Microcellular Injection Molding E-Book

Jingyi Xu

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Sprache: Englisch

Veröffentlichungsjahr: 2011

Beschreibung

This book presents the most important aspects of microcellular injection molding with applications for science and industry. The book includes: experimental rheology and pressure-volume-temperature (PVT) data for different gas materials at real injection molding conditions, new mathematical models, micrographs of rheological and thermodynamic phenomena, and the morphologies of microcellular foam made by injection molding. Further, the author proposes two stages of processing for microcellular injection molding, along with a methodology of systematic analysis for process optimization. This gives critical guidelines for quality and quantity analyses for processing and equipment design.

Leseprobe

Table of Contents

Cover

Table of Contents

Half title page

Series page

Title page

Copyright page

FOREWORD

PREFACE

1 INTRODUCTION

1.1 HISTORY OF MICROCELLULAR PLASTICS

1.2 ADVANTAGES AND APPLICATIONS OF MICROCELLULAR PLASTICS

1.3 PATENTS AND PUBLICATIONS COVERING MICROCELLULAR INJECTION MOLDING TECHNOLOGY

1.4 OUTLINES OF THE BOOK

2 BASICS OF MICROCELLULAR INJECTION MOLDING

2.1 BASIC PROCEDURES OF MICROCELLULAR INJECTION MOLDING

2.2 SUPERCRITICAL FLUIDS (SCF)

2.3 GAS SOLUBILITY AND DIFFUSION CAPABILITY IN POLYMER MELT

2.4 NUCLEATION OF CELLS

2.5 CELL GROWTH

2.6 SHAPING IN THE MOLD

3 MORPHOLOGY OF MICROCELLULAR MATERIALS

3.1 MORPHOLOGY DIFFERENCES BETWEEN BATCH PROCESS AND INJECTION MOLDING PROCESS

3.2 MORPHOLOGIES OF DIFFERENT MATERIALS

3.3 CHARACTERIZATION OF CELL ARCHITECTURE

3.4 INFLUENCES OF CELL ARCHITECTURE ON MICROCELLULAR QUALITY

3.5 OTHER SPECIAL CELL STRUCTURES

3.6 CONCLUSIONS

4 MATERIALS FOR MICROCELLULAR INJECTION MOLDING

4.1 STRUCTURE AND CHARACTERISTICS OF POLYMERS

4.2 CRYSTALLINE MATERIALS

4.3 AMORPHOUS MATERIALS

4.4 FILLER-FILLED MATERIALS

4.5 FIBER-REINFORCED MATERIALS

4.6 NANOCOMPOSITE-REINFORCED MATERIALS

4.7 BLEND MATERIALS AND COMPOUNDS

4.8 METAL POWDER

4.9 BIOPOLYMERS

5 DESIGN OF MICROCELLULAR INJECTION MOLDING

5.1 PART DESIGN

5.2 MOLD DESIGN

5.3 MATERIAL PROPERTIES VERSUS WEIGHT REDUCTION

5.4 SURFACE QUALITY IMPROVEMENT FROM MOLD AND PART DESIGNS

6 PROCESS FOR MICROCELLULAR INJECTION MOLDING

6.1 GAS DOSING

6.2 GAS MIXING AND DIFFUSING

6.3 NUCLEATION AND INITIAL CELL GROWTH

6.4 MOLD FILLING ANALYSES

6.5 COMPARISON WITH GAS-ASSIST MOLDING

6.6 COMPARISON WITH STRUCTURAL FOAM MOLDING

6.7 COMPARISON WITH REGULAR INJECTION MOLDING

6.8 COMPARISON WITH MICROCELLULAR EXTRUSION

6.9 COMPARISON WITH MICROCELLULAR BLOWING MOLDING

7 EQUIPMENT AND MACHINES FOR MICROCELLULAR INJECTION MOLDING

