Binder Design for Fabricating Internal Crack-Free Injection-Molded Si 3 N 4 -Based Ceramics
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BINDER DESIGN FOR FABRICATING INTERNAL CRACK-FREE INJECTION-MOLDED Si 3 N 4-BASED CERAMICS Sophia R. Su GTE Laboratories Incorporated 40 Sylvan Road Waltham, MA 02254, U.S.A. ABSTRACT Binder design is an important issue in ceramic injection molding technology. The binder decomposition mechanism, which involves thermodynamics, kinetics, as well as heat and mass transfer, controls the binder removal process. This process, in turn, is governed by the thermal and physical characteristics of the organic waxes used, and is the most critical step in injection molding ceramics. In this paper, we present the binder design philosophy and the method of binder selection. A systematic binder removal study focusing on heating rate, setter powder, and sublimable materials was carried out with the selected compositions. As a result of this study, we concluded that the fluid wicking controls the binder removal at the molten temperature of the binder, and the diffusion and permeation-controlled mechanism dominate at the decomposition temperature range of the binder. With the right binder selection, it is feasible to produce internal and external crack-free large cross-section injection-molded ceramic parts. INTRODUCTION Injection molding has been a major shape-forming technique used to fabricate complex net shape ceramic parts in recent years. This process provides economical, rapid production of highquality precision parts from a wide selection of ceramic materials. The injection-molding process consists of softening the material in a heated cylinder and injecting it under pressure into a relatively cold mold, where it resolidifies to produce a part in the desired shape. 1 Major variables, which affect the final products, are the morphology and the surface chemistry of the ceramic powder and physical characteristics of the organic vehicle. 2 These variables, in turn, govern powder-binder interactions, including dispersibility, wettability, solids loading, and binder removal mechanism. The binder removal process, which involves thermodynamics, kinetics, heat, and mass transfer, is considered to be the most critical step in this technology. Extensive studies have been made at GTE Laboratories 3 ,4 to understand the role of binder removal parameters, including setter powder, thermal cycle, and pressure on fabricating large crosssection parts. Theories of thermal debinding of organic binders from tape-cast sheets, including5 fluid wicking, diffusion, and permeation, have, been proposed and modeled by several authors. In this study, we have adopted the proposed debinding mechanisms as guidelines to develop two sets of binder compositions which are able to provide adequate rheological properties and are insensitive to the variations of Si3 N 4 powders. We present the systematic approach leading to this conclusion in detail. As a result, these binder compositions not only provide defect-free molded parts but also lead to crack-free debinded and sintered final products. The effects of binder components on compounding and molding are not in
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