Establishment of Gas Counter Pressure Technology and Its Application to Improve the Surface Quality of Microcellular Injection Molded Parts

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.