7.1 TWO STAGES FOR MICROCELLULAR INJECTION MOLDING

7.2 RECIPROCATING SCREW INJECTION MOLDING MACHINE

7.3 EXTRUDER WITH INJECTION PLUNGER MACHINE

7.4 SCF DELIVERY SYSTEM DESIGN

7.5 SINTER METAL SLEEVE OF GAS DOSING (OPTIFOAM®)

7.6 DYNAMIC MIXER OF GAS DOSING (ERGOCELL®)

7.7 GAS FEED IN A SEALED HOPPER FOR GAS DOSING (PROFOAM®)

7.8 RETROFIT MACHINE FOR MICROCELLULAR INJECTION MOLDING

7.9 GAS DOSING MIXER OF LIQUID SILICON

7.10 ACCESSORIES FOR MICROCELLULAR INJECTION MOLDING

8 SPECIAL PROCESSES

8.1 CO-INJECTION (SANDWICH) MOLDING FOR MICROCELLULAR PART

8.2 GAS COUNTERPRESSURE

8.3 OVERLAPPING (ALSO CALLED OVERMOLDING)

8.4 REVERSAL COINING (ALSO CALLED EXPANDABLE MOLD OR BREATHING MOLD)

8.5 A PROCESS WITH THE COMBINATION OF OVERMOLDING AND REVERSAL COINING

8.6 HOT AND COLD PROCESS (ALSO CALLED ALTERNATING MOLD TEMPERATURE, THERMAL CYCLING)

8.7 SUPER-MICROCELLULAR (OR ULTRA-MICROCELLULAR) WITHOUT WEIGHT REDUCTION

8.8 LOWEST GAS DOSAGE WITH MINIMUM FOAMING

8.9 CHEMICAL BLOWING AGENT IN MICROCELLULAR FOAM

8.10 WATER BLOWING AGENT

8.11 STRESS FOAMING

8.12 MICROPOROUS METAL PART

8.13 LOCAL MICROCELLULAR FOAM

8.14 THIN-WALL MICROCELLULAR FOAM

9 MODELING OF MICROCELLULAR INJECTION MOLDING

9.1 RHEOLOGY AND PVT DATA FOR GAS ENTRANCED MATERIAL

9.2 MOLDFLOW MODELING OF MICROCELLULAR INJECTION MOLDING [7–9, 17–20, 25, 26]

9.3 SIMPLE MODELING OF MICROCELLULAR INJECTION MOLDING

9.4 MOLD-FILLING SIMULATION GUIDELINES FOR MUCELL® PROCESS

10 POSTPROCESSING AND PROPERTY TEST OF MICROCELLULAR INJECTION MOLDING

10.1 WELDING FOR MICROCELLULAR INJECTION MOLDING

10.2 SURFACE POLISH AND PAINTING

10.3 POST-COOLING

10.4 DE-GASSING PROCESS

10.5 PROPERTY TEST FOR MICROCELLULAR PARTS

11 MARKETS AND APPLICATIONS OF MICROCELLULAR INJECTION MOLDING

11.1 MARKET ANALYSES FOR MICROCELLULAR INJECTION MOLDING PRODUCTS

11.2 TYPICAL APPLICATIONS: CASE STUDIES

11.3 FUTURE RESEARCH TOPICS AND POTENTIAL APPLICATIONS

12 COST SAVINGS FOR MICROCELLULAR INJECTION MOLDING

12.1 COST SAVINGS OF MATERIAL

12.2 MOLD

12.3 EQUIPMENT

12.4 PROCESS

12.5 DIMENSIONAL STABILITY

12.6 PROPERTY IMPROVEMENT FOR MICROCELLULAR FOAM

12.7 ANALYSIS OF RETURN ON INVESTMENT (ROI)

12.8 COMPARISON OF THE COSTS WITH DIFFERENT PROCESS METHODS

12.9 CASE STUDY OF ENERGY SAVING

NOMENCLATURE

LOWERCASE ROMAN CHARACTERS

UPPERCASE ROMAN CHARACTERS

LOWERCASE GREEK CHARACTERS

UPPERCASE GREEK CHARACTERS

APPENDIX A (CHAPTER 7)

PRESSURE DROP RATE dp/dt FORMULA

APPENDIX B (CHAPTER 7)

CLAMP LOAD W VERSUS SUPPORT DISTANCE L FOR THE PLATENS AT THE SAME DEFLECTION

APPENDIX C (CHAPTER 5)

TENSILE STRENGTH RATIO OF FOAM TO SOLID

APPENDIX D (CHAPTER 5)

REAL WEIGHT REDUCTION CALCULATION

APPENDIX E (CHAPTER 5)

FLEXURAL STRENGTH RATIO OF FOAM TO SOLID

APPENDIX F (CHAPTER 6)

