Specialized Molding Techniques

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Edited

by

Hans-Peter

Svcic~j' (Jf,Dlu:'.IIC,~ l"ngmeer.1

Heim

H. Potente Plastics Design Library

Copyright © 2001, Plastics Design Library. All rights reserved. ISBN 1-884207-91-X Library of Congress Control Number: 2001091835

Published in the United States of America, Norwich, NY by Plastics Design Library a division of William Andrew Inc. Information in this document is subject to change without notice and does not represent a commitment on the part of Plastics Design Library. No part of this document may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information retrieval and storage system, for any purpose without the written permission of Plastics Design Library. Comments, criticism and suggestions are invited and should be forwarded to Plastics Design Library. Plastics Design Library and its logo are trademarks of William Andrew Inc.

Please Note: Great care is taken in the compilation and production of this volume, but it should be made clear that no warranties, express or implied, are given in connection with the accuracy or completeness of this publication, and no responsibility can be taken for any claims that may arise. In any individual case of application, the respective user must check the correctness by consulting other relevant sources of information. The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use.

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One-shot Manufacturing: What is Possible with New Molding Technologies James F. Stevenson GenCorp Technology Center, Akron, OH 44305, USA

INTRODUCTION New molding technologies1 together with a revolution in thinking about how to design and manufacture products2-3 have merged to open exciting new possibilities in polymer part manufacturing. The new technologies offer versatile, cost effective forms of materials and more unified, efficient production methods. Even greater benefits, especially from consolidation, exist for parts previously made of metal. These processes move forward and, in some cases, realize the goal of forming a complex product in a single manufacturing step, or one-shot manufacturing. Conventional manufacturing processes generally employ homogeneous materials and simple primary shaping processes to form components which were then assembled by various joining methods. These subassemblies then go through various secondary operations and ultimately are combined to form the finished product. This sequential process is labor intensive, time consuming, and costly; it requires large inventories and long changeover times and is prone to produce scrap. This paper presents and analyzes common features of ten of the new material-process technologies. These technologies, along with chapters on predicting orientation and warpage by T.A. Osswald and on lean molding by Colin Austin are presented in greater depth in a recent book Innovation in Polymer Processing: Molding.1 All of these process and material innovations are based on antecedent technology and well-known physical principles. The key to realizing these innovations was conceptual.

NEW TECHNOLOGIES The innovative molding technologies are summarized in Tables A1-A10 which also include a listing of advantages/disadvantages, applications, and materials. These tables are self contained; readers are referred to them as independent sources of information and as descriptive material for the processes cited in Tables 1-3.

2

Special Molding Techniques

PRODUCT ELEMENTS The framework given in Tables 1-3 facilitates classification and comparison of the new molding technologies. It serves both as a guide to applications and as a means of locating opportunities and gaps in the new technologies. Technologies at similar locations in the tables can be considered as potential alternatives for each other. In the tables materials are divided into polymers, consisting of rubber and plastics, and nonpolymers, primarily solids or gases. Solids, typically fibers, serve as reinforcements, whereas gases reduce density or increase stiffness for a given cross-sectional area by distributing material to increase the moment of inertia. The term macroscopic refers to dimensions that are on the order of the thin part dimension, e.g., a 2-mm diameter void in a 3-mm rod. Microscopic means dimensions two orders of magnitude or more smaller (e.g., a 300-layer laminate) than the part thickness. Achieving one-shot manufacturing requires an optimal combination of MATERIALS, PROCESS, and GEOMETRY. MATERIALS In terms of MATERIALS, the innovative molding technologies are classified according to COMPOSITION, one or more POLYMERS with or without NONPOLYMERS, and SCALE, MICROSCOPIC or MACROSCOPIC, as shown in Table 1. Table 1. Materials: composition and scale Composition Scale

Polymers combined with Other polymers

Gas

Solids

Macroscopic

Blow Molding Coinjection Molding (MMP)* Multimaterial Molding (MMP) In-Mold Coating (MMP) Dual Molding (LPM)*

Gas-Assisted Molding Liquid-Gas Molding (LPM) Blow Molding

Laminate Molding (LPM)

Microscopic

Lamellar Molding

Microcellular Plastics Controlled Density (LPM)

Sheet Composites Reactive Liquid Molding

* Processes designated LPM (Low Pressure Molding) are described in Table A4; those designated MMP (Multimaterial Multiprocess) are given in Table A10.

One-shot Manufacturing

3

Two of the innovative processes shown in Table 1, lamellar molding and microcellular plastics, serve primarily as the means of generating a unique material on the microscopic scale and secondarily for shaping the material. Other processes listed in the table combine polymers and nonpolymers on a microscopic or macroscopic scale to from products where specific properties of the multiple materials meet local functional needs. This localized optimization ultimately enhances overall product performance. PROCESSING PROCESSING is the bridge between the unshaped raw MATERIALS and the GEOMETRY (MACROSCOPIC STRUCTURE and SIZE & SHAPE) of the product as shown in Table 2. Table 2. Processing: materials and geometry Geometry Materials

Macroscopic structure Laminate

Polymers

Polymers and nonpolymers

In-Mold Coating Coinjection Blow Molding Dual Molding (LPM)

Segmented Multimaterial Molding (MMP), Blow Molding

Size & shape Large Sheet Composites In-Mold Coating (MMP) Injection-Compression (MMP)

Solid

Laminate Molding (LPM) Reactive Liquid Molding

Laminate Molding (LPM) Sheet Composites Reactive Liquid Molding

Gas

Blow Molding Gas-Assisted Molding Liquid Gas Molding (LPM) Fusible Core

Blow Molding Controlled Density Molding (LPM)

Hollow Dual Molding (LPM) Blow Molding

Blow Molding Gas-Assisted Molding Liquid Gas Molding (LPM) Fusible Core

Many processes under MACROSCOPIC STRUCTURE in Table 2 allow the combination of a polymer with other polymer(s) or nonpolymers on a macroscopic scale to give

4


which have uniform properties over the surface of the part but variable properties through the thickness. SEGMENTED PARTS which exhibit a variation of properties along the surface but are uniform over a given cross-section. Combinations of laminated and segmented parts are possible, for example gas-assisted parts with hollow sections only in certain regions of the part. In terms of part SIZE & SHAPE, innovative technologies are generally needed when the part is LARGE, especially when a cosmetic surface is required, or when the part has a COMPLEX, often hollow shape, particularly when it is load bearing. LAMINATES

GEOMETRY Part GEOMETRY can be considered in terms of FUNCTION where the part is located on a scale ranging from ENCLOSURE (containers or panels, often with cosmetic surfaces) to LOAD BEARING, and COMPLEXITY which allows for geometric complexity ranging from SYMMETRIC (planar or axisymmetric) to fully THREE DIMENSIONAL. Table 3. Geometry: function and complexity


5

Table 3 suggests that molded parts and the associated innovative processes to make them generally range from more or less symmetric enclosures in the upper left to three dimensional load bearing parts in the lower right.

REFERENCES 1 2 3

Stevenson, J.F. (Ed.) Innovation in Polymer Processing: Molding, Hanser, Munich, Hanser-Gardner, Cincinnati (1996). Womack, J.P., Jones, D.T., and Roos, D., The Machine that Changed the World, Macmillan, New York (1990). Gooch, J., George, M., and Montgomery, D., America Can Compete, Institute of Business Technology, Dallas (1987).

6


APPENDIX Table A1 and A2. Gas-assisted injection molding: process [1.1] and simulation [2.1] PROCESS: Nitrogen gas under high pressure is injected through the nozzle or mold wall into plastic partially filling a mold. The gas flows preferentially through local thick sections with hot interiors and pushes the plastic ahead to fill the mold. SIMULATION: Commercial software now available to predict gas flow paths, polymer thickness, clamp force, and contraction during cooling for various geometries and process variables including gas pressure, injection time, and prefilled polymer volume. Simulations and experiment generally show • increasing gas pressure decreases fill time, gas penetration distance, and (by conservation of mass) polymer wall thickness, • melt temperature has a variable effect on gas penetration length, • increasing delay time before the start of gas injection increases wall thickness and gas penetration length, • increasing gas injection time increases gas penetration distance, • decreasing the prefilled polymer volume fraction increases the penetration length until a critical level when gas blows through. • increasing gas pressure level and time decreases shrinkage. Simulations are generally able to predict undesirable air traps and gas penetration into thin sections. Simulation of a freezer bottom part converted to gas-assisted molding showed a 70% reduction in packing pressure, feasibility of using a less expensive material, and reduced warpage due to lower, more uniform pressure and higher part stiffness. Eight design guidelines are given based on both experiment and computer simulation [2.1]. Advantages/Disadvantages

Applications

Materials

PROCESS: Part weight and cooling time can be reduced up to 50%. Sink marks are eliminated. Warpage is reduced. Clamp force and injection pressure are lower. Part stiffness is increased because of the higher moment of inertia. Licensing is necessary. SIMULATION: Simulation helps identify optimal process conditions including runner layout and size, and location and timing of gas introduction. Software is available but new.

Handles, Panels with Ribs, Appliance/machine Housings (TV benzels) Automotive Parts (Clutch Pedals, Mirror Housings)

ABS, PA, PE, PP, PS, PPO, PC, PBTP, PC/PBTP, SAN, TPE, TPU


7

Table A3. Fusible core injection molding [3.1] Complex hollow parts are formed by injection molding plastic around a fusible alloy core which is subsequently removed by melting. The fusible (or lost) core typically is cast form a bismuth-tin alloy with a eutectic melting point of 138oC. The molten metal fills a split steel mold from the bottom and then cools for 2 min to produce a heavy core with a mirrorlike surface. The still-hot core is positioned by a robot in a steel mold and plastic is injected. Flow channels are designed to balance forces around the core during filling to prevent core movement. For thermoplastics the injection temperature, e.g. 290oC for polyamide, can be well above the melting point of the core since the relatively high thermal diffusivity of the metal maintains a low interface temperature. After demolding, cores are melted out in a large bath or by induction heating or by injecting heat transfer fluid inside hollow cores. Advantages/Disadvantages

Applications

Materials

Plastic parts made by fusible core technology have a weight and cost advantage over metal parts. Fusible core molding eliminates the need for mechanically complex molds or joining separately molded parts. Interior surfaces of fusible core parts are smooth which increases gas flow. Disadvantages are loss or oxidation of expensive core metal and need for robots to handle heavy cores.

Air intake manifolds, tennis racquets, pump parts

PA Poly(etherarylketone)

Table A4. Low pressure molding [4.1] Low pressure molding, as developed by Siebolt Hettinga, enables a number of other molding technologies.In Low Pressure Molding (LPM) the mold cavity is filled at low speed through large gates with a controlled pressure profile in the shape of a broad inverted U. LPM has no packing stage and no cushion. The melt temperature profile is controlled by adjusting screw speed and flow resistance during plastication. LPM works better with low viscosity semicrystalline materials and is not suitable for thin-wall parts. Slow injection, lower melt and higher mold temperatures reduce residual stress to allow demolding at a higher temperature to maintain or reduce cycle times. For larger parts, low clamp force can be achieved using multiple valve gates with programmed opening [4.2]. Lower clamp force allows use of self clamping molds and multistation injectors. Laminate Molding involves molding plastic at low pressure directly behind textile, film, or metal. In Liquid Gas Injection Molding, a volatile liquid is injected at low pressure into the melt and then vaporizes to form hollow channels in the part. The liquid condenses and is absorbed in the part. Dual Molding, similar to Bayer’s Multishell Molding, forms an integrated hollow part by overmolding at low pressure an assembly formed from separately molded parts. In Controlled Density Molding the mold is partially opened once a skin has formed to give a low density interior.

8


Table A4. Low pressure molding [4.1]

Advantages/Disadvantages

Applications

Materials

Substantial capital costs savings result from the use of presses with a lower camp force or self clamping molds. Laminate Molding saves on assembly and adhesive costs in fabric/plastic laminates.

Low Pressure Molding: Interior Vehicle Panels, Bumper Fascia. Laminate Molding: Fabric/Plastic Seats, Vehicle Trim Panels. Liquid-Gas Assist Molding: Large Chairs, Chair Bases. Dual and Shell Molding: Manifolds, Pump Bodies, Valves, and Fittings. Low Density Molding: Fittings, Electronic Enclosures, Table Tops.

Thermoplastics, especially polyolefins, thermosets

Table A5. Advanced blow molding [5.1] The advanced blow molding technologies described below have greatly extended the versatility and facilitated product design. Deep-Draw Double-Wall Molding employs a mold with four hinged slides and an advancing core which close in a programmed manner around a partially inflated parison to shape a deep draw part. Press Blow Molding is used to form panels between shallow male and female mold halves which press together certain sections and inflate other sections to form hollow stiffening ribs.Three Dimensional (3D) Blow Molding forms serpentine three-dimensional parts without excessive scrap by manipulating the parison and positioning it in a convoluted mold cavity. Positioning the parison can be accomplished by (1) translating in two directions the mold which is titled at an angle, (2) movement of the parison by robotic arms in a mold with multiple sections which close sequentially, and (3) guiding the parison through the mold by sucking air along the length of the mold. Multimaterial Blow Molding employs multiple materials sequentially along the part length, in layers over the part thickness, or on opposite sides of the parison. New material developments include molding of 0.3-in fiber reinforced materials and foam layers [5.2]. Computer simulation of blow molding has been developed by A.C. Technology, Ithaca, NY in cooperation with G.E. Advantages/Disadvantages

Applications

Materials

Deep Draw Technology increases draw (depth-to-length) ratio from 0,3 to 0.7 and allows forming of parts with undercuts, ribs, and noncircular crosssections. Multimaterial applications allow soft surfaces on structural parts, flexible conduits with rigid connectors, or parts with opposite sides of different properties. 3D Blow Molding consolidates complex parts and enhances function.

Insulated containers with foam, Planters, Conduits, Air Ducts, Bumpers, Equipment Panels, Instrument Panels, Portable Toilets, Golf Cases, Arm Rests, Gasoline Filler Tubes, Gas Tanks.

