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According to characteristics of powder injection molding process, mathematical models with proper initial and boundary conditions of multiphase flow are given for powder injection molding filling process.Three-dimensional numerical simulations of multiphase flow are performed for a tensile test speci men and a gear parts using the CFD software CFX.Based on the variation of velocity of powder and binder at different nodes in the cavity, it can be seen from the simulated results that the powder-binder separation may occur at some nodes in the cavity during powder injection molding filling process.A number of pow der-binder separation phenomena at different nodes in the cavity are compared and analyzed.The results show that phase segregation is more possible to occur at the nodes located away from inlet and in the nar row regions.The powder-binder separation phenomenon only lasts for a short time in the initial stage of the filling process.The duration of the phase segregation depends on the location of the node.