Advanced Injection Molding Technologies

Advanced Injection M

olding TechnologiesChenTurng

Shia-Chung Chen (Editor)Lih-Sheng Turng (Editor)Musa R. Kamal (Series Editor)

Advanced Injection Molding Technologies

POLYMERPROCESSINGSOCIETY

Chen / Turng (Eds.) Advanced Injection Molding Technologies

Hanser Publishers, Munich Hanser Publications, Cincinnati

Advanced Injection Molding Technologies

Shia-Chung Chen (Ed.) Lih-Sheng Turng (Ed.)

With Contributions by:A. Ameli, E. Cabrera, J. M. Castro, R.-Y. Chang, S.-C. Chen, F. Gao, C.-T. Huang, W.-R. Jong, L. J. Lee, S.-J. Liu, R. Mulyana, C. B. Park, P.-J. Wang, Y. Yang, A. Y. Yi

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© Carl Hanser Verlag, Munich 2019 Editor: Dr. Mark Smith Production Management: Jörg Strohbach Coverconcept: Marc Müller-Bremer, www.rebranding.de, Munich Coverdesign: Stephan Rönigk Typesetting: Kösel Media, Krugzell Printed and bound by Druckerei Hubert & Co GmbH und Co KG BuchPartner, Göttingen Printed in Germany

ISBN: 978-1-56990-603-3 E-Book ISBN: 978-1-56990-604-0

The Editors:

Prof. Shia-Chung Chen, Chair Professor, Department of Mechanical Engineering, Chung Yuan Christian University, Taoyuan City, Taiwan

Lih-Sheng (Tom) Turng, Kuo K. and Cindy F. Wang Professor and Co-Director of the Polymer Engineering Center, University of Wisconsin–Madison, Madison, WI

Jean-FranÇois AgassantCEMEF, École des MinesFrance

M. Cengiz AltanUniversity of OklahomaU.S.A.

Patrick AndersonEindhoven University of TechnologyNetherlands

Satinath BhattacharyaRMIT UniversityAustralia

Mosto BousminaHassan II Academy of Science and TechnologyMorocco

Philippe CassagneauUniversité Claude BernardFrance

Shia-Shih ChenChung Yuan Christian UniversityTaiwan

Phil CoatesUniversity of Bradford United Kingdom

José CovasUniversity of MinhoPortugal

Furong GaoHong Kong University of Science & TechnologyHong Kong

Frank HenningFraunhofer-Institut für Chemische Technologie (ICT)Germany

Andrew HrymakWestern UniversityCanada

Sadhan JanaUniversity of AkronU.S.A.

Dilhan KalyonStevens Institute of TechnologyU.S.A.

Samuel KenigShenkar CollegeIsrael

Takeshi KikutaniTokyo Institute of TechnologyJapan

Progress in Polymer Processing (PPP) Series

Musa R. Kamal, Series Editor McGill University, Canada

Editorial Advisory Board

Ica Manas-ZloczowerCase Western Reserve UniversityU.S.A.

Masami OkamotoToyota Technological InstituteJapan

Chul ParkUniversity of TorontoCanada

Luiz A. PessanUniversidade Federal de São CarlosBrazil

Changyu ShenDalian University of TechnologyChina

Mark SmithCarl Hanser Verlag GmbH & Co. KGGermany

Giuseppe TitomanlioUniversità degli Studi di SalernoItaly

Lih-Sheng (Tom) TurngUniversity of WisconsinU.S.A.

John VlachopoulosMcMaster UniversityCanada

Injection molding is arguably one of the most important polymer processing meth-ods, accounting for approximately one-third of thermoplastics processed, and has been ranked No. 1 in terms of total product value, number of machines built and sold, and overall number of jobs. Many injection molding-related books published in the past have extensively covered the core topics of injection molding, such as material selection, process optimization, part and mold designs, tooling, special injection molding processes, and computer modeling and simulation, just to name a few.

However, since the beginning of the 21st century, due to growing environmental concerns, green molding and lightweight molding have received a lot of attention from the injection molding industry. As a result, novel engineering advancements (technology push) and increasing customer and societal demands (technology pull) have dramatically changed the technological landscape of this important process. Driven by calls to minimize the environmental footprint and save energy, the injec-tion molding industry has witnessed the widening adoption of intelligent control and the growing applications of microinjection molding, microcellular injection molding, water-assisted foaming, and water-assisted injection molding, as well as variable mold temperature technologies. It has also seen emerging applications of conductive polymers and injection-molded optical plastic components. As special injection molding processes are being adopted, modeling and simulation of those novel processes have also been developed and become commercially available to aid with design, processing, and optimization. Finally, as the time-to-market con-tinues to be compressed and knowledge management becomes one of the most important corporate assets, an automated mold design navigation system is ex-pected to become a common tool for the injection molding industry. Given all of these changes, this book strives to educate current and future generations of scien-tists and engineers on the latest injection molding techniques and processes, which have not been comprehensively covered in any other textbooks to date.

