Parameters affecting the full-shot molding of gas-assisted injection-molded parts

Full-shot gas-assisted injection-molding has the advantage of eliminating the switchover mark that usually occurs on the surface of short-shot gas-assisted molded parts. The purpose of this report was to study the effects of processing parameters on the moldability of the full-shot gas-assisted injection-molding process. Experiments were carried out on an 80-ton injection-molding machine equipped with a high-pressure nitrogen-gas injection unit. The materials used were general-purpose polystyrene and polypropylene. A plate cavity with a gas channel of various geometries (trapezoid, semicircle, and rectangle) across the center was used to mold the parts. After molding, the lengths of gas penetration were determined. The hollowed core ratio by the gas was also determined. A numerical analysis was carried out to find out the temperature distribution of the polymer melt inside the gas channel. It was found that the sink mark of molded parts decreases with the length of gas penetration. Molded parts using trapezoidal gas channel had the longest gas penetration length. In addition, a thermal contraction model was proposed to predict the gas penetration volume inside the parts. Good agreement was reached between the experimental data and the calculated result.