that image into an electrical signal. In a properly designed system, the signal has a reasonably stable relationship with linear density and is often regarded as a measure of it. In these and other devices, the active sensor element is usually of the order of 1 mm, as measured along the length of the material being measured. Consequently there is no problem in resolving the data containing the complex spectrum. More detailed discussion of electrode width is given by Furter [3]. Some electrical circuitry in the system as sketched in Fig. A4.4(a) has been omitted for simplicity. Also, as a precaution against the effect of stray electrical fields, it is normal to surround the active electrodes with guard elements. These guard elements are set into the same plane as the active ones and are earthed, or held at a controlled voltage. If we take each of these in turn: capacitance is controlled by length, L, the dimensions of the gap, the cross-sectional shape of the strand, the position of the strand within the gap, and the dielectric constant of the material in the gap. Within limits, L can be adjusted electronically. It is important to use the correct set of electrodes with the proper gap size for a given material. Several gap sizes (or slot numbers) are available on the commercial testing machines. Air has only a negligible effect, but moisture in the fibers has a powerful effect. Thus, it is important to control the moisture content of the fibers if an accurate result is required. The effective electrode length can be changed by integrating the results over time. Such integrated values are used for inert tests in which the higher frequency variations are damped out and the signal is smoothed to give a moving-average value. One form of record (Fig. A4.4(b)) relates the deviation of linear density to time elapsed. Since both the textile material and the chart travel at known speeds, it is a simple matter to translate time into length of material. These charts are known colloquially as strip charts and technically they are expressed in the time domain.

Advanced topics II: Testing of textile materials 357 L

Electrodes Bridge

Yarn

Output signal

Amplitude

Yarn (a)

Amplitude

Time Error wavelength, log scale (inches) (b) 2 4 8 16

2 4 8 16 Error wavelength, log scale (inches) (c)

Fig. A4.4

Capacitive transducer system

Strip charts have their main value in making long-term variations visible. In this regard, long term relates to the length of textile material tested. Extra long-term trends cannot be seen. The charts are also valuable for detecting irregular yarn faults and disturbances that, because of their non-repetitive nature, do not show up on a spectrogram. A second form of diagram is the spectrogram, which is a chart expressed in the frequency domain where repeating patterns of deviations are resolved by frequency or wavelength rather than time (Fig. A4.4(c)). For example, a simple sine wave can

358

Appendix 4

be expressed as a single line at the appropriate frequency or wavelength in the frequency domain. A complex wave made up of sine waves gives a spectrum of lines, but the rendering of the data is more economical and understandable in the frequency domain. To be effective, a sufficient length of textile material has to be tested and a large amount of raw data is generated. As implied in the previous paragraph, a way of compressing data is to convert what, in essence, is a long time series into the frequency domain. To further compress the error wavelength scale, it is usual to use a logarithmic basis. In the terminology of textile processing, the chart expressing evenness data in the frequency domain is known as a spectrogram although, technically, it is a periodogram. The spectrograms in Fig. A4.4(c) show reasonably good and bad examples of roving evenness. The first is an example of a spectrogram of a very bad yarn, which shows two humps (probably caused by the interaction of a bad front roll on a roving frame, combined with improper roll settings). The second shows a spectrogram that would often be regarded as satisfactory. In the spectrograms, the ordinate is often merely referred to as amplitude, but this needs a little more discussion. The purpose of such equipment is to record mass variations or so-called ‘evenness’, ‘regularity’, or the negatives of these. Based on the work of Furter [4] and others, one may define the parameter in two ways. The first is: Mean deviation or U% = [100/(xmT)]

∫

T

| x i – x m | dt

[A4.2]

0

where xi is the instantaneous value of linear density, xm is the mean value, | xi – xm | is the deviation of the instantaneous value from the mean, T is the evaluation time, and t is time in compatible units. If s is the standard deviation, s2 = ∑ (xi – xm)2/(ns – 1), and CV = s/xm, the second definition is:

CV % = [100/ x m ]

√

  (1/T ) 

∫

T

0

 ( x i – x m ) 2 dt  

[A4.3]

Subject to the data being Gaussian or normally distributed, the relationship between U% and CV% is stated to be CV = 1.25 × U%. Furter discusses in detail the meanings of various strip charts and spectrograms, as well as the measurement and importance of yarn faults. Turning to online monitoring, suppose a device is measuring, online, the linear density of sliver emerging from a draw frame at 600 m/min, and that the output signals relate to successive 1 mm lengths. Each measurement requires a calculation and there would have to be 1000 × 600/60 = 10 000 calculations/sec. A central processing unit in a computer adequate for this traffic thus becomes essential. In a 24 hr day, there would be 864 million calculations for each measurement position. There has to be some means of filtering the output, otherwise the analyst would be overwhelmed. One way of filtering is to use periodic measurement, but if the measurements are spaced too far apart there might be difficulties with under-sampling. Another way consists of the equivalent of a control chart and provides a warning only when the parameter concerned moves outside the control limits. An example of a control chart is given in Fig. A4.5, in which three values with round shaded plot symbols represent out-of-control points or outliers. Only these three points would be reported and the rest would be ignored except, perhaps, for a

Advanced topics II: Testing of textile materials 359 6 Upper control limit 5

Trend

Value

4 3

Lower control limit

2 1 0 0

5 10 Time (arbitrary units)

Fig. A4.5

15

Control chart

trend analysis that, in this case, shows the variables going steadily towards the upper control limit. As an aside, it should be noted that a record may be kept in the computer memory only for a limited period. When the data generated from the transducer is used in a control system, there are a number of ways to conserve computing power and keep the system under stable control. A transducer is a device that converts the variable to a usable signal. The digital data stream can be compressed to facilitate the transmission of signals to the controllers. Signals can be restricted to outliers, trend analysis can be used to modify the control, and other schemes can be applied to keep the volume of transmitted data within bounds.

A4.4

Measurement of twist

A4.4.1 Untwisting to zero twist method With ring and twisted filament yarns it is possible to untwist yarns until the fibers are approximately parallel, and this condition can be determined with a fair degree of accuracy. With filament yarns, the condition of zero twist can be determined by placing a thin blade between the filaments and then sliding the blade along the length of the yarn within the gage length [2]. When the blade can be moved without resistance from one end to the other, the yarn is at the zero twist condition. However, the use of such techniques becomes difficult with staple yarns. Ring yarns pass through what is essentially zero strength as they are untwisted, to zero twist. However, this is not true for rotor yarns. Zero twist staple yarns have little or no strength unless a bonding agent is present (which is highly unusual at this stage of processing). Consequently, the gage length is normally set below the staple length so that a sufficiency of fibers is gripped at both ends. It is desirable to test yarns under some tension to prevent the yarn from snarling when twisted. The tension applied when the yarn is at or near zero twist should not be so great as to cause the weakened yarn to break. The apparatus for such a test consists of two clamps attached to the yarn and some means of creating a sufficient controlled tension in the yarn between the clamps. One of the clamps rotates and untwists the yarn and the other is fixed. The test is fairly labor intensive and is usually carried out in the laboratory.

360

Appendix 4

A4.4.2 Twist contraction in staple yarns Twist is measured in turns per unit length and, as the twist is changed, the yarn changes length. The change in length from the untwisted to twisted condition is known as twist contraction and it can be used to help measure twist. This should not be confused with the contractions that take place when filament yarns are textured.

A4.4.3 Reversed twist method used in staple yarn testing A second technique requires one to carry on with what was the untwisting into the zone where the yarn becomes reverse-twisted [5]. Reverse-twisting continues until the gage length regains its original value, it being assumed that the twist contraction in the reverse direction is the same as the original direction. The reversed twist method is often known as the twist/untwist method (even though it would be more accurate to say ‘untwist/twist’ method). As shown in Fig. A4.6, a counterweight controls the yarn tension and the twist counter reads the change in the number of turns from the beginning to the end of the test. The twist indicated is assumed to be twice the twist in the yarn; consequently the twist density is half the indicated twist change/gage length. The initial tension in the twisted yarn should be maintained at 0.25 + 0.05 g/tex (or 2.45 + 0.49 mN/tex). The tension at the end of the reversetwisting should be the same as the initial value.

A4.4.4 Twist testing rotor spun yarns Rotor spun yarn never achieves a state during untwisting where all the fibers are approximately parallel. Thus, although the reversed twist method just described is normally used, there is an error due to the structure of the yarn. The measured values differ from those calculated from the machine parameters, as shown in Fig. A4.7 (for a polyester/cotton blend in this case). In this diagram, there should have been no change as the percentage polyester was altered. In the case of the so-called machine value calculated from the known rotational speeds of the machine components, this was true. The machine value was then used to normalize the other results. With the reversed twist method, changes in blend did affect the measured twist and this implies differences in yarn structure. This error is normally ignored because a standard of Hinged clamp with counterweight

Twist counter

Yarn

Rotating clamp with motor

Fig. A4.6

Twist tester


Twist testing factor

1.0

Machine

Reversed twist 0.5 Surface fiber

12/1 Polyester/Cotton OE yarn 0

Fig. A4.7

0

50 100 % Polyester in the blend

Various methods of twist measurements for rotor yarns

judgment different from that used with ring yarns is applied. The effect varies with the fiber being used. If one judges the twist by the fibers on the surface of the yarn, there is even more error, and so this method is little used. The population of bridging fibers in rotor spun yarn usually varies from 10 to 25% and most of these produce the wrapper fibers on the yarn surface. The fibers behave as if they are shorter than they really are and, as mentioned in Section 7.2.12, a higher TM is required than would be used in ring spinning.

A4.5

Visual examination of yarns

A4.5.1 Yarn board One factor in assessing staple yarns is that of yarn grade. Fairly long samples of the subject yarn (which are normally white) are wrapped on a black yarn board (Fig. A4.8) or sleeves are knitted from it. These techniques are used to telescope the errors and make them more visible; they are useful for determining fault rates, short and mid-term errors of both staple and filament yarns. However, it cannot show errors longer than a fraction of the sample length, which is about 100 yards or so. A yarn blackboard is either a rectangular or trapezoidal board onto which yarn is wound, closely spaced to simulate fabric. Periodic errors produce patterning to appear on the yarn blackboard, which indicates a mechanical error. Slubbiness indicates drafting or drawing defects, and reflective differences indicate changes in yarn luster, or yarn Yarn

Slot to hold yarn end

Fig. A4.8

Yarn board

362

Appendix 4

hairiness, or both. For dyed filament yarns, differences in color, luster, and bulk can be assessed. The eye quickly becomes skilled in assessing the error characteristics and in judging the quality of the yarn. A yarn board is a very successful and simple device. However, it cannot show errors longer than about 100 yards or so because sample length is insufficient. The example of use in checking for variation in luster and/or bulk in a textured yarn has already been mentioned. Textured yarn has to be wound at constant tension because the structure of the yarn is such that the bulk changes with tension; thus surface reflectivity and yarn diameter also change. A knitted sleeve can be used to achieve a similar objective. In this case, the knitted material is robust enough to be dyed and, as explained in earlier discussions, this dyeing may well reveal faults created in previous processes. Faults caused by different temperatures or mechanical stresses would otherwise not be visible until it is too late unless differences in dye affinity can be harnessed in the test procedure. This is why the dyed knitted sleeve is valuable.

A4.5.2 Electronic yarn boards Electronic equivalents to the yarn board have been developed (e.g. CyrosTM) in which evenness data are used to produce a raster.1 The brightness or thickness of the line being painted on the computer screen or paper is proportional to the diameter of the element of yarn being portrayed. The data can be acquired at the yarn testing stage or by online sampling using capacitive or optical sensors. In the latter case, no yarn would be consumed for testing. Each long series of data is divided into sub-series by deliberate choice and the selected series are plotted in the x and y directions on a monitor or print-out, to simulate a woven fabric. Such equipment is beginning to be used for fabric simulation and quality control in yarn manufacture. To be able to project what a given yarn will be like in fabric form is a useful addition to the means available to be able to control quality and assist sales. Furthermore, it can be done without the expense of actually making the fabric. It is likely that such an arrangement will permit recognition of very long-wavelength faults that are often missed with the present technology.

A4.6

Yarn hairiness in staple yarns

For the purpose of discussion, a fiber projecting from the surface of a staple yarn will be called a hair. It is, of course, sometimes difficult to define the surface of the yarn and the definition of hair is not very precise either. Error in measurement is thus inherent in some methods but such errors are better than no measurement or control. There are a large number of techniques [6] for measuring hairiness which range from microscopy to online measurement of projected optical or electronic images. Error in measurement is inherent in some methods but such errors are better than no measurement or control. For example, referring to hair A in the plan view of Fig. A4.9, the projecting hair length, when viewed in elevation, is foreshortened as shown. In addition, part of the hair length may be obscured by the body of the yarn, as also is shown. To increase 1 A common example of a raster is a television set where a light beam is made to oscillate across a screen in a two-dimensional pattern and the brightness is modulated to produce a picture.

Advanced topics II: Testing of textile materials 363 Elevation

W

Projected length

A Plan view

Fig. A4.9

Yarn hairiness

sensitivity of measurement, a mask of width W is often added; only the variations outside the shaded area are measured. If the width is smaller than the yarn diameter, signals from the body of the yarn are added to the signal as shown by the black areas bounding the shaded rectangle. This too produces an error. If W is too large, another error is introduced. Since yarn diameter is variable, some error is inevitable if such masks are used, but the increase in sensitivity reduces other errors in measurement and the net result is that it is worth using them. Often the information required relates to the outer surface of hairs, and the masking width is then increased to eliminate the portions of hair close to the yarn body. In these cases, the mask width, W, is a multiple of the yarn core diameter. Hairiness is variously described by the number of hairs/ unit length at a given radius, or by the length of the hairs. The hairiness is dependent on the yarn diameter and has a roughly linear relationship with fiber length and twist over a reasonable interval. Many different types of hairiness measuring equipment are used in laboratories and online means of testing are available. Outputs give hairiness data in both the time and frequency (wavelength) domains. Spectrograms of hairiness are becoming as common as spectrograms of linear density. Much of the comment made earlier about variability of linear density also applies to variability in yarn hairiness.

364

Appendix 4

A4.7

Tensile testing of strands

A4.7.1 Testing single strands A tensile testing machine consists of two yarn clamps that grip the yarn specimen and move apart to induce load in the specimen. Usually one clamp is held stationary and the other moves. The stationary one usually holds the transducer or load cell, which measures the load applied, and the movement of the other clamp reacts with a sensor to give the elongation of the specimen. Electrical or other circuits control the elongation of the specimen by changing the movement of the moveable clamp. The design of the load cell is an important part of the specification of the machine. Most load cells require a finite elongation in the direction of load to produce a signal. It is highly desirable that the system should be stiff so that the deflection of the machine is very small in comparison with the strain in the specimen. A discussion of the design of transducer is given by Furter [7]. Care has to be taken in interpreting the results because of the length factor. A long specimen has a low strength because of the number of weak spots in its length, whereas a short sample has more variable results. The short-term coefficient of variation of tenacity is of considerable importance in determining the efficiency of the ring spinning and subsequent operations. Despite the arguments for using singleend yarn testing, it is more expensive than skein testing and involves high skill by the technician. For these reasons, one finds mostly skein testing in the mills and singleend testing in research laboratories. Also, the materials are visco-elastic, which means that the results are dependent on the rate of loading of the material. This becomes an important matter when results from different test facilities have to be compared. A sample should be tested in a representative fashion. One implication of this is that the correct length of specimen should be tested. The tests often yield the breaking strength and the elongation at break. Because most textile fibers and yarns are viscoelastic, the stress–strain curve is not linear. Consequently, if one wishes to characterize the behavior in normal use, where the loading is not high, attention has to be paid to the slope of the stress–strain curve near zero load. Examination of elongation at break gives data that helps one assess the visco-elastic nature of the yarn or fiber. For complete information about the visco-elastic behavior, it is necessary to cycle the load to determine the hysteresis, but such testing rarely enters into commercial mill practice. Tensile test machines fall within one of three categories [8], namely: 1 2 3

Constant rate of extension (CRE); Constant rate of traverse (CRT); or Constant rate of load (CRL).

The test category should be cited with the test results because the type of test affects the numerical results. Selection of the category depends on the end use of the product being tested. It follows that inter-laboratory comparisons must be for like categories. In strength testing, the values are nearly always measured offline because the test is destructive. The shortest effective sample in measuring strength is where the yarn clamps touch one another at the clamping points and the gage length is zero (Fig. A4.10(a). Two problems arise. The first is that the yarn clamps induce a stress in the yarn around the clamping zones. This influences the characteristics of the yarn actually tested, and usually yields a lower breaking force than should have been obtained. The


Clamp

Clamp

Gage length

Gage length

Clamp

Yarn

Yarn

Variations in yarn thickness are exaggerated Clamp (a)

(b)

Fig. A4.10

Tensile testing of yarn

second problem is related to weak link theory and yarn variance. A chain always breaks at its weakest link and a yarn sample can be regarded as a chain of infinitesimally short links. A zero gage length test produces a variance amongst the results similar to that existing in the length of yarn from which the test specimens were taken. A long gage length contains a distribution of link strengths (Fig. A4.10(b)) and the weakest one fails; an erroneous conclusion would be that the single result is typical of the whole length tested. With insufficient testing to establish the variance, the single result would be no more than an approximation without any knowledge of the probable range of error. Clearly a long gage length has a greater probability of including a weak link and producing a low result than does a short gage length for that single test. Often a long sample is wrongly preferred because it is thought that the results are less variable and, therefore, the results are more reliable. There is a wide range of stress–strain characteristics of fibers and yarns. Space precludes more than a sample, but Fig. A4.11 shows a wide range, especially between untextured filament and staple yarns. It will be noted that there are also wide differences in the shapes of the curves that reflect their visco-elastic character. It must be emphasized that the curves shown are single-end tests and that there are considerable variations within any one series of tests with a given yarn.

366

Appendix 4 500

Nylon filament

Polyester filament

Stress (mN/tex)

400

300

Cotton yarn 200

Polyester staple yarn 100

0 0

Fig. A4.11

10 Elongation (%)

20

Stress–strain curves

A4.7.2 Testing skeins With the testing of single yarns as sketched in Fig. A4.11, the results should be averaged after sufficient tests have been made. An alternative method to obtain a value for yarn strength is skein testing [9]. However, if a skein is tested, many parallel portions of yarn are loaded simultaneously. Since all fibers gripped by both clamps experience the same extension, the load suffered by each gripped fiber depends on its longitudinal stiffness. A stiff fiber may take more load than less stiff parallel portions, with a result that certain fibers will fail before others have reached their breaking stress. Thus, the skein may appear weaker than might be expected. On the other hand there is an averaging effect between the parallel portions which reduces the effects of weak spots in the yarn. Long specimens of yarn in the form of skeins give strength values that are some sort of average; because of that, they are frequently used. Skeins of 120 yd are commonly used. The strengths of both single yarns and skeins are often expressed in terms of tenacity. For a single yarn, the tenacity (T ) often employs the units mN/tex. For a skein, the mathematical product of cotton count and breaking load in lbf (CSP – count-strength product) is customarily used. ASTM D1578 suggests that: CSP = KT However, varying characteristics of yarns and skeins cause the factor K to alter from case to case. ASTM quotes K = 21.23 for cotton.


Force

Skein

Force

Fig. A4.12

Skein test

Variation from skein to skein gives no information about short-term errors because these have been averaged out, but does give information about the earlier processing stages. Practice varies somewhat but an example typical in the USA is that 20, 40, or 80 turns of yarn are wrapped on a reel of 1.5 yd circumference to form the skein. The skein is usually elongated on a tensile testing machine at 12 inches/min. A sketch of a skein mounting portion of a testing apparatus is given as Fig. A4.12.

368

Appendix 4

The result is quoted as the lea strength or CSP. The skein test is considered unsuitable for yarns that stretch more than 5% during the test. Some low cost testing machines have a pendulum arm fitted with a ratchet. When the moveable crosshead drops, the load on the pendulum lever causes it to rise along a quadrant on to which the ratchet mechanism is fixed. The weight on the pendulum rises with load, but when the skein breaks it is unable to drop back and so it records the breaking load.

A4.7.3 Multi-function fiber measurement Because of the number of tests needed to control the blending of natural fibers, attention has been turned to assembly line layouts reminiscent of the automotive industry. This is particularly so with cotton testing. The so-called high volume instruments (HVI) consist of several working stations situated along a console. At each working station, one or more fiber attributes are measured that differ from the attributes measured elsewhere in the HVI. Data from every transducer is stored and processed in a computer. Thus, the testing of a sample is carried out at virtually the same time, under the same atmospheric conditions, with the sub-samples needed to make fiber beards taken from zones in close proximity. This arrangement expedites the flow of work and reduces the probability of error. Current HVI machines measure trash content, short fiber concentration, fiber fineness (micronaire), upper half mean length, strength, elongation, color, and reflectance. The acronyms used are trash, SFC, MIC, UHL, STR, ELO, +b and Rd. An earlier discussion (Section 11.2.1) deals with the definitions and importance from a quality control standpoint, whereas some other attributes will be measured in the future, and there certainly will be developments in the techniques of measurement. The Advanced Fiber Information System (AFIS) device measures fiber properties while the fibers are being carried by an airstream. A scanning laser illuminates the moving fibers and transducers pick up the reflections. The fiber fineness, length, and color attributes can be assessed in this manner. The device is not in such wide use as the HVI and it does not measure fiber strength and elongation.

A4.8

Filament yarns

A4.8.1 Linear density of filament yarns Normal untextured filament yarn has a linear density, which is virtually invariable along its length. Consequently, it is sufficient to reel off a certain length, and weigh it under standard conditions of temperature and relative humidity of the workspace. However, when the yarn is textured, the mass may no longer be evenly distributed along the length. Also, the modulus of elasticity of the yarn is low due to the texturing (i.e. it is ‘stretchy’). Thus, textured yarns have to be tested under constant tension conditions. Because of the variability, methods similar to those explained for staple yarns may be employed.

A4.8.2 Strength of filament yarns For industrial uses, filament yarns are usually tested for strength. This is a straightforward tensile test; providing there has been proper sampling, it is a simple exercise in


quality control. For apparel yarns, tensile tests are also used on occasion. The purpose in this case is usually to check for degradation of the filaments by overheating in texturing, or by some other cause. The tenacity loss can be up to 10% with polyester and nylon. Proxies for poor quality can be found by observing the manufacturing equipment. Excessive ‘snow’ (powdered oligomer and finish) around a texturing unit and excessive production of fumes from a heater are two such examples. Also, tests of evenness can be used to show errors. The problem is sometimes in the quenching after extrusion. Typically, a false twist textured nylon or polyester has a tenacity of 350 to 500 mN/tex (4 to 6 gf/den) and an elongation at break of between 25% and 35%.

A4.8.3 Definitions relating to the bulk of textured yarns There are a number of terms concerning normal usage of textured thermoplastic filaments that need to be mentioned. Bulk is generated when filaments are caused to coil, to take up a zigzag shape, or to be deformed in any micro-convolution. Some early forms of texturing involved a zigzag texture, and bulk was then described in terms of crimp. This terminology has been extended by use to cover other sorts of texture. Thus, the ability for a yarn to contract under tension is called crimp contraction and the ability to recover is called crimp recovery. Bulk shrinkage is a term relating to the potential stretch and ‘power’ of stretch yarns, or a measure of bulk in textured yarns [10]. (The term ‘power’ is used to convey the recovery properties of so-called elastic yarns and fabrics; the term ‘elastic’ means the ability of the material to withstand large deformations, and does not relate to the engineer’s definition as the relationship between stress and strain.) The term ‘crimp’ is used as a proxy for bulk, which is hard to define. A rough equivalent to bulk is yarn diameter, but the diameter is transient in the sense that it changes under load; also, when assembled into fabric, the cross-sectional shape changes according to the loads applied by the intersecting yarns. ASTM standard D4031 [10] defines crimp contraction, in this context, as ‘an indicator of crimp capacity or a characterization of a yarn’s ability to contract under tension’. Crimp recovery is defined as ‘a measure of the ability of a yarn to return to its original crimped state after being subjected to tension’. When a textured yarn develops bulk, it shrinks, even under load. Some yarns have a structure that favors ease of elongation (so-called ‘stretch yarns’) and others favor yarn bulk. The elongational behavior and the hysteresis loss (ability to recover from deformation) can be measured on tensile testing machines, as described earlier.

A4.8.4 Measurement of bulk and crimp in textured yarns Fabrics made from bulked yarns are intended to provide cover and insulation. The bulking processes are varied in nature and produce a wide range of yarn structures. Also the polymers available cover a wide range. Because the range of performance varies widely, there are several tests and sets of conditions available. ASTM suggests several loading options, which may be summarized as (a) 0.04 to 0.98 mN/tex, and (b) 8.8 mN/tex, the first being sufficient to extend the yarn without removing crimp and the second to remove crimp without significantly elongating the filaments. The recommended protocol prescribes that the low loading should be kept in place during the test, and that an extra load be added and removed when necessary to adjust the

370

Appendix 4

load between the two levels. Also, bearing in mind that the materials are visco-elastic, times of heating and loadings are strictly detailed as are other aspects of this series of tests. Information is kept up to date by ASTM, to whom the reader is referred for more information. During tensioning of the yarn under test, it can be immersed in water (Fig. A4.13), subjected to dry oven heat, or steamed. It is recommended that textured polyester yarns be tested first at the low load condition and then at the higher level of load. With textured nylon or polyester yarns, the water bath method can be used. For polyester yarns the bath should be at 97°C and a low stress level used, whereas for nylon a temperature of 82°C and a stress level of 0.13 mN/tex is recommended. The skein size is usually determined from the reel diameter (usually 1m) and the linear density of the yarn. ASTM standard D4031 quotes the numbers of turns on the reel varying between 25 and 63, according to the linear density of the yarn. Crimp contraction is quoted as the percentage change in length between the high and low load conditions. Similar measurements are made before and after heating to develop crimp. Let the length of the skein at low load be X and at high load be Y. Also, let the condition before heating be designated subscript 1, and after heating by subscript 2. Thus, the length before heating at low load would be written X1, etc. After heating, (a) the skein length at low load is measured, (b) the extra loading is applied to bring

Water-filled glass measuring cylinder

Skein

Weight

Fig. A4.13

Yarn bulk


up the load to the high level, (c) the skein length is measured again, (d) the extra loading is removed, and (e) the skein length is measured for the last time. Let X3 be the length after heating and removal of the heavy load in the stage (d) just mentioned. 1 2 3 4 5

Skein shrinkage before heating Skein shrinkage after heating Skein shrinkage Bulk shrinkage Crimp recovery

= 100 (Y1 – X1)/Y1 = 100 (Y2 – X2)/Y2 = 100 (Y1 – Y2)/Y1 = 100 (X1 – X2)/X1 = 100 (Y2 – X3)/(Y2 – X2)

Items (1) and (2) give indications of how much mechanical and molecular forces play in the total relaxation and this information is useful for diagnostic purposes. Items (3) and (4) give the total shrinkage at high and low loads respectively. The crimp recovery, item (5), is the difference in length of the skein after heating caused by the final removal of the heavy load, which characterizes the hysteresis in the system. A standard test for bulk is to use a modified skein shrinkage test (Fig. A4.13). The main difference between (a) the shrinkage test as a measure of potential stretch, and (b) the test as a measure of bulk, lies in the applied load. Care has to be taken when measuring bulk to ensure that all samples are tested with the same degree of ‘lag’. A freshly textured yarn behaves differently from a yarn that has stood for an hour or so; this is because of stress decay in the thermoplastic material. Fresh yarns generally have higher skein shrinkage and lower strength than aged ones. The properties of the textured yarns can continue to change even over a period of 100 hours, but the change rate diminishes with time and eventually stabilizes. The highest change rate is in the first hour. In fabric form, finishing and other processing can cause further bulk to be generated and this is usually associated with perceptible shrinkage. An alternative method of measuring bulk is to measure the volume taken up by a piece of fabric of a known mass, M grams. If the thickness is t mm and the area is A m2, then the bulk, 1/ρ, can be quoted in m3/g, and: 1/ρ = tA/1000M

[A4.4]

The structure of the fabric affects the cover factor and ‘basis weight’ (mass/unit area of fabric); for comparative purposes, the test is quite useful. The thickness is measured by a standard fabric thickness tester, which compresses the fabric slightly during the measurement (therefore there is some error).

A4.8.5 Stretch yarns Stretch yarns fulfill a function different from that of bulked yarns. The objective of using a bulked yarn is to cover and insulate. A stretch yarn is designed to permit extraordinary extensions in fabrics made therefrom, thus the high load described in the previous section may not fully extend the yarn, and different levels may be necessary for testing some yarns. However, the procedures are similar to those described. For stretch yarns, some use a standard weight of 20 grams acting on a skein of 12 500 denier, to give a specific stress of 0.141 mN/tex, whereas with a bulked yarn, a weight of 2 grams is used to give a specific stress of 10% of that just quoted. These figures are quoted merely to emphasize the difference between stretch and bulked yarns.

372

A4.9

Appendix 4

Visual tests of fabrics

The simplest form of dye test is to knit a sleeve, dye it, and then make a visual assessment. There are standard procedures for the dyeing of the samples. The most sophisticated form of this sort of test is to dye a long knitted sleeve and subject it to automatic color testing. In this, the color is analyzed for its tri-stimulus components. The data can be recorded by a computer and related to fixed standards. Analysis of the averages and variances permits diagnostic work to be performed, and this, in turn, permits good control of quality. The test fabrics are usually inspected visually for filamentation, tight spots, etc., at the time the color tests are performed. Continuous fabric inspection systems are useful for error diagnosis. Equipment using two scanning lasers may be used; a computer is programmed to recognize various types of faults and the data can be used to improve the quality of the yarn. Of course, it is far too late to wait until the fabric has been made to measure yarn faults. Nevertheless, there is hope that the techniques can be exploited to examine the yarn directly at an earlier stage.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Anon, Uster News Bulletin, No 35, Zellweger Uster AG, Uster, Switzerland, 1988. D1425, Annual Book of ASTM Standards, Section 7, ASTM, Philadelphia, USA. Furter, R. Manual of Textile Technology, Evenness Testing in Yarn Production: Part II, The Textile Institute, Manchester, UK, 1982. Furter, R. Manual of Textile Technology, Evenness Testing in Yarn Production: Part I, The Textile Institute, Manchester, UK, 1982. D1422, Annual Book of ASTM Standards, Section 7, ASTM, Philadelphia, USA. Barella, A. Yarn Hairiness, Text Prog, 13, 1, 1983. Furter, R. Strength and Elongation Testing of Single and Ply Yarns, Mannual of Textile Technology. The Textile Institute, Manchester, UK, 1985. D76, Annual Book of ASTM Standards, Section 7, ASTM, Philadelphia, USA. D1578, Annual Book of ASTM Standards, Section 7, ASTM, Philadelphia, USA. D4031, Annual Book of ASTM Standards, Section 7, ASTM, Philadelphia, USA.

Appendix 5 Advanced topics III: Staple yarn structures

A5.1

Theoretical yarn structures

DeWitt Smith [1] stated that the basic geometrical features of a yarn determine the resolution of fiber tensions into components parallel and normal to the yarn axis. The summation of the components parallel to the axis provides the yarn strength and the normal components produce compressive forces that provide frictional cohesion. If a bundle is made up of parallel fibers and then twisted, it produces a helical structure somewhat similar to that shown in Fig. A5.1(a). The fibers are under tensions TL and TR, which vary according to the load applied. Interfiber friction can cause dissimilarity between TL and TR. The component tensions acting on the fiber at the various positions along it produce resultants directed towards the center because the fiber is wrapped around the core of the yarn. The radial compression arising from reactions of the many fibers, like the one shown in Fig. A5.1(b), causes each layer to compact the layers underneath. This increases the frictional restraints acting on the fibers in the core. Staple fibers could not survive in such a structure because the surface would abrade leaving the next layer vulnerable. This, in turn, would abrade and the whole structure would fail in short order. In fact, the staple fibers migrate radially during processing so that a single fiber occupies many different radial positions along its length. This phenomenon is known as (lateral) fiber migration; it causes the structure to interlock so that it retains its integrity over a surprisingly wide range of conditions. A simple experiment can be made with a piece of string. Helically wrap the string round a person’s bare arm, and apply moderate tension. The subject will feel the inward pressure referred to and the string will be seen to bite into the flesh. In Fig. A5.1(c), the same situation is depicted with a fiber. If the yarn is sliced along the plane XX, the view normal to that plane in direction Z is as shown in Fig. A5.1(d). For a small angle of wrap, the fiber may be considered to lie in the plane of the cut. Thus we consider the tensions T1 and T2 rather than T3 and T4. A section of the core, in the plane XX, is elliptical as shown. Portion AB of the yarn projects as A′B′ in the lower part of the diagram; this is the small arc of contact between the fiber and yarn considered here. The length AB measured along the yarn is δL. The fiber lies at a

374

Appendix 5

(a) Pressure between fiber and core acts towards the yarn axis p TL

F

F

TR

p (b) Z X

T3

T4 X′ dθ

A′

X B′

T1

(c)

dθ

Fy (d)

Fig. A5.1

X′

T2

Fibers in yarn

radius r with respect to the center line of the core. The twist is τ tpi and the force/unit length is: FR /δL ≈ K(T1 + T2)/R

[A5.1]

where R is the radius of curvature of an ellipse which is a function of (r, τ); K is a factor; and δL ≈ 2R δθ.1 Equation (A5.1) shows that the compressive force is a function of fiber tension, yarn diameter (or count), and twist level. The greater the twist, the smaller is the radius of curvature of the yarn surface, and the larger is the compressive force. Thus, the higher the twist, the tighter (or leaner) is the yarn.

1 Radius of curvature = [d 2 y/dx2]/ζ where ζ = {1 + (dy/dx2}3/2. The equation of an ellipse is p2 = ax 2 + by2, differentiating w.r.t x, 0 = 2ax + 2by (dy/dx) and differentiating again 0 = 2a + 2b[y(d2y/dx2) + (dy/dx)2]. When dy/dx = 0, d 2y/dx 2 = – a /by and R = –by/a. The ratio of the major and minor axes is b/a and b/a = 1/sin α, y = r, |R| = r/sinα and the helix angle α is controlled by the twist.

Advanced topics III: Staple yarn structures

A5.2

375

Actual yarn structures

A5.2.1 Fiber migration in ring spinning Consider fibers traveling in the direction shown within the zone afdeh in Fig. A5.2(a). Fibers similar to the one marked abcd are typical of migrated fibers inherited from the roving, which pass into the twist triangle, def. Other fibers have lesser amplitudes of lateral displacement such as the fiber marked jg. Fibers along the nip line are squashed by the front drafting rolls such that it is very difficult for the fibers to slip with respect to the rolls. The strand is then translated into a roughly circular crosssection from the point d downwards by the application of torque. Fibers in or near the selvages of the twist triangle, such as the one marked de and df, take up much of the load created by the departing yarn and become quite highly tensioned. Many of the central fibers, such as that marked dg, cannot be taken up as quickly as they are delivered; they bear little or no load, and they buckle. In buckling, they tend to move to the outside of the newly formed yarn. Fibers passing down near the selvage of the twist triangle have the highest tension and tend to migrate into the center of the forming yarn structure, to relieve some of the tension. Twisted roving is usually used and the roving twist provides a supply of fibers that periodically change their lateral positions across the ribbon of fibers approaching the triangle in a roughly sinusoidal manner. The portion abc of the yarn represents part of one of the fiber sinusoids. Also we consider fiber abc because it is one with a maximum lateral displacement amplitude and will, therefore, show the periodic effect most clearly. The parts are marked a′, b′, and c′ in Fig. A5.2(b) at a later time. (Meanwhile the portion in the vicinity of c that was highly tensioned now passes into the center of the yarn near d.) When the portion b′ of the same fiber eventually passes into the yarn, it is likely to be slack and to migrate to the outside of the newly made yarn. Comparing Figures A5.2(a) and (b), it will be realized that a single sinusoidal fiber moves across the nip line as it flows towards d. Even if the lateral displacement is not sinusoidal, lateral movement will still occur and produce a similar effect. Thus, some parts of a fiber are taut as they pass through the twist triangle and some parts are slack. Consequently, the fiber migration is periodic. Changing the roving twist can alter the characteristics of the yarn. The result of this is that the structure interlocks into a stable structure. The phenomenon is known as (lateral) fiber migration. A typical result is shown for a single fiber in Fig. A5.2(c). Imagine many of such fibers in a yarn. It becomes clear that the structure loses some of its order and that fibers pass from layer to layer, causing the structure to interlock as just mentioned. The loss of order results in more volume being required to accommodate the fibers; in other words, the yarn becomes more bulky. The enhanced insulation properties given by the extra airspaces in the yarn give fabrics made from the yarn a warmer, softer feel. Also, the fibers have room to deflect more easily, which improves the hand.

A5.2.2 Yarn hairiness in ring yarn A good reference in the matter of yarn hairiness is given by Barella [2]. The geometry of the twist triangle not only controls fiber migration but also helps determine hairiness. Some of the buckled fibers extend from the surface of the yarn as hairs and loops. Air currents in the exit nip of the rolls also affect the process of creating hairiness, although this is more important in high speed spinning systems like air-jet spinning. Further hairiness is created in ring spinning as the yarn passes through the traveler,

376

Appendix 5 Roving input

a

j

h

b

f

x

c g e

Nip line

Taut d Taut Slack Yarn output (a)

a′ b′ Nip line Taut c′ Taut Slack Yarn output (b) Typical migrated fiber (c)

Fig. A5.2

Fiber migration at the twist triangle


377

and again when the yarn is rewound onto cones or cheeses. A photomicrograph of portions of ring yarn showing the structure and hairiness is given in Fig. A5.3. There are single hairs and loops standing out from the surface. (It might be noted that the lighting of the yarns shown was adjusted to show each fiber structure most clearly, and, in consequence, the backgrounds vary.) (a) Ring yarn

(b) W

W

W

W

Air-jet yarn

(c)

W

Rotor yarn

(d)

Ring yarn (e)

ST yarn

Fig. A5.3

Yarn micrographs

378

Appendix 5

A5.2.3 Air-jet yarn The twist triangle in air-jet spinning is short, the tape of entering fibers is wide, and the emerging yarn is temporarily very hairy. These hairs are then rearranged and embedded by a second twister to give the structure peculiar to air-jet yarns. The hairs are wrapped around the core and provide the forces that give the yarn cohesion. Hairs have to be long enough to give a reasonable probability of the outermost ends becoming anchored in the structure during the second twisting process so that they act as binders. With 1.5 inch 1.5 denier (or finer) man-made fibers there is little difficulty in this respect, but there are problems with short cottons. Longer cottons can be used, and a photomicrograph is shown in Fig. A5.3(b). The yarn was made from 1.125 inch of Californian cotton; the wraps can be seen clearly where marked W.