PE, PS, PP, POA, ABS, PPE elastomers


9

Table A6. Thermoplastic sheet composites [6.1] Production of thermoplastic sheet composites involves two steps: (1) FORMATION of fiber reinforced sheets (prepreg) by polymer impregnation and sheet consolidation and (2) SHAPING of the sheets. Large volume competitive sheet FORMATION processes are continuous Melt Impregnation (e.g. Azdel sheet by extruding polypropylene onto a continuous “swirled’ fiber glass mat), and Slurry Deposition in which long fibers and polymer powder with dispersing agents are deposited on a moving screen similar to paper making. Other processes are Powder Impregnation (powder and fiber consolidated by pultrusion, double belt press, or compression molding), Reactive Pultrusion, and Commingling (intertwining different fibers) and Coweaving polymer fibers, and reinforcing fibers. SHAPING techniques for consolidated prepreg include Melt-Phase Stamping (prepreg covering the mold cavity is heated with infrared and shaped in a fast closing press), Fast Compression Molding (thick charge flows during mold closing), and Solid State Stamping (semicrystalline plastics below their melting point are stamped into parts with simple geometries in 15 sec. Other shaping methods include Pultrusion of prepreg tapes, Diaphragm Molding (preform between plastically deformable diaphragms shaped by hard tooling), Rubber Pad Molding, Hydroforming (rubber bladder inflated hydraulically, Vacuum Forming in an autoclave, and Flexible Resin Transfer Molding (sheets of resin and fiber between elastomeric diaphragms are consolidated, then shaped). Advantages/Disadvantages

Applications

Materials

Extrusion melt impregnation allows high fiber contents and longer fibers which give improved mechanical properties. Slurry deposition employs shorter fibers which allow greater flow and more complex parts. Cycle times are short.

Automotive Body Panels, Other Components, Aircraft Components

PP, PE, PA, PBT, PET, PVC, PC, PEEK, PSU, PPS

10


Table A7. Reactive liquid composite molding [7.1] Reactive Liquid Composite Molding (RLCM) proceeds in two steps: (1) PREFORM FORMATION by organizing loose fibers into a shaped preform, and (2) IMPREGNATION of the fibers with a low viscosity reacting liquid. The reacting material may be thermally activated by heat transfer in the mold or mixing activated by impingement of two reactive streams. Simulations of flow and reaction, a recent innovation in RLCM, allow determination of vent and weld line locations, fill times, and control of ‘racetracking’ in terms of gate locations, mat permeability, and processing conditions. Commercial success requires (1) fast reaction and (2) efficient preform formation. Cycle time for thermally active systems can be decreased by using higher mold temperatures and heating the preform. Innovative processes for PREFORMING include: Thermoformable Mat heated by IR to melt the binder and pressed into shape by one or two moving platens while supported by a hold/slip edge clamp to reduce wrinkling. Automated Directed Fiber Performers employ multiple delivery systems to create a surface veil, a chopped roving layer, and continuous roving with loops, all of which are fused by hot air. The SCRIMP process channels resin flow between layers of fibers or along internal networks. Water Slurry Deposition positions fibers by water flow through a contoured screen and sets them with hot air. Innovations to reduce costs by combining process steps include: Direct Part Forming combines sheet formation and shaping, e.g. heating porous sheet and then consolidating and shaping in a compression mold. The Hot Air Preformer produces performs by either directed fiber or thermoplastic mat forming. The Cut-N-Shoot process combines preforming and molding steps consecutively in the same tool. Bladder inflation inside a mold shapes the preform and forms the mold wall during filling. Advantages/Disadvantages

Applications

Materials

Low pressure and temperature processing by RLCM allow the use of inexpensive light-weight tools, especially for prototyping. RLCM allows customizing reinforcement to give desired local properties and part consolidation via complex 3D geometries.

Marine and Poolside Products, Sanitary ware, Caskets, Automotive Panels, Vehicle Suspension Links

Isocyanate based resins (mixing activated) Unsaturated polyester and styrene (thermal activated)


11

Table A8. MIcrocellular plastics: formation and shaping [8.1] Extremely small closed cells from 0.1 to 10 microns in diameter can be formed in most plastics by dissolving gas in the plastic, typically supercritical CO2, and then rapidly reducing the pressure and increasing temperature in a controlled manner to cause homogeneous and likely heterogeneous nucleation and growth of gas bubbles. The bubbles are to be smaller than naturally occurring flaws in the polymer so mechanical properties are not compromised. Particles in PS(HI) can be sites for heterogeneous nucleation [9.2]. Short diffusion paths, elevated temperatures, and gases in the supercritical state are necessary to achieve the high diffusion rate and high gas concentration needed for commercial use. Extrusion with gas injection is an efficient process to saturate polymer with gas. Manufacturing issues are determining sequence of pressure, temperature, and shaping geometries to nucleate and form cells without disruption and to shape product without distortion. Microcellular technology is covered by several patents and is offered for licensing by Axiomatics, Woburn, MA Advantages/Disadvantages

Applications

Materials

The extremely small bubbles give weight reductions of 10-90%, no reduction in specific mechanical properties, appearance of a solid opaque surface, and foaming of thin sections. Fatigue resistance is observed to increase. Environmental advantages are use of atmospheric gases and lower material use

Siding, Pipes, Aircraft Parts, Athletic Equipment, Machine Housings, Automotive Components, Food Containers, Artificial Paper, Thermal Insulation, Fibers for Apparel and Carpets

ABS, PE, PET, PMMAPS, PS(HI), PP, PUPVC, SMC, Fluoropolymers, Poly(methylpentene)

Table A9. Lamellar injection molding [9.1] In Lamellar Injection Molding (LIM), two (or three) materials are extruded separately and combined with a 3 (or 5)-layer feedblock with multipliers to form a melt stream with hundreds of layers. This stream is injection molded to from parts with an irregular lamination pattern. The third material may be an adhesive. The layer structure, as assessed by oxygen permeability, shows (1) undesirable high permeability when too few layers allow easy passage around barrier layers, (2) low permeability (300-fold reduction) at 60-600 layers equal to theoretical minimum for lamellar structure, and (3) increased permeability as extremely thin laminates break up to from discontinuous domains (blends). LIM technology is offered for licensing by the Dow Chemical Company

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Table A9. Lamellar injection molding [9.1]


Applications

Materials

Only machine modifications needed are addition of feedblock and multipliers. LIM does not require multiple channels or sequenced valving used in coinjection molding and can easily be applied to complex parts or multicavity molds. Parts can be molded with high barrier properties to gases and hydrocarbons at lower costs than monolayer materials. Scrap can be recycled by incorporation into the major component or by conventional methods since LIM materials are compatible. Optical clarity (reduced haze) is improved compared to blends because the ordered LIM morphology reduces light scattering. LIM structure, with sheetlike continuous component selected for specific properties (controlled thermal expansion, increased load bearing, and temperature resistance), offers distinct property enhancements compared to blends.

Structural Parts (dimensional stability, temperature/ chemical resistance) Housewares/Durables (clarity, temperature and solvent resistance) Containers for Food and Chemicals (gas. hydrocarbon barriers) Automotive Reservoirs (fluid/heat resistance).

PC/PET, PC/ PBT, PO/ad/ EVOH, PET/ PEN, PO/ad/ PA, PS/PA6, PC/TPU, TP/ T-LCP, filled/ unfilled, brittle/ductile, virgin/recycle

Table A10. Multimaterial multiprocess (MMP) technology [10.1] The use of multiple materials and processes is the overarching technology in achieving one-hot manufacturing for large and/or complex parts. Material thermal expansion differences can be dealt with by flexible joints, (adhesives), process sequence to minimize distortion, sliding at interfaces (incompatible materials), and design for minimum distortion. Common multimaterial multiprocess technologies include Injection Compression Molding in which resin is injected into a partially open mold which closes, requiring less clamp force and producing less residual stress in the part. Multimaterial Molding in which a material is shaped, the mold is altered, and a second (or subsequent) material is shaped. Shaping processes are combinations of injection and compression molding and stamping. In-Mold Coating in which a thin thermoset coating is injected onto an injection or compression molded part in a closed mold. ‘Mono-Sandwich’ Coinjection Injection Molding in which a small extruder, operated intermittently, pumps a skin layer into the front of the main injection unit for subsequent coinjection. The Alpha 1 machine at GE Plastics, with two injection units, a long stroke vertical press, and shuttle table, allows combinations of compression molding, (gas-assisted) injection molding, and stamping. Other MMP technologies are described in tables on Low Pressure Molding, Advanced Blow Molding, and Lamellar Molding.


13

Table A10. Multimaterial multiprocess (MMP) technology [10.1]


Applications

Materials

The advantages of more than one material and/or process include design flexibility, tailored performance, effective material use, lower labor costs, improved quality through automation, reduced secondary operations, less auxiliary equipment, and more recycle use. Multiple materials allow advantageous combinations such as multiple colors (automotive lens), flexible/rigid (conduits with connectors), and consolidated/strong (plastic/metal composite) and cost /barrier or strength (laminate structure).

Telephone booth molded from 132 lbs of structural foam on a 2500-ton press with three injection units Air vent with molded movable louvers made from incompatible materials. Automotive bumper with injection molded fascia over a stamped beam. Multicolor automotive taillights.

Combinations of thermoplastic, thermoset, and reinforcements subject to constraints of product performance, limitations on distortion, and interface requirements (adherent or incompatible)

TABLE ABBREVIATIONS ad ABS EVOH PA PBT PC PE PEN PET PMMA PO PPS PP PS PSU PPO PPE SAN SMC T-LCP TPU TPE

adhesive Acrylonitrile-butadiene-styrene copolymer Poly(ethylene-co-vinyl alcohol) Polyamide Poly(butylene terephthalate) Polycarbonate Polyethylene Poly(ethylene 2,6-nathalenedicarboxylate) Poly(ethylene terephthalate) Poly(methylmethacrylate) Polyolefin Poly(phenylene sulfide) Polypropylene Polystyrene Polysulfone Polyphenyleneoxide Poly(phenylene ether) Poly(styrene-co-acrylonitrile) Sheet Molding Compound Thermotropic Liquid Crystal Polymer Thermoplastic Polyurethane Thermoplastic Elastomer

TABLE REFERENCES 1.1 2.1 3.1 4.1 4.2

Eckardt, H., “Gas-Assisted Injection Molding,” Ref. [1]. Turng, L.S. “Computer-Aided Engineering for the Gas-Assisted Injection Molding Process,” Ref [1] Hauck, C. , Schneiders, A., “Injection Molding with Fusible Core Technology,” Ref. [1]. Hettinga, S., “Controlled Low Pressure Injection Molding,” Ref[1]. Turng, L.S., Chiang, H., Stevenson, J.F., Plast. Eng., p.33, Oct. 1995.

14

5.1 5.2 6.1 7.1 8.1 8.2 9.1 10.1


Sugiura, S., “Developments in Advanced Blow Molding,” Ref. [1]. Myers, J., Mod. Plast., p.64, June 1995. Bigg, D.M., “Manufacturing and Formation of Thermoplastic Sheet Composites,” Ref. [1]. Castro, J.M., “Reactive Liquid Composite Molding,” Ref. [1]. Suh, N.P., “Microcellular Plastic,” Ref. [1]. Campbell, G.A. and Rasussen, D.H., U.S. Patents 5,369135, 5,358,675 (1994). Barger, M.A., Schrenk, W.J., “Lamellar Injection Molding Process for Multiphase Polymer Systems,” Ref. [1]. Avery, J.A., “Multimaterial Multiprocess Technology,” Ref. [1].

Preface

Over the next few decades, the development of our society will be characterized by two particular factors: 1. the increasing importance of the medical sector, coupled with growing requirements on medical engineering products. 2. a continuing key role for mobility within society, with growing emphasis on finding a solution to the associated ecological problems. Plastics will play a crucial role in both cases. With properties ranges that can be widely adjusted and ease of processing, plastics can be used to produce highly-integrated, customized product solutions for medical-engineering applications, as well as products for the automotive sector and telecommunications. The plastics sector is far from having exhausted the innovation potential that exists. What is required are material, process engineering, and mechanical engineering-based approaches to innovation which will make it possible to respond to ever more demanding applications or the substitution of other materials by plastics. The development trends currently emerging in plastics engineering are essentially the combination of different processes (e.g., injection molding and surface refinement), the combination of different materials (e.g., plastic/metal composites), the integration of a wide range of functions within a single component, the improvement of surface and/or optical properties, and also reduced material consumption and the recyclability of the materials employed. Much of the innovation potential lies in the injection molding process. Special injection molding processes such as the thin-wall technique, micro-molding, hybrid processes and multi-component processes, are playing an increasingly large role in the processing of plastics. At the same time, the requirements placed on product quality, production quality and also part precision are rising, while costs are expected to be reduced wherever possible. This combination of objectives can only be achieved by an appropriate increase in productivity from process-engineering innovations and a reduction in the number of process steps. From the microeconomic angle, the challenge facing a plastic-processing company today is one of retaining its market position through readiness and ability to innovate. This applies particularly to the opening up of new application potential and the application technology that this requires, as well as to the crucial aspect of employee skills. Specialist know-how is essential in order to survive on the market – in other words, injection molding

vi

Preface

companies are called upon to acquire know-how on new materials and material combinations as well as on process variants and special injection molding processes. It is at this point that the book commences. It constitutes a collection of experimental and theoretical studies in the field of special injection molding processes. The papers that have been brought together in this book were presented at International SPE (Society of Plastic Engineers) Conferences over the period 1996 to 2000. The presenters of the papers are scientists and representatives from eminent institutes and companies who have made a key contribution towards the continuing development of injection molding technology through their work. The manuscripts printed in the book represent an extract of their current work and research results. This book is not therefore intended as a textbook but rather as specialist reading, progressing far beyond the basic principles of application technology in injection molding. A broad range of specialized subjects is covered, including process engineering, material science, mechanical engineering, and mold engineering. The topics treated are divided into the following subject areas, which follow on from an initial overview of the special injection molding processes that are currently in use: Gas Assisted Injection Molding (GAIM) • influence of process parameters • molded part design and mold layout • the special processes of powder injection and reaction injection molding Thin Wall Molding • influences on molded part quality • avoidance of errors during application • influence on cycle time and costs Molding of Micro Parts and Micro Structures • reproduction of high-precision surface structures • optical applications • simulation techniques and mechanical engineering Improving Material Properties • modified plastics for automotive applications • improved molded part properties through vibrational molding Molding of Composites • compression molding • vacuum assisted liquid molding • resin transfer molding • inmold decoration