Under the encouragement of the Taiwanese Ministry of Education (MOE) Man-power Cultivation Program for Advanced Manufacturing, many members of the

Preface

VIII Preface

Society of Advanced Molding Technologies (SAMT), and Professor Musa Kamal, Series Editor of the Polymer Processing Society (PPS) book series Progress in Poly-mer Processing (PPP), we decided to take on this initiative and work with experts in the field to develop a new textbook that incorporates and covers industry-rele-vant topics so that it can meet the training requirements of the injection-molding industry.

We are truly grateful to the many chapter authors for their efforts and contribu-tions. We are also deeply indebted to staff members at Carl Hanser Verlag GmbH & Co. KG; in particular, Mark Smith, Julia Diaz Luque, and Cheryl Hamilton for their technical assistance in every stage of this book development project. We are also grateful for our colleagues; especially Mrs. Jane Lai and Ms. Chris Lacey, for their administrative and editorial assistance. Finally, we hope that this book will bring useful information to readers like you and your organizations, and we look forward to hearing your constructive criticisms and suggestions, so that we will be able to improve its content in future editions.

Shia-Chung Chen and Lih-Sheng (Tom) Turng

Contents

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VII

1 Introduction to Injection Molding . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Shia-Chung Chen

1.1 Injection Molding and Molding Machines . . . . . . . . . . . . . . . . . . . . . . . . . 11.1.1 Brief Overview of an Injection Molding Machine . . . . . . . . . . . . . . 31.1.2 Machine Setup for Process Conditions . . . . . . . . . . . . . . . . . . . . . . . 4

1.1.2.1 Injection Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.1.2.2 Melt Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.1.2.3 Injection Speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.1.2.4 Mold Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.1.2.5 Other Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.2 Characterization of the Injection Molding Process . . . . . . . . . . . . . . . . . . 71.2.1 Injection Cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71.2.2 Mold Filling Stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.2.2.1 Process Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71.2.2.2 Flow Velocity and Melt Front Advancement . . . . . . . . . . . 81.2.2.3 Pressure Variation and Distribution . . . . . . . . . . . . . . . . . . 91.2.2.4 Melt Temperature Variation and Distribution . . . . . . . . . . 101.2.2.5 Shear Stress and Shear Rate . . . . . . . . . . . . . . . . . . . . . . . . 12

1.2.3 Mold Packing/Holding Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121.2.3.1 Process Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121.2.3.2 Packing Pressure Variation and Distribution . . . . . . . . . . 131.2.3.3 Packing Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

1.2.4 Mold Cooling Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141.2.4.1 Process Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141.2.4.2 Coolant Temperature/Mold Temperature . . . . . . . . . . . . . . 141.2.4.3 Cooling Time/Ejection Temperature . . . . . . . . . . . . . . . . . . 151.2.4.4 Melt Temperature Distribution . . . . . . . . . . . . . . . . . . . . . . 15

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1.3 Influence on Part Properties/Qualities . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151.3.1 Effect of Processing Conditions on Part Properties . . . . . . . . . . . . 151.3.2 pvT Path and Thermal-Mechanical History . . . . . . . . . . . . . . . . . . . 161.3.3 Shrinkage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171.3.4 Molecular Orientation/Fiber Orientation . . . . . . . . . . . . . . . . . . . . 171.3.5 Residual Stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191.3.6 Warpage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211.3.7 Other Property/Quality Concerns . . . . . . . . . . . . . . . . . . . . . . . . . . 23

2 Intelligent Control of the Injection Molding Process . . . . . . . . . . 25Yi Yang and Furong Gao

2.1 Introduction of Injection Molding Machine Control . . . . . . . . . . . . . . . . . 25

2.2 Feedback Control Algorithms: Adaptive Control . . . . . . . . . . . . . . . . . . . . 272.2.1 Model Estimation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292.2.2 Model Predictive Control (MPC):

Generalized Model Control (GPC) . . . . . . . . . . . . . . . . . . . . . . . . . . . 302.2.2.1 Basic Principle of MPC and GPC . . . . . . . . . . . . . . . . . . . . . 302.2.2.2 Parameter Tuning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332.2.2.3 Adaptive GPC Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342.2.2.4 Adaptive GPC with Different Conditions . . . . . . . . . . . . . . 36

2.3 Fuzzy System in Injection Molding Control . . . . . . . . . . . . . . . . . . . . . . . . . . 372.3.1 Fuzzy Inference System Background . . . . . . . . . . . . . . . . . . . . . . . . 372.3.2 Fuzzy V/P Switchover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 382.3.3 Fuzzy V/P System Experimental Test . . . . . . . . . . . . . . . . . . . . . . . 432.3.4 Further Improvement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

