Update on Troubleshooting in Thermoforming

Update on Troubleshooting in Thermoforming

Natamai Subramanian Muralisrinivasan

Smithers Rapra Update

Update on Troubleshooting in Thermoforming Natamai Subramanian Muralisrinivasan

iSmithers – A Smithers Group Company Shawbury, Shrewsbury, Shropshire, SY4 4NR, United Kingdom Telephone: +44 (0)1939 250383 Fax: +44 (0)1939 251118 http://www.iSmithers.net

First Published in 2010 by

iSmithers Shawbury, Shrewsbury, Shropshire, SY4 4NR, UK

©2010, Smithers Rapra

All rights reserved. Except as permitted under current legislation no part of this publication may be photocopied, reproduced or distributed in any form or by any means or stored in a database or retrieval system, without the prior permission from the copyright holder.

A catalogue record for this book is available from the British Library.

Every effort has been made to contact copyright holders of any material reproduced within the text and the authors and publishers apologise if any have been overlooked.

ISBN: 978-1-84735-137-1 eISBN: 978-1-84735-470-9

Typeset by Integra Software Services Pvt. Ltd. Printed and bound by Lightning Source Inc.

C

ontents

Preface ..........................................................................................v 1.

Introduction ......................................................................... 1

2.

Plastic Sheet, Sheet Materials, Physical Properties and Plastics – an Overview .................................. 5 2.1

Thermoforming Materials – Physical Properties ...................................................... 8

2.1.1

Acrylonitrile-butadiene-styrene (ABS) ..........8

2.1.2

Polyvinyl Chloride (PVC) ..........................10

2.1.3

Low-Density Polyethylene (LDPE).............12

2.1.4

High-Density Polyethylene (HDPE) ...........13

2.1.5

Polypropylene (PP) ....................................14

2.1.6

Polystyrene (PS) .........................................16

2.1.7

Expanded Polystyrene (EPS) (Foam) ..........18

2.1.8

Polyethylene Terephthalate (PET) ..............19

2.1.9

Polymethylmethacrylate (PMMA)/ Acrylic .......................................................20

2.1.10 Polyetheretherketone (PEEK).....................21 2.1.11 Polycarbonate (PC)....................................22 2.1.12 Thermoplastic Olefin (TPO) ......................23 2.1.13 Polyphenylene Oxide (PPO) ......................23 2.1.14 Thermoplastic Elastomers .........................24 2.1.15 Biodegradable Polymer ..............................24 2.1.16 Cyclic-olefin Copolymer (COC) ................25 i

Update on Troubleshooting in Thermoforming

2.1.17 Polyacetal (POM) ......................................26 2.1.18 Composites................................................26 3.

Machine Specifications – Effect of Parameters .................... 35 3.1

3.2

4.

3.1.1

Straight Vacuum Forming ..........................39

3.1.2

Vacuum (Thermo) Forming .......................39

3.1.3

Pressure Forming .......................................41

3.1.4

Plug-assist Pressure Forming......................42

3.1.5

Plug-assist Vacuum Forming ......................42

3.1.6

Pressure-bubble Plug-assist Vacuum Forming .......................................46

3.1.7

Free Forming .............................................46

3.1.8

Matched-die Forming ................................46

3.1.9

Drape Forming ..........................................48

3.1.10 Vacuum Snap-back Forming Process ......................................................49 Effect of Parameters .................................................. 51 3.2.1

Raw Material (Plastic Sheets) ....................51

3.2.2

Moulds ......................................................53

3.2.3

Vacuum .....................................................56

3.2.4

Vacuum Holes ...........................................56

3.2.5

Plug ...........................................................57

3.2.6

Sheet Temperature .....................................57

3.2.7

Heaters ......................................................58

3.2.8

Daylight ....................................................59

3.2.9

Process Parameters ....................................60

Problem Identification – Classification– Troubleshooting – Flow Chart............................................ 67 4.1

ii

Methods of Vacuum Forming ................................... 39

Troubleshooting........................................................ 71

Contents

4.1.1

Blisters or Bubbles .....................................71

4.1.2

Webbing or Bridging .................................73

4.1.3

Excessive Sheet Sag....................................74

4.1.4

Pinhole or Rupturing .................................75

4.1.5

Excessive Post Shrinkage ...........................76

4.1.6

Uneven Sag ................................................77

4.1.7

Part Sticks to Mould..................................77

4.1.8

Stretch Marks on Part ...............................78

4.1.9

Nipples on Mould Side of Thermoformed Part ...................................78

4.1.10 Pock Marks ...............................................79 4.1.11 Poor Wall Distribution ..............................80 4.1.12 Uneven Edges ............................................81 4.1.13 Tearing of Sheet When Forming ................82 4.1.14 Bad Definition at the Edges of the Forming Area ..................................83 4.1.15 Glossy Spots ..............................................84 4.1.16 Part Warpage .............................................85 4.1.17 Cracking in Corners ..................................86 4.1.18 Raised Corners ..........................................87 4.1.19 Surface Markings ......................................88 4.1.20 Parts in the Corners Too Thin ...................................................89 4.1.21 Folds, Webbing or Wrinkles ......................90 4.1.22 Part Deforms During De-moulding .............................................90 4.2

4.1.23 Poor Part Detail.........................................91 End-product Problems .............................................. 91 4.2.1

Migration of Additives ..............................91

4.2.2

Melt Failure During Thermoforming .........93

iii

Update on Troubleshooting in Thermoforming

4.2.3

Blue Coloured Dots ...................................93

4.2.4

Shrinkage ..................................................94

4.2.5

Sheet Pull-out ............................................94

5.

Optimisation of the Production .......................................... 97

6.

Case Studies ..................................................................... 103

7.

6.1

Case Study 1 ........................................................... 104

6.2

Case Study 2 ........................................................... 106

6.3

Case Study 3 ........................................................... 107

Conclusion ....................................................................... 109

Abbreviations ........................................................................... 113 Author Index............................................................................ 115 Subject Index............................................................................ 121

iv

P

reface

This book is written to serve as a general purpose source both for experienced plastics users and for those who are new to plastic processing. It should be of interest to non-designers and managers who need a general overview of the concepts and critical issues related to thermoforming. Although the book is not a guide to thermoforming, much of the troubleshooting is based upon forming criteria involved in the manufacturing and thermoforming of plastic parts. Most thermoforming books deal with specific topics. This book deals with issues common to all thermoforming thermoplastic resins, with associated troubleshooting methods. Tables, figures and other descriptive methods are used to help illustrate significant troubleshooting differences. The book, which is divided into seven chapters, gives a thorough overview of troubleshooting in the thermoforming process and provides information for the experienced user of polymers who wishes to use thermoforming. Problems of surface quality in thermoforming are dealt with, and the complex interrelationship between the mould and the processing is recognised. The goal is to assist with the practical aspects of thermoforming, and the factors affecting the quality of thermoformed parts, such as the process, the machinery and the mould, are considered. Consideration is also given to the detection, classification and troubleshooting of defects. Case studies are included, the effects of thermoforming parameters are discussed and machine specifications are examined.

v

Update on Troubleshooting in Thermoforming The book will serve both as a reference and as a ‘how-to’ guide for those new to thermoforming processes. Information about every aspect of troubleshooting in the thermoforming process is provided in a reader-friendly form. The advantages of working with thermoplastics are discussed, and practical comments on processing are provided. The book begins with a general background overview and describes and reviews processing issues and conditions for the wide range of techniques used to optimise the processing. It is an indispensable resource for anyone working within the thermoforming industry. The book looks at different problems that can occur during thermoforming of extruded plastics, and how to solve them. An update on thermoforming troubleshooting, rather than a complete survey of the literature, is the main subject of the book. The main emphasis will be placed on troubleshooting, and the plastic materials and machinery will be discussed only briefly.

vi

1

Introduction

The term ‘thermoforming’ originally meant ‘post-process technology’, and it was that part of processing technology that has arisen from dealing with thermoplastic sheets. Thermoforming is an industrial process in which plastic sheets are heated and then formed into useful parts. It consists of three consecutive phases, namely heating, forming and cooling [1]. Thermoforming is a relatively simple processing technology. According to McKelvey [2], plastics processing is an operation carried out on polymeric materials or systems to increase their utility. Thermoforming is inexpensive when compared to other plastic moulding and forming methods. Although thermoforming is often associated with manufacturing of packaging items such as blister packs and disposable coffee-cup lids, the cost and time advantages are realised in a broad spectrum of products in an equally broad range of industries. Thermoforming is one of the oldest plastic moulding techniques, with a history dating well back into the nineteenth century, and is the process of forming thermoplastic sheet into discrete parts. Although contributions have been made in the USA, most methods of plastic processing originated in Europe. Polymers have been with us from the beginning of time; they form the very basis (building blocks) of life. Animals and plants – all classes of living organisms – are composed of polymers. However, it was not until the middle of the twentieth century that we began to understand the true nature of polymers. This understanding came with the development of plastics, which are true man-made materials and are the ultimate tribute to man’s creativity and ingenuity [3]. 1

Update on Troubleshooting in Thermoforming With cellulosic sheet materials, and later acrylic and vinyl, the growth of thermoforming became very lively and interesting. Baby rattles and teething rings were thermoformed in the 1890s. Around 1920, the growth was quite slow. The process saw major growth in the 1930s with the development of the first roll-fed machines in Europe and in the early 1950s the thermoplastic fabrication process began when the dairy industry began to use containers and lids formed from high-impact polystyrene and continued to gain strength with the manufacture of signs and displays, toys and other packaging applications. For many years, the injection and compression moulding techniques were of particular importance and the thermoforming industry was relatively small because it lacked suitable sheet materials and forming equipment capable of production operations [4]. The process of thermoforming plastic sheets has advanced quickly in recent years due to the advantages of low machine cost, low temperature requirement, low mould cost, low pressure requirement, large parts being easily formed and fast mould cycles. Worldwide, thermoplastics cover 85% [5] of polymers; 70% is accounted for by thermoplastics production for the large-volume, low-cost commodity resins such as polyethylene, polypropylene, polystyrene and polyvinylchloride. Engineering plastics such as acrylics, acrylonitrile-butadiene-styrene and high-impact polystyrene are based on their performance and cost. Because of the requirement for higher performance, engineering plastics such as polyacetals, polyamides, polycarbonate, polyethylene terephthalate, polypropylene oxide and their blends are increasingly being used. In the advanced technology areas, polymers such as liquid crystalline polymer, polysulfones, polyimides, polyphenylsulfide, polyetheretherketone and fluoropolymers are well established for high-temperature applications [6]. For today’s demanding applications, any thermoplastic sheet material can be thermoformed and sheet material has the dimensional stability and the impact strength required. Many resins and combinations of resins are available to meet any application requirements, and every year new thermoforming techniques are being developed. Thermoforming has become an increasingly important method of plastic sheet processing, especially in the case of larger parts. 2

Introduction Thermoforming has many advantages over other methods of thermoplastic fabrication: •

Parts with a large surface area can be formed;

•

Relatively low mould and equipment cost due to its low pressure requirement;

•

Very thin-walled parts can be formed;

•

Higher production rates with high-volume thin-walled products;

•

Low-volume heavy-gauge products through lower tooling costs.

References 1.

S. Yang and B. Boulet in Proceedings of the World Congress on Engineering and Computer Science (WCECS), 2008, San Francisco, CA, USA, 2008.

2.

J.M. McKelvey, Polymer Processing, John Wiley, New York, NY, USA, 1962, p.1.

3.

R.O. Ebewele, Polymer Science and Technology, CRC Press, Boca Raton, FL, USA, 1996.

4.

D.O. Kazmer, Simulation of the Blow Moulding and Thermoforming Processes, GE Plastics, Pittsfield, MA, USA.

5.

M. Xanthos and D.B. Todd in Kirk–Othmer Encyclopedia of Chemical Technology, 4th Edition, Volume 19, Ed., J.I. Kroschwitz, John Wiley & Sons, New York, NY, USA, 1996, p.290.

6.

M. Xanthos, Functional Fillers for Plastics, Wiley-VCH, Weinheim, Germany, 2005.

3

2

Plastic Sheet, Sheet Materials, Physical Properties and Plastics – an Overview

Polymers are composed of many simple molecules. These molecules are present as repeating structural units called monomers. Hundreds of monomers may be present in a single polymer and the polymer may have a linear, branched or network structure. Plastics are polymers with long chains of molecules composed of various elements such as carbon, hydrogen, oxygen, nitrogen, chlorine and sulfur. These polymers are synthetic, and the long chain is made up of repeating units joined together with covalent bonds that hold the atoms in the polymer synthesis by either addition or condensation polymerisation processes. Copolymers are composed of two or more monomers. The linear molecular structure is formed during the reaction of a monomer in such a way that it reacts and binds together. The molecular weight is dependent upon the increase in monomer contents in the polymer. Plastics are soft and approach a melt condition during any plastics processing; they become solids in their finished state. Plastic materials cover a wide range of synthetic or semi-synthetic products, which are processed using processing equipment. Additives and modifiers improve performance or economics. Plastics can be classified into two groups: thermoplastics and thermosets. Thermoplastics are generally long-chained polymers with a polymer backbone of carbon, which is the main component and forms covalent bonds with other materials. Thermoplastics can soften during heating and harden when cooling without any considerable degradation even on repeating the process.

5

Update on Troubleshooting in Thermoforming The two categories of thermoplastics according to their transition temperatures are amorphous and crystalline [1]. An amorphous material has a random molecular arrangement in the bulk polymer whereas a crystalline one has an orderly arranged molecular structure. Crystalline polymers have a sharp and defined melting point. Thermoplastics with amorphous nature will soften over a wide range of temperatures. Thermoplastics such as polyethylene, polyvinyl chloride (PVC), polystyrene (PS), polyamide, cellulose acetate, polyacetal, polycarbonate (PC), polymethylmethacrylate (PMMA) and polypropylene are fusible, melt when exposed to sufficient heat and can be recycled and reused indefinitely. Plastics can be reformed due to their structure; they can be heated, processed and covalently bonded several times. The physical properties of polymers are directly related to the length and combination of the bonds between the polymeric chains. They can be modified by alloying and blending with various polymers and reinforcement materials and it is also possible to modify them with additives and modifiers. Thermosets, which are polymers with crosslinks, can not be reprocessed. These materials are produced by a chemical reaction and the bonds which are crosslinked will have difficulty in recombining themselves. Because of this difficulty in recombining, these polymers are not reusable. In thermosets, the interlinking of the polymer takes place at the point of use and often under the application of heat and pressure. Once crosslinking has taken place, the material is stronger and is resistant to softening upon heating. Thermosets such as phenol formaldehyde, melamine formaldehyde, urea formaldehyde, epoxies and some polyesters can only be made once and can not be recycled. Another important physical property is glass transition temperature (Tg), a temperature above which the modulus decreases rapidly and the polymer exhibits liquid-like properties; amorphous thermoplastics are characterised by Tg and are normally processed at temperatures well above their Tg [1]. 6

Plastic Sheet, Sheet Materials, Physical Properties and Plastics – an Overview In thermoforming the temperatures are moderate and the material undergoes mainly viscoelastic deformation rather than pure flow. Thermoforming is a batch process, but is rather quick and controlled automatically. Common polymers that are fabricated by this method are polypropylene (PP), polyethylene (PE), high-impact polystyrene (HIPS), acrylonitrile-butadiene-styrene (ABS), PVC and acrylics, as well as many others. Thermal and mechanical properties of semi-crystalline polymers are strongly dependent on molecular weight (MW), molecular weight distribution (MWD), branching content and density [2–4]. Controlled variations in these structural parameters result in a broad family of products with wide differences in thermal and mechanical properties. Shear modified low-density polyethylene (LDPE) samples are also available commercially [5, 6]. Long-chain branching has a strong influence on MWD, and hence on resin properties such as processability, melt strength and film optical properties [7]. Above Tg, the thermoplastics show a high mobility leading to the high elasticity and deformity required for the thermoforming process. Below Tg, the polymers behave like rigid solids with reduction of extensivity with impact properties and are brittle in nature. Thermoplastics like PS, PVC, PMMA, PC and others exhibit relatively high Tg, whereas the polyolefins and rubber have low Tg, and behave like high-strength rubber or leather. Most engineering polymers will have superior short- and long-term thermal stability (higher melting point, glass transition temperature, heat deflection temperature and continuous use temperature), chemical and radiation resistance, resistance to burning and improved mechanical properties (stiffness, strength, toughness, creep, wear and fatigue). Thermoplastic systems tend to have a narrower glass transition between the glassy state and the rubbery plateau. When related to the physical properties of the system, this means that they will change properties fairly quickly. Thermoset systems possess a Tg, but it may have a very wide transition where the properties at various temperatures may be very similar [8, 9].

7

Update on Troubleshooting in Thermoforming

2.1 Thermoforming Materials – Physical Properties Thermoforming consists of warming a plastic sheet and forming it into a cavity or over a tool using vacuum, air pressure and mechanical means. The process begins by heating a thermoplastic sheet slightly above the glass transition temperature, for amorphous polymers, or slightly below the melting point, for semi-crystalline materials. As the final thickness distribution of the part is drastically controlled by the initial temperature distribution inside the sheet, it is very important to optimise the heating stage. In most thermoforming machines, this step is performed using an infrared oven consisting of long-wave infrared emitters [10].

2.1.1 Acrylonitrile-butadiene-styrene (ABS) Acrylonitrile-butadiene-styrene is a graft copolymer made by dissolving styrene-butadiene copolymer in a mixture of acrylonitrile and styrene monomers by chain transfer reactions with a monomer in an emulsion polymerisation process. It was patented in 1948 and introduced commercially by the BorgWarner Corporation in 1954. ABS is a tough, durable thermoplastic with weather and heat resisting properties. Heat resistance comes from acrylonitrile, rigidity from styrene and impact nature from butadiene present in the polymer. Table 2.1 shows the transition temperature of acrylonitrile content. ABS with larger amounts of hard rubber particles exhibits more melt elasticity and better thermoforming performance. Strain hardening in ABS provides simple, reliable indicators of the thermoforming performance [11].

Table 2.1 Physical properties of ABS (26–27% acetonitrile) Physical property

Value

Reference

Density (g/cm3)

0.92

[16]

Tg (K)

212

[17]

8

Plastic Sheet, Sheet Materials, Physical Properties and Plastics – an Overview In general, ABS has high impact strength and rigidity, exhibits resistance to chemicals and may easily be processed with good as-moulded surfaces. Properties of specific grades of ABS resin are determined by several factors including acrylonitrile-butadienestyrene mix ratio, manufacturing method, rubber particle size and polymer chain length [12]. The rubber particles included in the ABS and the impact modification of the particles play an important role in determining the rheological properties, processing behaviour and mechanical properties [11]. Large amounts of hard rubber particles exhibit more melt elasticity, stronger strain hardening and better thermoforming performance [11]. Lau and co-workers studied the ABS that can be easily processed during thermoforming. ABS generally had the highest melt strength in the low extrusion temperature region approaching the thermoforming region, indicating that it has a good sagging resistance during thermoforming [13]. ABS can also be blended with other plastics, such as polycarbonate or nylon, to produce alloys that combine the best of both resins [12]. ABS may be modified or alloyed to offer flame resistance and low smoke toxicity, good UV resistance, higher impact resistance, resistance to high heat distortion, good conductivity and/or static dissipation and moreover to satisfy food grade requirements. ABS is stiff, tough and typical applications include instrument enclosures, functioning parts, vending displays, ice dispensers and medical enclosures. ABS has a good balance between resistance to sag and ease of flow and better dimensional stability [14]. In ABS the mono- and multi-layer extruded or co-extruded sheets provide quality performance in terms of design, technology and optical properties and guarantee problemfree processing and short cycle times during thermoforming; sheets with embossing to gloss or matt finishes or UV protection finishes for outdoor applications are also available. Pre-drying is essential before thermoforming; it forms a product with high quality and high definition. ABS suffers with little stress and does not need to be stress released due to the rubber present in 9

Update on Troubleshooting in Thermoforming it; during formation it gives off a hot rubber smell due to butadiene evaporation. ABS is a versatile thermoforming material. Forming techniques in use are positive and negative mould vacuum forming, bubble and plugassist, snap-back and single- or twin-sheet pressure forming [15].

2.1.2 Polyvinyl Chloride (PVC) Polyvinyl chloride is the most versatile of all plastics because of its blending capability. It can be used to manufacture products ranging from heavy-walled pressure pipes to crystal-clear food and beverage bottles and some containers for detergent powders. PVC has the properties of good clarity and chemical resistance (which is important for holding household detergents and other harsh materials). PVC bottles make up less than 6% of plastic bottles typically found in the home. Compared to PP, PVC has good resistance to chemical and solvent attack. Its vinyl content gives good tensile strength and it can be changed from rigid to flexible by the use of additives and modifiers. Its resistance to fire is due to its release of chlorine. Because of its impact resistance, it is the leading thermoplastic in the construction industry. Plastics are based on resins made by polymerisation of vinyl chloride or copolymerisation of vinyl chloride with other unsaturated compounds, the vinyl chloride being in greatest proportion by weight. Pure PVC is a hard, brittle material, which is difficult to process. For most applications it requires the addition of a plasticiser or to be copolymerised with vinyl acetate. Phthalate plasticisers are used in PVC products for medical and pharmaceutical applications of PVC and in problems associated with their migration [18]. PVC normally has a 55% chlorine content and can also be characterised by its K value. PVC with K value 65 is used in sheet form for thermoformed applications. PVC is the main material used

10

Plastic Sheet, Sheet Materials, Physical Properties and Plastics – an Overview for thermoformed display package blisters. It is also thermoformed into portion packaging and point-of-purchase displays. PVC films have good clarity and can be made into cling-type films. Some plastic bottles requiring good clarity or resistance to hydrocarbon solvents are made of PVC. PVC film materials are also used to produce tamperevident bottle-neck bands. Chlorinated PVC (CPVC) is superior to unmodified PVC as a thermoplastic for use in thermoforming, if improved heat resistance and dimensional stability are required. Sheets for thermoforming CPVC with 65% chlorine content, based on its various formulations, are obtained by calendering [19]. Table 2.2 explains the differences in the physical properties of PVC and CPVC.