A5.2.4 Rotor yarn The structure of rotor yarn is controlled mainly by the lying of fibers on a core that has both real and false twists. Thus, the structure contains a twisted core but the twist level varies from core center to the outside sheath. The difference in helix angle can be seen by the shape of fiber cross-sections such as the one shown in Fig. A5.4(a). In this case, fibers in the central area (circled at B) were almost round, which indicates that they were almost parallel to the yarn axis. Those on the outside were decidedly elliptical (indicated at A) because the fibers were cut at an angle. Wrapper fibers are produced when the yarn intersects the ingoing fiber stream and an example of the effect can be seen in Fig. A5.4(a). In general, cross-sections vary according to spinning conditions and the yarn being spun. The structure needs higher twist levels than those used for comparable ring yarns. This reduces productivity and makes the yarn harsher. The percentage difference Wrapper fibers

Migrating fiber

A Rotor yarn Rotor yarn Slice = 0.035″ thick

B

(a)

(b)

Rotor yarn

Ring yarn (d)

(c)

Fig. A5.4

Cross-sections of various yarns


379

between the twist in the core and that of the sheath is a measure of the change in structure. According to Deussen [3], the difference in twist for cotton yarns ranges between 0 and 20%, whereas for polyester yarns it ranges between 10% and 45%. Fibers migrate in rotor yarn, but not so strongly as with ring yarn because of the lack of a well-defined twist triangle. Figure A5.4(b) shows a sample view along a piece of rotor yarn in which sections [4] were made at 30 micrometers apart. To make presentation easier, readings have been plotted as a polar graph and only a portion of the total is shown. The thickness of the slice of yarn shown was 0.035 inches. The total fiber traced a spiral path, had seven coils, and migrated between 1.00 and 0.26 of the yarn radius. Its end was hooked. Of course, this is only one fiber among millions, but it is hoped that it helps to convey the idea of a typical shape. Figures A5.4(c) and (d) show a comparison of the cross-sections of rotor and ring yarns made from the same batch of fiber and spun to the same count. The rotor yarn section shows peripheral fibers at a relatively large diameter but these are part of a loose wrapper fiber system rather than hairs. Close examination of the packing density of the fibers shows that the center of the rotor yarn is more tightly packed than the outside, whereas the ring yarn is relatively uniform in this respect. Wrappers are created when fibers entering the rotor are laid on the false twisted yarn at the take-off point near the rotor groove. The yarn passes in the region of the fiber entry stream once per revolution of the yarn tail. Thus, there are periodic wrappers along the length of the yarn. Fibers are collected in a roughly triangular groove and the prism of fibers collected there is subjected to a high twist as it is removed from the collecting surface. There is a sort of three-dimensional twist triangle in which there is some fiber migration and a relative movement of the fibers, which modify the structure when the wrappers become overlaid on the surface [5]. This refers to the wrapper fibers mentioned earlier. The zone is diffused, it lies inside the rotor, and is difficult to recognize. Following that, the whole structure is untwisted as the false twist is removed. The result is a complex structure with a rather roughfeeling surface due to the wrappers. This affects the hairiness, as illustrated in Fig. A5.3(c). The picture also shows the wrapper fibers and it should be noted that, although in general, rotor yarn is more bulky than ring yarn, the hairiness is less. The structure is readily seen by attempting to untwist some rotor yarn. It will be found that there is never a state when all fibers are parallel in the untwisted yarn, as will happen with ring yarns. Either the core of the rotor yarn has some twist when the outer layers are untwisted, or the outside has reversed twist when the core is untwisted. Furthermore, when one section is untwisted, the neighboring portion may not be, as shown in Fig. A5.5; the ends of the portion of yarn shown were restrained from untwisting by wrapper fibers, whilst those in the center of the picture were almost completely untwisted to form a ribbon. Differences in yarn structure such as these make measurement difficult; hence, it is usual to rely on the calculated machine twist. The reversed twist method (i.e. twist–untwist) is sometimes used and it is also possible to measure the angle of the surface fibers, although this is a tedious process. Additionally, elongational straining of the yarns produces changes in characteristics. Strained yarns perform badly in weaving.

A5.2.5 Self-twist (ST) yarn Referring back to Fig. A5.3(e), two staple yarns which have been self-twisted are shown. One yarn was made from black fibers and the other from white ones. The

380

Appendix 5

Fig. A5.5

Composite micrograph of untwisted rotor yarn

section shown is near a zone in the white yarn, which originally had zero twist, with S twist on one side and Z twist on the other. The yarn tries to relieve the torque by rotating and lessening the twist on either side. As it rotates about its axis, it is likely to ensnare fibers from the other yarn and wrap them about itself. Black fibers wrapped about the white yarn can be clearly seen. Also, white fibers were wrapped around the black yarn but these are difficult to see in the micrograph, it being remembered that the zero twist zones of the black and white yarns need not coincide.

A5.2.6 Comparing hairiness of various yarns Many measurements have been made regarding yarn hairiness but the matter is complicated because any friction acting on the surface of a staple yarn raises hair from the surface. In ring spinning, the yarn is usually less hairy in the balloon than it is when it is laid on the surface of the bobbin. This is because the yarn is scraped over the traveler. Centrifugal force, arising from the rotation of the bobbin, causes fibers to stand out from the rotating surface. This creates a bed of outstanding hairs (rather like the surface of a pile carpet), onto which is wound the newly made yarn. The yarn twists as it is later removed in unwinding, and the surface is again modified. This is because some of the hairs become entangled with others from different coils of yarn on the same bobbin. Movement of the yarn over guides in the winder has a further effect. Thus, it is not surprising that the hairiness is sometimes quite variable and that research results vary. Some results from Salah [6] (Fig. A5.6), show that rotor yarns have a larger body than ring yarns. This supports the finding that the average packing density of rotor yarns is less than that of ring yarns. With blend yarns the results became more scattered and this is possibly a reflection on the blending. Other studies have shown that the population of fibers at various D √Ne is the normalized diameter of the surface bounding the yarn 45 Rotor Rotor 40 40 45

AV D√Ne

D√Ne

Ring 35 Ring 30

35 30 25

25 100% Polyester

20

20 3.8

4

4.2 TM

Fig. A5.6

4.4

0

25 50 75 100 Blend, polyester/cotton (%)

Variations in normalized yarn ‘diameters’


381

stages of processing varies much more than many people expect. The percentage of polyester in a polyester/cotton blend had some effect but a trend line could only be established by taking averages over the range of twist multiple. However, it is quite clear that the fiber packing density, and therefore the hand and cover, vary substantially from yarn to yarn. Yarns from different machines of various designs differ, but the trends are similar. It might be noted that, in presenting results, a source of variance is normalized by multiplying the diameter of the theoretical cylinder by √Ne. There is still a weak dependence on count even after normalization. This implies that there is a small error in assuming that yarn diameter is inversely proportional to √Ne. Comparison of the cross-sections of ring and rotor yarns shows the ring yarn to be more densely packed. Despite this, the rotor yarn has a looser sheath structure, which is bound tightly in various places by fiber wrappers.

A5.3

Yarn behavior

A5.3.1 Frictional behavior Structure affects the frictional behavior of a yarn. It is known that a hairy yarn has a different coefficient of friction from a less hairy one. Chattopadhyay and Banerjee [7] showed that a rotor yarn running over a ceramic guide had up to 20% lower coefficient of friction as compared to a ring yarn. Increased running speeds sometimes reduced the coefficient of friction. The particular polyester tested showed reductions in friction, whereas viscose rayon yarns showed an increase. For a given fiber, the frictional behavior is affected by the finish applied, or, in the case of cotton, the natural finishes removed. In the case of wool, natural finish is removed in scouring and the fibers are oiled; the quantity and lubricity of these oils affect the frictional behavior. Also, experience has indicated that winding and other processes affect the surface characteristics of the yarn. For these reasons, no more than an approximate guide can be given to the frictional characteristics of specific yarns.

A5.3.2 Shrinkage in yarns Untextured filament yarns shrink very little unless the temperature is caused to rise above the glass transition point for the particular polymer. As discussed in Appendix A4.8, thermoplastic textured filament yarns can shrink, and the degree of shrinkage is determined by how well the yarns are relaxed. Shrinkage and yarn bulk are related, and this fact is evident in the fabrics made from the yarns. All staple yarns suffer a small percentage twist contraction, and variations in fabric finishing and laundering can produce shrinkage. Cotton yarns can be treated chemically to reduce shrinkage of woven fabric in service, and woven fabrics made from popular polyester/cotton yarns are reasonably stable in this respect. However, in single-jersey knitted fabrics, the twist liveliness of a yarn affects the fabric structure significantly. Relaxing the yarn by steam or water treatments prior to knitting helps to control this problem. Wool is a special case. It is a scaly, visco-elastic fiber that has many superior crease recovery properties but is vulnerable to post-spinning shrinkage. Shrinkage has been a problem for many years because of the ratcheting mechanism of the scaly surfaces of wool fibers. One method of reducing the shrinkage is to coat the fiber with a polymer to smooth over the scaly surface. Rosa et al. [8] showed a micrograph

382

Appendix 5

of a treated wool fiber that illustrates the character of a surface smoothed by a chemical additive. Of course, care has to be taken not to interfere with the other very desirable properties of wool. Henshaw [9] mentions several authors who have worked on the problem of shrinkage and he cites high drafting forces as being another cause. Fiber crimp tends to be most pronounced in the trailing ends of fibers. The contribution to bulk and hand varies according to the position of the fiber in the yarn. Fiber migration is extensive.

References 1. 2. 3. 4. 5. 6. 7. 8. 9.

Smith, De Witt W H. Textile Fibers: An Engineering Approach to their Properties and Utilization, ASTM Proc, 44, 543, 1944. Barella, A. Yarn Hairiness, Text Prog, 13, 1, 1983. Deussen, H. Rotor Spinning Technology, Schlafhorst Inc, Charlotte, NC, USA, 1993. Patel, J R. TX 591 Project Report, NC State Univ, USA, 1970. Lord, P R. The Structure of Open-end Spun Yarn, Text Res J, 41, 778–84, 1971. Salah, H A. Bulk and Hairiness of Open-end Spun Yarn, MS Thesis, North Carolina State Univ, USA, 1972. Chattopadhyay, R and Banerjee, S. Frictional Behaviour of Ring, Rotor and Friction Spun Yarn, 1990+. Rosa, M J, Gómez, L, Coderch, L and Erra, P. A New Process for Exhausting a Permethrinbased Mothproofing Agent on Wool Fibers, J Text Inst, 82, 4, 1991. Henshaw, D E. Worsted Spinning, Text Prog, 11, 2, 1981.

Appendix 6 Advanced topics IV: Textured yarn structures

A6.1

Yarn hysteresis

A6.1.1 Internal friction in the yarn The twisting of the filaments under heat can affect both the real and apparent frictional behaviors of the yarn. If the surface of the filaments is overheated, the fiber finish might deteriorate by oxidation or some other process and this can change the actual coefficient of friction between the filaments. Of course, the heating affects the morphology of the visco-elastic polymer and this affects both the elastic and the viscous forces acting within the material. External lateral forces acting on a softened polymer can cause flats to be formed on the filaments as shown in Fig. A6.1. All the time that lateral forces persist, it is difficult for the individual fibers to rotate about their own axes. The filaments try to rotate about their own axes when twist is removed from (or added to) the yarn, but the flats cause an impediment to this untwisting (or twisting) process. The next result of these factors is to cause the torque/twist1 characteristic of the

Flattened filaments

Fig. A6.1

Filament yarn cross-section

1 Torque may be regarded as the torsional analog of extension. Thus, a torque/twist curve has many similarities to a load/extension curve.

384

Appendix 6

yarn to have a distinct hysteresis loop, as shown in Fig. A6.2. Some of the energy used in distorting the material in normal use is dissipated in overcoming friction and is not available to return the structure to its original shape; this makes the hand of the yarn feel crisper. Furthermore, it affects the development of bulk as is explained in the next sections. In addition, the flattening of the filaments changes the luster of the material and may produce a sparkle. Overheating the fiber causes it to shrink and to change polymeric structure. The change alters dye affinity and is sometimes associated with polymer discoloration. Some fibers, such as acrylics, are prone to yellowing if overheated in an atmosphere containing oxygen. Deterioration in fiber finish is also expected, and there may be damage to the surface of the yarn. On the other hand, insufficient heating leads to improper heat setting and poor yarn performance. Thus, careful control of temperature, and sometimes atmosphere, is needed to ensure high quality and adequate performance of the yarn. A non-oxidizing gas, or steam, may be used to control the atmosphere. Within the thermoplastic classification there are a number of yarn manufacturing methods available of which one is the false twist method. Also, there are a number of variations within the false twist category. Systems have undergone a great deal of development over the years.

Torque

A6.1.2 Visco-elastic effects in the yarn Consider false twist yarn. Yarn becomes heat set when the twisted yarn is at a temperature above Tg. (The glass transition temperature is the temperature at which polymer softens.) As the yarn leaving the false twister is cooled and untwisted, the individual filaments become stressed. If the filaments are separated and then relaxed, they occupy a greater volume than formerly. They try to go individually into one of the minimum energy shapes (e.g. Figures A6.4(a) and (b)). However, they will not completely succeed in doing so because of interfiber friction and viscous effects within the polymer. The effects of fiber migration and interference between various helices and snarls tend to magnify the effects of friction. Frictional and viscous effects cause the torque/twist curve to take the form of a hysteresis loop. The coercive torques and residual twists vary throughout the process, as shown in Fig. A6.3. In stage (a) of the process, the filaments are taken from their original stress-free, straight

Coercive torque

Twist

Residual twist Assuming no filament buckling

Fig. A6.2

Hysteresis in twisting yarns

Advanced topics IV: Textured yarn structures

385

B Zero torque

Twist

Torque

A

Phase:

Twist

Heat treat (b)

(a) New zero torque O

C

O

G

Torque

Torque

Twist

F E

D Phase:

Untwist

Relax

Shape: (c)

Fig. A6.3

(d)

Hysteresis in the false twist process

condition to a helical, stressed condition (curve AB on the hysteresis loop). The yarn is then heated above Tg and cooled again to remove the stress so that the filaments reach a stress-free, helical condition at stage (b). The yarn is next untwisted and the filaments go from the stress-free helical condition to a stressed but straight condition (curve CD on the hysteresis loop in stage (c)). In the last stage, the yarn is relaxed under conditions that allow filament separation and movement towards the new minimum energy condition (curve EF). The units for the abscissa become coils/inch (or coils/ meter) instead of turns/inch (or turns/meter). Frictional and viscous forces determine how far up the curve is the final point. At point E, there is a large torque OE acting, and this tends to produce a snarl rather than a helix, if the yarn is completely relaxed (Fig. A6.4(b)). If, however, the yarn is heat treated under proper tension and the stress is removed at some point F, the tendency will be to produce a helical minimum energy shape rather than a set snarl (Fig. A6.4(a)). It follows that a second heat treatment under proper conditions in the zone EF produces a bulky yarn. Lack of such a second heat treatment tends to produce a stretch yarn.

386

Appendix 6

(a)

(b)

Coil separation

Filament diameter = d

(c)

Coil pitch = D + d

Fig. A6.4

A6.2

Coil diameter = D

Filament shapes

Yarn bulk

Yarn bulk depends on the geometry of the helices and how closely the coils pack together. Consider the case of adjacent coils of similar diameter, packed as shown in Fig. A6.4(c). Assume that: 1 2 3 4

There are sufficient lateral forces to keep adjacent filaments in contact. The fibers exist as coils. All helices are closely packed. Adjacent helices differ in geometry, with frequent helix reversal points (which makes it unlikely that one helix will intermesh with another).

At first assume that the enclosing box for a single helix is a unit of unshared volume and we will refer to this as a unit standard cell. The average height of the cell is designated as L (i.e. the coil separation) and the pitch is (D + d ). Hence the average standard unit cell volume is (D + d )2 L. If, however, some coils do not touch within the cell, or they are intermeshed, a factor K may be introduced to take this into account.

Advanced topics IV: Textured yarn structures

387

Thus, the average volume of a practical unit cell is: Volc = K(D + d )2L

[A6.1]

One coil contains a filament of about πD units in length. If the coil height is h, the volume per coil is approximately K(D + d)2 h Volume/unit length of filament ≈ K(D + d )2 h/πD ≈ Z′D(1 + d/D)2

[A6.2]

but (1 + d/D) ≈ 1 and volume/unit length of filament ≈ Z′D. Volume/unit length of filament is related to the specific volume (volume/unit mass). The factor Z′ is intended to take into account the obliquity of the coil as well as the value of K. As an approximation: bulk ∝ (D cos θ)

[A6.3]

This parameter can be related to the final state on the hysteresis loop (G in Fig. A6.3). The abscissa in Fig. A6.3 might be thought of in terms of (bulk)–1. The final position depends on the original torque, the tension and temperature during the second heating phase, and the characteristics of the filaments themselves. If the cells intermesh, K < 1 and the bulk might be affected drastically. There is not likely to be a great deal of similarity between adjacent helices and severe collapse is unlikely. The model is reasonably valid providing the coils do not flip into the snarled state.

A6.3

Fiber migration in textured yarns

A6.3.1 Filament migration Filament yarns, when false twisted, theoretically have no net twist when they emerge from the process. However, it is quite possible for there to be alternating twist with zero mean. Most of the points along the yarn might have some twist in the filaments, S twist, or Z twist, or some combination thereof. A typical filament shape is shown Fig. A6.5, wherein the twist reversal points are indicated. Fiber migration tends to prevent intermeshing of the coils and keep them separated. Consequently, the parallel coil structure is realistic. This is important because such a structure entraps large volumes of air, which greatly improves insulation and compressibility of the yarn. To reiterate, this gives improvements in hand that are perceived as the warm, soft feel. An interesting collection of micrographs of textured yarns is given by Lodge [1]; a variety of coiled, zigzag, and buckled filament shapes produced by the various texturing systems are shown.

Helix reversal points

Fig. A6.5

Fiber migration in textured yarns

388

Appendix 6

A6.3.2 Snagging and pilling An undesirable facet of some of the yarn structures when assembled into fabric, is the tendency for them to snag and pill. A snag relates to yarn withdrawn from the surface of a fabric to make an unsightly fault in the material. Single-jersey knitted fabrics are particularly susceptible to this problem because yarn can be withdrawn from a course with relative ease. The very strength of the filaments ensures that, if a protruding end is caught on an external object, yarn is withdrawn from the fabric instead of breaking off at the surface. This might cause a collapse of several adjacent loops of yarn, and any such distortion spoils the surface of the fabric. One solution to this problem is to modify the structure of the knitted fabric; another is to degrade the strength of the filament. Pilling is the formation of many tiny balls of fiber on the surface of the fabric. Abrasion of the surface tends to texture the protruding fiber ends or loops, causing them to form into tiny balls. This problem is common to both textured and staple yarns made from such man-made fibers. Again, an important factor is the strength of the fiber. If such pills form with a weaker fiber, they break off during laundering, or wear away, and the fault goes relatively unnoticed. For this reason, many man-made fibers and filaments used for apparel are deliberately de-rated in strength to combat the problem. For this sort of end use, the strength of the filaments is more than adequate.

Reference 1.

Lodge, R M. How Continuous Filament Bulked Yarns will be Made, 4th Shirley Int Seminar, The Hague, Netherlands 1971.

Appendix 7 Advanced topics V: Blending of staple fibers

A7.1

Introduction

A7.1.1 Introduction to blending In the present context the errors are assumed to be random. Blending smoothes random errors but is not an acceptable solution to the problem of periodic errors. Most periodic errors are man-made and a reliable solution is to eliminate the cause of the error. Consider first the everyday case of blending ingredients in a bowl. With sufficient mixing it is possible to create an almost perfect blend of the materials in the bowl. Consider next mixing a series of bowls from a continuous supply containing longterm variations in the proportions of the ingredients. Whilst each bowl might be perfectly mixed, the mix would vary from bowl to bowl. In a system in which the said ingredients flow through a single mixing chamber, the output would be smoothed to varying extents and the mass of ingredients in the mixing chamber would control the variance in blends from sample to sample. Small samples would be well mixed but large ones would not be. If the sample were smaller than the mass contained in the chamber, the variance within the sample would be very low but there would be a variance from sample to sample. If the sample mass were much larger than that of the mixing chamber, there would be appreciable variance within the sample. The irregularities in the blend would be smoothed only for a certain length along the stream of ingredients. Forwarding this idea to the textile field, one could consider the supply from the bale storage, the mill itself acting as a mixer. If we make the impractical assumption that the mill is a perfect mixer and the very practical assumption that there are longterm errors in the supply, we could come to the conclusion that all the yarn produced during the consumption of a bale laydown would be acceptably smoothed over something of the order of a billion yards or meters of yarn. There would be some smoothing over this length, but it might not be acceptable. There certainly might be unacceptable laydown-to-laydown variation, which the mill would be unable to cure by any internal blending scheme. Next consider a drawframe, where the supply would be the cans of

390

Appendix 7

sliver in the creel. Assume the mass of fiber in the creel is, say, 1000 lb (≈ 454 kg), and the amount of yarn produced from that creeling to be of the order of a million yards (or meters). Then the mass constant would be related to the 1000 lb (≈ 200 kg) and the limiting long-term error that could be smoothed would be related to the million yards (or meters). In rotor spinning there is blending inside the rotor, the mass in that mixing chamber is of the order of, say, 5 mg. The associated mass constant is a tiny quantity and the limiting long-term error might be only, say, 6 inches (roughly 150 mm). In the vernacular of this chapter, mass constant is proportional to the mass contained in the mixing chamber under working conditions and the lengths quoted as proportional to the maximum error wavelengths that can be smoothed by that mixer.

A7.1.2 Problems in defining a blend If, say, polyester and cotton are blended, it is easy to define the blend because a micrograph of a cross-section shows the two sorts of fiber as having distinctive shapes. If two fibers of the same type are blended, the question is no longer simple. For example, take the case of blending two cottons. Not only is it difficult to discriminate between the blend components in a micrograph, but often the input materials are variable. In the early stages of production, fibers exist in clumps; the average value of any attribute varies from clump to clump and so does the variance within the clump. The material is not very homogenous and it is difficult to define. For a good estimate, it is necessary to measure enough samples to calculate the average and CV of each attribute. Processing reduces this sort of macroscopic variation by dividing the clumps into smaller portions and mixing them. However, processing produces its own variations and it becomes difficult to characterize a blend with absolute accuracy. The attributes of a fiber do not vary in synchronism and the CVs also vary in unexpected ways. Some machines have the function of fractionating the fibers and removing some of the fractions. An obvious example of this is combing, where the fraction removed, called noil, has a high proportion of short fibers. This obviously changes the blend. For example, in cotton processing it is desirable to remove the short fibers, which have a great variability. In removing fibers in the fractionating process, the distributions of other attributes also change but not necessarily in synchronism. Thus, for example, changes in fiber fineness are not necessarily related to alterations in the percentage of short fibers.

A7.1.3 Definition of efficacy of blending It is known that ‘blending’ can mean different things, according to which is the fiber property of interest. Each fiber attribute probably exists in a given zone in the fiber flow line at a percentage that differs from that of the other zones. Furthermore, the spectrum of percentages changes along the direction of flow. If sufficient samples are taken at various times from the flow line, the CV of those data provides a good estimate of the efficacy of the blending process. If the process is perfect, the CV would be zero, but if the clumps are incompletely separated or the process changes the order of certain blend components with respect to others, the CV changes. The CV of a particular blend attribute indicates how well the blending process has worked for that attribute.

Advanced topics V: Blending of staple fibers

A7.2

391

Bale management

A7.2.1 Warehouse management Consider a case where a fiber is bought for a period and stored in a warehouse (which might be in a broker’s facility or elsewhere). If bales were to be used to meet the production requirements without thought to the remaining stock of fiber, there is a strong possibility that the best fibers would be skimmed off first. The stock would then degenerate in quality as time continues. In the case of man-made fibers, the fiber makers often take care of the problem by making fibers in large ‘merges’. The attributes of the fibers are made to change very slowly from merge to merge. With natural fibers the case is different. For one thing, the raw material is inherently variable within and between seasons. Any skimming could lead to an inadequate stock at some point through the year (especially if the fiber that is bought is, on average, only just good enough for the end use). Buying fiber of minimum quality in the name of an economical purchasing policy can sometimes be a false economy. From this, it is clear that the laydowns must be based on the technical figures of merit of the fibers left in the warehouse. Furthermore, the variance within the stock must be minimized always, otherwise there would be an impairment of ability to furnish bale laydowns with acceptably low variance at all times Natural fibers, such as cotton, are seasonal and large quantities of the material have to be stored for periods in the order of a year. Thus, there are several factors concerning the blending of such fibers. Factor (a) is to manage the stocks of fiber in the warehouse so that those laydowns withdrawn on a daily basis are reasonably consistent. Lack of control in the warehouse leads to ultra long changes; this causes problems if the old and new fibers become mixed. The time frame of the drift in properties is measured in months. Factor (b) is the consistency of fiber quality within a bale. This is determined by gin practice for cotton, sorting practice for wool, and industrial practice for man-made staple fiber. The time frame is in days. Factor (c) is to minimize the CV of the various fiber parameters in a laydown so that periodic removal of fiber from the bales in turn does not produce error. The time frame here is in minutes. Even the shortest time mentioned covers a period during which, perhaps, 100 000 yd of yarn are produced – a time that would still fall in the category of longterm error as far as yarn is concerned. It is common to divide the store of bales into attribute categories to make the management problem more tractable, because the range of values of the various fiber attributes is very large. Each bale is sampled, the samples are tested, and the bale is assigned to one of the attribute categories by using some formula that the mill operator thinks best serves his or her business. The number of categories depends on the complexity of the business. Bales of fiber do not have equal value, and storage of poor quality or high cost bales with little chance of their being used is a financial burden. It is desirable that the rate of movement of bales of each category be logged so that slow moving categories can be eliminated, unless there is special reason for having them. Management of the fiber warehouse requires a knowledge of the technical figures of merit and their mean values over long periods. It also requires the use of financial figures of merit that take into account the fiber and storage costs, as well as the value in yarn form. Warehouse management becomes a problem of maintaining constant proportions of the fibers that fall in carefully defined categories. Typically, each category contains only fibers of a certain range in fiber fineness, length, and color attributes. Other

392

Appendix 7

fiber attributes may be added but the number of categories expands exponentially with the number of attributes controlled. A large number of categories make commercial application very difficult. Hence, the bales issued for a given laydown can only be defined within limits and there is bound to be some variation within bales drawn from a given category. Blend proportions can be defined for any single fiber characteristic, but they do not all behave in the same way. For example, if one is interested in fiber strength above all else, then the blend must be arranged to minimize CV of fiber strength. If, as is more likely, the need is to reduce barré, then the blend should concentrate on the reduction of the CV of factors that affect color. These factors include not only the normal color measurements such as yellowness (+b) and reflectance (Rd), but also factors that affect dye affinity such as micronaire. A further factor is the surface structure, which can affect the perception of color.

A7.2.2 Bale laydown The quality of a blend is determined by (a) the fiber buying policy (cost and fiber quality), (b) the testing and application of the incoming raw material, and (c) the treatment of reworkable waste being returned from downstream processing. It should be pointed out that the affordable cost depends on what the end product will bear. The bales selected for the laydown should depend on the application. As mentioned, the pattern of laydown is important to get not only good performance of the mill but to maintain a consistent inventory of fiber in the warehouse. Apart from variations inherent in normal production, there are particular changes that have to be managed with care. The latter are referred to as merge changes and they occur at crop change time in natural fiber production. Merge changes from the production of fibers in a synthetic fiber maker’s plant occur when significant alterations are made to the process or product. Every care has to be taken to minimize the possibility of a customer mixing lots of yarns containing dissimilar fibers. The result of uncontrolled fiber application and flow will be barré in the final fabric. This is especially true when significant step changes in important fiber properties are involved. Many complaints from customers relating to finished fabrics have their origins in yarn production. Settlement of claims often involves the yarn maker paying for fabric production and finishing of defective material if the yarn is faulty. An expensive error! To help in reducing the variability of the blend components, it is now possible to use (HVI) and AFIS testing to measure fiber properties on a mass production basis. Discussions on AFIS and HVI are given in Section A4.7.3, and also further discussion on HVI appears in Section 11.2.1. Software programs to manage the laydowns appropriately can further augment results from such testing. A leading example of this is the EFS software produced by Cotton Incorporated. Table A7.1 gives examples that compare the values in bale material to those measured in sliver from the same laydown slice. The comparison shows that the CV, and therefore the blend evenness, was not always better after carding. In this particular case, sliver samples were taken at 10 yard intervals (equivalent to perhaps 1000 or 2000 yards of yarn) but these cannot be regarded as ‘long term’ relative to the bale samples. Consequently, the data should be interpreted to mean that the relatively short-term variation in many of the blend attributes, usually deteriorates in carding. The idea of a single ‘blend efficiency’ to describe performance is seen to be misleading because one must concentrate on the blending of components that matter for the given product. Also, the short- and

Advanced topics V: Blending of staple fibers Table A7.1

Bale* Sliver

393

Percentage coefficients of variation MIC

UHM

STR

ELO

Rd

+b

SFC%

3.3 2.4

0.7 1.5

3.1 4.3

4.7 5.4

4.6 8.0

4.8 6.7

11.1 17.3

Note: * = between bales. The acronyms are defined in Section 5.8.2.

long-term variations have to be balanced to give minimum trouble in year-long processing. Of course, there is a wide range of variability in natural fibers according to the type of fiber, growing area, climate, and seasonal changes, and some bales are more variable than others. The best summary of the position is that variation within the bales is not negligible. Here, it is only possible to cite a given case and no representation is made that it is typical; the purpose is to demonstrate that the problem is significant. The case in point resulted from a study in which the bales were specially selected to give as low a CV in micronaire as possible. The choice was made using test results from many bale samples. Space precludes giving full details but suffice it to consider a sample of nine bales, as shown in Table A7.1. Variances were averaged for each horizontal slice of all the bales in the laydown (the reader is reminded that variance is proportional to the square of CV). The sliver figures are based on the average variances in samples of card sliver taken systematically over the period over which the bale slices were consumed. The sliver data include the variances between bales, within bales, and any effects caused by processing. Upper half mean length varied little in this case. However, short-fiber content was high and there was a substantial within-bale variance, or processing had produced an extra variation, or both apply. This is important because short fibers cause instabilities in roller drafting, which add to the CVs generated in later processes and thus degrade the final product. The CV of micronaire was variable despite attempts to control it by selecting bales on the basis of the cotton broker’s data. Micronaire is important not only because of varying cross-sectional size, but because of varying wall thickness in immature fibers that sometimes occupy the low micronaire portion of the distribution. It is possible that carding exercised a fractionation function and removed some fibers of high or low micronaire values and thus reduced the CV in that attribute. High CV of micronaire has come to be recognized as one of the causes of barré. The substantial difference in the reflectance of the fibers (Rd) is difficult to explain by processing. Perhaps the values are linked to other attributes vulnerable to change by mechanical processing or perhaps some fiber crimp is removed and this affects the reflective capabilities of the fiber. Again, this is only anecdotal and the values quoted should not be taken as typical for all cases. The points are that the within-bale values are not negligible compared to the between-bale figures and processing can affect the results. On a number of occasions the author has observed variations of the different fiber within a bale that are of the same order of magnitude as those between the bales. Another set of circumstances sometimes confronts a yarn supplier to the knitting industry. The product is often judged in the greige state and fiber color becomes important. In such cases, variability in the +b and Rd values assume greater importance. (Greige refers to fabrics in the state that they leave the loom or knitting machine [1], and by extension, it refers to the yarn used in making such fabrics.) In the case of acrylic yarns, yellowing of the fiber might be a factor.

394

Appendix 7

Obviously, a prime requirement is to assemble a laydown with as little variance between bales as possible. Since the variability of one fiber attribute does not necessarily match that of the others, it is necessary to set up a priority system. Each fiber attribute is given weighting calculated on the end use of the yarn, so that there results a figure of merit customized for the particular product. To be workable, the figure of merit must have many combinations of fiber attributes that satisfy the requirements for a given end use.

A7.3

Mixing in the blow room

A7.3.1 Mixing the fibers flowing through the opening line In a bale laydown, only the fibers from a limited number of contiguous bales can be mixed before the fibers pass to the cards. Assume that there is a moving zone that includes a portion of one or more rows containing (m = m1 + . . . + m2) bales shown shaded in the simplified bale laydown in Fig. A7.1. Also, assume that only the fibers removed from the bales in this moving zone are intermixed. Bales from outside the zone are assumed not to participate in the mixing until the moving zone encompasses them. Participation stops when the moving zone passes by. The bale plucker moves slowly down the line of bales in the direction shown and the zone trails the cutting head. When the cutting head reaches bales 1/2 or Y/Z,1 it reverses. It slowly reciprocates between bales until the demand temporarily ceases, or until the fiber flow system calls a halt, or until the laydown is consumed. The moving zone always consists of the horizontal slices taken from the last m bales passed by the bale plucker as it moves to and fro. Some systems take slices at an angle, but the idea expressed is still similar. The fiber passes into a series of mixing zones that consist of several elements. Each machine in the opening line causes a degree of mixing between adjacent volumes of moving fiber and even turbulence in the ductwork contributes to the process. Where laydowns are formed with several bales set side by side to form a pattern w bales wide and m rows long, averages are taken for each row to yield m average rows. Fiber from these rows progresses through the system in line astern along the m direction. In theory, it is assumed that a step change exists in fiber characteristics as the cutter of the bale plucker leaves one row of bales and passes to the next. It is further assumed that the fibers from all the bales in a row are adequately mixed. The plucking head moves on a fairly regular, periodic basis, but there are occasional

1 3 5 .....

2 4 6 .....

Bale plucker head movement m1 ..... UWY

..... m2 Bale laydown

Fig. A7.1

VXZ

Bale laydown

1 Z often is greater than 40, and the number tends to grow as the technology improves.


395

dwells to keep the fiber flow in synchronism with the demand by the cards. However, the flow may be considered to be more or less continuous and the characteristics of the fiber delivered can be considered to vary periodically.

A7.3.2 Errors due to fiber removal from the bales The characteristics of the fibers can vary cyclically along the fiber flow path because of the reciprocation of a bale plucker over a bale laydown with varying bale to bale fiber characteristics. The effect is worsened by any inadequacy of a mixing machine(s) in the opening line. A peak in the spectrum occurs typically at an error wavelength related to one traverse of the bale plucker. It is useful to express this error in terms of a length of card sliver delivered from one of the cards connected to that opening line. Here, this length is defined as the amount delivered during the time it takes for the plucker to complete one cycle of its travel. This value is of the order of 1000 yards with current machinery. The result may be represented in the frequency domain in the manner of a spectrogram. Some examples of practical results are given later. A further problem exists. Bales are not uniform throughout. The profile of fiber characteristics from one cut across the laydown is not the same as another taken at a different time. The profile varies continuously as the laydown is consumed. This means that there can be some extraordinarily long errors from the system. Even if the order of presentation to the spinning machine is scrambled, unlike yarns will be produced on adjacent spindles. This is a recipe for barré.

A7.4

Theory of blending capacity

A7.4.1 Dispersion in the flow through mixers The foregoing discussion has indicated that variations can occur over a range of processing times varying from a few minutes to a year. The opening and blending line can only deal with changes that occur at less than a certain characteristic time peculiar to a given installation. As an analogy, consider water flowing down a river into a lake, which discharges into another river and out to sea. Under steady conditions, certain levels become established between the two rivers and the lake. If a sudden deluge causes a rise in the river entering the lake, the effect is not passed on in an unchanged way to the downstream (discharge) river. This is because the volume of the lake absorbs some of the sudden rise. The discharge river rises much more slowly. A similar situation occurs in the opening line; a sudden change in one of the fiber parameters in the bale laydown is not immediately passed on to the sliver emerging from the card. The intervening volume and the degree of variability in the fiber flow control the output. Consider the flow of fiber passing into a reservoir, as shown in Fig. A7.2(c). Assume an incremental volume of new fibers (which we will call fiber1) enters the mixing volume Q and is immediately mixed with all the other fibers (i.e. fiber2) in that main volume. The excess is ejected and contains the same proportions of each fiber as exists in the fixed volume Q. Let the total volume of fiber1 derived from a single pass over one bale be Qin; the rate of bale plucker movement be V bales/unit time, and the fixed volume be Q. When the bale plucker passes from one bale to the next, a front is created by the step change

396

Appendix 7 m bales

1 bale Fiber1 (a) Input

m bales 1 bale

Fiber1 (b) Output

Volume Q Input pulse

Output pulse (c)

% Mixing

100 80 60

Q = (1 × Q in)

40

Q = (2 × Q in)

20

Q = (4 × Q in)

0 0 1 2 Volume delivered to mixer (units of S) (d)

Fig. A7.2

Volume delivered to mixer in units of S (S = normalized fiber volume supplied to the mixer).

in fiber characteristics. The flow of fiber between successive fronts is called a fiber pulse. If the normalized flow of fiber is constant at S, then it can be shown that the volume of fiber1 in the mixer increases exponentially as the new front passes through it. Normalized fiber flow = S = V Qin /Q. The percentage value can be expressed by the equation: Qm1 = Q(1 – e–s)100%

[A7.1]

where Qm1 is percentage of fiber1 in the mixer. The greater the volume Q, the smaller is S, the slower is the rise in percentage of fiber1 in the output and the longer it takes to approach the limiting value of 100%. Thus, a sudden change of fiber, such as is met when the offtake from the bale laydown passes from one bale to the next, causes the mix to change. The output pulse leaving the mixing zone is modified by the mixing process and is diffused. In one of the theoretical examples plotted in Fig. A7.2(d), the mixing volume is four times the proportion of the bale slice removed as the offtake mechanism passes over a single bale. It would barely reach 50% of fiber1 before the offtake moved past the bale. Equation [A7.1] is applicable only while fiber1 is being supplied. When the


397

supply of fiber1 is replaced by that from the next bale in the laydown, the percentage of the fiber that had originated from the bale just passed declines exponentially, and the contribution from the new bale begins to rise. Two diagrams illustrate the idea. Figure A7.2(a) shows a rectangular input pulse, which represents the bale slice being removed. Figure A7.2(b) represents the percentage of fiber from that slice appearing in the output. Mixing blurs the boundaries and elongates the volume that contains some component of fiber1. The rate of change and the length over which the fiber is distributed depends on the size of the mixing volume and the efficiency of mixing. For our purposes, the larger the volume Q, the better, because it blurs the boundaries between consecutive zones in the flow line and thus improves the local blending. The length over which distribution occurs may be thought of as a sort of mass constant. In practice, all machines in the opening line contribute a mixing volume, each of which works similarly. Each machine contributes its quota to the mass constant of the whole line. A practical opening line would have no difficulty in blending a single bale slice with its neighbors. The point of the exercise was to show how fibers from a subject bale are spread amongst its neighbors in a mixing operation. More to the point is how far fibers from a subject bale can be spread. A modern bale plucking machine travels over a laydown causing a cyclic removal of fiber. The corresponding period in the sliver is usually related to twice the number of bale rows in the laydown. To eliminate periodic variations in fiber attributes, it is necessary to mix the fiber from a bale being worked with all the others. This implies that the mass constant of the opening line should equal or exceed the amount of fiber removed in a single pass over the laydown. For example, if the mass constant of an opening line is 100 lb and the bale plucker removes 2 lb per bale slice, the size of the laydown should be no larger than 50 bales. Many operators try to achieve synchronism of making a new laydown correspond with the work shift schedule by increasing the number of bales in a laydown. The result of this might be the introduction of a periodic variation in fiber characteristics with a period equal to one round trip of the bale plucker. Also, if the bale slices are heavier, a similar situation will arise. Either circumstance is to be avoided if possible. It has to be recognized that there are limitations to the possible courses of action. For example, the mass of an average bale slice might be (say) 2.5 lb (assuming a throughput of 800 lb/hr and a maximum rate of bale plucker movement of over 320 bales/hr). Continuing the example, let there be 100 bales in the laydown. If we use the criteria just suggested, the mass constant (blending capacity) would have to be increased to 250 lb. The discussions above have assumed a steady delivery from the bale pluckers. However, most bale pluckers stand idle at times, waiting for demand to catch up with output. Anything that can be done to keep the bale plucker in full use, or to decrease cutting depth, helps blending efficacy.

A7.4.2 Dispersion in drawing and combing The fiber flow through the mill might be considered analogous to flow through a pipeline. The system starts with a reservoir containing, perhaps, 25 000 lb of fiber, and flows through an opening line at roughly 1000 lb/hr. This assumes that 50 bales were used in the laydown. The flow branches to supply the cards and each card works at roughly 100 lb/hr. The card sliver is gathered in cans and forms a secondary reservoir behind each drawframe. The secondary reservoirs might each contain up to

398

Appendix 7

500 lb. Before combing, there is yet another reservoir, which we will call the tertiary reservoir, which contains, say, 1500 lb. Ingoing material is doubled in drawing and lapping which reduces the variance. Consequently, according to some practitioners, there is a reduced need for good blending at the early stages. With all the doubling that occurs up to finished sliver, fewer than four or five bales/creel are involved. Whilst it is true that the doubling and mixing at these stages helps within that compass, it certainly cannot be completely effective, especially when viewed against the 25 000 lb or so in the bale laydown. The idea of mass constant may be applied similarly to that applied to the blowroom. Also, it should be noted that the ratio of creel mass in the drawframe to the laydown mass is only of the order of 5:100. Clearly the value is so low that the creeling can only have a very small impact on extra-long-term errors. Even with multiple-sliver processes, the effect of step changes in bale laydowns still cannot be offset and this explains why consistently good blending is needed in the early stages.