Preface

vii

Mold Making and Plasticisation automation technology mold engineering for special injection molding processes twin-screw injection molding screw optimization This book presents the state of the art in all the areas presented. The authors of the individual contributions give their views on the uses and limitations of new injection molding technologies. They thus offer the know-how that is required to exploit the innovation potential of plastics engineering. • • • •

Helmut Potente and Hans-Peter Heim Paderborn, June 2001

Table of Contents

Preface Helmut Potente and Hans-Peter Heim One-shot Manufacturing: What is Possible with New Molding Technologies James F. Stevenson

v 1

Chapter 1: Gas Assisted Injection Molding

15

Gas Assist Injection Molding. The North American Legacy Jack Avery Flow Directions in the Gas Assisted Injection Molding Technology Young Soo Soh and Chan Hong Chung Gas-assisted Injection Molding: Influence of Processing Conditions and Material Properties Kurt W Koelling and Ronald C Kaminski Cover Part as an Application Example for Gas-assisted Injection Molded Parts Michael Hansen Molded Part Design for the Gas Injection Technique H. Potente and H.-P. Heim Design Optimization of Gas Channels for an Air Cleaner Assembly Using CAE Simulations D.M. Gao, A. Garcia-Rejon, G. Salloum and D. Baylis The Occurrence of Fiber Exposure in Gas Assist Injection Molded Nylon Composites Shih-Jung Liu and Jer-Haur Chang Saving Costs and Time by Means of Gas-assisted Powder Injection Molding Christian Hopmann, Walter Michaeli Gas-assisted Reaction Injection Molding (GRIM): Application of the Gas Injection Technology to the Manufacturing of Hollow Polyurethane Parts I. Kleba, E. Haberstroh

15 27

35 43 51

57

65 73

79

Chapter 2: Thin Wall Molding

89

Thin Wall Processing of Engineering Resins: Issues and Answers Larry Cosma

89

ii

Table of Contents

Effects of Processing Conditions and Material Models on the Injection Pressure and Flow Length in Thinwall Parts A. J. Poslinski 10 Common Pitfalls in Thin-Wall Plastic Part Design Timothy A. Palmer Flow Instabilities in Thin-wall Injection Molding of Thermoplastic Polyurethane Christian D. Smialek, Christopher L. Simpson Pressure Loss in Thin Wall Moldings John W. Bozzelli, Jim Cardinal, and Bill Fierens Integrating Thin Wall Molder’s Needs into Polymer Manufacturing W. G. Todd, H. K. Williams, D. L. Wise Thinning Injection Molded Computer Walls Lee Hornberger and Ken Lown

Chapter 3: Molding Micro Parts and Micro Structures

99 107 113 121 127 133

143

Transcription of Small Surface Structures in Injection Molding – an Experimental Study 143 Uffe R. Arlø, Erik M. Kjær Injection Molding of Sub- µ m Grating Optical Elements 149 R. Wimberger-Friedl Process Analysis and Injection Molding of Microstructures 157 Alrun Spennemann and Walter Michaeli Simualtion of the Micro Injection Molding Process 163 Oliver Kemmann, Lutz Weber, Cécile Jeggy, Olivier Magotte, and François Dupret

Chapter 4: Manufacturing of Composites Melt Compression Molding (MCM) a One-shot Process for In-mold Lamination and Compression Molding by Melt Strip Deposition Georg H. Kuhlmann In-mold Lamination Back Compression Molding Thomas Huber Analysis and Characterization of Flow Channels during Manufacturing of Composites by Resin Transfer Molding R. V. Mohan, K. K. Tamma, S. Bickerton, S. G. Advani and D. R. Shires Optimization of Channel Design in VARTM Processing Roopesh Mathur, Suresh G. Advani and Bruce K. Fink

171 171 187

193 209

Table of Contents

Injection Compression Molding. A Low Pressure Process for Manufacturing Textile-Covered Mouldings Carsten Brockmann, Walter Michaeli Kurz-Hastings Inmold Decoration Roy Bomberger

Chapter 5: Improving Material Properties High Impact Strength Reinforced Polyester Engineering Resins for Automotive Applications Mengshi Lu, Kevin Manning, Suzanne Nelsen, and Steve Leyrer Control of Internal Stresses in Injection Molded Parts Through the Use of Vibrational Molding, “RHEOMOLDINGSM”, Technology Akihisa Kikuchi, Marc Galop, Harold L. Brown, and Alexander Bubel Experimental Determination of Optimized Vibration-assisted Injection Molding Processing Parameters for Atactic Polystyrene Alan M. Tom, Akihisa Kikuchi, and John P. Coulter Vibrated Gas Assist Molding: Its Benefits in Injection Molding J.P. Ibar

Chapter 6: Mold Making and Plasticisation

iii

215 223

229 229

237

245 253

259

Advances in Stack Molding Technology 259 Vincent Travaglini and Henry Rozema Advanced Valve Gate Technology for Use in Specialty Injection Molding 267 John Blundy, David Reitan, and Jack Steele In-mold Labeling for High Speed, Thin Wall Injection Molding 273 Gary Fong Advances in Fusible Core Technique 281 E. Schmachtenberg and O. Schröder Processing Glass-filled Polyethylene on a Twin-screw Injection Molding Extruder 287 David Bigio, Rajath Mudalamane, Yue Huang and Saeid Zerafati Injection Molding by Direct Compounding 295 Bernd Klotz Improvement of the Molded Part Quality: Optimization of the Plastification Unit 301 S. Boelinger and W. Michaeli

iv

Table of Contents

Non-return Valve with Distributive and Dispersive Mixing Capability Chris Rauwendaal

307

Index

313

Chapter 1: Gas Assisted Injection Molding Gas Assist Injection Molding The North American Legacy

Jack Avery GE Plastics, USA Let's look at some of the specifics. What impact will the expiration of the original Frederich patent have? As you would expect, there are a variety of opinions. After having discussed this with a variety of people, it is my opinion that it will have little actual impact on the development of gas-assist injection molding. Why? Most applications use in-runner or in-article options. These variations provide the most flexibility to introduce gas into the part and consequently the most flexibility to optimize both the design of the component and the utilization of the process. Design will dictate the best solution for gas injection. But there will always be some applications where through the nozzle technology is the best choice. Typically these are handles, symmetrical components, and similar type applications. Typical applications of gas-assist injection molding include: • Business machine chassis/ housings • Material handling pallets • Furniture: chairs, tables • Handles • Automobile bumpers • Automobile trim • Television housings • Golf club shafts Not only has progress been made in licensing and utilization of gas-assist injection molding technology, advancements in computer simulation, design and variations in the technology continue.

GAS ASSIST INJECTION MOLDING TECHNOLOGIES Several variations of gas-assisted injection exist. Most are patented and require licensing to practice.

16


Commercial Technologies License Required Cinpres Yes GAIN Yes Johnson Controls No Airmould (Battenfeld) No Helga (Hettinga Industries) Yes Another trend is that machine manufacturers are entering into licensing agreements with Cinpres or GAIN or both. This enables them to integrate the control system for gas-assist injection into their machines as an option. Companies having such agreements include: • Cincinnati Milacron Cinpres • Engle Machinery - Worldwide GAIN • HPM Industries GAIN • Husky Cinpres Gas assist-injection molding is a global process. One of the drivers is that global OEM's recognize its value and are beginning to apply the technology on a broad base. Examples are: • Samsung • Mitsubishi Consumer Electronics • Ford • Xerox Another significant factor in the globalization of gas-assist injection molding is that the technology suppliers have a global presence. Battenfeld, Cinpres and GAIN all have relationships that enable them to serve the worldwide market, either directly or on a primary licenser basis. In addition, Cinpres is opening an office in Singapore to serve Asia (Table 1). Table 1. N. America

Europe

Japan

Airmould

Battenfeld

Battenfeld

Tsukishima

Cinpres

Cinpres Ltd.

Cinpres Ltd.

Mitsubishi Gas Chemical

GAIN

GAIN

GAIN

Asahi

Helga

Hettinga

Hettinga

Toray

Johnson Controls

JCI

JCI

JCI

Gas Assist Injection Molding

17

LICENSING GAS-ASSIST INJECTION TECHNOLOGIES A significant difference between the use of gas-assist injection technology in Europe and the rest of the world is the need for licensing. Two companies, Cinpres and GAIN hold a variety of patents on this technology. To use any of these patented technologies anywhere in the world, and to ship a product manufactured using gas-assist technology anywhere in the world, a license is required. Three licensing options are available: • Patent only license • Development license • Full technology license A patent only license provides protection to use technology patented by the licenser. An example of a patent only license is Engle who requires that a GAIN Technologies patent only license be taken prior to or in conjunction with purchase of Engle equipment. Machine manufacturers with these agreements include: Cinpres GAIN Engle X X Husky X Klockner Ferromatik Desma X Mannesman Demag X A development license can be taken for the development phase of an application. Under this arrangement, the technology is in your facility and access to support from the technology supplier is available. In some cases, this license is used to evaluate the technology for a specific application. The development license must be converted to a full technology or full manufacturing license prior to going into production. Each technology supplier has variations on the type of license and fees. A summary of the license fees and "hardware" cost associated with using gas assist technology in production is in Table 2. It is necessary to review each individual case with the technology licenser to determine actual costs and conditions.

18


Table 2. Gas-assist injection molding licensing information(1) Manufacturing license fee

"Gas injection" equipment

Additional costs "royalties"

Airmold (Battenfeld)(6)

None

Single Machine, Single Injection Point, Base Price $110 000, Expandable(7)

None

Cinpres(5)

$60,000

Single $35,000, Multiple $58 - 95,000

Based on: Material Usage or Tooling Fee or Flat Fee for Parts

Epcon

-

Single $55,000 Multiple $77,500

None

GAIN

Per mold $1.5 -15,000/yr per facility $25 - 250,000/yr(2)

Single $25 - 50,000 Multiple $35 - 85,000

None

HELGA (Hettinga)(3)

None

HELGA Package $70 - 75,000

None

Johnson Controls Multinozzle/ Sequential Gas Assist(4)

None

Integrated into machine controls $30 -50,000

None

Nitrojection

$25,000

$45 - 85,000

None

(1) (2) (3) (4) (5) (6) (7)

Licensing fees and details vary depending upon each application and supplier. It is essential to obtain from each supplier relative to a specific application Per mold license is also available on a lifetime basis: $12.5 - 75,000 "HELGA" Package includes required equipment and rights to practice Available only as an option on new or as a retrofit on existing Johnson Controls' machines Patents pending Airmold process per Airmold specification, no license required Includes pressure generator which can be used with additional machines

APPLICATION DEVELOPMENT What is different in the application development process? The most important step is to determine if the application is appropriate for the gas-assist injection process.


19

How is this accomplished? The first step is to complete a thorough assessment of the performance requirements. Then the material, process and design of the component can be determined. Factors which make gas assist injection the process of choice include: • High stiffness to weight ratio is required • Part design allows for hollow rib geometry • Tight tolerances are required • A hole in the part surface can be tolerated, or the hole can be sealed • Improved surface is desirable vs. structural foam molded parts Once the performance requirements have been determined and gas assist injection has been selected as the appropriate process, part and tool design proceed. It is important to take into consideration details which are different from standard injection molding. They are: PART DESIGN CONSIDERATIONS • • •

Sizing of gas channels Gas channel layout Location of gas injection point(s) TOOL DESIGN CONSIDERATIONS

• •

Gate size for the through-the-nozzle or in runner gas injection Gas nozzle location - for in-runner and in-article gas injection, material must cover the gas nozzle prior to gas introduction • Location of gas nozzles in the tool to prevent interference with cooling lines, slides, ejector systems, etc. Where can you obtain assistance? Four primary sources of assistance exist: 1) Gas-assist injection technology supplier (Cinpres, GAIN, Battenfeld, etc.) 2) Material supplies may provide assistance if the application is a fit for their materials 3) For OEM's the third source may be a molder who has experience with gas assist technology 4) Consultants who specialize in gas-assist technology (such as Caropreso Associates) or firms that can provide modeling assistance, i.e. Plastics and Computer As a general comment, licensees find that even though assistance is available, the final test is to learn through the experience of putting parts into production. A useful recommendation of many molders is to do a prototype tool for the first few applications. This will provide you with some leeway for changes prior to the production tool. Also, in many cases, the prototype tool can be used for process validation, preproduction or initial parts while the production tool is being completed. Another option is to prototype a section of the part, i.e.

20


one-quarter. This could provide information critical to design and construction of the mold but would not provide parts for evaluation. Each application needs must be considered.

ADDED COST USING GAS-ASSIST TECHNOLOGY One factor often overlooked in the development of a program using gas-assist injection molding technology is the added cost involved. In addition to the licensing fees and royalties, equipment costs plus the nitrogen used in the process must be taken into account. Also, the cost of the mold may be higher than for standard injection molding since, except for through the nozzle technology, gasinjection nozzles must be integrated into the mold. These added costs must be recovered. Some factors which may contribute to recovering costs are: • Parts consolidation resulting in fewer molds, less machine utilization and reduced or elimination of assembly • Use of lower tonnage machines • Improved part quality • Reduced cycle time • Less scrap • Lower weight (lighter and less material) Delphi Interior and Lighting Systems had significant challenges to overcome to meet GM requirements for door systems for new vehicles. Some of the requirements were: • Reduced systems cost • Systems mass reduction • Reduced assembly time • Improved system quality Gas-assisted injection molding was selected as the process technology to meet these demanding requirements due to the benefits it offered: • High strength-to-weight ratio • Low molded-in stress provides excellent dimensional stability • Parts consolidation opportunities • Weight reduction • Design flexibility • Reduced part cost When Delphi initiated the program in the early 1990's, gas-assist injection molding had not demonstrated the capability to deliver all of these benefits in a production environment. What did Delphi do to reduce the risk of employing a new technology in this high visibility program? They brought in a gas-assist technology supplier and a material supplier who was actively developing gas-assist design and process technology for their materials.


