Minimum Amount of Binder Removal Required during Solvent Debinding of Powder-Injection-Molded Compacts
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NTRODUCTION
POWDER-INJECTION molding (PIM) is a netshaping powder metallurgy process by which parts with complicated shapes can be produced economically. This process starts with the kneading of powders with multicomponent binders, followed by molding of the kneaded feedstock into shaped parts, removal of the binder, and, finally, sintering.[1] The removal of organic binders, or debinding, is a critical step in the PIM process because of the difficulties in removing the binder without introducing defects such as cracking and blistering. The most commonly applied debinding techniques include thermal debinding,[2] vacuum debinding,[3] catalytic debinding,[4] and solvent-thermal debinding,[5] which is currently the most widely used process. Solvent-thermal debinding is a two-stage process in which the soluble binder components in the compact are first extracted by solvent; the rest of the binders are then removed during the subsequent thermal-debinding step. The detailed debinding mechanisms have been discussed previously. For the straight thermal debinding of PIM compacts that contain two or more binder components, two-step debinding is usually practiced. Cima et al. noticed that, during the low-temperature thermalYANG-LIANG FAN, SHIAU-HAN WU, and YAU-CHING LIAU, Graduate Students, and KUEN-SHYANG HWANG, Professor, are with the Department of Materials Science and Engineering, National Taiwan University, Taipei, 106, Taiwan, Republic of China. Contact e-mail: [email protected] Manuscript submitted July 22, 2008. Article published online January 30, 2009 768—VOLUME 40A, APRIL 2009
debinding step, in which the low-molecular-weight binder was removed, there was no planar binder-vapor interface that receded as debinding proceeded. Instead, pore channels formed, starting at the surface and extending to the interior.[6] These pore channels provide conduits for the decomposed gas molecules of the remaining binders to escape to the compact surface during the second debinding step at high temperatures.[6–8] These pore channels can also increase the debinding rate, because the diffusion distance to transport the low-molecular-weight binder from the binder melt to the binder-pore interface is much smaller than if the binder has to diffuse to the part surface. In the case of the solvent-thermal-debinding process, Hwang and Hsieh used mercury porosimetry data and scanning electron micrographs to analyze the evolution of the binders inside the part during debinding.[5] As molded parts are immersed in the solvent bath, solvent molecules penetrate into the compact and dissolve soluble binders, leaving pore channels.[9] As debinding continues, new fine pores form, while existing pores increase in size. Similar to the case of straight thermal debinding, these pores can shorten the diffusion distance that the binder molecules must travel to reach the vapor-liquid interface and provide the conduits for the permeating gas to escape during the subsequent thermal debinding; thus, the cycle time is decreased. If the pore channels are not avail
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