A7.4.3 Channeling Under certain circumstances, differences in fiber supplied to one card compared to that supplied to another can cause barré problems, and so can differences in performance of one card compared to another. The problem arises when a drawframe and the following processes are fed exclusively from a dedicated set of cards. If the mean of the product from one set of cards differs from that of the others, the yarns differ, and when they are mixed in winding there can be undesirable step changes in yarn properties. This phenomenon is known as channeling. A solution is to cross-feed the drawframes and combers in a pattern likely to minimize the variations. Often color banding of the cans is used to determine their destination.

A7.4.4 Periodic variations Assuming that relative short wavelengths of periodic error are controlled by good inspection and maintenance, there still remains the question of ultra long-term errors. Difference from top to bottom of a laydown and differences between laydowns produce a mixture of random and periodic errors. The doubling is effective in reducing the errors within the limits already described, but doubling is ineffective in cases where all cards within a set produce a periodic error of the same frequency. Ultra long-term errors arising from the bale laydown could affect all slivers produced by that opening line. Doubling of periodic errors in the creels of subsequent machines produces a vector addition of errors of similar frequency and amplitude but of random phase. Phase is determined by the relative longitudinal positions at which the sliver ends are laid in the creel. Such additions of periodic errors can result in output error components varying from zero to m times the periodic error originally in the sliver, where m is the number of doublings. The problem is that these errors are so long that they are rarely detected. The probability of such an error at a significant level is of the order of one in several hundred creelings. This means the problem shows itself rarely, but when it does, the error can be significant for the duration of that one creeling. When there are periodic variations of different frequencies in the slivers being creeled, the waveform of variation is complex. The various frequencies beat together and produce a difference frequency, which may be expressed as a very long wavelength.


399

Thus, there are infrequent periods when the vectors are aligned. At these times, the amplitudes of the component errors add together algebraically on a temporarily regular basis. Errors at these times are large and similar in amplitude to the case just mentioned. Doubling does not diminish periodic errors reliably; thus it is prudent to avoid this sort of error. Hence, sources of periodic error should be diagnosed and the fault eliminated as far as possible.

A7.5

Fiber migration and blending

A7.5.1 Longitudinal fiber migration If the fiber flow consists of large fiber clumps, the blend will be uneven because of the variations between them. At the beginning of the process, there are large migrations and very considerable blend irregularities. Unevenness in mass distribution is created when the blend is uneven and there is fiber migration. The effects of this are offset by the doubling that occurs at each condensation stage. Consequently, quite large errors might be generated, even if they are later reduced by doubling. Doubling will not reverse the migrations and the composition of fibers flowing through the system will show effects of both types of process. The equation of flow can be considered as a spectrum of sinusoidal variations. Let one of these sinusoids have a wavelength of λ, and let the fiber migration be m, where both variables are measured in consistent units of length. Figure A7.3 shows a simple

Short fiber Remaining fiber

(a)

Constant linear density

Short fiber (b) Varying linear density Remaining fiber Short fiber (c)

(d)

Error amplitude

Fig. A7.3

Effect of longitudinal fiber migration

Linear density

Remaining fiber

400

Appendix 7

sinusoidal blend error in a perfectly leveled strand. In Diagram (a), the leveled strand has a constant overall linear density, but the two fiber components vary sinusoidally in a complementary fashion. The two components are shown as short fibers and the remaining fibers respectively. The short fiber has a linear density of n1 + A sin α, where A is the amplitude of variation in linear density of the component and α is the length along the flow line expressed as an angle. For explanation purposes, let the curves be transposed as in Fig. A7.3(b). To account for the longitudinal migration, the remaining fiber portion is moved horizontally to the right as in Fig. A7.3(c). Diagram (d) shows the addition of the ordinates for short and remaining fibers, and this represents the overall linear density after migration. Clearly migration has changed the overall linear density so that it is no longer level. The amount of error introduced by this mechanism depends on the changes in the blend ratio; if the blend had been perfectly even, no amount of migration would have produced an effect. In practice, there are many wavelengths of error and the amount of error produced varies; more complex analysis is required but the exercise has demonstrated how the interaction occurs. Expressing the results mathematically, the phase change, φ, due to the migration is: φ = 2π (m/λ) radians

[A7.2]

The fiber moves (φ/2π)λ length units. The top curve in Fig. A7.3(c) is represented by: n1= y1 + A sin α

[A7.3]

and the bottom one by: n2 = – y2 + A sin (α + φ)

[A7.4]

The curve in Fig. A7.3(d) is the difference between Equations [A7.3] and [A7.4]. nm = k + 2A(cos(α + φ/2) sin φ/2)

[A7.5]

where n1 is the linear density of the top portion, n2 is the linear density of the bottom portion, and nm is the linear density of the strand after migration. The values y1 and y2 represent the mean values of a long length of the two portions and k = n2 – n1. This means that the error introduced in linear density is a function of the error wavelength of the interface and the phase change referred to earlier. One of a number of critical phase changes is when the fiber migration equals half the error wavelength of the interface. The effect is worse when sin φ/2 = 1.0 (for example, when the phase change is 180°). However, there is no effect on linear density when the phase change is zero or any multiple of 2π radians (360°). To repeat, the practical importance of this is that (a) there has to be significant fiber migration, and (b) there have to be differences in the blend proportions along the strand, before errors in linear density are caused by the phenomenon. Generally, the greater the gradient of the change in a blend component along the length of the strand, the greater is the change in linear density of the strand. Therefore, sudden changes in blend should not be allowed to happen; changes should occur very slowly if problems are to be avoided. As a practical matter, this means: (a) keeping the fiber clump sizes as small as possible at every stage, (b) arranging to avoid dissimilar bales being in proximity in the bale laydown, and (c) promoting as much mixing as possible in the largest mixing volumes possible. There is also a requirement that the average value of the moving zone shown in Fig. A7.1 should vary by a minimum amount and this requirement may conflict with others.


401

A7.5.2 Incremental effects of migration on staple fiber blends For simplicity, consider a two-component system, shown in Fig. A7.4, in which Component B contains fibers that all have a common fiber attribute and Component A contains the rest of the fibers. However, let the proportions of the two components vary along the length. A migration of one component with respect to the other along the line of flow causes blend changes with respect to all fiber attributes. For instance, if fiber length is a major variable, then the fiber migration will change the distribution of fiber lengths along the strand. In Fig. A7.4(a), the two components are labeled A and B. The masses of fiber at any position x from the zero point in the central zone are described by two equations. This zero point is shown by a black dot on the o–o line. The ordinate reference line is o–o and the total mass at position x is ( y + y1). The local blend proportions are b = y/(y + y1) of Component B and b1 = y1/(y + y1) of Component A. In Fig. A7.4(b), Component B has migrated to the left by an amount φ and we shall consider only the zone Z. The equation for y1 is unchanged but the shift of origin causes the other equation to become: y = m(x + mφ) + b The blend proportion for component B after migration becomes: bm = (y + mφ)/(y + y1 + mφ) and when mφ is small with respect to ( y + y1) bm ≈ b + mφ/(y + y1)

[A7.6]2

A

y1 = m1x + b1

b1 O

y = mx + b

O B

b (a) Direction x Direction y

A Z O

O B φ (b)

Fig. A7.4

2 Differentiating, db/dφ → m(y + y1).

y = m(x + φ) + b

Fiber migration gradient

402

Appendix 7

Equation [A7.6] may be interpreted as implying that the rate of change of the blend proportion due to migration depends on the gradient of the components. In other words, if the entering blend component is of variable mass along its entering length, there will be changes in blend proportions in the output due to the migrations caused by drafting. This is distinct from the linear density of the total strand. As before, if the entering blend components are level, no amount of migration will cause a blend change.

A7.5.3 Blending by migration If the blend proportions vary along the length of the strand, the migration causes changes. Thus, the drafting of an irregularly blended but perfectly even strand not only causes irregularity in output linear density but also causes changes in blend proportions. Fibers leaving a machine that drafts the fiber are in a different order from those arriving. Sequential cross-sections contain samplings along the flow path of the input and output material and the cross-sections vary in content from one to another. Calculating the linear densities of the two components from Equation [A7.3] and taking the ratio of the two components, gives us the blend proportions. The average original blend proportion is taken as 0.5 in this example and φ is the phase angle representing the longitudinal fiber migration. The effects are cyclic and at certain phase angles the variation in blend becomes a minimum, as shown in Fig. A7.5. An actual blend contains a spectrum of cyclic components and, for a given fiber migration, there is a spectrum of relative values of φ. The result of this is that some cyclic components are emphasized in comparison to others and there is a sort of resonance pattern. Thus, while the overall effects of migration tend to improve the blend, strong patterns of variability can arise which can have deleterious effects in the subsequent processes and products.

A7.6

Real blend variation

A7.6.1 Variations in mechanical attributes of fibers An illustration of the levels of variability of mechanical attributes was given by a set of experiments carried out at Cotton Incorporated. About 7000 yards of sliver were measured at 10 yard intervals along that length for the various fiber parameters. For the examples quoted here, it was preferred to use periodograms (similar to spectrograms) Relative blending ratio

1.0

φ = 0°

0.5

0

φ = 30° 0

Fig. A7.5

360 α (degrees)

720

Phasing of blend components


403

0 400 200

leng

th,

100 le ( yds ) (a) Short-fiber content

log

sca

0

Wav e

Leng

200 100

2000 4000 th fr o slive m the s r sa mple tart of th e (yds )

FFT amplitude

rather than frequency based curves. Each parameter yielded a time series to which a fast Fourier transform (FFT) was applied to change it into the frequency domain (or wavelength domain). To get the range of wavelengths to describe the phenomena, it is necessary to use a log scale in a fashion similar to that used with spectrograms. To lessen the confusion from the multitude of results, only short-fiber content and micronaire are quoted here. Variations in the short-fiber content are shown in three dimensions in Fig. A7.6(a). The calculated value for error wavelength of the bale plucker cycle was about 700 yards; that compared well with one of the major peaks in the practical values. There were quasi-random variations denoted by the ‘mountains’ in the diagram. These illustrate the danger of applying an FFT to a limited time series; often-anomalous results can be obtained. However, there were also periods when a strong FFT component was noted at the wavelength corresponding to the bale plucker movement. The strength of these peaks varied along the length of the material and this, in part, might be due to the intermittent demand on the supply to the mixer.

FFT amplitude

2000 Card sliver Drawn sliver

1000

0 100

1000 Error wavelength, log scale (yds) (b) Micronaire Notes: Input micronaire is the average of the values from 8 creel slivers. Error wavelength for drawn sliver expressed in terms of card sliver.

Fig. A7.6

(a) Three-dimensional view of short-fiber content; (b) Comparison of micronaire at the input and output of a drawframe

404

Appendix 7

The length of sliver depicted took about an hour to produce, during which time about 5% of the bale laydown was consumed. Micronaire values affect the dye performance of the yarn. A typical pattern measured with card sliver is given in Fig. A7.6(b). As expected, a distinct peak was present at the bale plucker cycle frequency. These variations showed as color barré in fabrics made from the yarn produced from the sliver mentioned. The barré had a periodicity, which related exactly to the cycle frequency just mentioned. The FFT analysis was meaningful and accurate in this case. A blend can be judged in a number of ways. Consider the real case of the 100% cotton sliver shown in Table A7.2, which has been selected because it was a rare, almost worst case scenario. The scenario arose from a badly organized commercial bale laydown and it was compounded by the lack of a blending machine in the line within the particular industrial plant. Card sliver was sampled every 10 yards and the samples were tested for various fiber attributes. The drawframe was creeled with 10 card slivers in the input and the draft ratio was 10. It may be recalled that, according to doubling theory, the ratio should have been √10 in favor of the output. As might be expected, the actual output sliver had considerably larger values of CV than the theoretical values (Table A7.2). The output did not vary in sympathy with the input, despite the synchronization of the testing. Similar experiences have been had in carding and opening. Correlation between input and output has been determined to be statistically insignificant. This is because of the differential longitudinal migration of fibers in the fiber stream within the process concerned.

A7.6.2 Coefficients of variation It is possible to use coefficients of variation (CV) or variance in a specific fiber characteristic, for example, if one is trying to control fiber micronaire or short-fiber content (because of drafting problems). However, each of the spectra for different fiber attributes exhibits a different characteristic. Figure A7.6(b) compares some spectra before and after the first passage of drawing. The draft in drawing was moderate, and the degree of fiber migration small. The resulting effects were also small and thus drawing was found to produce only a modest effect on wavelength distribution of each of the many fiber parameters actually tested. Opening and carding produced irregular peaks, which sometimes corresponded to the bale plucker movement.

A7.6.3 Color variations Standard colorimetry can be used to measure color of loose fiber, sliver, roving, yarn, and fabric. In modern mill practice, fiber color is measured on HVI equipment and it is expressed as yellowness (+b) and reflectance (Rd). In staple yarn mills, it is rare Table A7.2

Variations in sliver (%CV)

Card sliver (drawframe input) Theoretical output value Drawn sliver (drawframe output)

MIC

UHM

UI

STR

ELO

SFC

3.0 1.0 3.1

1.7 0.5 1.8

1.1 0.3 1.1

3.4 1.1 3.6

5.2 1.6 4.2

11.4 3.6 11.9

Note: The acronyms are defined in Section 5.8.2.


405

to measure the color of yarn or the intermediates in this respect, although this might change because of the growing availability and use of optical measuring devices. In the following text, some preliminary data relating to measurements made on yarn and intermediate products is given in the hope of providing some guidance on the possibilities.

A7.6.4 Color measurements on greige fabric Some knitted fabrics were made from the carded sliver referred to in Fig. A7.6(b). Sliver-to-yarn spinning was used to avoid any distortions from the processes of drawing and roving. Also, the samples of sliver actually knitted were contiguous to the samples used for HVI testing. A sample of the results at a color wavelength of 700 nm (i.e. red) is given in Fig. A7.7. Similar curves were produced for other color wavelengths. The error wavelength were determined by applying a Fast Fourier Transform (FFT) to the time series of measured results. The similarity of the spectra implies that micronaire is related to the color response in fabric form. It is known that the dye uptake of cotton fibers is affected by the micronaire values and therefore it might be expected that the changes noted would show up even more clearly in dyed fabric. The exaggerated data shown here imply that bale-to-bale variation of fibers in the bale laydown affected the color of the product cyclically and that blending in the blow room had been inadequate. This latter fact was true for the particular case but it must be emphasized that it was a departure from normal, modern practice.

A7.6.5 Color measurements on sliver Further blending reduces variations in blends but complete homogenization is not possible. Thus, it should be expected that some variations occur in all the fiber attributes when measured in yarn or in any of the intermediate products. This observation

FFT amplitude (arbitrary scale)

1500

Greige knitted fabric 1000

500

0 100

Fig. A7.7

Carded sliver

Error wavelength, log scale (yds)

1000

Comparison of color is card sliver and fabric made therefrom

406

Appendix 7 Cyan Mean = 4.2% St dev = 1.2%

Yellow Mean = 12% St dev = 3.3%

Y C

Magenta Mean = 2.2% St dev = 0.7%

M

Gray Mean = 3.9% St dev = 1.0%

K

200 250 Energy absorption (arbitary scale)

Fig. A7.8

Comparison of color components in sliver

applies to the color spectra as well as to the other attributes. It is possible to measure the color spectra fairly easily and this provides a useful view on the efficacy of blending. The color spectrum of a fiber assembly may be expressed as color separations, such as those used in the printing of color photographs. One method is to use the cyan, yellow, magenta, gray (CYMK) system of color values and to quote the color depths of the textile material, such as that of some sliver as illustrated in Fig. A7.8, instead of the Hunter scale of +b. Examination of scanned images of various sliver and yarn specimens indicates that there are micro-variations in yellowness, which are likely to arise from incomplete homogenization of the fibers within the blend. Even combed sliver shows striations in the yellow separation, which are not very visible in the other color components. More work is needed to comprehend the longer-term ramifications of such anomalies.

Reference 1.

Beech, S R (Ed). Textile Terms and Definitions, The Textile Institute, Manchester, UK, p 114.

Appendix 8 Advanced topics VI: Drafting and doubling

A8.1

Theories of drafting

A8.1.1 Purposes of drafting The purposes of drafting are to elongate the strand (a) to change the linear density, and (b) to improve the fiber orientation within the strand. Two types of drafting are common, namely roller and toothed drafting. Roller drafting will be dealt with first, but many of the remarks apply to toothed drafting as well. The combing roll used in rotor spinning typifies toothed drafting. Roller drafting is known to create irregular fiber flows, particularly when there is a large variation in effective fiber length. Thus, errors are introduced into the output strand by the act of drafting. The means of control for this irregular flow include the use of aprons, pressure bars, and rolls. All of these control elements attempt to restrain the forward movement of the fibers within the draft zone until the last feasible moment. More detailed discussion of this is given in Chapter 3. The ideal control element acts to keep the floating fibers at the speed of the back rolls. It only permits the fibers to accelerate to the front roll velocity when the leading end nears the front roll nip. This means that the control element is in contact with some fibers traveling at the rapid front roll speed and some at slower velocities. The manner of drafting in one stage affects the performance of the next. Consequently, there is a chain reaction along the stages of production, which can culminate in a very poor performance of the spinning frame. Poor performance at this point directly affects the efficiency of spinning and subsequent processes. It also adversely affects the quality of the product.

A8.1.2 Error wavelength and amplitude A machine-created error is often sinusoidal (i.e. its amplitude and wavelength are characterized by a sine wave). An error wavelength is merely the distance along the strand between repeats of the sine wave. Amplitude is the size of the maximum error due to that sinusoid.

408

Appendix 8

A8.1.3 Mechanical errors In practice, errors from the early stages of drafting are elongated by the drafting and have long error wavelengths at the output of the final draft zone. Thus, there is a spectrum of errors of varying wavelength, components of which come from the various drafting zones. A repetitive error that is not sinusoidal can be expressed as a Fourier series of sine waves of different wavelengths, and harmonic analysis can reveal these various components. This is a valuable diagnostic tool for finding machine errors and it is extensively used in the textile industry. Furthermore, random errors typical of drafting waves caused by fiber-borne variations produce recognizable patterns on a spectrogram. It might be recalled that the spectrogram is a diagram that gives the error wavelengths and amplitudes. Consider the effect of an eccentric front roll, where the roll is round but off-center. Assume that the remaining rolls are perfectly true, all the rolls are of the same radius (r), and the rotational speeds are ω1 and ω2 radians/sec for the back and front rolls respectively. Let the eccentricity be ε inches. At the extreme position (a), the surface speed of the roll is ω2(r + ε), but at the other extreme position (b), the surface speed is ω2(r – ε) length units/sec. The linear velocity, Vo, of the delivery varies cyclically between these extremes but the input velocity, Vi, remains constant; the result is that the draft, ∆, varies cyclically. This, in turn, causes the linear density to vary. For an eccentric front roll, the error wavelength is equal to the circumference of the faulty roll. For cotton spinning this is often about 5 inches. (Bad aprons also produce cyclic errors related to the length of the apron.) In position (a), the mechanical draft is Vo /Vi = (ω2(r + ε))/(ω1r) = (1 + (ε/r))∆ In position (b), the mechanical draft = [1 – (ε /r)]∆ Amplitude of the error = ± (ε/r) × 100%

[A8.1]

The mechanical error is magnified because meshing eccentric rolls have a nip line that oscillates along the direction of the flow of fibers. This oscillation causes periodic drafting wave activity and the physical movement of the nip itself creates an additional error ‘spike’ in the spectrogram. If the faulty material is drafted again, the original error is further elongated. Drafting reduces the absolute error amplitude but the percentage value either remains constant or increases. The input error wavelength is increased by drafting in proportion to the draft ratio. The general case is: Error Wavelength = π × roll diameter × intervening draft

[A8.2]

Intervening draft is that draft which exists between the point of origin of the error and the point of measurement of the strand. If there is no drafting between these points, the intervening draft is 1.0. An error in the back roll of a drafting system produces an error in the strand, but the error is then elongated as it passes through the draft zone. The wavelength of the error in the material delivered by the front roll is equal to the circumference of the back roll multiplied by the draft ratio between the back and front rolls. Where several machines are involved, the appropriate draft is calculated by multiplying all the intervening drafts together before applying the result to Equation [A8.2].

Advanced topics VI: Drafting and doubling

409

A8.1.4 Fundamental theory of roller drafting Much of the theory concerning roller drafting has treated individual fibers as long, thin rods lying parallel to the flow direction, the length of the rods being the only independent variable. The simple basis for the theory is that the fibers accelerate to the front roll speed out of phase with one another. This causes migration of the short fibers with respect to the longer ones [1]. Consider two fibers approaching a drafting zone, as shown in Fig. A8.1(a), and consider a worst case scenario. The long fiber approaching the input is of the same length as the roll setting and it is regarded as the reference fiber. The short fiber is of length S and its leading end is level with the reference fiber, of length L, as it passes into the back nip. The long reference fiber cannot accelerate from velocity Vi to velocity Vo until the leading end reaches the Draft zone

Reference fiber

Vi

Vo

Short fiber

S L

L (a) Time t = 0

Vi

Vo

V

(b) Time t = x

xi Stage 1

Vo

Vi U

2 3 4

l L xo

(c)

Fig. A8.1

a Back nip Front nip Floating fiber zone

Fiber flow in a drafting zone

410

Appendix 8

front nip, unless there is slippage between it and the roll surfaces. However, the short fiber can accelerate before this and the acceleration point can vary. The short fiber under these circumstances is called a ‘floating fiber’. Figure A8.1(b) depicts the situation after the trailing end of the short fiber has left the nip of the back roller. The fiber travels at a velocity V that is greater than Vi, and could be as high as Vo, or somewhere in between (i.e. Vi > V > Vo). Fast moving fibers already nipped by the front rolls but also in contact with the short fiber just described, create a force tending to accelerate it (the short fiber). The more numerous fibers in contact with the back rolls try to restrain this acceleration. The accelerating force increases as the floating fiber moves towards the front rolls and the restraining reaction decreases. Thus, there is a point at which the floating fiber accelerates to the higher output velocity. From then on, until the leading end of the long reference fiber reaches the front roll nip, the two fibers travel at different speeds.1 In the worst case, the short fiber travels a distance (L – S) at velocity Vo after the acceleration. The long fiber travels (L – S) at Vi, and the time for transit is t = (L – S)/Vi. Taking the draft ratio, ∆, as numerically equal to the velocity ratio, the short fiber moves: Vot = Vo(L – S)/Vi = ∆(L – S)

[A8.3]

The long fiber moves: Vit = (L – S)

[A8.4]

Not all fibers migrate as much as this. The typical short fiber moves k(∆ – 1) (L – S) relative to the reference fiber, where k is a factor < 1. Grishin [2], like many others, assumed that the fibers are straight and oriented in the direction of movement. Furthermore, he termed the longitudinal distance between the centers of the fibers as shear; also he pointed out that the shear increases as the fiber stream is drafted. Starting from the position of the leading ends of fiber, the shear changes from xi (when both the short and long fibers are both gripped by the back roll pair) to xo (when the long fiber passes from the control of one roll to the next). The scheme is shown in Fig. A8.1(c), where the long fibers of length L are shown as heavy black lines and the short fibers of length l are shown cross-hatched. The zone in which the short fiber can accelerate is shaded in gray and is called the ‘floating zone’. Fibers change velocity from Vi to U in the shaded zone. Most theoreticians assume that U = Vo; the exact point where the events occur is open to interpretation but clearly not all fibers accelerate at the same distance from the back nip. Grishin assumed that xo = ∆xi and that the maximum deviation of a(∆ – 1) occurred at the front nip. ∆ is the draft and a is the width of the floating zone. This implies a uniform probability distribution of the fiber acceleration point over the distance a. In this theory, a systematic displacement of all fibers of the same length plays no role in the unevenness of the strand delivered. However, deviations from the random do cause irregularity. Fujino and Kawabata [3] suggested that the wrongful acceleration of the fibers in this zone is influenced by the fiber speeds. They deduced that the probability distribution of the fiber acceleration point was an exponential function of distance from the front nip. Goto et al. [4] used a normal distribution. All 1 According to some authorities, the short fiber passes through several steps of speed change, perhaps sometimes accelerating during the intermediate stage, but, for the present purpose, a simple one-step model will be used.


411

these theories assume that the fibers are straight, aligned and act independently. Other authors recognize that fibers sometimes travel in groups. Whichever theory might be closest to the truth, it is evident that the smaller the value of a, the less chance there is for irregularity to develop. It is quite clear why a proper setting of the aprons has such a beneficial effect on the evenness of the strand. The main limiting factor in the use of aprons is the wear at high linear densities and high speeds. It is also clear that the magnitude of the draft plays a large part in determining the errors produced. Thus, one would expect that the ring frame would produce the largest errors because the draft there is the largest of the roller drafting systems used in a mill. Table A8.1 confirms this expectation. Following Grishin’s lead, let the fiber shear be defined as the average distance between the ends of various fibers denoted by X. Providing there has been no extra disturbance, shear after perfect drafting is ∆X. If a fiber accelerates earlier than it should, it becomes displaced relative to the others; it swims downstream because its average velocity is increased by the early acceleration. The relative movement of this fiber makes the strand thicker in the place to which it has swum, and thinner in the place from whence it has come. Hence, the displacement of any fiber from its proper position creates an irregularity of linear density. Such fiber migrations alter the shear in the output. There is always some irregularity in the positions of the fiber ends and the shear after drafting, X′, may be expressed as: X ′ = ∆X + s

[A8.5]

where X is the shear, ∆ is the draft, and s is the standard deviation in shear. Shear may be regarded as a form of longitudinal fiber migration. Consider two fibers traveling at the same time, one of the correct length and one shorter. The first travels through the draft zone at the back roll velocity until the leading end almost reaches the nip of the front roll. The short one accelerates early and travels distance y at the higher front roll velocity. The velocity difference during the time that the short fiber is passing through the front nip is (V2 – V1) and the relative draft is (∆ – 1). The scale of all the deviations is changed by the same factor. Standard deviation in shear = (∆ – 1)y

[A8.6]

Johnson [5] used a computer to simulate fiber movement in a random sliver where the rolls were eccentric and the fibers were elastic. He also allowed the fibers to group. Lamb [6] claimed that use of this mathematical model produced a good result for wool processing. This demonstrates that, although the use of models using rigid rods as the moving elements might be deficient, the models suggest that grouping of the fibers might not be as important as some suggest.

Table A8.1

Typical changes in evenness of linear density due to drafting

Yarn count Ne Theoretical CVth% 5% Uster values CVact% Irregy index = CVact/CVth

Card

1st Draw frame

2nd Draw frame

Roving frame

Ring frame

0.14 0.65 2.7 4.2

0.15 0.67 2.8 4.2

0.18 0.74 2.8 3.8

2.0 2.45 5.2 2.1

60 13.4 14.9 1.1

412

Appendix 8

All the theories deal with the short-term errors, and even if some of these errors are elongated by successive draftings, they deal with only one aspect of the problem.

A8.1.5 Drafting using toothed components (Staple fibers) There is another class of drafting in which the one or more pairs of rollers is/are replaced by moving toothed components. (A sketch of a typical toothed drafting system is shown in Fig. A8.2(a).) These components may have saw-teeth, pins or even just a roughened surface capable of gripping a fiber. The toothed components usually create a grip on the leading portions of the fiber elements being drafted and the trailing ends are often restrained by a roll and feed plate combination. Other combinations are possible. In this class of drafting, it is necessary to define the fiber element being drafted. The element might be a fiber clump of some size or it might be a single fiber.

Nose

Toothed roll

Feed roll

N

Vin Feed plate C

Vout

(a)

Fiber clump F

F Strength at weakest link = s (b) Front nip

Force

Back nip Reaction force at N for a strong strand CN FS Reaction force at N for a weak strand CN FW

Applied force

X

Y Distance along the flow path (c)

Fig. A8.2

Toothed drafting


413

Consider a sliver used in rotor spinning. There might be some very small fiber clumps embedded in a matrix of a larger number of single fibers. However, the average exit clump size nears that of a single fiber. In the opening line processes, the clump size is much larger. These clumps are also embedded in a matrix of fibers and it is difficult to characterize the clumps. Generally, the fibers within the clump have a greater mutual cohesion than that which exists between them and those in the surrounding matrix. The clump tends to retain its identity until sufficient force is applied to break it apart. As before, the material enters at velocity Vi and is delivered by the toothed element at Vo; the draft ratio is Vo/Vi. Many theories of roller drafting relate to indivisible fiber elements, whereas with toothed drafting the fiber elements (i.e. fiber clumps) divide in the process. Theories of roller drafting assume that the length of element is virtually unchanged by the drafting, whereas with toothed drafting it is almost certain that the average length changes substantially. Consider a fiber clump being restrained by the rearward portions of the mechanism (such as N in Fig. A8.2(a)). The teeth in the forward part of the mechanism apply a force, F, to the leading portion of the fiber clump, which tends to stretch the clump. This force increases as the leading end of the clump approaches the virtual nip zone in the front of the drafting zone. As the clump moves forward, the force rises until either (a) the trailing end is released, or (b) the clump divides at its weakest point (Fig. A8.2(b)). One possibility is that, when the acceleration force, F, equals the strength, s, at the weak spot, the frontmost portion of the tuft accelerates to the delivery velocity. Later, a second weak spot might fail during the drafting process. Then a further portion of the clump might accelerate to be followed by the remainder when the new accelerating force and the reaction come into equilibrium. Alternatively, the second weak spot may not fail before the equilibrium between the accelerating force and reaction is reached, in which case the clump will have been divided only into two pieces instead of three. Other possibilities also exist. The statistical distribution of the daughter clump sizes differs from the mother distribution. Generally, drafting reduces the range of clump lengths. The length of the clump is always measured in the direction of flow. Performance of the toothed system depends upon the design, among other things; the shape of the zone between the toothed roll and the feed plate is particularly important. An aggressive design tends to break down the clumps more quickly and to straighten fibers more, but at an increased risk of fiber breakage. The design and condition of the teeth or pins are also considerations. In roller drafting, rule of thumb indicates that error wavelength of the major drafting errors is about three fiber lengths. If this were applied to toothed drafting, it would imply that the error wavelengths at the exit of the drafting system would be about three clump lengths. As the tufts get shorter, so would the distances that the tufts migrate due to the drafting process. The greater the strength of the weak link, the smaller is the distance X in Fig. A8.2(c), since the leading end of the tuft has to penetrate nearer to the nip to accumulate a sufficient force. The strengths of the weak links that fail in drafting are variable and the value of X is also variable. The value is equivalent to the value (L – S) in roller drafting. It controls the migration of the daughter tuft with respect to a reference one that does not divide. There is a distribution of fiber migrations and each component contributes to the irregularity of the fiber stream emanating from the draft zone. The complex and varying interactions between clump lengths, longitudinal migrations and blending irregularities makes any deterministic solution extremely difficult. The

414

Appendix 8

distributions of these components are unknown and therefore only generalizations can be made. Fiber migration is determined by draft ratios (in the order of 100). Thus, it might be realized that relative fiber movements in the order of several yards are obtained at each stage. After the various drafts of intervening machinery are taken into account, cumulative relative motions in the order of 100 yards in card sliver are possible. Such migrations might be significant when the blend is irregular.

A8.2

Roller drafting

A8.2.1 Draft distribution The simplest practical roller drafting system is a ‘three-over-three’ system. Too small a break draft does not fulfill its function of breaking down fiber clumps in the input material and too great a break draft introduces unacceptable error in the first stage. More attention to the quality of the input material pays a larger dividend than ultra fine tuning of the break draft value. SKF [7] recommended that the break draft should be between 1.1 and 1.4 for a total draft between 12 and 25; for higher total drafts the break draft could be higher. Values up to 4 were suggested.

A8.2.2 Roll setting There is always a margin allowed over the theoretical setting of the rolls. The exact margin depends on the linear density of the strand and the variance in the length of the fibers. For sliver, the setting can be as much as 10 mm more than the upper quartile length (UQL) for cotton fibers. For finer strands, the margin is reduced. However, other factors intervene and many machine manufacturers recommend settings that are 1 or 2 mm less than the UQL. This is because natural fibers have only a small percentage of long fibers; the majority of them are short. The breakage of a few long fibers is regarded as a worthwhile sacrifice to obtain a better performance with the bulk of the fiber population. Even with man-made fibers, there are fibers shorter than the original cut length because of breakage and fiber convolution. Also, there are a few that are longer due to stretching during processing. An example of the effect of varying the setting for a drawframe when drawing polyester sliver is shown in Fig. A8.3(c). The machine maker’s recommendation was for a setting of between 35 and 37 mm. The example was chosen because there is less variation in fiber length with a man-made fiber than with cotton. When the setting was reduced below 27 mm, the evenness of the sliver deteriorated sharply and it became progressively more difficult to run the machine as the setting was further reduced. In theory, the relationship between the setting and the fiber length is important. The settings theoretically should be changed to match the fibers being run, otherwise the evenness of the output strand suffers. However, in practice, it is rare for changes to be made in the mill except for merge changes and during maintenance.

A8.2.3 Errors in drafting Each draft zone produces its own error, which is superimposed on the errors already present. Short-term periodic errors produce moiré effects, and cloudiness in a fabric if the errors are non-periodic; long-term errors produce streakiness and barré. In roller drafting, the top rolls are covered with a rubber material and, if the load

100% 38 mm Polyester Fiber

8

4 25

50 Roll setting (mm) (a)

75

0

1 2 Length along the strand (m)

Roller neck motion

12

415

C A B

20

30

40 50 60 Frequency (Hz) (b)

70

180 Roll angle (degrees)

360

12 10

Roll separation (arbitrary units)

CV of linear density (%)

Uster CV, log scale (%)


8 6 4

0

(d)

(c)

Fig. A8.3

Some drafting errors

is left on while the machine is still, flats develop because the rubber is visco-elastic. These cause periodic errors, which were a function of the ratch setting as in Fig. A8.3(a). It is common for top rolls to have several minor flats and Fig. A8.3(b) shows a periodogram of the results found with a damaged top roll meshing with a true bottom roll. The several peaks in the profile can be seen. When the data is expressed in CVs as in Fig. A8.3(c), it is evident that the value of CV is considerably higher at start-up than after a few seconds of running. Changes occur as the rubber of the top rolls warms up. The variations in roll separation are closely tied to the irregularities in the strand. A frequency of just over 30 Hz corresponds to the front roll speed (Peak A) and Peak C corresponds to the second harmonic. Peak B comes from elsewhere. When the separation of the rolls was plotted against angle of rotation of the bottom roll for a very large number of rotations, there was a unique pattern dictated by the roll error as illustrated in Fig. A8.3(d). Clearly, mechanical errors can be made to show up distinctly, in contrast to the random errors (For more discussion see Section A8.2.6). An investigation by Keyser et al. [8] showed a linear relationship between yarn unevenness and roll eccentricity, as shown in Fig. A8.4. The data are old but the result clearly demonstrates the importance of roll eccentricity. The authors also found that yarn strength was diminished and the appearance of the yarn deteriorated. Foster and Tyson [9] carried out a similar experiment and found that the slope of the curve of standard deviation vs. roll setting was a function of draft. It follows that there is an interaction between what happens in the draft zone and the cyclic change in velocities. This aspect will be discussed later, but before that, other forms of error have to be considered. All significant mechanical errors in drafting arise from poor maintenance or improper setting. Out-of-true rollers, rolls with uneven rubber hardness, and damaged roll necks (i.e. bearings), gearing, aprons, or other components can cause product errors. Slack bearings can also give problems. Cots or cushion rolls (rubber-covered top

416

Appendix 8

Unevenness (%)

140

Front bottom drafting roll was made eccentric

y = 839x + 105 r2 = 0.846

130

120

110

100 0

0.01 0.02 0.03 0.04 Roll eccentricity (inches)

Fig. A8.4

Bottom roll eccentricity

rolls) must be buffed periodically to true them up; worn or damaged elements must be replaced when necessary. The combination of a good maintenance program with a proper quality control system to identify the sources of error is essential to the running of a modern mill.

A8.2.4 Pneumafil and reworked fibers Production of yarn temporarily ceases when an end breaks in a ring frame. An end refers to the yarn emerging from the delivery rolls of the drafting system. Repair has to wait until an operator (or robot) gets around to it. During these times of interrupted production, the drafting system continues to deliver fiber, which is sucked away by a pneumafil system. The fiber removed is also referred to as pneumafil. The fiber removed is that which would have gone into the yarn, and has good characteristics of length and strength; but fiber crimp and elongation characteristics have been changed as compared to the virgin fiber. Nevertheless, the pneumafil waste is reused or ‘reworked’ because it is too good to throw away. The reworked fiber has to be blended with the virgin fiber very carefully so that the percentage does not exceed, say, 3% anywhere in the blend. Failure to control the blend leads to problems throughout the process line. Besides the pneumafil produced during the time of an end down, there is a continuous loss of fiber from the twist triangle and balloon during spinning. This is only a fraction of a percent of the total volume being processed, but it is significant in terms of the amount of pneumafil produced. Consequently, there is not a unique relationship between the ends down rate (in breaks/1000 spindle hour), machine productivity (in lb/spindle hour), and the pneumafil production rate (in lb/spindle hour) as might be expected, although there is a rough correspondence. Mill trials produce a range of values depending on the product, machine, and operators. A very rough rule of thumb is that the percentage of pneumafil is about one-tenth of the end-breakage rate. Excessive deviations in the ratio between the pneumafil production rate and the theoretical value given below are a sign of inefficient repairs of the broken ends. Theoretical pneumafil production rate, Pp, is given by Pp = K + {Py × (m r /ms)}

[A8.7]

where K = a factor dependent on the production of pneumafil created whilst the spindle is working


Pp = Py = mr = ms =

417

average production of pneumafil in lb/hr for 1 spindle the production of yarn in lb/hr for 1 spindle number of spindles idle in a set because of end-breakage number of spindles in a set = (usually) the spinner’s assignment.

A8.2.5 Hairiness Errors exist other than changes in linear density or creation of thick and thin spots, slubs, and nep. If the surface of the yarn has a differing structure along its length, it can produce customer complaints because of shading, barré, and moiré in the fabric. Change in hairiness of the yarns is one of the factors responsible. Variations in hairiness can come from several sources. One source is wear in yarn guides and other running surfaces, which rough up the yarn as it runs through the machine. Sometimes lack of careful maintenance will allow deep cuts to be produced on surfaces, especially when running with a fairly abrasive fiber such as polyester. A second source is in the twist triangle, where conditions leading to a ragged and varying construction can also lead to undesirable changes in the surface of the yarn. A third cause arises from varying quantities of short fiber arriving at the twist triangle which produce changes in the yarn surface.