2.4 Learning Type Control for Injection Molding . . . . . . . . . . . . . . . . . . . . . . . 482.4.1 Learning Type Control Background . . . . . . . . . . . . . . . . . . . . . . . . . 482.4.2 PID-Type ILC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 502.4.3 Time-Delay Consideration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 512.4.4 P-Type ILC for Injection Velocity . . . . . . . . . . . . . . . . . . . . . . . . . . . 522.4.5 P-Type ILC for Packing Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

2.5 Two-Dimensional Control Algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 552.5.1 Two-Dimensional Control Background . . . . . . . . . . . . . . . . . . . . . . 552.5.2 Two-Dimensional Dynamic Matrix Control . . . . . . . . . . . . . . . . . . . 59

2.5.2.1 Problem Formulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 592.5.2.2 Controller Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

2.5.2.2.1 2D Equivalent Model with Repetitive Nature . . 602.5.2.2.2 2D Prediction Model . . . . . . . . . . . . . . . . . . . . . . 612.5.2.2.3 Cost Function and Control Law . . . . . . . . . . . . . 622.5.2.2.4 2D-DMC Design Procedure . . . . . . . . . . . . . . . . . 64

2.5.2.3 Analysis of Convergence and Robustness . . . . . . . . . . . . . 65

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2.5.3 Simulation Illustration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 732.5.3.1 Case 1: Convergence Test . . . . . . . . . . . . . . . . . . . . . . . . . . . 752.5.3.2 Case 2: Repetitive Disturbances . . . . . . . . . . . . . . . . . . . . . 782.5.3.3 Case 3: Nonrepetitive Disturbances . . . . . . . . . . . . . . . . . . 80

2.5.4 Experimental Test of 2D-DMC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

2.6 Summary and Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

3 Water-Assisted Injection Molding . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89Shih-Jung Liu

3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 893.1.1 Molding Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 903.1.2 Advantages and Disadvantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . 923.1.3 Water versus Gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 933.1.4 Molding Resins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

3.2 Tooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

3.3 Process Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 973.3.1 Water Penetration Behavior in Molded Parts . . . . . . . . . . . . . . . . . 973.3.2 Water Channel Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 993.3.3 Part Fingering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1003.3.4 Unstable Water Penetrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1023.3.5 Molding of Fiber-Reinforced Materials . . . . . . . . . . . . . . . . . . . . . . . 103

3.4 Morphology Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

3.5 Modeling and Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108

3.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

4 Foam Injection Molding of Conductive-Filler/ Polymer Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115Amir Ameli and Chul B. Park

4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

4.2 Conductive-Filler/Polymer Composites (CPCs) . . . . . . . . . . . . . . . . . . . . . 116

4.3 Foam Injection Molding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

4.4 Foam-Injection-Molded CPCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1204.4.1 Microstructure of CPC Foams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

4.4.1.1 Fiber Interconnectivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1234.4.1.2 Fiber Orientation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1244.4.1.3 Skin Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1264.4.1.4 Fiber Breakage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

4.4.2 Conductivity of CPC Foams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1294.4.2.1 Through-Plane Conductivity . . . . . . . . . . . . . . . . . . . . . . . . 129

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4.4.2.2 In-Plane Conductivity and Anisotropy . . . . . . . . . . . . . . . . 1334.4.2.3 Uniformity of Conductivity . . . . . . . . . . . . . . . . . . . . . . . . . 135

4.4.3 Impact of Processing Conditions on the Conductivity of CPC Foams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1364.4.3.1 Degree of Foaming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1364.4.3.2 Injection Flow Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1424.4.3.3 Gas Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1444.4.3.4 Melt Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145

4.5 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146

5 Water-Assisted Foaming: A New Improved Approach in Injection Molding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149Rachmat Mulyana, Jose M. Castro, and L. James Lee

5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149

5.2 Need for Water-Carrier Particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1515.2.1 Evaluation of Water-Carrier Particles . . . . . . . . . . . . . . . . . . . . . . . . 1515.2.2 Pressurized Water inside the Pellet . . . . . . . . . . . . . . . . . . . . . . . . . 1565.2.3 Residual Water and Drying after Molding . . . . . . . . . . . . . . . . . . . . 1575.2.4 Shell Life of Pressurized Pellets . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158

5.3 Injection-Molding Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1625.3.1 Molds and Molding Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1625.3.2 Experimental Observations during the Filling Stage . . . . . . . . . . . 1635.3.3 Packing and Cooling Stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166

5.4 Mechanical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1705.4.1 Mechanical Property Comparison at Minimum Cycle Time . . . . . 1705.4.2 Effect of Packing Time on Mechanical Properties . . . . . . . . . . . . . 1715.4.3 Effect of Water Level on Mechanical Properties . . . . . . . . . . . . . . . 172

5.5 Warping and Surface Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1735.5.1 Warpage Improvement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1735.5.2 Flow Marks and Surface Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . 1755.5.3 Hiding the Flow Marks Using In-Mold Coating . . . . . . . . . . . . . . . 1775.5.4 Method of Weight Saving . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178

5.6 Accelerated-Aging Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1825.6.1 Effect of Water-Carrier Particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1825.6.2 Effect of Residual Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183