Table 2.2 Physical properties of PVC Physical property

Experimental condition

Value

Reference

100 °C

1.352

[23]

120 °C

1.338

140 °C

1.332

Tm transition (K)

Calorimetric

485–583 473–573 (decomposition)

[24]

Sub Tg transition (K)

Dynamic mechanical

223

[25]

Tg (K)

Dilatometry

344

[26]

DSC, 20°C/min

344

[27]

DSC, 32°C/min

371

[28]

PVC Tg (°C)

65

[29]

CPVC Tg (°C)

107

[29]

Density (g/cm3)

Tm: melting temperature DSC: Dynamic scanning calorimetry

11

Update on Troubleshooting in Thermoforming PVC is a generic term for any of the vinyl resins, or for film, or other products made from them. Plastics are based on resins made from vinyl monomers, except those specifically covered by other classifications such as acrylic and styrene plastics. Typical vinyl plastics are PVC, poly(vinyl acetate), poly(vinyl alcohol) and poly(vinyl butyral), and copolymers of vinyl monomers and unsaturated compounds. PVC can be broadly divided into rigid and flexible materials. It is also versatile and has clarity, ease of blending, strength, toughness and resistance to grease, oil and chemicals. PVC can also be made flexible, chemically resistant, flame retardant and applications include medical packaging. In order to improve thermoforming, acrylic processing aids have been used in rigid PVC. Acrylic processing also improves cell structure in foam extrusion [20]. Inter-material substitution is a common trend based on the product delivering improved performance over the incumbent material at comparable costs, such as to replace flexible PVC with soft thermoplastic olefins in automotive interior thermoformed sheeting [21]. The technology for extruding rigid cellular PVC has grown substantially over the past decade. Formulation additives, tooling and processing expertise have made possible a variety of applications, e.g., relatively complex profiles, foam-core pipes, thermoformable foam sheet and house siding [22].

2.1.3 Low-Density Polyethylene (LDPE) LDPE is widely used in applications requiring clarity, inertness, processing ease, strength, flexibility and a moisture barrier. Its largest end use is as film for bags (e.g., bread bags or rubbish bags). LDPE side branches are relatively long, while linear-low-density polyethylene (LLDPE – a PE copolymer typically with octene or hexene) has many, but very short, side branches. LDPE and LLDPE are characterised by good clarity, high elongation and the lowest

12

Plastic Sheet, Sheet Materials, Physical Properties and Plastics – an Overview

Table 2.3 Physical properties of LDPE obtained using DSC and TMA Physical property 3

Density (g/cm ) Tg (K)

Value

Reference

0.910–0.935

[30–32]

140–170

[33–35]

TMA: Thermomechanical analysis

melting point of packaging polymers. Major applications include heat-sealable coatings and films, stretch- and shrink-wraps and retail and industrial bags. Table 2.3 shows the Tg of LDPE measured by DSC and TMA. LDPE, LLDPE and blends or copolymers with other modifiers are usually regarded as one family with similar applications. LDPE and LLDPE are both branched hydrocarbon polymers differing from high-density polyethylene (HDPE) in that they have significant side branching that prevents dense packing of the molecules. LDPE has long-chain branching, which causes the polymer to exhibit melt strain hardening, an important attribute in several processes. LDPE has some compatibility with PP and thus has an influence on the melt behaviour. This blend approach has also been extended to thermoforming applications.

2.1.4 High-Density Polyethylene (HDPE) High-density polyethylene is characterised by its stiffness, strength and toughness, low cost, ease of forming and processing, gas permeability and resistance to chemicals, moisture and breakage. HDPE has a variety of uses such as milk, water and juice beverage bottles, bleach and detergent bottles, motor oil bottles, margarine tubs and some grocery sacks and represents over 50% of the plastic bottle market. 13

Update on Troubleshooting in Thermoforming

Table 2.4 Physical properties of HDPE obtained using DMA Physical property Tg (K) 3

Density (g/cm )

Value

Reference

146–155

[37]

0.935–0.975

DMA: Dynamic mechanical analysis

High-density polyethylene is a hydrocarbon polymer that has linear chains allowing for dense packing. Table 2.4 shows the density between 0.94 and 0.96 or more. High-density polyethylene is economical, can be processed easily by most methods, has good moisture barrier properties and has good chemical resistance. It is soft, and has a comparatively low melting point, high elongation and poor gas barrier properties. Even though it has poor gas barrier properties, it is used for most household product bottles, many plastic bags and for injection-moulded dairy crates and beverage carriers, among other applications. Polyethylene offers a wide range of properties due to differences in structure and molecular weight [36]. Polyethylene is somewhat flexible, chemically resistant, tough even at low temperatures and inexpensive and applications include bottles, truck-bed liners, bowling pin dispensers sport utility vehicles and tanks. The advantages of HDPE in thermoforming, such as its toughness at temperatures as low as -118 °C, the ability to withstand steam sterilisation at 121 °C and excellent resistance to many chemicals, have long been recognised.

2.1.5 Polypropylene (PP) In the global market, 37% of PP is used for flexible and rigid packaging, followed by 21% for automotive, electric/electronic and appliance purposes, and 18% for textile applications. Biaxially oriented polypropylene, with a growth of about 7.5%, approaches close to 10% in the global demand for PP [38]. New opportunities in packaging applications are now opening up for PP, as a result of 14

Plastic Sheet, Sheet Materials, Physical Properties and Plastics – an Overview a crop of new additives – clarifiers and nucleators – that improve clarity, stiffness, heat deflection temperature and also processing rates [39]. PP has a high softening point, has good chemical resistance, is strong and has a high melting point; it is also very tough, very flexible, chemically resistant and has low heat distortion. It is found in packaging and consumer products, and other applications include living hinges, surgical helmets and automotive roof supports. Because it has a higher softening point, it is used to make bottles where elevated temperature will be a factor such as in hot-filling. Extrusion blow-moulded and injection-moulded PP does not have good lowtemperature performance. PP has superior live-hinge properties. It has a barrier to moisture and advantages such as low density and resistance to fat, salty and sugar-based foods; it is also resistant to heat, grease, oil and chemicals. PP continues to expand its existing properties with new diversified features, making it ideal for replacing traditional materials in rigid packaging applications. PP has gained wide acceptance in the production of food packaging containers [40]. With excellent food contact properties such as resistance to contributing off-flavours of foods, PP is safe for microwave cooking due to its temperature resistance and is also attractive due to its transparency. PP is a polymer with low specific gravity and good resistance to chemicals, delaminating (good mechanical strength) and fatigue. Table 2.5 shows some of the physical properties required for thermoforming of polypropylene.

Table 2.5 Physical properties of polypropylene obtained using DMA Physical property

Value

Reference

0.90–0.91

[45–47]

Tg (K)

283.7

[48]

Tm (K)

~459*

[49]

Density (g/cm3)

*: Crystalline

15

Update on Troubleshooting in Thermoforming PP is one of the popular materials among thermoplastics and is most extensively used for thermoforming due to its high tensile strength, stiffness, chemical resistance and increased significance as packaging material. This has led to a more widespread use of this semi-crystalline material as film, thermoformed packaging and in blow mouldings. Due to its desirable performance/cost properties, PP is a suitable alternative for ABS, PVC and PS in packaging and automotive thermoformed articles [41–43]. Extruded clarified polypropylene sheets in the form of thermoformed end-use markets are used in pots and tubs for dairy products, and other types of thin-wall packaging [44]. Oriented PP has excellent clarity, low elongation, good moisturebarrier properties and good low-temperature performance. It has a barrier to moisture and is also resistant to heat, grease, oil and chemicals. It has successfully replaced engineering polymers in automotive and interior applications due to its low cost. With PP, thermoformed parts can be produced that have transparency, stability, good side-wall stiffness, low thermal conductivity, good cooling behaviour, possibility to seal the trays by appropriate covering foil, and gloss; parts also feature minimum taste and odour and optimum processing and thermal stability for superior colour. PP with typical melt temperature is processed on in-line, narrow web thermoforming with solid phase pressure forming equipment. PP thermoforming sheets are processed into packaging media of various shapes and sizes.

2.1.6 Polystyrene (PS) Polystyrene (PS) is an easily processed resin for extrusion thermoforming applications. It has outstanding thermoforming properties. Polystyrene is hard, very clear and has good resistance to water, alkali chemicals, acids and detergents, but it is brittle unless modified. It has a low melting point. Table 2.6 shows some of the

16

Plastic Sheet, Sheet Materials, Physical Properties and Plastics – an Overview

Table 2.6 Physical properties of polystyrene obtained using DSC Physical property

Value

Reference

1.04–1.127

[51–53]

Tg (K)

373

[51, 52, 54]

Tm (K)

513

[51–53]

3

Density (g/cm )

physical properties required for thermoforming of PS. It is used for various applications, including impact polymers with equally wide-ranging properties in terms of fire resistance, appearance, processing and cost. However, high temperature or tensile stress may diminish its chemical resistance and it is attacked by aliphatic and aromatic hydrocarbons and derivatives. Due to its tremendous ease of processing and its low cost it is one of the most commonly used polymers in food packaging, audio/video, household appliances and construction industries. PS forms a good base sheet for thermoforming into portion cups, point-of-purchase displays, merchandising units and internal product supports. It is injection-moulded into a variety of boxes and shapes used for cosmetics, jewellery, compact discs, hardware and other items. Polystyrene’s properties assist greatly in many medical devices and it is perhaps no surprise that 70% of the world’s medical devices are injection moulded, thermoformed or extruded. Electrically charged PS microspheres stimulate the body’s own wound repair cells making recovery quicker [50]. Oriented polystyrene forms a good base sheet for thermoforming. PS can be readily expanded with blowing agents to make cellular plastics of varying densities. These can be formed into containers or used as shapes for protective and shock-absorbing applications. Polystyrene is proving to be the ideal material for these breakthrough technologies due to its rigidity and ease of processing.

17

Update on Troubleshooting in Thermoforming

2.1.7 Expanded Polystyrene (EPS) (Foam) Polystyrene and polystyrene copolymers are supplied as a compound with physical blowing agents, which can be processed into low-density (11.2 to 160.1 kg/m3) foamed articles. One of the major end uses is for protective packaging. Most properties of these foams are related to density. This permits the processor to fine-tune the exact required performance by a simple processing change, without redesign of tooling. Low moulding pressures and economical tooling make EPS moulding an inexpensive method of producing foam shapes. Unexpanded PS beads with pentane in solid solution in each bead are produced by the petrochemical industry. These beads are heated to about 100 °C to soften; the pentane rapidly expands the beads by up to 30 times in volume. The beads are heated with steam to make EPS beads. After a steamed pre-expansion process that produces foamed beads, the beads are put into a mould again and heated with steam where they expand further until they fuse together, forming the finished product in the packaging industries. The finished product is approximately 95–98% air. Hence, transport of large volumes of EPS beads over long distances is cost prohibitive and expansion using locally available resources is preferred [55]. This process is used for hot beverage and soup containers, as well as shape-moulded packaging such as electronics packaging. Extruded solid polystyrene is made from polystyrene resin pellets without a blowing agent mixed in. The sheet is reheated and thermoformed into various shapes. When no colouring is added, transparent products are produced. The material thickness of products produced in this manner is typically thinner than that made with blowing agents, but the products themselves may be heavier than comparable foamed products. This process is used for clear covers, cold drink cups, clear salad containers, plates, trays, bowls and hinged-lid containers, including hamburger ‘clamshells’. Polyethylene terephthalate (PET) foam has a maximum working temperature of ~200 °C, whereas polystyrene foam can stand a working temperature of 70 °C, and polyurethane and PVC foams 18

Plastic Sheet, Sheet Materials, Physical Properties and Plastics – an Overview ~100 °C. PET foam’s dimensional stability is better than for the other foams. Polystyrene foam has a complex microstructure and its diffusion properties are not completely understood. Polystyrene films are being used as a starting block to build a multi-scale model of diffusion through the foam. The foam is made from expanded beads and this creates a varying structure and varying wall thicknesses. This creates difficulties for accurately modelling the diffusion, because of the variability throughout. PS can be readily expanded with blowing agents to make cellular plastics of varying densities. These can be formed into containers or used as shapes for protective and shock-absorbing applications.

2.1.8 Polyethylene Terephthalate (PET) Polyethylene terephthalate, commonly known as PET, is a plastic material with simple long-chain polymers belonging to the generic family of polyesters. PET has found increasing application in the packaging industry due to its chemical inertness together with other physical properties such as high tensile strength, stiffness and use temperature of the commodity packaging polymers. PET was developed as a film in the late 1950s. In the early 1970s, PET was stretched by blow-moulding techniques which produced the first oriented three-dimensional structures initiating the rapid exploitation of PET as a lightweight, tough and unbreakable material. Table 2.7 shows some of the physical properties required for thermoforming of PET. PET is clear, tough and notch sensitive and applications include mannequins and chemical trays. It can readily exist in either an amorphous or a highly crystalline state [56]. It also has good clarity and good all-round barrier properties. In its pure form, PET is an amorphous, glass-like material. The crystalline state is necessary for extruding the material, and the amorphous state permits it to be oriented. Crystallinity is developed under the influence of an additive modifier or heat treatment of the 19

Update on Troubleshooting in Thermoforming

Table 2.7 Physical properties of PET obtained by using DSC Physical property

Value

Reference

1.41

[58, 59]

Tg (°C)

342–388

[58, 60–63]

Tm (K)

538

[58, 60–63]

3

Density (g/cm )

polymer melt. Today many requirements are placed on packaging materials. PET withstands handling, abuse and also protects the contents of the packaging without affecting them. Carbonated drinks have also proven difficult to package in plastic. PET bottles can be formed, which address many of the issues, but further developments are still required. Plastic materials used to produce the finished products in thermoforming are characterised by large deformations, non-isothermal conditions and non-linear material making the performance of the final part difficult to predict [57]. Trays made from amorphous PET sheet are crystallised for high-temperature service. Over the last decade, the volume of recycled PET has greatly increased, and is expected to grow further [54].

2.1.9 Polymethylmethacrylate (PMMA)/Acrylic Acrylics are very clear, are brittle, have excellent weathering properties and applications include skylights, windows and light diffusers. Acrylic polymers can be repeatedly softened and melted on heating and hardened on cooling with insignificant property changes and without considerable degradation. Table 2.8 shows some of the physical properties for thermoforming.

Table 2.8 Physical properties of PMMA Physical property Tg (K) Density (g/cm3)

20

Experimental condition

Value

Reference

318

[65, 66]

1.17–1.20

[67]

Plastic Sheet, Sheet Materials, Physical Properties and Plastics – an Overview Acrylic sheet ensures exceptional surface finish, optical quality (colourless) and thickness uniformity characteristics. The advantages include breakage resistance (in-shop, in-transit, during installation and in-service), excellent clarity and weathering properties, good stiffness, desirable thermoforming characteristics and enhanced solvent craze resistance. It also exhibits weathering advantages such as outstanding clarity and gloss retention properties and an unyielding resistance to yellowing. The advantages over alternative plastic sheet materials used in thermoforming are that it has shorter thermoforming cycles, lower forming temperatures and wider forming temperature windows. Acrylic sheet has successfully penetrated a variety of markets and applications based on its unique characteristics. It has significantly greater toughness and breakage resistance, has outstanding thermoforming definition and flexural modulus features for designers, is economically attractive and easily applied, and has exceptional quality, durability and appearance for end users. Acrylic is best known for its high transparency, good optics, surface hardness, chemical resistance and ability to withstand weathering. PMMA is the most important homopolymer in the acrylic family. When blended with vinyl, butadiene, acrylic rubbers and polyesters, improved physical properties are achieved. For applications that require breakage resistance, impact-modified pellets are used to produce extruded sheet. Since PMMA has good elastic memory, PMMA sheets can be formed using unmodified thermoforming equipment [64].

2.1.10 Polyetheretherketone (PEEK) Polyetheretherketone (PEEK) thermoplastic resin is characterised by a high Tm and high Tg (Table 2.9). It forms a partly crystalline polymer morphology that has a high resistance to chemical attack, radiation and thermal oxidation.

21

Update on Troubleshooting in Thermoforming

Table 2.9 Physical properties of PEEK Physical property Density (g/cm3) Tg (K)

Value

Method of determination

1.263–1.265

Amorphous

1.400–1.401

Crystalline

425

PVT data

[72]

410

DSC quenched

[73]

415

DSC annealed

[73]

Reference [69–71]

PVT: pressure, volume, temperature

PEEK-based film provides a comprehensive range of properties for high-performance applications including speaker diaphragms for mobile phones and consumer speakers, electrical wire insulation and cable wrapping, sensor membranes, industrial and electronics wear surfaces, electrical substrates and aerospace insulation blankets [68].

2.1.11 Polycarbonate (PC) Polycarbonate is very tough, has good heat resistance, has good weathering properties and is used in applications demanding a high degree of impact resistance, such as vandal-resistant glazing and sign faces in high-traffic, high-vandalism areas. Table 2.10 shows its physical properties which help in thermoforming. General-purpose polycarbonate sheet preserves impact properties significantly longer, resists yellowing and maintains a high degree of light transmittance over time.

Table 2.10 Physical properties of polycarbonate Value

Experimental method

Reference

Density (g/cm3)

1.2

ASTM D792

[74]

Tg (K)

423

Dielectric measurement

[74]

Physical property

22

Plastic Sheet, Sheet Materials, Physical Properties and Plastics – an Overview Polycarbonate requires special techniques to thermoform and it differs from most thermoplastic sheets. PC loses rigidity very rapidly above 188 °C. It has a very narrow high-temperature forming range. At higher temperatures entrained moisture can result in bubbling. In thin-sheet material, the water vapour usually escapes to the surface before it develops a bubble in the hot sheet. In the case of thicker sheet, it must be dried in an oven to ensure that bubbling will not occur during thermoforming.

2.1.12 Thermoplastic Olefin (TPO) Thermoplastic olefin (TPO) is very tough, flexible, has outstanding resistance to low temperatures, has low thermal expansion, has excellent UV resistance and is used in applications that combine the properties of high-quality polyurethane with the processing efficiency of thermoplastics. TPO has emerged as one of the most viable substitutes for PVC in automotive interior trim applications and this replacement continues to be a major objective of the automotive industry. It has not been an easy switch as initial TPO candidates lacked grain retention, were high in cost and had narrow processing windows [75]. Thermoplastic polyolefins (TPO) based on polypropylene (PP) are desired for cut sheet thermoforming applications for their superior physical properties, weathering performance and chemical resistance. However, their acceptance has been limited due to poor processing performance [76].

2.1.13 Polyphenylene Oxide (PPO) Polyphenylene oxide (PPO) is flame resistant, has good chemical resistance, has good electrical properties and applications include electrical enclosures and medical enclosures. Table 2.11 shows some of the physical properties required for thermoforming.

23

Update on Troubleshooting in Thermoforming

Table 2.11 Physical properties of PPO obtained using DSC Physical property 3

Density (g/cm )

Value

Reference

1.407 ± 0.01

[77]

363

[78]

535 ± 10

[78]

Tg (K) Melting point, Tm (K)

2.1.14 Thermoplastic Elastomers Thermoplastic elastomers are one of the fastest growing segments because they offer additional options and benefits on priceperformance. Thermoplastic olefins are expected to grow at higher rates than many styrenics and thermoplastic vulcanisates in automotive, durables and construction market sectors [79].

2.1.15 Biodegradable Polymer The history of thermoplastics began with the development of a thermoplastic celluloid in 1869. The demand for natural polymers is expected to grow by 7.1% annually to 2012, and opportunities are bright in packaging applications as a result of increased availability and cost effectiveness of polymers such as polylactic acid (PLA), according to Freedonia Group [80, 81]. Prices are stable due to it not being linked to petrochemicals and a profusion of new PLA varieties. The plastics are made from renewable carbon chain resources rather than fossil carbon from oil or gas [82–84]. Biodegradable packaging materials are renewable-resource-based biodegradable polymers and some of the potential biopolymers are cellulosic plastic, corn-derived plastics and polyhydroxyalkonates. These have the potential to be big players in the global packaging industry. In combination with nanoclay reinforcement, nanocomposites can be produced for a variety of applications with expectation to possess improved strength and stiffness with little sacrifice of toughness and reduced water/gas permeability. 24

Plastic Sheet, Sheet Materials, Physical Properties and Plastics – an Overview

Table 2.12 Physical properties of poly(lactic acid) Physical property 3

Density (g/cm ) Tg (K)

Value

Reference

1.248

[89]

326–337

[90–95]

PLA is a bio-resin that is rapidly expanding into a variety of applications. It requires a range of processing techniques including extrusion, calendering, blow moulding and thermoforming. The brittleness and low melt strength of the polymer provide challenges in processing and in the final product performance and the use of additives can improve processing, melt strength and impact strength of PLA [85]. It provides a lower coefficient of thermal expansion and an increased heat deflection temperature. Table 2.12 shows some of the physical properties required for thermoforming of PLA [86]. PLA is synthesised from carbon and other elements extracted from sugars, which have been broken down from starches from plants such as maize. The polymer is therefore sourced totally from renewable resources, and is said to be fully compostable in municipal and industrial compost facilities [87]. Polylactic acid is derived from sustainable carbon harvested from plants such as corn [88].