A8.2.6 Continuous measurement of sliver properties Grover [10,11] discussed the problem of dynamic measurements of linear density in a flowing sliver. Amongst many sorts of transducers assessed was a thermocouple device (which measured the temperature of the throat of the trumpet), a pneumatic system (which measured the pressure at the throat), and a force reaction system (which measured the drag force acting on the active part of the trumpet). Compressed sliver sliding through the throat of the trumpet dissipates energy. Friction at the throat heats the sliver as well as the trumpet and it is possible to measure the sliver temperature. The time response is slow for the relatively massive machine components in the heat dissipation path. However, the speed of the sliver causes it to respond to the changes in the resistance of the sliver passing through the trumpet with a time response better than 0.05 sec. Significant differences in temperature even over just a few inches of sliver running in a drawframe could be detected. When running cotton, temperatures of up to 30°C were typical whereas with acrylic fiber the value would rise to the region of 40°C. Drawn slivers produced lower temperatures than carded ones. Figure A8.5 shows a series of scans with an infra red movie camera recording the temperature of the sliver as it left the trumpet. Each scan showed an increase in temperature as it passed over the sliver. About 70 mm of sliver passed during one scan cycle. The three-dimensional graph shown has x, y, and z axes; temperature is along the y axis, length along the sliver is along the x axis, and distance across the sliver is along the z axis. All three axes are mutually perpendicular. The temperature profile on each scan is an indication of structure of the sliver, and the distance apart of the shoulders is an indication of the sliver diameter (as shown in the diagram). Tests showed good results but the expense of the system was a problem. Until a cheaper means of measuring the radiated heat is available, the method is unlikely to be developed commercially. Pneumatic trumpets are often used in which the air pressure at the throat of the trumpet is measured as a proxy for linear density. The flowing fibers carry air into the

Appendix 8 Sliver temperature at trumpet exit

418

y Distance perpendicular to strand axis z

30°C 26°C

0 Le pa ng ss th ed of 1 th sliv ro er ug t h ha th t h e tru as j 2 m us pe t t

x Sliver diameter

Fig. A8.5

Continuous sliver temperature measurement

trumpet and this creates a pressure in the throat. It also creates air turbulence due to backflow at the entrance to the throat. The relationship between the air pressure and the linear density depends on the bulkiness of the ingoing sliver and the speed at which it travels. Trials were made with a trumpet in which a sliver was run and pressure measurements were recorded. The sliver was cut into consecutive one-inch lengths of sliver, weighed and matched to the corresponding portions of output signal from the transducer. The comparison showed only a 0.64 correlation coefficient. Longer-term errors yielded better results, but it was clear that the pneumatic trumpet could not be relied upon to provide an accurate measure of very short-term error. A trumpet was equipped with one or more diaphragms, each containing an orifice through which the sliver passed. In the simplest one, a single orifice was mounted on a diaphragm and strain gages were used to measure the reaction forces. A test with consecutive one-inch samples, similar to the one just described, was used, and this produced a correlation coefficient of 0.75. Although this was an improvement in performance in the measurement of very short-term error, there were problems with signal drift caused by uneven heating of the strain gage elements in a bridge network. (A Wheatsone bridge measures and compares the electrical resistance of the four arms; variations in strain or temperature of the material in any one of the arms will unbalance the bridge and give a signal.) This emphasizes the difficulty of getting a reliable signal to be used for control. At a given roll pressure and a given fiber ribbon width, the linear density of the sliver is given by the separation of the rolls through which the material travels. A tongue-and-groove measuring system works on this principle and it is very successful in yielding accurate results providing the operating surfaces are true and clean. However, there is a tendency for the meshing of tongue and rolls to cut fibers that are not pressed into the groove. Grover [10] and Lord and Govindaraj [12] used drafting roll separation as a proxy for linear density of the ribbon of drawn slivers passing through the front rolls. The measurements were found to be sufficiently accurate to use as a control signal to adjust the ratch setting although some errors due to changes in density at the selvages of the ribbon became evident. Mechanical errors in the rolls came into sharp focus. The use of an encoder permitted the exact position of any flaw in a rotating roll to be identified without stopping the frame. It was also found that


419

when a frame first starts up there are minor flats on the rubber cots but that these flats disappear as the rubber warms up in use. Thus, there is a temporary increase in the irregularity of the sliver when a machine is started after a certain rest time. The regularity of the sliver improves over the first minute of running. Another approach by the researchers just mentioned was to optically measure the thickness of the fiber ribbon that passes through the draft zone of the drawframe. There are difficulties in getting access to the material because of the space taken up by the rolls. One of the conclusions from this phase of their work was that the input slivers retained their autonomy through the drawing process, each sliver behaving independently of the others. Traditionally, m input slivers are considered doubled before drafting, and the variance of the material input to the drafting process is expressed as (1/m)th of the average. The variance of the output material from the drafting process is then given as: (σout)2 = (σin)2 + (σadded)2

[A8.8]

where the variance due to drafting is added to the variance of the input material. However, if the slivers behave independently during drafting, it is more correct to say that the variance added by drafting is added to each input sliver and the doubling takes place after drafting. The equivalent relationships are then: m

m

m

j =1

j =1

j =1

(1/m ) Σ ( σ out, j ) 2 = (1/ m ) Σ ( σ in, j ) 2 + (1/m ) Σ ( σ added, j ) 2

[A8.9]

This may be interpreted as: average of the output variances = average of the input variances + average of added variances In this situation, drafting adds even more irregularities than is usually believed. Furthermore, the result implies that their neighbors influence fibers when the packing density is high, and that they behave as groups rather than as individual fibers. Hence, the theoretical CVs should be calculated on the number of fiber groups in the cross-section rather than the number of fibers.

A8.2.7 Automatic control of drafting errors In 1962, Ishikawa and Shimuzu [13] proposed a device for sliver drawing in which the drafting force was used to generate a signal to control the draft ratio. It was found that the time constant of the system was an important parameter and that errors could be amplified at wavelengths shorter than dictated by the time constant of the system. In practice, they were able to control evenness down to about 3 inches (8 cm in the original paper) but instabilities at smaller wavelengths were troublesome. They succeeded in reducing the errors in their test apparatus but the importance of the paper was more in directing attention to the regions of stability in a control system than in developing a viable machine. As mentioned previously, separation of the rolls in a drawframe has been used to provide a signal, which was used to control the ratch setting rather than the draft ratio. The idea was based on watching an expert set up a drawframe for minimum drafting wave error. These errors are caused by variations in fiber attributes that react with fixed roll settings. The expert first judges the drafting wave activity by the height of

420

Appendix 8

the hill on a spectrogram, then changes the roll setting, makes another test, re-judges and so on. This process continues until the setting is optimized for minimum drafting wave activity. In the automatic control system, the middle and rear pairs of drafting rolls were mounted on a platform, the rolls were driven by stepping motors, and the setting of the front drafting zone was also controlled by a stepping motor. A computer program was written that sampled the output from the roll separation transducer within a specified error waveband, which included a typical drafting wave. If the magnitude of the signal was outside a control zone, the ratch setting was adjusted by 1 mm and the error was sampled again. If the error was still out of control, the process was repeated, but if it was now in control, the process ceased until the signal went outside the control limits again. Roll errors made sharp peaks enclosing only a small error in a periodogram whereas the drafting waves made a diffuse spectrum that encompassed a considerable variance. The system performed satisfactorily and could be seen to change settings from time to time as the fiber population changed. A cotton sliver was spliced to one made of longer polyester fibers and, as the splice passed through the system, there was a permanent adjustment to the new fiber length. Despite this, the system was not successful because it was so slow in reacting to changes.

A8.3

Avalanches in roller drafting

A8.3.1 Slub production As stated earlier, a short fiber in the draft zone not being nipped by either set of rollers is called a floating fiber. The floating fiber is under the influence of others in contact with the front and back nips but it is not directly controlled by either set of rolls. Consider what happens just before and after the acceleration of the floating fiber. Just before the acceleration, the subject fiber may be in contact with other floating fibers. For explanation purposes, assume the following. Three fibers in contact with the subject floating fiber are completely controlled by the back nip. Two are adjacent floating fibers traveling at the speed of the back nip but they are not directly connected to it. Another fiber is completely controlled by the front nip and is also in contact with the subject. No other fiber controlled by the front roll is in contact with the subject. If, at that moment, the fiber in contact with the front roll generates not quite enough force to overcome the frictional resistance between the subject and those in contact with the back roll, the subject will not yet accelerate. If, however, one of the three which had been in contact with the back roll is later released, and accelerates, it might cause the subject also to accelerate because there is now one fiber less restraining it. If the subject does accelerate, there is a possibility that the other floating fibers in the vicinity will be induced to go with it. A small avalanche has been created. Obviously, larger avalanches are possible, especially where the subject fiber and neighbors are nearing their normal acceleration points anyway. Where fiber clumps rather than single fibers are involved, the greater number of neighboring fibers can have a greater effect because the circumference of the clump is larger than that of a single fiber. Consequently there are differing chances of avalanches. The effect of such avalanches is to magnify the size of the defects and to undesirably increase the variability of output. If a group of fibers accelerates early, then the population of fibers in the rearward


421

section is depleted. Also, the group to be accelerated is not formed until Z inches of strand have passed. After drafting this becomes ∆Z. Concentrations of short fiber create weak zones, which are interspersed with good fiber. The body of the avalanche can become a slub, which carries a concentration of short fiber with it when it accelerates. These concentrations of short fiber are likely to cause more avalanches (and slubs) in later processing. After drafting, there are sometimes long portions of good material between the outbreaks of slubs (which are often processed without trouble). Under these conditions, there can be outbreaks of this type of fault in each of the drafting systems through which the concentrations pass. A steady concentration of short fiber can break up into pulses of short fiber; these, in turn, break up in subsequent drafting into intermittent bursts of activity. Relatively short lengths of yarn may be subject to heavy production of slubs and then there can be quite long periods without activity. The final slubs can be very heavy and the following thin spots are very prone to breakage in spinning. To minimize such effects, the roll settings must be properly controlled and high concentrations of short fiber should be avoided.

A8.3.2 Variations in fiber population Lord et al. [14] point out that a fiber (or fiber clump) accelerates when the force exerted on it by fibers being pulled into the front nip exceeds the reaction from the more slowly moving fibers under the influence of the rear gripping medium. In roller drafting, this would be the back roll. The acceleration point is dictated not only by the relative speeds of the fibers, but also by the number of fibers in contact with the subject fiber (clump) and their degree of entanglement. This may lead to unstable conditions and avalanches [15]. Table A8.1 makes it clear that the actual CVs of linear density are much larger than the theoretical values for the draw- and roving frames where the strand cross-sections are large. It is not until the cross-section is reduced to the order of 100 fibers that the theoretical and empirical values approach one another. This suggests that the fiber flow in the first stages of the process line is not composed of independent fibers moving singly in relationship to one another; rather that fibers move in groups, and this causes a deterioration in the expectation of evenness. It also suggests that the populations of fibers entering a machine may vary considerably along their length. If this is so, then there will be corresponding variations in drafting performance. Lord and Johnson [15] suggested a system of fiber population changes that would carry errors from one machine to the next in line, to make complex patterns of spinning performance. Grover [16] made many cross-sections of polyester/cotton yarns. The percentages of each sort of fiber were noted at one-inch intervals. The work gave CVs of about 20% in polyester content. The few gaps were filled by interpolation. The resulting time series (shown graphically in Fig. A8.6(a)) was subject to a fast Fourier transformation to give the periodogram shown in Fig. A8.6(b). The CV in the one component was considerably higher than that of the linear density (c 14%). There seemed to be a distinct probability that the main variability detected was from drafting waves in the roving frame. Since then, some work in measuring the characteristics of the cotton fiber delivered from the drafting system of a ring frame has shown a CV of linear density over 17% in long-term variability. The high CV probably arose from variations in the first drawframe. It is very difficult to measure the short-term variability except for short lengths. The

422

Appendix 8 40/1 P/C yarn

Amplitude (arbitrary scale)

Polyester in X section (%)

40 35 30 25 20 15

500

FFT of % polyester

400 300 200 100 0

10 0

25 50 75 100 Length along the yarn (inches) (a)

Fig. A8.6

1 10 100 Wavelength, log scale (inches) (b)

Variation in polyester content

implication of this work is that changes in fiber population interact with the successive drafting zones to cause varying performance. The characteristics of batches of yarn might well be associated with these changes in fiber population. Changes in the attributes of the fiber passing through the system undoubtedly affect the CV of linear density of a yarn emerging from the front rolls of a ring frame. These effects might be even larger than those arising from changes in roll settings or machine condition. We finish this section on a practical note. Sampling the production and testing the CV gives a sample value that is good only for that time. A number of samples taken over an extended period needed to characterize a drafting system.

A8.4

Doubling associated with roller drafting

A8.4.1 Reduction of variance by doubling Associated with many drafting systems is the idea of doubling. Several strands of input material are fed to the drafting system in an attempt to reduce the errors. A typical machine in which this occurs is the drawframe. Here it is not practical to use aprons; only the fixed control surfaces and the effects of doubling mitigate the irregularity caused by drafting. A quite usual assumption is that the variance in linear density among the strands is averaged. The resulting variance of the output is taken as (1/m)th of the variance of any of the m slivers used in the creel as input (i.e. m doublings). This neglects the variance added by drafting and the variations between the input slivers. An idea of this can be gleaned from an old study by Ozgur [17]. Data were re-plotted to give Fig. A8.7. Plotting the output irregularity in terms of 1√m and extrapolating towards zero, the irregularity does not reduce to zero. In fact, some studies have found that drawing more than about three times not only reduces sliver cohesion but also causes the regularity to deteriorate. Also, Bowles and Davies [18] showed that the improvement in CV due to doubling is reduced as the wavelength of error increases. No improvement will occur if the input error wavelength is much larger than the length of sliver in the supply cans. Very long variations can be induced by a poor bale laydown and doubling at drawing or elsewhere after carding might have little effect on these longer components. Dyson [19] considered a model in which the variability of fiber extent was introduced.


3

2

2.5

1

0

Sliver irregularity (CV%)

5

y = 6.861x + 0.777 r 2 = 0.986 r = correlation coefficient Draft ratio = 8

Doubling = 8

Sliver irregularity (U%)

4

423

0 0.2 0.3 0.4 1/√Doublings NB Irregularity is a synonym for unevenness. 0

0.1

Fig. A8.7

Effect of doubling

(Fiber extent is the distance between the extremities of a crimped fiber.) The minimum irregularity, CVi, is then expressed as: CVi = {100/( mk )}(1 + 0.0001 CVd2 + 0.0001 CVk2 )

[A8.10]

Where CVd is the CV of fiber linear density, CVk is the CV of fiber extent, m is the number of fibers in the cross-section, and k is a factor. This assumes that fiber fineness and fiber extent vary independently of one other. Dyson quoted work by other authors yielding k = 0.95 for carded cotton ring spun yarns and k = 0.8 for rotor yarns. With k = 0.95 and 0.8, the equations simplify to 109/√m and 119/√m, respectively. One way of expressing irregularity is by using an index, which relates the actual CV to the theoretical value. Experience shows that the index of irregularity decreases as the material moves down the process line, with the index varying from roughly 4 at the card to about 1.1 at the ring frame. Lamb [6] argues that doubling does not affect the index because both the denominator and numerator are affected by the factor √m. Undoubtedly, the movement of aggregations of fiber rather than single ones has an effect.

A8.4.2 Effects of between-stream variance Passing reference was made earlier regarding the need for similarity in the means and variance in the streams of material being doubled. An example in staple yarn production illustrates how the between-stream variance can be included in the estimate of the expected total variance or CV. Figure A8.8 illustrates how variations in the means of strands A, B, and C broaden the probability distribution of the combined strand shown as D. The means are a, b, and c; and the total unevenness in the product stream is calculated by adding variances between the means to the variance within the samples. In symbols:

CVt ≈ √ ( CV 2 / m + CVb2 )/100%

[A8.11]

where CVb is between the means, CV is the mean of the components, and m is the

424

Appendix 8 a

Statistical frequency

CVa

A

Components

CVc

CVb C B

b CVt

c Total

D O Linear density

Fig. A8.8

Variability caused by differences in means

number of strands. Doubling of periodic errors can present problems, particularly if the error frequencies are similar for all input strands. The relative longitudinal positions of the streams then play a part in determining the error. Any such difficulties in this respect can be avoided by making sure that the equipment is in good condition and does not produce periodic error. A normal drawframe has input slivers of varying mean values of linear density. Thus, finisher drawn slivers provide variations in yarn not only from the variance within each sliver and the variance produced by spinning but also from the variance between the slivers.

A8.4.3 Doubling mass constant As discussed elsewhere, the flow of material through a mill is not really continuous; rather batches of material are processed in sequence. When traditional doubling is used, it is within a batch. Thus, for example, when we double sliver in a drawframe, we double within the batch defined by the mass of fiber in the creel. This mass may be thought of as a mass constant. The system is not able to significantly reduce error for wavelengths greater than that represented by about twice the mass constant of the machine involved.

A8.4.4 Effects of overdrawing Whilst drawing sliver improves the orientation of the fibers and applies some doubling, too great a number of drawframe passages can adversely affect the product. Sliver tenacity falls off rapidly with multiple drawings. Klubowicz [20] determined the effect of multiple drawings (up to 36 drawings) on the yarn strength and strand evenness. He found that the yarn strength reached an optimum at somewhere between four and eight drawings, but then decreased with further drawings. The strand uniformity and yarn appearance improved with increased number of drawings, while the yarn elongation decreased. There was difficulty in handling overdrawn sliver because of the low sliver cohesion. This makes it clear that there is a limit to the benefits derived from drawing and doubling. Improvements in fiber orientation are similarly limited.


425

Many of the simple ideas of doubling and drafting are insufficient to explain the whole set of problems.

A8.4.5 The combined effects of drafting, doubling, and twisting Cavaney and Foster [21] found, from empirical studies, that the variance of the output strand from a drafting system was: (ANe(∆ – 1)/m] + b

[A8.12]

The factor A was a figure of merit but was not a constant and the factor b was almost zero; m was the number of ends fed to the system. Speed of the frame was found to have little effect. They recognized that fibers did not necessarily travel through the draft zone independently; they commented that the variance depended, in part, on the number of fibers in the fiber groups involved. Further, they pointed out that twist in a strand had a stabilizing effect on drafting. The performance of a roving frame may also be expected to differ from that of a drawframe on that account.

A8.5

Doubling and toothed drafting

A8.5.1 Opening line and carding The process of dividing fiber clumps, which is a form of drafting, was described in some detail in Chapter 5. As outlined in Section A8.1.5, the basic ideas are fairly clear but the idea has not been widely recognized. There are large drafts applied to fiber clumps and the division of the clumps is irregular. Draft is applied to the whole stream, and the flow becomes irregular. However, few attempts have been made to assess this irregularity because (a) it is difficult to do so without impeding the operation, and (b) the effect of the irregularity is obscured by the massive doubling that occurs in devices like mixers and chute feeds. Perhaps someone will realize that there are potential gains in better controlling the fiber flow in the opening line, and then we shall see a further step in the continuing trend of improved yarn quality.

A8.5.2 Rotor spinning Separation of fibers in a strand supplied to an open-end spinning machine is the essence of the process. It is necessary to separate the fibers almost into separate entities to make the system work. Open-end spinning, in the form of rotor spinning, has become very successful. Toothed drafting is an essential part of that success. Damage to the feed and combing rolls produces periodic errors as one might suspect and that sort of error can be detected by conventional testing and corrected by proper maintenance. Random variation in the fiber stream delivered to the rotor is reduced by the massive doubling that occurs when the many layers of fiber are laid inside the rotor to build up the necessary linear density of yarn (see Sections 3.4.1 and 7.2). Short-term random errors are low in rotor spinning. Perhaps the main lesson to be learned is that adequate doubling reduces the random errors. For this to be effective the mass constant must be large enough and, of course, there must be a sufficient number of doublings.

426

Appendix 8

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

Lord, P R and Grover, G. Roller Drafting, Text Prog, 23, 4, 1993. Grishin, P F. A Theory of Drafting and its Practical Applications, J Text Inst, 45, T167, 1954. Fujino, K and Kawabata, J. Method of Analyzing Problems in Drafting, J Text Mach Soc Japan, 8, 3, 1962. Goto, H, Ichino, S and Kurozaki, J. Fiber Motion in Roller Drafting, J Text Inst, 48, T389, 1957. Johnson, N A G. A Computer Simulation of Drafting, J Text Inst, 72, 2, 69, 1981. Lamb, P R., The Effect of Spinning Draft on Irregularity and Faults, Parts I and II, J Text Inst 78, 88 and 101, 1987. SKF Bulletin No 3, Skefko Ball Bearing Co Ltd, Luton, UK, Technical notes, 1957. Keyser, W R, Middleton, J O and Dougherty, J E. The Effect of Roll Run-out in Spinning on Yarn Quality, Text Res J, 1956. Foster, G A R and Tyson, A. The Amplitudes of Periodic Variations Caused by Eccentric Top Drafting Rollers and their Effect on Yarn Strength, Trans Text Inst, T385–T393, 1956. Grover, G. Dynamic Measurement of Sliver Properties, Ph D Thesis, North Carolina State Univ, Raleigh, USA, 1989. Grover, G and Lord, P R. Measurement of Sliver Properties on the Drawframe, J Text Inst, 1992. Lord, P R and Govindaraj, M. Dynamic Measurement of Mechanical Errors in Sliver-Drawing, J Text Inst, 81, 195, 1990. Ishikawa, S and Shimuzu, J. Automatic Control of Short Term Sliver Irregularities by the Detection of Drafting Force, Text Mach Soc Japan, 8, 3, July 1962. Lord, P R Stuckey, W C, Yu, X and Grover, G. Deblending in Roller Drafting, J Text Inst, 76, 5, 339, 1985. Lord, P R and Johnson, R H. Short Fibers and Quality Control, J Text Inst, 3, 145, 1985. Grover, G A. Periodic Variations in some Fiber Properties along Strands Drafted in a Roller System, MS Thesis, NC State Univ, USA, 1984. Ozgur, N. Relationship between Input and Output Variations at Drawing, MS Thesis, NC. State Univ, 1965. Bowles, A H and Davies, I. Description of the Evening Action at the Drawframe. Part III, Extremely Long Wave Irregularity, Shirley Inst Bull, 42, 1, 19, 1969. Dyson, E. Some Observations on Yarn Irregularity, J Text Inst, 65, 215, 1974. Klubowicz, A MS Thesis NC. State Univ, USA, 1965. Cavaney, B and Foster G A R. The Irregularity of Materials Drafted on Cotton Spinning Machinery and its Dependence on Draft, Doubling and Roller Setting, J Text Inst, 46, 8, 1955.

Appendix 9 Advanced topics VII: Yarn balloon mechanics

A9.1

General observations

The whirling length of yarn between the pigtail guide and the bobbin produces yarn tension. Too high a tension above the pigtail guide leads to a high frequency of endbreaks, which reduces spinning efficiency and yarn quality. Too high a tension below the traveler makes unwinding at the next process stage more difficult and increases the number of interventions in winding, which reduces both winding efficiency and yarn quality. Too low a yarn tension in the balloon leads to collapse that produces similarly undesirable results. The behavior is usually analyzed by considering the forces involved, but there is also the possibility of using energy balances as a means of description. Forces are vector quantities whereas energy is a scalar quantity and this provides some relief from the mathematical rigor needed for acceptable solutions. An explanation can be derived from a consideration of the energy dissipated, stored, and/or transformed at the various parts of the rotating system. We will look at both approaches. Technicians observe the balloon shape as a measure of the yarn tensions. Sophisticated means of measurement are rarely available in a mill and the normal means of judgment is whether the balloon is long and thin, whether it is fat, or whether it is bottle shaped. What is being observed is the surface area swept by the rotating yarn. Theoreticians have used vector mechanics to explain the complex phenomena and a reference point frequently used is the node formed at the pigtail guide. However, the energy to sustain the balloon derives from the bobbin, which is rotated by a mechanical drive system. As far as the present discussions are concerned, the subject will be divided into several divisions. These will deal with various aspects of ballooning, and then go on to discuss (a) the lower zone between the traveler and the winding point, (b) the central zone, and (c) the upper portion above the pigtail guide. The central zone is a powerful tension producer but it is the reactions at the traveler and pigtail guide that produce the consequences.

428

A9.2

Appendix 9

A rotating plane balloon – a very simplified case

As matters of definition, let the surface swept by the generator OBP (Fig. A9.1) be called the balloon and that swept by the more or less horizontal generator QP be called the base. To introduce the ideas, first consider the problem at the lower of two levels of simplification; the simplifying assumptions reduce the obscurity of the topic without enormously affecting the central idea. In this simplified case, the yarn rotates in a vacuum about an axis OQ, as shown in Fig. A9.1. This involves a rather unrealistic assumption that the plane OPQ rotates about the axis OQ at a speed of ω radians/second and that the yarn is confined to this plane. The theory ignores the effects of bending and torsional stiffness of the yarn. The yarn is treated as a string of beads, each element of which is δs units long. The action of the distributed centrifugal force and the restraints at O and P cause the yarn to become curved similarly to a hanging cable. Several radii may be defined. The radius with respect to the spindle axis at any height is denoted by r. The maximum value (re) is at the equator at B. The radius rg is that at which the mass of the yarn length OBP may be considered to be concentrated (i.e. the distance to the centroid G). The length of yarn in the balloon (S) above the traveler may be estimated by calculating the length of the line OBP. The mass of yarn in that length is Sn (where n is the linear density of the yarn) and the centripetal acceleration acts on that mass through the centroid G. During a single chase, the length S might change by up to 10% but, in this rough analysis, the change will be ignored. (The meaning of the word chase is defined in Section A9.3.2.) Ft and Fho are the horizontal components of the forces acting on the yarn at the traveler and pigtail guide, respectively. Ft includes the centrifugal force acting on the traveler. The centrifugal force Fcf = nω2 rgS. Horizontal force components must balance and the moments within the plane should also balance. Taking moments about O, Fcf × hg = Ft × H

[A9.1]

Fho O

hg H

Ft

ω G

B Q

Fcf P

rc

Fig. A9.1

Rotating plane

Advanced topics VII: Yarn balloon mechanics

429

Ft is equal and opposite to the difference between the horizontal components of centrifugal force acting on the traveler and the reaction of the traveler with the ring. Distribution of the forces between O and P also alters with the position of the centroid, which changes during the building of the bobbin. The centrifugal force acting on the yarn changes with the radius of the centroid, rg. The effect of these important reactions will be described later. A force of Fcf acting on an element δS is ω2rnδS. The total force acting on the portion OP is the summation of all such elemental forces between O and P. If the length of the yarn between these limits is S, then s

Fcf = Σ ω 2 r n δ S

[A9.2]

o

This force is divided as in Equation [A9.1] to give the reactions at the ends. The vertical components of force for the simplified model are shown in Fig. A9.2. A force acting along the yarn is called yarn tension, T, and a force in any other direction is denoted by F. In a portion of the yarn at the equator B, the downward tension is Fv and the upward reaction at O is Fvo. For vertical equilibrium, Fv = Fvo. A similar argument can be made if the yarn had been cut at U. The vertical component of T at that point also equals Fv for equilibrium. Thus, in the oversimplified model, the vertical component of tension in the yarn balloon does not vary at all as a function of height; it is solely determined by the end conditions. In ring spinning, the forces at the traveler and the pigtail guide determine the end conditions. The resultant forces at these points press the yarn against metal and friction creates tension gradients across these items. The frictional forces cause the tension below the pigtail guide to be more than that just above it because the yarn moves downward. The tension in the yarn leaving the traveler is higher than that just above it because there is a tension gradient due to friction. These effects will be further discussed later. These rough analyses set the scene and establish how the distribution of the applied force alters the reactions at the end points. Even when the balloon is considered in Fvo

To

O

hg

φ

φ

Fcf

H

x

B

U

re

Z

Ft

y Q

Horizontal components (a)

Fig. A9.2

O

Yarn tension

Fho

To

Fv

Q

Vertical components (b)

Force components in a simple theoretical balloon

430

Appendix 9

three rather than two dimensions this is still true, although the analysis becomes more complex.

A9.3

Energy distribution in the balloon

A9.3.1 Energy taken from the bobbin Figure A9.3(a) shows a compound view of the yarn between the bobbin and the ωy

B

Yarn

Bi

ωt ωb

Bo

(a)

Rail traverse

Bobbin Traveler

Ring

αo

ωt Bo

Bi

αt O

A ωb

rBo

rr

rBi Plan view

(b)

T1 Fy T2 A

Yarn traveler

Fa

B

Ring flange Reaction View perpendicular to plane containing the yarn (c)

Fig. A9.3

The lower portion of the balloon


431

traveler. The torque supplied to the rotating yarn system is the mathematical product of the horizontal component of yarn tension at B and the winding radius. The energy supplied is torque × rotational speed. Let this energy be designated E. Conservation of energy dictates that the energy available at B is absorbed or dissipated by (a) kinetic energy of the yarn between the pigtail guide and the point B in Fig. A9.3, (b) kinetic energy of the rotating traveler, (c) strain energy stored in the yarn under tension and torque, (d) losses due to airdrag acting on the yarn, (e) friction losses caused by the traveler sliding on the ring, and (f) energy arising from forces generated by balloon instability. Potential energy changes are negligible; energy changes due to balloon instability are ignored.

A9.3.2 Energy balance in the base The first component to consider is the more or less horizontal portion of yarn lying between the bobbin surface and the traveler. Figure A9.3 shows oblique and plan views of the ring and traveler system. The plan view at the bottom shows only the yarn departing from the traveler on its way to the bobbin and does not show the yarn arriving. This is to make it clear that the angles between the center line and the yarn change. As the winding point reciprocates between the lay point on the bare bobbin at Bi and that of the full bobbin radius at Bo, the angle changes from αi to αo. Due to this motion, vector components of the yarn tension acting along OA vary from TBo sin αo to TBi sin αi, as the chase moves from bottom to top. (The chase describes the reciprocating movement of the lay point of the yarn onto the conical portion of the yarn already on the bobbin. The lay point, or wind point, means the point on the bobbin surface where the yarn is laid.) Periodic changes in geometry of the yarn, as the winding point moves through the chase, are reflected in the yarn tensions. Kinetic energy stored in this portion of yarn is: Ek1 = I1 ω2/2

[A9.3]

where I1 is the second moment of mass of yarn in the base about the axis of the bobbin and ω is the rotational speed in radians/second. The subscript ‘k’ refers to kinetic energy and ‘1’ refers to the base. The winding radius changes cyclically through B0 and B1 as the ring rail moves through the chase motions. Further, the length of yarn changes cyclically through AB0 and AB1 (Fig. A9.3(b)); consequently, the kinetic energy in AB changes cyclically because of alterations in length, mass, radius of gyration, and winding radius. Ek1 is a factor that depends on the geometry and mechanical arrangements of the short-term ring rail motion (or chase). The torque available changes cyclically in sympathy with the rail movement. The kinetic energy of the traveler Ekt = Itω2/2, where It is the second moment of mass about the spindle axis. Since I t = M t k t2 , where Mt = mass of the traveler, and kt is its radius of gyration (note: radius of gyration is a special term used in mechanics to describe not only the position of the mass, but also its shape and size). The subscript ‘t’ refers to the traveler and ‘kt’ to the kinetic energy of the traveler. E kt = M t k t2 ω 2 / 2

[A9.4]

The elongational strain energy is Tε/2, where T is the yarn tension, and ε is the elongation of the length of yarn involved. Yarn is visco-elastic and thus there is also a non-recoverable energy loss associated with the extension which is proportional to

432

Appendix 9

the length of the yarn segment concerned. However, the length of yarn in the segment now being considered alters. Consequently, the strain energy (Es1) changes in sympathy. Thus, it follows that the energy level has a component, which is affected by the chase motion, but the changes are small and may be ignored. Among other factors, airdrag depends on the length of yarn in the airflow. The fact of changes in length of AB means that there is a cyclic change in airdrag on the particular segment of yarn that is synchronous with the chase movement. Consequently, the energy dissipated (Ea1) has a dependence on the chase similar to those just discussed. The subscript ‘a’ refers to airdrag. Airdrag losses in this segment of yarn are minor compared to the kinetic component; there is no need to complicate the analysis further. There is also a yarn–metal frictional energy loss at the traveler, but the sliding velocity is so low that this item, too, may be neglected. The total energy absorption between A and B (which we may denote as E1 where E1 = Ea1 + Es1 + Ek1) has a cyclic component that is dependent on the chase motion. Thus, the energy available to the traveler and the yarn above it is E – E1. It might be realized that E is a variable by virtue of the changes in yarn tension and, as has just been discussed, that E1 has a component related to the cyclic chase motion. A9.3.3 Friction between ring and traveler The traveler is pulled round the ring by the yarn. Drag on the traveler due to the friction between it and the ring causes the yarn in the balloon to rotate slower than the bobbin. The difference in speed causes the yarn to ‘wind on’ the bobbin. Referring to Fig. A9.3, the relative rotational speed of the bobbin in relation to the traveler is (ωb – ωt), and the relative linear winding speed is (ωb – ωt)rb. The rotational speeds of the yarn and traveler are the same except for occasional local excursions in portions of the yarn above the traveler. The fiber is delivered to the system at constant linear speed and the winding system adjusts itself accordingly. Movement of yarn along its own axis proceeds at constant velocity, Vy. Values of ωb and Vy are fixed, but rb changes with the position of the winding point within the chase. Thus, the rotational speed of the yarn, ωy, changes with ring rail position within the chase but it is normally marginally less than the bobbin speed. Microwelds between the ring and traveler, unstable air conditions, and perhaps some other causes result in occasional deviations from the normal cycle of events. Figure A9.3(c) shows portions of the traveler and the sliding track on the ring; the traveler is shown cut at the level of yarn contact for clarity. Components of the yarn tensions in contact with the traveler (T1 and T2 in Fig. A9.3(c)) produce a resultant Fy. The tension at A is not the same as at B. A component of Fy tends to hold the traveler away from the ring. Centrifugal force acting on the traveler (Fct) acts horizontally along a radius centered on the spindle axis and it tends to force the traveler into harder contact with the sliding track on the ring (i.e. in a direction roughly opposite Fy). Forces Fy, Fcf, and Fa adjust themselves to provide equilibrium by causing the traveler to tilt as necessary. The force normal to the sliding track, Fn, is the vector sum of the appropriate components of these and the tangential friction force along the sliding track is µFr (see Section A9.5.5 for the full analysis of forces.) The energy dissipated is Eft = µFr × ωrr, where the subscript ‘t’ refers to the traveler and ‘f ’ to friction. Since ωrr is virtually constant, the energy loss depends on the coefficient of friction, µ, and the normal force, Fn. The coefficient of friction is affected by the lubrication, or lack thereof; for cotton, lubrication is largely from crushed fiber debris deposited on the track on the ring.


433

The coefficient of friction depends on the state of wear of the sliding surfaces and the reaction force depends on the attitude of the traveler, yarn tensions, and centrifugal forces. Wear on the traveler alters the position of the center of contact area on the traveler. Also, the magnitude and directions of the yarn tension vectors vary. This is important because, not only do the forces applied to the traveler have to balance, but so do the first moments. Thus, if the direction and magnitude of Fy alters, the traveler tilts to correct the imbalance. This results in a modified value of Fn and a change in the energy absorbed in friction. In other words, there are reactions to changes in the system both above and below the traveler. There is also a long-term variation caused by wear in the components. In the case of the traveler after the initial break-in, this long-term change is measured in days, whereas the corresponding change to a properly run-in ring is measured in months, or even years. The kinetic energy of the traveler, Ekt, = Itω2/2 = M t k t2 ω 2 / 2 , where the second moment of mass about the spindle axis = It, the mass = Mt, and the radius of gyration of the traveler = kt. Thus, the mass of the traveler is an important factor in determining the yarn tension since ktω varies but little. Thus, summarizing: Eft = µFn × ωrr

[A9.5]

E kt = M t k t2 ω 2 / 2

[A9.6]

and

A9.3.4 Yarn above the pigtail guide Rotation of the balloon induces torsion in the yarn above the pigtail guide and this stores some energy as torsional strain energy; it dissipates some of this due to frictional losses. There is also a small amount of tensile strain energy involved. The strain energies Es reduce the energy available to the main balloon, but the quantity involved is small and may be neglected.

A9.3.5 Energy available to the main balloon The energy available to the yarn above the traveler is (E – E1 – Ekt – Eft – Es). Changes in the chase motion, the mass of the traveler, and the effects of wear are now seen to affect the energy available to the main yarn balloon. As before, the energy available is distributed over categories similar to those already recited. Kinetic energy of the upper yarn depends on the mass of yarn involved and its radius of gyration; strain energy depends on the length of yarn involved. Of these factors, kinetic energy is the most important and the integration implicit in the factors for airdrag1 may be left for later. Let Ea2 be the airdrag of the yarn in the balloon between the pigtail guide and the traveler. The subscript ‘2’ refers to the main balloon. The kinetic energy now available to the main balloon is: (E – E1 – Ekt – Eft – Es – Ea2) = Ek2

[A9.7]

1 Airdrag on an element of yarn may be taken as proportional as to Cd α √n (ωr)2δ, where Cd is the airdrag coefficient, α takes into account the attitude of the yarn element, n is the linear density of the yarn, ωr is that linear velocity which is tangential to the circular locus of the yarn element and δ is the length of the yarn element.

434

Appendix 9

Let Ek2 be the kinetic energy, n the linear density, S the length of yarn, and k the radius of gyration; each term referring to the yarn in the balloon rotating about the spindle axis, and the yarn referred to is between the pigtail guide and the traveler. Equation [A9.1] may be modified as: Ek2 = nSk2 ω2/2

[A9.8]

Where Ek2 is the kinetic energy, n is the linear density, S is the length of yarn, and k is the radius of gyration; each term referring to the yarn in the balloon rotating about the spindle axis. The yarn referred to is between the pigtail guide and point B. The radius of gyration is related to the maximum diameter of the balloon; the normal speed is assumed to be so slightly different from the spindle speed that it can be regarded as constant. Thus, if ω and n are treated as invariable, the changes in energy available must cause changes in S, or k, or both. In other words, the size and shape of the balloon changes with the energy available. The length, S, changes significantly as the yarn on the bobbin builds up from base to tip; also there are changes in k. The yarn spirals in the balloon depending on the airdrag and this can cause significant changes in length; there is no unique relationship between length and diameter. These changes in Ek2 are superimposed upon those arising from the right-hand side of Equation [A9.7]. Whatever combinations of these factors exist, there is a change in energy level that is distributed over the time it takes to spin a bobbin full of yarn.

A9.3.6 Instabilities It is possible for the traveler motion to become unstable. At these times, the attitude of the traveler oscillates and imposes an additional energy variation on the main balloon, which is reflected in the tension variations. If the yarn tension at the wind point drops below a certain level, there is insufficient energy available to maintain the normal, single-noded balloon. The balloon will then change such that the length of yarn is accommodated in a different shape, which permits the radius of gyration (or, approximately, the distance of the centroid) to be reduced. Sometimes the change in shape involves wrapping part of the yarn around a revolving support; when this happens the unsupported length and the radii of gyration are reduced. There are then extra frictional losses. The balloon is said to collapse because of the reduced diameter. One cause of such an event is the use of a traveler of too low a mass. This will be discussed later. It is possible that subsidiary oscillations in the main balloon could absorb energy that would be subtracted from that available to shape the mean path of the yarn. There could be resonant vibrations at surprisingly low frequencies because the yarn in the balloon is restrained at the ends like a suspended cable; the effective modulus of elasticity of the system is low. Energy losses due to such vibrations would be one of these subtractions. Vibrations of this sort might be excited by intermittent slippages of torque and tension at the pigtail guide, mechanical vibrations, local air disturbances, etc. The system has a response time to force pulses imposed upon the balloon. Microwelds between the ring and traveler can create such pulses, particularly if the ring is not properly run in. Recovery of normal running conditions after such occurrences is dependent on the response time.


A9.4

435

Yarn tension gradients

A9.4.1 Tension gradients as a connecting factor A factor that appears in nearly all the energy items mentioned in the previous sections is yarn tension. There is a progressive change in yarn tension from the bobbin to the fiber delivery system, which is situated above the pigtail guide. A change in one segment affects all the rest; thus, the factors discussed earlier are mutually dependent and some discussion about yarn tension is necessary.