5.7 Comparison with Supercritical Fluid Molding (SCF Molding) . . . . . . . . . 1845.7.1 Experimental Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1855.7.2 Mechanical Property Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . 1885.7.3 Warpage and Surface Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189

5.8 Summary and Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191

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6 Variable Mold Temperature Technologies . . . . . . . . . . . . . . . . . . . . 195Shia-Chung Chen

6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195

6.2 Various Methods for Dynamic Mold Temperature Control . . . . . . . . . . . . 197

6.3 Variable Mold Temperature Control with Embedded Internal Heat Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1996.3.1 Hot Water Heating/Cold Water Cooling . . . . . . . . . . . . . . . . . . . . . . 1996.3.2 Oil Heating/Water Cooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1996.3.3 Steam Heating/Water Cooling (RHCM) . . . . . . . . . . . . . . . . . . . . . . 2006.3.4 Electrical Heater Heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2036.3.5 Pulse Cooling (Alternating Temperature Technology) . . . . . . . . . . 2046.3.6 Electrical Heating at the Mold Surface Using a

Two-Layer Coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205

6.4 Mold Heating Based on Electromagnetic Induction Technology . . . . . . . 2066.4.1 Principle and Characteristics of Induction Heating . . . . . . . . . . . . 2066.4.2 Induction Heating from Mold Surface (External Heating) . . . . . . . 2086.4.3 Induction Coil Design for Mold and Molding . . . . . . . . . . . . . . . . . 2096.4.4 The Challenges Facing EIHTC Applications and

Their Possible Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2126.4.5 The Real Application of EIHTC – Mold Exterior

Induction Heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2156.4.5.1 Elimination of a Weld Line and Floating Fiber Marks . . . 2156.4.5.2 Micro-Features Molding . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216

6.4.6 Mold Exterior/Induction Heating by an Externally Wrapped Coil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217

6.4.7 Induction Heating from the Mold Interior Using Embedded Coils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220

6.4.8 Mold Interior/Proximity Effect Induced by Internal Current . . . . 224

6.5 Other Mold Surface Heating Technologies . . . . . . . . . . . . . . . . . . . . . . . . . 2276.5.1 Hot Gas-Assisted Heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2276.5.2 Infrared Heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231

7 CAE for Advanced Injection Molding Technologies . . . . . . . . . . . . 235Rong-Yeu Chang and Chao-Tsai (CT) Huang

7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235

7.2 Multi-Component Molding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2367.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2367.2.2 Governing Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2387.2.3 Case Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2407.2.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248

XIV Contents

7.3 Long-Fiber Microstructure Prediction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2497.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2497.3.2 Governing Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2507.3.3 Case Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2527.3.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256

7.4 Microcellular Injection Molding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2567.4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2567.4.2 Governing Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2577.4.3 Case Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2597.4.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264

7.5 Gas-Assisted Injection Molding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2657.5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2657.5.2 Governing Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2667.5.3 Case Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2677.5.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271

7.6 Advanced Hot Runner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2717.6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2717.6.2 Governing Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2727.6.3 Case Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2737.6.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281

7.7 Conformal Cooling System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2817.7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2817.7.2 Governing Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2827.7.3 Case Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2827.7.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 289

7.8 Variotherm Molding Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2897.8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2897.8.2 Governing Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2907.8.3 Case Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2907.8.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300

7.9 Injection–Compression Molding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3017.9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3017.9.2 Governing Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3027.9.3 Case Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3027.9.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313


8 Injection Molding of Optical Products . . . . . . . . . . . . . . . . . . . . . . . . 317Pei-Jen Wang

8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317

XVContents

8.2 Optical Qualities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 318

8.3 Design Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321

8.4 Fundamentals of Optics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3228.4.1 Snell’s Law and Lens Images . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3238.4.2 Monochromatic Aberrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3248.4.3 Zernike Polynomials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3278.4.4 Abbe Number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 328

8.5 Material Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329

8.6 Mold Flow Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 331

8.7 Case Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3388.7.1 The Lens and the Mold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3388.7.2 CAE Simulation Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3398.7.3 Experimental Verification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343


9 Microinjection Molding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 349Eusebio Cabrera, Jose M. Castro, Allen Y. Yi, and L. James Lee

9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 349

9.2 Issues in Molding Parts with Microfeatures . . . . . . . . . . . . . . . . . . . . . . . . 350

9.3 Influencing Factors in Microinjection Molding . . . . . . . . . . . . . . . . . . . . . 354

9.4 Experimental and Numerical Studies of Injection Molding with Microfeatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355

9.5 Developments in Microinjection-Molding Technology . . . . . . . . . . . . . . . 363

9.6 Ultrathin Wall Case Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 368

9.7 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375

10 Mold Design/Manu facturing Navigation System with Knowledge Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 379Wen-Ren Jong

10.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 379

10.2 Knowledge Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 381

10.3 Four-Layer Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393

10.4 Redevelopment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 402

10.5 Stage Integration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 410

10.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 417

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 419

�� 1.1�Injection Molding and Molding Machines

Injection molding is an important thermoplastic processing technique for produc-ing plastic parts and products [1, 2]. Complete operation of injection molding re-quires an injection molding machine with a control unit, a properly clamped mold with a cavity or cavities that define(s) the part geometry, and a mold temperature control unit. The process begins with feeding plastic pellets of about 2 to 3 mm in size into the hopper of the injection molding machine (Figure 1.1). Before feeding, the plastic pellets are dried and cleaned to ensure low moisture content. Additives may be added to the pellets to be fed into the hopper to modify the plastic’s or final product’s properties.

