;Lab Report Powder Injection Molding at Rensselaer Polytechnic Institute Randall M. German and Robert Messler Rensselaer Polytechnic Institute Editor's Note: Rensselaer Polytechnic Institute will be participating in the Materials Applications and Service Exposition (MASE) to be held October 13-15, 1987, in Cincinnati, Ohio. The exposition is the first such show to bring together the areas of materials selection and specification, materials development and advanced technology, and materials quality control and testing.

INTRODUCTION There is rapid growth in advanced powder forming technologies, involving both ceramic and metal powders. A large portion of this growth is traced to the emerging technologies that use organic binders. Injection molding (1M) is one such manufacturing technique with merit for several of the new and difficult to fabricate materials. 1M is a high volume manufacturing method for small, complex shapes which can not be die compacted. The process relies on mixing powder with a binder to produce a pliable media which can be injected into a closed die. The hydrostatic pressure used in forming eliminates the density gradients inherent in uniaxial die compaction, enabling densification in sintering with uniform and predictable shrinkage. Advantages of forming by 1M include net shape geometries (with concomitant material and processing savings) while attaining improved performance. A further advantage comes from the overall flexibility of the process. Despite the recognized advantages, widespread acceptance of 1M is yet to come, mainly because current production practices are largely an art. As a result, product quality, property consistency and component costs are not optimal. Exploitation of the processing options inherent to 1M requires an understanding of the powders and the interrelations between the materials, processing and product structure. Modern technologies such as process modeling for optimization and adaptive control offer the opportunity to achieve improved properties and performance. However, the basic processes associated with 1M are far from being understood. The attainment of process economy and high performance prop-

JOURNAL OF METALS. August 1987

erties will rely on developing a fundamental understanding in the hands of a greater number of technologists. RPI'S INJECTION MOLDING RESEARCH In response to the need for both trained technologists and improved powder forming technologies, a program in powder injection molding has been initiated at Rensselaer Polytechnic Institute (RPI). The research program, entitled Advanced Powder Processing, is being conducted by the Center for Manufacturing Productivity and Technology Transfer. Initially, the research will focus on binder-assisted processing of advanced materials. Early experiments are on both metal (ferrous) and ceramic (alumina) powders with supplemental efforts in composites, intermetallics, cemented carbides and refractory metals. These materials were selected to establish generic materialprocessing-property relationships. Later, the program will undertake more specific research reflecting sponsor interests in various applications. The RPI research program is multidisciplinary, involving experts in materials science, powder processing, polymer processing, computer-aided design and analysis, and process and quality control. The key faculty members (and their areas of expertise) include Chan Chung (polymer science and engineering), Daeyong Lee (mechanical engineering), Randall German (materials engineering), and Robert Messler (man ufactu ri ng). The fabrication of affordable components with complex geometries, close tolerances, unique microstructures and improved properties is a major goal of the research. The expected output from the program will be both innovative technology and technologists to assure broader application of particulate materials processing techniques. The research includes studies in 18 categories: rheology studies, powder characteristics, binder development, part geometry and computer-aided design, process monitoring, mixing, molding and form-

ing, residual stress and strain, debinding processes, modeling of debinding, sintering, modeling of sintering, embedded nondestructive evaluation, process control, computerized tool design, integrated quality, property assessment, and database development. As an example of the research emphasis, consider the role of particle size distribution. In 1M, small particles are selected to give sintering densification to compensate for the low packing fraction of current 1M powder-binder mixtures. Also, a small particle size provides an advantage in forming intricate geometries. However, there are problems with small powders which include high cost, slow debinding, contamination, agglomeration, availability, and reactivity. Powder packing is one approach to the use of other particle sizes. A wide particle size distribution is most useful in practice because of a better fitting together of particles. In conventional powder processing, a wide particle size range is a problem because of size segregation. However, in binder-assisted molding, this problem can be avoided. The binder will maintain the mix homogeneity, thereby allowing forming of high powder packing densities at low applied pressures. It is expected that an optimal packing density can be found that represents a balance between packing, strength, mixture rheology, rapid binder removal and sintering densification. SPONSORSHIP The RPI program started in September, 1986, with support from Alcoa, Allied-Bendix, General Motors, Kaiser, and SCM Metals. Supplemental research on specific applications is being supported by the New York State Science and Technology Foundation, Metadyne, and Defense Advanced Research Project Agency. Several other companies are being solicited to join the program. Membership is $25,000 per year, with a three-year duration. If you want more information on this subject, please circle reader service card number 49.

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