A9.4.2 Dynamic and passive yarn tension gradients Tension gradients in the yarn may be classified into two categories. One has been called dynamic, because it arises from the centripetal accelerations acting on the yarn. The other has been called passive, because it does not depend directly on the rotation of the yarn about the spindle axis. Figure A9.4 illustrates one case and demonstrates how yarn tension forms a connecting thread in the control loop of the system. Remembering that the winding tension helps determine the available energy for the system, it can be seen that the behavior of any one segment of yarn is dependent on the tensions generated elsewhere. Yarn tension is at its highest at the winding point on the bobbin. The existence of a dynamic tension on the more or less horizontal portion of yarn sweeping the base means that the tension at the traveler is less than the maximum. Since the outer diameter of the balloon base is limited by the ring radius, this gradient does not vary much for a given spindle speed. Friction between the yarn and the traveler causes a passive tension gradient and the tension of the yarn entering the traveler is still lower. The yarn in the main balloon suffers dynamic tension gradients that vary along the length. A passive, frictionally induced tension gradient occurs at the pigtail guide with the result that the input tension to the twisting section is further reduced. In Fig. A9.4, the curve is shown for the simplest case, but the gradient can be multi-noded. When the balloon collapses, the lower end of the yarn might wrap around the bobbin (or crown, if one is used) and a further passive tension gradient will be introduced. It is also possible for the balloon generator to change from a roughly parabolic shape

Length along the yarn

Pigtail guide

Tension gradients due to frictional contact with the guides Yarn flow

Traveler Wind point Yarn tension

Fig. A9.4

Yarn tension profile

436

Appendix 9

into a sinuous one with a number of nodes. In some sorts of unwinding from a bobbin, the balloon base, ring, and traveler no longer exist and these sources of tension gradient are removed from consideration.

A9.5

The real balloon

A9.5.1 The central section of the real balloon When operating without balloon control surfaces (more about these later), any element of yarn is subject to the tensions and forces acting on it. The forces include airdrag, other frictional restraints, and the effects of electrical charging. Few of the forces arising from these phenomena act through a common point and thus there are moments that tilt, bend, and twist the yarn in the vicinity of the element. Figure A9.5 illustrates the forces acting on an element of yarn. The yarn in the balloon is curved; consequently the tensions acting on an element of yarn do not act along a common straight line; furthermore, the tensions differ from end to end. The forces acting approximately within a horizontal plane are shown with cross-hatched arrows. The light gray area represents the horizontal plane. Let us take the roughly horizontal forces one by one. The airdrag is due to the relative motion between the yarn element and the surrounding air. It does not act along the same line as the velocity vector because of the airflows caused by the pumping action of the bobbin and yarn [1]. The centrifugal force acts along a line in the horizontal plane that passes through the center of rotation of the yarn element. This may or may not be congruent with the center of rotation of the bobbin. The Yarn tension (T + dT )

Fm

Fa

Mechanical drag Air drag

Fcf Yarn

Centrifugal force

Horizontal plane Yarn

Velocity V Yarn tension T

Fig. A9.5

The central zone of a balloon


437

mechanical drag force, Fm, arises when the hairs protruding from the yarn element lash some machine part such as a separator plate, or when there is shear in the airflow. This produces minor periodic, false twist torques as repeated contact is made. The force system comprises components from the cross-hatched force vectors shown. The yarn element tilts and twists to balance the system, as sketched in Fig. A9.6. Effects of tension, twist, and mechanical abrasion can change the condition of the yarn and alter the airdrag characteristics and perhaps the propensity to react to mechanical disturbances, such as contact with separator plates. The magnitude of these various components varies from one level to another because of the changes in radius, linear speed, airflow, and physical condition of the yarn. The shape of the yarn in the balloon is curved in three-dimensional space; also the gradients of tension, torque, and geometric attitude vary from level to level within the balloon. The varying population of fibers in the yarn being spun produces varying linear densities and amounts of hairiness, and there are long-term variations in airdrag that alter the tension patterns in the balloon [2]. These, of course, influence the end-breakage rates. A real balloon is not confined to a rotating plane as was earlier assumed. The effects just discussed cause it to spiral, and if we use a rotating plane in any mathematical model, we can use it only as a reference. Thus, the real balloon rarely fits the simple theories; comprehensive equations of motion are needed. These give a better fit but they are not easy to manipulate without a computer and even these sophisticated programs do not completely account for all the vagaries of the balloon. The next step is to consider events with such a reference. Plane OPTr in Fig. A9.6(a) is assumed to rotate at the speed of the traveler about OP, and yarn in the balloon does not necessarily even touch it. (Tr is used rather than T to avoid confusion with tension.) Yarn streams behind the winding point due to the frictional forces and any element above the traveler lags. For example, an element at B lags the traveler by φ, with the yarn taking up an angle α, to a vertical plane OPBQ that passes through the segment being considered. Some drag may be from mechanical friction with Plane OPQ

O Wind and drive point

E Fv + Torquei

ω α

Fh + B

Yarn B

Fh Tr

Airdrag = Fa

P

α

φ Traveler

Q TorqueO (a)

Fig. A9.6

Fv

A non-plane balloon

(b)

438

Appendix 9

machine components, and there is an airflow caused by the moving parts. However, for the moment, we will deal only with airdrag caused by the motion of yarn through a fixed environment. Clearly there are three components in directions: (a) tangential to the locus of the element concerned, (b) vertical and parallel to OP, and (c) horizontal and parallel to QP.

A9.5.2 Airdrag Airdrag is a function of fluid friction caused by the yarn moving relative to the air. McAdams [3] quotes the Fanning equation, which indicates that drag is proportional to the square of the relative linear velocity and is a complex function of Reynolds Number.2 Figure A9.7 is based on McAdams’ data which refers to flow in pipes but is often used for fluids flowing outside, but parallel to, the pipe axis. Our case involves hairy yarn and the hairs stream behind the main body almost like a comet’s tail and affect the coefficient of airdrag. The relative airflow is usually neither parallel nor perpendicular to the comet’s tail. The tail is oriented away from the line of motion because the hairs are subject to centrifugal as well as airdrag forces. In calculating Reynolds Number for airplane wing sections and the like, the typical dimension normally used is the chordal width or length of the streamer rather than the thickness. Other researchers use results from flow perpendicular to the yarn, and the resulting graph has a somewhat similar shape but a different scale. The ‘comet tail’ of fibers is thought to have a significant effect on airdrag in ballooning. Some of the yarn near the top operates in the laminar region, with high drag coefficients. Other parts operate near the equator in the turbulent region with lower drag coefficients, and intermediate parts operate in the unstable region with variable drag coefficients. The point of the diagram is not the friction factor, but the range of operating conditions involved and the instability around 103 to 104 Reynolds Number. Fortunately, the highest drag coefficients occur at small radii and thus have only a small effect. Figure A9.8 demonstrates differences in the lag of the yarn due to airdrag. Not only do the theories give differing results for the portion of yarn below the equator, but experimental data show that considerable variation is possible. Most theorists assume the yarn to 1.0

Friction factor

Usual assumption Unstable 0.1

0.01

Laminar

0.001 2 10

103

Fig. A9.7

Turbulent

104 105 Reynolds Number

106

Airdrag coefficient

2 Reynolds Number = ρVD/ζ, where ρ = air density, V = relative velocity, D is a typical dimension, and ζ is the viscosity of the air. It is the ratio of viscous and inertia forces; at a critical value, the flow changes from streamline to turbulent.

Advanced topics VII: Yarn balloon mechanics Stationary r = –3/52h2 + 4.54h – 0.06 Non-stationary r = –3.77h2 + 4.63h – 0.03

Normalized height (h)

0

ω

270°

240°

Node 210°

0.25

Re

E 180°

0.50

439

300° Non-stationary = Stationary = Experimental = 0°

O B

Equator

Φ

0.75 150°

30°

A

Ring

1.00 0

0.5 1 1.5 Normalized radius (r) (a) Elevation

Fig. A9.8

60°

120°

90° (b) Plan view

Radius profile of a balloon

be a thin cylinder of yarn of diameter d, and calculate Reynolds Number and drag coefficient (Cd) accordingly. If the length of the hairs streaming behind the yarn is used rather than the yarn diameter, the Reynolds Number spans a range that includes laminar, unstable, and turbulent regimes.

A9.5.3 Balloon theory relating to the central section Batra et al. [4,5] quote the basic equations of motion of a quasi-stationary balloon, the adjective ‘stationary’ referring to the fact that they assumed the yarn was stationary relative to a rotating plane of reference. The plane of reference included the axis of rotation of the bobbin, and rotated about that axis. A balloon node was at the pigtail guide situated on the axis, and the center of the ring was also situated on the axis. The yarn in the balloon was treated as a quasi-stationary object with respect to the traveler and then the mathematical description was boiled down to a series of differential equations amenable to solution. The vector equation is: Absolute acceleration = A0 + Ar + 2ω × vr + ω × (ω × s) + a × s

[A9.9]

Some of these terms can be eliminated and the following comments apply: (a) the acceleration of the origin at the pigtail guide is zero, (b) the first term (A0) is zero, (c) the second term (Ar) is negligibly small, (d) the third term containing the Coriolis acceleration is negligible, and (e) the last term is assumed to be zero because it includes the factor a, which is the angular acceleration of the balloon. Thus, the equation reduces to absolute acceleration = ω × (ω × s), which can be translated as centripetal acceleration acting along the radius of rotation of an element = –ω2r. Furthermore, the rate of tension change along the rotating yarn is –nω2rdr, where n is the linear density of the element, ω is the rotational speed in radians per second, r is the radius of the element, and dr is the incremental change in radius over the element considered. From this, Batra et al. say that the tangential component of airdrag is negligible and that the tension in the yarn in the balloon is: To – T = nω2r2/2

[A9.10]

As the yarn is made heavier, the balloon enlarges, the spindle is run at a higher speed,

440

Appendix 9

the traveler weight increases, or any combination of them, the yarn tensions increase. A curiosity is the resemblance to the equation met in rotor spinning (centrifugal force = n ω2r2/2); the yarn inside the rotor rotates within a plane and in a balloon it occupies a three-dimensional space. The validity of using a stationary model is disputed by Lisini et al. [6] for cases where the balloon shape is subject to rapid variations. They point out that in ring spinning, the movement of the ring rail causes changes in traveler speed and this undermines the assumption of constant rotational speed of the inertial frame. The coil of yarn deposited on the bobbin forms a spiral rather than a circle. Relative motion between the traveler and the wind-on point is caused by changes in length and attitude of the yarn between the wind-on point and the traveler. These changes are related to the alterations in length in the yarn forming the balloon above the traveler. Consequently, the traveler speed tends to change cyclically, but the variation is small. Thus, it is true that an error is involved in using the traveler to anchor the inertial frame even if the effect is small. These authors favor the finite element method of calculation over the iterative Runge-Kutta solution. (The finite element theory assumes that the yarn is made up of very small straight segments.) A comparison of the two methods shows that they give similar results above the equator of the balloon, as shown in Fig. A9.8(a). However, the plan views in the top left quadrant of Fig. A9.8(a) make more visible the differences between the two theories relating to the yarn lying below the equator. It is interesting to note that the shape of the elevation of the balloon is very near to parabolic, confirming the data of the present author. The angular lag of elements in the balloon relative to the traveler varies in the two theoretical cases. The use of stationary solutions greatly simplifies the analysis but at the cost of some accuracy. Theoretical models involve non-linear equations and a computer is required to obtain a solution in a reasonable time span. However, the solution is only as good as the assumptions made in respect of airdrag, coefficients of friction, and the flow of torque and tension in the system [2]. At point B back in Fig. A9.6, there is a system of forces that includes those shown, but there are others perpendicular to the plane of the paper. Curvature of the element results from the application of these forces. The normal to the yarn at B no longer intersects OP. The center of curvature is in space outside the balloon. Tension gradients across the yarn segment make the forces at the upper terminal of the segment differ from those at the bottom one; the segment is forced to tilt until the moments are in equilibrium. At different heights above the traveler, the inclination of the yarn, α, changes with respect to the center line. The element of yarn shown does not lie in the plane of the paper but at an angle that varies. Yarn tension is the resultant of all the components acting on a segment terminal and as α alters, the yarn tension changes. There are sometimes multiple solutions to the equations under unstable spinning conditions. Instability is often the result of the use of too light a traveler. Figure A9.8 also shows a plan view of some yarns in a balloon. As previously mentioned, the top left quadrant contains theoretical data based on the work of Lisini et al. [6]. An adjustment was made to the angular positions of the stationary and nonstationary curves to bring them as nearly as possible into congruence. It is interesting to note that there is little difference between the results for that part of the yarn that lies above the equator (which normally includes the majority of the yarn in the balloon). In the bottom right quadrant there are two sets of new experimental data


441

gathered within a few seconds of each other, with the spinning machine running at constant speed and a fixed rail height. The data in the two quadrants should not be compared because different conditions prevailed, however curves A and B in the bottom right quadrant should be nearly identical but they are not. Obviously, variations in the yarn altered the shape of the balloon. One candidate for suspicion is the airdrag coefficient, which is normally modeled as a constant for a given yarn.

A.9.5.4 Balloon control rings The purpose of a balloon control ring (see Fig. A9.9) is to reduce yarn tension; the device works for a range of conditions but it is not universally effective. Balloon control rings cannot be effective under the conditions of incipient collapse and they are rarely used for fine counts. From a practical point of view, the surfaces can become poisoned by accumulations of fiber finish or oligomer and these accumulations lead to difficulties in spinning. The control rings also impose an extra drag on the yarn that increases the spirality with effects similar to those discussed above. The control rings also tend to make the yarn more hairy. As a first step, one can use a fairly superficial explanation of their mode of operation. The control rings reduce the surface area of the balloon. When the spiral angle of the yarn in the balloon is small, the yarn tension is roughly proportional to the surface area of the balloon. Thus, the control rings pinch the balloon to form a waist, which reduces surface area and thereby reduces the yarn tension. Mathematical models confirm that control rings reduce the tension for stable balloons and promote stability; the rings also reduce the destabilizing influence of slubs passing through the balloon. The reduction in yarn tension permits the use of higher speeds, weaker yarns, or both. A higher speed improves productivity and permits the spinner to spread the fixed costs over a larger poundage, which reduces the cost/lb. The possibility of using

Node

β

Control ring movement Drive Main ring movement Yarn removal

Fig. A9.9

Main ring not shown

Balloon control ring

442

Appendix 9

weaker yarns means that, sometimes, lower twist can be used and this also increases productivity. Advantages are balanced by disadvantages. Summarizing the problems with balloon control rings: (a) they make the yarns more hairy, (b) they accumulate spin finish, (c) they add slightly to the cost of the machine, (d) they interfere with the doffing and piecing operations, and (e) they produce a torque in the yarn within the balloon. Of these, the first two are the most important.

A9.5.5 The traveler The balloon size and shape vary as the yarn builds up on the bobbin, and this is associated with changes in yarn tension. Consider the forces acting on the traveler as depicted in Fig. A9.10. Centrifugal force, Fct, acts through the center of gravity of the traveler and is balanced by the resultant yarn tension, Fy, also there is a reaction force, Fr, acting between the ring and the traveler. There is a sliding contact between the ring and traveler at A, and the friction due to this exerts a drag force which causes the traveler to lag behind the bobbin. The beauty of the system is that the speeds adjust automatically to the prevailing conditions; no mechanical complications are needed. Sliding contact can cause serious wear on the traveler and the life of the traveler is then measured in days. A normal practice is to judge the wear of the travelers by the number that are burned. According to Grishin [7], every 10% of burned travelers in the population increases the ends down rate by 5 per 1000 spindle hours. There is also some collateral damage to the ring and, over a much longer time, the ring too becomes unserviceable. For no damage, the vector sum of Fy, Fr, and Fct should be zero, but if we were to run under those conditions there would be traveler instability. Centrifugal force acting on traveler Fct

Resultant yarn tension Fy H Traveler Yarn

Yarn

T2

T1

Tilted traveler

A

Flange Centroid of traveler

x

Flange

Flange damage

x Ring Reaction force F r cross section

Traveler scar View in direction x – x (b)

(a) Yarn Y

Fy ωt

Ring X

Fr Fct Traveler

Bobbin ωb

Fcd

(c)

Fig. A9.10

Ring and traveler


443

Fy varies during the bobbin build and, if contact is to be maintained, the traveler weight has to be sufficient to control the tension over the whole range of conditions. The reaction force, Fr, is strongly influenced by the traveler mass, M, and consequently the tension, TB, is also dependent on it. Traveler mass has to be changed as the yarn count is altered within the normal spinning range (M/n is usually kept constant). Adjustments also have to be made for changes in ring size and shape. A properly run-in ring will last for years, whereas a traveler might only last, say, 10 days. The coefficient of friction between the two metal surfaces changes and this influences the drag force. The tensions T1 and T2 (Fig. A.9.10(a)) cause the traveler to tilt and the angle of tilt changes with balloon geometry. The reaction force is sufficient to cause transient metal to metal seizures of the poorly lubricated surfaces, although fiber debris and particles of fiber finish offer some lubrication. As a new traveler is put into use, there is a small contact area that runs at a high local temperature and creates fairly rapid wear of the surfaces. The damaged surface of the traveler is concentrated in a band and Fig. A9.10(a) shows a scar typical of a used traveler. Wear causes the area of contact to increase sharply at first, but the rate of wear then abates as the scar on the traveler grows. As the scar widens, the centroid of the reaction moves and changes the attitude of the traveler. Eventually, the tilt becomes sufficient to cause it to be thrown off. Before that happens, however, the yarn tensions become sufficient to cause a higher end-breakage rate than normal. It is important to change the travelers in timely fashion. If moments are taken about A in Fig. A9.10(b), the moment due to the resultant yarn tension must balance the moment due to the centrifugal force acting on the traveler. Any change in the geometry of the traveler alters the position of the centroid and causes the traveler to adjust its angle with respect to the horizontal until balance is achieved. Various factors determine the forces described. For a given ring diameter, the centrifugal force acting on the traveler is determined by its mass and speed. The linear density of the yarn, the balloon geometry, the rotational speed of the balloon, and the reaction between the ring and traveler define the resultant yarn tension. For a given speed, the centrifugal force acting on the traveler is theoretically constant whereas the forces transmitted by the yarn vary as the bobbin builds. Thus, the angle of tilt taken up by the traveler varies cyclically. With a poor design of traveler, slipstick conditions can lead to an unstable porpoising as it rides the flange and this either causes an end-break or throws off the traveler. Because of constraints in mass and size of the traveler, there is little space available for the yarn and it could become trapped near H. Consequently, the shape of the traveler is important and each type of yarn not only needs a traveler that has the required mass, but one which provides adequate space for the yarn. The yarn can also become trapped if the traveler tilts too much. If the traveler is too heavy, the friction between the ring and traveler soon destroys the traveler and might damage the ring. If it is too light, the balloon can collapse and cause high tensions with all the problems described earlier. Stability of the yarn package becomes a problem if the winding tension (related to traveler weight) is reduced too much; soft-wound packages occupy too much volume and are liable to become damaged in subsequent handling. Frazer [8] illustrated the instability of a balloon when the traveler is too light. At low traveler mass, there is an ambiguous tension at radius r, as indicated by the leftmost dark curve in Fig. A9.11. In that case, three tensions are theoretically possible at the lowest traveler weight shown. The other dark curves show stable relationships between the lay point radius and tension.

Appendix 9

Yarn tension

444

r Lay po int radiu s

Fig. A9.11

v Tra

r ele

ma

ss

Effect of traveler mass

A9.5.6 The lower portion of the balloon The factors determining the resultant force (Fy) can be visualized in three dimensions as indicated in Fig. A9.10(c). It will be seen that Fy is the vector sum of the forces in the upward pointing section of yarn at Y and the roughly horizontal section shown at X. The resultant force is balanced by the system containing: (a) centrifugal force acting on the traveler (Fct), (b) the reaction between traveler and ring (Fr), and (c) the drag force, Fd, acting tangentially to the ring. The drag force (Fd) is the result of the traveler sliding on the ring at a rotational speed of ωt. The bobbin (shown in truncated form) rotates at ωb. Reiterating previous statements, changing the mass of the traveler alters the yarn tensions. Indeed this is the only practical way a user can adjust the tension, given that the yarn count, balloon geometry, speed, and machine configuration are fixed by design or commercial considerations. The centrifugal force acting on the traveler is Fct = ω2rtM, where M = mass of the traveler, rt = radius of the locus of its centroid, and ω = rotational speed. The angles taken up by these portions of yarn are critical in determining the tensions TA and TB, which act at X and Y respectively. These tensions, in turn, help to determine the rest of the tensions in the balloon. There is a relationship between them that is dictated by the friction forces between the yarn and the traveler. Using Amonton’s Law as an approximation, the relationship is: TA = TB eµΨ

[A9.11]

However, any assumption that the coefficient of friction is a constant is imperfect. The coefficient varies with yarn hairiness, finish, and possibly rh. Another source of possible error in Equation [A9.11] arises from the fact that the yarn is bent to a small radius of curvature when passing round the traveler. If the radius is too small, bending stiffness begins to play a significant part and the normal force between the yarn and the traveler will be higher than estimated (which results in higher drag force). Thus, one might expect deviations in winding tension from those predicted by some mathematical models. Also, the greater the energy loss in overcoming the drag force acting on the traveler, the less is the energy available to inflate the balloon. During spinning, the yarn winding point is controlled by the ring rail motion. There is a fairly short oscillation period as individual cones of yarn are laid on the bobbin. Also, there is a much longer period as the bobbin is built from bottom to top, laying new cones over each of the previous ones. Each chase builds a new layer of


445

yarn and requires a small change in mean height of the ring rail. The upwards rate of change of the rail position during the chase is usually different from the downward one; this is to create an interlocking yarn package structure. A vector component of the tension along ABo in Fig. A9.3 helps to balance the centrifugal force acting on the traveler. Let this force be TB cos α and let Fd = kbµ Fr (where kb is a factor to take into account the forces omitted and Fr is the reaction to the forces acting on the traveler). The appropriate subscripts should be added. If the yarn above the traveler is nearly upright, k is almost 1.0. Substituting for Fd and cos α = rwo/r, we can write in functional form: Fr/TB = f{(rw/rr), kb, µ}

[A9.12]

The radius rw varies from rB1 to rB0; k and µ vary also. This relationship implies that as the ring rail moves, Fr /TB changes. The tension TB is related to the yarn tension above the traveler and it increases to a local maximum at the top of the chase where rw becomes a minimum. There is a limiting size to the bobbin diameter. As stated earlier, the bobbin size is normally about 40% of the ring size, because winding yarn on smaller diameter bobbins creates excessive yarn tension. Also, the bobbins are slightly tapered. As the bobbin builds, the wind-on radius, rw, is normally limited between about 0.4rr and about 0.9rr. For continuous control, Fr > 0 if undesirable instability is to be avoided. The energy available to the system = Twrwω, where the subscript w refers to conditions at the winding point. Except under the unlikely condition where tension Twrw is invariable and ω changes significantly, any changes in rw are associated with changes in energy available. Because of the changes in yarn angles at the traveler, the passive tension gradients can change markedly with changes in rw. Under stable conditions, an increase in rw is associated with a drop both in tension and energy available. The tension variations arising from the ring rail movement, which controls the chase, are measured at frequencies of less than 1 Hz.

A9.5.7 The upper zone of the balloon Yarn tension between the node at the pigtail guide and the front rolls of the drafting system is a critical factor in determining end-breakage rates in spinning. The friction of the yarn running through the pigtail guide situated at O in Fig. A9.12 affects the tension and twist of the yarn. As mentioned earlier, To ≈ Tieµε. If the guide is offcenter, or the yarn flow approaching the guide is not coaxial with the center line of the spindle, the angle ε varies within each revolution of the yarn in the balloon, with the result that Ti varies also. The point at which the strand is at its weakest usually lies in the twist triangle and Ti must be kept below that breaking strength. The term strand can mean either the yarn or the fiber flowing through the twist triangle. Not only is the value of To important, as previously discussed, but so is the angle ε because, if the variation is large, then ω2rr has to be kept lower to compensate (ry is the yarn radius in general). The tension variations from this source appear at the frequency of the rotation of the balloon (say, 200–300 Hz). Not only is there a tension gradient in the yarn passing through the pigtail guide, but there is also a torque gradient. The normal force acting on the yarn at the contact point produces a friction drag force, which has components (a) along the yarn, and (b) tangential to a normal cross-section of yarn. The former produces tension and the latter produces torque. It can be argued that:

446

Appendix 9

Tension Ti

Yarn flow O ε2

ε1

Tension T01

Fig. A9.12

τo ≈ τieµα

Tension T02

Upper portion of the balloon

[A9.13]

The symbol τ denotes torque and the other symbols have their previous meaning. If, for example, we assume that µε varies between the limits of 0.04 to 0.12 radians as the package builds up, then the ratio of torques would vary between 1.04 and 1.128. In other words, the average twist in the yarn above the pigtail guide would, in that case, be reduced by 4% to 13% compared to the value just below the guide. The twist is reduced in the very place where it might be an advantage to increase it. Attempts to use rotating guides to overcome the problem have not been successful; this is partly due to the extra costs involved and partly to the difficulties in piecing. False twist in the yarn leaving the pigtail guide is of some importance. The resultant of the input and output yarn tensions on either side of the guide has a horizontal component that presses the yarn against the inside surface of the pigtail. The yarn might roll, as well as slip, on that surface and the rolling action would produce false twist between the drafting system and the pigtail guide. Total twist in this region is the sum of the real and false twists but the false twist above the guide is negative. The net effect is another small reduction in twist in the yarn coming away from the twist triangle. This component changes cyclically. The twist triangle geometry is determined by the net twist, which affects a number of yarn properties such as hairiness and bulk as well as the end-breakage rate. If there are surges of twist at this point, then the balloon will be disturbed, the tension will fluctuate, and the yarn properties will vary accordingly. The surges are similar to those sometimes found in rotor spinning at the navel. Evidence of such phenomena has been gathered by illuminating the balloon with


447

horizontal thin sheets of light at different levels. The normal assumptions imply that the loci of the small segment of yarn are circular. In some cases seen in industry, the locus of the yarn elements just below the pigtail guide is badly distorted. Figure A9.13 shows a modest distortion arising from the pigtail guide, but it fades at distances remote from the guide. There are also distortions from other causes. The strata designated B through E were between the pigtail guide and the top of the bobbin. The stratum A was above the guide in the secondary balloon and the stratum F was below the top of the bobbin. The balloon control ring did not operate and the photographs were taken with the ring rail at a constant position in the chase.

A9.5.8 Stability of the speed of the yarn balloon There is a torque generated tending to change the rotational speed of the mass of an independent element of yarn, δm, if it changes radius. As a first model, consider a yarn to consist of a series of contiguous elements like a string of beads and the length of yarn in the balloon, S, is ∑δm. If elements of yarn do not follow a circular locus, they must change speed to conserve momentum unless a pattern of forces restrains the change. Momentum of each element = Iω and I = δm × r2 = nδs × r2, where n is the linear density of the element. In a balloon, the elements of yarn are not independent, but a change of radius still produces a system of forces tending to change the speed of the element and of its neighbors. A reduction of radius causes the elements to speed up and an increase in radius slows them down. Another cause of change arises from the lag of one element relative to another due to drag. Any change in drag alters the transient speed of the element with respect to the lower portions of yarn in the balloon. Once the stability of the balloon is disturbed, transient changes in speed and shape of the balloon are inevitable. These effects are not normally large unless the balloon is in or near the unstable region.

A Pigtail guide

B

C D

E

F

Fig. A9.13

Loci of balloon elements

448

A9.6

Appendix 9

Balloon collapse

A9.6.1 Energy variation in the balloon The relationships between the forces acting in a balloon are complex and distinctly non-linear. Most often, a perturbation causes an energy change that restores the system to its normal state, but under certain conditions the system is unstable. The system can go from one energy equilibrium state to another. Some of these energy states are stable within certain confines, but the operating zone can be induced to move from one local minimum to another. This is illustrated diagrammatically in Fig. A9.14, where the local equilibrium is illustrated as moving from B to A. Consider an example. A perturbation in kinetic energy available to the yarn in the balloon usually leads to a change in radius of the centroid and the yarn tension changes in sympathy. If there is a change in mode, there is also a change in height between nodes. The yarn is no longer roughly parabolic but assumes a sinuous shape. There are a variety of balloon shapes in which the balloon contains the same amount of yarn but has a different position of the centroid. If the perturbation acts perversely, the effects permeate the system. If, for example, there is a decrease in yarn tension at the surface of the bobbin due to a change in yarn shape similar to that discussed, there is a reduction in energy available. If the reaction force between the ring and traveler increases because of the reduced tensions, more energy will be dissipated due to friction. This leaves even less available to the kinetic energy of the main balloon, which then causes the balloon to deflate. The reduction in balloon diameter further reduces the tensions and the system is seen to be unstable.

Kinetic energy

A9.6.2 Vector analysis In standard ring spinning, where the machine designer does not intend collapse, the event causes difficulties. For example, when the balloon is long, a portion of it sometimes temporarily collapses on the top of the bobbin. When conditions verge on instability, the balloon collapses periodically as the wind-on point approaches the top of the bobbin. Often the result is that there is an end-break (which has economic repercussions) or there are periods of increased yarn hairiness while the balloon remains collapsed (which has quality repercussions). With coarse counts, the balloon may reach a size that is over double that of the ring diameter before collapse occurs. With a fine yarn, the balloon might collapse at a diameter roughly equal to the ring size. Let a yarn consist of a series of small elements and, for the present purpose, consider the middle element in a chain of three. Figure A9.15 shows the forces acting

B A

Balloon parameter (ωTrw )

Fig. A9.14

Various energy states


449

Length of element = δs

F + δ F1 F + δF 1

Fe 1

δγ

Fe 2 F δs / R O

F – δ F2

R

δγ

Fe

R

Fe3 F – δ F2

Fe Force diagram

Space diagram

δF and δγ exaggerated for clarity (a) Case 1

F + (–δF1)

F + (–δF1)

F e2 δγ

O F δs / R1

F – (–δF2)

δγ

Fe

Fe3

Fe F δs / R Force diagram

R R

F – (–δF2)

Fe 1

Length of element = δs

Space diagram δF and δγ exaggerated for clarity (b) Case 2 for negative δF

Fig. A9.15

Forces on an element of yarn

on the subject element of yarn (shown shaded in gray). Before discussing the meanings of these, let the symbols be explained. For example, in diagram (a), a force of F + δF is applied by the element immediately above it, and another force of F – δF is applied by the element immediately below. This is regarded as a positive tension gradient. The radius of curvature of the element is R and the forces all lie in the plane of curvature, but they have been rotated about the element to make Fe horizontal in the diagram.3 The vectors do not necessarily lie in the plane of the ring or in one including the spindle axis. The external forces acting on the element are almost horizontal. External forces are the centrifugal force acting on the element and the airdrag forces. The latter is true only if any secondary airflow produces negligible airdrag on the element. Thus by rotating the plane about a vertical axis to make Fe parallel to the resultant of the external forces, the element is brought into its correct attitude in the balloon. In Fig. A9.15(b) the tension gradient is negative (the value of δF is negative) and the attitude of the element has changed in consequence. Resolve the principal

3 Constraints are (a) Fe1 = Fe2 + Fe3 and (b) the sum of the moments about any point on the element has to be zero (which implies that Fe2 ≠ Fe3). The moment arm about which Fe acts is not δs/2, unless Fe and Fe1 are coincident.

450

Appendix 9

components shown in gray. In the right-hand diagrams in the direction O–Fe1, the left facing components Fe2 and Fe3 (shown in black and facing leftwards) are unequal. Consequently, there is a moment tending to tilt the element, which should be balanced by the moment generated by the application of Fe. Thus, the tension gradient along the yarn within the balloon affects the attitude of the element with respect to its neighbors. Clearly, Fe is greatly influenced by any change in the radius of curvature. The behavior of the balloon is influenced heavily by changes in radius of curvature and tension gradient. Equilibrium of the balloon occurs only when the outward forces balance the inward ones. The outward forces are a combination of the centrifugal and drag force acting on the yarn element. Drag forces are mostly tangential to the locus and have little direct effect on this balance when the balloon is fairly upright. However, collapse is initiated in the region just above the ring where airdrag causes the yarn in the balloon to incline almost to its maximum extent. A rough approximation in that zone is to treat the plane of curvature as the same as that of the ring, which implies that the radius of curvature is smaller than elsewhere in the balloon. The consequence is an increased tendency to reduce the radius of the locus in the lower regions near the traveler.

A9.6.3 Collapsed balloon spinning It is fairly obvious that if the radius of the yarn balloon could be reduced as a practical proposition, the yarn tensions could also be reduced. In long-staple spinning this is an option, but in short-staple work it is not. Generally, collapsed balloon spinning is used for heavy, long-staple yarns that are capable of withstanding high tensions. The friction tends to make the yarns hairy. The winding tension in such cases is partly determined by the friction between the sliding yarn and the machine surfaces. It is also partly determined by the end conditions, which are determined by the traveler weight and other parameters already discussed. Lubrication of sliding surfaces is also a factor. If the tensions are properly adjusted, it is possible to make the balloon collapse, or run at a reduced size, as shown in Fig. A9.16. Although the centrifugal component is much reduced by this, there is now a significant frictional drag as the yarn passes over the surface of the spindle or crown. The frictional drag may be calculated approximately from Amonton’s Law using Equation (A9.13); this implies that the ratio of tensions is a function of the angle of wrap and the coefficient of friction. For the system to pay off, the increase in yarn tension due to friction by the above mechanisms must be less than the increase caused by allowing the balloon to inflate. Usually, a crown is mounted on anti-friction bearings on top of the spindle to reduce the frictional forces. However, the yarn still has to slide over the crown in a direction along the length of the yarn.

A9.6.4 Unwinding The ring bobbins provide only temporary storage and the yarn has to be unwound from them in the so-called winding process as was described in Chapter 9. Winding machines usually pull yarn over-end from a stationary package. The package from which the yarn can be removed might be a ring bobbin, cone, or cheese, although the most common is the ring bobbin. The yarn is caused to balloon by the motion of the


451

From drafting system

Collapsed balloon

Fig. A9.16

Reduced balloon

Balloon collapse

wind-off point on the surface of the package. The take-off speed and the radius at which the departing element of yarn is removed from the bobbin determine the rotational speed of the balloon. Of necessity, the rotational speed is variable and the structure of the package causes the take-off point to oscillate rapidly. The balloon changes height, diameter, and shape as a result, and a chaotic balloon is created. There is usually no ring and traveler to help smooth out the fluctuations. Any instantaneous view of the yarn in the balloon shows a multi-noded sinuous shape rotating about the package axis. The presence of ballooning forces is important because they hold the yarn clear of the surface of the package. This avoids the removal of neighboring coils of yarns that would result in tangles being formed. It also reduces the amount of hairiness created by the over-end unwinding process.

A9.7

Balloons in two-for-one twisting

A9.7.1 Tension control by the use of cylindrical surfaces Two-for-one systems involve the high speed removal of yarn from large diameter packages stored inside the balloon. Consequently, the shape of the balloon has to be controlled to prevent the yarn just removed from rubbing the surfaces of the package(s). The tensions have also to be controlled because of the high speeds and diameters involved. Often there are two coaxial balloons involved and these have to be kept separate. For these reasons the balloons are frequently contained within cylindrical cans which act rather like balloon control rings.

A9.7.2 Tension control by friction devices An absence of any frictional type of control leads to balloon instability and it is normal to use a spring-operated tensioner or a governor operated by centrifugal

452

Appendix 9

forces to help control the tension. Changes in yarn length within the balloon are accommodated by a disk designed to dynamically store limited amounts of yarn. An extreme and undesirable case is that of the chaotic balloon just described in Section A9.6.4, which lacks any such a control.

References 1. 2. 3. 4. 5. 6. 7. 8.

Shintaku, S. Oda, J and Yamazaki, H. Airflow around the Rotating Pirn (or Cop) and Power Loss, J Text Mach Soc Japan, 43(1), T1–T9, 1990. Lord, P R, Rust, J P and Fenercioglu, F. Balloon Irregularities in Ring Spinning, J Text Inst, 1997. McAdams, W H. Heat Transmission, p 99, McGraw Hill, 1942. Batra, S K, Ghosh, T K and Zeidman, M I. An Integrated Approach to Dynamic Analysis of the Ring Spinning Process Part I, Text Res J, 59, 6, 309–17, 1989. Batra, S K, Ghosh, T K and Zeidman, M I. An Integrated Approach to Dynamic Analysis of the Ring Spinning Process Part II, Text Res J, 59, 7, 416–24, 1989. Lisini, G G, Toni, P, Quilghini, D and Di Giogi Campedelli V L. A Comparison of Stationary and Non-stationary Mathematical Models for the Ring-spinning Process, J Text Inst, 83, 4, p 550, 1992. Grishin, P F. Fundamentals of Spinning Ring Development, Whitin Review, 25, 3, p 34, 1968. Frazer, W B. Ring Spinning, Text Horiz, Benjamin Dent & Co. Ltd, pTH 37, 1996.

Appendix 10 Advanced topics VIII: Topics in rotor spinning

A10.1

Brief history of open-end spinning

The idea behind open-end (OE) spinning is almost as old as history itself. Farmers twisted straw into binders for stooks of corn and wheat by continuously adding new straw to the end of the binder and twisting it into the existing structure to make it ever longer. The industrial revolution saw some clumsy attempts at a mechanical solution, but it was not until the twentieth century that elegant solutions began to appear [1]. Derivatives of two of the systems then envisioned have become established, namely air-jet and rotor spinning. Early patents by Götzfried [2] disclosed the idea of using an air vortex to assemble fibers and twist them into yarn. Lord [3,4] worked on such vortex systems but fiber losses and yarn structure were unacceptable; the method was then commercially unattractive, despite the simplicity of the device. A design where the vortex was confined to the fiber assembly was offered for sale but did not achieve significant market penetration. It was left to Nakahara [5], Morihashi [6], and others to develop a system that used air-jets to twist but allowed the fiber assembly to be controlled by other processes. Although the idea started out as a sort of OE spinning, the successful system lost the essence of OE spinning because there was no longer an open end, merely a very ingenious way of manipulating twist and yarn structure. This was described in Chapter 10 and it is merely a matter of peripheral interest in this context. The origins of rotor spinning were in the work of Berthelson [7] in 1937 and Meimberg [8]. In the early 1960s, VUB [9] in Czechoslovakia, the Shirley Institute [10] and UMIST in Manchester, UK, SRRL in New Orleans, USA, and perhaps others, were experimenting with rotor spinning. VUB produced a working prototype designated the KS200, which was the predecessor to the BD200, which was offered for sale in 1966 at $200 per spindle. Eventually, the Czech BD200 gained a good market share. In the very early days, there seemed to be a limit of about 20 000 r/min in rotor speeds, because of bearing design. Also, calculations of that time suggested that a prudent speed limit might be about 25 000 r/min for a 3 inch (76 mm) diameter aluminum rotor with air pumping holes. (The pumping holes introduce stress

454

Appendix 10

concentrations that reduce the strength of the structure.) A number of experimenters found that rotors deformed or even burst when oversped. Somewhat later, Landwehrkampf [11], who was concerned with large rotors for long-staple rotor spinning, opined that plain 120 mm (4.75 inch) diameter rotors could run at 25 000 r/min. In a study by Wunsch [12] and Kerr [13], the energy consumption of a plain disk was found to be proportional to D3.8 ω2.5 and this implies that large rotors running at high speed will get very hot. Landwehrkampf published some curves that showed a 5.46 inch (138 mm) rotor running at 20 000 r/min required nearly 300 watts and approximately 1.5 inch (38 mm) rotors required about 120 watts to run at 80 000 r/min. However, by the time losses are included, a frame of 100 large rotors running at 20 000 r/min seemed to require well over 30 kW. The data of Landwehrkampf did not fit those of Wunsch and Kerr but that was not surprising because of the differences in shapes of the rotating member. For various reasons, the long-staple rotor spinner did not succeed commercially and it was the short-staple version that made a remarkable impact on the industry. From a consideration of the foregoing, it was estimated in the 1970s that a machine of 300 rotors of 1.5 inch (38 mm) diameter running at 100 000 r/min would require some 60 kW. This is a very large power demand and it was clear that the rotor size had to be reduced. The size of 38 mm had been picked on the basis that the diameter of the rotor ought not to be less than the fiber length for quality reasons. This idea was proved wrong as it turned out. In 1997, 28 mm rotors could be run at 130 000 r/min (the power consumption is not known to the author) and commercial speeds ranged between 85 000 and 110 000 r/min. The reduction in rotor size has continued over two decades and it certainly has been connected with the rising power demand at the ever higher speeds. In the modern rotor machine, the temperature of air leaving the rotor is very high. How much further these trends can go is another matter. The early experiments at UMIST [14] indicated that rotor spun yarns were weak in comparison to ring yarns. One reason was the incidence of bridging fibers that caused an enlarged population of hooked fibers in the yarn. For this reason it was concluded (wrongly) that, to make the yarn attractive to spinners, the circumference of the rotor had to be many times the fiber length, in order to reduce the proportion of bridging fibers. The elegance of the tapered rotor sliding wall, which conserves space for the assembling of fibers, was not appreciated then. The history of ring spinning shows the difficulties of getting a new process accepted, and one can find old articles opining that ring spinning would never replace the mule. Similarly, rotor spinning took time to become established and yarn weakness was one of the reasons for the reluctance. The market then began to accept the yarn for what it was, but it still took many years before it was fully accepted [15,16]. The relatively low cost of operating rotor spinning has always been one of its main attractions although the capital cost per rotor was initially four or five times that of a comparable ring spinning machine position. Consequently, the rotor had to be run faster to reduce the capital cost/lb of yarn produced to competitive levels. It is the history of this pursuit of speed that is so fascinating. Small rotors, new alloys, protective treatments to withstand wear, and new drive systems all made their contributions. The driving force behind this was to reduce the capital cost/lb of yarn by increasing rotor speed. It was necessary to increase productivity faster than capital cost in order to achieve this. Many of the doubts and reservations of the time are well expressed in a review of rotor spinning made in 1978 [17].

