Abstract: In order to simulate the melt flow and melt front more precisely, mathematic models of coupled melt and air flows were formulated in terms of two phase flow theory. A viscoelastic model rather than conventional viscous model was employed to represent the relationship between stress and shear rate. To simplify the melt front simulation, a uniform mathematic model for melt and air flows at melt front was established using dimensionless parameters, and an iterative method was proposed to solve the viscoelastic flow problem. The melt front was tracked using level set method. Based on the proposed mathematic models and numerical methods, three type of flows and filling modes for low-, medium-, and high-speed micro-injection molding were investigated. In addition, the mechanism of air-trapping was analyzed and the possible location of trapped air was predicted. To verify the validity of the proposed method, a skew gear was designed and manufactured using high speed micro-injection molding. The actual trapped air location is in good agreement with the simulated results. This study showed that the filling mode depends on injection speed, material characters, and cavity size. The air-trapping was likely to occur at the jetting mode.

Key words: micro-injection moldiing, tow phase flow, filling mode, viscoelastic model