Microcellular Injection Molding

Jingyi Xu

• Publisher: Wiley
• Year: 2011
• Print ISBN: 9780470466124
• eBook ISBN: 9781118057872
• Edition: 1
• Topic: Tecnología e ingeniería
• Subtopic: Ingeniería química y bioquímica

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.

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 Technology	Trade Name	Comment
Microcellular 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.	None	It 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 cel...

Table of contents

• Cover
• Half title page
• Series page
• Title page
• Copyright page
• FOREWORD
• PREFACE
• 1 INTRODUCTION
• 2 BASICS OF MICROCELLULAR INJECTION MOLDING
• 3 MORPHOLOGY OF MICROCELLULAR MATERIALS
• 4 MATERIALS FOR MICROCELLULAR INJECTION MOLDING
• 5 DESIGN OF MICROCELLULAR INJECTION MOLDING
• 6 PROCESS FOR MICROCELLULAR INJECTION MOLDING
• 7 EQUIPMENT AND MACHINES FOR MICROCELLULAR INJECTION MOLDING
• 8 SPECIAL PROCESSES
• 9 MODELING OF MICROCELLULAR INJECTION MOLDING
• 10 POSTPROCESSING AND PROPERTY TEST OF MICROCELLULAR INJECTION MOLDING
• 11 MARKETS AND APPLICATIONS OF MICROCELLULAR INJECTION MOLDING
• 12 COST SAVINGS FOR MICROCELLULAR INJECTION MOLDING
• NOMENCLATURE
• APPENDIX A (CHAPTER 7)
• APPENDIX B (CHAPTER 7)
• APPENDIX C (CHAPTER 5)
• APPENDIX D (CHAPTER 5)
• APPENDIX E (CHAPTER 5)
• APPENDIX F (CHAPTER 6)
• APPENDIX G (CHAPTER 9)
• APPENDIX H (GLOSSARY)
• Index