An investigation of active melt manipulation during polymer processing and its effects on part mechanical properties.

摘要

The primary objective of the proposed research study was to determine the mechanism by which polymer orientation is effected by active manipulation of the polymer melt during processing and how the orientation affects final mechanical properties of injection molded parts. Initially, a processing method known as vibration assisted injection molding (VAIM) was investigated through the study of polymer behavior within the mold during processing as well as through the observation of "frozen-in" molecular orientation and testing of mechanical properties of molded parts.; An experimental apparatus was developed to observe melt behavior within the mold cavity in real time under normal molding conditions. The apparatus consisted of a specially designed and constructed mold, optical train for observing melt birefringence within the mold and a high-speed video camera for recording the temporal birefringence patterns. Based on the observations of melt behavior it was concluded that the bulk of orientation induced by melt manipulation originates during the packing phase of the molding cycle, subsequent to the vibration, and not during the vibration phase as previously thought. These observations led directly to the proposition that similar results could be achieved by simply delaying the packing stage of the molding cycle and foregoing vibration entirely. A subsequent study using the custom mold and a delay packing molding process showed behavior similar to that seen with VAIM.; In general, parts with higher, more uniform levels of retardation throughout the specimen gage section exhibited higher tensile strengths. Failure of the specimens was a result of craze crack propagation normal to the direction of applied load. Crack propagation and patterns were affected by molecular orientation in that cracks tended to propagate in areas of lower molecular orientation and were arrested when they reached an area of higher orientation. Both polystyrene and Styrene acrylonitrile copolymer showed similar responses by both melt manipulation processes experiencing increases in ultimate tensile strength on the order of ten to fifteen percent. Melt manipulation processing using various molecular weight polystyrenes showed a significant impact on the UTS and toughness of the higher molecular weight grade relative to the lower molecular weight material.