EXPERIMENTAL-STUDY ON THE EFFECTS OF ADDING COMPRESSION TO INJECTION-MOLDING PROCESS

The quality of thermoplastic injection molded parts is mainly determin ed during the post-filling process. To enhance the control capacity ov er the pose-filling process, adding compression to the conventional in jection molding process opens a flexible channel for pressure control. This study experimentally investigated the effects of adding compress ion on the thermal-mechanical history and on the quality of molded par ts. An instrumented molding machine with a compression mechanism was u sed. Molds for rectangular plate and circular disk were equipped with a movable cavity wall and shut-off mechanism. The thermomechanical his tory during the injection compression molding was identified with the aid of a PC-based data acquisition system. The quality of parts was ev aluated by flatness, deviation from mold dimensions, and birefringence . It has been found that the thermomechanical history of the melt unde r injection compression molding follows a quadrilateral-shaped path ov er the p-v-T diagram. The control capacity over post-filling is enhanc ed. It thus can produce parts with better dimensional control with les s frozen-in orientation than those from conventional injection molding . (C) 1995 John Wiley and Sons, Inc.