The purpose of this study is to establish the Gas Counter Pressure (GCP) technology in combined with microcellular injection molding process. The application of gas into the mold cavity during the melt injection period provides a counter force against the melt front advancement, restricting the foaming process during the melt filling stage. Various gas counter pressures from 0 bar up to 300 bar and gas holding times of 0 to 10 s after filling completion were employed for investigating their relevant effect. It was found that when gas counter pressure is greater than 100 bar, foaming does not appear in the skin layer if no holding time is applied after melt-filling. As gas counter pressure increases, thickness of the solid skin layer without foaming also increases. Employment of holding time after melt-filling also assists foaming restriction in the melt core area. If a holding time of 10 s is applied combined with a counter pressure greater than 100 bar, foaming is completely restricted resulting in a transparent PS sample part. The part surface of those subjected to foaming-restriction in the skin layer also appears to be smooth and glossy. For the black PS parts molded via microcellular injection combined with gas counter pressure, it appears the same gloss quality as that achieved by conventional injection molding.

Characteristics of gas penetration and polymer melt flow in gas‐assisted injection molded spiral tubes was investigated by simulations and experiments. Distribution of the skin melt thickness along the gas flow direction was measured, and gas penetration in the primary and secondary stages was identified. An algorithm based on the control‐volume/tiniteelement method combined with a particle‐tracing scheme using a dual‐filling‐parameter technique is utilized to predict the advancements of both melt front and gas from during the molding process. The simulated distribution of gas penetration shows reasonably good coincidence with experimental observations. © 1995 John Wiley & Sons, Inc.

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Erperimental shrdies of polymer mekj7uw in the moldJiUing stage of the coinjection molding process have been carried ouf using sequenliol injection of lra~parenf and colored polystyrene resin. Simuhtions are also developed to predict the meN fronl uduancemertls of both skin and core meh. A control uolume/Jinife elemenl method employed within eoch grid hyer of the gapwise direction is applied lo trace the mek front advancements for both skin and core materiuls. Numerical sirnulolions show reasonnlily good consistency wifh experimenlal results in both skirr and core mulerid distribution. Howeuer, the sirnulolion accumcy can be improuedfurther i f edge eflect is &ken into m o u n t using the shape factor as a geometrical correction.

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