2.1.16 Cyclic-olefin Copolymer (COC) Cyclic-olefin copolymer (COC) blend can be extruded into thermoformable sheets, which can withstand steam sterilisation at 121 °C for 20 minutes [96]. Table 2.13 shows some of the physical properties required in thermoforming of COC.

Table 2.13 Physical properties of cyclic olefin polymer Physical property 3

Density (g/cm )

Value 1.02

Glass transition temperature (°C)

85

Thermoforming temperature (°C)

110–130

25

Update on Troubleshooting in Thermoforming COC can provide film producers and packaging converters with an opportunity to create thermoforming films. COC are amorphous thermoplastics with excellent moisture barrier, high temperature stability and stiffness. Mono- and multi-layer examples of LLDPEbased forming films, compared against commercially available products, clearly demonstrate how well the addition of COC improves physical properties, thermoforming and packaging performance. COC improves material distribution of LLDPE formed trays. These improvements enable the formed tray to withstand greater crushing force. Enhanced performance permits the possibility of down gauging [97].

2.1.17 Polyacetal (POM) Properties of thermoformable polyacetal, which is available in sheets and coils, include a high strength-to-weight ratio, high dimensional stability, scratch and wear resistance and good chemical and solvent resistance. In addition, the material has a surface that readily accepts paints and inks for decorating and printing applications [98].

2.1.18 Composites Composites are essentially a combination of more than one type of material. The final product obtains advantageous properties from each component. Sometimes a composite is the only material available

Table 2.14 Physical properties of polyacetal (POM) Physical property

Value

Density (g/cm3)

1.491

Melting Point Tm (K)

448

Tg (K)

198

26

Experimental method

Reference [99]

ASTM D2133

[100] [101]

Plastic Sheet, Sheet Materials, Physical Properties and Plastics – an Overview to obtain the properties needed. Composites can be combinations of metal–metal, metal–ceramic, metal–plastic, ceramic–ceramic, ceramic–plastic or plastic–plastic. There are three groups of composite materials: particulates, fibres and laminates. The length of the fibres is critical in fibre-reinforced composites. Thermoplastic composites made from polypropylene (PP) and long sisal fibres by using different processing techniques are thermoformed with small wall-thickness reductions to obtain a three-dimensional shape with very low forming energy, outstanding properties and excellent surface finish [102].

References 1.

M. Xanthos and D.B. Todd in Kirk–Othmer Encyclopedia of Chemical Technology, 4th Edition, Volume 19, Eds., Ed., J.I. Kroschwitz, John Wiley & Sons, New York, NY, USA, 1996, p.290.

2.

D.V. Bibee in Handbook of Plastics Materials and Technology, Ed., I.I. Rubin, John Wiley and Sons, NY, USA, 1990, p.317.

3.

R. Popli and L. Mandelkern, Journal of Polymer Science, Part B: Polymer Physics Edition, 1987, 25, 441.

4.

A.J. Peacock and L. Mandelkern, Journal of Polymer Science, Part B: Polymer Physics Edition, 1990, 28, 1, 917.

5.

J. Meissner, Pure and Applied Chemistry, 1975, 42, 551.

6.

F.C. Stehling, C.S. Speed and L. Westerman, Macromolecules, 1981, 14, 698.

7.

K.W. Doak and A. Schrage in Crystalline Olefin Polymers Part 1, Eds., R.A.V. Raff and K.W. Doak, Interscience Publishers, New York, NY, USA, 1965, Chapter 8.

27

Update on Troubleshooting in Thermoforming 8.

J.M. Dealy and K.F. Wissbrun, Melt Rheology and Its Role in Plastics Processing, Kluwer Academic Publishers, Norwell, MA, USA, 1999.

9.

P.C. Painter and M.M. Coleman, Fundamentals of Polymer Science, 2nd Edition, Technomic Publishing Co., Inc., Lancaster, PA, USA, 1997.

10. F.M. Schmidt, Y. Le Maoult and S. Monteix, Journal of Materials Processing Technology, 2003, 143/144, 225. 11. J.K. Lee, C.E. Scott and T.L. Virkler, Polymer Engineering and Science, 2004, 42, 7, 1541. 12. K. Dunay, Machine Design, 2002, 13, 65. 13. H.C. Lau, S.N. Bhattacharya and G.J. Field, Polymer Engineering and Science, 2004, 38, 11, 1915. 14. S.S. Morye, Polymer Engineering and Science, 2005, 45, 10, 1369. 15. Thermoforming Cycolac Brand ABS, Technical Publication P-406, GE Plastics, Pittsfield, MA, USA, 1989. 16. Physical Properties of Polymers Handbook, Ed., J.E. Mark, American Institute of Physics Press, Woodbury, NY, USA, 1996. 17. P.G. Kelleher, Advanced Polymer Technology, 1990, 10, 219. 18. F. Gay, Plast’ 21, 2002, 115, 96. 19. A.J. De Vries and C. Bonnebat, Polymer Engineering and Science, 2004, 16, 93. 20. W.J. Casey and K. Okano, Journal of Vinyl Technology, 2004, 8, 1, 37.

28

Plastic Sheet, Sheet Materials, Physical Properties and Plastics – an Overview 21. L.B. Weaver, P. Vercuski and D. Waszeciak, Journal of Vinyl and Additive Technology, 2004, 10, 3, 129. 22. E.C. Szamborski and T.E. Buterbaugh, Journal of Vinyl Technology, 1994, 16, 1, 11. 23. P.A. Rogers, Journal of Applied Polymer Science, 1993, 48, 1, 61. 24. H.C. Park and E.M. Mount in Encyclopedia of Polymer Science and Engineering, 2nd Edition, Eds., H.F. Mark, N.M. Bikkales, C.G. Overberger and G. Menges, John Wiley & Sons, New York, NY, 1987, Volume 7, p.89. 25. R.C. Stephenson and P.V. Smallwood in Encyclopedia of Polymer Science and Engineering, 2nd Edition, Eds., H.F. Mark, N.M. Bikkales, C.G. Verberger and G. Menges, John Wiley & Sons, New York, NY, USA, 1987, Volume 5, p.858. 26. G. Greiner and F.R. Schwarzl, Rheologica Acta, 1984, 23, 378. 27. P. Singh and J. Lyngaae-Joergensen, Journal of Macromolecular Science, Part B: Physics, 1981, B19, 2, 177. 28. G. Ceccorulli, M. Pizzoli and G. Pezzin, Journal of Macromolecular Science, Part B: Physics, 1977, B14, 4, 499. 29. P.G. Kelleher, Advanced Polymer Technology, 1990, 10, 219. 30. K.W. Doak and A. Schrage in Crystalline Olefin Polymers Part 1, Eds., R.A.V. Raff and K.W. Doak, Interscience Publishers, New York, NY, USA, 1965, Chapter 8. 31. D.V. Bibee in Handbook of Plastics Materials and Technology, Ed., I.I. Rubin, John Wiley & Sons, 1990, p.317. 32. Modern Plastics Encyclopedia, Ed., P.A. Toensmeier, McGraw-Hill, New York, NY, USA, 1996, p.185. 33. F.C. Stehling and L. Mandelkern, Macromolecules, 1970, 3, 242.

29

Update on Troubleshooting in Thermoforming 34. L. Mandelkern in Physical Properties of Polymers, 2nd Edition, Ed., J. Mark, American Chemical Society, Washington, DC, USA, 1993, p.189. 35. D.E. Klein, J.A. Sauer and A.E. Woodward, Journal of Polymer Science, 1956, 22, 455. 36. C. Vasile and M. Pascu, Practical Guide to Polyethylene, Rapra Technology Ltd, Shawbury, Shrewsbury, UK, 2005, p.176. 37. N. Alberola, J.Y. Cavaille and J. Perez, European Polymer Journal, 1992, 28, 935. 38. P. Ellis, J. Frogg, H. Maack and K. Scheidl, Chemical Week, 2002, 164, 35, 49. 39. L.M. Sherman, Plastics Technology, 2002, 48, 7, 44. 40. P. Roose, Popular Plastics and Packaging, 2007, 52, 1, 40. 41. W. Weng, E.J. Markel and A.H. Dekmezian, Polymer Preprints, 2000, 41, 2, 1924. 42. E. Winkel, H. Gross, U. Masberg and J. Wortberg, Advances in Polymer Technology, 2003, 2, 2, 107. 43. Flexible and Plastic Packaging Innovation News, 2008, 2, 5, 39. 44. S. Mohammadian-Gezaz, I. Ghasemi, M. Karrabi and H. Azizi, Polymer Testing, 2006, 25, 4, 504. 45. G. Natta and P. Corradini in Proceedings of the Conference Nuovo Cimento XV, 1960, p.40. 46. B. Wunderlich, Macromolecular Physics, Academic Press, New York, NY, USA, 1980, Volume 3, p.61. 47. Polymer Handbook, 3rd Edition, Eds., J. Brandrup and E. H. Immergut, John Wiley & Sons, New York, NY, USA, 1989, p.5.

30

Plastic Sheet, Sheet Materials, Physical Properties and Plastics – an Overview 48. P. Järvelä and L. Shucai, Journal of Applied Polymer Science, 1996, 62, 813. 49. K. Mezghani, R.A. Campbell and P.J. Phillips, Macromolecules, 1994, 27, 997. 50. J. Hockey, Plastics and Rubber Weekly, 2008, 14th March, p.11. 51. J. Brandrup and E.H. Immergut, Polymer Handbook, 3rd Edition, Wiley-Interscience, New York, NY, USA, 1989. 52. R.F. Boyer in Encyclopedia of Polymer Science and Technology, Eds., H.F. Mark, N.M. Bikales, C.G. Overberger, and G. Menges, John Wiley & Sons, New York, NY, USA, 1970, p.13. 53. D.W. Van Krevelen and P.J. Hoftyzer, Properties of Polymers: Correlations with Chemical Structure, Elsevier Publishing Company, Amsterdam, The Netherlands, 1972. 54. Physical Properties of Polymers Handbook, Ed., J.E. Mark, AIP Press, Woodbury, New York, NY, USA, 1996. 55. C.F. Curtis and J. Minjas, Parasitology Today, 1985, 1, 36. 56. Plastics and Rubber Asia, 2002, 17, 111, 40. 57. D.O. Kazmer in Proceedings of the Institute of Industrial Engineers, International Industrial Engineering Conference, Chicago, IL, USA, 1992. 58. Encyclopedia of Polymer Science and Engineering, Volume 12, Eds., H.F. Mark, N.M. Bikales, C.G. Overberger and G. Menges, John Wiley & Sons, New York, NY, USA, 1985, p.226. 59. I.H. Hall, Structure of Crystalline Polymers, Elsevier Applied Science Publishers, Barking, UK, 1984, p.39.

31

Update on Troubleshooting in Thermoforming 60. F. Rodriguez, Principles of Polymer Systems, 2nd Edition, McGraw-Hill, London, UK, 1983, p.432. 61. L.H. Sperling, Introduction to Physical Polymer Science, 2nd Edition, John Wiley & Sons, New York, NY, USA, 1992, p.199. 62. C.H. Palys and P.J. Phillips, Journal of Polymer Science, Part B: Polymer Physics Edition, 1980, 18, 829, 16. 63. Handbook of Plastics Materials and Technology, Ed., I.I. Rubin, John Wiley & Sons, New York, NY, USA, 1990, p.644. 64. P. Carfantan, Modern Plastics Worldwide, 2005, 87. 65. Materials for Microlithography: Radiation-Sensitive Polymers, Eds., L.F. Thompson, C.G. Willson and J.M.J. Frechet, American Chemical Society, Washington, DC, USA, 1984, p.266. 66. E. John and T. Ree, Journal of Polymer Science, Part A: Polymer Chemistry Edition, 1990, 28, 385. 67. F.W.J. Billmeyer, Textbook of Polymer Science, John Wiley & Sons, New York, NY, USA, 1984. 68. K. Boi, Asian Plastics News, 2007, November, p.14. 69. D.J. Blundell and B.N. Osborn, Polymer, 1983, 24, 953. 70. P.C. Dawson and D.J. Blundell, Polymer, 1980, 21, 577. 71. S.X. Lu, P. Cebe and M. Capel, Polymer, 1996, 37, 2, 999. 72. P. Zoller, T.A. Kehl, H.W. Starkweather and G.A. Jones, Journal of Polymer Science, Part B: Polymer Physics Edition, 1989, 27, 993. 73. J.E. Harris and L.M. Robeson, Journal of Applied Polymer Science, 1988, 35, 1, 977.

32

Plastic Sheet, Sheet Materials, Physical Properties and Plastics – an Overview 74. Physical Properties of Polymers Handbook, Ed., J.E. Mark, AIP Press, Woodbury, NY, USA, 1996. 75. R. Bragole and C. Yang in Proceedings of the SPE Automotive TPO Global Conference, Dearborn, MI, USA, 2002, p.49. 76. T.A. Hogan, S.M. Hoenig, B.W. Walther, K.L. Walton, M.F. Finlayson and S.C. Costeux in Proceedings of the SPE ANTEC Conference, 2007, Cincinnati, OH, USA, p.1127. 77. J. Boon and E.P. Magré, Makromolekulare Chemie, 1969, 126, 130. 78. W.J. Wrasidlo, Journal of Polymer Science, Part B: Polymer Physics Edition, 1972, 10, 1, 719. 79. L.B. Weaver, P. Vercuski and D. Waszeciak, Journal of Vinyl and Additive Technology, 2004, 10, 3, 129. 80. C. Bonten, Bioplastics Magazine, 2008, 3, 5, 28. 81. Plastics and Rubber Weekly, 2008, 5th December, p.13. 82. Plastics Technology, 2007, 53, 2, 60. 83. J.H. Schut, Plastics Technology, 2008, 54, 11, 66. 84. Packaging Review South Africa, 2009, 35, 1, 47. 85. Z. Cygan, M. Singh and S. Seshadri in Proceedings of the SPE ANTEC Conference, 2007, Cincinnati, OH, USA, 2007, p.3071. 86. A. Taylor, Brand, 2006, 5, 3, 34. 87. British Plastics and Rubber, 2002, 26. 88. Packaging Magazine, 2002, 5, 15, 24.

33

Update on Troubleshooting in Thermoforming 89. E.W. Fischer, H.J. Sterzel and G. Wegner, Kolloid-Zeitschrift und Zeitschrift für Polymer, 1973, 251, 980. 90. I. Engelberg and J. Kohn, Biomaterials, 1991, 12, 292. 91. D.K. Gilding and A.M. Reed, Polymer, 1979, 20, 1, 459. 92. K. Jamshidi, S-H. Hyon and Y. Ikada, Polymer, 1988, 29, 2, 229. 93. A. Celli and M. Scandola, Polymer, 1992, 33, 2, 699. 94. D. Cohn, H. Younes and G. Marom, Polymer, 1987, 28, 2, 18. 95. P. Van De Witte, P.J. Dijkstra, J.W.A. Van Den Berg and J. Feijen, Journal of Polymer Science, Part B: Polymer Physics Edition, 1996, B34, 2, 553. 96. Plastics Technology, 2003, 49, 12, 17. 97. P.D. Tatarka in Proceedings of the SPE ANTEC Conference, 2007, Cincinnati, OH, USA, 2007, p.1149. 98. Plastics Engineering, 2003, 59, 10, 9. 99. B. Wunderlich, Macromolecular Physics, Volume 1, Crystal Structure, Morphology, Defects, Academic Press, New York, NY, USA, 1973, p.118. 100. A.G. Serle in Engineering Thermoplastics: Properties and Application, Ed., J.M. Margolis, Marcel Dekker, New York, NY, USA, 1985, p.151. 101. Y. Aoki, A. Nobuta, A. Chiba and M. Kaneko, Polymer Journal, 1971, 2, 502. 102. L.M. Arzondo and C.J. Perez, Polymer Engineering and Science, 2005, 45, 7, 976.

34

3

Machine Specifications – Effect of Parameters

The thermoforming process is different from other plastic-processing techniques. The thermoplastic sheet is the most important element of the thermoforming process. Without it, thermoforming would not be possible. Thermoforming deals with the changes that can be effected in the sheet materials. In thermoforming the material is not melted, but raw thermoplastic sheet materials are converted into finished products at their softening point and, moreover, low pressure is required to thermoform, because the mould material is less sturdy compared to other plastic processing. Relatively low forming pressures are needed and large sizes can be economically fabricated. Since the moulds are exposed to relatively low forces, they can be made of inexpensive materials. Mould fabrication time is therefore very short, minimising lead times. Thermoforming is often selected for fabricating prototype and display parts due to its low tooling costs. However, as part volumes increase, processes such as injection moulding become more economical. Packaging from thermoforming is penetrating the narrow web market with great results because the printers and converters are constantly developing new trends in packaging to attract consumers [1]. Figure 3.1 shows the major steps involved in the thermoforming process from heating to trimming. Thermoforming has become one of the fastest, if not the fastest, growing methods of processing plastics. With the development of the thermoforming technique, more and more relevant theoretical

35

Update on Troubleshooting in Thermoforming

Heating

Clamp up

Pre-stretch

Optional plunger assist

Trimming

Final forming and cooling

Forming

Figure 3.1 Main steps for thermoforming process Reproduced with permission from L-T. Lima, R. Auras and M. Rubino, Progress in Polymer Science, 2008, 33, 838. ©2008, Elsevier Ltd [2] research on thermoforming control is carried out. Since the repetitive production of sheets meeting standards is very demanding in the industry, the implementation of cyclic control on the machine has naturally become an important objective for researchers [3]. Each thermoforming process has its own characteristics. The parameters and the process will determine the suitability of the thermoforming conditions. It is necessary to establish the process conditions to ensure the quality of the finished products. Acrylonitrile-butadiene-styrene (ABS) can be easily processed during thermoforming due to the fact that its melt strength increases with decreasing temperature and increasing extrusion rate and it has a good sagging resistance. It has the highest melt strength in the low extrusion temperature region approaching the thermoforming region [4]. The thermoforming process is widely used in the packaging, pharmaceutical, toy and automotive industries to form numerous containers of different sizes, from small food trays to automotive dashboard skins and inner door panels, among others [5]. The thermoforming process consists of heating an extruded sheet of thermoplastic material and applying a pressure differential to force 36

Machine Specifications – Effect of Parameters the material into some mould cavity. There are many variations of this process in which the sheet may be heated by conduction, convection or radiation; forced with plugs or slides; and inflated with positive air pressure or vacuum [6]. The basic principle involved in thermoforming is to heat the thermoplastic sheet until it softens and force the hot and pliable material against the contours of a mould by using vacuum, air or mechanical forces. Then the sheet will be allowed to cool against the mould and the plastic retains its shape. Thermoplastics can be thermoformed according to the principles of positive, negative or free forming, with or without the use of air pressure or vacuum. Male forming gives a thicker bottom, whereas female forming implies thicker walls. Free-formed thermoplastics need to be kept in their desired shapes until they have reached a temperature lower than 70 °C. Figure 3.2 shows the schematic basic configuration of a thermoforming machine. Advances in thermoforming technology have been made through improved machinery and the limitations are being challenged almost daily. Innovations and improvements in machinery include: •

Improved ways of clamping the polymer sheet material;

•

Improved heaters and heat distribution patterns and controls;

•

Sensors for monitoring and controlling the sheet residence time in the oven;

•

Better control of stretching forces and pressures;

•

Improved trim dies;

•

Improvement in moulds and mould materials;

•

Improved plug-assist materials.

Annealing is not needed when parts are formed. If stress cracks occur on a thermoformed part, the part can be reconditioned at 70 °C. 37

Update on Troubleshooting in Thermoforming

Rolled sheet stack Pre-heating unit Main heat “Tunnel”

Granulation

Former and in situ trim press

Component collation and stacking

Figure 3.2 Schematic diagram of the thermoforming process. Reproduced with permission from Polypropylene Sheet Extrusion and Thermoforming for Packaging Applications, Basell Polyolefins, Rotterdam, The Netherlands, 2005, 054 PP e. ©2005, Basell Polyolefins [7] Thermoforming is a collection of manufacturing methods that heat and form sheets of extruded plastic. Thermoforming processes include drape, vacuum and pressure forming. The techniques of the forming process can be grouped into several types, as described below.

38

Machine Specifications – Effect of Parameters

3.1 Methods of Vacuum Forming 3.1.1 Straight Vacuum Forming One of the oldest techniques is straight vacuum forming, i.e., where the sheet is held against the top of a female mould at the start of the heating cycle. However, this technique is seldom feasible with highdensity polyethylene due to the appreciable sag of the sheet while heating. Therefore, drape forming is the first practical technique using these materials.

3.1.2 Vacuum (Thermo) Forming In vacuum forming, after the plastic sheet is heated to soften it, it is drawn down onto a male or female tool that has vent holes around the periphery and in areas requiring crisp detail. The air is sucked out of the mould through the vent hole to form a vacuum, causing the moulding material to conform to the mould and assume its shape. Air is allowed in again to the remove the part. The application of a vacuum offers improved feature definition and greater wall thickness consistency. Vacuum forming also allows the use of thicker sheet stock and reduces forming time, when compared to drape forming. Figure 3.3 shows the straight vacuum forming process with variable configuration. Advantages: •

Requires low temperature and pressure;

•

Lower cost of mould and machine and these remain reasonable for large parts;

•

Used to manufacture large parts;

•

Faster mould cycles.

