MECHANISM OF CELL NUCLEATION IN HIGH-PRESSURE FOAM INJECTION MOLDING FOLLOWED BY PRECISE MOLD-OPENING

摘要

In the present study, the mechanism of cell nucleation in high-pressure foam injection molding with high expansion was investigated. An in-situ visualization technique was used which enabled the monitoring of the entire process of cellular structure formation from the mold filling to the final nucleation stage. It was observed that the cells which were nucleated during mold filling, the so called gate-nucleated bubbles, remained in the melt/gas mixture after the mold filling stage was completed. These gate-nucleated bubbles led to the presence of large bubbles in the final molded part, which caused a reduction in the foam cell density (number of bubbles per unit volume) and a reduction in the uniformity of the cellular structure. A foam injection molding protocol based on the application of melt packing pressure was then proposed to remove the gate-nucleated bubbles. In this protocol, a high packing pressure is applied on the melt/gas mixture to re-dissolve gate-nucleated bubbles before the mold-opening stage. When the mold opens, new cells nucleate uniformly due to the rapid reduction of pressure. Also, a numerical simulation was performed, and a comparison with the experimental result was made.