RELATIONSHIP BETWEEN UM AND UT

APPENDIX G (CHAPTER 9)

VISCOSITY MODEL FOR ANNULAR CHANNEL OF NOZZLE RHEOMETER

APPENDIX H (GLOSSARY)

APPENDIX H (GLOSSARY)

Index

MICROCELLULAR INJECTION MOLDING

Copyright © 2010 by John Wiley & Sons, Inc. All rights reserved

Published by John Wiley & Sons, Inc., Hoboken, New Jersey

Published simultaneously in Canada

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Library of Congress Cataloging-in-Publication Data:

Xu, Jingyi.

Microcellular injection molding / Jingyi Xu.

p. cm. – (Wiley series on plastics engineering and technology)

Includes index.

ISBN 978-0-470-46612-4 (cloth); 978-1-118-05787-2 (ebk)

 1. Molding (Chemical technology) 2. Foamed materials. 3. Microfluidics. I. Title.

TP156.M65X83 2010

668.4′12–dc22

2010004436

FOREWORD

It is a great privilege to write the Foreword for a book that provides so much detailed and insightful information about microcellular injection molding technology. Injection molding is one of the most versatile and important polymer-processing methods for mass production of complex plastic parts. In addition to thermoplastics and thermosets, injection molding has been extended to such materials as fibers, ceramics, and powdered metals, with polymers as binders. Among polymer-processing methods, injection molding accounts for one-third by weight of all polymeric materials processed. New variations and emerging innovations of conventional injection molding have been continuously developed to further extend the applicability, capability, flexibility, productivity, and profitability of this versatile, mass production process. These special and emerging injection molding processes introduce additional design freedom, new application areas, unique geometrical features, enhanced part strength, and sustainable economic benefits, as well as improved material properties and part quality that cannot be accomplished by conventional injection molding processes. Among these special and emerging techniques, microcellular injection molding holds great promise for improving process economics while enhancing the characteristics and functionality of molded parts.

The author, Mr. Jingyi Xu, is a world-renowned expert in microcellular injection molding due to his early and continuous involvement in the developments of this novel process that originated from the work of Dr. Suh and his colleagues at MIT in the early 1980s. The potential benefits of microcellular injection molding go beyond the apparent and attractive savings of material, cycle time, and energy, as it also greatly boosts dimensional stability and facilitates part consolidation and innovation. A wide range of commodity and engineering resins ranging from amorphous, semicrystalline, and thermosetting materials to thermoplastic elastomers and bioplastics have been successfully used with the microcellular injection molding process.

While there are many publications and patents related to new developments and applications of microcellular injection molding, there remains a huge gap between process fundamentals and real-world applications. This book bridges this gap by providing comprehensive coverage in a format that is easy to understand and apply. It also touches on a wide range of subjects, including historical developments, fundamental principles, applicable materials, design guidelines, process comparisons, equipment design, modeling, material properties, and commercial applications. I admire the painstaking commitment and sheer effort of Mr. Xu in making this book a reality. His contribution to the illumination and dissemination of process know-how and guiding principles will be applicable for years to come. This book will undoubtedly assist readers of various backgrounds and levels of expertise to better understand, implement, and benefit from this novel and promising technology.

LIH-SHENG (TOM) TURNG

Polymer Engineering Center

University of Wisconsin—Madison

PREFACE

Microcellular polymers can replace solid polymers with 5% or more material reductions without compromising a significant amount of material properties. In addition, this technology yields even more benefits of microcellular foam such as dimensional stability, short cycle time, elimination of sink marks and warpage, and stress-free parts. Therefore, microcellular injection molding provides a revolutionized way to save materials, and protect the environmental. Consequently, microcellular injection molding has become the fastest developing technology in all microcellular processes. However, most published papers and books on foaming are not often concerned with the practical aspects of applying this technology. This book is intended to bridge these gaps and provides everybody engaged in the design, research, and professional training with a reference book that covers the design and production of microcellular processing in a comprehensive manner.

This book takes the available respected literature into account as well as the real results from extended research and development works in the world. It is my intent to include sufficient detailed discussions for the student pursuing, or just beginning to pursue, a career in the broad microcellular processing arena.