39

Update on Troubleshooting in Thermoforming Radiant Heater

IR Heat Clamp Heated Thermoplastic Sheet Volume of entrapped air

1

Vacuum Channel

Mold Temperature Sensor Lead

Vacuum Drawn

2 Cooling Heating Channels

Figure 3.3 Schematic of the vacuum forming process for thermoplastic resin in sheet form (straight vacuum forming). Reproduced with permission from D. F. Walczyka and S. Yoo, Journal of Manufacturing Processes 11, 2009, 9. ©2009, Elsevier Ltd [8] Disadvantages: •

High cost of plastic sheet and scraps;

•

Part shapes limitation;

•

Part defined by mould on only one side;

•

Non-uniformity with inherent wall thickness;

•

Residual stresses.

Any thermoplastic extruded sheet may be used in simple and straightforward vacuum forming when prototyping or manufacturing plastic parts. Unlike injection- or blow-moulding processes, wall thickness can range from foils to thick gauge stock. There are a few considerations to be made before choosing vacuum forming. This process does not support variable wall thicknesses. The geometry of the part must allow a straight pull and no undercuts or side action are allowed. Unlike injection moulding, vacuum forming can not manufacture strengthening ribs or mounting bosses. The side

40

Machine Specifications – Effect of Parameters of the part that does not contact the tool surface will lack detail and definition when using forming equipment designed for prototyping. For large production runs, vacuum forming tools are machined from aluminium. Dual-sided texture and feature definition are only available with matched tools, which are used with production vacuum forming equipment. Packaging is the leading application for vacuum forming. Blister packs are produced by this method. For vacuum-formed products, the packaging industries are the main consumers and the applications extend across many product types, which include automotive, aerospace, marine industry, electronics and many more. The list of applications is virtually unlimited. Vacuum forming delivers plastic parts for prototyping and short-run manufacturing and the thermoformed products are delivered quickly and cost effectively.

3.1.3 Pressure Forming In pressure forming (vacuum or pressure) (see Figure 3.4), the positive air pressure is to be applied between 99,974 and 2,068,427 Pa. Lower temperatures with higher forming pressure and faster mould cycles use air pressure, often with the addition of a plug-assist in pressure forming. The plug-assist forces the material deeper into the mould cavity. Even though it is possible to have fine detail and surface finish in injection moulding with pressure, it usually requires much higher annual volume than thermoforming to be economically feasible. Injection moulding becomes a more serious contender for the application as its volume increases. Then the piece part cost differential can be applied to the substantially greater cost of the injection moulds. The cost of the tooling for injection moulding rises substantially with increasing size, and the payoff volume, the point at which the additional tooling cost is offset by piece part savings, goes 41

Update on Troubleshooting in Thermoforming Pressure Clamp

Clamp

Polymer sheet Deformed shapes Mould

Figure 3.4 Pressure forming. Reproduced with permission from M.K. Warby, J.R. Whitemana, W-G. Jiang, P. Warwick and T. Wright, Mathematics and Computers in Simulation, 2003, 61, 210. ©2003, Elsevier Science BV [9] up accordingly. Thermoforming can, however, make substantially larger parts than can injection moulding.

3.1.4 Plug-assist Pressure Forming In plug-assist pressure forming, a plug forces the hot sheet into a female cavity. The plug further forces the plastic sheet against walls of the mould while air pressure is applied. To optimise the material distribution, plug design and plug speed can be varied. For plug-assist thermoforming, plugs manufactured from syntactic foam are used and coated with a slip coating to prevent sticking in some deep draw applications. Plug shape has had more impact on part quality than has the material of construction. Plug shape is dependent upon the particular part being moulded.

3.1.5 Plug-assist Vacuum Forming Plug-assist forming pulls additional material into the female cavity in the manufacture of thin wall thickness articles. The plug should

42

Machine Specifications – Effect of Parameters be around 20% smaller than the mould. The mould is heated just under the forming temperature of the sheet. The plug forces the hot sheet into the mould cavity and air is drawn from the mould to form the part. Both pressure and the vacuum plug-assist process allow deep drawing and short cycle time with good wall thickness control, require close temperature control and are more complex than straight vacuum forming. Figure 3.5 shows a typical process involved in plug-assist vacuum forming processing. Plug-assist vacuum forming creates better wall thickness uniformity, especially for cup or box shapes. The plug materials include wood, metal and thermoset polymer and the plug is 10–20% smaller than the cavity. The temperature of the plug is important in plug-assist vacuum forming. The deep draw shortcoming of the female mould can usually be overcome by the use of plug-assist. Plug-assist is commonly used in deep drawn articles, or where the distribution of materials needs to be altered. In using complicated moulds with grooves, pockets or recesses, the plug should be designed to carry more material into these areas. A temperature below the suggested range can result in chill marks and thin spots in the material. Pettersen and co-workers made a comparison of oxygen barrier properties and wall thickness distribution of different thermoformed trays manufactured with three drawing depths and two different thermoforming methods, with and without plug-assist [11]. They found that temperature had more influence on the oxygen transmission rate (OTR) than humidity. In the packages, the OTR increased with increasing drawing depth. OTR may have influence by orientation other than thinning. Plug-assisted thermoforming had an effect on the OTR in a thick laminate of polypropylene (PP)/polyethylene (PE), which was probably due to exceeding the maximum drawing depth of this material. The influence of plug design was investigated, namely plug volume, plug taper, plug depth and plug temperature on the wall thickness distribution, weight and compression strength, in thermoformed 43

Update on Troubleshooting in Thermoforming

Mould cavity

(a)

(b)

Compressed air

(c) Compressed air

Vacuum (d)

Figure 3.5 Plug-assist vacuum forming. Reproduced with permission from G. Sala, L. Di Landro and D. Cassago, Materials and Design, 2002, 23, 21. ©2002, Elsevier Science [10] PP cups and it was observed that the plug volume was the most importance factor for part shape. Plug depth had a significant effect on the bottom and corner thicknesses and part weight. Plug temperature and plug taper had a significant effect only on the compression strength [12].

44

Machine Specifications – Effect of Parameters In plug-assist thermoforming, surface friction strongly affects the final part thickness distribution. The effect of the plug material, plug temperature and sheet temperature on the coefficient of friction between the plug and the sheet with three different plug materials (epoxy syntactic, engineering thermoplastic non-syntactic and engineering thermoplastic syntactic) with a range of friction coefficients affects the thickness of the thermoforming polypropylene sheet [13]. Prototype thermoforming trials of a soy protein isolate-based sheet material were conducted to evaluate the effect of moisture level, draw ratio, plug-assist and vacuum rates on the forming of cupshaped containers. The forming behaviour of the soy sheet was compared with that of PVC and PP materials. Cycle time and draw ratio comparisons were made. The soy protein isolate sheet material exhibited similar thickness reductions to PP and PVC for comparable draw ratios. Plug-assist forming did not improve the wall thickness reduction of soy protein isolate as it did for PP and PVC. Moisture change, from a high of 15% to 10%, did not significantly affect the forming behaviour of soy protein isolate sheet material [14]. The stretching operation is often performed by the movement of a mechanical plug, which contacts some areas of the sheet. During contact it is known that conductive heat transfer between the plug and sheet material is an important factor in determining the process output. The process is characterised by large deformations, nonisothermal conditions and non-linear material behaviour, making final part performance difficult to predict. There are many variations of this process in which the sheet may be heated by conduction, convection or radiation; forced with plugs or slides; and inflated with positive air pressure or vacuum [6]. The selection of plugs for thermoforming of cups for products such as yogurt is related to the problem of pre-stretching with plug assists. Empirical plug design is considered with reference to plug geometry and plug penetration, triggering of forming pressure, plug material (polyoxymethylene or syntactic foam) and plug temperature [15].

45

Update on Troubleshooting in Thermoforming An instrument has been developed to obtain the friction coefficients between sheet and plug in the plug-assist thermoforming process, to facilitate process simulation. Discs of the polymer and the plug material are pressed together under a known load, at a controlled temperature, and one disc is rotated whilst the torque is measured on the other. This enables determination of the friction coefficient as a function of temperature and shear rate [16].

3.1.6 Pressure-bubble Plug-assist Vacuum Forming When deep articles with good thickness uniformity are required, the pressure-bubble plug-assist vacuum is used. The sheet is placed in the frame and heated. Controlled air pressure is used to create the bubble. The bubble has been stretched to a pre-determined height; the normally heated male plug-assist is lowered to force the stretched sheet into the cavity. The variation in the plug speed and shape can improve the distribution of the material. The plug has to be made as large as possible to stretch the plastic material close to the shape of the finished part. The plug penetrates around 80%. The air pressure is applied from the plug while a vacuum is drawn on the cavity. The female mould helps to escape the trapped air by venting.

3.1.7 Free Forming In free forming, very low pressure, about 2.76 MPa, is used to blow a hot plastic sheet through the contour of a female mould. The sheet forms a smooth bubble-shaped article with the help of air pressure. Since the air touches only the side of the pad, there will be no markoff unless a stop is used to form a profile in the bubble. Figure 3.6 shows a photograph of free forming.

3.1.8 Matched-die Forming Matched-die forming is similar to compression moulding. Figure 3.7 shows a schematic representation of the matched-die forming process. 46

Machine Specifications – Effect of Parameters

Sheet

Lamp

Photoelectric eye

Bubble

Clamp Mold

Inflating air

Figure 3.6 Free forming. Reproduced with permission from J. L. Throne, Technology of Thermoforming, Carl Hanser Verlag, Munich, Germany, 1996, 18. ©1996, Carl Hanser Verlag [17]

Inner mould Outer mould Outlets (a)

(b)

Final shape (c)

Figure 3.7 Matched-die forming. Reproduced with permission from G. Sala, L. Di Landro and D. Cassago, Materials and Design, 2002, 23. ©2002, Elsevier Science [10] 47

Update on Troubleshooting in Thermoforming The sheet is trapped between the male and the female moulds. The mould is made up of wood, plaster, epoxy, etc. The water-cooled matched moulds produce more accurate parts with close tolerances, although the cost is higher. In the matched-die forming method, the material is heated to the sag point, and then transferred to the moulding station, where the plug and cavity moulds are brought together to squeeze the material. During the process no vacuum or air pressure is applied. The material is mechanically pressed into the shape defined by matched moulds, and allowed to cool while the mould continues to press against it.

3.1.9 Drape Forming In drape forming, the sheet is framed, heated and mechanically stretched and a pressure differential is then applied to form the sheet over a male mould. This process is comparable to straight vacuum forming, however drape forming is more complex. The sheet touches the mould and remains close to its original thickness in drape forming. Male moulds are cheaper and easier to build than female moulds. Male moulds are very susceptible to damage. Gravitational force can also be used alone in drape forming. In female moulds, multi-cavity forming is possible and they do not require as much space as male moulds. Drape forming relies on gravity to pull the sheet against the tool. Vacuum forming, as the name implies, draws the heated sheet against the tool with the assistance of a vacuum. Pressure forming combines vacuum and pressure to simultaneously pull and push the plastic sheet to the contours of the tool. Figure 3.8 shows a schematic diagram of drape forming. Uniaxial bent parts can be achieved by drape forming. Moulds can be made out of wood or aluminium covered with felt. Slight pressure is sufficient to drape the sheet over the positive mould. The recommended sheet temperature for drape forming is 130 °C. The masking tape should be removed before putting the sheet into 48

Machine Specifications – Effect of Parameters Heat source

Frame Sheet

Mould

Vacuum (a) Airtight seal

(b) Thick area Thin area Moulded area (c)

Figure 3.8 Drape forming. Reproduced with permission from G. Sala, L. Di Landro and D. Cassago, Materials and Design, 2002, 23, 21. ©2002, Elsevier Science [10]

the heating oven. The sheet is placed on the mould immediately after heating. The sheet should be allowed to cool down at room temperature, not force cooled with air. Cool draughts should be avoided during processing, as these may cause distortion/stress in the draped part.

3.1.10 Vacuum Snap-back Forming Process In the vacuum snap-back forming process, the rheological behaviour of the ABS polymer is related to the hard rubber particles associated with it. Figure 3.9 shows a schematic representation of the vacuum snap-back forming process.

49

Update on Troubleshooting in Thermoforming

Vacuum

Vacuum

Frame Switch

Vacuum (a)

Room pressure (c)

Vacuum (b)

Final shape (d)

Figure 3.9 Vacuum snap-back forming process. Reproduced with permission from G. Sala, L. Di Landro and D. Cassago, Materials and Design, 2002, 23, 21. ©2002, Elsevier Science [10] Strain hardening is a reliable indicator of the thermoforming performance of ABS polymers [18]. In vacuum snap-back forming, the influence of the initial temperature distribution over the sheet on the part thickness distribution, the effect of temperature on the material rheology and the polymer/mould friction coefficient are primarily responsible for the changes in the thickness distribution. During the bubble growth stage, even small temperature differences over the sheet greatly influence the bubble shape and pole position and play a critical role in determining the part thickness distribution [19]. In the vacuum snap-back forming process, the influence of both rheological properties and processing parameters on the part 50

Machine Specifications – Effect of Parameters distribution shows a predominant role over the other processing parameters in uniform wall thickness. Rheological properties include uniaxial and biaxial elongational viscosity and strain hardening and/or softening, while processing parameters include friction coefficient, heat transfer coefficient and sheet and mould temperatures [20]. In this study we investigated the influence of initial temperature distribution over the sheet on the part thickness distribution of a vacuum snap-back forming process. The effects of the temperature distribution on the material rheology and polymer/mould friction coefficient are primarily responsible for the changes in the thickness distribution. Even small temperature differences over the sheet greatly influenced bubble shape and pole position during the bubble growth stage and played a critical role in determining the part thickness distribution [21].

3.2 Effect of Parameters 3.2.1 Raw Material (Plastic Sheets) Stretching, orientation and thermoforming involve the ability of a plastic film or sheet to withstand high mechanical stress and permit considerable elongation without failure; they are therefore best at quite high molecular weight. Figure 3.10 shows a graphical representation of thermal history during thermoforming processing. Thermoforming usually begins with plastic sheet or film; sheet thickness tends to be 250 μm and greater; film thicknesses are normally less than 250 μm. Extrusion is the most common method of producing sheet and film for thermoforming; very small amounts are cast or calendered [23]. Ease of forming depends on material characteristics; it is influenced by minimum and maximum thickness, pinholes, the ability of the material to retain heat gradients across the surface and the thickness, the controllability of applied stress, the rate and depth of draw, the 51

Update on Troubleshooting in Thermoforming

Temperature of sheet (C)

200 In the oven

180 160

Cooling Stretch- with ing mold closed

Cooling with mold open

Part being removed

140 120 100 80 60 40 20 0 0

10

20

30

40

50

60

70

80

90

100

Figure 3.10 Graphical representation of thermal history during thermoforming. Reproduced with permission from X. Haihong, J. Wysocki, D. Kazmer, P. Bristow, B. Landa, J. Riello, C. Messina and R. Marrey in Proceedings of the ANTEC Conference, New York, NY, USA, 1999, 872. ©1999, SPE [22] mould geometry, the stabilising of uniaxial or biaxial deformation and, most importantly, minimising the thickness variation of the sheet. Another component of successful thermoforming that was mentioned earlier is crosslink density. Systems that will thermoform readily are thermoplastic. Films such as PVC, polyethylene terephthalate, PE and PP are thermoplastic and will readily deform upon heat and pressure. Pre-drying is not required; however, if sheets are in stock for a very long period, moisture may be absorbed, and pre-drying would be required. Residual stress is an inherent consequence of plastic manufacturing processes, and it occurs when molten polymer is cooled and shaped. In most cases these stresses are an invisible and insidious problem that can result in field failures [24]. Sagging of polymer sheet is observed during the heating phase of the thermoforming process. Sometimes sagging is catastrophic, leading

52

Machine Specifications – Effect of Parameters to the sheet falling over the heaters. The behaviour of large polymer sheets during the sagging phase is highly unpredictable [25]. There can be the problem of thickness variations in extruded sheet – a problem which can have equipment-related causes, material-related causes or process-related causes [26].

3.2.2 Moulds Wood moulds, particularly pine and sappy woods, should be used only for prototypes. Aluminium is the best material for permanent moulds. Aluminium must be maintained at 93–99 °C with an oil heat exchanger to prevent chill lines and other distortions. While epoxy and polyester moulds last longer than wood moulds, their heat transference is much poorer than aluminium, and they require long cooling cycles. Epoxy and polyester moulds must be cooled to prevent high temperatures from destroying the surface. A 5° draught angle is recommended on all vertical surfaces to allow easy removal. Moulds may be constructed inexpensively with simple materials, such as wood, plaster, epoxy or reinforced polyester. The life expectancy of such moulds is short, however, and the low heat-transfer coefficient of the construction materials, coupled with the lack of mould temperature control, reduces thermoforming efficiency. Chromium steel moulds give the best optical results. Cast aluminium moulds with moulded-in heat control channels are more efficient and last longer, but require greater initial investment and are costly to modify. Thermoforming moulds can be made of most common materials. If a series is small, or if you are working on a prototype, a wooden mould is acceptable. The problem with wood is the accumulation of heat in the mould. The cooling time will be long and the productivity low in a continuous production. If the radius is below 400 mm, a female mould should be used. It could either be vacuum bagged or match mould formed. The latter

53

Update on Troubleshooting in Thermoforming should be used if the radius is small, the thickness or density is high and a high load is required. Fixed stops must be used to avoid compression of the core. Steel or aluminium moulds are preferable due to their high thermal conductivity and stability. Plastic moulds could be used but they also accumulate heat. Single curved products with a radius greater then 400 mm are best formed on a male mould. This could either be sweep forming with a thin steel foil or vacuum bagging. Vacuum bagging is a simple operation with low tooling costs, but it has some disadvantages. It takes time to apply the vacuum bag, and the sheet might cool down too much. This can be avoided if the vacuum bag is assembled on a cold sheet and mould. The mould is then placed in a hot air oven. The temperature inside the sheet is registered with a thermal gauge. When the correct temperature is reached the vacuum is applied. The mould is then taken out of the oven and allowed to cool down with the vacuum. Consideration has to be given to time, temperature and vacuum to avoid a creeping effect. Uniform mould temperature is essential to the production of minimally stressed parts. Typically, this temperature should be in the range of 60–70 °C. The mould interior must be channelled, and should have enough vacuum holes to allow fast, uniform evacuation during thermoforming. The heart of the vacuum-forming process is tooling. Processes such as blow moulding and thermoforming offer the potential to manufacture large parts with much lower tooling costs than injection moulding [27]. Prototypes and tooling can be fabricated from sheets of a thermoplastic alloy that readily shapes via thermoforming [28]. Normally, mould temperatures should be below the heat distortion temperature of the sheet material. Mould temperatures in the range of 20 °C to 60 °C are commonly used. The complete repeating sequence of operation, i.e., heating, thermoforming, cooling and ejection is dependent upon cycle time, relative humidity, sheet thickness and temperature. Cycle time ranges from seconds to many minutes. The forming cycle in 54

Machine Specifications – Effect of Parameters the thermoforming process begins by sealing the clamped, pre-heated sheet to the mould. Mould temperature in this range allows for excellent material distribution, good production rates and production of blisters with good dimensional stability. Without mould temperature control, results may vary due to heat variations of the mould. Moulds used in the thermoforming process are of three basic designs: 1. The male mould where all the mould surfaces are raised; 2. The female mould where all the mould surfaces are recessed; 3. Combinations of 1 and 2 where the mould surfaces are either raised or recessed. Figure 3.11a shows the operation of thermoforming with male and Figure 3.11b with female moulds. HEATING

STRETCHING

COOLING

TRIMMING

EXTRUDED SHEET

Q

Figure 3.11a Basic operations in thermoforming process - male mould. Reproduced with permission from F.G. Torres and S.F. Bush, Composites: Part A, 2000, 31, 1293. ©2000, Elsevier [29] Heating zone Polymer sheet Plug

Mould Infrared emitters

Figure 3.11b Plug assist thermoforming - female mould. Reproduced with permission from F.M. Schmidt, Y. Le Maoult and S. Monteix, Journal of Materials Processing Technology, 2003, 143–144, 226. ©2003, Elsevier Ltd [30] 55

Update on Troubleshooting in Thermoforming In general, the male moulds allow deeper draws than the female moulds since the plastic can be draped (pre-stretched) over it. Male moulds are also easier to fabricate and are cheaper to produce than female moulds. Male moulds are more popular and are more commonly used for blister packaging because they produce a stronger blister package. A mould that is too cold may cause tension in a thermoformed piece, depending on thickness and complexity of the formed piece.

3.2.3 Vacuum The basic requirements of vacuum pump, surge tank and required lines are very critical. The application of the vacuum during the cycle should be consistent. The sheet being drawn must contact the mould (and thus begin cooling) uniformly and quickly so that highly stressed thin sections in the part do not occur. Normally, a vacuum of 736 mm of mercury is used, and the reading on the gauge should not drop below 508 mm during forming. The size of the surge tank and the number and size of holes in the mould must be designed with these conditions in mind.

3.2.4 Vacuum Holes Vacuum holes should be kept to the minimum needed to provide quick and uniform air evacuation between the plastic web and the mould. These holes are generally between 0.04 and 0.08 mm in diameter to avoid any visual marks being left on the thermoformed blister package. The larger holes are generally recommended for thicker gauged materials. Careful placement of these holes and counterboring them from the backside will reduce the pressure drop during evacuation and decrease cycle time. Further cycle time reduction can be achieved if the part design allows the use of long slots for quick air removal.

