In the Metal Injection Molding industry, the so-called black lines are frequently observed in the surface inspection of the final sintering products. Such a critical appearance defect is attributed to powder-binder phase separation for academia. Recently, we have performed that powder concentration is used as an indicator for phase separation. At present, it is significant to numerically predict the powder concentration distribution during mold filling and then discuss the effect of powder size and bulk concentration on phase separation. Therefore, this will be significant understanding for MIM mold designers.