Visualization Analysis on Reduction Effects of Bubble Breaking and Gas Cloud Generation at Flow Front by Counter Pressure Injection Molding with Low Pressure Level

Abstract

Counter pressure injection molding is a type of gas-assisted molding technology for preventing the generation of swirl marks by adding N₂ or CO₂ gas at high pressures of 1-15 MPa to the cavity. In this study, using a laser-light-sheet visualization mold, we visualized the movement of gas clouds and the bubble breakage phenomena at the flow front. The resin and gas used were general purpose polystyrene and N₂. First, we confirmed that the generation of gas cloud (as a result of gas condensation) can drastically be suppressed in the conventional injection molding process, even at a very low pressure of 0.7MPa. Secondly, in microcellular foam injection molding using supercritical fluid, the counter pressure method was found to reduce not only the nucleation of voids in the filling core layer and the subsequent fountain flow area, but also gas cloud generation with N₂ even at a low counter pressure of 1.0MPa. These visualization results clarify that low counter pressure is effective for empirically reducing both the gas cloud generation and the voids nucleation in supercritical foam injection molding.