56

Machine Specifications – Effect of Parameters Part design: To prevent stress concentrations and cracking problems, sharp corners should be avoided. All radii should be at least equal to the starting sheet gauge. Deep draw parts should be designed with at least 3° of draught and the part stiffness and appearance can be enhanced by using ribs.

3.2.5 Plug In order to prevent chill marks and sticking, plug assists should be made from low heat-transfer materials, have a smooth surface finish with no imperfections and possess good release characteristics. If a plug-assist is used, the plug temperature should be maintained within 25 °C of the sheet temperature, or be made from a low thermal conductivity material, in order to assure minimal stress or chill marks in the part. Polytetrafluoroethylene (PTFE), polysulfone and syntactic foam are often selected for plug-assists because they do not require heating. Heated aluminium plugs coated with PTFE can be used, but they are difficult to maintain at constant temperature due to their sensitivity to air draughts and hot spots from heaters. Polymer composite known as syntactic foam has also been used. It is a composite of polymer with hollow spheres provided by hollow particles such as glass.

3.2.6 Sheet Temperature In the thermoforming process, heating is a critical and key step in reaching the optimal temperature distribution over the surface and across the thickness of the plastic sheet. The process of choosing a heater is difficult due to the complexity of the systems and the variety of proposals made by the original equipment manufacturers [31]. Overall average sheet temperature is more important than surface temperature, because surface temperature does not consider the wide temperature gradient throughout the sheet.

57

Update on Troubleshooting in Thermoforming

3.2.7 Heaters Electric heaters are safer, more controllable and require less maintenance, but gas may be dramatically cheaper than electricity in some areas. Air ovens are recommended for low-temperature heating and forming, because closer temperature control prevents mark-off on optical parts. Most sandwich-type infrared heaters are appropriate for thermoforming operations. Cal rod heaters can be used, but ceramic or quartz elements offer improved control of sheet temperature and cavity-to-cavity part uniformity. Manufacturers using IR heating realise the lower processing temperature benefits in terms of shorter heating times. Figure 3.12 shows the temperature distribution at the sheet centre and a constant temperature gradient across the thickness using the

90 sheet centre

t = 40s 75 t = 35s 70 t = 25s

65

heating

Temperature (°C)

80

cooling

t = 65a

85

sheet front face 60

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Thickness (mm)

Figure 3.12 Numerical temperature distribution through the thickness. Reproduced with permission S. Monteix, F. Schmidt, Y. Le Maoult, R. Ben Yedder, R.W. Diraddo and D. Laroche, Journal of Materials Processing Technology, 2001, 119, 1–3, 95. ©2001, Elsevier Science [32]

58

Machine Specifications – Effect of Parameters numerical modelling during heating. The temperature decreases as soon as the heating is switched off. The best way to ensure uniform heating is to heat the sheet from both sides. Ideally, this would be done in a closed air-circulating oven. A semi-automated thermoforming machine usually provides top and bottom radiant heating for sheet held in a moveable metal clamp frame. Ovens which heat the sheet from the top only run the risk of blistering the top surface of the sheet with too much heat or inducing stress through uneven or inadequate heating. Several heating techniques are currently in use for production: •

Ceramic or quartz panels: this is the best type of unit for uniform heating and longevity and offers a good method for individual or zone-controlled heating;

•

Metal-sheathed tubular rods containing heating elements also are common. The rod surface gradually oxidises, reducing thermal efficiency which may be partially restored by cleaning with steel wool or emery cloth;

•

Coiled nickel/chrome wire is frequently used. It has a quick response time and loss of efficiency may lead to inconsistent or inadequate performance and, in the long run, higher energy costs;

•

Quartz tube: due to brittleness, they are better suited for upper heating surfaces than on the bottom.

3.2.8 Daylight The thermoforming machine should have ample ‘daylight’ (distance between clamping frame and mould) to allow for a deep sag, and have fast-operating platens to minimise cooling time. A photoelectric cell is used to monitor the sheet sag during heating.

59

Update on Troubleshooting in Thermoforming

3.2.9 Process Parameters The influence of process parameters in the final properties of a thermoformed part showed the delay time as the most important parameter to obtain good quality parts [33]. The sag point is the point (temperature, time, thickness, etc.) at which a sheet begins to sag inside the thermoforming oven. There are three basic thermoforming techniques to force the hot, flexible sheet against the mould: vacuum, mechanical and positive pressure. Thermoforming can range in complexity from manual operations to highly automated, large-scale ones. The wall thickness of the formed part is a function of initial thickness, depth of draw, type of mould and the shape of the part. Pre-stretching the sheet prior to forming yields more uniform wall thickness. The deeper the draw, the thinner sidewalls are produced. There are many variations of this process in which the sheet may be heated by conduction, convection or radiation; forced with plugs or slides; and inflated with positive air pressure or vacuum [6]. Unfortunately, many thermoformers have allowed themselves to fall behind in technology, design and engineering with regards to thermoforming. The end-user, designer and thermoformer have no choice but to become and stay deeply involved in the specifications of the raw materials, as well as the finished part, quality control and costing all the way through the operation. Therefore, in today’s market there are consultants and other resources dedicated to helping companies make the necessary internal improvements and modifications. •

60

Costing – The thermoformer must have a complete understanding and knowledge of their costing. They must know every aspect of their pricing structure – from labour, to materials, to machinery to secondary operations, to packaging.

Machine Specifications – Effect of Parameters •

Design requirements – Thermoforming starts with the part. A company has to design its products for efficiency, forming, trimming, assembly, limitations, teamwork and prototypes. With thermoforming, the design of the parts can either make or break the project. The wrong design of a product can cause high reject rates, poor quality products and dissatisfied customers.

•

Resins – The correct resins will help a company make better and stronger products.

•

Polymer companies today are developing new materials almost daily. However, the almost-daily increases in raw materials have to be considered and factored in to cost.

•

Sheet material – Thermoformers have to have written material specifications for production of their extruded sheet material. Specifications have to be established and agreed upon with their sheet extruder, but more importantly have to be checked, validated and enforced.

•

Environment – A clean facility with no breezes or draughts blowing over heating ovens or forming stations is ideal. An airconditioned, humidity-controlled plant is by far the best, though for most companies this is impractical.

•

Heating – Outside of a top-quality sheet, the single most important factor in thermoforming is repeatable, controllable, uniform (properly profiled) heat. The core of the sheet must be heated uniformly to its particular processing temperature.

•

Tooling/moulds – Epoxy, wood and polyester fibreglass should be used only for special forming projects and very short runs. However, to achieve quality and consistent parts, at an economical cost, temperature-controlled aluminium moulds must be used. There can be no compromise on this requirement. Moulds with cast-in cooling tubes are essential. The ability to control the mould temperature throughout the thermoforming run is essential. In addition, composite and syntactic foam materials have

61

Update on Troubleshooting in Thermoforming dramatically improved and provided thermoformers with options and solutions to problems that previously did not exist. •

Vacuum – Drape and vacuum forming are common ways of stretching polymer materials onto the surface of a mould. A vacuum is used to remove the trapped air between the mould surface and the polymer sheet material. A proper vacuum is essential to form and achieve consistent products. For most applications, the surge tank volume needs to be a minimum of six times and preferably ten times the volume of the part being thermoformed. Lack of or improper vacuum is a major issue with most thermoforming companies.

•

Thermoforming machines – There are three distinct phases of thermoforming:

•

1.

Heating the polymer sheet above its glass transition temperature or its melting temperature and the material should not be too sensitive so that it creeps, in order not to fail under its own weight;

2.

Forming in a mould through vacuum or compressed air or gas or by means of a metallic punch;

3.

Cooling down by conduction in the case of thin film for about 1–2 second cycle time and for thick walls using fans with 40–60 seconds cycle time [10].

Team – The right team and partners are essential for a successful thermoforming operation. Thermoforming companies must keep up with the latest technology and advances being made in order to be competitive.

The complex thermoforming system requires a perfect synchronisation of machine and tool (format set). This is an essential condition for smooth production, especially with respect to the relatively short cycle times. The required negative undercut of the bottles alone was a remarkable challenge for tool and machine designers. Bottles

62

Machine Specifications – Effect of Parameters with such pronounced undercuts can only be manufactured using moveable lower tool parts.

References 1.

J.E. Goodrich, New Raw Materials for Thermoformable UV/EB Inks and Coatings, Sartomer Company, Inc., Exton, PA, USA, 2008.

2.

L-T. Lima, R. Auras and M. Rubino, Progress in Polymer Science, 2008, 33, 820.

3.

S. Yang and B. Boulet in Proceedings of the World Congress on Engineering and Computer Science, San Francisco, CA, USA, 2008.

4.

H.C. Lau, S.N. Bhattacharya and G.J. Field, Polymer Engineering and Science, 2004, 38, 11, 1915.

5.

R. Christopherson, B. Debbaut and Y. Rubin in Proceedings of the SPE ANTEC 2000 Conference, Orlando, FL, USA, 2000, 46, p.773.

6.

J.L. Throne, Thermoforming, Hanser Publishers, New York, NY, USA, 1984.

7.

Polypropylene Sheet Extrusion and Thermoforming for Packaging Applications, Basell Polyolefins, Rotterdam, The Netherlands, 2005, 054 PP e.

8.

D.F. Walczyka and S. Yoo, Journal of Manufacturing Processes 11, 2009, 9.

9.

M.K. Warby, J.R. Whitemana, W-G. Jiang, P. Warwick and T. Wright, Mathematics and Computers in Simulation, 2003, 61, 210.

10. G. Sala, L. Di Landro and D. Cassago, Materials and Design, 23, 2002, 21.

63

Update on Troubleshooting in Thermoforming 11. M.K. Pettersen, M. Gällstedt and T. Eie, Packaging Technology and Science, 2004, 17, 1, 43. 12. S. Khongkruaphan, J. Mead, S. Orroth, N. Tessier and T. Murray in Proceedings of the SPE ANTEC 2006 Conference, Charlotte, NC, USA, 2006, p.2631. 13. A. Tulsian, J. Mead, S. Orroth and N. Tessier in Proceedings of the SPE ANTEC 2004 Conference, Chicago, IL, USA, 2004, p.904. 14. L. Reifschneider in Proceedings of the SPE GPEC 2006 Conference, Atlanta, GA, USA, 2006, Paper 15B. 15. E. Haberstroh and J. Wirtz, Kunststoffe Plast Europe, 2003, 93, 12, 24. 16. B. Hegemann, P. Eyerer, N. Tessier, K. Kouba and T. Bush in Proceedings of the SPE ANTEC 2003 Conference, Nashville, TN, USA, 2003, p.791. 17. J.L. Throne, Technology of Thermoforming, Carl Hanser Verlag, Munich, Germany, 1996, p.18. 18. J.K. Lee, C.E. Scott and T.L. Virkler, Polymer Engineering and Science, 2004, 4, 7, 1541. 19. J.K. Lee, T.L. Virkler and C.E. Scott, Polymer Engineering and Science, 2004, 41, 10, 1830. 20. J.K. Lee, C.E. Scott and T.L. Virkler, Polymer Engineering and Science, 2004, 41, 240. 21. J.K. Lee, T.L. Virkler and C.E. Scott, Polymer Engineering and Science, 2004, 41, 10, 1830. 22. X. Haihong, J. Wysocki, D. Kazmer, P. Bristow, B. Landa, J. Riello, C. Messina and R. Marrey in Proceedings of the ANTEC Conference, New York, NY, USA, 1999, p.872.

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Machine Specifications – Effect of Parameters 23. D.V. Rosato, Plastics Processing Data Handbook, 2nd Edition, Chapman & Hall, London, UK, 1997. 24. J.M. Feingold, Plastics Technology, 2005, 51, 12, 50. 25. A.V. Gupta and D. Misra in Proceedings of the SAMPE ‘06 Conference: Creating New Opportunities for the World Economy, Long Beach, CA, USA, 2006, 51, p.166. 26. T.W. Fisher, Plastics Technology, 2005, 51, 8, 49. 27. K.L. Walton, M.K. Laughner, L.J. Effler and E.S. Gisler in Proceedings of the 63rd SPE Annual Conference, Boston, MA, 2005. 28. Machine Design, 2006, 78, 3, 87. 29. F.G. Torres and S.F. Bush, Composites: Part A, 2000, 31, 1293. 30. F.M. Schmidt, Y. Le Maoult and S. Monteix, Journal of Materials Processing Technology, 2003, 143–144, 226. 31. F. Wilson in Proceedings of the 16th Annual SPE Thermoforming Conference - Creative and Innovation in Thermoforming, Nashville, TN, USA, USA, 2006. 32. S. Monteix, F. Schmidt, Y. Le Maoult, R. Ben Yedder, R.W. Diraddo and D. Laroche, Journal of Materials Processing Technology, 2001, 119, 1–3, 95. 33. R. Morales and M.V. Candal in Proceedings of the 64th SPE Annual Conference, Charlotte, NC, USA, 2006, p.2641.

65

4

Problem Identification – Classification – Troubleshooting – Flow Chart

Thermoforming is a stretching process which permits thinner wall thicknesses than machining, for which walls must withstand machining forces and heat, or the moulding processes that require wall thickness thick enough to permit melt flow. For products that do not require thicker walls for strength, this can result in lower piece part prices. There are actually several thermoforming process variations, and they can be considered according to the thickness of the sheet used (‘thin gauge’ or ‘heavy gauge’ thermoforming), the manner in which it is supplied (roll or sheet) or the contour of die used (male or female). Thin-gauge (thickness less than 1.5 mm) thermoforming uses material supplied in a roll. It is the high-volume variety of this process and is generally associated with packaging. With the exception of disposables, such as cups and plates, it is not generally used for product manufacturing. In order to analyse the problem in terms of quality of the product, the variables can be divided into four categories: •

Manufacturing process, including feeding, pre-heating, force used to form, pressure applied and rate of cooling;

•

Design variables used in input and output;

•

Material and mould design, including plug size, design pattern, drawing ratio and type of material;

•

Control process of the system such as heating, punching and ejection [1].

67

Update on Troubleshooting in Thermoforming Developments in thermoforming machines and the effects of equipment improvements on the process itself have been made in the heating-up stage, flexible production units, advantages of servo motors and trimming. It is shown that, together with measures to optimise the heating process, the consistent use of servo drives and valves will represent a further step towards part optimisation through improved machine engineering, because all the parameters that influence the thermoforming process can be precisely coordinated to each other and automatically optimised. This will make shorter cycle times possible and enable the operator to be guided specifically through the process. Any problems can thus be quickly recognised, permitting much better diagnosis support [2]. The thermoforming process is one of the most popular techniques in polymer processing due to its interesting capabilities [3]. The large number of applications of thermoforming are due to its high performance, simplicity, compactness and relatively low-cost equipment [4–6]. In the competition between injection moulding and thermoforming for the production of rigid packaging items such as yoghurt cups, the ability to apply labels and other decoration using in-mould labelling often tips the balance towards injection moulding. The biggest problem on larger and faster machines is getting the air out of the cavity from under the labels. The industry is looking at ways of putting small holes in the labels to prevent bubbles from forming [7]. Drying or keeping moisture content at designated low levels is important, particularly for hygroscopic resins, polyamide, polycarbonate (PC), polyurethane, polymethylmethacrylate, acrylonitrile-butadiene-styrene (ABS). Most engineering resins are hygroscopic and must be ‘dry’ prior to processing. In most materials, small amounts of moisture will cause a change in the polymer’s melt viscosity, which, in turn, will affect the way it processes. When polymers, such as PC and polyester, are heated above the melt temperature, small amounts of moisture in the pellets or on the

68

Problem Identification – Classification – Troubleshooting – Flow Chart surface will cause a chemical reaction. This reaction can degrade the polymer, changing its molecular weight, melt viscosity and mechanical strength. Figure 4.1 shows a schematic representation of the problems that occur during the thermoforming process. Market trends, coupled with materials science advances, are opening the door for thermoformers to participate in large-part application opportunities that are currently dominated by metal and fibreglass [8].

Blisters/bubbles

Incomplete Forming, poor Details

Blushing or color intensity change

Webbing, bridging, wrinkling

Excessive sag

Sag variation between sheet banks

Figure 4.1 Problems occurring during the thermoforming process [1]. Reproduced with permission from S.I. Jalham, The Journal of King Abdul Aziz University, Engineering Science, 2005, 16, 1, 17. ©2005, King Abdul Aziz University

69

Update on Troubleshooting in Thermoforming

Chill marks, striations

Shiny streaks on part

Post-forming shrinkage, distortion

Poor material allocation

Very thin corners

Part sticks in mould

Sheet sticks to plug

Sheet tears while Forming

Figure 4.1 Continued

70

Problem Identification – Classification – Troubleshooting – Flow Chart

4.1 Troubleshooting 4.1.1 Blisters or Bubbles Compression of the entrapped air by the rapidly introducing sheet or entering sheet in the automatic machine can cause blisters or bubbles and also moisture present in the sheet material emerges as bubbles. Table 4.1 explains how to eliminate this problem. •

Summary of Table 4.1

The heating is marked by wide vibrations and softening and the material will start to form small ripples also known as oil canning. Smoothened sag and ripples are an indication that most stresses have been removed and the material will start to smooth out. Thermo labels or an infrared pyrometer are ideal for determining sheet surface temperature. In PVC, plastisols and the fused product can absorb moisture from the air. The water absorption can be minimised in the fused thermoformed sheet by selecting plasticisers and PVC resins having low moisture [9]. An increase in the distance between the heaters and the sheet can eliminate the problem. Be sure that all heaters are functioning and use the screening to balance heat. If there is excess moisture, then the sheet material must be pre-dried or pre-heated. Larger amounts of moisture may result in a rough and scaly surface finish and even bubbles and voids in the product. Sheets to be thermoformed must be stored properly to avoid moisture absorption, which reduces the peak temperature at which blistering may occur. Variables include heating either on one or on both sides, rate of heating, thickness, mould design, material of construction and heating pattern, evacuation flow, control and vacuum level. Bubbling is almost always caused by moisture in the sheet. Bubbling can sometimes be eliminated by heating faster, rather than having to pre-dry the sheet. A lower forming temperature also eliminates bubbling. 71

Update on Troubleshooting in Thermoforming

Table 4.1 Elimination of Blisters or Bubbles Probable causes

Remedy

Moisture

Pre-dry the sheet

Higher temperature

a. Optimise the temperature b. Reduce heating time c. Reduce the intensity of heating

Material too hot

Eliminated by reducing heating time or intensity

Form ejection too fast

Decrease de-moulding speed, optimise heat distribution

Irregular heating

Reduce heating time or intensity

Heating too intensive

Exchange material

Forming temperature window of material too narrow

Roughen or sand tool to assist in air escape

Trapped air between sheet and tool, dust or dirt or small particulate matter on tool

a. Clean tool b. Increase the tooling temperature or repair leaks c. Condensation on tooling d. Water dripping on sheet

Notes: Higher temperature leads to sheet blisters and/or

excessive smoke. In such cases the sheet is being heated too quickly, hence the heat should be reduced and the dwell (cycle) time increased. The uniform sag and lack of rippling is an indication to look for thermoforming. In thermoformed foam PVC sheets, water absorption can result in defects in the product appearing as wrinkles. At higher temperature above the process condition, blistering may appear due to vaporisation of absorbed moisture and leads to degradation of the polymeric sheet. Insufficient heating of the sheet or excessive cooling prior to, or during, the formation may lead to problems caused by the induced stress. Blisters or bubbles can be due to heating too rapidly or lower temperatures; uneven heating may also create the problem.

72

Problem Identification – Classification – Troubleshooting – Flow Chart As the material starts to heat, it will begin to soften and bulge up slightly from the heat below it. When it begins to approach thermoforming temperature, the material will start to form ripples. Ripples are a result of inherited stresses from the extrusion process. They are common in any extruded plastic. As the ripples start to smooth out, the material formed into a part would have a lower quality or higher internal stresses and possibly creates poor part definition and thinning in the side walls. The surface temperature should be at the proper thermoforming temperature, and may be heating too quickly; lower the oven settings and adjust the dwell time, and hence the material will be smooth and free from ripples along the clamping frame and sagging slightly. This indicates the proper surface temperature and core temperature is the key to achieve better parts with fewer rejects and better cost savings. Shop cleanliness is an important factor determining the quality of the parts obtained. It is vital that the thermoforming area is kept clean at all times. Dust and other particles will greatly affect surface quality of the finished product.

4.1.2 Webbing or Bridging Webbing or bridging occurs when the sheet is too hot at the centre. Even wrinkling occurs, when the melt strength of sheet is too low. Table 4.2 will give guidance on eliminating the problem. •

Summary of Table 4.2

Webbing is a common thermoforming problem caused by excess material. An air pillow is sometimes used to prevent sagging of the sheet during heating, which minimises pre-stretching. However, this technique will not allow the sag method of determining sheet temperature and limits the heating source to one-sided heating, which takes longer. Take-up blocks and pushers are more effective methods of taking up excess material around the web. The degree of sag is directly proportional to the combined effects of oven temperature and the dwell time of the sheet in the oven.

73

Update on Troubleshooting in Thermoforming

Table 4.2 Webbing or Bridging Probable causes

Remedy

Material too hot

Improve part design or moulding technique Use plug- or ring-assist

Inadequate vacuum to evenly draw sheet

Use web blocks to minimise the sag

Excessive draw

Redesign mould to improve cavity spacing or balance draw

Notes: In the thermoforming process, higher sheet temperature can result in lower internal stress in the thermoforming packages. Overheating will cause the sheet to sag, resulting in bridging (webbing).

