The critical requirement for high-pressure foam injection molding with supercritical fluid

Abstract

The limitations of the traditional low-pressure foam injection molding technique (such as cell nucleation at the gate, coupled growth and filling stages, and active coalescence) often lead to poor morphology and properties. In this work, the critical, but often overlooked, packing/holding stage that decouples the foaming and filling steps in high-pressure foam injection molding was studied. A series of systematic experiments was conducted with the aid of a visualization mold to study the effect of various processing parameters (i.e., gas concentration, packing pressure, packing time, injection speed and melt temperature) on the packing efficiency to determine the time required to fully dissolve the gate-nucleated cells. It was found that a drop in gas concentration or an increase in packing pressure could greatly expedite the dissolution process. On the other hand, the cell dissolution time was not sensitive to changes in packing time. Interestingly, the effect of injection speed and temperature implied that competing mechanisms existed on their influence on the cell dissolution. An attempt was also made to predict the evolution of cell size during packing using a model in order to optimize the packing stage, with a view to achieving the desired foam structures and properties in a more efficient way. A sensitivity analysis was conducted to show the model's response to the changes in various parameters. The predicted response echoed findings from the experiments.