Advanced topics VIII: Topics in rotor spinning

A10.2

455

Yarn evenness

A10.2.1 Number of doublings inside the rotor Fibers are laid into the vee-shaped collecting surface inside the rotor, and enter as a thin stream of fibers. It takes many layers of fiber to make up sufficient linear density; in other words, there are many doublings. These doublings tend to even out any short-term irregularities in the yarn and OE yarns tend to be surprisingly even. Also, there are no errors carried forward from a roving frame, and many errors created by the combing roll drafting system are smoothed. However, longer-term errors arising from the sliver still remain, and these are usually neither worse nor better than with ring yarn. Referring to Fig. A10.1, let n = linear density of the fiber, V = velocity, M = mass flow, m = number of fibers in the cross-section, and the subscripts f and y refer to fiber and yarn, respectively. Also let ω = rotational speed in rad/sec, r = radius of collecting surface of rotor, and τ = twist/unit length of yarn. Mass flow/unit time at input = Mf = mf n Vf Mass flow/unit time at output = My – my n Vy But M f = My , from which: my/mf = Vf/Vy. If Vf = ωr and Vy = ω/2πτ, then my/mf = 2πrτ

[A10.1]

= number of internal doublings in the process Thus, for a 30 tpi yarn running in a 1.5 inch diameter rotor, there are approximately 140 doublings in the rotor groove. This, then, is why the short-term unevenness is so good in comparison to ring yarns. my Fibers in cross-section of yarn

mf Fibers in cross-section of flow

My Vy ω

r Vr

Fig. A10.1

Vf

Mf

Conservation of mass flow in the rotor

456

Appendix 10

A10.2.2 Short-term blend evenness Multiple doublings inside the rotor improve the short-term evenness and the intimacy of the blend [18]. In theory, the per unit CV of linear density of the yarn should be √(my/mf) for lengths up to (my/mf) × rotor circumference. Figure A10.2 is from the work of Deshpande [19], who blended dyed viscose rayon with polyester fibers at a single passage of drawing, before spinning the blend on an OE machine. The machine used is now obsolete but the work shows that the multiple layering inside the rotor ensures a good blend. However, careful examination of Fig. A10.2 shows several spots where there are concentrations of similar sorts of fiber, and the homogeneity is not as perfect as might be hoped. Good dispersion of the components requires that the slivers be properly prepared and that the combing rolls in the OE machine be maintained and operated correctly.

A10.2.3 CV of linear density With staple yarns, the number of fibers in the cross-section can vary considerably. If we assume that there is a Poisson distribution in this number, mav is the average number of fibers in the cross-section, m is the actual number, and s is the standard deviation then: [A10.2]

CV = s/mav

The standard deviation for this type of distribution is estimated to be a function of m; hence, if the value of CV is not too large: CV = 100/√m %

[A10.3]

But m is related to the linear densities of yarn and fiber. Thus it will be realized that the CVs of blend, strength, and count vary with linear density. A 36s cotton yarn made from 4.5 micronaire fibers only has about 93 fibers in the cross-section and we

Fig. A10.2

Cross-section of a blended rotor yarn


457

would expect 10.4% CV due to randomness of the fibers. If the yarn had been made of 2.5 micronaire cotton, the number of fibers would have increased to about 167 and the CV would reduce to 7.7%. Components due to organized errors arising from malsetting of the machines and variations in fiber properties should be added vectorially to these figures. The effect of doubling is to bring the actual CVs closer to the minimum values. Since the rotor doubles over a length equivalent to the rotor circumference, one can expect that errors shorter than, say, 6 inches (150 mm) will be sufficiently doubled and the shortterm evenness should be improved. However, the actual short-term error is still significantly above the theoretical values predicted by Equation (A10.3) (but is normally better than with ring yarns). Some reasons for this are discussed in the next section. The long-term errors are little affected by the doubling in the rotor and are dependent on the doubling at the drawframe and other preparatory machines. If a single passage of drawing is used with eight slivers in the creel, there might be only eight doublings there. This is much less than is found within the rotor. The point being made is that preparation has a larger relative impact on long-term yarn evenness with OE yarn as compared to ring yarn. It will be recalled that poor preparation can induce high error production in ring spinning.

A10.3

Toothed drafting

A10.3.1 Combing roll clothing Combing rolls pull fibers from the beard of a sliver that is continuously fed by a feed roll and plate system. The combing roll usually rotates between 500 and 9000 r/min, it is clothed with either saw-teeth or needles, and its function is to detach fibers from the advancing fiber beard. If the sliver is not well prepared, fiber breakage can ensue because the entanglement of fibers in a clump increases the withdrawal force per fiber and more are caused to break than is desirable. Some measurements were made with rayon fibers on an old OE spinning machine. Undyed rayon fibers can be made almost invisible in a bath of liquid methyl salicytate so that a dyed tracer fiber within the structure of the yarn can be seen among the surrounding fibers. Dyed tracer fibers were placed carefully in the sliver entering the OE machine and the yarn produced was studied under a microscope. In the yarn, the fiber extent (the distance between the extremities of a folded fiber embedded in a yarn) was greatly reduced as the fibers took up a variety of hooked and looped shapes. Sometimes the original fiber was found to exist in two or more pieces and often only a shortened piece of tracer fiber would be found. When the fiber placed on the sliver was greatly crimped or relaxed into a very convoluted shape, the fiber almost invariably broke. The condition of the combing roll wire, its speed and its shape, all affected breakage rates; it also affected the ejection rate for trash. Siersch [20] showed that helicoidally arranged teeth on the combing roll split fiber tufts into roughly parallel fibers separated by contiguous teeth (Fig. A10.3). This beneficial separation was accompanied by cyclic fluctuations in fiber flux (number of fibers per unit area of flow) and yarn tension, which were related to the pitch of the tooth helix. CVs of the fiber flux in his experiments varied between 8.9% and 9.6%. This, then, accounts for one of the reasons why the short-term CV is greater than the theoretical value. The larger the number of tooth helices, the smaller was the variation, and the higher were the yarn strength and breaking elongation. Too fine a

458

Appendix 10

Fiber

Motion of teeth

Penetration of combing roll teeth into a fiber beard

Fig. A10.3

tooth pitch (< 2 mm) created increased nep production with cotton fibers and a deterioration in yarn CV (Fig. A10.4). If the front angle of saw-tooth clothing (Fig. A10.4) was increased above about 20°, the drafting force increased and so did fiber damage. Various investigators have shown that combing roll damage produces yarn irregularity. The most usual damage is to the teeth; sometimes careless handling causes this, sometimes it is caused by large particles in the feed sliver, and sometimes by fiber jamming. The latter can be caused if a loop of sliver is lifted from the can and a double, or triple, thickness of sliver is ingested by the feed roll. A common time for this to happen is when a can is being emptied of the last length of sliver. However, it can happen when a sliver piecing has just been performed, or if a can has been damaged. The life of the combing roll clothing is finite and the use of dusty fiber, or of fiber with abrasive fiber finish, increases the wear rate on the teeth. Consequently, not only are the metal surfaces hardened, but they are also surface treated to improve their wear resistance. Like card wire, the body of the tooth has to be tough to prevent brittleness; thus, despite the hardness of the cutting edge, the teeth can be bent. Bent teeth result in a loss of evenness in the yarn.

A10.3.2 Combing roll bearings Combing roll bearings become damaged in service. Slippage in the tape drive can cause the bearings to become overheated, which causes the grease to fail. Typically, the grease hardens and blocks further lubricant from reaching the ball track. 1.5

17

1.0 16

CV yarn 0.5

Nep/sq inch

Yarn CV (%)

α

Nep/sq in 15 –20 –10

Fig. A10.4

0 10 20 Tooth angle (α)

30

0 40

Effect of comber roll tooth angle on yarn performance


459

Shock or overloading can cause the balls to indent the ball track. The race becomes noisy and consumes more power, which, in turn, leads to lubricant failure. Tests [21] using accelerometers to measure the vibrational accelerations at the combing roll bearing housings showed unusually high values for worn units. The use of an encoder driven by the combing roll enabled the vibration pattern to be resolved, and a sample is shown in Fig. A10.5. Cutting the bearing housings open revealed damage to the ball tracks.

A10.4

Fiber assembly – the formation of wrapper fibers

Once per revolution, the laying of the fibers on the collecting surface and the peeling of the yarn from the collecting surface interferes. Fibers laid at these times are called bridging fibers and, during removal, portions of these fibers become bent back and wrapped around the body of the yarn. Consider Fig. A10.6(a). A fiber is shown sliding on the inside of the conical portion of the rotor. One end is already trapped in the yarn leaving the rotor groove. The peeling point is where the yarn leaves the rotor groove. As yarn is pulled from the rotor, this peeling point should move in the same direction as the rotor. The dotted line represents a sliding path of a fiber in the recent past. In Diagram (b), events occurring very shortly after the first are portrayed. A small amount of yarn has been removed, carrying the entrapped fiber with it; meanwhile, the trailing end of the fiber has slid nearer the rotor groove. Eventually, the trailing end must be folded back on the core of the yarn, as depicted in notional form in Diagram (c). However, the yarn rotates about its axis because of the false twist, and this causes the folded back fiber to become wrapped around the core of the yarn as indicated in Diagram (d). Variation in inclination of the fibers within the yarn is typical of the structure. Portions of the bridging fiber are wrapped around the outer surface of the yarn and carry very little load when the yarn is in the free state (i.e. not assembled into fabric). The remaining portions of the bridging fibers are buried in the yarn structure and, although they carry some load, they behave like short fibers. Consequently, the yarn is weaker than Pulses due to the indentations in the tracks and balls Ball damage

Outer track

Shaft

Inner track

Periodic track damage

Fig. A10.5

Polar diagram of acceleration at the bearing housing

Combing roll bearing damage

460

Appendix 10 Fiber

Fiber

Fiber pulled from the rotor by the departing yarn

Original fiber path Yarn

Yarn

Fibers sliding inside the rotor Tail slides

(a) (b) Hooked fiber

Yarn Twisted hooked fiber

(c)

(d)

Fig. A10.6

Bridging fibers

ring yarn. Wrapper fibers increase the pressure on the enclosed fibers and this gives some local resistance to failure, although it produces an unwanted waisting in the yarn. The chance of a bridging fiber depends on the rotor diameter and the projected length of the sliding fiber approaching the rotor groove. The term ‘projected fiber length’ must be explained. The fiber does not approach the rotor groove with its length parallel to a tangent of the rotor groove; rather, it approaches obliquely. Furthermore, the fiber may not be straight but might be convoluted in some way. Thus, if viewed perpendicular to the direction of slide, the distance between the extremities of the fiber is less than the real length. This distance between the extremities is referred to here as the projected length. To repeat, it is always less than the actual fiber length. Consider an example where the circumference of the rotor is 3 inches and the projected fiber length is 1 inch. In such a case, two out of every three fibers will be assembled inside the rotor groove without intersecting the path of the outgoing yarn. One in three will intersect the outgoing yarn and might be entrapped by it. The first, and larger, category of fibers becomes the core of the yarn and the second category become wrappers. The structures of the yarn are discussed in Appendix 5. Figure A10.7 shows some micrographs in which the core has been shaded to highlight the wrapper fibers. The wrappers are shown as dark fibers.

A10.5

Twist distribution

A10.5.1 False twist control by use of a rotating navel Causing the navel to rotate can change the false twist created at the navel. Lünenschloss [22] showed that using a rotating navel increased the minimum TM at which one could spin (Fig. A10.8). He also showed that a soft yarn with a low twist could be spun. This was attractive not only because of the hand of the yarn but also because a low twist multiple gives a potential productivity increase. However, the rotating navel was an extra complication and it has not proved acceptable in practice.


461

(a)

(b)

(c)

Fig. A10.7 Rotating

Minimum twist multiple

6

Fixed

5 4 3 Rotor diameter = 50 mm Linear density = 50 tex 2 20 000

Fig. A10.8

Wrapper fibers

40 000 60 000 Rotor speed (r/min)

80 000

Effects of fixed and rotating navels in rotor spinning

A10.5.2 False twist distribution in the rotor vee The geometry of the navel affects the performance. As explained in the main text, the yarn rolls on the navel and creates false twist in the yarn inside the rotor. It should be noted that the twist of the yarn arm inside the rotor can be significantly higher than that in the emerging yarn. The torque of the yarn in the rotor vee is relieved at or near the peeling point of the yarn. A length of incipient yarn lying in the rotor vee adjacent to the peeling point has a varying level of twist, as indicated in Fig. A10.9. The shape of the rotor groove affects this twist propagation and a sharp vee tends to restrict the propagation more than a rounded one. An approximate distribution of twist in the incipient yarn lying in the rotor groove is: T ≈ To ekµθ

[A10.4]

where µ = coefficient of friction, θ = angle subtended by the incipient yarn measured from the peeling point, k = ω2r2n/2 sin α and 2α = angle of the rotor groove. Thus, the groove angle has a strong effect on performance. If α is too small, the yarn jams in the groove. The distribution of forces acting on the yarn lying in the rotor groove is shown in

Appendix 10 Total twist in rotating yarn arm

Yarn twist

462

Twist in emerging yarn Length along the yarn inside the rotor

Twist distribution inside a rotor

Fig. A10.9

Fig. A10.10. A rounded groove gives a different distribution of forces, reduces the total lateral force acting on the yarn from F to F′, and modifies the coefficient k. Wear can sometimes convert one shape of vee to another; this causes changes in yarn characteristics. In the 1970s, before adequate wear protection treatments had evolved, cases were known where the wear was sufficient to penetrate to the outside of the rotor. Build-up of dust and trash inside the rotor also changes the shape of the vee and affects the yarn characteristics.

A10.5.3 Twist surges At very high speeds, difficulties begin to appear in retaining the twist in the rotating yarn arm. False twist can surge forward through the navel, leaving a transient depletion in twist inside the rotor, which causes an end-break near the navel inside the rotor. Twist traps in the yarn withdrawal tube become a necessity at high speeds, especially when spinning polyester or similar fiber. Many twist trap designs can be recognized by the cranked doffer tube, which causes the yarn to leave at an angle to the rotor center line. This is an important device in controlling the twist surges. The effect of such surges can be recognized by the presence of portions of yarn inside the rotor after an end-break. Without significant surges, there is only fiber and dust present because the failure under non-surging conditions occurs where the yarn is peeled δF = ω2r2nδθ δFt = δF/2 sin α δF t

δF t α α

α α

µ δF t

µδFt

δF

δFt = δF/2 Sin α Force diagram

Rotor vee (a) δ Ft ′

δ Ft ′

µΣ δ Ft

µ Σ δ Ft

Fouled or damaged rotor (b)

Fig. A10.10

Forces involved in rotor wear


463

from the rotor groove. The angle of the cranked yarn withdrawal tube is important. Normally it is cranked at about 45°. The smaller the angle, the lower the spinning tension, but a torque-stop effect can be produced if the angle is increased. There are also other designs of twist trap to fulfill a similar function. The design and condition of the navel, as well as the character of the fiber and the yarn count, play important parts in determining the nature of the yarn. The navel also plays a part in determining the effectiveness of the operation. Yarn tension creates forces between the orbiting yarn and the stationary navel. Normal forces between yarn and metal create friction; the frictional forces act tangentially on the yarn and produce torque. The mean tangential force is a function of the normal forces referred to and the coefficients of friction between the surfaces. The normal force is proportional to ω 2 rr2 n , where ω is the rotational speed, rr is the radius of the rotor, and n is the linear density of the yarn. The false twist depends on the fiber finish, the type of navel surface, as well as on rotor speed and size. An increase in coefficient of friction decreases yarn strength but improves end-breakage rates. The flare radius connecting the bore to the front surface of the navel plays a significant part in determining both the false twist and the properties of the output yarn [13,14,15]. With a large flare radius, it is possible to increase the false twist at the expense of the winding tension. Lünenschloss et al. [23,24,25] wrote that the output yarn tension can exceed the yarn strength when using a rotor diameter of 60 mm (2.36 inches) at rotor speeds above 70 000 r/min. This gives some idea of the problems at high rotor speeds.

A10.6

Conclusion

In high speed rotor spinning, attention to the design and condition of the combing roll clothing is needed to preserve yarn quality. Attention to the design and state of the rotors is important, not only to obtain high quality, but also to minimize the costs associated with unnecessary end-breaks, and the consumption of power. Cleanliness and maintenance are of great importance.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

Lord, P R. The Prospects of Break-spinning, 11th Canadian Textile Seminar, Kingston, 1971. Götzfried, K. USP 2 926 483, 1955, German Pat 1115163, 1961 and BP 880239, 1961. Lord, P R. Spinning from the Card using an Air Vortex, Textile Inst Ind, Jan 1967. Lord, P R. Application of the Air Vortex Method of Spinning, Czech Sci and Tecn Symp, Prague, Czechoslovakia, 1967. Nakahara, T. USP 4 142 354, Mar 1979. Morihashi, T. USP 4 183 202 Jan 1980. Berthelson, S E. BP 477 259, 1937. Meimberg, J. BP 695 136, 1953 and several others up to 1965. Rohlena, V. Open-end Spinning, Elsevier , Oxford, UK, 1975. Catling, H. The Comparative Merits of the Principal Break-spinning Systems, Textile Inst Ann Conf, 1968. Landwehrkampf, H. General Purpose or Special Machine for Open-end spinning, Melliand Textilberichter, 4, 11, 1976. Wunsch, H. Frictional Torque in Small Ball-bearings at High Speeds, NEL Report No 25 1962. Kerr, J. Private communication, NPL.

464

Appendix 10

14. 15.

Lord, P R. Developing Rotor Break Spinning, Text Ind, Feb 1970. Lord, P R. Commercial Developments in Open-end Spinning, Textile Inst Ann Conf, Harrogate, UK, 1976. Coll-Tortosa, L. Neue Aspekte der Technologischen Spinngrenzen des OE-Rotorspinnens, Vorträge anlässlich der 3. gemeinsamen Tagung der Aachener Textilforschunginstitute, 1977. Hunter, L. The Production and Properties of Staple-fibre Yarns made by Recently Developed Techniques, Text Prog, 10, 1/2, 1978. Lord, P R. Yarn Evenness in Open-end Spinning, Text Res J, 512–15, 1974. Deshpande, S V. Open-end Spinning as a Means of Blending, MS Thesis, NC State Univ, NC, USA, 1972. Siersch E. Ein Beitrag zum Mechanismus der Fasertrennung und des Fasertransporte beim OE-Rotorspinnen, Fortschrift-Berichte der VDI Zeitschriften, 3, 56, 1980. TE402 Project, NC State Univ, Raleigh, NC, USA, 1995. Lunenschloss, J. Private Communication. Lunenschloss, J. Seminar Series at NC State Univ, Research into, and Design of, Rotor Spinning Machines, NC, USA, 1975. Lunenschloss, J, Coll-Tortosa, L and Phoa, T T. Die Einfluss der Faserlange und der Faserlangenverteilung auf die Eigneschaften Rotorgesponnener OE-Baumwollgarne, Textil-Praxis, Sept 1974. Lunenschloss, J, Coll-Tortosa, L and Phoa, T T. Die Untersuchung der Faserstromung im Faseleitkanal einer OE-Rotorspinnmaschine, Chemiefasern/Textil-Industrie 24/76, 355–485, 1974.

16. 17. 18. 19. 20. 21. 22. 23. 24. 25.

Index

Abbreviations fil = filament HOK = normalized productivity ls = long staple m.m. = man made m/c = machine r.h. = relative humidity spg = spinning ss = short staple 2-for-1 2-for-1 2-for-1 2-for-1 2-for-1 2-for-1 2-for-1 2-for-1 2-for-1 2-for-1 2-for-1 2-for-1 2-for-1 2-for-1 2-for-1 2-for-1 2-for-1

co-axial balloons, 451 twisting, 220 twisting, armature, 256 twisting, balloon rail, 255 twisting, cylindrical pot, 255 twisting, fly & dust, 257 twisting, high speed, 66 twisting, package rotation, 65 twisting, piecing, 66 twisting, prep costs, 305 twisting, rotor-spun yarn, 255 twisting, space available, 255 twisting, storage disk, 257, 452 twisting, tension control, 451 twisting, yarn tension, 256, 257 twisting, yarn unwinding, 257 twisting, yarn waxing, 257

abrasion, 128 abrasive additives, 48 abrasive fibers, 417 acceleration zone, 70 access to capital, 313 accumulations of grease, 128 acetate fibers, 38 acid-cracking, 212 acrylic fibers, 21 acrylonitrile, 21 actual draft ratio, 321 added variance in drafting, 353 additive tensioner, 69 adequate quality of product, 350

advanced fiber info system (AFIS), 368 aerobic bacterial action, 212 AFIS, 285, 392 aggregations of fibers, 423 agricultural fiber production, 11 AGV, 184, 185 air conditioning, 148, 151, 283, 288, 289, 341, 346, 348 air ducting 128, 148 air permeability of sample, 353 air quality monitoring, 348 air vortex spg, 186 air wash, see air conditioning air/steam mixtures, 343 air-drag, 438 air-jet spg m/c, 106, 107, 109, 262–267 air-jet spg ply yarn, 265 air-jet spg wide ribbon, 266 air-jet spg, fiber requirements, 264 air-jet spg, perforated surfaces, 262 air-jet texturing, 106, 266, 267 air-jet texturing, wetting process, 107 air-jet yarn, 89, 261, 267 air-jet yarn, polyester/cotton, 267 air-jet yarn, structure, 260, 267, 378 air-jet, yarn character, 107, 265–267, 378 air-jet/mechanical twister comb, 266 airstream study, 289 alternating self-ply, 68 alternating twist systems, 67 amino acid side chains, 31 amortization costs, 8 animal excretions, 208 anthrax, 32 anvil roller, 51 aperiodic nubs or loops, 109 aperture in cover plate, 262 aphid, 25 apparel market, 205 apron drafting, 79, 80, 174 aramid fibers, 22 artificial lighting, 349

466

Index

assignment specifications, 306 atmosphere, temperature control, 384 autoclave, 50, 90, 342, 343, 344 autodoffing, 175, 217, 305, 313 autoleveler, 86, 135, 136, 158, 159, 217 autoleveler, CPU, 136 autoleveler, echo, 135 autoleveler, temporary sliver storage, 136 automated guided vehicles, see AGV automatic creeling, 175 automatic doffing, see autodoffing automatic fiber transfers, 87 automatic handling, 9 automatic piecing, rotor spg, 310 automatic ratch setting, 420 automatic traveler changing, 313 automatic weighing system, 213 automation, 6, 184, 253, 303, 310, 313, 316 availability of capital, 5 availability of product, 301 average transit time, spindle-spindle, 308 axiflow machine, 123, 127 back-leakage in extruder barrel, 42 back-pressure of filter pack, 42 backwashing, 216 bacteria & viruses, 25 bad bale laydown, 404 balance, labor cost & capital, 7 balanced plied yarns, 60 bale blooming, 120, 121, 123 bale conditioning, 120, 216 bale cutter, 121, 122 bale decompression, 31 bale density, 31, 121 bale height, 121 bale income, seasonal, 391 bale laydown, 72, 121, 122, 131, 392, 394 bale laydown, withdrawal consistency, 391 bale lots, 119–121 bale management, 118, 120, 122, 129, 391, 392, 394 bale milling, see bale plucker bale mixing, 397 bale plucker movement, 131, 167, 395, 397, 403, 404 bale plucker, 117, 121, 394 bale plucker, depth of cut, 131, 397 bale plucker, productivity, 131 bale preparation, 119 bale profile, 395 bale size, 13, 31 bale storage, mill, 392 bale ties, 31 bale wrapping, 31, 113 bale, assignment to category, 391 bale, compressed, 117, 120 bale, fiber tags, 122 bale, selection from warehouse, 119

bale-bale variation, 116 bale-plucker, cyclic removal of fiber, 397 bales issued, category limits, 392 bales, between-lot variations, 120, 391 bales, fiber attributes, 391, 395, 405 bales, fiber removal, 395 bales, moisture content, 120 bales, selection for laydown, 392, 393 bales, ss fibers, 14 bale-to-bale variation, fiber attributes, 131 baling fibers, 53 balloon collapse, 181, 299, 441, 448 balloon control ring, 178, 436, 441, 442 balloon geometry, 178, 427 balloon, 2-for-1 twisting, 451 balloon, common axis, 65 balloon, real, 436 balloon, yarn tension, 427 ballooning, length of streamers, 438 bargaining, supplier and customer, 276 barré, see fabric barré barrel/screw clearance, 43 barrel/screw cooling seizure, 43 basis weight, 371 bast fiber spg, 231, 232 bast fiber spg, gilling, 232 bast fiber spg, hackling, 231 bast fiber spg, roughing, 231 bast fiber spg, shives, 232 bast fiber spg, spreading frames, 232 bast fiber tow, 231, 232 bast fiber wet-spg, 232 bast fiber, cuts (hanks), 232 bast fiber, dry-spg, 232 bast fiber, grist (count), 232 bast fiber, leas (hanks), 232 bast fiber, rove (roving), 232 bast fiber, sampling procedure, 279 bast fiber, spread sheet, 232 bast fiber, working, twist & untwist, 232 bast fibers in stem bark, 2, 35 bast fibers, history, 2 beat frequencies, 398 beaters, 123 beetle larvae, eggs, 32 belt transmission calc, 332–334 between-stream variance, 423 bi-component sheets, slit, 114 bicomponent yarns, 85, 110, 111 bimli (hibiscus cannabinus), 36 biochemical oxygen demand (BOD), 212 biological control, 32 blend component discrimination, 390 blend CV, 390 blend evenness, 164, 389, 392, 399, 402, 406 blend proportions, 117, 121, 400, 404 blend, fractionation, 390 blend, intimacy, 117, 129, 131, 155 blend, sudden changes, 400

Index blending machine, 116, 117, 131, 132, 133 blending man-made fibers, 13 blending product streams, 84 blending, 13, 51, 116, 389 blending, bowl analogy, 389 blending, mixing before carding, 129 blending, slivers, 129 blends, wool/m.m.fiber, 14 blowroom waste disposal, 150 blowroom, 116–118, 150 blowroom, fiber losses, 150 bobbin (slubbing) transport, 230 bobbin (slubbing) wind structure, 230 bobbin feed to winders, 255 bobbin flow, 327 bobbin transfer systems, 155, 255 bobbin, 168, 169, 185, 230, 299, 444, 445 bobbin, random mixing, 133, 134 bobbin, unwinding, 185 bobbins, deformed, 185 boiler leaks, 344 boiling point, 342 bonds, molecular, 85 bottom roll fluting, 156 break draft, 72, 414 breast works, 213 brighteners, 48 Btu/hr, 342 bulk, see yarn, bulk bulked continuous fil (BCF), 106 burr picker, see card, ls, burr beater byssinosis, 154 cabling machine, 66 calculations, 317 calibration cottons, 355 camel hair, 31 can-changers, 155, 310 cans of sliver, transport, 310 cap spg, 218 capital & fixed costs, 309 capital cost per stream, 87 capital cost, 6, 50, 87, 291, 301, 303, 309, 310, 313, 348 capstan friction, 46, 75 capstan tensioner, 69 card autolevelers, see autoleveler card clothing, 145 card licker-in waste, 153 card productivity, 324, 325 card sets, 15, 215, 219, 230 card waste, 149, 152 card, feed roll, 154 card, high inertia, 154 card, licker-in, 187 card, ls, blending, 214, 215, 224 card, ls, burr beaters, 210, 213 card, ls, carding actions, 213–215, 219, 224, 227, 228

467

card, ls, element sizes, 224 card, ls, element speeds, 227 card, ls, fettling, 227 card, ls, multiple worker/strippers, 215 card, ls, nep creation, 216, 228 card, ls, producion rates, 214, 224 card, ls, wire, 227 card, parallel streams, 398 card, roller-top, 15, 213–228 card, woolen, 224 carding m/c, ss, 118, 136, 137, 139, 141–147 carding prep, ss, 116 carding, ss, 136–139, 143, 144 –146 carding, ss, card wire see card clothing carding, ss, clothing, 139, 141, 143, 145–147 carding, ss, control chart of nep, 147 carding, ss, cotton cleaning, 142–145 carding, ss, doffing, 136 carding, ss, dynamic testing of setting, 147 carding, ss, fiber condensation, 136, 142, 143 carding, ss, fine fibers, 147 carding, ss, grinding, 139, 141, 145 carding, ss, knife edge, see cleaning edge carding, ss, nep, 136, 139, 141, 146, 147 carding, ss, performance deterioration, 141 carding, ss, re-clothing, 139, 148 carding, ss, rewiring see reclothing carding, ss, screens, 144 carding, ss, segments, 139 carding, ss, separation of fibers, 136 carding, ss, settings, 146, 147 carding, ss, sliver can, 143 carding, ss, sliver, 142, 143 carding, ss, sticky cotton, 143 carding, ss, trailing fiber hooks, 143 carding, ss, trash separating devices, 144 carding, ss, trumpet or condenser, 142 carding, ss, wire see clothing carding, ss, worn teeth, 139 carding, worsted, 213 cards in parallel, 119 cards, draft ratios, 73 cards, input fiber variation, 395 cards, ls, multiple-doffer split-web, 220 carpet backings, 113 carpet market, 205 carpet yarn manufacture, 219 caterpillar drafting, 216 ceilings, 349 cellulose tube, 22 cellulose xanthate, 4, 40 central signal processing, 350 centrifugal force, 169 ceramic rings, 182 cessation of flow, 43 change in state, 342 change the X-sectional area of strand, 70 changes in fiber content to variation in linear density, 86

468

Index

channeling, 44, 84, 398 characteristic times, 395 charges that the market can bear, 309 chase, 177, 181 cheese, 114, 234, 237 chemical contamination, 349 chemical precipitation, 40 chemistry, 11 choice of fibers, 307 choice of solvent, 38 chokes, 76, 128, 174, 182, 224 Churka gin, 27 chute feed, 118, 122, 129 claim settlement costs, 392 clamping of hooked fibers, 353 classers’ length, 280 classes of economies, 5 classifying spinners, 286 clean air in mill, 348 cleaning machines, 117, 128 cleaning means, 127, 128 cleaning points, number, 127 cleaning, process, 125 clogging, spinneret, 38 clump-clump, fiber attributes, 390 coagulation, 4, 38, 40 coefficient of air-drag, 438 coefficients of variation (CV), 83 coercive torques, 384 co-extrusion, curl, 113 cohesion curve, 59 cohesion, shared fibers, 68 coil geometry, 386 coiler, 136, 149, 157 collapsed balloon spg, 219, 450 color matching, 216 color measurements, 404–406 color tests, knitted fabrics, 405 comb overloading, 163 combed wool sliver, 217 comber lap, ss, 159 comber noil, 162, 163, 286 comber production, 324 comber roll, OE clothing, 188 comber speed, 162 comber webs, 159 comber-roll, OE, 186–191, 425, 457–459 comber-roll, OE, abrasive fiber finish, 458 comber-roll, OE, life, 458 comber-roll, OE, loop of sliver fed, 458 comber-roll, OE, performance, 189, 191 comber-roll, OE, use of encoder, 459 combing elements, damage, 162 combing m/c, ss, 159, 161, 162, 163 combing process, 161, 161 combing, cost, 161, 301 combing, doubling, 163 combing, fiber dispersion, 397 combing, fiber orientation, 159

combing, maintenance, 162 combing, short fiber removal, 159 combing, sliver appearance, 159 comfort zone of operatives, 348 commercial nylon, 1940, 4 common principles, 56 compact spg, 261 competition between suctions, 262 competition, 87, 303 complaints, 277, 300 component balance, 162 component HOK & OHP, 304 composite yarns, 15, 260 compressed fibers buckle, 50 compression (nip) zone, 81 computers, local, 350 concentricity of drafting elements, 73 condenser, 81, 122 cone pulleys, 169 cone winding, 239, 240 cones or cheeses, damaged, 198 cones, 114, 234, 239 confined & non-confined systems, 67 coning oils, 95 conservation of computing power, 359 conservation of flow, 71, 119, 313, 320 continuous fabric inspection, 372 continuous fil, 11, 18 continuous heating systems, 50 continuous measurement, 356, 417 control & autoleveling, 135 control charts, 285, 286, 358 control limits, 351 control mechanisms, 71, 82, 342 control of flowing material, 71 control signals, 350 controlled climate, 344, 347, 349 controllers, 71, 133, 134 controlling number of categories, 392 conversion between count systems, 319 conversion cost, 301, 302, 311, 312 conversion of stems to sliver, 231 cored slivers, 81 corrosion in boilers, 344 cost & price, 301 cost & quality of product, 9 cost considerations, 209 cost data, 8 cost estimates, transitory, 309 cost minimization, 308 cost of fibers, 7 cost of labor, 308 cost of waste, 128, 150, 308 cost proportions, 301, 310 cost savings, end-break repairs, 306 cost, quality penalties, 306 cost, spg, 315 cost, what the market will bear, 392 cost, winding, 315

Index costs & sales, 8 costs by percentage, 304 costs, historical, 303 costs, supplier, 276 cots, 75, 156, 174, 415 cotton & flax, 2 cotton & synthetic fibers, 3 cotton bale labeling, 279 cotton cleanliness, 189 cotton count X breaking load (CSP), 366 cotton fiber merges, 391 cotton fibers, cost, 5 cotton ginning, 27, 143 cotton growing, 25 cotton hank, 318 cotton module, 26, 279 cotton seed, 22, 26 cotton, 3, 22 cotton, cleaning, 150 cotton, color grade, 130, 281 cotton, cost, 28 cotton, cultivation, 23, 25, 26 cotton, dirty, 30 cotton, dye uptake, 166, 167 cotton, fiber attributes, 23–27, 30, 130 cotton, flattened tube, 23 cotton, harvesting, 24, 26 cotton, infestation, 25 cotton, irregular collapse of walls, 23 cotton, primary wall, 23 cotton, reflectance (Rd), 355, 404 cotton, secondary wall, 23 cotton, sticky, 25, 143, 175 cotton, warehouse categories, 391, 392 count & twist determined by market, 327 count spectrum, 304 count systems, 318 cover, 12, 15, 260, 371 creel blending, 129, 164 creel, 156, 168, 397, 398 creel, doubling, 164 creel, lap-winder, 159, 161 creel, power, 156 creel, roving bobbins, 175 creel, sliver combination, 164 creel, slivers, 216 creeping build motion, 177 crimp contraction, 369, 370 crimp recovery, 369, 371 crimp, fil, 12 crimp, ls, 206 crimped fiber, 78 cross link, 20, 21 crutching, 33 crystallinity, 42 crystallization rate, 46 cumulative draft, 72 cumulative frequency curves, 280 cuprammonium solvent, cellulose, 4

cushion of stock, 258 cushion roll, see cots customer complaints, 129, 257, 277, 392 customer goodwill, 258, 306 cut tow, 48 cuticle, 31 cutting edges, 51 cutting, mill based, 13 CV of particular attributes, 130, 280, 314, 358, 390, 393, 411 cyclic errors, 73, 408 cyclone filters, 150, 348 CYMK system, 406 Cyros™, 362 dangers of extrapolation, 1 data obsolescence, 304 daughter tufts, 413 deburring, 219 defect levels, 17 defective fibers, 284 defining a blend, 390 definition of shear, 84 degradation of yarn quality, 81 delayed quality factors, 306 delivery speed, 276, 324 denier, 318 depreciation costs, 303 detergent scouring, 209 development of bulk, 384 development of machinery, 5 development of POY, 87 dew point, 344 dicotyledenous plants, 35 difference frequency, 398 differences between laydowns, 398 differences in fiber color, 284 differences within laydown, 398 differences, cost & price, 276 differential gearing, 169 differential shrinkage of fibers, 50 differential stress, 85 difficulties in processing, 47 digital data stream, 359 direct labor cost, 311 direct labor force, 309 direct number, 317, 318 direct winding, 242 dirt removal. tuft surface, 127 displacement of centers of production, 3 dissimilar bales in proximity, 400 dissolved gases, 344 distribution of pressure in nip zone, 81 distribution of the polymer flow, 44 diversity in m/cs, 205 divisions in testing, 350 doffer rail, 185 doffer, automatic, ring-frame, 185 doffer, grasping device, 185

469

470

Index

doffer, peg belt, 185 doffing, 142, 185 double creeling, 176 double fleeces, 34 doubled & mixing, 398 doubling in blending machines, 83 doubling & toothed drafting, 425 doubling mass constant, 424 doubling, 83 doubling, 83, 216, 398, 399, 422, 424 doubling, periodic errors, 399 down-twisting, 65 draft distribution, 414 draft magnitude, 411 draft ratio changes, 158 draft ratio of mill, 72 draft system, OE, 186 draft zone, 70 drafted fiber ribbon width, 418 drafting & doubling, 407 drafting errors, automatic control, 419 drafting or drawing, 71 drafting roll separation, 418 drafting system time constant, 419 drafting system, 70, 80, 84, 156, 168, 294 drafting systems, ls, size, 206 drafting theory, 407, 409–411 drafting waves, 78, 85, 282, 351, 410, 411 drafting zones, successive, 422 drafting, 14, 71, 74, 174, 321, 331, 408, 413– 415 drafting, added frictional forces, 79 drafting, aprons, 77, 79, 411, 415 drafting, CV at start-up, 415 drafting, damaged roll necks, 415 drafting, early staple processing, 83 drafting, error source identification, 413, 416 drafting, roll setting, 414 drafting, rotor spg, 413 drafting, setting, 84, 415 drafting, short-term errors, 412 drafting, sliver autonomy, 419 drafting, state of maintenance, 84 drafting, toothed, 83 drafting, uneven rubber hardness, 415 drafting, variance, 425 drafting, variance added, 422 drafting, variance, autonomous slivers, 419 drag coefficient, 439 draw zone, 156 drawframe & dedicated card in series, 398 drawframe autolevelers, 135 drawframe creel, 83 drawframe, 155, 156, 324, 325 drawframe, cross-feed, 398 drawframe, ls, 220 drawing & condensation, 83 drawing (high speed), start up, 47 drawing elements, 70, 75, 79, 49

drawing error, 164 drawing fil, 47, 75 drawing head, 155 drawing operation, 156 drawing POY, 42 drawing sliver, 75, 397, 77 drawing tow, 13 drawing, ls, front roller system, 217 drawing, m.m.fibers, 77 drawing, number of slivers in creel, 457 drawn fibers, 11 draw-off aprons, ls, 218 draw-off cylinders, ls, 218 draw-texturing, 42, 46, 102, 103 draw-texturing, crimp-resilience, 102 draw-texturing, degree of setting, 102 draw-texturing, economics, 102 draw-texturing, fil flats, 102 draw-texturing, logistical problems, 102 draw-texturing, sequential, 102 draw-texturing, simultaneous, 102 draw-texturing, speeds, 103 draw-texturing, surges, 103 draw-texturing, yarn properties, 102 dry air, slubs and fishes, 288 dry spg, 38 dry steam, 342 dryness fraction, 342, 343 ducting, fiber strings, 148 ducting, 148, 346 ducting, egress of dust and fiber dust emission, 154, 156 dust filteration, 154 dust house, 150 dust removal, 117, 123 dust, fiber debris in air discharge, 150 dye affinity, 11, 21, 23, 78, 114, 284, 384 dye mandrels, 258 dye package, 234 dye sprigs (porous package centers), 234 dye streaks, 129 dye take-up see dye affinity dyed fil yarns attributes 362 dyed knitted sleeves, 361, 372 dyeing and setting operations, 344 dyeing operation, 114 dyeing, autoclave, package density, 257 dyeing, errors, 115 dyeing, hank, 115 dyeing, knitted test sleeve, 114 dyeing, peg frame, 234 earthing equipment, 349 eccentric front roll, 408 economic effects of automating, 313 economic mill operation, 9, 301 economics, storage, low value bales, 391 economy changes, 277 edge crimp in staple yarn processing, 85