The greatest appreciation should go to everyone who worked hard for this technology. I thank Professor Lih Sheng Turng for reviewing part of my book and for providing the Foreword for this book. Special thanks also go to Professor Chul B. Park for providing valuable information; Levi Kishbaugh for providing courtesy information from Trexel Inc.; and Peter Kennedy, Sejin Han, and Xiaoshi Jin for simulation information, My co-worker Ben Keur read this manuscript and offered excellent opinions, and many others provided their expertise.

I wish to express my appreciation to editor Jonathan T. Rose at John Wiley & Sons, Inc., particularly for his initial idea to write this book and for his splendid efforts during this difficult time to publish this book.

Additionally, I thank my daughter Jiayun Xu as a first general reader with many excellent suggestions for readability of this book. My wife Jufen Guan also gave me unconditional support, as always.

JINGYI XU

Engel Machinery

York, Pennsylvania

1

INTRODUCTION

1.1 HISTORY OF MICROCELLULAR PLASTICS

Historically, microcellular plastics are not new: They existed more or less in the thin transition layer of structural foams. It can be found partially in sections with thin thickness, as well in the high shearing zone of structural foam parts. However, as an idea to develop microcellular plastics, Dr. Nam Suh and his students at the Massachusetts Institute of Technology invented micro­cellular processing in the early 1980s. This technology proposes two goals: One is to reduce the material, and another is to promote the material toughness by tiny spherical cells that act as crack arrestors by blunting the crack tip [1]. Furthermore, the rigidity of the material in resisting the buckling of the cell walls has been improved through the formation of spherical closed cells. Concentrated research and development efforts of microcellular foams began in the late 1980s, with a focus on the batch process and the topics mentioned above.

The microcellular batch processing technology was invented at the Massachusetts Institute of Technology (MIT) from 1980 to 1984 [1], and the first U.S. patent on microcellular technology was issued in 1984 [2]. Jonathan Colton showed a heterogeneous nucleation mechanism from the effects of additives in the polymers at certain levels of solubility [3]. Jonathan Colton also investigated the methodology of foaming for semicrystalline polymers such as polypropylene (PP) [4]. The gas can be dissolved into the amorphous structure because raising the temperature beyond its melting point eliminates the crystalline phase of PP. This heterogeneous nucleation is now dominating today’s industry processing. On the other hand, the crystalline material, such as PP, has been used for microcellular foam by Jonathan’s method in the industry practice now. Chul Park and Dan Baldwin studied the continuous extrusion of microcellular foam. Chul Park investigated both (a) the dissolution of gas at the acceptable production rate and (b) the application of a rapid pressure drop nozzle as the nucleation device [5]. Dan Baldwin studied the microcellular structure in both crystalline and amorphous materials [6]. Sung Cha investigated the application of supercritical fluid, such as CO2, to dissolve the gas faster and to create more cells [7, 8]. With supercritical fluid, the cell density was increased from 109 cells/cm3 to 1015 cells/cm3. Vipin Kumar also used thermoforming supersaturated plastic sheets to study the issues of shaping three-dimensional parts [9]. Sung Cha also found that the large volume of gas in polymers decreases significantly with the glass transition temperature of plastics. Therefore, simultaneous room temperature foaming is possible. All of these pioneer contributions are fundamental to microcellular foam technologies. Through many people’s creative research, this technology has completed the laboratory stage and transitioned to industry application.

The commercial application of microcellular technology began in 1995 by Axiomatics Corp., which was later renamed Trexel Inc. Trexel continued to develop microcellular technology through extrusion first. Then, the first injection molding machine with plunger for injection and extruding screw for plasticizing and gas dosing was developed in Trexel Inc. with the help from Engel Canada in mid-1997. After successful microcellular injection molding trials were carried out in this plunger-plus-extruder injection molding machine, the first reciprocating screw injection microcellular molding machine was built by Trexel and Engel together in 1998 [10]. This machine marks the milestone of the commercialization of microcellular injection molding and is now the most popular microcellular injection molding machine in the world. Trexel also modified a Uniloy Milacron machine to the first microcellular blow-molding machine in 2000.

One important term, supercritical fluid, is abbreviated as SCF. SCF is the name of the state condition of a gas when the gas is above both its critical pressure and critical temperature; this is discussed in more detail in Chapter 2. It is critical to use SCF to describe a gas if the gas is at a supercritical state. Otherwise, use the general term, gas, if the gas is at any condition from normal atmospheric to supercritical state. Unless otherwise specified, the term of SCF and gas will be used with the conditions above in the entire book.

