Two-dimensional simulation of mass transport in polymer removal from a powder injection molding compact by thermal debin
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Two-dimensional simulation of thermal debinding in powder injection molding based on mass and heat transfer in deformable porous media is proposed. The primary mechanisms of mass transport, i.e., liquid flow, gas flow, vapor diffusion, and convection, as well as the pyrolysis of polymers, and their interactions, are included in the model. The simulated results revealed that polymer removal process is primarily affected by liquid flow, which is mainly dominated by pressure-forced flow rather than capillary-driven flow. A significant phenomenon, enrichment with liquid polymer in the outer surface regions of the compact, is explained.
I. INTRODUCTION
Powder injection molding (PIM) is an important netshape manufacturing process and has received much attention. One of the most critical steps in PIM process is debinding. It consumes a major part of the processing time. Failure of the powder compact often results if the process is accelerated. Thermal debinding is a common methodology for the final removal of residual polymer from a PIM compact prior to sintering. During debinding, the polymer is heated thermally, melted into liquid, and decomposed into vapor. The overall removal of residual polymer is an intricate combination of evaporation, liquid and gas migration, pyrolysis of polymer, and heat transfer in porous media. A successful modeling of thermal debinding provides the potential for optimization of the process to prevent the formation of defects during the decomposition of the polymer. As summarized in Table I, several researchers attempted to model thermal debinding by considering various mass transport mechanisms.1– 14 German1 modeled isothermal debinding by two separate controlling processes: vapor diffusion and vapor permeation in the one-dimensional porous outer layer of compact under steady-state conditions. The pyrolysis of binder and the liquid transport processes were neglected. The binder–vapor interface was modeled as a planar front, which receded into the compact as removal progressed. The effects of particle size, porosity, and component size on debinding times were assessed. Tsai5 analyzed the gas pressure buildup and the stresses on the one-dimensional cylinder powder skeleton during binder burnout on the basis of gas transport in a porous medium 2436
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J. Mater. Res., Vol. 16, No. 8, Aug 2001 Downloaded: 15 Apr 2015
combining with the pyrolysis of binder and elasticity theory. Evans et al.6 modeled the removal of polymer from a molded ceramic body in the shape of an infinite cylinder as an unsteady-state diffusion of degradation product in solution in the parent polymer, on the basis of the partial differential equation of diffusion, and with degradation of polymer. The critical hearing rate of thermal debinding was estimated using the criterion of bubble formation. The model was valid only for the initial stage of the polymer removal process. The critical heating rate was underestimated. To overcome these limitations, Matar et al.7 dealt with the evolution porosi
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