Figure 1.1 Schematic of the injection molding process [3]

1 Introduction to Injection MoldingShia-Chung Chen

2 1 Introduction to Injection Molding

The injection screw then rotates and conveys the pellets to the heated barrel of the injection molding machine. The barrel consists of three zones—the feeding zone, the compression zone (transition zone), and the metering zone—all of which may be set to different heating temperatures (Figure 1.2). As the pellets pass through the feeding and compression zones, they gradually melt until they finally become a hot melt within the metering zone. The rotation of the screw provides shear heat-ing resulting from the mechanical crushing of the pellets between the screw flights and the barrel. The shear heating acts as the major heat source for pellet melting. Additional heat is also transferred from the barrel to the pellets via contact, which assists in melting the pellets. The rotation of the screw stops when the proper amount of melt is accumulated from the tip of the screw to the nozzle. The accu-mulation and conveying of melt within the barrel builds up pressure in front of the screw tip. The pressure pushes back the screw until enough melt for one shot is accumulated. This melt preparation stage is also known as the plasticating stage. In front of the screw tip, there is a shut-off valve to prevent melt entering into the mold during the plasticating stage.

Figure 1.2 Schematic of the three main zones within the injection molding barrel [3]

Once the screw stops rotating, the hydraulic pressure or electrical motor acts to move the screw forward at the profiled speed and push the plastic melt to flow through the nozzle, the sprue, the runner system, and into the cavity. The stage in which the melt begins to flow through the sprue until the cavity is entirely filled is called the mold filling stage. Once the cavity is filled, the screw continues to push additional melt into the cavity under very low speed (pressure-activated) to com-pensate for the subsequent shrinkage due to melt solidification. The slow advance of the screw under the profiled pressure is called the mold packing/holding stage. The packing stage ends when the gate is completely frozen and no more melt can be pushed into the cavity. The filled melt within the cavity is cooled until the part surface is sufficiently solidified. Then the mold is opened and the part is ejected. This stage of melt solidification is known as the mold cooling stage. The plastic

31.1 Injection Molding and Molding Machines

melt begins to cool as soon as it touches the cold surface of the mold cavity. How-ever, the serious cooling begins when the filling of the cavity ends. The cessation of hot melt flow also indicates that no more heat is being conducted into the cavity. Although cooling continues throughout the entire injection cycle, the cooling stage is usually recognized as the period between the end of the packing stage and the beginning of part ejection.

When the gate is frozen, the screw can begin rotating and plasticating the melt for the next injection cycle. Thus, one injection cycle (Figure 1.3) includes mold clos-ing, mold filling, mold packing, mold cooling, and mold opening. Plastication be-gins during the mold cooling stage and may last until mold opening or even before the end of mold closing of the next cycle. Plastic shrinks as it cools from the melt temperature to a solid state. The relationship between pressure, temperature, and specific volume (discussed later), as well as the thermal-mechanical history of material elements, determine the volumetric shrinkage of the parts. During the injection molding process, pressure and temperature gradients exist within the melt and the solidified parts, resulting in non-uniform part shrinkage and associ-ated warpage, which are important issues to be resolved and controlled within acceptable limits during the molding process.

Figure 1.3 Injection molding cycle

1.1.1�Brief Overview of an Injection Molding Machine

The most popular type of injection molding machine at present is the reciprocating screw injection machine, prototypes of which first appeared in the 1940s and 1950s. The two essential components of an injection molding machine are the injection unit and the clamping unit, whose operation relies on the hydraulic or electrical servomotor system and the associated control system.


1.1.2�Machine Setup for Process Conditions

The major parameters of the injection molding process include the injection pres-sure, injection speed, melt temperature, and mold temperature, any of which may vary with time at different stages in the molding process. The settings of these parameters can be achieved via the control system, which actuates the hydraulic or servomotor devices, injection unit, clamping unit, and other heating/cooling elements.