The injection molding aspect of microcellular foam processing has developed the fastest. The main developed technologies of microcellular injection molding are listed in Table 1.1. The most popular trade name for this technology is MuCell® and is licensed by Trexel Inc. since 2000 (MuCell® is a Registered Trademark of Trexel Inc., Woburn, Massachusetts). Several other injection molding companies and research groups in the world were developing this technology prior to Trexel’s announcement of MuCell®. However, they did not finish the commercialization of their technologies for real applications. The MuCell® technology uses a reciprocating screw as the SCF dosing element, and the SCF is injected into the reciprocating screw through the barrel. It makes full use of the shearing and mixing functions of the screw to quickly finish the SCF dosing and to maintain the minimum dosing pressure in the barrel and screw for the possible continuing process of microcellular injection molding. In addition, two other trade names of this technology were found later on: (a) Optifoam® licensed by Sulzer Chemtech [11] and (b) Ergocell® licensed by Demag (now Sumitomo-Demag in 2008) [12]. Optifoam® is a microcellular technology that uses a nozzle as the SCF dosing element. It is a revolutionary change to the traditional SCF dosing method, which adds gas into the barrel. This unique, innovative idea has a special nozzle sleeve made of sintered metal with many ports to let gas go through as tiny droplets. On the other hand, the melt flow through the nozzle is divided into a thin film between the nozzle channel and the sintered metal sleeve. As a result, the gas can diffuse into the melt in a short amount of time. The gas-rich melt is then further mixed in a static blender channel that is located in the downstream of the nozzle dosing sleeve. The advantage of this technology is that the regular injection screw and barrel do not need to be changed. The regular injection molding machine in existence can be easily changed to use the Optifoam® process. However, only some of these applications have been successful [11]. At K2001, Demag Ergotech introduced its Ergocell® cellular foam system [12]. Ergocell® technology has reached an agreement with Trexel to have their customers pay a reduced price to the MuCell® license when using Ergocell® technology legally. The Ergocell® system is essentially an assembly of an accumulator, a mixer, a gas supply, and a special injection system that is mechanically integrated between the end of the barrel and the mold to put gas into the polymer and create the foam upon injection into the mold. A special assembly needs to be created for each screw diameter. Additional hydraulic pumps and motor capacity must be added to operate the mixer and accumulator injection system. The system only uses carbon dioxide as the blowing agent.

TABLE 1.1 Main Developed Microcellular Injection Molding Technologies

Type of TechnologyTrade NameCommentMicrocellular plasticizing unit with special reciprocating screw and barrel to carry out the SCF dosing and injection.MuCell®Most popular technology was developed by Trexel, Inc., and has been widely applied worldwide.Microcellular equipment with special nozzle sleeve for SCF dosing; regular reciprocating screw for injection.Optifoam®It was developed by IKV and has been commercialized by Sulzer Chemtech. There are some applications worldwide.Microcellular dynamic mixer for SCF dosing plus plunger for injection, later modified with reciprocating screw for injection.Ergocell®It was developed by Sumitomo-Demag; it has not been common usage on the market yet.Microcellular equipment with special gas dosing unit in hopper of the regular reciprocating screw for injection.ProFoam®It has been invented and tested fully by IKV, and it is still is in the development stage.Microcellular extruder for SCF dosing plus plunger for injection.NoneIt was developed by Trexel and Engel in 1997, and it is not available on market yet.

The latest developing foam technology from IKV is the ProFoam® process [13]. It is a new and cheap means of physically foaming injection molding technology. The gas, either carbon dioxide or nitrogen, as the blowing agent is directly added into the hopper and diffuses into the polymer during the normal plasticizing process. The plasticizing unit of the molding machine is sealed off in the feeding section of screw for gas adding at pressure, but feeding of pellets of material occurs at normal conditions without pressure. With this ProFoam® process the part can reduce up to 30% weight via the foaming.

