Primary Experimental Study on Microcellular Injection Molding Using Newly-developed Equipment

Microcellular injection molding using supercritical fluid (SCF) as blowing agent attracts more and more interests. There are still many difficulties in microcellular injection molding, such as decreasing equipment cost, controlling cellular morphology, and increasing feasibility. In this work, primary microcellular injection molding experiments were carried out on newly-developed equipment, which includes a conventional injection molding machine, SCF conveying unit, specially designed nozzle, and mold. SCF from syringe pump was injected into the polymer melt between the screw and nozzle during the injection phase. Then, polymer/SCF melt was pushed by screw through a mixing element, and was injected into mold cavity after forming homogeneous phase. Polystyrene (PS) was selected to mold the standard tensile and impact specimens using the equipment. The required minimum filling pressures during the microcellular and conventional injection molding were compared. The densities of microcellular injection molded specimen and its solid counterpart were measured. The cellular morphologies at different positions of microcellular injection molded specimen were observed using scanning electron microscope (SEM). The tensile and impact strengths of standard specimens were tested. The results showed that introducing SCF during molding decreased the filling pressure and material usage. There were different cellular structures along and perpendicular to the direction of melt flow. The newly-developed equipment has several advantages, such as low cost, easy operation, and extensive use.