Decreasing the oven dwell time and increasing the oven temperature allows a higher sheet temperature without excess sagging. Decreasing the dwell time has the advantage of cycle time reduction. This enables the use of a smaller oven on the thermoforming machine and decreasing energy needs. For blank-fed male mould thermoforming machines, forming can be improved and bridging can be decreased or eliminated by using wire-helper grids.

4.1.3 Excessive Sheet Sag Overheated sheet material can cause sagging, due to crystallisation and can even cause whitening, embrittlement and excessive sag with resultant webbing can also occur. Table 4.3 gives measure that will help to eliminate the problem and the observations that have to be made to reduce the wastage that can occur during thermoforming. •

Summary of Table 4.3

This problem can be eliminated by reducing the heating time or intensity or by increasing the air pressure, reducing the tool

74

Problem Identification – Classification – Troubleshooting – Flow Chart

Table 4.3 Excessive Sheet Sag Probable causes

Remedy

Sheet too hot

Reduce oven temperature or cycle time

Hot oven too wide

Reduce melt flow

Notes: Excessive sheet sag is mainly due to the overheating

of the polymeric sheet. Appearance is also defective due to stretching of the material or sagging and can be controlled by reducing the heating time or intensity; if moulding is insufficient, change the material. A shrinkage difference over the width or excessive transverse direction growth lead to a defective appearance and can be changed by changing the defective material. Materials being too hot, insufficient compressed air, tools being too hot, sagging of the material, the space between the plug and the mould being too narrow or wide, tool construction and the cycle time being too long or short can cause bridges or webs. temperature or reducing the heating time or intensity; if moulding is insufficient, change the material, optimise plug and/or ring dimensions, optimise tool geometry, install a technical aid or optimise the cycle time.

4.1.4 Pinhole or Rupturing In thermoforming, the sheet material deforms, and while it reforms uneven heating may cause rupture. Rupture of polymeric sheets is one of the practical problems during plug-assist thermoforming. This defect may occur both in the stage of mechanical stretching with a plug, and in vacuum or pressure thermoforming [10]. Table 4.4 will give guidelines to resolve the problem of rupturing. •

Summary of Table 4.4 75

Update on Troubleshooting in Thermoforming

Table 4.4 Problem - Pinhole or Rupturing Probable causes

Remedy

Vacuum holes too large

Partially plug up holes with wood or solder or completely plug and re-drill

Uneven heating

Attach baffles to the top clamping frame

Notes: Pinholes are due to the presence of either contamination

or dirt particles over the sheet. The dirt attraction may be due to electrostatic charge developing on the sheet surface. This dirt and contamination can be avoided by storing in a proper place. The electrostatic charges can be eliminated by using anti-static additives during the sheet manufacturing.

4.1.5 Excessive Post Shrinkage Post shrinkage dominates along both length and the width of a newly produced part. Table 4.5 gives the troubleshooting measures to minimise the problem. •

Summary of Table 4.5

Optimum heating and proper cooling will avoid the excessive post shrinkage.

Table 4.5 Excessive Post Shrinkage Probable causes

Remedy

Sheet removed from mould while still hot

Increase cooling time

Notes: Shrinkage is a process of dimensional change from the original form of the part, which occurs over time. This may be due to the mould not having been made with proper shrinkage as per the material data recommendation.

76

Problem Identification – Classification – Troubleshooting – Flow Chart

4.1.6 Uneven Sag Sagging, thermal buckling and inhomogeneous temperature will arise in the sheet material after heating of the sheet, before pressure is applied. Use of Table 4.6 will help to eliminate the problem. •

Summary of Table 4.6

This can be eliminated by checking the heaters and taking corrective action to avoid much of the wastage. There may be a possibility that regrind present in the material may not match during the heating process and may cause uneven sag.

Table 4.6 Uneven Sag Probable causes

Remedy

Sheet temperature not uniform

Eliminate air draughts in oven and fix faulty heaters

Notes: Uneven sag is due to uneven heating or the heater not working properly.

4.1.7 Part Sticks to Mould Sheet material sticks to the mould upon contact as the hot polymeric material is supposed to ‘freeze’ instantly when touching the cold surface. This will reduce the production speed. Table 4.7 gives guidelines to eliminate the problem. •

Summary of Table 4.7

Defects can be eliminated by checking the pressure of the compressed air, adjusting the geometry of the part, polishing the surface of the tool, exchanging the material or using a release agent, reducing the tool temperature or reducing the heating time or intensity.

77

Update on Troubleshooting in Thermoforming

Table 4.7 Part Sticks to the Mould Probable causes

Remedy

Part temperature too hot for proper release

Reduce part temperature a. Increase cooling cycle b. Reduce sheet temperature Reduce mould temperature Draught angle too small, increase draught angle Reduce mould temperature Use female mould Reduce heating time or temperature Remove part from mould as soon as possible

Notes: The formed part sticking to the tool can be caused by ejection air, i.e., it is missing or too low, the edges or corners or undercut of the mould are too sharp, the surface of the tool is too rough (material does not slide), bad de-moulding of the film or poor mould release, the tool is too hot or the material is too hot.

4.1.8 Stretch Marks on Part The stretching of the plastic sheet in thermoforming process involves large strains and large thickness variations. Table 4.8 will help to eliminate the problem. •

Summary of Table 4.8

Internal stresses should be monitored using a polarised light table to remove the stretch marks.

4.1.9 Nipples on Mould Side of Thermoformed Part Due to vacuum holes and moreover because the vacuum applied to the sheet can cause nipples (an elevated portion in the thermoformed sheet)

78

Problem Identification – Classification – Troubleshooting – Flow Chart

Table 4.8 Stretch Marks on the Part Probable causes

Remedy

Plug-assist sticks to sheet and causes

Eliminate sticking

Freeze-off lines

a. Change heated plug temperature to equal sheet temperature b. Apply release coating to plugassist c. Use lower stick material for basic plug construction

Cold mould causes curved chill lines around lip of part

Increase mould temperature or increase air cushion as part is formed

Mould temperature varies between cavities

Increase number of water channels or clean out plugged channels as necessary

Notes: The mould surface should maintain a consistent

temperature of 40 °C or 60 °C to prevent chill line formation or sticking. on mould side. Table 4.9 gives guidance on how to eliminate formation of nipples on the mould side of the thermoformed product. •

Summary of Table 4.9

Sheet temperature should be reduced; this may also reduce the nipples on mould side.

4.1.10 Pock Marks Pock marks are due to entrapped air and the solution in Table 4.10 will help to eliminate the problem. •

Summary of Table 4.10

79

Update on Troubleshooting in Thermoforming

Table 4.9 Nipples on the Mould Side of the Thermoformed Part Probable causes

Remedy

Sheet too hot

Reduce sheet temperature

Vacuum holes too large

Plug holes and re-drill with necessary drill bit

Notes: Too much vacuum creates nipple on mould side of

thermoformed part. It may also be due to a vacuum hole.

Table 4.10 Pock Marks Probable causes

Remedy

Air entrapment between part and mould

Eliminate trapped air a. Slightly roughen large, flat mould surfaces with very fine grit blasting or glass beading of problem area b. Clean out plugged vacuum holes c. Add vacuum holes or vents as required

Notes: Temperature developed in the air trapped in between the

part and mould will create burn or pock marks. Use a temperature below 175 °C. Apply a soft rubber coating to the mould surface or form the part out of a material having a matte finish, keeping the matte side next to the mould at a temperature greater than 175 °C.

4.1.11 Poor Wall Distribution Sheet materials with severe non-linearity associated with large stretches and strains leads to uneven thickness. Thickness variation also depends

80

Problem Identification – Classification – Troubleshooting – Flow Chart

Table 4.11 Poor Wall Distribution Probable causes

Remedy

Sheet temperature not uniform

Use screens, additional heaters or eliminate air draughts in oven

Poor mould design

Reduce severe areas of draw, increase draught angles and reduce undercuts in mould

Sheet drags on mould lip during plug-assist forming

Increase air cushion under sheet during plug-assist travel a. Increase plug speed b. Reduce vacuum bleed rate c. Reduce vacuum hole size Raise mould surface around lip of part so sheet drag on lip is reduced

Notes: Pressure or the vacuum not working properly will cause

poor wall distribution. on time, due to the short duration of the forming stage. Table 4.11 will give trouble shooting ways to eliminate the problem. •

Summary of Table 4.11

Correction of the vacuum or pressure will solve the problem.

4.1.12 Uneven Edges During the thermoforming, the sheet material is hot and the vacuum applied too early may cause the uneven edges and Table 4.12 will give the trouble shooting ways to eliminate the problem. •

Summary of Table 4.12

Even at the stage of forming, this will be clearly seen and the temperature can be adjusted to correct the problem.

81

Update on Troubleshooting in Thermoforming

Table 4.12 Uneven Edges Probable causes

Remedy

Excessive forming temperature differential

Preheat clamping frame, use slip clamp frame (low/high)

Notes: A temperature difference in the polymeric sheet will form

uneven edges.

4.1.13 Tearing of Sheet When Forming Tearing at the mould edge is sometimes caused by inadequate clearance between the frame and the mould. Table 4.13 will help to eliminate and minimise the problem. •

Summary of Table 4.13

Thermoforming consistent parts depends on knowing and controlling several material and process variables, the most important being the quality of the sheet feedstock. The extruded sheet of the same material

Table 4.13 Tearing of Sheet When Forming Probable causes

Remedy

Sheet may be too hot or too cold

Reduce or increase heating time Reduce or increase heater temperature

Notes: Effects of polymeric contamination, thickness, thermal

stresses and amount of regrind volatiles, colour, gloss and grain manifest during actual thermoforming result in tearing, wall thinning, shape distortion, fading, pinholes and grain distortion [11]. Materials becoming too hot, too much pre-stretching, the upper plug being applied too late or too slowly, the material being too thin, uneven thickness profile over the width, the material having inclusions, the vacuum being applied too early or too much pressure can cause material tears on the bottom of the formed part or the formed cavity to be too thin.

82

Problem Identification – Classification – Troubleshooting – Flow Chart could vary in terms of polymeric contamination, thickness, thermal stresses and amount of regrind, volatiles, colour, gloss and grain. Mould material and mould temperature could also vary. The problem can be eliminated by reducing the heating time or intensity, decreasing the level of pre-stretching, optimising timing or speed, choosing a thicker material, checking the thickness profile over width, checking the material specification and adjusting the vacuum timing or pressure. The sheet or film becomes thinner as it stretches around the returns on a form; the steeper the draught, the thinner the film. The depth of draw should be kept to a minimum. Again, a deeper draw thins the film even more. The material tears or blows holes when contacting the tool or during forming, when the material is too hot or cold, and the heating time or intensity has to be reduced or increased, respectively. Sometimes compressed air or the vacuum being applied too early also create the same problem and by adapting the delay the problem can be eliminated. The temperature of the tool should be increased when the tooling is too cold. The geometry of the formed part should be changed when the edges and corners of the tool are too sharp. the tool surface should be polished if the tool is too rough, i.e., if the tool is rough, the material will not slide. Excessive material sag or tool dragging on index is controlled by reducing the heating time or increasing the line speed. Too little clearance between tooling components can be adjusted by adjusting the tool for greater distances between male and female. The speed in the forming press or the plug speed can be adjusted if they are too fast. In the case of the material gauge or review tooling draw ratios being insufficient for the designed draw, the material gauge can be increased.

4.1.14 Bad Definition at the Edges of the Forming Area Bad definition at the edges is the thermoformed sheet material with uneven thickness. Due to hot sheet material, while apply vacuum, the

83

Update on Troubleshooting in Thermoforming

Table 4.14 Bad Definition at the Edges of the Forming Area Probable causes

Remedy

Material too cold

Increase heating time or intensity

Ejected part too hot

Reduce tool temperature or increase cooling time.

Draught over the film

Protect machine or film against draught

Not enough space between cavities

Increase space between cavities

Clamping frame too narrow

Enlarge frame out

Pressure and vacuum timing not optimised

Modify vacuum and pressure settings

Pin chain rails cooling edges of material

Increase temperature of pin chain rails

Notes: The material being too cold or the part being ejected with

less dwell time may cause the problem. bad definition at the edges of the forming area appears. Table 4.14 will help to eliminate the problem. •

Summary of Table 4.14

This can be eliminated by increasing the cooling and adjusting the heating.

4.1.15 Glossy Spots Glossy spots are due to overheated melt particle present in the thermoformed sheet material and appearance of the product looks ugly. Table 4.15 helps to eliminate the problem, •

Summary of Table 4.15

A slight failure in the heat may cause the degradation of polymeric material and produce glossy spots. 84

Problem Identification – Classification – Troubleshooting – Flow Chart

Table 4.15 Glossy Spots Probable causes

Remedy

Material too hot

Reduce the heating time or intensity

Unequal material distribution

Add vacuum holes around this area

Non-uniform tool surface

Bead or polish tool

Notes: A temperature above the thermoforming temperature will

create the problem.

4.1.16 Part Warpage The fundamental defect in the thermoforming technology is warpage of the products during their application which becomes particularly apparent under high temperatures. When large internal stresses are released by heating of the sheet through thermoforming operation, it could result in warpage and deformation. This is a process of non-uniform change of the geometric dimensions of products. Table 4.16 gives remedy to reduce the problem. •

Summary of Table 4.16

The warpage is understood as the process of non-uniform (heterogeneous) change of the geometrical dimensions of products in time resulting in a change (distortion) of their original form. Thereby, it is possible to work out and provide valid recommendations for partial and, in some cases, complete elimination of the defect [12–14]. New high-melt-strength PP products, in addition to delivering higher melt strength than conventional PP, exhibit less sag or warpage and allow for new wall thickness options, all of which provide thermoformers with the ability to optimise process efficiency [15]. Uniform cooling is desirable so that design integrity is maintained, warpage is minimised and residual stress content is minimal and uniform.

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Update on Troubleshooting in Thermoforming

Table 4.16 Problem - Part Warpage Probable causes

Remedy

Mould too hot or too cold

Try setting mould temperature at 38 °C

Uneven mould cooling

Clean plugged water channels or add channels as needed

Poor part wall distribution

Improve wall distribution as suggested in previous troubleshooting section above

Part not cooled adequately

Increase cooling cycle time, reduce mould temperature or reduce sheet temperature

Poor mould contact

Improve vacuum on part (see section under ‘Poor part detail’)

Part design not rigid enough

Add ribs and additional detail where possible

Notes: The fundamental defect inherent to the thermoforming

technology is warpage of the products during their application, which becomes particularly apparent under high temperatures. Conventional polypropylene (PP) has been injection moulded into large parts, but not thermoformed, because of its melt strength deficiency. The part should not be cooled for a long time on the mould. When the part cools to 121 °C, it should be removed so that air can be directed to both surfaces at the same time to even the cooling process. Heated frames should also be used. To ensure a flat flange, frames should be heated to at least 121 °C.

4.1.17 Cracking in Corners The cracking in corners is due to the high stress formed during the heating and thermoforming operations. Table 4.17 give solutions to help eliminate the problem. •

86

Summary of Table 4.17

Problem Identification – Classification – Troubleshooting – Flow Chart

Table 4.17 Problem - Cracking in Corners Probable causes

Remedy

Stress concentration

Heat sheet evenly Preheat frames or use heated frames Add supplemental heat to corners

Notes: The sheet should not be heated too fast; heat

accumulation will damage the sheet and cause it to become brittle on the formed part. The formed part should not be cooled too fast, since this may generate stress, resulting in cracking of the formed part. Insufficient heat or loss of too much heat during forming leads to uneven and high residual stress in the yield parts. This stress in the part after production can cause cracking and warping during the reinforcing process or over a long period of time. In thermoforming, the sheet is to be heated to appropriate forming temperatures to draw uniformly into the mould at a rate and manner which balances the accurate mould reproduction with adequate wall thickness and minimum stress in the thermoformed parts.

4.1.18 Raised Corners Raised corners are due to the frame temperature, stress created in the plastic sheet materials. Table 4.18 helps to eliminate the problem.

Table 4.18 Problem - Raised Corners Probable causes

Remedy

Excessive stress

Heat frames to proper temperature before inserting sheet Add supplemental heat to corners

Notes: Frames being heated up too much, this creates stress on

the sheet.

87

Update on Troubleshooting in Thermoforming •

Summary of Table 4.18

There may not be sufficient heating available in the middle of the sheet and additional heat may solve this problem. There may not be sufficient heating available in the middle of the sheet and additional heat may solve this problem.

4.1.19 Surface Markings Surface markings are mainly due to the mould, because the thermoformed sheet carry impression of the mould surface. Table 4.19 will help to eliminate the problem. •

Summary of Table 4.19

Table 4.19 Surface Markings Probable causes

Remedy

Mould surface too rough

Use proper mould covering (foam, felt, flocking)

Mould surface too smooth

Change mould material; the mould has to be grit blast mould surface

Poor vacuum

Check vacuum system or add vacuum holes

Mould too hot or too cold

Reduce or increase the mould temperature

Dirt on sheet or mould

Clean with deioniser air

Notes: The tool temperature being too low, too much silicone on the

surface or film defects cause cloudy areas or marks on the surface. The sheet or film being too hot (crystallisation), the temperature being too low (white break), the surface of the mould being too rough, the tool or plugs being too cold, the plug or tool speed being too fast, the material surface being scratched or having abrasions can cause bad transparency, haziness or blushing of the formed part. 88

Problem Identification – Classification – Troubleshooting – Flow Chart Surface markings can be eliminated by increasing the tool temperature or changing the material until cloudy areas disappear. Defects can be eliminated by reducing or increasing the sheet temperature, polishing the surface of the mould, increasing the tool or plug temperature, decreasing the plug or tool speed and inspecting the material transport through the machine. With a glass transition temperature (Tg) higher than room temperature, the thermoforming system would have very good scratch and scuff resistance, since it would still be in the glassy state. But if the Tg is too high, then the system may not have thermoplastic character at deformation temperatures. Scuffs and blemishes transferred from the mould surface are referred to as mark-off to the forming sheet during processing. This is a frustrating problem; unless high-cost tooling is specified, otherwise perfect mouldings have to be scrapped [16].

4.1.20 Parts in the Corners Too Thin Application of vacuum to the hot sheet material can cause the thinning and Table 4.20 will help to eliminate the problem.

Table 4.20 Parts in the Corners Too Thin Probable causes

Remedy

Material too hot

Reduce the heating time or intensity of heating

Radius of the mould too small

Increase radius or use thicker material

Tool too hot or too cold

Adjust tool temperature (cooling or heating)

Flanks too flat

Redesign the part

Notes: Cold or hot areas in the heating station, not enough

space between cavities, the tool close or the plug speed being too fast or the material gauge being insufficient for the designed draw can cause the walls to be too thin.

89

Update on Troubleshooting in Thermoforming •

Summary of Table 4.20

This problem can be eliminated by checking the radiator or heating plate, increasing the space between cavities, adjusting the tool close or plug speed, increasing the material gauge or reviewing the tooling draw ratios.

4.1.21 Folds, Webbing or Wrinkles The folds, webbing or wrinkles problem occurs basically the introduction sheet material in the machine and temperature applied. Table 4.21 helps to eliminate the problem. •

Summary of Table 4.21

Excessive heat can cause the sheet or film to degrade and may result in a change of colour or premature failure when exposed to the outdoors.

Table 4.21 Folds, Webbing or Wrinkles Probable causes

Remedy

Material too hot

Reduce heating time or intensity of the heating

Notes: To control the appearance, the temperature of the

radiator (oven) or heating has to be checked. If any bent or damaged heaters are present they should be replaced.

4.1.22 Part Deforms During De-moulding Deformation of the part is particularly due to the stress generated on the sheet material during the thermoforming operation. Table 4.22 helps to eliminate the problem. •

90

Summary of Table 4.22

Problem Identification – Classification – Troubleshooting – Flow Chart

Table 4.22 Part Deforms During De-moulding Probable causes

Remedy

Material ejected too hot

Increase cooling time

Tool too hot

Reduce tool temperature (cooling)

Ejection air too high or ejection time too long

Optimize ejection

Notes: Overheating can create the problem of deformation in

the part after de-moulding. Reducing the temperature and use of proper sheet material and thickness will solve the problem.

4.1.23 Poor Part Detail Poor part detail is mainly on the temperature introduced to the sheet material during thermoforming operations and other causes and the solutions provided in Table 4.23. •

Summary of Table 4.23

If the problem occurs in the same area every time, raise the heater temperature. Fine detail and imperfections in the mould (such as undercuts) can lock the part to the mould. Use generous radii with adequate draught, smooth all surfaces, and do not use a mould (such as wood) that can easily chip out during thermal cycling at high temperatures. Also, remove the part from the mould as it cools.

4.2 End-product Problems 4.2.1 Migration of Additives In plastic packaging with PVC, overall migration increases most significantly after microwave cooking, and it is also affected by aqueous stimulants coming into contact with it [17]. 91

Update on Troubleshooting in Thermoforming

Table 4.23 Poor Part Detail Probable causes

Remedy

Cold sheet

Increase sheet temperature

Sheet temperature not uniform

Eliminate air draughts in oven, screen oven heat and add clamp rail heaters as needed

Inadequate vacuum on part

Improve contact with mould a. Fix vacuum leaks b. Clean plugged vacuum holes and vents c. Add vacuum holes or vents in problem areas d. Add moat or ring assist to ensure good seal around perimeter or part e. Make sure surge tank and vacuum pump are large enough to quickly evacuate the mould

Sheet too cold

Increase the heating time Raise the heater temperature Check the heaters are functioning

Clamping frame not hot enough

Heat the frame to the recommended temperature

Insufficient vacuum or pressure

Check the vacuum or pressure at the mould Avoid 90° bend in vacuum system Check vacuum holes for clogging Increase number of holes

Vacuum draw not enough

Check vacuum design Back relieve vacuum holes Use slots instead of holes in vacuum

Notes: Uneven sheet temperature causes uneven forming detail.