Trexel continues to develop and support the microcellular injection molding process worldwide. There are already over 300 MuCell® injection microcellular molding machines in the world. Through the efforts of many more organizations, more and more advances are being made for the microcellular injection molding process. These organizations include not only original equipment manufacturers (OEMs) licensed from Trexel but also numerous unlicensed organizations, such as universities, and university/industry consortia. All of them are contributing to further advances in microcellular technology.

1.2 ADVANTAGES AND APPLICATIONS OF MICROCELLULAR PLASTICS

The microscopic cell size and large number of cells in microcellular material can reduce material consumption as well as improve the molding thermodynamics, which results in a quicker cycle time. Additionally, the process is a low-pressure molding process and produces stress-free and less warped injection molding products. The major differences between conventional foam and microcellular foam are cell density and cell size. The typical conventional polystyrene foam will have an average cell size of about 250 microns, and a typical cell density in the range of 104–105 cells/cm3. Microcellular plastic is ideally defined with a uniform cell size of about 10 µm and with a cell density as high as 109 cells/cm3 [1]. It is possible to make this kind of microstructure cell density with microcellular injection molding if material and processing are controlled very well. The scanning electron microscope (SEM) morphology of glass-fiber-filled PBT is an excellent example of microcellular injection molding that almost matches the ideal definition of microcellular plastics made by batch process. It is made by using 30% glass fiber and reinforced polybutylene terephthalate (PBT) with a 15% weight reduction (see Chapter 3, Figure 3.12). The cell density is about 8 × 108 cells/cm3, with an average of 15 µm of uniform cell distribution. However, this microstructure is not always the result of microcellular injection molding. The SEM picture in Figure 1.1 is a more typical microcellular unfilled polystyrene foam made by injection molding that has an average of 25 microns, and has a cell density of about 8.1 × 107 cells/cm3. The microstructures of industrial parts from microcellular injection molding are characterized by an average cell size on the order of 100 µm, although the real cell size can be varied from 3 µm to 100 µm. However, the cell structure of the microcellular part with microcellular injection molding might not necessarily be defined as the cell density of 109 cells/cm3. The microstructure of ABS has a cell density of about 106 cells/cm3, and it definitely shows a microcellular structure with an average cell size of about 45 µm. The comparisons of average cell sizes between microcellular foam and conventional foam are summarized in Table 1.2. The data in Table 1.2 show that the minimum cell size of conventional foam is about the same size as the maximum cell size of microcellular foam; the maximum cell size of conventional foam is about twice as large as the maximum cell size of microcellular foam. Usually the cell density of the conventional foam is about 102 to 106 cells/cm3. However, the cell density of the microcellular foam is 106 cells/cm3 or higher.

TABLE 1.2 Comparisons Among Conventional Foam, Microcellular Foam, and Regular Solid

aNA, not available.

Figure 1.1 Morphology of polystyrene microcellular foam (white bar indicates 100 µm). Average cell size: 25 µm. Cell density: 8.1 × 107 cells/cm3.

The cell size in the foam mainly determines the property differences between conventional foam and microcellular foam. Table 1.1 shows the comparisons among injection molding parts made by conventional foam, micro­cellular foam, and regular solid. It is clear that microcellular foam has more advantages than conventional foam. Microcellular foam overcomes the major disadvantages of conventional foam, such as a long cycle time and a thick wall. The most important advantages of microcellular foam can be summarized as follows:

The main advantage of structural foam molding (one of the conventional foams) is to increase stiffness without increasing the weight of the component. Microcellular foam can be made for this target as well, by redesigning thin wall structures and by creating a nice cell structure to save material (weight reduction by a thin wall) and cost (shorter cycle time).The microcellular process can be used for thin-wall solid parts that are difficult to make full mold filling from flow restrictions, which results in either clamp tonnage shortage or injection pressure limit.Microcellular technology allows mold filling without foaming because the gas-rich melt reduces viscosity significantly.The microcellular process almost eliminates all dimension stability problems, such as sink mark, flatness defects, warp, and residual stress after molding due to the elimination of pack and hold phases during molding.The microcellular process dramatically reduces cycle time if the part is designed properly.Microcellular processing equipment can be designed to save more energy since the peak of injection pressure is not necessary and also saves up to 50% of clamp tonnage.

The disadvantages of microcellular foam are the same as conventional foam, such as poor surface finish, strictly balanced runner system for multi­cavity mold, nontransparent application only, and complicated processing technology.