1.1.2.1�Injection PressureThe dynamic pressure development during an injection cycle at different locations in the machine and cavity is illustrated in Figure 1.4 [4]. There is a delay in the pressure development between the hydraulic pump and the injection cylinder. The delay depends primarily on the function of the hydraulic valve and the circuit de-sign. The pressure profile in the hydraulic cylinder is basically similar to the pres-sure profile in the screw tip. The difference between them results from the differ-ence in friction between the screw and the barrel, and the friction between the hydraulic cylinder and the piston. Pressure losses range from roughly 3–10%, and the higher the pressure, the lower the loss. The pressure at the front of the screw tip is the pressure that actually pumps the melt into the mold cavity. This pressure is usually not readable from the control panel unless a pressure transducer is em-bedded in the screw tip. This pressure value is also the pressure predicted in com-puter-aided engineering (CAE) simulations. The employed force at the front of the screw tip is usually 10 to 14 times the force in the hydraulic cylinder, depending on the ratio of the cross section areas of the hydraulic cylinder to that of the screw. Within the mold cavity, there is little relevance between the pressure profile either near the gate or near the end of the flow path and the pressure profile in the run-ner or in the screw tip. This is because the pressure loss mainly depends on the cavity geometry and the flow characteristics when the melt flows through the rela-tively thin walled cavity. In a regular injection filling stage, the screw forward speed is profiled and the required hydraulic pressure will be built up to maintain the profiled speed unless the hydraulic pressure reaches the maximum value that the machine can supply. In the packing/holding stage, screw advancement is basi-cally driven and determined by the profiled pressure. Normally, the hydraulic pres-sure and screw tip pressure will reach their peak values during the filling-to-pack-ing switchover. Soon after the peak pressure value is reached, the pressure begins to drop because the melt is cooling down. Once the gate is frozen, no more melt (and no more heat) is pushed into the cavity and the pressure decreases even more rapidly. The deflection point in the pressure drop is usually identified as the time when the gate is completely frozen. The magnitude and duration of the packing/holding pressure strongly affect the dimensional accuracy and cosmetic quality of the part. They also affect the residual stress and the associated warpage.

51.1 Injection Molding and Molding Machines

Figure 1.4 Pressure histories at different locations in the machine and mold cavity [4]

1.1.2.2�Melt TemperatureThough plastic pellets become melt due to the shear heating resulting from the rotation of the reciprocating screw during the plasticating stage, the heating ele-ment of the barrel can fine-tune the melt temperature. It is common for the barrel temperatures to be gradually increased as the melt moves from the feeding zone to the compression zone and to the metering zone. The plastic material supplier will usually provide recommendations for the range of temperature settings for each zone. Finally, the nozzle temperature is set to be equal to the desired melt tempera-ture. The recommended melt temperatures generally fall within a window of 30 to 40 °C. The higher the melt temperature, the lower the viscosity and also the lower the required injection pressure. High melt temperatures frequently lead to longer cooling times and sometimes cause degradation in the plastic material.

1.1.2.3�Injection SpeedThe injection speed is inversely proportional to the filling time. If melt is injected at very high speeds, the high shear rate and the associated high shear stress will result in high injection pressure. If the injection speed is too slow, the melt will cool significantly, thus increasing the viscosity. Hence, the required injection pres-sure also increases. There is usually an optimum filling time corresponding to the lowest injection pressure. The U-curve-like variation of pressure with filling time (Figure 1.5) enables molding operators to choose the optimal filling time. During the cavity filling stage, when pressure loss determined by taking measurements near the gate area and the end of the flow path increases with decreasing injection speed, this reflects the cooling of the melt and the formation of a solidified skin layer near the cavity surface.


Figure 1.5 Injection pressure needed for different fill times

1.1.2.4�Mold TemperatureCoolants circulate in the cooling channels to maintain the cavity surface tempera-ture. Cavity surface temperature is determined by coolant temperature, coolant thermal and rheological properties, coolant flow rate, and the corresponding cool-ing channel dimensions and layout. The mold materials, mold size, and the dura-tion of melt within the cavity, as well as factors such as initial melt temperature, ejection temperature, and part thickness, may also influence the cavity surface temperature. During molding operation, the heat from the hot melt flow should be approximately equal to the heat removed by the coolants. Thus, the coolant flow rate in each branch of the cooling channel should have a high Reynolds number to ensure that the flow is turbulent. Cavity surface temperature is not only intimately related to the cooling time but also significantly affects the part quality. Mold temperature varies in a transient cyclic manner (Figure 1.6) and also varies from location to location within the mold. Thus, the design of cooling channels to achieve uniform cooling and to maintain a relatively constant mold temperature is crucial.

Figure 1.6 Mold temperature variation versus injection cycle [5]

71.2 Characterization of the Injection Molding Process

1.1.2.5�Other ParametersIn addition to the parameters stated above, other parameters such as back pres-sure, speed of screw rotation, and mold clamping/opening speed must be properly set for a smooth molding operation. Mechanical operations such as mold clamping, alignment, and pressure buildup between the sprue bushing and the nozzle, as well as safety measures, are also important for successful molding.

