Comparative study of pore structure evolution during solvent and thermal debinding of powder injection molded parts
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INTRODUCTION
AMONG the manufacturing routes to precision parts with complicated shapes, powder injection molding (PIM) has become an attractive process. This process starts with the mixing of metal powder with a polymer, followed by molding the mixed feedstock into shaped parts, removal of the polymer, and finally sintering. It combines the advantage of easy shaping of plastic injection molding and the advantage of superior mechanical properties of powder metallurgy components. However, in spite of these advantages, progress in PIM has been impeded by the slow and diffficult-to-control debinding process. Thermal debinding was the first process developed and is still widely used in the PIM industry because of its simplicity and the low equipment investmentY ,2,3]In this process, some binder components are decomposed first at low temperatures, leaving pore channels behind. After the pore network is formed, parts are further heated to pyrolyze high molecular weight polymers. This process is time-consuming because the debinding rate must be slow in order to avoid internal pressure buildup from decomposed gas, which causes cracking, blistering, and exfoliation during debinding.I4.5.6] Solvent debinding[7,8] and other alternatives, such as catalytic debindingtg] and vacuum debinding,t~o] were developed later to overcome the aforementioned problems in thermal debinding. Among these new processes, solvent debinding has been widely accepted by the industry, because as it replaces thermal debinding, only small changes in binder design and a small equipment investment are re-
K.S. HWANG, Professor, and Y.M. HSIEH, Graduate Student, are with the Institute of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan, 106, Republic of China Manuscript submitted January 4, 1995. METALLURGICALAND MATERIALSTRANSACTIONS A
quired. In the solvent debinding process, soluble binder components, such as wax and stearic acid, are removed first by solvent extraction in a few hours. The remaining binder is pyrolyzed in the subsequent thermal debinding. Thus, a complete debinding process is essentially a two-stage process, i.e., solvent debinding followed by thermal debinding. The main advantage of this process is that the long debinding cycle in conventional thermal debinding is now shortened significantly. Considerable efforts have been made to understand the transport of binder vapor and binder fluid in the molded compact during thermal debinding.t4,5,11-141 In contrast, the solvent debinding mechanism has not been thoroughly explored. Postulations have been made that solvent debinding occurs in two stages, i.e., dissolution of the binder followed by interdiffusion of dissolved binder and solvent.[15,~61 As soluble binders are extracted, pore channels are developed, and they serve as escape paths for decomposed gas during subsequent thermal debinding for insoluble binders. Under the assumption that the rate-controlling step is the interdiffusion of binder and solvent, equations similar to that of decarburization
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