1.3 PATENTS AND PUBLICATIONS COVERING MICROCELLULAR INJECTION MOLDING TECHNOLOGY

There have been many patents issued for microcellular injection molding since 1998. The major patents, directly or indirectly related to microcellular injection molding technology, are listed here:

Pierick, D. E., et al., International Patent Application WO 98 31 521 A2 (1998)

Park, C. B., et al., U.S. Patent No. 5,866,053 (1999)

Pierick, D. E., et al., International Patent Application WO 00 26 005 A1 (2000)

Xu, J., International Patent Application WO 00 59 702 A1 (2000)

Michaeli, W., et al., German Patent DE 19 853 021 A1, (2000)

Anderson, J. R., et al., International Patent Application WO 01 89 794 A1 (2001)

Xu, J., U.S. Patent No. 6,322,347 (2001)

Burnham, T. B., et al., U.S. Patent No. 6,284,810 (2001)

Anderson, J. R., et al., U.S. Patent No. 6,376,059 (2002)

Gruber, H., et al., U.S. Patent Application No. 0,056,935 A1 (2002)

Pierick, D. E., et al., International Patent Application WO 02 090 085 A1 (2002)

Kim, R. Y., et al., International Patent Application WO 02 081 556 A1 (2002)

Vadala, J. P., et al., International Patent Application WO 02 026 484 A1 (2002)

Kishbaugh, L. A., et al., International Patent Application WO 02 026 485 A1 (2002)

Kishbaugh, L. A., et al., International Patent Application WO 02 072 927 A1 (2002)

Xu, J., U.S. Patent No. 6,579,910 B2 (2003)

Anderson, J. R., et al., U.S. Patent No. 6,593,384 (2003)

Dwivedi, R. K., U.S. Patent No. 6,759,004 (2004)

Cardona, J. C., et al, U.S. Patent No. 6,926,507 (2005)

Anderson, G., et al., U.S. Patent No. 7,172,333 (2007)

Xu, J., U.S. Patent No. 7,267,534 (2007)

Xu, J., et al., U.S. Patent No. 7,318,713 (2008)

Kishbaugh, L.A., et al., U.S. Patent No. 7,364,788 B2 (2008)

Xu, J., et al., U.S. Patent No. 7,615,170 B2 (2009)

There are many publications regarding the technology behind microcellular injection molding. They cover both the fundamentals and real practices in industry. However, it is well known a huge gap exists in fundamentals and realities. Hopefully, this comprehensive coverage in the book will help bridge this gap and will enable readers to apply the concepts in a straightforward manner.

1.4 OUTLINES OF THE BOOK

This book presents the microcellular history and a specific short history of microcellular injection molding in Chapter 1. Then, in Chapters 2 and 3, the fundamental knowledge of microcellular injection molding is covered. With the understanding of the principles of microcellular processing, a review of materials and details of design for microcellular injection molding are well discussed in Chapters 4 and 5. Moreover, injection molding makes the foaming process more complex. Therefore, both theory and experiments are needed for good analyses of microcellular process. Chapter 6 uses the fundamental guidelines in previous chapters to analyze the specific processing procedures one by one with a combination of theory and empirical data. Some comparisons among different gas-entrained processes, such as gas assistant, micro­cellular extrusion, microcellular blow molding, and structural foam molding are discussed in Chapter 6. It is also important to know the differences between regular injection molding and microcellular injection molding, which is discussed briefly in Chapter 6. To realize the processing requirements in Chapter 6, the equipment designing rules are introduced in Chapter 7. It will generate further insight on both the future development and the efficient operation. After understanding normal microcellular injection molding, more specialized microcellular injection molding processes are discussed in Chapter 8. All commercialized special processes and most developing special processes are covered in this chapter. In addition, the modeling of microcellular injection molding is also presented in Chapter 9. Some PVT data and rheology data of the gas-laden polymer melt are given in Chapter 9. The necessary postprocesses and basic test procedures are briefly introduced in Chapter 10. Finally, application in the market is covered in Chapter 11, and cost analyses are presented in Chapter 12.

REFERENCES

1. Suh, N. P. Innovation in Polymer Processing, edited by James F. Stevenson, Hanser/Gardner Publications, Cincinnati, 1996, Chapter 3, pp. 93–149.