�� 1.2� Characterization of the Injection Molding Process

1.2.1�Injection Cycle

The injection cycle consists of the mold closing, injection (mold filling), packing/holding, cooling, and mold opening stages. Ejection occurs during the mold open-ing period. Plastication usually begins at the end of the holding stage and finishes during the mold cooling stage. For typical parts, the cooling stage takes about two thirds of the total cycle time. In the beginning, the mold is cold and the first twenty or more shots warm up the mold. At this stage, the heat transport from the hot melt is greater than the heat removed by the coolants due to the small temperature difference between the cooling channel surface and the coolants. As the mold temperature rises, the difference becomes greater, and the heat removal rate also increases. When the heat entering the mold from the hot melt is equal to the heat transferred to the coolant, the injection cycle reaches a cyclic steady state. After that, the mold temperature fluctuates slightly within each cycle. Stable mass pro-duction can then be performed provided that the ambient temperature and the ma-chine conditions can be maintained. The final molded part properties are deter-mined by the filling, packing/holding, and cooling stages. In addition to proper machine setup, understanding of melt processing properties such as viscosity and shrinkage, as well as appropriate mold design, are equally important for achieving high-quality parts.

1.2.2�Mold Filling Stage

1.2.2.1�Process OverviewThe mold filling stage (phase) begins when the screw moves forward and pushes the melt into the mold. It ends when the cavity is filled with the melt. Screw ad-vancement is controlled by a profiled velocity in normal molding operation unless the set injection pressure is insufficient to drive the screw at the profiled velocity.


If the required injection pressure exceeds the set pressure in the machine, then the screw will be driven by the set pressure and the velocity becomes a priori unknown and generally slows down. The injection speed or screw forward speed determines the volumetric flow rate and the filling time of the cavity. In most oper-ations, the injection speed begins at a lower velocity, then accelerates to a more rapid velocity during most of the filling stage, and finally reduces to a slower veloc-ity near the end of filling to enable the switchover to packing stage to be smoothly performed. Since a fixed amount of melt should be pushed into the mold, the oper-ation should be accompanied by a preset screw traveling distance (injection stroke). Generally speaking, injection pressure, melt temperature, velocity profile, and injection stroke are the most important mold filling operation parameters for the injection molding process.

1.2.2.2�Flow Velocity and Melt Front AdvancementThe velocity of the melt front advancement, although proportional to the injection speed, varies with the geometry of the mold. Figure 1.7 depicts this situation. At any given time, the melt front velocity times the melt front area (i. e., melt front length times the cavity thickness) is equal to the melt volumetric flow rate. Even if the screw forward speed, the associated volumetric flow rate, and the part thick-ness are kept constant, the melt front advancement velocity will change as the melt front area changes. The melt front velocity becomes very high when the melt first enters the cavity since the melt front length is small. The velocity also be-comes very high when filling the last portion of the cavity, where the melt front length again becomes small. Generally, the melt front velocity slows around the halfway point of melt filling. This is why injection speed should be reduced when the melt flows through the gate area and at the end of filling. It is important to keep the melt front velocity as constant as possible throughout the entire filling process. An improper or non-constant melt front velocity sometimes results in flow marks and uneven molecular orientation and/or fiber orientation.

Figure 1.7 Melt front advancement and the associated melt-advancing velocity

It is also important to examine the gapwise flow velocity. Near the cavity entrance, hot melt continuously streams into the mold. As a result, the frozen layer is thin. Furthermore, because some of the melt contacts the cooler cavity surface, a solidi-


fied layer forms near the cavity surface. The slower the melt velocity, the thicker the frozen layer. At the melt front area, the fountain flow effect dominates the hot melt and the melt is turned over from the gap center to the cavity surface [4]. Therefore, the frozen layer is also thin at the melt front area. In addition, the gap-wise velocity is roughly uniform in the melt front area due to this effect. Behind the melt front, about a few times the gapwise thickness away, a laminar velocity profile will develop with a maximum velocity at the gap center and a zero velocity (nonslip condition) at the interface of the frozen layer. A schematic of this is shown in Figure 1.8. Accompanying these velocity characteristics, polymer molecules are also subjected to different amounts of orientation throughout the gapwise direc-tion, which are described later, that can result in differential shrinkage and/or variable properties throughout the part. A typical shear rate curve associated with this gapwise velocity profile can be seen in Figure 1.9.

Figure 1.8 Gapwise melt velocity profile and fountain flow at the melt front [4]

Figure 1.9 A typical shear rate profile along the gapwise direction

1.2.2.3�Pressure Variation and DistributionAs melt flows into the cavity, the pressure required to maintain the screw velocity will increase with time. Provided that the melt temperature during the filling pro-cess remains unchanged, the pressure gradient required for melt advancement in a constant thickness cavity is also roughly constant. The pressure is then linearly proportional to the flow path length. The pressure loss in the runner system should be kept as low as possible; typically, less than 20% of the injection pressure is a


common rule of thumb. The injection pressure is also sensitive to the thickness variation; the thinner the wall, the higher the required pressure. The entrance has the highest pressure while the pressure around the melt front is usually assumed to be identical to atmospheric pressure if the venting design works properly. When-ever there is a hesitation in the flow (e. g., due to rapid cooling and restrictive flow), pressure will build up around that area. If possible, the gating system should be designed to achieve a pressure distribution which is as uniform as possible to minimize non-uniform shrinkage. The influence of gate design on pressure distri-bution is illustrated in Figure 1.10.