Index edge crimping, 111 edge crimping, asymmetric quenching, 111 edge crimping, disoriented polymer, 111 edge crimping, quality control, 111 effect yarn, 19, 109 effective coefficient of friction, 59 effective electrode length, 356 effective fiber length, 78, 351, 352, 407 effects of shear, 84 efficacy of blending, 390 efficient light sources, 349 elastic & viscous forces, 383 elastic yarns, recovery properties, 369 elastomeric core, 112 electrical charges, 349 electrical energy, 342 electrified fibers coagulate, 349 electronic yarn boards, 362 electrostatic precipitators, 348 elongated plant cells, 23 elongation of specimen controlled, 364 elongational capabilities, 22 emerging fil cooled, 12 emerging fil drawn, 12 encoder, 418 end break, patrolling, 182 end break, roving frame, snow storm, 288 end breaks vs thins spots, 307 end breaks vs weak spots, 307 end breaks/cheese or cone, 315 end-break patterns, 291 end-break rate X length of down time, 307 end-break repair, 182 end-break, 69, 87, 174, 177–183, 183, 299, 308, 314, 314, 437, 442, 443, 448, 457 end-breaks & economics, 314 end-breaks & operator assignment, 307 end-breaks & quality, 298 end-breaks vs intolerable yarn faults, 315 ends breaks during patrol, 306 ends-down rate & defects/bobbin, 298 end-use, 20 energy balance in enclosed space, 346, 347 energy dissipation, 346, 347, 384 energy in joules, 342 energy input, electric motors, 346 energy removal from workspace, 347 energy source, bobbin, 177 energy, 341 English cotton system, 318 equalization of wages, 7 equation of fiber flow, 399 equipment maintenance, 424, 425 error amplitude, 407 error analysis, 294 error correlation, encoder, 292 error elongation, 408 error length limits, 362 error signal, 158

471

error error error error

source, 293, 294 spectrum, 291, 408 wavelength from bale plucker, 395 wavelength, 73, 77, 114, 166, 399, 407, 408 error wavelengths carried fwd, 293, 294 error, bobbin to bobbin, 294 error, fiber-related, 294 errors in drafting, 414 errors, non-periodic, 414 errors, short wavelength, 83, 355 errors, start up, 419 errors, very long, 361 eveness tester, 283 evenness, 79, 358 evenness, sliver 155, 158, 299 exceptions reporting, 158 experimental plan, 281 expert system, 419 extensibility, 12 external forces on softened polymer, 383 extruder auger (screw}, 41 extruder barrel heating, 44 extruder die, debris at exit, 46 extruder screw, 42, 43 extruder spinneret, 44 extruder start-up, 44 extruder, 11, 12, 43 extruder, tapered metal screw, 44 extruders, ganged, 13 extruding polymer, 38 extrusion as drawing,71 extrusion thro’ same nozzle, 112 extrusion, pressure in barrel, 42 eye protection, 180 fabric appearance, 19, 22, 276, 282, 284, 295 fabric attributes, 18 fabric barré, 19, 78, 84, 96, 97, 106, 114, 129, 172, 175, 284, 294, 299, 355, 392, 393, 395, 398, 404, 417 fabric blotchiness or streakiness, 78 fabric cloudiness, 19, 414 fabric cover, 322, 369 fabric defects, demerit points, 289 fabric defects, see fabric faults fabric durability, 276, 277 fabric faults, 289, 296 fabric filters, 348 fabric hand and appearance, 18, 101 fabric manf. problems, 254 fabric moiré, 294, 299, 414, 417 fabric patterning, 274, 275, 297 fabric shading, 114 fabric streaks, 114, 175, 284, 414 fabric weights calc, 321 fabric, burling & mending, 220 fabric, heat-set, 112 fabric, nature, 18

472

Index

fabric, seam pucker, 253 fabric, streaky, 274 fabric, woolen, felted, 220 fabric, woolen, milled, 220 false twist at rotor navel, 314 false twist spindle, 96 false twist spindle, suspension, 96 false twist spindle, tires, 96 false twist spindle, yarn drag, 96 false twist texturing, process stages, 384 false twist, 56, 62, 63, 92, 261 false-twist yarn, 384 false-twister, pin type, 94 fancy yarns, 19 fasciated yarn, false twist, 263 fasciated yarn, torque removal, 263 fasciated (wrapped) yarn, 263 fast Fourier transform (FFT), 166, 403 fault frequency, 283, 290 feed arrangements, 123 feed ribbons width, 264 feed roll damage, 425 feed systems, OE, 188 felting, 2 FFT analysis, 404 FFT of specific color wavelengths, 405 fiber-purchasing agent, 79 fiber & m/c interactions, 301 fiber, ls, damage, 206 fiber acceleration, 407 fiber accumulations, 123, 288 fiber acquisition policy, 281 fiber ageing, 47 fiber alignment, OE, 186 fiber array, 352 fiber attribute categories, 391 fiber attribute variability within flow, 164, 390, 392, 402 fiber attributes, 17, 50, 129, 280, 390, 394, 405 fiber avalanches, 420, 421 fiber batt, 117, 127 fiber blending, 48 fiber bonding along cuts, 51 fiber breakage during specimen prep, 353 fiber breakage, 28, 283, 413 fiber breaking stress, 353 fiber buckling, 104 fiber bulk, 50, 10 fiber buying policy, 301, 391, 392 fiber chokes, see chokes fiber classification, 21 fiber cleaner, inclined, 127 fiber cleaning, OE, 186 fiber cleaning, re-entry into stream, 126 fiber clump division, 116, 117, 122, 390, 413, 421, 425 fiber clump flow, irregular, 425 fiber clump size, 122, 123, 124, 400

fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber fiber

clumps, 117, 148, 412, 413 cohesion, 116, 413 color, 24 condensation as doubling, 117 condensation, 83, 117, 123 condensation, OE, 186 condenser, 75, 124 condenser, thin spots, 125 control, 262 control, chute feed, 134 control, error signal, 135 control, instability, 135 control, lag time, 135 control, linear density, delivery, 135 control, mechanical restraint, 79 control, on/off, 135 control, reserve box, 134 control, set point, 135 cost, 16, 301, 302 costs/yarn costs, 8 crimp, 4, 10, 116, 53 crimp, trailing ends, wool fiber, 382 damage, 128, 208, 221, 384 debond, 51 defects & drafting, 288 diameter variable, 205 distribution length, 397 dressings, 51 extent, 422, 423 feed systems, 117 fineness CVs, 166 fineness, 18, 19, 23, 24, 25, 280, 353 fineness, immaturity, 354 finish & treatments, 4, 47, 48, 51, 95, 116, 190, 191, 381 finish deposits, 288, 289 finish deterioration, 383, 384 finish, lighter fractions, 349 finish, ls, 206 finish, volatile fractions, 95 finishes as a size, 47 finishes formulations, 47 flats, 50, 383 fleeces, 117 flow control, 117, 133, 136, 407 flow, air turbulence, 148 flow, blockage, 133 flow, clumps, 399 flow, fractionation, 148 flow, shutdown cost, 133 flux, 455 handling, 128 helix angle, 323 homogenization, 117 hook removal, 81 hooks, 158, 189, 260, 353 in transit, specific volume, 123 laps, 174, 175 leading end, 407

Index fiber length determination, laboratory, 279 fiber length variability, 293 fiber length, 23, 25, 59, 79, 205, 206, 279, 351 fiber length, sampling, cotton bales, 279 fiber loss, 182, 327 fiber lubrication, ls, 207 fiber mass, specific volume, 126 fiber migration & blend, 399–401 fiber migration in twist triangle, 85 fiber migration vs evenness, 86 fiber migration, 58, 75, 84, 262, 264, 375, 382, 384, 400, 411, 457 fiber migration, longitudinal, 164 fiber migration, yarn bulk, 375 fiber mixing process, 396 fiber mixture, step change, 395 fiber movement control, 216, 407 fiber movement, draft zone, 407 fiber movement, groups, 421 fiber oiling, 216 fiber orientation, 155, 161 fiber passing through fan blades, 128 fiber percentages in mixer, 396 fiber populations, 164, 421, 422 fiber prices, 6 fiber processing, 71 fiber pulse, 396 fiber purchase, see fiber buying fiber quality control, 278 fiber reflectance, 284 fiber removal, layer by layer, 131 fiber restraining reaction, 410 fiber ribbon, optically measurement, 419 fiber rolls, accumulations, mill floor, 289 fiber selection, 312 fiber shuffling, 51 fiber slippage control, 56 fiber slippage, driving surfaces, 81 fiber stream, 123 fiber stream, specific volume, 122 fiber strength, 22, 25, 353, 388 fiber strings, 128 fiber structures, mechanics, 10 fiber tangles, 12 fiber tension, 374, 375 fiber testing, mass production basis, 392 fiber transfers, 87 fiber transport, 116, 123, 126, 128 fiber transport, OE, 186 fiber values in yarn, 391 fiber variability, 17 fiber yellowing, 281, 384 fiber, condensation, 123 fiber, consistent inventory in storage, 392 fiber, normalized flow, 396 fiber, physical characteristics, 114 fiber, re-cycled, 121 fiber, swollen & unswollen, 354

473

fiber, technical figures of merit, 391 fiber-crimp removal, 81 fiber-crimping, 50 fiber-finish level, 175 fiber-packing density variation, 381 fibers condensation, 128 fibers & fil, 18 fibers blend components lubricated separately, 211 fibers clumps, 390 fibers from moving zone, intermixing, 394 fibers migrate in drafting, 71 fibers swelling, caustic soda, 354 fibers, buckled, 50 fibers, carbon, 113 fibers, carpet, 10 fibers, detach from beard, 187 fibers, hooks, 158 fibers, short staple, 116 fibers, volume occupied, 10 fibrillation, 113 fibrils, 23 fibrogram, 352 fil curl, coil, or loop, 85 fil acceleration between godets,72 fil aspirator, 47 fil breaks, 46, 77, 96 fil changes due to age, 40 fil cooling before tension release, 49 fil cross-sectional shapes, 39 fil deformation, 89 fil drawing, 38, 42, 47, 114 fil dyeability, 4 fil flatting, 82, 384 fil lubrication, 47 fil minimum energy shapes, 384 fil processing, insufficient heating, 384 fil production, 11, 88 fil separation, 385 fil shrinkage, solidification, 39 fil spg speeds, 39 fil stability of POY, 42 fil strength, 42 fil take-off & drawing, 46 fil temperature control, 384 fil texturing process stages, 384, 385 fil texturing, edge-crimp, 85 fil texturing, torque, snarls & helices, 385 fil winding, 41 fil yarn production, 88 fil yarn, linear density, 368 fil yarns, 368 fil yarns, dye affinity, 282 fil yarns, polymer morphology, 282 fil yarns, twisted, 61 fil yarns, twisting & folding, 111 fil, aramid polymers, 113 fil, controlled conditions, 11 fil, dye affinity, 46

474

Index

fil, extruded, 11 fil, fibrillated, 113 fil, fibrillation process, 113 fil, glass, 113 fil, high-modulus, 113 fil, high-tensile, 113 fil, industrial, 113 fil, linear density, 45 fil, luster, 61 fil, minimum energy shape, 91 fil, multi-lobed, 113 fil, partially oriented (POY), 102 fil, setting, 90 fil, ultra-fine, 113 fil, winding, 90 fil, yellowing, 90 fil, 12 filtered polymer derivative, 40 filtration of flowing polymer, 38 filtration of signal output, 358 filtration, 348 finish & fiber deposits, 48, 114 finisher drawing, 159 finite element method of calculation, 440 fire and explosion risks, 348 fire code, 348 fixed costs, 308, 309, 312, 316 flammable airborne material, 348, 349 flatted fibers, constrained rotation, 383 flat-topped wire, 213 flat-waste, 287 flax (linum usitatissimum), 35 flax fiber fineness, 354 flax fibers, air permeability tester, 354 flax fibers, fiber division, 354 flax history, 1, 2 flax stricks, 231 flax use, 2, 35 flax, baling, 35 flax, bark removal, 35 flax, cells, 35 flax, curly top, 35 flax, fiber strength, 35 flax, hackled fiber stricks, 231 flax, hackling band, 231 flax, hackling tooth size, 231 flax, hackling, root end first, 231 flax, harvesting, 35 flax, linen cloth, 2 flax, lumen, 35 flax, natural drying, 35 flax, pulling machines, 35 flax, retting, 35 flax, scotching, 35 flax, seeds, 35 flax, stalks de-seeding, 35 flax, stooks, 35 flax, straw, 35 flax, wigwams, 35

flax, wilt (pathogenic fungi), 35 fleece of sheep, 31 floating fiber, 407, 410 flow charts, 13 flow of fiber passing thro’ reservoir, 395 fluted metal roll, 75, 125 fly deposited on textile material, 288 fly discharge, spg frame, 289 fly, 50, 156, 173, 282, 288, 346, 348 flyer speed, variable, 169 flyer twisting, 169 flyer, 168, 169 flyer, false twist, 169 flyer, presser arm, 169 flyer, roving support, 169 flyer, rubber grommet, 169 flyer, winding tension, 169 fog, 344 folding, 112, 220 folding, cable twisting, 111 folding, forming twist, 111 folding, see doubling folding, torque balance, 111 force equation, rotor spg, 440 force equation, stationary model, 440 force equation, yarn balloon, 440 foreign fibers, 31, 120, 278 fork frame, 208 formation of necks, 77 fractionation of fibers, 162, 164, 390 freedom from faults, 276 frequency domain, 357, 358 friction spg, 186 friction twister, 98 friction twister, torque generated, 99 friction twisting, cumulative torque, 100 friction twisting, damage, drive rollers, 100 friction twisting, disk changes, 101 friction twisting, disk penetration, 100 friction twisting, disks pumping yarn, 100 friction twisting, drive surfaces, 100 friction twisting, fiber finish, 101 friction twisting, fil breakage, 100 friction twisting, filamentation, 101 friction twisting, grooved ball, 102 friction twisting, humidity, 101 friction twisting, limits to speed, 100, 101 friction twisting, productivity, 101 friction twisting, run-off angles, 100 friction twisting, run-on angles, 100 friction twisting, stacked disks, 100 friction twisting, thread line angles, 100 friction twisting, urethane surfaces, 100 friction twisting, yarn damage, 100 friction twisting, yarn tension, 100 friction, ring & traveler, 432 frictional bonds, 72 frictional restraints, 373 front roll nip, 407

Index front roll velocity, 407 front roll, partial wrap of fibers, 174 fundamental wavelength, 291 funding, proceeds of sales, 309 g/9 km (denier), 318 gage pressure, 344 garnet, 221 gas equation, 343, 344 gear crimp, 53 gearing calc, 334 general mill expenses, 304 gilling (drawing, ls,), 216 gilling frames, productivity, 217 gilling, faller bars, 216 gin practice for cotton, 391 gin stand, 27, 28, 29 gin, ‘stick’ machines, 27, 28 gin, 24 gin, automatic feed control, 28 gin, bale presses, 28 gin, bales, 31 gin, boll traps, 28 gin, conveyor-distributor, 28 gin, disk-like saws, 29 gin, dryers, 28 gin, feed roller, 30 gin, feeder, 28 gin, fiber cleaning, 28, 29, 30 gin, fiber damage, 27, 29, 30, 31 gin, financial return, 28 gin, lint cleaners, 28, 31 gin, moisture content, 29, 30 gin, mote bars, 30 gin, out-turn, 30 gin, roller surface, 27 gin, saw blades, 27 gin, trash content, 31 ginning as a blender, 27 ginning byproducts, 27 ginning process, 25, 26, 117 glass fibers, 4 glass transition temperature, 20, 49, 381 goat hair, 31 godets, 46, 75, 82 godets, inclined pins, separate wraps, 82 Gossypium, 23 grains & yards, 318 greasy wool, 205 grids, 123 grin through, 16, 112, 260 growth rings, 23 guide pins, 75 hackling see flax, hackling hand, 10, 22, 206 handling cost, 309–312 handling vs fixed costs, 313 hank length, 318

475

hard ends, 69, 288 hardening of rubber coverings, 73 harmonic analysis, 358, 408 harmonic errors, 73 harvesting, 23, 26 head-to-tail package connections, 258 heat energy, 341, 342 heat flow, 342 heat flow from newly extruded fil, 42 heat from m/c directly exhausted, 347 heat loss, conduction, 346 heat loss, radiation, 346 heat removal by water washing, 347 heat setting, 384 heat transfer, 39, 42, 346 heating system, 346 heat-setting of fil, 85 heat-stretching, 49 Heilman (French) comb, 217 helical minimum-energy shape, 385 hemp (cannabis sativa), 36 hemp cultivation, 36 hemp history, 2 hemp, fiber strength, 36 high draft, 176 high volume instrument (HVI), 279, 351, 355, 368, 392 high-capital-cost machines, 309 high-friction materials, 100 high-speed equipment, 6 high-speed texturing, 42 high-volume fans, 128 history, 1, 5, 119 HOK & productivity, 7, 9, 10 hollow spindle spg, 66, 106, 270 home furnishings market, 5 homogenizing multiple streams, 83, 131 honeydew, 25 horsepower, 342 hot pin, 77 hot-fluid texturing, 106 human exploitation, 5 human intervention, 9 humidity, workspace, 344 hump magnets, 126 Hunter scale of +b, 406 HVI calibration, 355 hydrogen bonding, 341 hysteresis, 371, 384, 385 image analyzers, 354 immature fibers, 284 immigrants carry their skills, 5 imperfect gas or vapor, 343 indirect labor force, 309 induction of lateral forces, 56 industrial case study, 165 industrial materials, 22 industrial practice, m.m. staple fiber, 391

476

Index

industrial revolution, 1, 5 industrial yarns, 250 information revolution, 1 input electrical energy to heat energy, 346 insect secretions, 25 insects, destructive, 25 inspection tours, 299 insulation properties, 89, 12 integrated values, 356 integration and automation, 86, 87 interest rates, 316 interference between helices & snarls, 384 interference, tube/balloon, 178 interfiber entanglement, 76 interfiber friction, 59, 85, 383, 384 interlocking structure, 58 international competition, 315 interpretation of data, 277 intersecting breaker bars, 50 intersecting faller bars, 216 inventiveness, 5 inventory control, 47 inventory storage, 258 investments in equipment, 8 irrecoverable low-grade heat, 347 irregular fiber flows, 77, 407 irregular input slivers, 81 irregular polymer flow in drawing, 77 irregular yarn faults, 357 irregularity index, 411, 423 irregularity or unevenness, 282 irreversible fiber migration, 399 joy riders, 184 justification of extra capital cost, 313 just-in-time (JIT) shipping, 258 jute & polypropylene, 2 jute (Corchorus), 22, 36 jute history, 2 jute substitutes, 36 jute, (abutilon theophrasti), 36 jute, bark, 36 jute, cultivation, 36 jute, fiber luster, 36 jute, fiber strength, 36 jute, overlapping cells, 36 jute, retting, 36 jute, stripping, 36 keratin, 31 labor cost/unit weight, 7 labor costs by process, 303 labor costs, 87, 118, 301, 303, 312 labor costs, ring spg vs rotor spg, 305 labor costs, spg vs count, 304 labor needs, 309 laboratory (or off-line) testing, 350 laboratory testing, 158, 351

lanolin, 208 lap quality, 161, 163 lap ribbon, doubling, 161 lap size, 163 lap winder speed, 161 lap winding, ss, 159 lappet guide, see pigtail guide laps, split, 161 lap-up, 262 large package inside yarn balloon, 65 latent crimp, 92 latent heat, 342 lateral fiber migration, 85, 157, 260, 373 lateral fiber migration, see fiber migration lattice apron, 117, 123, 125 lattice card feed, 213, 223 lay gear, 65 laydown pattern, 392 laydown variations, 129 laydown-laydown variation, 389 lea strength = count-strength product, 368 leading & trailing hooks, 353 lean yarns, 262 length data series by choice, 362 length variable, 205 length/mass, 317 length-biased sample, 280 lighting, good reflectors, 349 limited total fiber denier, 49 linear density CV/theoretical values, 421 linear density errors, 400 linear density transducer, 157 linear density variance among strands, 422 linear density vs time elapsed, 356 linear density, 24, 57, 58, 317, 321, 324 linear density, capacitance of strand, 356 linear velocity of yarn, 57 linen thread, plying, 232 linkage, 155 linked spg, 118, 185, 255 linking & automation, 87 lint & linters, 24 lint and harmful dust removal, 348 lint collection systems, 174 liquefying by melting, 38 liquefying by solvents, 38 liquefying cellulose, 4 liquid flow rate, 45 liquor degradation, 208 liquor evaporation, 212 liquor flocculation, 212 liquor make-up, 208 liquor pH, 208 liquor temperature, 208 liquor troughs (or bowls), 208 load cell, 353, 364 load disribution among fibers, 375 load distribution between feeds, 82 long fibers, 15, 205, 207

Index long knitted dyed sleeve, color testing, 372 long staple, cutting tow, 51 long-chain molecules, 4, 20 longitudinal fiber migration, 85, 86, 165, 399, 400 long-staple combing, 217 long-staple roving & spg, 218 long-staple spg, 205, 218 long-staple yarn production, 15 long-term errors, 83, 118 long-term variables, 310 loss of production, 182, 307 lost production vs pneumafil levels, 308 lower portion of balloon, 444 lumen, 22 m.m. carpet yarns, 15 m.m. fiber bales, 216 m.m. fibers, 22, 38 m.m. fibers, history, 4 m.m. fibers, oligomers, 157 m.m. fibers, process r.h., 212 m.m. fil, 4 m.m. staple fiber production, 48 m.m. staple fiber, 4 m.m. fiber damage, 283 m.m. fiber, sampling procedures, 279 m.m. fibers, 8 m.m. fibers, drawn fiber, 284 m.m. fibers, finish concentrations, 284 m.m. fibers, oligomers, 284 m/c clogging, 128 m/c component speed calc, 339 m/c created errors, 407 m/c design, 309 m/c dimensions/fiber length, 218 m/c element eccentricities, 282 m/c element velocity, 321 m/c element, abrasion, 123 m/c element, maintenance, 123 m/c idle, 313 m/c maintenance, 174, 299 m/c productivity calcs, 324 m/c setting, 128, 299 m/c speeds calcs, 334, 335 m/c tooth size, 122 m/c wear, 283 m/c, OE, sliver input, 186 magnet to remove ferrous material, 126 maintenance costs, 304 maintenance cycles, 155 maintenance of cutter, 51 maintenance of roll settings, 79 maintenance staff costs, 308 management costs, 304 management of repairs, 291 market size, 15 markets, spun yarns, 9 married fibers, 46, 282

mass constant, 397, 398 mass distribution unevenness, 399 mass flow control by passive devices, 70 mass flow, 321 mass testing of cotton bales, 279 mass transfer, 39 mass, 341 mass/length, 317 matching productivities, 324 materials handling, 9 measurement of twist, 359 measurements on staple fibers, 351 measuring r.h., 346 mechanical & thermal history, 114 mechanical cleaning function, 117 mechanical draft ratio, 321 mechanical energy, 342 mechanical errors, 408 mechanical feeds, 125 mechanical properties, 10 mechanical working of melt, 43 mechanics of false twist, 62 mechanics of fiber wraps, 70 melting point (Tm), 85 melt-spg, 41 merge changes, 13, 392 meshing eccentric rolls, 408 metallic (saw-toothed) wire, 213 metering device, 45 metering pump leakages, 45 microbial pathogens, 32 micronaire index, 24, 280, 353, 354, 403 micronaire variations, dye affinity, 404 micronaire, error wavelength, 166 microscopic examination, 354 migration of solvent, 40 migration of the textile industry, 8 mill application of incoming bales, 392 mill as mixer, 389 mill balance, 183, 184, 313, 327 mill environment, 283, 346 mill management reports, accuracy, 307 mill performance, 310 mill pipeline, 397 mill processing, 17, 344, 345, 397 mill productivity target, 325 mill testing of incoming bales, 392 mill, fiber flow branches, 397 mineral & vegetable particles, 125 minimizing mill imbalance, 314 minimum cost, 316 minimum energy condition, 385 minimum irregularity, 423 minimum processing trouble, 350 mixed noil, 216 mixers, dispersion of fiber flow, 395 mixing machines, 395 mixing volume, 395, 397, 400 mixing within machines, 116

477

478

Index

modacrylics, 21 modified skein shrinkage test, 371 modified twist, 261 modified yarn structures, 260 mohair, 31 moiré effects, 114 moist air, 2, 344 moisture content of fibers, 212, 356 molecular dislocations, 85 molecular orientation, 42 molecular structure, polymers, 11, 12, 38 monitoring, 11, 158, 350 monitoring, computer linked, 158 Morel beaters, 213 Morel clearance, 213 morphological structure, 42, 114 moth & beetle larvae, 31 mothproofing, 32 moving interval, 166 moving zone variation, 400 moving zone, bale laydown, 394 moving zone, horizontal slices, 394 mule spg, 218, 230 multi-function fiber measurement, 368 multiple doublings, 205 multiple drawings, 424 multiple parallel fiber flow paths, 324 multiple weak spots, 413 multiple, consecutive draw zones, 72 narcotics & fiber, 2 natural draw ratio, 47, 77 natural fibers, 22, 393 natural polymers, 20 navels, ceramic, 195 neck position, stabilize, 77 needle melts, 16 nematode, 25 nep control, 147 nep creation, 25, 123, 128, 458 nep, 10, 19, 23, 281, 283, 284, 285 nep, OE combing roll, 285 neps in card web, 281 neutralization of acid, 41 new technology, 303 Nickerson-Hunter colorimeter, 130 nip line oscillation, 408 nitrocellulose fil, 4 noil, 151, 162, 163, 287, 390 noise level, 181, 216 non-fibrous material, 284 non-lint material, 122, 150 non-oxidizing atmosphere, 384 non-reworkable waste, disposal, 150 non-wovens, 22 normalized productivity. see HOK, 7 normalized variance, 381 noxious chemicals, 348, 349 nubs, 19

number of bobbins w piecings, 315 number of m/cs needed, 324, 325, 327, 328 number of piecings, 315 number of spinnerets, 13 numbering ply yarns, 319 nylon, 21 obliquity curve, 58 OE history, 186 offtake passage over a bale, 396 OHP, 304, 308 OHP/count, 306 oiled wool fibers, coeff friction, 381 oily soiling, 4 oligomers accumulations, 349 on line monitoring, capital cost, 291 on-line monitoring yarn defects, 351 on-line monitoring yarn hairiness, 351 on-line monitoring, 290, 291, 300, 350, 351, 358 on-line sampling, 362 open-end (OE) spg, 67, 185 open-end spg, brief history, 453, 454 opening & carding, trash removal, 283 opening & cleaning, 125, 153 opening line & carding, 425 opening line layout, 119 opening line, 116, 122, 153, 310 opening line, mixing, 394 opening of new markets, 5 opening, 117, 118 operation without undue disruption, 50 operational factors, 313 operational flexibility, 117 operational phases, 118 operator assignment & OHP, 309 operator assignment, 299, 306, 307 operator efficiency, 299 operator hrs/kg product (HOK), 303 operator hrs/lb product (OHP), 303 operator hrs/task, 303 operator patrolling, 306 operator perception of delayed quality factors, 306 operator productivity, 304, 305 operator productivity, history, 303 operator training, 298, 299 operator wage rate, 306 optical character, 355 optical masks, 363 optical testing of flowing strand, 356 optimum cost, 312, 313 orbital movement, 70 order book, 314 organochlorine compounds, 32 orientation, 70 oscillating rolls, 68 oscillation of the neck, 78 overall draft, 72, 321

Index overall draft ratio, OE, 188 overdrawing, 424 overfeeding, 92 overhead costs, 301, 304 overhead rail systems, 184 oversupply, 4 package build, 181, 235 package chains in feed, 258 package density, 234, 235, 257 package dyeing, 234 package moisture content, 257 package shoulders, 239 package size, 250 package size, OE, 186 package storage, 234 package structure, 234, 235 package structure, periodic change, 237 package transport, 234 package, cross-wound, 234, 235, 237 package, density variation, 238 package, inter-yarn forces, 237 package, ribboning, 238, 239 package, stability, 237 package, traverse motions/revolution, 239 package, unwinding, 234, 257 package, yarn interlace, 237 papilla, 31 parallel portions of yarn loaded, 366 partial pressure of moisture in air, 344 partial pressures, 343 partially oriented yarn (POY), 42, 47, 102 partial-wrap design to improve grip, 81 particulate level in air, monitoring, 349 passage of drawing, 76, 159 pathogenic fungi, 25 patrol events, 253 patrolling sensor, 291 payback time, 309 pendulum tester, 368 perfect commodities, 8 perforated apron, 262 perforated hollow front roll, 262 performance correlations, 307 performance of machine in serial line, 293 performance vs spindle assignments, 306 periodic errors of random phase, 398 periodic shearing, 33 periodic variations, 76, 114, 115, 281, 397, 398, 408, 425 periodogram, 358, 402 permeability, 12 personnel costs, 308, 310 Peruvian Tanguis, 23 pesticide, 25 pests, 23 phase change of blend components, 400 phases, OE, 186 phasing errors, 398

479

physical basis of texturing, 89 piecer, mechanical, 185 piecing 1 for each bobbin used, 315 piecing, 179, 182, 282, 283, 287, 309, 310, 315 piecing, automatic, 190 piecing, fault source diagnosis, 287 pigtail guide, 177, 178, 268, 427, 446 pigtail guide, rotating, 446 pilling, 4, 388 Pima cotton, 23 pin drafters, 216 pin twisting, economic limitations, 98 pinned feed systems, 208 pinned rolls, 125 pinning density, 216 pin-twister machine, limitations, 96 pin-twister, 93, 97 planning, 9 plied ply yarns, 61 plied yarns, 253 ply direction, 60 ply twist, 60 ply yarn numbering, 332 ply yarns, 250 Plyfil system, 265 Plyfil yarns, index of irregularity, 270 Plyfil yarns, twist balance, 270 plying (doubling), 65 plying 2 fold yarns, 251 plying by two-for-one twisting, 65 plying, unwanted three-fold yarn, 251 pneumafil collection, 175, 182, 308, 346 pneumafil devices, 262 pneumafil production, 291, 416 pneumafil waste, 150, 287 pneumafil, fiber crimp & elongation, 416 pneumatic separators, 126 pneumatic trumpets, 417 polyacrylonitrile fibers, 38 polyamide fil, 4 polyamides, 21 polyester, 21 polyester/cotton, 5 polyester/wool, 5 polyethylene, 22 polymer cross-linking, 44 polymer debris, 282 polymer degradation, 42 polymer derivative ripening, 40 polymer discoloration, 38, 384 polymer drip, 45, 282 polymer feed, continuous molten, 42 polymer filtering & pumping, 41 polymer filtration, 43, 45 polymer melted in barrel, 12, 41 polymer metering, 12 polymer morphology & dyeing, 114 polymer morphology, 84, 277

480

Index

polymer over-heating, 42 polymer oxidation, 38 polymer pressurize, 12 polymer pumps, 42 polymer solidification, 38 polymer solution, 40 polymer supply, 12 polymer transport, 41 polymer viscosity, 41, 42, 45 polymer, liquefying, 38 polymer, orientation, 38 polymer, visco-elastic properties, 85 polymeric materials, 20 polymeric structure, elongation, 112 polymerization/spg systems, 42 polyolefins, 22 polypeptide, 31 polypropylene, 22 polypropylene, UV degradation, 113 polyurethanes, 22 poor cleaning procedures, 288 poor spg, poor winding, 315 portable hygrometer, 346 post-spg processes, 234 power cost, 301, 303, 311, 312, 342 POY, 47, 102 poymer filtering, 12 precision winding, 242 premature fiber acceleration, 81 preparation costs, 304 pressure of the steam, 344 pressure on inner fibers, 57 pressure on stationary cushion rolls, 73 prickle, 10 probability distribution of tenacity, 314 probability distribution, applied stress, 314 probability of piecings, 315 process costs, 212 process efficiency, 324 process integration, 86, 87 process of elongation, 71 processing error, 281 processing variations, 390 processing, ls, large diameter rolls, 206 product delivery, 8 product judgment in dyed state, 393 product judgment in the greige state, 393 product labeling, 300 product price, 306 product sampling, 300 production calcs, 336–338 production of polymers, 9 production of viscose rayon yarn, 4 production speeds, 42 production systems, 10 production, 24 hr/day, 155 production, m.m.fibers, 38 productivity gains, 1 productivity, 15

productivity, 303 productivity, normalized, 303 productivity, opening line, 324, 326 productivity/spindle, 308, 309, 327 profit (or loss), 301 profit margins, 276 prompt delivery, 258, 316 proper ventilation, 349 prosperity variable, 1 prudent recycling, 287 psychrometric charts, 344 publicly financed companies, 5 pumps, 38 purpose of twist, 56 purposes of drawing, 70 purposes of texturing, 89 quality & price, 276 quality & quality control, 276 quality and economics, 306 quality assurance, customer protection, 287 quality audits, 159 quality control & testing, 350 quality control program, 278 quality control, 17, 185, 276, 350 quality control, intermediate products, 281 quality deficiencies, market, 306 quality factors, 278 quality of product, 8, 315 quality, determined by the user, 276 quality, fabric attributes, 276 quality, fiber attributes, 276 quality, yarn attributes, 276 quasi-random variations, 71, 403 quasi-stationary balloon, 439 quench, unequal cooling, 46 quenching, 46 r.h., 344 radial compression, 373 radius of curvature, on yarn surface, 374 rag picker (shredding), 221 ramie degumming, 36 ramie (B. Tenacissema), 36 ramie (Boehmeria Niva), 36 ramie decortication, 36 ramie, fiber removal from stalks, 36 ramie, fiber strength, 36 ramie, thick-walled cells, 36 random error, 76, 83, 293, 389, 408, 425 random speed varying devices, 109 rapid technological change, 3 rare-event stoppages, 155 ratch setting, 49, 51, 206 rates of improvement, 303 ratio of linear densities, 321 real & false twist, 61 real blend variation, 402 reason for testing, 350

Index re-break long fibers, 50 re-condensation, 67 recording mass variations, 358 rectilinear motions, 162 recycled blended fibers, 286 recycled polymers, 286 recycling waste, max percentage, 133 recycling, fiber identification, 221 recycling, limited by regulation, 221 refrigerated air-conditioning system, 347 regenerated fibers, 20 regional balance of technology, 304 regularity, see evenness reinforcement of composites, 113 relative conditions undergo change, 316 relative costs, 302 relative movement of clamps, 353 relative movement of segments of molecules, 85 relative rotational speeds, 177 reliability, 316 remnants of sliver & roving, 150 reputation, 276 research, 8 residual fault rate, 287 residual twists, 384 residual vegetable matter, 210 residues of salt, calcined, 212 resultant forces acting on fibers, 57 return air systems, 346 returned shipments, 258 review of processes, 16 re-workable waste, 151, 392, 416 Reynolds number, 438, 439 ribbon breaking, package lift, 236 ribbon breaking, package oscillation, 236 ribbon breaking, slippage, 236 ribbon breaking, variable drive speed, 236 ribbon lap machine, 161 ring & traveler, 65, 178 ring & traveler, force analysis, 433 ring & traveler, micro-welds, 432 ring & traveler, run-in, 433 ring & traveler, sliding track, 432 ring & traveler, wear, 433 ring bobbin wind, 243 ring bobbins, temporary storage, 450 ring bobbins, unwinding, 450 ring burn, 442 ring damage, 180, 442 ring flange, 178 , 179 ring frame productivity, 327 ring frame, monitoring, 291 ring frame, winder head balance, 255 ring life, 443 ring rail lift, 178 ring rail, 177 ring run in, 180 ring size, 182, 183

ring spg, twist triangle, see twist triangle ring spg, 168 ring spg, costs, 304 ring spg, forces acting, 181 ring spg, m/c, 205, 218, 230 ring spg, real twist, 62 ring spg, yarn above pigtail guide, 433 ring spindle productivity, 177 ring tube storage, 180 ring tube, 175, 234 ring tube, package structure, 451 ring yarn packing density, 381 ring yarn, self-locking structure, 85 ring, lubrication, 179 ring, micro-welds, 179 ring, run-in, 179 ringframe limitations, 182 ring-rail, 65 rings centered, 179 ring-spg m/c, 175 ring-spun yarn, structure, 260 rise in wage costs, 6 risk & cost, 258 roll ageing, 175 roll buffing, 156, 174, 416 roll circumference/fiber length, 174 roll damage, 49 roll eccentricity, 291, 408, 415 roll laps, 206, 349 roll layouts, 80 roll pairs, 125 roll separation transducer, 420 roll setting, 78, 420 roll size, 175 roll squeeze on soft fil, 82 roll weighting, 174, 175, 217 roller drafting system, 73, 171, 407, 414 roller drafting, doubling, 422 roller errors, 73, 76 roller gin, 30 roller or godet defects, 76 rolls, deposits of oil or grease, 175 ropes & cables, 113 rotary gills, 216 rotating condenser, 117, 123 rotating rings, 179 rotating sliver passage, 157 rotating spikes or teeth, 121 rotational speed of twister, 57 rotor bearing systems, 196 rotor cleaning, 190, 198 rotor collects fibers, 67 rotor deposits, 192 rotor doubling, 425 rotor fiber flow, 192 rotor groove lapped by rotating yarn, 193 rotor groove, 193 rotor groove, fiber ring, 193, 198 rotor input, fiber flow, 186