Check the temperature of the heaters and the spacing of the heaters from each other versus the spacing from the sheet. Do not bring the sheet closer than the heater spacing. Any air draughts originating from open doors can chill the sheet before it can be formed. Screen areas that are too hot and stretch too much.

92

Problem Identification – Classification – Troubleshooting – Flow Chart

4.2.2 Melt Failure During Thermoforming The effects of temperature, strain rate and rheological properties on ABS failure and the relationship between failure modes in uniaxial extension and failures occurring during thermoforming are to be considered [18]. Factors such as resistance to sag, ease of flow, mould replication, deep draw capability, sensitivity to thermoforming temperature and speed, uniformity of thickness distribution and post-forming shrinkage and dimensional stability are different important factors to ascertain the thermoforming of polymeric materials [19].

4.2.3 Blue Coloured Dots Blue coloured inclusions (blue dots) occur on thermoformed ABS, PC and most other types of thermoplastics. Blue dots are not visible on flat sheets. These inclusions are found on the surface layer of the material. The intensity and size of the dots increase as more heat is applied during the forming process. They are more visible in light-coloured thermoformed parts, usually in white-coloured parts. Independent laboratories identified the blue dots as a blue dye, most likely from the copper phthalocyanine blue family. The cause of the blue dots is fibres that have had intimate contact with the surface while the sheet is being heated for thermoforming. As the sheet is heated, the dye from the dark fibres transfers the colour to the surface of the sheet. To minimise the occurrence of blue dots, the colour of production uniforms should be changed from blue to beige or white, and coloured rags replaced with white rags. The mezzanine should be quarantined from the extruders. A wall should be constructed on the mezzanine to minimise possible contamination from loose fibres or particles that may fall onto the surface of the sheet as it is processed by the texture rollers.

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Update on Troubleshooting in Thermoforming

4.2.4 Shrinkage Shrinkage is dominated by linear behaviour; it is very economic to decrease the part size to a certain extent to reduce the manufacture and measurement of cost. Other combinations of materials, sheet thickness and mould configurations are being considered for more advanced empirical and analytical shrinkage study. When the temperature drops, the density of the polymer will increase or the specific volume will decrease. Shrinkage can be estimated according to the thermodynamic theory on volumetric change [20].

4.2.5 Sheet Pull-out Sheet pull-out is usually caused by trying to form a sheet that has cooled below its forming temperature. After locking on clamping devices and the frames, the sheet should be heated to a higher temperature. The sheet will then form faster. The thick sheet will hold the heat longer.

References 1.

S.I. Jalham, The Journal of King Abdul Aziz University, Engineering Science, 2005, 16, 1, 17.

2.

J. Beine and B. Braun, Kunststoffe Plast Europe, 2003, 93, 10, 53.

3.

H. Hosseini, B.V. Berdyshev and A. Mehrabani-Zeinabad, European Polymer Journal, 2006, 42, 8, 1836.

4.

H. Hosseini and B.B. Vasilivich in Proceedings of the SPE Conference - ANTEC 2006, Charlotte, NC, USA, 2006, p.707.

5.

B.V. Berdshev and H. Hosseini in Proceedings of the SPE Conference - ANTEC 2006, 2006, Charlotte, NC, USA, p.2618.

6.

H. Hosseini, B.V. Berdyshev and A. Mehrabani-Zeinabad, European Polymer Journal, 2006, 42, 8, 1836.

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Problem Identification – Classification – Troubleshooting – Flow Chart 7.

A. Clark, European Plastics News, 2004, 31, 7, 24.

8.

K. McPhillips, Modern Plastics Worldwide, 2005, 82, 10, 16.

9.

R.A. Marshall, Journal of Vinyl Technology, 2004, 12, 4, 195.

10. B.V. Berdshev and H. Hosseini in Proceedings of the SPE Conference - ANTEC 2006, Charlotte, NC, USA, 2006, p.2610. 11. A. Dharia in Proceedings of the SPE Conference - ANTEC 2006, Charlotte, NC, USA, 2006, p.2605. 12. H. Hosseini, B.V. Berdyshev and A. Mehrabani-Zeinabad, European Polymer Journal, 2006, 42, 8, 1836. 13. H. Hossein and B.B. Vasilivich in Proceedings of the SPE Conference - ANTEC 2006, Charlotte, NC, USA, p.707. 14. B.V. Berdshev and H. Hosseini in Proceedings of the SPE Conference - ANTEC 2006, SPE, 2006, Charlotte, NC, USA, p.2618. 15. K. McPhillips, Modern Plastics Worldwide, 2005, 82, 10, 16. 16. A. Adam, Fabrication and Plastics Machining IAPD Magazine, 2005, February/March. 17. M.J. Galotto and A. Guarda, Packaging Technology and Science, 2000, 12, 6, 277. 18. J.K. Lee, S.E. Solovyov, T.L. Virkler and C.E. Scott, Rheologica Acta, 2002, 41, 6, 567. 19. S.S. Morye, Polymer Engineering and Science, 2005, 45, 10, 1369. 20. J.L. Throne, Technology of Thermoforming, Hanser Publishers, Munich, Germany, 1996.

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5

Optimisation of the Production

The thermoforming process differs from traditional plastic processing techniques such as injection moulding and compression moulding as they produce parts in which wall thicknesses may vary significantly with both design and processing. The process requires large deformation of the material which may result in significant nonuniform thinning, material waste and often ineffective designs. The most common materials are polymeric sheet materials and the sheet material quality is characterised as having consistent gauge, low contamination, low cosmetic defects and consistent inherent viscosity or molecular weight. Thermoforming is a polymer processing technique in which an extruded sheet is heated to its softening temperature and subsequently formed to the required shape by forcing the hot and pliable material against the contours of a mould by using mechanical methods, air or vacuum pressure; it is then held against the mould and allowed to cool. The plastic retains its shape and, after it is removed from the mould, excess plastic is trimmed from the formed part. Uniaxial tensile stretching experiments have been used to identify an optimum window for thermoforming. Comparison of the thermoformability of a polyphenylene ether/polypropylene blend with thermoformable acrylonitrile-butadiene-styrene is discussed in by Morye [1]. Thermoforming is a versatile, relatively inexpensive shaping method used extensively for packaging applications, which also involves two-dimensional extensional deformation. Stretch shaping, as the name implies, involves shaping of a preformed polymer by stretching. Thermoforming is a stretch-flow-type shaping method,

97

Update on Troubleshooting in Thermoforming which involves deformation of previously shaped polymer sheets or films into a desired shape. The principles of thermoforming are very similar to those of parison inflation [2]. The production of high-quality thermoformed parts is critically dependent on the standard of extruded sheet feedstock used. Thermal history imparted on the sheet material during the extrusion process dictates its subsequent thermoforming process [3]. In the last decade, new materials and more critical applications with tight tolerances made thermoforming very difficult. Improved heaters, accurate process controls, easy thermoforming polymers and trimming techniques greatly assisted the thermoforming industry. In addition, because of the greater attention gained by packaging industries, the manufacturers of the raw materials had to improve the grade of their materials, particularly for thermoforming applications. Heavy-gauge material (thickness greater than 1.5 mm) can produce relatively flat parts with rounded corners, known as vacuum form. However, its pressure forming version can produce detail that can rival injection moulding. It has very large part capability. It allows combination of several parts into one in many cases, thus eliminating some assembly operations completely. The optimum heating cycle required to reach forming temperatures depends upon the sheet thickness, machine setting and ambient conditions of the forming area. Such variables make process controls vital in thermoforming. To establish the cycle time effectively, a quick response pyrometer will serve to measure the sheet surface temperature and is used occasionally to check and indicate potential stress problems due to insufficient heating below the surface of the sheet. Temperature-sensitive tapes can be used to confirm that the process is under the correct conditions and sheet surface temperatures can be reproduced consistently. Thermoforming processes are characterised by large deformations, non-isothermal conditions and non-linear material behaviour, making final part performance difficult to predict. A computer simulation is 98

Optimisation of the Production presented which accurately determines the thickness distribution of finished plastic mouldings, thereby reducing the design cycle while optimising material usage [4]. It is important to understand the effect of material and processing parameters on the thermoforming process since it is critical to optimise the processing conditions to develop better materials for higher quality products [5, 6]. The material behaviour of polymers is totally controlled by their molecular structure. This is true for all polymers, synthetically generated polymers as well as polymers found in nature (bio-polymers), such as natural rubber, ivory, amber, protein-based polymers or cellulose-based materials. A temperature above the glass transition temperature (Tg) is necessary to allow sufficient softening and avoidance of distortion in shape. For every material, there is a heating range or working range where the material can be properly formed with minimal changes in the physical properties [7]. Knowledge of the properties of materials is essential for several purposes: design, specification, quality control, failure analysis and for understanding the structure and behaviour of new materials [8]. Important factors to ascertain in thermoforming include resistance to sag, ease of flow, mould replication, deep draw capability, sensitivity to temperature and speed, uniformity of thickness distribution and post-forming shrinkage and dimensional stability [9]. Heating profiles are controlled to produce uniform temperature across the top and bottom sheet surfaces. Thickness variation, incidence of webbing and minimum part thickness are measured to qualify the overall quality of the product. The optimum conditions are to minimise the variation in part wall thickness and are not the same as to minimise formation of webs or to minimise thinning in certain localised areas. The pre-stretch bubble height, sheet temperature and mould temperature are found to have high validity and the delay time between pre-stretch and forming, stretching rate and heating rate are found to have less validity [10].

99

Update on Troubleshooting in Thermoforming Thickness distribution is an important quality criterion for the performance of the end-product. Mechanical, optical or sometimes permeability properties may depend on this distribution as well as on the extension undergone in different parts of the sheet [11]. To optimise heat transfer and part production rates, machined aluminium is typically used for thermoforming. To maximise part clarity, mould cavities should be highly polished. Grit-blasted mould surfaces will cause a significant loss of clarity. Grit-blasted surfaces can be used in combination with polished ones to provide highlighted areas of reduced clarity and create a unique look in the thermoformed part. The mould must be well vented to prevent air becoming trapped. Air entrapment will cause a slight dimpling of the part surface and reduce part clarity. If venting is achieved with vacuum holes, holes should be made clear. Flat surfaces can be ‘glass beaded’ to ensure good contact for part cooling. Mould temperatures are normally set between 21 and 66 °C. Deep draws and sharp detail require high temperature thermoforming from 188 °C to 213 °C. Sheets with less than 0.04% moisture are essential to form successfully without incurring bubbles. In simple drape forming, a temperature between 177 °C and 188 °C can be used. Pre-drying is not necessary at these temperatures. Infrared technology is the main choice for heating plastic sheets prior to the forming operation. Today, several technologies, both electric and gas, are in use in the thermoforming industry [12]. Although there are ways of measuring surface temperature to achieve the proper forming temperature (infrared temperature sensors, paper thermometers, etc.), the most difficult aspect is achieving the proper core temperature as well. By achieving the proper core temperature, internal stresses in the material are reduced, improving part quality and consistency. In co-extruded film thermoforming, to understand the effect of interlayer adhesion, a peel test has to be performed after the

100

Optimisation of the Production material has been stretched. Before the peel test, both planar and biaxial stretching are performed at an optimum temperature and controlled stretch rate. Loss of interlayer adhesion at the optimum temperature is related to thickness draw-down as result of stretching. A significant amount of reduction of interlayer adhesion is due to stretching [13]. The relationship between heating conditions, rheological behaviour and thickness distribution with the aim of optimising the latter in thermoforming showed that under practical processing conditions the effect of differences in thermal properties was predominant over that of the rheological ones [14].

References 1.

S.S. Morye, Polymer Engineering and Science, 2005, 45, 10, 137.

2.

C.J.S. Petrie and K. Ito, Plastics & Rubber Processing, 1980, 5, 68.

3.

N.J. Macauley, E.M.A. Harkin-Jones and W.R. Murphy, Polymer Engineering and Science, 2004, 38, 4, 662.

4.

D.O. Kazmer, Simulation of the Blow Molding and Thermoforming Processes, GE Plastics, Pittsfield, MA, USA.

5.

J.K. Lee, C.E. Scott and T.L. Virkler, Polymer Engineering and Science, 2004, 41, 240.

6.

J.K. Lee, T.L. Virkler and C.E. Scott, Polymer Engineering and Science, 2004, 41, 10, 1830.

7.

A.P. Patel, A.J. Goldberg and C.J. Burstone, Journal of Applied Biomaterials, 2004, 3, 177.

8.

R. Brown, Handbook of Polymer Testing, Rapra Technology Limited, Shawbury, Shrewsbury, UK, 2002.

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Update on Troubleshooting in Thermoforming 9.

S.S. Morye, Polymer Engineering and Science, 2005, 45, 10, 1369.

10. C.M. Bordonaro, T.L. Virkler, P.A. Galante, B. Pineo and C.E. Scott, Thermoforming Quarterly, 2004, 23, 3, 8. 11. R. Christopherson, B. Debbaut and Y. Rubin in Proceedings of the SPE Conference - ANTEC 2000, Orlando, FL, USA, 2000, p.773. 12. N. Bedard and S. Marchand, Thermoforming Quarterly, 2004, 23, 1, 10. 13. H. Zhou, Polymer Engineering and Science, 2004, 44, 5, 948. 14. F.M. Duarte, V.C. Barroso, J.M. Maia and J.A. Covas, Journal of Polymer Engineering, 2005, 25, 2, 115.

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6

Case Studies

These case studies present typical scenarios that challenge those involved in the manufacture of thermoformed end-products. The goal is to develop the process to optimise the conversion of raw materials into thermoformed end-products by quantifying the operating conditions in each step in the processing. This can be done by quantifying the processing conditions and product design. This is called the trial-anderror method. Since polymers are macromolecules and do not have uniform characteristics, they have widely varying end-use properties. To minimise wastage in the trial-and-error approach, the process engineer approaches the problem in an intellectual way, according to the complex nature of the process and product. Case studies can be approached by the intelligent approach, the modelling approach or the open literature approach. The intelligent approach requires the following details to proceed: •

Research data;

•

Material datasheet;

•

Statistical analysis;

•

Fundamental process method;

•

Assessment of advantages and disadvantages.

There are advantages and disadvantages in each intelligent approach. In this approach, qualitatively inferring the necessary research data is

103

Update on Troubleshooting in Thermoforming the easiest and quickest way of getting to the answer. However, the answer is limited in situations where there is little or no data. In the modelling approach the product is qualitatively analysed with the models. It is a process with a systemised first approach and it requires considerable time to generate the model and to proceed, but it suffers by not analysing the qualitative point of view. The third approach is to assess qualitatively by referring to the open literature regarding the product and process and to start from the fundamental model. Though this model requires a significant upfront investment of time, it is the approach that promises the most benefits. Brief details of some of the case studies are given next.

6.1 Case Study 1 Challenge: Improving performance, safety and cost of automotive interior parts. Requirements: •

Reduced manufacturing cycles;

•

Increased toughness and impact resistance;

•

Post-forming possibilities such as corrections and forming in multiple steps;

•

Unlimited shelf life;

•

Recyclability, both during manufacturing (recycling scrap) and after service life.

The surface of interior automotive parts, particularly the instrument panel, is required to have a very low gloss finish. Using

104

Case Studies male thermoforming techniques does not have the benefit of a moulding surface to provide a finish. Thus, the inherent property of the sheet stock must provide the necessary gloss to the finished part [1]. Engineered polyolefins offer improved weathering and recycling over traditional thermoformable polymers used for automotive applications. The extrusion and thermoforming characteristics of several engineered polyolefin products can be utilised for applications ranging from soft skin top-coats to structural designs. Engineered polyolefins can also be used in all-olefin, multi-layer combinations for lower system cost and improved recycling compared to existing multi-material combinations [2]. Hard thermoplastic olefins (TPO) have grown rapidly in the automotive industry due to their favourable cost/performance characteristics and thermoforming offers the potential to manufacture large parts with much lower tooling costs than injection moulding. High-melt-strength polypropylenes are being used in hard TPO applications requiring high melt strength. The impact modifier plays an important role in the thermoforming characteristics of hard TPO compounds [3]. Soft thermoplastic polyolefin materials alone have not been successful in meeting many of the requirements for automotive soft skin, for a variety of reasons including grain retention during forming, chemical resistance and softness. Improved performance and cost-effective materials for positive thermoforming will help this technology retain its position versus competitive technologies such as polyurethane and polyolefin slush moulding. In the area of negative vacuum forming, these new products have shown promise to date. This technology is currently more expensive, but shows great promise in delivering good performance skins with excellent grain definition [4]. Thermoformable engineered polyolefin composites with low temperature ductility and various surface effects such as high gloss, low gloss and soft touch are designed for durability, weathering,

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Update on Troubleshooting in Thermoforming colour matching and other customised properties. Substrate materials provide structural rigidity, fracture toughness and low temperature ductility. All composite structures are fully recyclable and compatible with other polyolefins to provide utilisation of process scrap and trim [5].

6.2 Case Study 2 Challenge: Finding an environmentally safe and cost effective substitute for automotive fuel tanks. Requirements: To replace automotive fuel tanks with plastic sheet material to reduce the cost, and improve the environmental safety, recycling and ease of processing. •

Easy processing;

•

Environmental safety;

•

Cost reduction.

Steel fuel tanks are currently in use in the automotive industry. Plastics can be used in the areas of hydrocarbon emissions. Efforts are being made by the thermoforming industry to reduce evaporative tank emissions in order to meet more stringent airquality regulations. Increasing the use of plastic fuel tanks through thermoforming solves recycling problems [6]. Polypropylene (PP) can be used in automotive fuel tanks since it has resistance to oil and petroleum products. Although general-purpose PP has a low melt strength, high-melt-strength grade PP is available for thermoforming. It is possible to recycle this material, hence it is safer for the environment.

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Case Studies

6.3 Case Study 3 Challenge: To develop a thermoformed product for a display enclosure with product protection, theft-resistance, a visually pleasing appearance, easy-to-read display, low cost and recyclability. For display products, the use of thermoformed products is increasing in the retail environment. Properly designed thermoformed products can be theft resistant and display friendly. Plastic enclosures are preferred due to their anti-theft characteristics. The thermoformed package can only be opened using a knife or scissors. Products are displayed in thermoformed plastic enclosures. PP, polyethylene and polyethylene terephthalate as commodity plastics or polystyrene as non-commodity resin are the preferred materials to satisfy the above requirement with pre-formed or pretextured sheet. These materials have advantages such as the ability to be fabricated on-site, they use low-cost moulds, they require minimal start-up costs and have low tooling costs, design flexibility and good cosmetic appearance.

References 1.

K.L. Walton, L.B. Weaver and D.P. Waszeciak in Proceedings of the SPE Conference - ANTEC 2006, Charlotte, NC, USA, 2006, p.2636.

2.

C. Reid and M. Mahan in Proceedings of the SPE Automotive TPO Global Conference 2001, Dearborn, MI, USA, 2001, p.213.

3.

K.L. Walton, M.K. Laughner and E.S. Gisler in Proceedings of the SPE Automotive TPO Global Conference 2004, Dearborn, MI, USA, 2004, p.12.

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Update on Troubleshooting in Thermoforming 4.

R. Butala and M. Balow in Proceedings of the SPE Automotive TPO Global Conference 2004, Dearborn, MI, USA, 2004, p.5.

5.

C. Reid, M. Mahan and H. Tavakoli in Proceedings of the SPE Automotive TPO Global Conference 2002, SPE, Dearborn, MI, USA, 2002, p.193.

6.

M. Tolinski, Plastics Engineering, 2005, 61, 6, 18.

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7

Conclusion

Today’s thermoforming industry has the equipment, technology and materials to continue strong growth. Thermoforming offers processing advantages over competitive processes such as blow moulding and injection moulding. The process of thermoforming has advanced as applications have become increasingly challenging and demanding, and thermoformers are now seeking out higherperformance resins [1]. With today’s global economic challenges, the thermoforming industries have to be more competitive and make changes and improvements. Thermoforming is currently one of the most suitable production technologies for processors in operating industries, as it enables them to realise new products in reduced time and with low investments in moulds and equipment [2]. Thermoformed products are usually categorised as permanent or industrial products and disposable products. Rigid or semi-rigid packaging is the most common type of disposable thermoformed product. Thermoforming, being the art and engineering of fabricating functional plastic parts from sheet, is maturing into a viable, competitive technology in packaging and structural parts [3]. Despite economic recovery in the United States driving strong demand and with capacity additions few and far between, high feedstock prices have put profits on hold for makers of high-volume polymers such as polyethylene, polypropylene, polystyrene (PS) and polyvinylchloride. Strong demand has boosted operating rates at all kinds of polymer plants, which had been running well below their capability [4].