21

A four year collaboration produced the following results: Parts consolidation: 61 parts to 1 part Assembly time reduction from 330 sec to about 60 sec Lighter weight (up to 1.5 Kg/vehicle) Reduced tooling requirements and investment costs Improved material handling Better assembly ergonomics Improved quality (fewer squeaks and rattles) Reduced operational noise Improved corrosion resistance Best-in-class serviceability Up to 10% piece price savings This sounds like a successful program. The most important lesson is that the key participants were involved from the inception and worked through the development program together. • • • • • • • • • • •

TECHNOLOGY DEVELOPMENTS Technology developments continue. Design guidelines have been developed and published. The lead has been taken by material suppliers who have developed this information for use with their customers in developing applications that employ gas-assist injection molding technology. In addition, the Structural Plastics Division of the Society of the Plastics Industry has a compilation of papers relating to gas-assist injection molding technology which have been presented at their annual conferences. Process technology development continues. Mitsubishi Gas Chemical has developed a variation of the Cinpres process - the "full shot" process. Instead of short shooting the mold and packing it out with gas, in the "full shot" process, the mold is filled with polymer and the gas is used only for packing. Nozzle design is one of the critical areas of gas-assist injection molding. For this to be a commercially viable process, injection of the gas through the nozzle must be as trouble free as injecting the material. This is an area of ongoing development by a wide variety of suppliers. Cinpres has introduced a new "directional" nozzle design that ensures the flow of gas into the mold in the same direction as resin flow. A nozzle with a 90 degree tip whereby the gas exits at right angles, ensures the flow of nitrogen into the mold in the same direction as resin flow. This configuration is claimed to prevent blemishes on the surface of the part that appear opposite the nozzle.

22


Xaloy Inc. has introduced an upgraded nozzle that is claimed to increase durability, and reduce cost. This redesigned nozzle uses a hydraulically or pneumatically actuated needle to shut off melt flow when the hold pressure is released at the end of the injection cycle. This prevents gas in the sprue and runner system from flowing back into the nozzle. Nitrogen source and recovery play a critical role in the cost and quality of components produced using gas-assist injection technology. Studies underway indicate that nitrogen purity is critical, especially when using engineering thermoplastic resins. Impurities (i.e., oxygen) can result in oxidation and burning in the mold. Several on-site nitrogen separation systems are available for prices ranging from $10,000 - $70,000 depending on volume required. Gas recovery is a topic of intense discussion, as it offers cost reduction opportunities. Recovery rates of 70-90% are attainable with some resins. However, volatiles can be picked up by the gas flowing through the molten materials resulting in contamination or fouling of the recovery system and/or clogging of gas injection needles. Much more work needs to be done in this area. Design, analysis and material optimization are also critical elements. The full potential of gas-assist technology will not be realized without continuing developments in these areas. Dow, DuPont, GE Plastics all are working to optimize design for their materials. A.C. Technology, Plastics and Computers Inc. and Moldflow provide specialized software for design optimization and process simulation. Work continues as new releases of software incorporating more sophisticated approaches to the gas-assist technology continue to be commercialized. In addition, gas-assist injection technology development continues. Cinpres and GAIN continue their developmental work, and variations of the basic technology continue to be introduced. Another approach was introduced in late 1995 by EPCON Gas Systems Inc. This approach is based on three key elements: • Shot control • Simplified gas pins • Pressure control EPCON has applied for patents on their approach to shot control. It is based upon "strategically" located thermocouples at points in the mold where the material flow front needs to stop before gas pressure completes the mold filling. The thermocouple simultaneously signals the gas injection unit to begin the gas flow and the molding machine to stop its hydraulic pumps which end the injection forward sequence. Their approach to gas pins is similar to the concept used for mold vents - the holes are small enough so that the material skins cover but allows the gas to pass through. According


23

to EPCON, "Microscopic holes have been incorporated to allow enough volume of gas to pass through." To date, I am not aware of any EPCON systems in commercial production. It remains to be seen how well this technology performs. Their approach to gas pins is similar to the concept used for mold vents - the holes are small enough so that the material skins cover but allows the gas to pass through. According to EPCON, "Microscopic holes have been incorporated to allow enough volume of gas to pass through."

EQUIPMENT Variations on gas-assist injection molding continue to evolve. The latest is Battenfeld's Monomodule. This unit is for use when gas is to be injected at a single point. Then only one pressure regulator is required. The handheld unit provides the capability to be programmed directly. The data is stored in the Monomodule.

SUMMARY We all know that gas-assist injection molding technology languished for several years in North America due to litigation threats. How was this "fear" overcome? As with any industry, there are leaders and there are followers. The leaders were convinced that the upside for productivity and design flexibility were worth the risk. They stepped forward and developed applications using gas-assist injection molding technology. The initial applications took longer to develop than standard injection molding or with structural foam. But once they have developed the technology, they have an advantage. This "confidence factor" is a competitive advantage that can be exploited and translated into a bottom-line advantage over the followers. My opinion as to why gas-injection molding technology is developing so rapidly in North America, has been the collaboration of the members of the supply chain involved in developing an application. In Europe, the machine suppliers have been the primary drivers of gas-assist technology. Even though they had an earlier start, I feel that North America is moving faster and has applied the technology to more challenging applications. How have we been able to do this? The OEM's and material suppliers recognized the potential this technology offered and joined forces with technology suppliers to apply it to applications that offered a high return. The GM-Delphi Super Plug application is an excellent example of this. For this process technology to become mainstream, progress must be made in several areas.

24


1) The "knowledge gap" must be overcome. It must be able to be done right the first time. The development cycle for components molded using gas-assist technology must be no longer than for a standard injection or structural foam molded component. 2) Recognition that gas-assist injection is not a "fix-it" for existing problems. As with any process, to optimize value, the product must be optimally designed for a material and process. 3) Additional costs of using gas-assist injection must be understood and minimized. The value of components molded using this technology must be recovered. 4) Equipment consistency and reliability issues (gas injection control and nozzle) must be eliminated. 5) "Fear of litigation" must be eliminated! Commercialization of gas-assist injection technology can take several paths. Three of which might be: 1) Technology becomes widely understood and low cost to implement. Most converters become "experts" and utilize. 2) A group of converters specialize in gas-assist injection molding. Their expertise enables them to develop components quickly and cost effectively, positioning them as the suppliers of choice for gas-assisted injection molded components. 3) Captive converters become the largest users of gas-assist injection. They have the large volume applications which enable them to amortize the added cost. What is needed for this technology to prosper? Higher levels of collaboration. The OEM must involve the processor, material supplier and tool maker at the program inception. This will ensure that all required input is provided early and that costly changes and/or compromises are eliminated or minimized. This will also develop an increased sense of ownership by the members of the supply chain. Another area of need is increased education of the design community. For proper application of gas-assist injection molding, the designer must understand the process, its strengths and weaknesses in order to minimize design flaws which result in process, performance and economic issues. Will the rapid growth continue? Not unless the technology becomes widely understood and is demonstrated to provide cost effective solutions. Designers must become confident not only in designing components, but also that mold makers and converters have the capability to implement the technology.

SOURCES Battenfeld of America, Inc., 31 James P., Murphy Industrial Highway, W. Warwick, R.I. 02893, Phone (401) 823-0700, Fax (401) 823-5641


25

Cinpres Limited, Waterworks Plaza, Building, 3135 South State Street, Suite 108, Ann Arbor, MI 48108, Phone (313) 663-7700, Fax (313) 663-7615 Epcon Gas Systems, Rochester Hills, MI, Phone (810) 651-9661, Fax (812) 650-8293 Gain Technologies, Inc. 6400 Sterling Drive, North, Sterling Heights, MI 48312, Phone (810) 826-8900, Fax (8I0) 826-8906 Hettinga Technologies, Inc., 2123 N.W., 11th Street, Des Moines, IA 50325, Phone (515) 270-6900, Fax (515) 270-1333 Johnson Controls, Inc., 10501 Highway, M-52, Manchester, MI 48158, Phone (313) 428-8371, Fax (313) 428-0143 Kontor- JPI Technologies Inc., 35 Gibson, Lake Dr., Box 220, Palgrove, ON, LON 1PO, Canada Phone (905) 880-2600, Fax (905) 880-2599 AC Technology/C-Mold, 11492 Bluegrass, Parkway, Suite 100, Louisville, Ky 40299, Phone (502) 266-6727, Fax (502) 266-6654 Moldflow Pty, Limited, 4341 S. Westnedge, Suite 2208, Kalamazoo, MI 49008, Phone (616) 345-4812, Fax (616) 345-4816 Plastics & Computers, 14001 Dallas, Parkway, Suite 1200, Dallas, TX 75240, Phone (214) 934-6705, Fax (214) 934-6755

Flow Directions in the Gas Assisted Injection Molding Technology

Young Soo Soh and Chan Hong Chung Department of Chemical Engineering, Kyungbook, 712-714 Korea

INTRODUCTION Gas assisted injection molding process produces parts with many advantages including Class A finish with no sink marks, reduced cycle time, lower injection pressure, lower clamping tonnage, reduced part warpage, greater design freedom, and more. In the gas assisted process, an inert gas is injected into the center of the flow of plastic. A combination of the high surface tension of the plastic and the lower viscosity of the hotter molten material in the center of thicker sections, such as ribs and bosses, confines the gas to form hollow area in the thicker sections of the parts. Most gas and assisted injection molded parts may be categorized into two types: The parts consist merely of single thick section through which the gas penetrates, and the parts consists of a nominal thin wall with gas channels traversing the parts. The latter are more difficult to design and process because the gas may not just flow through the channel but penetrate into the nominal thin walled. These parts are expected to be designed such that the gas cores out all the channels without penetrating into the thin walls. To design molds such that the gas cores out all the thick sections and not the thin walls, one needs to predict and understand the preferred direction of gas in the process. In this paper, we use a method to relate the preferred gas direction with the process variables. The method requires a knowledge on the relations between resistance for the gas flow and processes variables such as resin flow length, cross section area of cavity, melt temperature, and existence of short shot. A simulation package was used to confirm the method. Commercial packages simulate the flow of gas. At a mold design stage, the commercial package plays a very important role to prevent blow through or fingering phenomena by simulating the gas flow. At the pilot production or first mold trial stage, the package also plays an important role to make perfect parts. If, however, the packages are not available at the molding shop or instant solution to prevent blow-through or fingering is necessary at the mold trial stage, the equations in this paper are very useful to treat the troubles. When a trouble shooting engineer modify the virgin mold without a flow analysis package, initially

28


he needs qualitative information described in the theory, not quantitative. After qualitative questions are answered, the quantitative solution comes from trial and error. For example, if the theory tells that the channel size be enlarged, the shop mold maintenance technician will enlarge the channel diameter a little and try the molding cycle, which then will be followed by another small channel enlargement, if necessary, until satisfactory parts come out of the mold.

THEORY Although the process is unsteady state, steady state flow equations may be used to explain the gas flow directions. The equation for the steady state flow of a Newtonian fluid between infinite parallel flat plates is given by1 ----------------∆p = 12µVL 2 a

[1]

where L a

length of plate in direction of flow distance between plates ∆p pressure drop across the distance average velocity V neglecting end effects. The equation for the steady state flow of pseudo plastic liquids between infinite parallel flat plates is given by Q ( 3n + 1 ) n m  ∆p ------- =  -------------------------  -----------------   ( 3n + 1 ) πn 2L a

[2]

where m, n power law indices Q flow rate The steady state flow of a Newtonian liquid through conduit with diameter D is given by1 ----------------∆p = 32µVL 2 D

[3]

The steady state flow of pseudo plastic liquids through conduit with radius R is given by Q ( 3n + 1 ) n m  ∆p ------- =  -------------------------  ------------------   ( 3n + 1 ) πn 2L R

[4]

When the direction of gas path is discussed for the gas assisted injection molding, the term "the direction of least resistance" is commonly used. When more than two paths are

Flow Directions

29

competing for the direction of gas, the gas prefers the direction of less resistance. Very often, this resistance is explained by temperature, length, or distance between plates. The degree of resistance is proportional to the pressure drop requirement in equation [1] through [4]. Using these equations, the preferred gas directions can be predicted. Equations [1] and [3] are easier than equations [2] [4] to use, and equations [1] and [3] are as correct as equations [2] and [4] to answer qualitative questions such as "which one is the least resistance path?" Theoretically, there exists pressure drop both along the gas phase and along the polymer melt phase. However viscosity of gas is less than 0.1% of apparent viscosity of polymer resin, and the pressure drop along the gas phase can be considered negligible. Thus the pressure of the gas may be considered the same for all regions of gas not only in the stage of stationary packing stage, which comes from fluid statics theory, and but also in the phase of first dynamic stage. Hence, only the pressure drop of the resin is necessary for the discussion of the degree of the resistance. The resistance increases with the increase of viscosity, path length between gas front and melt front, velocity of the melt, and decreases with the increase of path cross section area. The path cross section area here excludes the frozen layer of mold cavity

RESULTS AND DISCUSSIONS Consider that a polymer melt was injected in the middle of a 8 mm diameter pipe to a total melt length of 60 mm, followed by gas injection at the point 0.5 mm left of the melt injection point, and we now try to pick the preferred direction of gas. With the equation 1, one can calculate that the resistance to the left hand side for the gas to move with velocity, V, is proportional to 32µVL ( ∆p ) L = -------------------L2 D

where ( ∆ p)L is pressure drop requirement to the left hand side direction and LL is distance between gas injection point and left melt front. The resistance to the right hand side for the gas to move with velocity, V, is proportional to 32µVL ( ∆p ) R = -------------------R2 D

One only needs to compare LL with LR as all the remaining variables are the same. LL is less than LR and ( ∆ p)L is less than ( ∆ p)R. Thus the resistance of the flow to the left hand side direction is smaller, and the left hand side direction is the preferred direction for gas flow.

