EXPERIMENTAL-STUDY ON GAS PENETRATION CHARACTERISTICS IN A SPIRAL TUBE DURING GAS-ASSISTED INJECTION-MOLDING

Characteristics of gas penetration in a spiral tube during gas-assiste d injection molding were investigated experimentally. It was found tha t the coating melt thickness, which decreases with increasing gas pres sure, is uniformly distributed behind the gas front during the primary penetration period. When the gas pressure reaches a critical value, t he coating melt thickness becomes constant. Skin melt thickness also i ncreases with decreasing melt temperature and mold temperature. Near t he gas front the coating melt thickness varies significantly, and the variation shows two types of characteristics. At low injection gas pre ssures and longer delay times, the coating thickness at the gas front first decreases and then increases significantly, resulting in a melt- thinning region of a gas-bulge shape. This indicates the existence of a secondary fountain flow around the gas front tip. At high gas pressu res and short delay times, the skin melt thickness around the gas fron t increases dramatically as a result of melt shrinkage during the seco ndary penetration stage.