A statistical analysis of the effect of a mixture component on the rheology of alumina feedstocks
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INTRODUCTION
POWDER injection molding (PIM) is an advanced technology for processing ceramic powders. This technology combines the plastic injection molding technology for forming desired shapes and the conventional powder technology for sintering the molded parts. Therefore, it is capable of producing parts with complex shapes at a relatively low processing cost5 u Suitable fluidity and plasticity of feedstocks provided by binders are critical in PIM. It helps to sustain the powder and make a homogeneous mix free from agglomerates.P] Most importantly, it is essential to make moldings without defects. Failure to disperse the powder uniformly or poor rheological behavior of material will cause molding defects such as distortion, cracks, or voids that will further lead to nonuniform shrinkage, blisterings, or warping in the final sintered compact. I31Consequently, both characterization and control of the rheological properties of feedstocks are necessary. The most common approach in characterizing the rheological properties is to measure the viscosity of feedstocks through a capillary viscometer over a range of temperatures and shear rates. Fluidity, pseudoplasticity, flow energy, and yield stress are the rheological parameters that can be determined or generated from viscometer measurements. Some criteria about these parameters and molding behavior have been established. The viscosity is suggested to be less than 1000 Pa s at a shear rate ranging from 100 to 1000 s -~ for feedstocks to be injection molded successfully/31 With respect to the dependence of viscosity on shear rate, pseudoplastic flow is preferred for PIM mixtures in the molding process to ease mold filling, minimize jetting, and
K.C. HSU, Associate Professor, and P.C. TSAI, Graduate Student, are with the Department of Chemistry, National Taiwan Normal University, Taipei, Taiwan 11718, Republic of China. Manuscript submitted Jtme 15, 1995. METALLURGICALAND MATERIALS TRANSACTIONS B
help to hold component shape. Small flow energy or yield stress is desired to minimize problems due to a fluctuating molding temperature, thereby minimizing stress concentrations, cracks, and shape distortionY-71 Binder is known to be a multicomponent mixture in fulfilling many requirements in PIM. Is,91 Usually, binder is composed of polymer, wax, and processing aids. Thermoplastic polymer such as polyethylene or polypropylene offers the general rheological property. Waxes help to lower the viscosity of feedstocks. Stearic acid is commonly used as a lubricating agent in promoting the flow of material especially at high powder loading.VO.lq Besides, coupling agents or other flow modifiers are sometimes included in binder formulations to modify some propertiesY 2.131 It is clear that binder is responsible for providing the rheological properties and determining whether the resulting feedstock can be injection molded without any defects. Indeed, many works have obtained useful results concerning the influence of binder composition on the theological properties of the resu
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