30


In Figure 1, a Mold Flow simulation results are shown, which is consistent with the method given here. GAS INJECTION Consider another case where a pipe with 8 mm diameter is connected to a pipe with 4 mm diameter. The melt length from the center is the polymer polymer same for both directions. The gas is injected at the center and we now try to pick the preferred Figure 1. Gas injection - Case I. direction of gas. With the equation 1, one can calculate that the resistance to the left hand side is proportional to POLYMER SHUT OFF

D = 8mm

D = 8mm

29.5mm

30.5mm

32µVL ( ∆p ) L = -------------------L2 DL

and the resistance to the right hand side is proportional to 32µVL ( ∆p ) R = -------------------R2 DR

Thus ( ∆p ) L = 32µV ( 30 ⁄ 16 )

and ( ∆p ) R = 32µV ( 30 ⁄ 64 )

( ∆ p)L is less than ( ∆ p)R. and the resistance of the flow to the right hand side direction is smaller, and the right hand side direction is preferred direction of gas flow. In Figure 2, Mold GAS INJECTION Flow simulation is shown for the case, which is consistent with the method given here. Consider the third case where a pipe with 8 polymer polymer mm diameter is connected to a pipe with 4 mm diameter, where gas is injected at the point Figure 2. Gas injection - Case II. where two pipes are connected. The melt length from the gas injection point is different at each side. We now try to pick the preferred direction of gas. With the equation 1, one can calculate that the resistance to the left hand side is proportional to POLYMER SHUT OFF

D = 4mm

30mm

D = 8mm

30mm

32µVL ( ∆p ) L = -------------------L2 DL

and the resistance to the right hand side is proportional to

Flow Directions

31

32µVL ( ∆p ) R = -------------------R2 DR

Thus ( ∆p ) L = 32µV ( 10 ⁄ 16 )

and ( ∆p ) R = 32µV ( 100 ⁄ 64 )

( ∆ p)L is smaller than ( ∆ p)R and the resistance of the flow to the right hand side direction is greater, and the left hand side direction is preferred direction of gas flow. In Figure 3, Mold GAS INJECTION Flow simulation results is shown for the case, which is consistent with the method given here. Consider case 4, where a pipe with diamepolymer ter 7 mm is connected to a cavity of 7 mm thickFigure 3. Gas injection - Case III. ness formed by two parallel plates, where gas is injected at the point where two cavities are connected. The melt length from the gas injection point is the same at each side. With the equation 1, one can calculate that the resistance to the left hand side is proportional to POLYMER SHUT OFF

D = 8mm

D = 4mm

10mm

100mm

32µVL ( ∆p ) L = -------------------L2 DL

and the resistance to the right hand side is proportional to 12µVL ( ∆p ) R = -------------------R2 a

Thus ( ∆p ) L = 32µV ( 20 ⁄ 49 )

POLYMER SHUT OFF

and ( ∆p ) R = 12µV ( 20 ⁄ 49 ) 20mm 20mm

GAS INJECTION

polymer

polymer

Figure 4. Gas injection - Case IV.

( ∆ p)L is greater than ( ∆ p)R and the resistance of the flow to the left hand side direction is greater, and the right hand side direction is preferred direction of gas flow. In Figure 4, Mold Flow simulation is shown for the case, which is consistent with the method given here.

32


Consider case 5, where a 7 mm diameter pipe is connected to a cavity of 7 mm thickness formed by two parallel plates, where gas is injected at the point where two cavities are connected. The melt length from the gas injection point is 40 mm at the plates and the same for the pipe. With the equation 1, one can calculate that the resistance to the left hand side is proportional to 32µVL ( ∆p ) L = -------------------L2 DL


Thus ( ∆p ) L = 32µV ( 20 ⁄ 49 )

and ( ∆p ) R = 12µV ( 40 ⁄ 49 )

( ∆ p)L is greater than ( ∆ p)R and the resistance of the flow to the left hand side direction is greater, and the right hand side direction is preferred direction of gas flow. In Figure 5, Mold Flow simulation is shown for the case, which is conGAS INJECTION sistent with the method given here. Consider case 6, where a 7 mm diameter pipe is connected to a cavity of 7 mm thickness polymer formed by two parallel plates, where gas is injected at the point where two cavities are conFigure 5. Gas injection - Case V. nected. The melt length from the gas injection point is longer at the plates with 60 mm. With the equation 1, one can calculate that the resistance to the left hand side is proportional to POLYMER SHUT OFF

D = 7mm

20mm

40mm

32µVL ( ∆p ) L = -------------------L2 DL


Thus ( ∆p ) L = 32µV ( 20 ⁄ 49 )

Flow Directions

33

and ( ∆p ) R = 32µV ( 60 ⁄ 49 )

( ∆ p)L is smaller than ( ∆ p)R and the resistance of the flow to the right hand side direction is greater, and the left hand side direction is the preferred direction of gas flow. In figure 6, Mold Flow simulation is shown for the case, which is consistent with the method given here.

REFERENCES 1

W. L. McCabe, J. C. Smith, and P. Harriot, Unit Operations of Chemical Engineering, 4 th Ed. McGraw-Hill, 1986.

Gas-assisted Injection Molding: Influence of Processing Conditions and Material Properties

Kurt W Koelling Dept. of Chemical Engineering, The Ohio State University, Columbus, Ohio 43210, USA Ronald C Kaminski The Geon Company, One Geon Center, Avon Lake, Ohio 44012, USA

INTRODUCTION In general, gas-assisted injection molding can be described by a simple three-step process.1 A short shot of molten polymer initially fills 75-98% of the mold cavity under the ram speed control of the injection molding machine. After a short delay period, compressed nitrogen gas cores out the molten polymer, filling the remainder of the mold. The third step, or the gas packing stage, occurs as a result of the volumetric shrinkage of the polymer melt. As the plastic solidifies, the gas expands into the volume created by shrinkage, locally packing out the part. In 1935, Fairbrother conducted the first experiments investigating this flow phenomena using a viscous Newtonian solution. He found that m, the fractional coverage or fraction of liquid deposited on the walls of the tube after bubble penetration, is a function of the capillary number, Ca, for capillary numbers up to 0.009.2 The fractional coverage, m, is defined for tube-shaped geometries as: m = Ap/ At = 1 - (rb/R) 2 [1] where Ap is the polymer cross-sectional area, At is the tube cross-sectional area, rb is the radius of the gas bubble, and R is the radius of the tube. The capillary number is defined as the product of the bubble velocity, Ub, and the viscosity of the fluid, η , divided by the fluid surface tension, Γ , or: Ca = η Ub / Γ [2] Taylor investigated this problem further in 1961 and ran experiments that extended to capillary numbers of two.3 Cox found that the fractional coverage reached an asymptotic value of m = 0.60 for capillary numbers greater than ten for viscous Newtonian fluids.4 With interest renewed in this problem because of gas injection molding, Poslinski and Stokes conducted similar isothermal experiments using silicone liquid pastes that behaved as Bingham fluids.5,6 Capillary numbers of up to 800 were obtained and showed good cor-

36


relation with previous work at low capillary numbers. However, the fractional coverage ultimately approached a value of m = 0.564 at high capillary numbers.5 An isothermal computer model was developed to simulate the gas-liquid dynamics to validate the experimental results. It was discovered that the hydrodynamic layer of molten polymer deposited by the passage of the gas bubble was much larger than the associated frozen layer developed at the mold-melt interface. Polymer solutions, including a viscous, Newtonian fluid, an elastic Boger fluid with Newtonian shear viscosities, and a shear-thinning polymer solution have been used in isothermal experiments to characterize the role of melt rheology in the gas-assist process.6 Fractional coverage data from the shear-thinning fluid compared well with the Newtonian data at very low capillary numbers, but dropped abruptly at a capillary number of two and approached a limiting value much lower than m = 0.60. The Boger fluid also showed good agreement at low capillary numbers, but began to climb at a capillary number of approximately six, ultimately reaching a value of m = 0.75. Simple correlations for the actual gas-assisted injection molding process have also been developed.7-11 It was shown that the wall thickness of the molded part has a dependence on the residual time of the gas bubble.7,8 The residual time is defined for each point on the flow path of the gas bubble as the difference in time between the passing polymer front and the moving bubble tip. This research indicates that increasing residual times result in higher fractional coverage. Others, however, have performed research to demonstrate a capillary number dependence by accounting for the solid wall thickness build-up as a function of residual time.9-11

EXPERIMENTAL BACKGROUND CCD video camera

75 ton injection molding machine

High resolution monitor,S-VHS video recorder, and 486 DX2 PC with data acquisition board

Controlled volume gas injection unit

Figure 1 - Experimental apparatus for gas-assisted injection molding trials.

A spiral tube mold cavity of 0.0127 m (0.5 in) diameter with a flowlength of 0.585 m (23 in) was utilized for the molding experiments. Six Kistler melt pressure transducers, model 6159A and 6157 type, were mounted flush in the full spiral mold cavity. The placement of the first four transducers allowed measurements of the advancing gas bubble in the region filled during the polymer injection step of the process. The fifth and sixth transducers were situated in a region filled during the gas filling step of the cycle.

Influence of Processing Conditions

37

η (Pa-s)

The trials were conducted at the laboratory facilities of the Ohio State University Engineering Research Center for Net Shape Manufacturing (ERC/NSM). The equipment used to mold the test parts is shown in Figure 1. An all-electric 75 ton ACT-B Cincinnati Milacron injection molding machine was utilized in conjuction with a single cylinder Cinpres gas injection unit equipped with a Cinpres II gas nozzle for a constant volume GIM process. The melt pressure transducer measurements of the gas bubble advancement were verified in preliminary trials by replacing the moving side core plate with a two-inch thick, circular, borosilicate glass plate, fixtured into a protective steel frame. This produced a crosssection that was one-half of the full tube. A mirror positioned at a 45 degree angle was placed directly behind the glass window. Mounted above the mirror was a Cohu CCD video camera, model 4915-2001. The video image captured by the camera was recorded by a high resolution, S-VHS videotape recorder at a rate of 60 frames/second. While the video images from the GIM process were recorded, the signals from five melt pressure transducers, amplified by Kistler model 5012 charge amplifiers, were fed to a data acquisition system. A Keithley MetraByte DAS - 1600 series data acquisition board, installed in an IBM-compatible 486 DX2 computer, sampled the amplified signals at a rate of 200 samples/second per channel. Comparisons between the videotaped images and the pressure profiles, were then performed to verify the measurement technique. During the molding experiments, an electronic Mettler AE-100 balance was used to weigh each set of full spiral parts, ensuring as much repeatability as possible during the trials by monitoring the shot weight. A Sheffield model RS30 co-ordinate measuring machine (CMM) evaluated the gas bubble area of each cross-section from the molded spirals. Three points were measured on the part exterior, along with one point near the gas bubble center. Twenty-five points around the circumference of the bubble surface were then measured. These points were connected to the initial probe position to calculate the bubble area by dividing it into twenty-five small triangles. Three transparent injection molding grade 10000 compounds were utilized in this study: a general PS PVC purpose polystyrene (Dow Styron 685 D), a rigid 1000 poly(vinyl chloride) (GEON 87781), and a high PC 100 viscosity polycarbonate (GE Lexan 101). Each 10 compound provided the opportunity to examine the effects of polymer rheology on the resulting 1 0.01 0.1 1 10 100 1000 10000 100000 fractional coverage or wall thickness of the γ (sec ) molded test spirals. Figure 2 displays the shear Figure 2. Cross-exponential viscosity characterizations rate dependent viscosity of each material at its' of experimental materials. base melt temperature, as predicted by the Cross-1

38

Figure 3. Fractional coverage vs. flowlength for three gas piston speeds using polystyrene (Tmelt = 487 deg F, Gas delay = 2.4 sec).


Figure 4. Fractional coverage vs. flowlength for three gas piston speeds using polycarbonate (Tmelt = 624 deg F, Gas delay = 2.4 sec).

exponential model. Model parameters were taken from the material selection database of AC Technology's injection molding simulation software.

RESULTS AND DISCUSSION The role of the melt rheology and the gas bubble velocity during the deposition of the hydrodynamic polymer layer is shown by the response of the wall thickness to changes in the gas compression rate. Figures 3 and 4 show the fractional coverage as a function of distance down the spiral for three different piston speeds of the gas cylinder. In Figure 3, a large change in wall thickness is shown for the polystyrene depending on the piston speed of the gas cylinder. The slowest gas compression rate created a more uniform wall thickness distribution, before dropping off in the region filled during gas bubble penetration. Conversely, the fastest gas piston speed caused a non-uniform, decreasing wall thickness distribution. In contrast to the polystyrene, the response of the polycarbonate to the changes in piston speeds was negligible until the end of the gas bubble advancement. As Figure 4 shows, there was no change in wall thickness until a flowlength of 0.375 m was reached. After that point, slow piston speeds cause slightly thicker walls and fast piston speeds cause the walls to thin, as with the polystyrene. Subsequent trials were performed with pressure transducers mounted in the spiral mold. Since the times at which the polymer and gas fronts reached each transducer could be determined from the melt pressure profiles, residual times as a function of flowlength were obtained. The time delay between the end of polymer injection and the beginning of gas injection was the same for all three gas piston speeds for polycarbonate and polystyrene. The residual times, however, change as the gas compression rate changes. The lowest gas piston speed produced the highest, most uniform distribution of residual times along the spiral flow path. In contrast, the highest setting lowered the residual times, with a resulting distribution that is less uniform than either of the other two settings.

39

100

100

10

10

Ub (m/sec)

Ub (m/sec)


1 1.8 in/sec 3.1 in/sec 5 in/sec

0.1

0.01 0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

1 1.8 in/sec 0.1

0.50

Flowlength (m)

Figure 5. Bubble velocities vs. flowlength for three gas piston speeds using polystyrene (Tmelt = 492 deg F, Gas delay = 2.65 sec).

0.01 0.00

3.1 in/sec 6.2 in/sec 0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

Flowlength (m)

Figure 6. Bubble velocities vs. flowlength for three gas piston speeds using polycarbonate (Tmelt = 637 deg F, Gas delay = 2.05 sec).

