Using differential mold temperatures to improve the residual wall thickness uniformity around curved sections of fluid assisted injection molded tubes

Fluid assisted injection molding technology, including gas assisted injection molding and water assisted injection molding, has been used to manufacture plastic tubes in recent years, due to the light weight of molded parts, relatively lower resin cost per part and faster cycle time. However, the non-uniform residual wall thickness distribution usually occurs around curved sections and can significantly affect the molded product quality. This aim of this report was to improve the uniformity of residual wall thickness distribution at curved sections of fluid assisted injection molded tubes by adopting differential mold temperatures. Experiments were carried out on an 80-ton injection molding machine equipped with gas and water injection units. The material used was semi-crystalline polypropylene. It was found that the water assisted molded parts exhibit a more uniform thickness distribution at curved sections than the gas assisted molded parts. The uniformity of residual wall thickness in fluid assisted injection molded parts could be improved by adopting differential mold temperatures. In addition, a numerical simulation using commercially available software was carried out to simulate the melt temperature distributions during the filling process, so as to better interpret the fluid penetration behavior in fluid assisted injection molded parts.