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In sandwich injection molding, two polymeric materials are sequentially injected into a mold to form a multilayer product with a skin and core structure. Different properties of these polymers and their distribution in the cavity greatly affect the applications of the moldings. In an ideal situation, the core material should be entirely encapsulated in the skin material. When the flow front of the core material overtakes that of the skin material, breakthrough occurs, resulting in a defective part. The focus of this study is to determine the effect of molding parameters on the skin/core material distribution. The commercial simulation package (Moldflow) has been extensively compared with experiments. Both simulated and measured results suggest that in order to obtain the optimum encapsulated skin/core structure in the sandwich injection molded parts, it is necessary to select a proper core volume fraction and suitable processing parameters. A good agreement between simulation and experimental results indicates that the Moldflow program can be used as a valuable tool for the prediction of melt-flow behavior during the sandwich injection process.

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Push-pull-processing (PPP) is a live in-mold manipulation method of co-injection molding used for enhancing the orientation of the polymer molecules and reinforcing fillers. This technique implements an alternating shear field induced by a coordinated action of two injection units to create multiple oriented layers across the thickness of molding during the packing phase. In this investigation, the PPP is employed to enhance the weldline strength of short-glass-fiber reinforced polycarbonate (SFRPC) with respect to the fiber orientation in the weldline areas. The effects of glass-fiber concentration and processing parameters including the number of push-pull strokes and the holding pressure differences between both of the injection units have been studied. In addition the degradation of the fiber length caused by the oscillation is also investigated.