Figures 5 and 6 demonstrate gas bubble velocities which were calculated from the residual times discussed above. Recalling the large changes in fractional coverage and the shear-thinning behavior of the polystyrene, the velocities for the three settings in Figure 5 are not surprising. The interaction between the polymer melt rheology and the advancing gas front can be seen, as the velocities for the high setting diverge from those for the low and base settings. As the gas bubble accelerates due to the decreasing resistance associated with polymer deposition on the mold walls far in front of the bubble tip, the melt shearthins. This further decreases the resistance to gas bubble acceleration, causing faster bubble acceleration, higher shear rates and continually decreasing viscosities and resistances to flow. Since the polycarbonate has a large, upper Newtonian viscosity before a transition to a shear thinning regime and the polystyrene displays shear-thinning behavior over the entire range of interest, it is reasonable to expect that the polycarbonate would show little response to changes in gas piston speed and bubble velocities until they were large enough to cause the melt to shear-thin. As expected, the polycarbonate bubble velocities in Figure 6 showed very different rates of gas front acceleration from those of the polystyrene. Comparing well with the bubble velocities, the fractional coverage did not begin to change as a function of the gas piston speed until the bubble velocities began to diverge and rapidly accelerate. The issue of the relative importance of the gas bubble velocity in comparison to the thermal influences in the mold cavity may be addressed through plots of Fourier number vs. fractional coverage. The Fourier number, Fo, is defined as: Fo = α tr / Rt2 where α is the thermal diffusivity, tr is the residual time, and Rt is the tube radius. Figures 7 and 8 show the dependence of the wall thickness on the Fourier number. These plots are divided into two regions where the polymer deposition on the mold walls is either as a result


0.8 0.75

F illed u n d er G a s C on t r ol

0.8

F illed u n d er P olym er C on t r ol

0.7

0.65 0.6

0.55

F illed u n d er G a s C on t r ol

F illed u n d er P olym er C on t r ol

0.7 0.6

0.5

0.5

0.45 0.4 0.001

F r a ct ion a l cover a ge

F r a ct ion a l cover a ge

40

0.01

0.1

1

Figure 7. Depende nce of wall thickn ess on Fou r ier nu mber u sing polycarb ona te.

0.4 0.00001

0.0001

0.001

0.01

0.1

Figure 8. Dependence of wall thickness on Fouri er nu mber us ing polystyr ene.

of a polymer or a gas bubbl e dr iving for ce. In th e r egion filled under polymer contr ol where small bubble velocities are common, ther mal inf luen ces cluster th e fr act ional coverage r esults in a genera lly incr easing tr end . In th e r egion filled under gas contr ol where th e gas velocities are highly varied, it is appare nt t hat the wall thi ckness shows litt le or no de pen dence on t he res idual time.

CONCLUSIONS Gas -assisted injection molding experi ment s wer e per for med u sing a spir al tub e mold and thre e comm on injection mol ding gr ade compo unds : po lystyr ene, polyvinyl chlori de, a nd polycar bona te. By meas uri ng th e wall thickne ss along the flow path of the gas bubble, the r esidua l time, gas bubbl e velocity, and m ateri al pr opert ies were found to be int er r elated and r esponsible for c han ges in fr actional wall coati ng t hickn ess of as muc h as 20%. The sha pe of the shear r ate dep ende nt viscosity curve for ea ch ma ter ial was found to p r ovide an i ndi cation of the ability to cha nge the wall th ickness cover age by direc t manipul ati on of the gas bu bbl e velocity thr ough pr oce ss pa ram eters su ch as gas pi ston spe ed and pr e-cha rge gas pr essure . Polymer melts which b egin to shear -th in at low shear rates ar e mor e sensitive to chang es in gas pr essure and gas piston speed, while those polymers th at ha ve significan t up per Newtonian r egions ar e r elatively insensitive to the se cha nges.

ACKNOWLEDGMENT Thi s wor k was sup por ted b y the En gineer ing Researc h Ce nte r for Net Shape Manufacturing at the Ohio State Univer sity and The Geon Company . The aut hor s would like to thank D r. Clive Cops ey and Mr . Scott Weir for the ir help in pr oviding t echni cal ad vice and a ssistanc e and Mr . Alfr ed Geiger f or hi s effor ts in collecting and summarizi ng resul ts fr om the PVC exper iments .


41

REFERENCES 1 2 3 4 5 6 7 8 9 10 11

Zheng, T., Glozer, G. and Altan, T, ERC for Net Shape Manufacturing at OSU Report, 1992. Fairbrother, F. and Stubbs, A.E , J. Chem. Soc., 1, 1935, pp. 527-529. Taylor, G.I., J. Fluid Mech., 10, 1961, pp. 161-165. Cox, B.G., J. Fluid Mech., 14, 1962, pp. 81-96. Poslinki, A.J. and Stokes, V.K , ANTEC Proceedings, 39, 1993, pp. 68-73. Huzyak, P., and Koelling, K., 1994 International Conference on Gas-assisted Injection Molding Technology, Ohio State University, November 1994. Findeisen, H., Diploma thesis at the IKV Aachen, 1991, Betreuer: A. Lanvers. Findeisen, H., 1994 International Conference on Gas-assisted Injection Molding Technology, Ohio State University, November 1994. Brockmann, C., Zheng, T., and Altan, T., ERC for Net Shape Manufacturing at OSU Report, 1994. Zheng, T., Koskey, J., and Altan, T, 1994 International Conference on Gas-assisted Injection Molding Technology, Ohio State University, November 1994. Zheng, T., and Altan, T., ERC for Net Shape Manufacturing at OSU Report, 1994.

Cover Part as an Application Example for Gas-assisted Injection Molded Parts

Michael Hansen Mack Molding Company, 608 Warm Brook Road, Arlington, VT 05250, USA

INTRODUCTION The gas -assisted inj ection m olding pro cess is in use now for sev er al year s offer ing new t echnical and c r eative possibilities for inj ection mol ding. After a bri ef surve y of th e pr inciple sequenc e of the process and ba sic pr ocess phy sics this paper c omm ents on Injection of an applicat ion example for a cover part and pr ovides solutions for the melt the pr oblems found d uri ng t he pr ocess of fixing existing issues on thi s tool. In Figur e 1, the pr inciple sequen ce of the pro cess is shown. In this pr ocess, the pr essur ized nitrogen gas is injected into the Gas injection melt to penetrat e the part via a network of thicker cro ss-sectioned and subsequent gas-chann els. The pr ocess consists of ever ything from a partial to follow-up pressure a volumetric filling of a cavity with polymer melt, as in compact inj ection mol ding. This phase is followed by the inj ection of compressed ga s, usual ly nitr ogen, be cau se of its availability, cost a nd iner tn ess. Ther e is a vari ety of gas-assisted injection molding pr oVenting and part release cesses. In most of the processes, the gas is inj ected into th e hot cor e of the melt through the nozzle an d th e sprue /r unne r s ystem, Figur e 1. Sequence of th e gas- or dire ctly int o the cavity via one or more gas needles. Due to an assisted injection molding pr o- almost consta nt pre ssure in a ll areas with g as p enetr at ion, a good cess. and even pr essur e distri but ion a nd t r ansmission is gua ranteed across the molded parts. After the end of the gassing phase, the pressure is released either by gas r ecycling or blowing the gas into th e at mosphere . As soon as ambient p r essur e is r eached , th e molded p ar t can be ejected . The r e ar e thre e main basic cat egori es of appl icat ions and some combi nat ions of t hese. The categories are as in the following. Th e first gro up in cludes tube - or r od-sha ped pa rt s such as for example clothes hangers and grab handles. The second category consists of large

44


-Design and layo11t of the part based on e1."perlence . .CoRstJ11ction of a prototype

tool

. -Te6iing or tht pl'Ototype tool

~

... -Detection

of weaknesses

. -Rem011'lll ofweaknesies tltronglt modiflcat1on I adaptation (gfomcil'Y , illjcction point locatiOlls, 2})5 ~ectlon locatIon)

Figure 3. Influential factors for the production of gasassisted injection molded parts.

.Layout or a nlM~produdiOlltool bum on thefmdingsfium the prototype tool n,-outs

cover-shaped structural parts with a network of gas channels often combined with the rib struc. ture of those parts such as e.g. business machme Figure2. Partandtooldesignfor gas-assisted injection housings, automotive panels and outdoor furnimoldedparts. ture. The third group encircles complex parts consisting of both thin and thick sections where the process is often used mostly for part integration by consolidating several assembled parts into a single design. Typical examples are armrests or mirrors as designed for automotive applications. The characteristic of the gas-assisted injection molding process leads to both various advantages and inherent problems associated with the design and processing. The key factor in being successful in producing with one of the gas-assisted injection molding processes is to use the advantages to your benefits and to avoid the difficulties by optimizing part design, material selection for the application as well as to choose the optimum technique.

STRATEGIES

FOR PART AND TOOL

DESIGN

In Figure 2, the particular steps for the optimization of tool and part design are listed. The building of a mass production tool is based on the knowledge and experience gained from experiments. In many cases,the use of programs for mold filling simulation is a good help to optimize the final design. INFLUENTIAL

FACTORS

There are four different basic groups of influential factors involved in the production of gasassisted injection molded parts as shown in Figure 3. These groups are material properties, processing parameters, part design and the gas-assistedinjection molding technique. Substantial is a part design adapted to the gas-assisted injection molding process. It is very important, that designers take the specific process conditions and requirements associated

Cover Part as an Application Example

45

with these techniques into account in advance while developing new part designs. The necessary adaptation work and the costs associated herewith can be reduced to a minimum. Furthermore the knowledge about interdependencies/interactions between part design, runner/gate-type and location, course and size of the gas-channels in connection with the filling pattern after melt injection and processing parameters are a very important factor for achieving an optimized and stable production process. The above-described procedure can be used for all application examples and offers solutions for all the process-related phenomena/problems in each category of parts. The course to optimize every single part differs a little bit from application to application. But the key principle is always to analyze the influential factors and adjust and improve those as much as necessary. With this better understanding of the process more and more of the advantages can be used in order to utilize the high and still growing potential of the gasassisted injection molding process in daily production and for new future developments. In the following the experiences in fixing an existing production tool is discussed. The analysis starts in this case with a try-out to determine the problems associated with tool/ design. After the weaknesses are determined the necessary adaptations are will be implemented and tested in a subsequent try-out. In order to achieve the best possible result for the production all the above-mentioned influential factors were investigated while testing and analyzing the tool.

GAS–ASSISTED INJECTION MOLDED COVER PART Because of the physics of the gas-assisted injection molding process cover parts require a network of gas-channels to provide the guidance for the gas entering into the hot melt through nozzle or gas pin. The more than one-dimensional flow path of cover-shaped structural parts ask for an excellent balancing of the injection point location, the gassing location and the melt filling pattern before gas injection. These are only some of the boundary conditions in connection with a cover application, which need to be taken into consideration. The size of the part reviewed in the following is 985 mm x 560 mm with a 3 mm wall thickness. The material used in this application is a glass-fiber filled PPE/PS resin. The material was chosen for this application because of structural requirements, heat and chemical resistance. The cavity surface of the finished part is a class A textured surface. The problem with the glass-fibers is to create an acceptable surface finish for the subsequent texture painting process without showing visible sink, hesitation or flow marks. The material solidifies quickly once the flow front stagnates. In areas with weld lines problems can occur to produce an acceptable surface without glass-fibers showing up at the surface. If those areas show up another secondary filling/sanding operation would be necessary.

46


PROBLEM DESCRIPTION

Figure 4. Core side of cover part with gas-channel network.

In case of this cover application the main issues are as described in the following. The melt filling pattern before gas-injection has to match the gas-channel network along the structural ribs. The task is to determine the appropriate degree of pre-fill with polymer before gas-injection, which makes possible to produce parts without the above-mentioned surface imperfections. The configuration of the injection sprue, gas pin and the network of gas- channels are shown in Figure 4. This con-figuration of gas and melt injection creates certain boundary conditions for the pre-fill with melt, which are illustrated in the following chapter. Figure 5 shows the injection sprue and the gas pin location as well as one of the filling stages before gas injection. Very important for the production of the part is to avoid sink marks on the cavity as occurring in areas around screw bosses and thick ribs running across the length of the part in center and on the side opposite to the screw boss locations.

FINDINGS FROM PRACTICAL MOLDING EXPERIMENTS In the try-outs with the existing tool the next step was to find weaknesses/necessary adaptations for Figure 5. Filling pattern showing the location of sprue correcting the problems occurring in the mass and gas pin. production. Prerequisite for running a successful production is to establish a stable molding process. One important prerequisite for the production of gas-assisted injection molded parts are the filling pattern before gas injection. This needs to be adapted to the planned or existing course of the gas-channel network. The part design is dependent upon size and shape, as well as the course and length of the gas-channels. The information for the final layout can be obtained from short shot studies as well as from mold filling simulation programs.