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Update on Troubleshooting in Thermoforming Thermoforming does have some disadvantages such as the generation of scrap, the higher cost of sheet materials because of the separate sheet-forming step, limited design parts because parts with sharp bends and corners are difficult to produce and the process results in internal stresses; however, these problems are minimised in the modern techniques, and thermoforming is better capable of producing thin mouldings than other processing techniques. Thermoforming has evolved technically to become the process of choice for many applications across a broad range of products. The expansion of thermoforming into a wide range of markets is pushing innovation in both equipment and materials as applications become more varied and parts more complex [5]. New developments in the thermoforming machinery, in tooling and in materials are helping the sector to increase its market share as a processing method. Thermoforming remains a favoured process for thin-wall packaging, where it offers material savings on high-volume parts at a unit cost, which is unachievable with injection moulding. Although injection moulders have taken substantial thermoforming business, due to their ability to offer high-end in-mould graphics, developments by thermoforming toolmakers are reported to be enabling thermoformers to provide commercial-scale processing of in-mould labelled thermoformed containers [6]. Design is essentially the process of devising a product that fulfils as completely as possible the total requirements of the user, and at the same time satisfies the needs of the fabricator in terms of cost effectiveness. The efficient use of the best available material and production process should be the goal of every design effort [7]. The microthermoforming process, a microscopic adaptation of trapped sheet forming, is used in the manufacture of chips for capillary electrophoresis, using impact-resistant, biaxially-oriented PS, as well as in the manufacture of chips for cultivation of cells in the medical field [8].

110

Conclusion

References 1.

M.J. Gehrig, S. Kelly and M. Carr, Plastics Technology, 2005, 51, 5, 78.

2.

C. Celata, Popular Plastics and Packaging, 2007, 52, 12, 49.

3.

J.L. Throne and P.J. Mooney, Thermoforming Quarterly, 2005, 24, 2, 19.

4.

A.H. Tullo, Chemical and Engineering News, 2004, 82, 25, 12.

5.

C. Goldsberry, Modern Plastics International, 2004, 34, 11, 40.

6.

M. Defosse, Modern Plastics World Encyclopedia, 2006, p.106.

7.

Reinforced Plastics, 2005, 49, 4, 30.

8.

R. Truckenmueller and S. Giselbrecht, Plastics Engineering Europe, 2006, 4, 1, 14.

111

A

bbreviations

ABS

Acrylonitrile-butadiene-styrene

ASTM

American Society for Testing and Materials

CPVC

Chlorinated polyvinylchloride

COC

Cyclic-olefin copolymer(s)

DSC

Differential scanning calorimetry

DMA

Dynamic mechanical analysis

EPS

Expanded polystyrene

Tg

Glass transition temperature

HDPE

High-density polyethylene

LLDPE

Linear low-density polyethyelene

LDPE

Low-density polyethylene

Tm

Melting temperature

MWD

Molecular weight distribution

OTR

Oxygen transmission rate

POM

Polyoxymethylene

PC

Polycarbonate

PEEK

Polyetherether ketone

PE

Polyethylene

PET

Polyethylene terephthalate

PLA

Polylactic acid

PMMA

Polymethylmethacrylate

PPO

Polyphenylene oxide

PP

Polypropylene

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Update on Troubleshooting in Thermoforming PS

Polystyrene

PTFE

Polytetrafluoroethylene

PVC

Polyvinylchloride

PVT

Pressure, volume, temperature

TMA

Thermomechanical analysis

TPO

Thermoplastic olefin(s)

UV

Ultraviolet

114

A

uthor Index

A Adam, A., 89 Alberola, N., 14 Aoki, Y., 26 Arzondo, L. M. 27 Auras, R., 36 Azizi, H., 16

Brandrup, J., 17 Braun, B., 68 Bristow, P. 52 Brown, R. 99 Burstone, C. J., 99 Bush, S.F., 55 Bush, T., 46 Butala, R., 105 Buterbaugh, T. E., 12

B Balow, M., 105 Barroso, V. C., 101 Bedard, N., 100 Beine, J., 68 Ben Yedder, R., 58 Berdyshev, B. V., 68, 85 Bhattacharya, S. N., 9, 36 Bibee, D. V., 7, 13 Billmeyer, F. W. J., 20 Blundell, D. J., 22 Boi, K., 22 Bonnebat, C., 11 Bonten, C., 24 Boon, J., 24 Bordonaro, C. M., 99 Boulet, B., 1, 36 Boyer, R. F., 17 Bragole, R., 23

C Campbell, R. A., 15 Candal, M. V., 60 Capel, M., 22 Carfantan, P., 20 Carr, M., 109 Casey, W. J., 12 Cassago, D., 43 Cavaille, J. Y., 14 Cebe, P., 22 Ceccorulli, G., 11 Celata, C. 109 Celli, A., 25 Chiba, A., 26 Christopherson, R., 36, 100 Clark, A., 68 Cohn, D., 25 Coleman, M. M., 7

115

Update on Troubleshooting in Thermoforming Corradini, P., 15 Costeux, S. C., 23 Covas, J. A., 101 Curtis, C. F., 18 Cygan, Z., 25

D Dawson, P. C., 22 De Vries, A. J., 11 Dealy, J. M., 7 Debbaut, B., 36, 100 Defosse, M., 110 Dekmezian, A. H., 16 Dharia, A., 82 Di Landro, L., 44, 47, 49, 62 Dijkstra, P. J., 25 Diraddo, R. W., 58 Doak, K. W., 7, 13 Duarte, F. M., 101 Dunay, K., 9

E Ebewele, R. O., 1 Effler, L. J., 54 Eie, T., 43 Ellis, P., 14 Engelberg, I., 25 Eyerer, P., 46

F Feijen, J., 25 Feingold, J. M., 52 Field, G. J., 9, 36 Finlayson, M. F., 23 Fischer, E. W., 25 Fisher, T. W., 53 Frogg, J., 14 116

G Galante, P. A., 99 Gällstedt, M., 43 Galotto, M. J., 91 Gay, F., 10 Gehrig, M. J., 109 Ghasemi, I., 16 Gilding, D. K., 25 Giselbrecht, S., 110 Gisler, E. S., 54, 105 Goldberg, A. J., 99 Goldsberry, C., 110 Goodrich, J. E., 35 Greiner, G., 11 Gross, H., 16 Guarda, A., 91 Gupta, A. V., 53

H Haberstroh, E., 45 Haihong, X., 52 Hall, I. H., 20 Harkin-Jones, E. M. A., 98 Harris, J. E., 22 Hegemann, B., 46 Hockey, J., 17 Hoenig, S. M., 23 Hoftyzer, P. J., 17 Hogan, T. A., 23 Hosseini, H., 68, 75, 85 Hyon, S-H., 25

I Ikada, Y., 25 Immergut, E. H., 17

Author Index

J Jalham, S. I., 67, 69 Jamshidi, K., 25 Järvelä, P., 15 Jiang, W. -G., 42 John, E., 20 Jones, G. A., 22

K Kaneko, M., 26 Karrabi, M., 16 Kazmer, D. O., 2, 20, 52, 99 Kehl, T. A., 22 Kelleher, P. G., 8, 11 Kelly, S., 109 Khongkruaphan, S., 44 Klein, D. E., 13 Kohn, J., 25 Kouba, K. 46 Kroschwitz, J. I., 2

L Landa, B., 52 Laroche, D., 58 Lau, H. C., 9, 36 Laughner, M. K., 54, 105 Le Maoult, Y., 58 Lee, J. K., 8, 9, 50, 51, 93, 99 Lima, L. -T., 36 Lu, S. X., 22 Lyngaae-Joergensen, J., 11

Mahan, M., 105, 106 Maia, J. M., 101 Mandelkern, L., 7, 13 Maoult, Y. Le, 8, 55 Marchand, S., 100 Markel, E. J., 16 Marom, G., 25 Marrey, R., 52 Marshall, R. A., 71 Masberg, U., 16 McKelvey, J. M., 1 McPhillips, K., 69, 85 Mead, J., 44, 45 Mehrabani-Zeinabad, A., 68, 85 Meissner, J., 7 Messina, C., 52 Mezghani, K., 15 Minjas, J., 18 Misra, D. 53 Mohammadian-Gezaz, S., 16 Monteix, S., 8, 55, 58 Mooney, P. J., 109 Morales, R., 60 Morye, S. S., 9, 93, 97, 99 Mount, E. M., 11 Murphy, W. R., 98 Murray, T., 44

N Natta, G., 15 Nobuta, A., 26

M

O

Maack, H., 14 Macauley, N. J., 98 Magré, E. P., 24

Okano, K., 12 Orroth, S., 44, 45 Osborn, B. N., 22 117

Update on Troubleshooting in Thermoforming

P Painter, P. C., 7 Palys, C. H., 20 Park, H. C., 11 Pascu, M., 14 Patel, A. P., 99 Peacock, A. J., 7 Perez, C. J., 34 Perez, J., 30 Petrie, C. J. S., 98 Pettersen, M. K., 43 Pezzin, G., 11 Phillips, P. J., 15, 20 Pineo, B., 99 Pizzoli, M., 11 Popli, R., 7

R Ree, T., 20 Reed, A. M., 25 Reid, C., 105, 106 Reifschneider, L., 45 Riello, J., 52 Robeson, L. M., 22 Rodriguez, F., 20 Rogers, P. A., 11 Roose, P., 15 Rosato, D. V., 51 Rubin, Y., 36, 100 Rubino, M., 36

S Sala, G., 44, 47, 49, 62 Sauer, J. A., 13 Scandola, M., 25 Scheidl, K., 14 118

Schmidt, F. M., 8, 55, 58 Schrage, A., 7, 13 Schut, J. H., 24 Schwarzl, F. R., 11 Scott, C. E., 8, 9, 50, 51, 93, 99 Serle, A. G., 26 Seshadri, S., 25 Sherman, L. M., 15 Shucai, L., 15 Singh, M., 25 Singh, P., 11 Smallwood, P. V., 11 Solovyov, S. E., 93 Speed, C. S., 7 Sperling, L. H., 20 Starkweather, H. W., 22 Stehling, F. C., 7, 13 Stephenson, R. C., 11 Sterzel, H. J., 25 Szamborski, E. C., 12

T Tatarka, P. D., 26 Tavakoli, H., 106 Taylor, A., 25 Tessier, N., 44, 45, 46 Throne, J. L., 37, 45, 47, 60, 94, 109 Todd, D. B., 2, 6 Toensmeier, P. A., 13 Tolinski, M., 106 Torres, F. G., 55 Truckenmueller, R., 110 Tullo, A. H., 109 Tulsian, A., 45

Author Index

V Van De Witte, P., 25 Van Den Berg, J. W. A., 25 Van Krevelen, D. W., 17 Vasile, C., 14 Vasilivich, B. B., 68, 75, 85 Vercuski, P., 12, 24 Virkler, T. L., 8, 9, 50, 51, 93, 99

W Walczyka, D. F., 40 Walther, B. W., 23 Walton, K. L., 23, 54, 105 Warby, M. K., 42 Warwick, P., 42 Waszeciak, D. P., 12, 24, 105 Weaver, L. B. 12, 24, 105 Wegner, G., 25 Weng, W., 16 Westerman, L., 7 Whitemana, J. R., 42

Wilson, F., 57 Winkel, E., 16 Wirtz, J., 45 Wissbrun, K. F., 7 Woodward, A. E., 13 Wortberg, J., 16 Wrasidlo, W. J., 24 Wright, T., 42 Wunderlich, B., 15, 26 Wysocki, J., 52

X Xanthos, M., 2, 6

Y Yang, C., 23 Yang, S., 1, 36 Yoo, S., 40 Younes, H., 25

Z Zhou, H., 101 Zoller, P., 22

119

S

ubject Index

A Acrylics 2, 7, 12, 20–21 Acrylonitrile-butadiene-styrene 7, 8–10, 36, 68, 97 blending with other plastics 9 dimensional stability 9 physical properties 8 pre-drying 9–10 processing 9,36 strength and rigidity 9 Additives 5–6, 10, 12, 15, 19, 25, 76 anti-static 76 clarifiers and nucleators 15 formulation 12 migration of 91 modifier 15

B Biodegradable polymer 24–25 polylactic acid, physical properties 24 Blisters or bubbles 11, 23, 55, 71–73

C Case studies 103–107 Cellulosic sheet materials 2, 23 Co-extruded film thermoforming 101 Composites 26–27 polyacetal, physical properties of 26

121

Update on Troubleshooting in Thermoforming Copolymers 5, 12, 13, 18 Cracking in corners 86–87 Cyclic-olefin Copolymer 25–26 physical properties 25

D De-moulding, part deformations during 90–91 Drape forming 39, 48–49, 100 Dynamic mechanical analysis 14 Dynamic scanning calorimetry 13, 20, 24

E End-product problems 91–94 blue coloured dots 93 melt failure during thermoforming 93 migration of additives 91–92 sheet pull-out 94 shrinkage 94 Engineering plastics 2 Expanded polystyrene (EPS) (foam) 18–19 extruded solid polystyrene 18 films 19 polyethylene terephthalate foam 18–19, 52, 107

F Fluoropolymers 2

G Glass transition temperature 99

H Heaters 58–59 heating techniques 59 temperature distribution 58 Heating, infrared technology 100 Heating, optimum heating cycle 98 122

Subject Index Heavy-gauge material 98 Heavy-gauge thermoforming 67 High-density polyethylene 13–14 physical properties of 14 High-impact polystyrene 2, 7

I Infrared emitters 8, 55 Infrared heaters 58 Infrared oven 8 Infrared pyrometer 58 Infrared technology 100 Infrared temperature sensors 100

L Liquid crystalline polymer 2 Long-chain branching 7, 13 Low-density polyethylene 7, 12–13 advantages of 15 physical properties, of 13

M Matched-die forming 46–48 male and female moulds 48 plug and cavity moulds 48 Melamine formaldehyde 6 Melt failure 93 Migration of additives 91–92 Modifiers 5–6, 10, 13 Molecular weight 5, 7, 14, 51, 69, 97 Molecular weight distribution 7 Mould surfaces, grit-blasted 100 Moulds 35, 37, 53–56, 61 aluminium/steel/plastic 54 basic designs/operations 55 complicated, with grooves 43 epoxy/polyester/chromium steel/cast aluminium 53–54 123

Update on Troubleshooting in Thermoforming Moulds (continued) injection 41 low-cost 101 male and female 48 part sticking 77–78 plug and cavity 48 tooling 54 vacuum bagging 54 water-cooled matched 48

N Nipples, prevention of 78–80

O Oil canning 71 Optimisation of production co-extruded film thermoforming 101 grit-blasted surfaces 100 heating profiles 99 thickness distribution 100 Oxygen transmission rate 43

P Phenol formaldehyde 6 Phthalate plasticisers 10 Plastics classification of 5 sheet, materials/physical properties 5–27 sheet processing 2 Plug 57 polytetrafluoroethylene 57 Plug-assist plug 57 pressure forming 42, 44 thermoforming, female mould 55 Plug-assist vacuum forming 42–46

124

Subject Index pressure-bubble 46 influence of plug design 43–44 prototype thermoforming 45 selection of plugs 45–46 stretching operation 45 surface friction 45 Polyacetals 2, 5–6, 26 physical properties of 26 Polyamides 2, 6, 68 Polycarbonate 2, 6, 9, 22–23, 68 physical properties 22–23 Polyetheretherketone 2, 21–22 physical properties of 22 Polyethylene 2, 6–7, 7, 14, 18, 39, 43, 52, 107, 109 Polyethylene terephthalate 2, 18, 19–20, 52, 107 crystallinity 19 foam 18–19 physical properties of 20 Polyimides 2 Polylactic acid 24–25 Polymers 2 physical properties 6 semi-crystalline, thermal and mechanical properties 7 Polymethylmethacrylate 5, 6–7, 20–21, 68 acrylic 20–21 advantages 21 physical properties 20 Polyphenylene oxide 23–24 physical properties, using DSC 24 Polyphenylsulfide 2 Polypropylene oxide 2 Polypropylene 2, 5, 7, 14–16 extruded clarified polypropylene 16 oriented 16 physical properties of 15

125

Update on Troubleshooting in Thermoforming Polystyrene 2, 5, 16–17 oriented 17 physical properties of 17 Polysulfones 2, 57 Polytetrafluoroethylene 57 Polyvinyl chloride 2, 5, 6–7, 16, 18, 23, 45, 52, 71, 91, 109 acrylic processing 12 chlorinated polyvinylchloride 11 films 11 K value 10 phthalate plasticisers 10 physical properties 11 resistance to chemical/solvent attack 10 rigid cellular 12 vinyl monomers 12 Post-process technology 1 Pressure forming 10, 16, 38, 41–42, 48, 98 equipment 16 injection moulding 41 plug-assist 42 snap-back and single- or twin-sheet 10 Problems categories 67 end-product problems 91–94 occurring during thermoforming 69–70 troubleshooting 71–91 Process parameters 60–63 internal improvements and modifications 61–63 thermoforming techniques 60

R Raised corners 87–88 Raw material 51–53, 60, 61, 98, 103 plastic sheets 51–53 residual stress 40, 52, 85 sagging of polymer sheet 52 Residual stress 40, 52, 85

126

Subject Index

S Sheet pull-out 94 Sheet sag 74–75 Sheet temperature 45, 48, 57, 58, 73–74, 79, 89, 98–99 Shop cleanliness 73 Shrinkage 94 excessive post 76 post-forming 93 scuffs and blemishes 89, 99 Straight vacuum forming 39–40, 43, 48 Stretch marks 78, 79 Surface markings 88–89

T Tearing of sheet 82–83 Thermal history, graphical representation 51–52 Thermoformed sheet folds 90 pinholes 75–76 rupturing 75–76 webbing 90 wrinkles 90 Thermoforming, 36, 38 acrylonitrile-butadiene-styrene 7 acrylics 7 advances in 37 advantages of 2 co-extruded film 101 design 110 disadvantages of 110 end-product problems, melt failure during 93 growth of 2 heavy gauge 67 high-impact polystyrene 7 machines 62

127

Update on Troubleshooting in Thermoforming Thermoforming (continued) melt failure 93 new developments in machinery 110 plastic sheets 2 polyethylene 7 polypropylene 7 polyvinylchloride 7 post shrinkage 76 problems occurring during 69 prototype 45 temperatures 7 thermal history of 52 thin-gauge 67 versatility 97 Thermoforming materials, physical properties acrylonitrile-butadiene-styrene 8–10 biodegradable polymer 24–25 cyclic-olefin copolymer 25–26 composites 26–27 expanded polystyrene (foam) 18–19 high-density polyethylene 13–14 low-density polyethylene 12–13 polycarbonate 22–23 polyetherether ketone 21–22 polyethylene terephthalate 19–20 polymethylmethacrylate/acrylic 20–21 polyoxymethylene 26 polypropylene 14–16 polypropylene oxide 23–24 polystyrene 16–17 polyvinyl chloride 10–12 thermoplastic elastomers 24 thermoplastic olefins 23 Thermomechanical analysis 13 Thermoplastics 5 amorphous 5 cellulose acetate 5

128

Subject Index crystalline 5 polycarbonate 5 polymethylmethacrylate 5 polyacetal 5 polyamide 5 polyethylene 5 polypropylene 5 polystyrene 5 polyvinyl chloride 5 sheet material 2 Thermoplastic elastomers 24 Thermoplastic fabrication process 2–3 Thermoplastic olefin 23–24, 105 Thermosets 5–6 glass transition temperature of 6 reprocessing of 6 Thin-gauge thermoforming 67 Troubleshooting bad definition edges of 83–84 blisters or bubbles 71–73 bubbling 71 cracking in corners 86–87 folds, webbing or wrinkles 90 glossy spots 84–85 nipple formation 78–79, 80 part deformation 90–91 part sticks to mould 77–78 part warpage 85–86 pinhole or rupturing 75–76 pock marks 79–80 poor part detail 91, 92 poor wall distribution 80–81 post shrinkage, excessive 76 raised corners 87–88 sheet sag, excessive 74–75 stretch marks on part 78, 79 surface markings 88–89

129

Update on Troubleshooting in Thermoforming Troubleshooting (continued) tearing of sheet 82–83 thinning of corners 89–90 uneven edges 81–82 uneven sag 77 webbing or bridging 73–74

U Urea formaldehyde 6

V Vacuum forming drape forming 48–49 free forming 46, 47 matched-die forming 46–48 plug-assist 42–46 plug-assist pressure forming 42, 44 pressure-bubble plug-assist vacuum forming 46 pressure forming 41–42 straight 39 vacuum snap-back forming process 49–51 Vacuum holes 54, 56–57, 78, 100 Vacuum snap-back forming process 49–51 Vacuum thermo forming 39–41 advantages/disadvantages 39–40 packaging 41 process, schematic of 40

W Warpage, part 85–86 Webbing or bridging 73–74, 90, 99 degree of sag 73

130

Published by iSmithers, 2010

This book provides thermoformers and part designers with an insight into the problems that can occur during the production of a product of the desired quality. It is often difficult to recognise the cause of problems that occur in thermoforming and thus correct them. Problems can be classified as uneven shapes, blisters or bubbles, poor detail, incomplete forming, sagging (excessive sheet sag, uneven sag), webbing, stretch marks, poor wall distribution, tearing the plastic sheet when forming, part sticking to the mould, part warpage, surface marking, uneven edges, raised corners, and cracking in corners. Also discussed are the different types of surface defects, their identification in plastic parts and the ways to solve the problem. All these problems and their remedies are covered in detail, in this book. This book will be of use to those producing thermoformed items whether they are experienced thermoformers or new to the technique, and to ­designers, designing items which will be produced by thermoforming.

Shawbury, Shrewsbury, Shropshire, SY4 4NR, UK Telephone: +44 (0)1939 250383 Fax: +44 (0)1939 251118 Web: www.iSmithers.net