2. Martine-Vvedensky, J. E., Suh, N. P., and Waldman, F. A. U.S. Patent No. 4,473,665 (1984).

3. Colton, J. S. Ph.D. Thesis, Massachusetts Institute of Technology, Cambridge, MA, 1985.

4. Colton, J. S., and Suh, N. P. U.S. Patent No. 4,922,082 (1990).

5. Park, C. B. Ph.D. Thesis, Massachusetts Institute of Technology, Cambridge, MA, 1993.

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2

BASICS OF MICROCELLULAR INJECTION MOLDING

The fundamental theory for microcellular injection molding has been developed for a decade and is still one of major research topics in the plastics industry. The basics of microcellular plastics introduced in this chapter will serve as the general guidelines to both fundamental research and technology development for the microcellular injection molding.

2.1 BASIC PROCEDURES OF MICROCELLULAR INJECTION MOLDING

Typical analyses of gases as the supercritical fluids (SCF) focus on the solubility and dissolution capability in different plastics. To make a nice mixture of gas–polymer solution is a real challenge in the industrial plasticizing unit. After this solution is ready, the nucleation will be the next key technology for success of microcellular injection molding. Finally, how to control the cell growth and distribution throughout the molding part becomes the value of the microcellular part. If the injection time is too long, the bubble collapse and coalescence in the flow front may occur. However, the injection time usually is very short for microcellular injection molding. Therefore, the bubble collapse and coalescence will not be considered as the normal defects in injection molding. To summarize the key issues for successful microcellular injection molding, there are four basic steps of microcellular injection molding: SCF mixing and dissolution in the melt of polymer; nucleation of cells; cell growth; and shaping in the mold [1–3].

The concept of the continuous process was successfully tried in an extruding process [4]. It basically needs to create the gas–polymer solution first in the barrel. The gas at the supercritical fluid state is metered and injected into the barrel and then dissolved into molten polymer, as shown in Figure 2.1a. The gas is pressurized up to 20.7 MPa before flowing into the barrel. As the gas flows into molten polymer, it forms big gas droplet in the molten polymer since the flow of the gas is briefly interrupted every time the screw flight wipes over the barrel. The size of the gas droplet in the molten polymer is determined by five major factors: gas pressure and molten polymer pressure; gas flow rate; viscosity of molten polymer; wiping frequency of the flights (screw rotation speed); and diameter of orifice in the gas injector. Then, the large gas droplet is elongated in the barrel through the shear deformation induced by the screw rotation. The elongated gas droplet will be broken up forming many small gas droplets above a critical value of the Weber number We, which is a ratio of shear force to the surface force (refer to Chapter 7). These gas droplets may be stabilized in the screw channels to form bubbles in the molten polymer matrix. These bubbles, in turn, undergo elongation with additional shear deformation that increases the area-to-volume ratio of each gas bubble. Then, the gas in the bubble diffuses quickly into the molten polymer due to the increased polymer–gas interfacial area and decreased striation thickness of polymer between the gas bubbles.

Figure 2.1 Schematic of the gas–polymer solution. (a) Gas injected into polymer melt. (b) Mixed gas-polymer solution.

However, the shearing rates are varied from the different layers in the channel of screw so that the bubble sizes are different from top (inside diameter of barrel) to bottom (root diameter in the screw) in the flow channel of the screw. The screw mixing section must be designed with the mixing elements to alter the positions of bubbles from top to bottom and vice versa, which will be discussed in Chapter 7. Eventually, the gas droplets must be small and uniformly distributed in the molten polymer matrix, as shown in Figure b. It may be defined as gas–polymer mixture ready for nucleation. Ideally, the final gas–polymer solution should become the so-called single-phase solution [3, 5]. In other words, there are no separate phases such as gas phase and polymer melt phase. However, the real practice in the injection molding machine can create the excellent gas–polymer solution with tiny bubbles in the molten polymer. The single-phase solution of gas and molten polymer may never truly form in such short recovery time in the plasticizing screw [1], and even shorter mixing time with new technology of injecting gas through the nozzle during injection [6, 7]. Therefore, the single-phase solution may be defined in this book as the gas–polymer solution with a uniformity distribution of many tiny bubbles, which has been proven to be a good mixture of gas–polymer solution ready for the next step of microcellular processing in the most current technologies of microcellular injection molding processes [1–7].