Filling time with poor gate design

Pressure distribution with poor gate design

Filling time of better design with gate moved

Pressure distribution of better designwith gate moved

Figure 1.10 Filling time and pressure distribution with different gate locations

1.2.2.4�Melt Temperature Variation and DistributionAs mentioned earlier, once the melt enters the mold it is subjected to cooling due to the cold cavity surface temperature. Besides the influence of the mold tempera-


ture, flow-induced convection heating and shear heating are also important factors affecting melt temperature distribution. Whenever possible, the melt flow should be balanced to avoid local flow hesitation. Because of the hesitation, the bulk tem-perature difference may be as high as 136 °C (see Figure 1.11). Hence, the bulk temperature distribution of the melt at the end of filling should be limited to 15 °C to minimize non-uniform shrinkage and warpage.

Temperature distribution of poor design Temperature distribution of better design with gate moved

Figure 1.11 Temperature distribution with different gate locations

At very high flow rates, or when the melt flows through a very narrow geometry, the shear heating effect may be significant and may lead to a rise in the melt tem-perature. The shear heating effect mainly depends on the shear rate value and may result in a non-uniform distribution in the gapwise direction. This may cause a great temperature gradient near the cavity surface and possible melt degradation. A typical temperature profile across the gapwise direction can be seen in Figure 1.12. A non-uniform temperature profile in the gapwise direction also contributes to a build-up in thermal residual stress.

Figure 1.12 Schematic of the temperature profile across the gapwise direction


1.2.2.5�Shear Stress and Shear RateShear stress can be considered as a supplemental measure of pressure and flow velocity. High shear stress will introduce extensive molecular orientation and flow-induced stress. This will lead to high residual stress if the melt is cooled to a temperature below its freezing temperature. Shear stress exhibits its highest value along the cavity surface or wall of the solidified layer. The shear rate is a measure of the velocity gradient in the gapwise direction. It is usually zero near the gap center and within the frozen layer on the cavity wall but exhibits a maximum near the interface of the frozen layer. At the end of the melt filling stage, the melt stops flowing and relaxation begins. However, because of the non-uniform melt temperature and uneven heat transfer gapwise, the residual flow-induced stress also exhibits a non-uniform nature; hence, the maximum shear stress exists some-where between the gap center and the cavity wall, usually closer to the cavity wall at the end of the cooling process. A typical curve via simulation is illustrated in Figure 1.13 [6].

Figure 1.13 Typical shear stress curve at different stages across gapwise direction ( dimensionless gapwise position Z/b, where Z is the height and b is the half thickness) [6]

1.2.3�Mold Packing/Holding Process

1.2.3.1�Process OverviewThe packing/holding stage begins when the cavity is completely filled. The screw should be switched over from being velocity-driven to being pressure-driven by this stage. Because of the compressibility and cooling of the polymer melt, the cav-ity can be filled with more melt at the end of the filling process. This should be carried out under low flow velocity and can be controlled more easily via pressure operation. Proper switchover is important in molding operation. A late switchover


will induce an instantaneous pressure increase and cause flashing problems. An early switchover may reduce the injection speed dramatically during the later stage of the filling process and introduce other molding problems, such as hesita-tion or excessive shrinkage. The switchover situation can be identified from the cavity pressure profile, as illustrated in Figure 1.14.

1.2.3.2�Packing Pressure Variation and DistributionWhen the packing process begins, the cavity pressure will rise to roughly the same peak pressure unless the flow path is long and the pressure loss is large (because the part has thin walls or there is significant melt cooling). Within a very short period of time, the melt is compressed into the cavity due to its compressibility. Soon after this period, the screw can push more melt only when the compressed melt has cooled and shrinks, enabling the cavity to allow more room for the melt to flow in. This stage is known as the holding stage. In the later part of the holding stage, and after it has ended, the cooling effect becomes obvious and the shrink-age-induced melt flow will meet great resistance (great pressure loss). Thus, the cavity pressure will drop faster at the end of the flow path than nearer the gate area. In the later part of the holding stage, most of the melt is squeezed around the gate area, which may result in high residual stress in the part. This is why the holding pressure is usually profiled with a lower holding pressure setting in the second stage. The packing/holding pressure affects part dimensions significantly and should be carefully selected.

Figure 1.14 Cavity pressure profile at various filling-to-packing switchover points [4]

1.2.3.3�Packing TimeThe packing/holding process ends when the gate is completely frozen. This can be estimated based on the gate dimensions. If the machine setting for packing time is too long, then the melt will be squeezed into the runners, resulting in overweight

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Advanced Injection Molding Technologies