Investigation of Powder-Binder Separation in Powder Injection Molding Process

Abstract

Powder-binder separation based on the governing equation combining the shear-induced particle migration and non-Newtonian viscosity models suggested by Phillips and Krieger, respectively in the powder injection molding is numerically and experimentally investigated. An essential condition for the steady-state of Phillips model to be valid is mathematically derived. The validity condition for Phillips model in Poiseuille flow is identified. It is also shown that the separation behavior is numerically examined by the separation index defined from the validity condition. The effects of rheological and processing parameters including powder law index, initial volume fraction, and critical solid loading are numerically analyzed. For the experimental analysis, three different powder sizes of SUS17-4PH powder and wax-polymer binder system are used. The critical solid loading and rheological parameters for non-Newtonian viscosity model based on power law and Krieger models are obtained by data treatment obtained from rheological experiments including torque and capillary rheometry experiments, respectively. To measure the powder distribution, the zig inserted in the capillary rheometer is specially designed. The extracted feedstock within the zig is encapsulated in the mounting resin without the pressure and temperature to avoid destroying the original powder distribution profile. After various attempts to distinguish powder from the binder, the surface on the cross-section of the injected feedstock is polished by MD-Dac plate with acetone. The ratio of diffusive coefficients for steady-state solution is obtained by the mean residual method. Given a value of the ratio of diffusive coefficients, a value of mean residual is determined. It is natural that the best estimator of ratio of diffusive coefficients would minimize the deviation, which is represented by the mean residual. The values of ratio are numerically verified through calculating the flux based on the experimentally obtained powder distribution profile. In addition, each term for governing equation is examined by the parameter study. The effects of material and processing conditions including powder size, injection temperature, and initial volume fraction are investigated though Variation defined in this thesis. In addition, Taguchi method is also employed to take the effect of shear rate into account.