47

Another important factor is to check the process for repeatability in production. To compensate for slight fluctuations in the injection stage with polymer it is very important to ensure the same conditions for the subsequent gas injection via gas pin. Changes in the short shot sequence can create a narrow processing window for the gas-assisted injection molding process and under certain circumstances it results in a borderline running process. Due to the lower flow resistance there is a significant foreflow visible in areas where gas-channels are running along ribs or across surfaces. This flow behavior determines the degree of pre-fill with polymer before gas injection. As a result of the gas pin position and the gas-channel connecting the gassing location the whole surface area around the gas pin location needs to be completely filled before gas injection. Otherwise a gas blow-through would occur. The height (51 mm) and a 3 mm wall thickness of the main high rib running almost across the whole center portion of the part results in a sink mark potential on the later texture painted surface. There is also a thick rib running almost all the way along the part on the side of the 6 hinges as shown in Figure 4. This rib is a bigger problem from a processing prospective because there are more than one gas–channel intersecting with this rib. A sink mark far away from the gassing location can only be avoided when the gas hollows out the area where the rib intersects with the main surface. To achieve this there are at least 3 different gas-bubbles necessary created via the different gas-channels attached to the rib. The most critical factor in producing these parts is to figure out a way to keep the gas bubble penetrating through the hot melt always in the same areas of the part. This is the key to a successful and stable serial production. Slight fluctuations in gas-bubble extension and path can lead to significant changes in the molded part. The ability to keep the gas-bubble penetration in certain limits is the boundary line between running a successful application or having a constant fight with a production problem. In connection with the processing there is a very important issue with gas-assisted injection molded parts, which is often even more critical than the process itself. It is the design of the molded part. In the case of this part there are 4 areas with screw bosses and a rib pattern as shown in Figure 6 a) that create a mounting plain. These create one of the production problems. The rib pattern around the screw boss can result in a significant visible sink mark on the surface. All those areas are located far away from the melt and gas injection and represent areas which are filled and packed late in the process of producing the part. The gas-channels need as shown in Figure 6 b) to be at least close enough to pack the areas around the boss areas. In Figure 7 a) the course of the gas penetrating the melt is shown. The gas flows from the gas pin in direction to the high center rib and the gas flow is diverted into 6 different directions following the gas-channel network of the part. The gas flow is split up is more

48


Figure 6. Screw boss areas with rib pattern for creating a mounting plane.

often diverted flowing towards the last filled areas of the part as shown in Figure 7 b). The Figure 7. Gas distribution from the gas injection point following the gas-channel network. more flow paths are provided for the gas, the more problem areas can occur on the molded part. The most critical areas from showing sink marks are the thick ribs running along the length of the part. Due to the wall thickness and height the gas pressure needs to prevent those areas from sinking in. The rib on the side opposite the boss areas is very critical for packing. As shown in Figure 8 a) and b) the gas-bubble splits up reaching the thick rib and flows in two opposite directions. Important here is to achieve a stable gas-channel pattern with a gas-bubble entering the root of the rib from both sides (see Figure 8 b) packing out the melt to avoid a sink mark. This can be achieved by extending the gassing time and pressure to allow the gas to compensate the shrinkage in those thick-walled areas.


49

CONCLUDING REMARKS The application example shows both the difficulties of distributing the gas along several different paths as well as the ability to pack areas far away from the gas injection location. To make sure that sink mark can be avoided it is necessary to provide a consistent compact injection molding phase. It also demonstrates that extending the pin length in those screw boss areas were the key to void sink marks along with an adapted gassing time and pressure. As long as the gas-channel can penetrate close or into those areas, visible sink marks can be avoided as shown in Figure 7 b). The nitrogen gas has the ability to pack ribs far away from the gas injection without leaving a visible sink mark on the surface. Prerequisite to achieve this goal is to provide the consistency of metering and constant conditions in the compact filling phase. Then the gas always follows the same flow Figure 8. Gas-channel course close to boss areas and paths and the gas-channel distribution along ribs along the rib parallel to main cross rib. and mass accumulations is nearly constant. Based on those results very little adaptations to the gas-channel network were necessary. Basically only one gas-channel needed to be tapered down at the very end to avoid slight sink marks. In other areas a slight chamfer helped to guide the gas in certain locations. The high rib running across the length of the part didn’t show any visible marks on the cavity surface after optimizing the gas processing parameters. This part is a very good example that in a lot of applications the sometimes so-called minor side issues are creating the biggest production problems and those can be identified by analyzing all the influential factors, which contribute to the production of a gas-assisted injection molded part.

REFERENCES 1

2

Hansen, M. “Anwendungsbeispiele fuer Gas-innendruckformteile,” (Topic: “Application Examples for the Gas-Assisted Injection Molding Process”), seminar “Processing Technology in Injection Molding”, at the SKZ in Wuerzburg, Germany, June 1997 Hansen, M. “Verfahrenstechnische Grundlagen zur Auslegung von Gasinnendruck-formteilen”, (Topic: “Processing Basics for the Design of Gas-Assisted Injection Molded Parts”), Ph.D. thesis, Shaker Publishers, Aachen, 1996

50

3

4 5


Potente, H. “Anwendung des GID-Verfahrens am Hansen, M. Beispiel eines Haltegriffes”, (Topic: Burgdorf, D. Application of the Gas-Assisted Injection Molding Technology exemplary for an Oven Handle”), Plastverarbeiter 46 (1995) 3, p. 40-51 Hansen, M. Application Examples for Gas-Assisted Injection Molded Parts Structural Plastics ‘99, Boston, p. 95-108 Hansen, M. Application Examples for Gas-Assisted Injection Molded Parts Journal of Injection Molding Technology, Vol. 3, No. 3, p. 141-153.

H. Potente lnstitut

fur Kunststofftechnik,

Universitiit-GH

and H.-P. Heim

Paderborn,

Germany

INTRODUCTION Over the past few years, a large number of variants of the injection moulding process have been introduced offering properties optimally tailored to the applications and targets in question. The gas injection technique (GIT) is a multi-component process, similar to the sandwich technique, which involves different raw materials being successively injected into the mould, leading to a layered moulding structure. By injecting inert gas into the molten plastic, hollow spacesare created in areas of the moulded part with higher wall thicknesses. The use of this process can be motivated either on purely process engineering grounds or for design reasons.In many cases,design requirements are placed on parts which only permit cost-efficient production by the gas injection technique and, at the same time, the process offers a number of technical advantages.Various drawbacks have to be set against these advantages -especially the considerably more difficult design of the part and the more complex process control.7 The different variants of the gas injection technique can be distinguished on the basis of the nature of the gas introduction or the type of melt injection. The gas can be introduced either via the machine nozzle or via a mould injector.8 Both processes have their specific advantagesand drawbacks for the particular application involved.7 To form the hollow spaces,melt is pushed out of the liquid centre of the moulded part and replaced by gas, and hence it is necessary for a similar-sized volume to be available or to be created in the moulded part in order to receive this displaced material. The mould cavity is only filled partially to begin with, by the short-shot process, and the residual filling with gas conducted in such a way that melt is conveyed into the as yet unfilled volume by the gas guidance geometry (thick-walled area of the moulded part).5 In the ancillary cavity process, by contrast, the moulded part undergoes complete volumetric filling and, when the gas is injected in, a connected volume is opened up by the machine control system. The melt is conveyed into this area by the gas guidance geometry as the gas is being introduced.7 Both casescall for highly elaborate balancing of the displaced volume and the volume to be filled.

52


The gas injection technique has found a large number of areas of application over the past few years in particular. The moulded part geometries for which the gas injection technique is used similarly vary over a broad range. It has become established practice to distinguish between .rod-shaped, thick-walled moulded parts .thin-walled moulded parts with gas guidance ribs and .thin-walled moulded parts with thick points in certain parts. The actual shape of a moulded part in these categories results from the requirements placed on the part design and function, making it necessaryfor the process employed to be adapted to the properties desired in the moulded part being produced, with consideration to design guidelines. The cause-and-effect correlations for the gas injection technique are correspondingly complex.

PROBLEM When it comes to the design optimization of a gas injection moulding, different publications contain information on the design of the thick-walled areas of the moulded part in respect of cross-section and wall thickness ratios as well as in terms of the radii employed. This provides design guidelines, which have already been incorporated in a systematic design procedure in the main.7 These guidelines relate to the gas guidance geometries, for which highly detailed design recommendations are given. They constitute the basis for what is set out below and ought to be borne in mind whenever a GIT moulding is being de-signed. POSITION OF THE MEL T AND GAS INJECTION POINTS In conjunction with the structural layout of the moulded part, it must be stated that the position of the melt and gas injection points is of particular importance, in addition to the design guidelines.2,6 The position of the melt gate must be optimized with allowance for the melt flow and coordinated with the position of the gas injection point.l Since the moulding compound can only be displaced to-wards the end of the flow path, i.e. towards unfilled areas of the moulding, the fosition of the melt gate and the gas injection point determine the course of the gas bubble. Apart from this, the associatedhigh pressure requirement causesmolten plastic to be displaced from the thick-walled gas guidance geometry to thin-walled areas of the moulding, which can lead to instabilities in the process sequence. In order to completely fill the thin-walled areas, a sufficiently high pressure must be provided with the gas. The problem here is the stagnation of the melt after its injection, while the gas pressure is being built up, which is inherent to the process. Figure 1 shows the pressure development at different measurement points in the moulded part. A pressure gradient is clearly evident along the flow path. This is only eliminated

53

Molded Part Design

Penetration of the gas in the flat area of the moulding and blow-through

gas guidance rib

residuallyfilled area

gas injection

Figure I. Pressure development over time in the gas injection process, from Michaeli, W., Lanvers, A.: Gasinjektionsverfahren, p. 248.

Figure 2. Schematic diagram of an unfavourable combination of gas guidance rib position and residually-filled area.

as the gas bubble advances, giving an isobaric state prevails along the length of the gas channel.4 Areas of the moulded part that are a long way from the gas injection point first have a relatively low pressure acting on them at the time the gas is introduced. If this is not sufficient to displace the melt, the flow front will freeze, producing a gramophone record effect on the surface. the moulded part will only be fully shaped with a high gas pressure. It can be concluded that this effect will be all the stronger the higher the pressure requirement in the residually-filled area is. The gas follows the course of least resistance. The higher the pressure requirement for residual filling, therefore, the higher the tendency for fingers to form in areas adjacent to the hollow space guidance rib. In extreme cases, the course followed by the gas can deviate completely from the gas guidance rib if there are other areas of the moulding that are easier to displace. The schematic diagram in Figure 2 shows an example of such a case. The position of the so-called residually-filled area, i.e. the part of the mould cavity that is not filled with melt prior to the injection of the gas, is thus of decisive importance. Prefilling thus constitutes a crucial aspect of moulded part design. In critical cases,just a slight variation in the amount of compound initially injected in can lead to the problems that have been indicated. Two requirements can thus be placed on the layout of GIT mouldings: .the residually-filled area must be located at the end of the desired gas channel .thin-walled areasmust be completely filled with melt before the gas is introduced.

54


PRESENTATION

OF THE PROBLEM TAKING A SAMPLE MOULDED PART

,\rea...fthec".jtynotIiIlcd priortothe.tartofg~rod..o:tjOn -/ ~ "

The moulded part shown in Figure 3 was developed for different studies of the gas injection ~echnique.Thi~ is a test mold whic.h de~ibera~ely mcorporates different process engmeenng diffi-

culties, such as the thick-walled annular region, and also the gas guidance geometry divided into two flow directions. In order to produce a lowwarpage and low-shrinkage moulded part, the (;as n..'.'dl" thick-walled areas of the moulding are to be holposition lowed-out by the gas injection technique. First of all, it is necessary to establish the Figure 3. Presentation of the residually-filled areas gas guidance geometry and the required position of the residually-tilled area(s), so that the melt and gas injection points can be determined. After different preliminary considerations, it was decided that gas injection should be performed in the annular area and that the gas bubble should propagate around the circular ring and in the two rod-shaped, thick-walled areas. It must be borne in mind here that it is, of course, not possible to achieve a ring fully surrounded by a gas bubble and, hence, apart from the two residually-tilled areas in the front domes, an unfilled area must also remain in the ring after partial filling. The target residually-tilled areas are shown in Figure 3. The requirements for the establishment of one or more appropriate melt injection points are thus: .partial filling with the unfilled areasthat are marked in Figure 3 .proportional distribution of the volumes of the un-filled areas in accordance with the volume of melt to be displaced (with different degrees of pre-filling) and .complete filling of the thin-walled area during partial filling.

ESTABLISHMENT

OF OPTIMUM MEL T INJECTION POINTS WITH A SAMPLE MOULDED PART

To determine the melt injection points, use was made of finite element calculations to simulate filling behavior with different gate positions. Different calculations were performed with melt injection in the central thin-walled area, inter alia. By contrast to injection in the edge area, this position also constitutes an advantage in mould engineering terms. Figure 4 shows the calculation results for three different gate positions.

55

Molded Part Design

Figure 4. Filling simulation for three different gate positions (marked by the arrow).

mc11injoction po;n",

0 impro,""

0

artick ~comctty

2

Figure 5. Presentation of the improved article geometry and melt injection points.

It is clear that these injection points will not give satisfactory results. The flow pattern shows Figure6. Presentation of differentdegrees of filling for that the desired division of the residually-filled theoptimised articlegeometry. areas is not possible on account of the flow direction dictated by the perforations. The left-hand picture shows an excessively high filling level in the thick-walled domes. When the injection point is moved towards the centre of the moulded part, disproportionately high filling of the ring area results. This effect can be attributed to the melt guidance through the bars in the thin-walled area of the moulding. Since a different gate position cannot be expected to improve the flow pattern, structural changes were made to the moulded part. Figure 5 shows the changed article geometry. As the Figure 5 shows, additional bars were incorporated in the region of the perforations in order to guide the melt. This produces a clearly improved flow front course which better meets the requirements on the flow behavior defined at the outset. As Figure 6 shows, virtually symmetric mould filling results in the thin-walled region. The depiction of different degrees of filling shows that the requirements in respect of the residually-filled areas can also be fulfilled. An estimate of the unfilled residual volumes for

56


different degrees of filling shows that the three unfilled areas are more or less proportional to the displaced volume. This ensures that the prefilling can be aligned to the different gas bubble cross-sections to be expected. CONCLUSIONS The example outlined illustrates the basic approach to the establishment of appropriate melt injection points for the gas injection technique. It becomes clear that even slight changes to the article geometry and the position of the gate can lead to changes in the course of the flow front, which have an extremely negative impact on application of the gas injection technique. The flow pattern is thus one of the decisive parameters for the process reliability that can be attained with the GIT and for the quality of the moulded parts produced. Filling simulations employing finite element calculations represent an appropriate method for obtaining information on the application of the GIT right at the development phase. Through appropriate balancing of the filling behavior and the unfilled areas of the cavity after partial filling, it can be ensured that the conditions for application of the GIT, i.e. the three main requirements on appropriate melt injection points, can be fulfilled. The results presented deal exclusively with the filling behavior of the moulded parts. The extent to which other results of an FE simulation, such as pressure and temperature distribution, can be used and interpreted specifically for the GIT is to be checked in further investigations.