481

482 rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor

Index m/c, productivity, 306 piecing, diagrams, 199 piecing, end conditioning, 198 piecing, introduce starter yarn, 198 piecing, sliver end conditioning, 199 piecing, start up, 198 piecing, timing, 198 productivity, 328 productivity, OE, 67 spg, 9, 56, 186, 347, 425 spg, abrasive fiber or dust, 203 spg, acrylic fibers, 202 spg, air bearings, 196 spg, airflow, 189, 192 spg, assembly of fibers in rotor, 192 spg, automatic doffing, 198 spg, automatic piecers, 187, 305 spg, automatic rotor cleaning, 198 spg, automatic start-up, 198 spg, blend evenness, 456 spg, blend yarns, 202 spg, bridging fibers, 200, 459 spg, built-in monitoring, 198 spg, bunch winder, 198 spg, capital cost /lb yarn, 187 spg, capital cost, 186 spg, centrifugal force, 193 spg, channel damage, 193 spg, chokes, 191 spg, cleaning aperture, 192 spg, cleaning capability, 189 spg, cleaning cycle, 191 spg, cleaning edge, 188, 189, 192 spg, combing roll housing, 192 spg, combing-roll clothing, 457 spg, conical transfer surface, 459 spg, cotton fibers, 190 spg, cranked doffer tube, 462, 463 spg, CV of linear density, 456 spg, debris in rotor, 190 spg, dirt box, 189 spg, doffing tube, 193 spg, doublings, 455 , 456 spg, end-breakage, 196, 201 spg, equipment maintenance, 456 spg, false twist, 459, 462 spg, feed roll & plate, 457 spg, fiber assembly, 459 spg, fiber breakage, 457 spg, fiber cleaning, 191, 202 spg, fiber crimp, 201 spg, fiber damage, 189 spg, fiber extent, 193 spg, fiber fineness, 201 spg, fiber finish, 201, 202 spg, fiber flux, cyclic variations, 457 spg, fiber length, 200, 201 spg, fiber peeling point, 459 spg, fiber requirements, 200

rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor rotor

spg, fiber tenacity, 201, 202 spg, fiber-transport channel, 192 spg, financial return, 191 spg, glazed belt, 191 spg, hooked fibers, 193 spg, hot rotor, 192 spg, incipient yarn in rotor V, 461 spg, ingoing fiber/outgoing yarn interfere, 193 spg, large yarn packages, 197 spg, load carrying ability of wrappers, 459 spg, long-term errors, 457 spg, low-friction finishes, 201 spg, m.m.fiber, 189, 190, 201 spg, m/c element abrasion, 189 spg, m/c element operating lives, 203 spg, m/c maintenance, 203 spg, m/c transmissions, 197 spg, navel, 194–196 spg, nep production, 190 spg, noil, yarn quality, 201 spg, number of fiber in X section, 456 spg, oligomer, 202 spg, organized errors, 457 spg, overheated whorl, 191 spg, piecer control, start up, 200 spg, piecing, 186, 198 spg, Poisson distribution, 456 spg, preparation errors, 457 spg, prepared starter tubes, 198 spg, random variations, 457 spg, real + false twist, 194 spg, recognition of end breaks, 462 spg, retention of twist in rotor, 462 spg, rotating navel, 194 spg, rotor debris, 191 spg, rounded groove, 462 spg, single tape drive, 191 spg, sliding contact, 193 spg, sliding path of a fiber, 459 spg, sliver feed, 189 spg, sliver loop in feed, 191 spg, sliver weight, 189 spg, slivers preparation, 456 spg, speed, 186 spg, spin limit, 201 spg, standard deviation, 456 spg, starter-yarn bunch, 198 spg, start-up devices, 200 spg, stationary navel, 194 spg, suitable fibers, 188 spg, tapered duct exit, 193 spg, transfer channel, see transfer duct spg, transfer duct, 189, 193 spg, transfer yarn tails, 198 spg, transient twist depletion, 462 spg, transport of full bobbins, 198 spg, twist multiple, 195, 201

Index rotor spg, twist propagation, 461 rotor spg, twist surges, 462 rotor spg, twist trap, 194 rotor spg, twist traps, 462 rotor spg, vector addition, 457 rotor spg, waste fibers, 202 rotor spg, wedge action of V groove, 193 rotor spg, winding, 197 rotor spg, wrapper fiber production, 459 rotor spg, yarn count, 202 rotor spg, yarn formation, 192 rotor spg, yarn handling costs, 197 rotor spg, yarn package, 197 rotor spg, yarn rolls in navel bore, 194 rotor spg, yarn take-up, 197 rotor spg, yarn torque, peeling point, 461 rotor spg, yarn withdrawal tube, 462 rotor suction, 189 rotor supports fibers, 67 rotor yarn packing density, 381 rotor yarn structure, 361, 378, 379, 381, 460 rotor yarn, 378, 459 rotor yarn, bridging fiber, 460 rotor yarn, bulkiness, 379 rotor yarn, calculated machine twist, 379 rotor yarn, ceramic guide, 381 rotor yarn, end-breakage, 189 rotor yarn, evenness, 189 rotor yarn, false twist, 460 rotor yarn, fiber helix angles, 378 rotor yarn, geometry of navel, 461 rotor yarn, hairiness, 189, 379 rotor yarn, nep production, 189 rotor yarn, projected fiber length, 460 rotor yarn, rotating navel, 460 rotor yarn, rotor V, 461 rotor yarn, tenacity, 189 rotor yarn, twist distribution, 460 rotor yarn, twist levels, 378 rotor yarn, untwisting, 379 rotor yarn, yarn waists, 460 rotor yarns, calculated twist, 360 rotor, air r.h. inside rotor, 192 rotor, air stream, 187 rotor, draft ratio at feed, 187 rotor, fiber condensation, 187 rotor, fiber sliding path, 193 rotor, OE, 67 rotor, real yarn twist creation, 193 rotor, sliding wall, 193 rotor, yarn twist runs back, 193 rounding results, 325, 327 roundness of drafting elements, 73 roving bobbin rotation, air pump, 289 roving costs, 304 roving count, 172 roving draft, 173 roving frame, 168 roving m/c, 218

483

roving m/c, bobbin-lead, 169, 171 roving m/c, break out supply, 173 roving m/c, end breaks, 173 roving m/c, fiber discharge, 173 roving m/c, fly, 173 roving m/c, flyer-lead, 169, 171 roving m/c, grommet, 172 roving m/c, headstock, 171 roving m/c, lay gear, 171 roving m/c, relative winding speed, 171 roving m/c, shut down, 173 roving m/c, size, 171 roving m/c, snowstorm, 173 roving m/c, spindle rows, 172 roving m/c, traverse, 171 roving m/c, traversing blowers, 173 roving m/c, traversing suction nozzles, 173 roving m/c, winding-on speed, 171 roving package damage, 169 roving productivity, 56, 325 roving quality, 299 roving size, 171 roving stop mechanism, 291 roving stop system, 182 roving testing, 281 roving twist, 69, 168, 169, 171, 173 roving, bobbin density, 169 roving, end breaks, 171 roving, hard ends, 173 roving, package, 171 roving, twist set, 173 roving, variable wind-on speed, 169 roving, winding tension, 169 rubber cots, flats, 415, 419 rubber covered roll, 75, 157, 216 rubber hardness, 174 running speeds, coeff of friction, 381 rust, 344 safety, 119, 129, 153, 154 sales & price, 8 sales system inertia, 258 sales transaction chains, 8 sample conditioning, 350 samples, 84, 276, 279, 350, 390, 422 sampling frequency, 287 sandwich blender, 131, 132 saw teeth, 127 scanning laser, yarn arrays, 372 schappe, (spun silk), 38 scouring liquor, 208 scouring, 15 Sea Island cotton, 23 seasonal changes, 392 seed & non-lint material, 30 seed coat fragments, 125 segmented polyurethane, 112 segments of long-chain molecules, 84 segregation, non-lint & usable fiber, 150

484

Index

self twist (ST) yarn structure, 379, 380 self twist m/c, 271, 273, 274 self-twist m/c, costs, 274 self-twist m/c, oscillating front roll, 274 self-twist m/c, productivity, 274 self-twist m/c, roller-drafting, 274 self-twist m/c, shuffling/twisting rolls, 274 self-twist spg, 271 self-twist spg, plied worsted rovings, 271 semi-worsted systems, 219 sensor transport systems, 291 sensors in machine, 350 sensors, 350 sensors, insensitivity to environment, 350 separation of fibers, 425 separation of winding and twisting, 65 separator plates, 177 sequential flow of batches, 424 service, 8, 276 settling tanks, 209 severe drafting action, 67 sewing m/c, needle melts, 252 sewing thread, cotton, gassed, 251 sewing thread, cotton, singed, 251, 252 sewing thread, 16, 250, 251, 252 sewing thread, cotton, plied, 252 sewing thread, dye affinity, 253 sewing thread, guide clogging, 252 sewing thread, linen, 252 sewing thread, mercerized, 252 sewing thread, needle eye clogs, 252 sewing thread, Nomex, 252 sewing thread, silk, 252 sewing thread, waxed, 252 sewing thread, yarn hairiness, 252 shear effects, blend, 85, 400 shear rate in extrusion zone, 42 shearing of two pinned surfaces, 207 sheep breeds, 33 sheep confined for clip, 33 shelf-life, 48 shipping costs, 8, 258, 316 short fiber acceleration, 410 short fiber content, 164, 353, 417 short fiber removal, 59, 161 short mechanical process, 11, 87, 303 short staple, cutting tow, 51 short-fiber content, 280, 281, 308, 403 shorthand twist notation, 60 short-staple cutter, 52 short-staple yarn production, 14 shrinkage, 321, 381, 384 shuffling rolls, 68 signals for information, 350 signals, outliers, 359 silk & denier, 37 silk & man-made fibers, 2 silk ‘books’, 37 silk attributes, 2, 37

silk cultivation, 37 silk glands of larvae, 37 silk harvesting, 37 silk history, 1 silk reeling, 37, 38, 113 silk, Bombyx, 37 silk, cottage industry, 113 silk, fibroin (amphoteric colloid), 37 silk, fibroin extrusion, 37 silk, fibroin ripening, 37 silk, fil, 37 silk, filature, 113 silk, gum as a size, 113 silk, hanks, 113 silk, long-chain molecules, 37 silk, sericin (natural gum), 37 silk, spg, 38, 113 silk, staple fiber, 37, 113 silk, throwing see silk spg silk, yarn production, 37 silk, yellowing, 37 silkworm (lepidoptera), 37 simple draft calculation, 320, 321 singeing, 251, 252, 349 single-end tests, 365 sinusoidal blend error, 400 Sirospun spg, 219, 268, 269 Sirospun spg, cost, 269 Sirospun yarn, 270 Sirospun yarn, twist balance, 270 Sirospun yarn, weavability, 268 sisal & manila hemps, 36 skein dyeing, 236 skein shrinkage, 371 skein testing, 366 skeins weights, 355 skirting, 33 Sl system, 341 slack fibers buckle, 375 slippage between rolls & fiber, 81 slippage, 321 sliver attributes, 76 sliver ball, 217 sliver can, 148, 149, 155, 156, 159 sliver can, crushed sliver, 149 sliver can, piston, 149 sliver cohesion, 75 sliver condensation, 157 sliver condenser, 156 sliver CV, fiber attributes, 84, 161, 404 sliver doubling, 156 sliver drafting, 156 sliver handling, 184 sliver lapper, 161 sliver output, 118 sliver preparation, 155 sliver ribbon, 161 sliver sampling, 158 sliver storage, 191, 148

Index sliver tension, 161 sliver testing, 281 sliver trumpet, 157, 418 sliver, 14, 117, 148 sliver, carded, 155 sliver, color striations, 406 sliver, combed, 155 sliver, ls, samples, transport, 281 sliver, mass sensors, 217 sliver, over-worked, 158 slivers, roll grip variation, 161 slow adoption of new technology, 313 slub creation, 283, 420 slubbing (roving), 216 slubbing, cheeses on mandrel, 230 slubbing, drafting, 230 slubbing, winding, 230 slubbings, rubbed for fiber cohesion, 229 slubs, 69, 288, 361, 421 slubs, periodic outbreaks, 421 sludge centrifuges, 209 sludge disposal, 212 smoothing length, 389 smoothing, drawframe creel, 389 snagging & pilling, 388 socialism, 6 softening point (Tg), 85 soiled wool, 33 soluble organic salts, 212 solvent distributions, 40 solvent removal, 39 solvent scouring, 209 sources of defects, 282, 288, 349, 399 space costs, 301 span length, 352, 353 spark-free motors, 348 specific enthalpy, 343 specific heat, 341, 342 specific volume of steam, 344 spectrogram array, 292, 294 spectrogram, 73, 158, 291–293, 296, 356–358, 408, 420 spectrum of sinusoidal variations, 399 spectrum of wage rates, 7 speed frame (roving frame), 216 spg bobbins, mixing, 294 spg cost vs yarn count, 305 spg efficiency, 300 spg end-breaks, 289, 298, 299 spg faults produced/hr, 300 spg faults, 278 spg frames, 15 spg m/c substitution, 312 spg productivity, 314 spg tension, 178 spg, cotton, 12 spg, individualized fibers, OE, 67 spg, long-staple, 12 spg, process, OE, 185

485

spg, short-staple, 12 spg, ss, 168 spg, wool, 12 spiky trash, 127 spin finish, 41, 47 spin limit, 316 spindle assignment, 253, 298, 308 spindle eccentricity, 180, 181 spindle life, 181 spindle productivity, 183 spindle speed, 176, 180, 181, 183 spindles required, 183 spindles/machine, 327 spinline, 11 spinneret blockage, 45 spinneret die, 45 spinneret, 12, 38, 39 spinners assignment, see spindle assignment spinner’s costs, 234 spinpacks, 42 spiral cutter, 51 splice tails, 288 splicing chamber, 248 squeeze rollers, 208 stable twist in flowing yarn, 63 stages of drafting, 321 stages of drawing, 216 stages of processing, 6 stages of production, chain reactions, 407 standard fabric thickness tester, 371 standard of quality, 277 standard tests, 279 standards of living, 7 staple fibers, 18, 294 staple fibers, man-made, 11 staple length, 59 staple vs fil, 22 staple yarn production, 13 staple yarn structures, 373 staple, drawing, 14 staple-yarn manufacture complex, 11 staple-yarn systems, 11 static electricity, 4, 47, 174 static loading, 75 statistical control techniques, 355 steam pressure & temperatures, 341 steam properties, 342 stepless drive motors, 217 stiffness & bulk, 10 stiffness of fiber loops, 18 stiffness of hairs, 18 stock control, 79 storage, r.h., 216 strand flow thro’ a torque zone, 62 strand(s) wrapped around a core yarn, 66 strap & bale covers removal, 120 streakiness, 21, 129 stream orientation, 46 strength of POY, 42

486

Index

strength of a twisted bundle, 58 stress decay, 371 stress removal by polymer softening, 385 stress-free, helical condition, 385 stretch breaking for m.m. fiber, 13, 217 stretch yarn, 369, 371, 385 stretch, 12 stretch-break, mill based, 13 stretch-breaking system, 15, 48 49, 50 stringing-up, 95 strip charts, 356, 357 stroboscope, 175 stuffer box, 50, 53, 105 stuffer-box texturing, 104, 105, 106 stuffer-box yarns, plied, 105 stump cotton, 23 substitution of capital for labor, 313 suceptability of m.m. fibers to pilling, 388 suceptability of m.m. fibers, snagging, 388 suction scour m/c, 210 sufficiency of samples, 390 suint liquors, evaporated, 212 suint, 32, 208 supervisory costs, 308 surface abrasion of fabric, 388 symbols, 319, 324 synthetic fiber, history, 4 synthetic polymer, 20 synthetic staple & natural fiber blends, 5 systematic series, thick & thin places, 282 take-up rolls, 157 take-up speed, 45 tape condenser, 229, 230 tape drive, slippage, 458 tariffs and quotas, 8 tastes & ability to buy, 7 technical analyses, marketing, 277 technical analyses, problem solving, 277 technical fil, 4 technical service, 8 technology of production, 10 temperature distributions, 40 temperature, 341, 344 temperature, abs zero, 341 tenacity, 58 tensile tester, stiff system, 364 tensile testing of strands, 364, 368 tension control, 69, 217, 451 test fiber immaturity, 354 test laboratory, air-conditioned, 350 test rotor yarn, 379 testing by observation, 369 testing fiber during transit, 368 testing fil yarns, 351 testing fil yarns, dyed knitted sleeve, 362 testing for yarn defects, 290, 354, 355 testing linear density, weighing, 355 testing natural fibers, 279

testing of textile materials, 350 testing rotor yarn, untwisted, 359 testing single strands, 364, 366 testing staple yarns, 351, 360 testing textured yarns, 369 testing yarn bulk, water immersed, 370 testing yarn hairiness, 363 testing, 8, 276 testing, organization, 300 testing, supplier & supplied agreed, 280 tests influenced by r.h., 350 tex, 318 textile materials, 1, 18 textile products & fiber production, 18 texture, 10 textured yarn production, 12 textured yarn properties, 91, 368, 369 textured yarn structures, 362, 369, 383 textured yarn, 12 texturing calc, 339, 340 texturing, 4, 21, 56, 89, 90, 92–98, 110 texturing, air-jet, 13 texturing, co-extrusion, 112 texturing, cooling length, 94 texturing, disk twister, 99 texturing, false-twist & air-jet, 106 texturing, false-twist, 13, 98 texturing, fil handling, 94 texturing, friction twisting, 98 texturing, heater length, 94 texturing, knit-de-knit, bulk, 112 texturing, theoretical model, 90 texturing, threadline, 94, 95 texturing, tight spots, 95, 97 texturing, twist surges, 99 texturing, two-heater machines, 96 texturing, wear rates, 95 texturing, wild fil, 98 texturing, yarn attitude, 99 texturing, yarn quality, 98, 110 theoretical assignment, 308 theoretical CVs, fiber groups, 419 theoretical twist, 61 theory of blending capacity, 395 theory of doubling, 83 thin jets of polymer, 12 thin spots, twist, 177 throwing, 88 tight spots, 81 time domain, 356 titanium dioxide, 48, 95 TM/tpi relationship, 323 tongue-and-groove system, 418 toothed drafting, 122, 407, 412, 413, 457 top making, 217 top roll hardness, 156 top, 49 torque/twist curve, 383, 384 torsional stiffness, 68

Index total variance, 281 tow and man-made staple, 13 tow knotting, 53 tow supplied to mills, 48 tow, 13, 15, 48, 49, 53 tow, drawing, 82 tow, stretch-break, 13 tow-to-top, 51, 220 tow-to-yarn, 51 tracer fiber, 457 tracing source of faulty bobbins, 255 transducer cost, 417 transducer time response, 417 transducer, 158, 159, 359, 364, 417 transfer costs of textile product, 6 transfer systems, automated, 7 transient can storage, 159 trans-oceanic shipping, 316 transport material, 184, 310 trash content, 281, 355 trash crushing, 127 trash in card flats, 284 trash reintroduced into fiber stream, 127 trash removal, 117 trash, 30, 117, 284, 286 traveler & balloon collapse, 443 traveler balance, 180, 181, 442, 443, 445 traveler change schedule, 180 traveler life, 180, 298, 442, 443 traveler mass vs yarn count, 443 traveler mass, see traveler weight traveler moment balance, 442 traveler numbering, 181 traveler porpoising, 443 traveler scar, 443 traveler vs yarn tension, 443 traveler weight, 177, 181, 433, 440, 443 traveler, 177–183, 427, 433, 442–444 traveler, centroid, 443 traveler, fiber build-ups, 179, 283, 289 traveler, kinetic energy, 433 traveler/ring, friction, 443 traveling cleaner, 288 trend analysis, 359 trumpet condenser, 162 tube goemetry, 178, 180 tuft curves, 280 turbulent airflow, 106 twist & flow, 57 twist angle, 323 twist balance, 268 twist calcs, 335, 336 twist carried by moving yarn, 62 twist contraction, 257, 360 twist density, 57, 323, 324 twist direction, 57 twist evenness, 68 twist factor (α), 324 twist gear, 57

487

twist twist twist twist twist twist twist twist twist twist

gradients, 177 in rotor spg, 193 insertion, 61 liveliness, 60, 68 migration, 68 multiple, 57–60, 206, 323, 324 projected by twister, 62 testing, 360, 446 to control fil, 61 triangle, 82, 174, 261, 262, 265, 314, 375, 378, 417 twist, 56, 61, 374 twist, staple yarn, 57 twist, thin spots, 69 twist/untwist method, 360 twisted self-twist yarns, 274 twisted yarn, compressive force, 374 twister speeds, 93 twist-gear, 169 twisting & doubling, 14, 220 twisting fil & frictional behavior, 383 twisting fil under heat, 383 twisting m/c, 250 two-for-one twisting, 63–67 two-heater machine, 93 typical process schedules, 15 ultra-fine fibers, 10 ultra-long errors in yarn, 391, 398 undercard waste, 287 undrawn polymers, 47 uneven breaking, 49 uneven build-up of finish on rolls, 47 uneven dye penetration, 114 uneven gripping of web, 163 uneven hardening, 73 uniform distribution, 346 uniformity index, 353 unit standard cell, 386 unopened (‘green’) bolls, 26 unraveled knitted fabric, 112 untextured fil, linear density, 368 untwisted yarn strength, 260 untwisting to zero twist, 359 unwinding, 243, 244, 451, 452 unwinding, balloon breakers, 244 unwinding, chaotic balloon, 244, 452 unwinding, end-breakage, 244 unwinding, hair plucked from adjacent yarns, 243 unwinding, tension controllers, 244 unwinding, yarn faults, 244 upper half mean length, 353 upper zone of balloon, 445 up-twisting, 65 USP, normalized comparisons, 295 Uster Classimat, 283 Uster evenness tester, 356 Uster hairiness values, 295

488

Index

Uster statistics, 295, 302 Uster statistics, percentile rankings (USP), 164, 295 utilities, 341 variable cost, 309–312 variable grip on fibers, 81 variable ratch setting, 81 variance, 69, 83, 281, 422, 424 variation period, 395 variation, skein-skein, 366 variations in hardness, 157 variations, annual, 393 vector alignment, 399 vegetable & mineral particles, 208 vegetable matter, 32, 34, 210 vinyl fibers, 38 virtual nip zone, 413 visco-elastic fibers & yarns, 365 visco-elastic polymer, 383, 384 visco-elastic rubber, 73 viscose rayon production, 40 viscose rayon solution, 4 viscosity, 39 viscous effects within polymer, 384 visual examination of fabrics, 372 visual examination of yarns, 361, 372 volume occupied, 12, 371, 343 volume of unit cell, 387 wage levels, elements of difference, 6 wage rates, 7, 316 wall-mounted hygrometers, 346 warehouse management, 391 warehouse stock, 391 warp beam defects, 289 warp wind, 169 waste disposal, scouring, 212 waste fiber, 126, 150, 151, 182, 286 waste, air currents, 152 waste, card, ss, 151, 152 waste, dust house, 151 waste, handled pneumatically, 151 waste, pepper trash, 152 waste, product flow calculations, 152 waste, reusable, 151, 152 wastewater disposal, 212 water vapor & steam, 341, 342 wavelength & frequency, 398 wax & sweat glands, 32 weak link, variable strengths, 365, 413 weaken fiber, 388 wealth flows, 7 wear of elements, 156, 417 wear resistant materials, 188 weathering, 26 weaving performance, 290 web doubling, 161, 163 weft wind, 177

weighpan feeder, 136, 123, 223 weight & mass, 317 wet & dry bulb temperatures, 344 wet spg, 38, 40 wet spg, extrudate, 40, 41 wet steam, 342 wet-spun fibers X-section, 40 wide band of technology, 219 widing & clearing, 235 wild fil, 61 willeyed blend, 220, 223 wind structure, 177 wind, precision, 242 winder productivity, 327, 328 winder, OE, 186, 197 winders, 15, 168, 234 winding & yarn dyeing, 257 winding complaints, prior processing, 278 winding costs, 304, 305 winding frames, ply, 251 winding heads/splicer, 246 winding m/c, 234, 235, 236, 241, 250, 254, 255, 315 winding m/c, air pollution, 246 winding m/c, automatic, 254 winding m/c, capital costs, 254 winding m/c, operator attention, 254 winding m/c, stable packages, 250 winding m/c, yarn joining, 254 winding m/c, wear, 250 winding performance, 327 winding tensions, 242 winding traverse mechanisms, 240, 242 winding traverse, pattern breaking, 240 winding traverse, ribboning, 240 winding, customer interface, 250 winding, damage resistance, 235 winding, defect removal, 245 winding, economics, 235 winding, faulty, 114 winding, hank, 236 winding, hard shoulders, 236 winding, invariable supply speed, 242 winding, nep, 250 winding, over-tension yarn damage, 242 winding, parallel w flanges, 236 winding, quality, 235 winding, reciprocating guide, 235, 242 winding, remove yarn faults, 235 winding, ribboning, 236 winding, skein, 236 winding, temporary yarn storage, 242 winding, tension control, 242, 245 winding, traverses per rotation, 236 winding, twisted yarn, 65 winding, withdrawal at will, 242 winding, yarn appearance, 250 winding, yarn clearing, 245 winding, yarn contraction, 246

Index winding, yarn hairiness, 250 winding, yarn lag, 240 winding, yarn moisture content, 246 winding, yarn tension, 235, 236 winding. grooved roll, 242 wind-off speed, 65 windows, 349 winnowing, 127 wire flyer, 65 within-bale consistency, 391 within-clump variance, 390 wool & stretch-broken yarn, 50 wool & synthetic fibers, 3 wool classer, 33 wool cleaning, 32, 208 wool color, 32 wool crimp, 205 wool delivery in cool state, 210 wool devil, 221 wool dryer, 210 wool dyeability, 208 wool felting, 210 wool fiber blending, 205 wool fiber cleaning, 205, 211 wool fiber crimp, 32 wool fiber elongation, 32 wool fiber oiling, 210, 212 wool fiber strength, 32 wool fiber tips, 32 wool fibers, preparation, 206 wool grease by-products, 208 wool grease separation, 209 wool grease, 14, 208, 212 wool harvesting, 33 wool history, 2 wool kempts, 33 wool moisture content, 210 wool opener, 221 wool prices, 34 wool production, 33 wool rinsing and drying, 210 wool scouring, 208 wool shoddy, 286 wool tops, 34, 281 wool use, 33 wool willow, 221 wool yarn, crease recovery, 381 wool, 20, 31, 33, 34, 205, 210, 212, 213, 382 wool, blended, 213 wool, carbonizing, 210 wool, core-boring, 279 wool, cotted, stained or muddied, 34 wool, crease-shedding properties, 3 wool, cuticle & cortex, 33 wool, deburred, 213 wool, fiber diameter, 280 wool, fiber length, 33 wool, fiber strength loss, 210 wool, fleeces classification, 34

489

wool, foreign matter, 208 wool, grease, 213, 221, 284 wool, greasy, 32, 208, 213 wool, history, 3 wool, locks, 221 wool, market size, 3 wool, matchings, 34 wool, medulated fibers, 33 wool, moisture absorption, 3 wool, murrain, 33 wool, natural finish removal, 381 wool, pieces, 221 wool, prickle, 32 wool, removal of foreign materials, 32 wool, sampling procedure, 279 wool, scour, 34, 212, 213 wool, shorn or pulled fiber, 33, 208 wool, shorn or sheared, 33 wool, slipes, 33 wool, suint, 284 wool, tender, 31 wool, variability, 34, 221 wool, vegetable matter, 284 wool, washed & rinsed, 208 wool, wax & suint, 32 wool/m.m.fiber blend, 205, 216 woolen blending, 220, 222 woolen card delivery, 229 woolen card set, 224, 228 woolen carder, 226 woolen carding, feed, 220 woolen carding, initial, 223 woolen spg m/c, damage, 212 woolen spg, autocount system, 228 woolen spg, balloon control rings, 231 woolen spg, blending procedures, 220 woolen spg, blending, 211, 223, 228 woolen spg, carder (finisher card), 225 woolen spg, chute feed, 224 woolen spg, clean & sterilize fibers, 211 woolen spg, cleaning system, 211 woolen spg, cleaning, 221 woolen spg, collapsed balloons, 231 woolen spg, color differences, 220 woolen spg, computer control, 224 woolen spg, condensed slubbing, 221 woolen spg, control signals, 228 woolen spg, cross-lapper, 225 woolen spg, crush roll set, 225 woolen spg, dancing roller, 224 woolen spg, decomposition of raw material, 221 woolen spg, doubling (plying), 231 woolen spg, draw (pause), 221 woolen spg, equipment groupings, 224 woolen spg, fearnought, 221, 222, 223 woolen spg, feed rolls, 225 woolen spg, fiber baling, 223 woolen spg, fiber degradation, 228

490

Index

woolen spg, fiber electrification, 211 woolen spg, fiber handling, 222, 223 woolen spg, fiber loss, 228 woolen spg, fiber oiling, 211, 222, 228 woolen spg, fiber pollution, 224 woolen spg, flexible wire, 226 woolen spg, fractionation, pinned rolls, 224 woolen spg, gamma ray thickness sensor, 224 woolen spg, garnet clothing, 225 woolen spg, hopper feeders, 222, 223 woolen spg, human monitoring, 224 woolen spg, lattice feed, 225 woolen spg, m.m. fiber, 223 woolen spg, manual blending, 222 woolen spg, mass flow control, 224 woolen spg, mechanical layering, 222 woolen spg, mechanical mixing, 222 woolen spg, necessity for blending, 220 woolen spg, oiling system, 223 woolen spg, opening, 221, 228 woolen spg, periodic dumping, 223 woolen spg, picker opener, 221 woolen spg, processes variable, 220 woolen spg, r.h., 211 woolen spg, reworking, 222 woolen spg, rotary spreader, 222 woolen spg, safety, 226 woolen spg, sampling & testing, 223 woolen spg, Scotch feed, 225 woolen spg, scribbler (breaker card), 225 woolen spg, sensors, 224 woolen spg, settings, 228 woolen spg, short process, 221 woolen spg, sliver, 229 woolen spg, slubbing condenser, 224 woolen spg, slubbings, 225, 229 woolen spg, spike size, 221 woolen spg, spin limit, 228 woolen spg, stock records, 222 woolen spg, stock-dye fibers, 222 woolen spg, swift loadings, 228 woolen spg, swift/worker/stripper, 225 woolen spg, synchronized production, 223 woolen spg, tape condenser, 225, 226, 229 woolen spg, teaser, 221 woolen spg, thickness sensor, 224 woolen spg, threads, 221 woolen spg, tooth size, 221 woolen spg, tumbling, 223 woolen spg, variety cleaning systems, 211 woolen spg, varying fiber cleanliness, 223 woolen spg, waste fibers, 220 woolen spg, web error, 228 woolen spg, workers & strippers, 225 woolen system, 15, 205, 219, 220 woolen yarns, 205, 231 work practices in mill, 291 worked examples, 329 worker exploitation, 6

workspace r.h., 282, 346 worsted hank, 318 worsted processing system variations, 213 worsted S-on-S or Z-on-Z ply yarns, 268 worsted spg, 15, 218 worsted spg, automatic winding, 219 worsted spg, bobbin length, 219 worsted spg, bobbin movement, 219 worsted spg, constant balloon length, 219 worsted spg, electronic clearing, 219 worsted spg, large packages, 219 worsted spg, m.m. fibers, 216 worsted spg, ratch settings, 216 worsted spg, ring size, 219 worsted spg, splicing, 219 worsted spg, tension, 219 worsted spg, twist levels, 216 worsted system, 205, 213, 216, 219 worsted, plied warp yarns, 268 wound packages, series of specimens, 287 woven fabric simulation, 362 wrap friction, 75 wrap spg bouclé yarns, 270 wrap spg false twist in staple, 270 wrap spg fil wraps core, 270 wrap spg fil yarn via hollow spindle, 270 wrap spg productivity, 270 wrap spg rotating hook, 270 wrap spg, 66 wrappings, 22 wrap-spun yarns, 270 wrap-spun yarns, core twist, 270 wrinkle resistance, 2 yard board, 355, 356, 361 yarn & strand numbering, 317 yarn amount to be tested, 290 yarn appearance, 278, 282, 361 yarn balloon base, energy, 445 yarn balloon control, 65 yarn balloon energy balances, 427 yarn balloon mechanics, 427 yarn balloon, 70, 177, 188 yarn balloon, air-drag, 432, 437 yarn balloon, air-flow, moving parts, 438 yarn balloon, ambiguous tension, 443 yarn balloon, balloon collapse, 435 yarn balloon, balloon shape, 435 yarn balloon, base, 428 yarn balloon, central zone, 427, 436, 439 yarn balloon, centroid, 429, 448 yarn balloon, chain of elements, 448, 449 yarn balloon, changes in energy level, 434 yarn balloon, chase motion, 432 yarn balloon, collapse, 434 yarn balloon, conservation of energy, 431 yarn balloon, crown, 450 yarn balloon, cyclical traveler speeds, 440 yarn balloon, drag coefficients, 438

Index yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn

balloon, elements tilt & twist, 437 balloon, end conditions, 429, 450 balloon, energy balance, 431 balloon, energy equilibrium state, 448 balloon, energy to inflate balloon, 444 balloon, energy, 430, 431, 433, 434, 448 balloon, equilibrium, 450 balloon, experimental data, 440, 441 balloon, force analysis, 429, 432, 436, 437, 440, 444 balloon, force analysis, 449, 450 balloon, forces, 427 balloon, friction, 429, 435, 445 balloon, frictional restraints, 436 balloon, geometry changes, 445 balloon, gradients, 437 balloon, hairy comet’s tail, 438 balloon, inertial frame, 440 balloon, instabilities, 434 balloon, insufficient energy, 434 balloon, kinetic energy, 431 balloon, lay point, 431 balloon, loci of segments, 447 balloon, lower zone, 427 balloon, multi-noded, 435 balloon, non repetitive, 440 balloon, offset pigtail guide, 445 balloon, periodic collapses, 448 balloon, pigtail guide, 445 balloon, plan view, 440 balloon, protruding hairs, 437 balloon, radius of gyration, 434 balloon, radius of locus, 450 balloon, radius of yarn curvature, 450 balloon, ring lubrication, 450 balloon, rotating plane, 428 balloon, rotating reference plane, 437 balloon, rotational speed, 451 balloon, separator plates, 437 balloon, shape changes, 447, 448 balloon, simplified, 428 balloon, single-noded, 434 balloon, size & shape, 434 balloon, speed stability, 447 balloon, strain energy, 431 balloon, subsidiary oscillations, 434 balloon, surface swept, 427 balloon, temporary collapses, 448 balloon, tension & twist of yarn, 445 balloon, tension at wind point, 434 balloon, tension gradients, 435, 449 balloon, tension variations, 445 balloon, tension, 435, 445 balloon, torque supplied, 431 balloon, torque, 445 balloon, torsional strain energy, 433 balloon, traveler centroid, 442 balloon, traveler instability, 434, 448 balloon, traveler mass, 434, 450

yarn yarn yarn yarn

491

balloon, traveler, 442 balloon, twist gradients, 446 balloon, upper zone, 427 balloon, vector analysis, 427, 431, 439, 448 yarn balloon, vertex, 428 yarn balloon, winding tension, 444 yarn balloon, wind-off point, 451 yarn balloon, yarn tension, 439 yarn blackboard, 361 yarn blend, 277 yarn bobbins, 175 yarn bulk, 89, 384, 386 yarn bulk, geometry fil helices, 386 yarn cheese, 65 yarn clearing vs winding efficiency, 255 yarn clearing, 234, 235, 245, 246, 247, 287 yarn clearing, automatic splicer, 245 yarn clearing, capacitive sensors, 246 yarn clearing, defect sources, 245 yarn clearing, nub plates, 245 yarn clearing, optical devices, 246 yarn clearing, patrolling piecer, 245 yarn clearing, performance, 247 yarn clearing, prescribed limits, 245, 246 yarn clearing, residual faults, 255 yarn clearing, settings, 246 yarn clearing, size of defect removed, 287 yarn clearing, wind at high tension, 245 yarn color, 114 yarn conditioning, steaming, 246, 257 yarn cone, 65 yarn contraction, 257 yarn cooling, 384 yarn cost, end-breakage, 314 yarn cost, spg a major portion, 314 yarn count, 314, 318 yarn cross-sections, ring and rotor, 381 yarn CV, 254, 295, 458 yarn damage, 180 yarn defect classification, 290 yarn defects, 173, 227, 243, 254, 277, 281– 283, 286, 288, 291, 348 yarn degradation, 153, 300 yarn diameter, 374 yarn dyeing performance, 258, 278 yarn economics, 301 yarn elongation, 424 yarn error, test length, 294 yarn errors, fiber flow, 291 yarn errors, mechanical, 291 yarn evenness, 277, 291 yarn evenness, OE, 455 yarn failure, fiber breakage, 58 yarn fineness, 317 yarn from rotating package, 65 yarn grade, 361 yarn hairiness, 19, 176, 178, 181, 263, 277, 281, 288, 295, 375, 377, 380, 381, 417, 448

492 yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn yarn

Index hairiness, m/c type, 381 hairiness, reduction, 262 hairiness, within-bobbin variation, 295 hand, 384 heated above Tg, 385 hysteresis, 383, 384 joining, 220, 245, 277 length on packages delivered, 284 length/sq yard fabric, 290 manufacturing problems, 254 moisture absorption, 257 number, 318 numbering calcs, 330, 331 or tow take-up rate, 45 package build, 65, 177 package density, 181 package diameter, 180 package inside the balloon, 65 package types, 234 processability, 277 properties, OE, 190, 194, 195, 260 quality & operator assignment, 306 quality, 17, 316 relaxation, 381, 385 removal from rotor groove, 314 removal, OE, 186 slub, 227, 282 sources, 302 splice strength, 246, 250 splice, dynamometer, 250 splice, splice efficiency, 250 splice, stiffness, 250 splice, tails, 248 splice, thick spot, 248 splicer, hand-held, 246 splicer, multiple, 246 splicer, settings, 248 splicing, 56, 235, 246–249 splicing, component wear, 249 splicing, end conditioning, 247–249 splicing, joint appearance, 248 splicing, lint accumlation, 249 splicing, rubber elements, 249 splicing, temporary false twist, 248 spun from card, 205 strength regularity, 296 strength, 163, 277, 281 strength, fiber force components, 373 structure, 89, 377 tails, packages, 258 tenacity CVs, 296 tension control, 69 tension gradients, 435 tension, 65, 177, 178, 180–183, 218, 243, 314 testing, 290, 365 thin spots, 306 transfers, 243 twist liveliness, 277, 381

yarn twist, 19 yarn twist, exit from twist triangle, 446 yarn twisting, OE, 186 yarn untwist, 384, 385 yarn value, reduction, 128 yarn variance, 365 yarn weak spots, 289 yarn winding point, ring rail motion, 444 yarn, air-jet interlacing, 275 yarn, air-jet, see air-jet yarn yarn, appearance, 89 yarn, blended (dissimilar fibers), 155 yarn, blended, 13, 155 yarn, bulk, 12, 61, 92, 93, 371, 385 yarn, co-extruded, 109 yarn, co-mingled, 109 yarn, core, 109 yarn, core/sheath, 112, 260 yarn, creel breaks, 173 yarn, cross-sectional shape, 321 yarn, CV, 188 yarn, elastomeric, 112 yarn, elongation, 89, 112 yarn, false-twisted, 61, 177 yarn, fault removal, 234 yarn, fibrillated, 109 yarn, fine linen, 232 yarn, flat fil, 109 yarn, frictional behavior, 381 yarn, hand, 89 yarn, hand, OE, 195 yarn, heat setting, 89 yarn, heat-set when twisted, 384 yarn, high-bulk, 50 yarn, interfiber friction, 89 yarn, linear density, 177 yarn, low-bulk, high-stretch, 91 yarn, ls, folding, 220 yarn, ls, spun from sliver, 220 yarn, mock ply, 261 yarn, open-end, 185 yarn, over-conditioning, 257 yarn, ply, 14 yarn, self twist, 261, 271, 273 yarn, self twist, piecing, 273 yarn, Selfil, 275 yarn, short-term errors, 297 yarn, slit, 109 yarn, slub, 173 yarn, soft-wound packages, 95 yarn, squashed, 321 yarn, staple/fil, 260 yarn, STT, character, 274 yarn, STT, composite ply yarns, 275 yarn, STT, modified, air-jet texturing, 275 yarn, STT, real twist addition, 274 yarn, textured, 61 yarn, theoretical diameter, 322 yarn, thermoplastic, 89

Index yarn, thick & thin spots, 173 yarn, twisted self-twist (STT), 274 yarn, twist-liveliness, 277 yarn, winds at differing diameters, 17 yarn, woolen, 220 yarn, worsted, 205 yarn, wrap-spun, 270 yarn-making technologies, modern, 11 yarns of complex structure, 260 yarns, cable, 61 yarns, carpet, 15 yarns, core/sheath, 109

yarns, fancy, 219 yarns, industrial, 16 yarns, ls, heavy, 206 yarns, modified twist, 261 yarns, plied acrylic, hand knitting, 253 yarns, plied aramid, ropes etc., 253 yarns, plied, costs, 253 yarns, silk, 113–114 yarns, viscose rayon, 381 yarns, world market, 9 yarns, worsted, 219 yellowness (+b), 355, 404, 406

493

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