Symmetry-Breaking Powder Concentration and Temperature Distributions in the Weld Zone of a Geometrically Symmetric A-Type Mold During Metal Powder Injection Molding

A-type mold flow analyses were performed to examine the effects of the feedstock flow behavior on the powder particle distribution in the weld zone following the forward collision of two flow fronts during metal powder injection molding. Although the A-type mold cavity and the two converging feedstock flow paths are geometrically symmetric about the central weld plane, the present numerical results indicate that the coupled shear-heating and viscosity variation during flow-front collision break this symmetry, resulting in an asymmetric powder concentration and temperature field at the weld zone. This local departure from mirror symmetry was quantified using a proposed Mirror Asymmetry Index (MAI). The complex rheological characteristics of the weld zone result in a separation of the particles and the binder, which leads to changes in the powder concentration distribution. Such predicted concentration non-uniformity may be relevant to local density variation after de-binding and sintering; however, the present study does not simulate sintering or experimentally measure density or mechanical properties. Thus, to enhance the powder concentration in the weld zone and ensure a uniform distribution of the particles, mold flow analysis simulations, combined with the Taguchi experimental method, were performed to identify the optimal material and processing parameters for the metal powder injection molding process. The results indicate that regions of the weld zone with higher shear rates exhibit lower powder concentrations. In addition, local stagnation occurs at the intersection of the two flow fronts, which causes radial flow and phase-separation effects in the weld zone. Among the four factors examined (melt temperature, mold temperature, particle diameter, and injection flow rate), particle diameter was found to exert the dominant effect on weld-plane powder concentration, while the injection flow rate had the least influence. The optimal material and processing conditions (a melt temperature of 220 °C, a mold temperature of 60 °C, a particle diameter of 5 micron, and a filling rate of 10 cm3/s) improved the powder concentration in the weld zone and achieved a more uniform distribution, raising the mean particle concentration by 63.3% and reducing its standard deviation by 74.2% relative to the default settings. These findings show that geometric symmetry alone does not necessarily guarantee perfectly mirror-symmetric predicted process fields under the numerical assumptions used here. Physical confirmation of the predicted asymmetry remains necessary.