REFERENCES 1 2 3 4

5 6

7 8

Eckardt, Helmut: Gas-Assisted Injection Molding, in: Stevenson, J. F.: Innovation in Polymer Processing: Molding, Carl Hanser Verlag, Munich Vienna New York, 1996. Jaroschek, Christoph: Elegant? Preiswert? Oder sogar beides?, Kunststoffe 87 (1997) 9, pp. 1172-1176. K1otz, B.: Voraussetzungen im Bereich der Formteilgestaltung fUr die Anwendung des Gasinnendruckver-fahrens, Transferzentrum Aachen Kunststofftechnik, SpritzgieBtechnisches Kolloquium 1990, pp. 36-57. Michaeli, W.; Lanvers, A.: SpritzgieBen transparent gernacht -Neue Entwick1ungen bei der ProzeBsimulation, Teil1: CAE- Techniken fUr das Zweikomponen-tenspritzgieBen und das Gasinjektionsverfahren, Plaste und Kautschuk 39 (1992) 7, pp. 241-248. Moritzer, Elmar: Ph1lnomenorientierte ProzeB-und Formteiloptimierung von thermoplastischen Gasinjektions-( GIT)SpritzgieBartikeln, Dissertation an der Universitlit-GH Paderborn, Shaker Verlag, Aachen 1997. Renger, M.: Das Gasinnendruckverfahren -eine SpritzgieBvariante mit besonderen Mt\glichkeiten, SUddeutsches Kunststoff-Zentrum WUrzburg, Fachtagung 18.-19. September 1990, pp. 101-136. Rennefeld, Christoph: Konstruktive Optimierung von Thermoplastformtei1en und SpritzgieBwerkzeugen fUr die Gasinnendrucktechnik, Dissertation an der Universitlit GH Paderborn, Shaker Ver1ag, Aachen, 1996. Shah, Suresh: Gas Injection Molding: Current Practices, ANTEC '91, pp. 1494-1506.

Design Optimization of Gas Channels for an Air Cleaner Assembly Using CAE Simulations

D.M. Gao, A. Garcia-Rejon, G. Salloum and D. Baylis Industrial Materials Institute - National Research Council Canada, 75, blvd. de Mortagne, Boucherville, Quebec J4B 6Y4, Canada

INTRODUCTION The typical gas-assisted injection moulding process (short shot process) can be subdivided into the following steps: a) polymer filling to a predetermined percentage cavity filling b) gas injection; and c) packing stage. During polymer filling the cavity is partly filled (up to 80%). Shortly after the end of the polymer injection, the gas is injected to hollow-out the gas channels until the cavity is completely filled. The relative melt/gas flowrate and the switch-over time between polymer and gas injections will determine the amount to be hollowed. Due to the gas penetration inside the polymer melt during the gas injection phase, the amount of material needed, the level of injection pressure as well as the clamp tonnage required, the resulting shrinkage/warpage and sink marks on the part can be greatly reduced. Due to its versatility in the production of mouldings of much greater complexity which combine thick and thin walls, hollow sections and elongated shapes, gas assisted injection moulding has resulted in the production of single parts replacing multipart assemblies and therefore a substantial reduction in manufacturing costs. Designers now have much greater freedom to incorporate thick and thin sections in the same moulding that will result in ribs and flow leaders; higher stiffness to weight ratios; reduced cycle times; and higher dimensional stability. The air cleaner assembly, analyzed in this project, is a good example of an open-channel part. These parts have a thin wall with the gas channels traversing the part similar to conventional ribs. These parts are more difficult to design and mould because the gas may penetrate into the thin walled sections of the part (fingering). The optimal layout of the gas channels - relative location of gas channels and gas injection points to polymer gates - within a cavity should create a polymer filling pattern in which the lowest pressure will be located near the end of the channels. In order to avoid fingering the channels should be oriented in the direction of the melt flow. Channel size and geometry have to be chosen in such a way to minimize race track effects while maintaining

58


the structural advantages offered by gas assisted injection moulding. In the case of multiple gas channels it is also very important to avoid an unbalanced gas penetration in the different channels. The use of computer aided flow analysis in the case of gas assisted injection moulding can be of great help to the part designer, mould maker and processor in the determination of parameters such as: i) gas channel design; ii) % of polymer to be injected and its optimal injection point; iii) gas injection locations; iv) preset volumes and pressures for gas injection; and v) filling patterns and operating conditions for optimal polymer wall thickness; best quality surface and minimal shrinkage and warpage. The primary objectives of this project are to determine, through computer aided flow analysis, the gas channel design (size and location) as well as the optimal moldability diagram (polymer injection speed, flow rate, temperature range, gas pressure, etc.) for an air cleaner assembly moulded using a 30% glass fibre reinforced polypropylene. In order to provide a useful information to the part designer, mould maker and moulder, several gas channel designs as well as different moulding conditions are considered. A number of important factors such as polymer flow pattern, gas penetration, tooling constraints, etc. are taken into consideration in the optimization of the moulding conditions.

NUMERICAL MODEL During the gas penetration into the polymer melt, three distinct flow regions during the gas injection stage can be identified: 1) gas penetration region, 2) polymer melt region, and 3) empty or unfilled region. Region 1 is initially filled by the polymer. The gas penetrates into the polymer melt and creates a gas core. During the gas penetration, the gas is transmitting the pressure required to advance the melt front. The polymer skin layer between the gas and the mould walls is stagnant. It is assumed that the skin layer consists of a solidified layer and an adhered layer. The solidified layer is formed by the polymer freezing upon contact with the cold mould. The adherence between the polymer flow and the solidified layer creates the adhered layer. Regions 2 and 3 are identical to those encountered in conventional injection mould filling except that two moving boundaries for the polymer melt region are present. In this study, the polymer melt is considered as a Generalized Newtonian Fluid, i.e. the viscosity is a function of shear rate and temperature. The flow is assumed to be quasi-steady state and the inertia terms are neglected due to the low Reynolds numbers encountered in molten polymer flow. Since most parts produced by gas-assisted injection moulding have a shell like geometry, i.e. the part thickness is much smaller than other part dimensions, the lubrication approximation (Hele-Shaw flow)1 can be used for modelling the global flow behavior in the mould cavity. A dimensional analysis of the energy equation shows that the

Design Optimization

59

heat conduction in the flow direction can be neglected since the thickness of the cavity is much smaller than the other two dimensions. The convection in the gapwise direction is also neglected. The pressure equation is solved using the Galerkin finite element method. A three node triangular element was chosen to approximate the pressure. The energy equation is discretized using the finite difference method. The time dependent derivative of the temperature is approximated by backward finite difference. A control volume approach has been employed to track both, the flow front advancement as well as the gas-polymer interface. A thickness fraction of polymer skin (Fs) is associated to each control volume in order to represent the three distinct regions present during the filling phase. Fs is defined as the ratio of the thickness of the polymer skin to the total thickness of the part. Fs = 1 represents an element completely filled with polymer, while Fs = 0 represents an empty element. 0< Fs 300 mm/s (< 4 in/s) (> 12 in/s) mould temperature upper/ 40°/40-70°C 10-20°/20°C lower half (100°/100-160°F) (50-70°/70°F) wall thickness achievable 2.5 - 3.5 mm 1.8 - 2.0 mm (0.10-0.14 in) (0.07-0.08 in) typical cycle time with 120 (105) s 67 (55) s manual (robotic) insertion and demolding

Melt Compression Molding

177

Multi-cavity moulds are common already e.g. 2 left + 2 right center console parts for BMW series 5 with pile carpet lamination i.e. 4 parts per cycle of 57 s or • 2 left + 2 right door trim inserts for BMW series 3 i.e. 4 parts per cycle of 60 s both with robotic coverstock placing and parts demolding. There are many valid reasons for the ready acceptance of the MCM-IML process by the industry like • numerous degrees of freedom of parts design • moulding of high/deep assembly fixtures including undercuts • elimination of 180° back wraps previously necessary for optical reasons • 90° back wraps also with recess • sensitive treatment of delicate coverstock e.g. fabrics with pile and plush finish, embossed foils, also material with foam back • genuine uniform soft touch also on large area parts • none or little protective backfinish of decorative coverstock • lamination in one heat (and cycle) i.e. IML resulting in • absence of adhesives i.e. of aggressive solvent matters • secure intimate durable bonding of lamination • low content of manual labour • low internal stresses because of the compression cycle • most rejects, waste, and used parts are recyclable or suitable for incineration i.e. no waste for classified disposal • lower cost of investment as with conventional process • uniform industrially reproducible high quality In fact there are few compelling reasons advising against a wide use of the MCM-IML process. Such reasons are • parts with areas too small for controlled deposition of the melt strip • significant undercuts at the fringes of the laminated area and • unavailability of space - even after tilting of the core - to securely deposit the melt strip. It is appropriate to say that a comparison of cost i.e. for investment and manufacturing of parts produced by conventional high pressure moulding with subsequent press lamination to parts produced by the integrated MCM-IML method will result in cost savings up to 40%. On average a cost reduction of 20 to 25% is a realistic assumption. •

178


MCM-LFT The request from within the car component industry to develop the MCM-LFT process is motivated by the intention to replace glass mat reinforced thermoplastics (GMT or Azdel) and to an extent SMC wherever possible for reasons like • cost reduction (material and processing) • simplification of process • reduction of thermal stress on the PP polymer • lower cost of investment • floor space requirements • more flexibility in regard to material analysis and characteristics etc. At present there are two methods which have matured to continuous production application. The processing of long glass fiber reinforced granules (LFG) which come as little rods and are readily available in Europe in lengths from > 10 mm (0.4 in) to 25 mm (1 in) from sources like (in alphabetical order) • Appryl (Elf-Atochem) of France, brand name "Pryltex" • Borealis of Finland, brand name "Nepol" • DSM of the Netherlands, brand name "Stamylan" and • Ticona (Hoechst) of Germany, brand name "Compel" The pertinent equipment will also process recycled GMT (Azdel) material or blends with LFG. Parts already produced are bumper carriers, undercovers, and battery holders. Instrument panel carriers are under development. In-line compounding and subsequent forming of LFT starting from roving. The process runs through the following stages: • Roving filaments are unwound from bobbins under rupture control and guided through tubes to a preheating station. • The rovings are pulled through a pultrusion head fed by a single screw extruder with PP melt for roving impregnation. • The impregnated roving is taken in by screws of a corotating twinscrew extruder at a point where the main matrix is already molten. • Fiber length is determined by screw rotation. • The LFT melt is extruded into preforms to be placed in a compression mould or discharged into an injection unit. Recycled material (GMT-Azdel, LFT) may be added by means of an additional single screw extruder at the end of the compounding phase. Fiber lengths are varying however come with a length concentration around 10 mm (0.4 in) to 80 mm (3.1 in).


179

The process is successfully applied for the production of VW Passat frontends since a longer period of time. A typical MCM-LFT cycle is identical to the MCM-IML cycle exclusive of the coverstock placing phase. The stock temperature at the orifice of the deposition die will be around 240°C to 260°C (- 460° to 500°F). However the cooling time aided by the better heat conductivity of glass fiber and the absence of coverstock will be about the same as with the MCM-IML technique. Based on a wall thickness of 2 to 3 mm (- 0.08 to 0.12 in) a total cycle time of 60 sec by manual demolding and 45 sec by robotic demolding can be achieved. Early demolding with subsequent postcooling will shorten the cycle and help to control internal stresses. The internal mould pressure required may vary from 80 bar (- 116 psi) to approx. 150 bar (- 2180 psi) depending on the structure of the part, fiber lengths, and fiber content and hence the flowing properties of the stock. For the MCM-LFT process most development and application efforts are directed to the preservation of the original fiber lengths. Glass fiber appears to be mainly affected by pressure resulting in immediate contact of the fibers. Detrimental pressure may result from pressure development in the final flights and at the tip of plastication screws i.e. backpressure and/or by narrowing of the melt passage for extrusion or injection. Adequate conditions during extrusion are provided for LFG (pellet) processing by preparing the melt on a single screw extruder with the following characteristics: • low compression, low friction screw of 30 D length, screw diameter min 90 mm (3.5 in) preferably 130 mm (5.1 in) with the low rotation speed of max 30 rpm • deep cut, thermolator controlled intake zone with tangential intake pocket • external heating by heater collars on the barrel supported by a screw core temperature control • axially movable screw for back pressure control and melt ejection. The above extruder is also capable of processing recycled material from GMT-Azdel or LFG. Due to the most careful plastication conditions output capacities are in the range of 160 kg/h (350 lb/h) for screw diameter 90 mm (3.5 in) to 240 kg/h (530 lb/h) for a 130 mm (5.1 in) screw. Extruders with larger screws are available. Twin screw extruders with corotating screw are designed for high mixing capacities at low stock pressure i.e. they are high efficiency compounding extruders. They are not suitable to process LFG or recycled materials.

180


MCM EQUIPMENT In principle the main units of a MCM-IML plant are identical to those for MCM-LFT, adapted only for specific application engineering requirements.

Figure 5. Schematic drawing of MCM-plant. 1. clamping unit, 2. injection unit, 3. melt strip deposition die, 4. hydraulic systems, 5. electrical and electronic control units.

CLAMPING UNIT MCM clamping units are designed as hydraulic vertical 4 column presses. As cycles start with an open mould for subsequent compression moulding special emphasis is laid on a mechanically sturdy design including platens with low deflection, generously dimensioned columns with extent and precise guidings of the upper moving platen. The guidings are about 2.5 times the length compared to conventional clamping units plus additional stiffening elements. During MCM-IML and most MCM-LFT applications none or moderate tilting momenta occur which will be absorbed by the mechanical design features. In rare cases i.e. involving parts with a complicated structure, melt with poor flowing properties, etc. requiring higher internal mould pressures along with critical tilting momenta a frame press with closed loop platen parallelism control may be necessary. MCM clamping units are available with two hydraulic systems (reference: 8000 kN i.e. 880 US-t):


181

Hydraulic system based on variable displacement pumps with the following data relevant to the process closing speed mm/s 500 in/s 19.7 opening speed mm/s 330 in/s 13.0 compression speed mm/s 25 in/s 1.0 pressure build-up s

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