Injection molding, mechanical properties, and morphology of a compatibilized polycarbonate blend in the presence of weld lines

Abstract Weld lines are known to present serious problems, especially in injection molding processes. They usually represent the weakest region in a molded part in terms of mechanical properties. They are also known to be even weaker in the case of blends. In this work, the mechanical properties and the morphology of injection molded polycarbonate (PC), high‐density polyethylene (HDPE), and their blends (PC/PE) were investigated. This typically immiscible blend was compatibilized by the addition of small amounts (<5% by weight) of ethylene vinyl acetate copolymer (EVA) containing 60% by weight vinyl acetate. The modified and nonmodified blends were injection molded in the presence of weld lines using a double‐gate dogbone‐shaped tensile bar mold. The weld line strength and its morphology were investigated as a function of injection temperature. In the case of pure polycarbonate, the weld line strength was found to be very sensitive to injection temperature whereas for polyethylene no difference was observed between the strength of the nonwelded and the welded specimens. For the 80/20 PC/PE blend, the weld line strength is lower compared to that of the blend components. The addition of less than 1% of EVA increased the strength of the weld line above that of the neat blend. The morphology of the nonmodified blend was similar to that of a typical immiscible blend (i.e., PE spheres dispersed in a rigid matrix of PC). However, addition of the copolymer to the blend resulted in a better phase dispersion and finer PE particles. In addition, the V‐notch in the middle of the sample was greatly reduced by the addition of the copolymer. This was experimentally investigated by scanning electron microscopy in the region of the weld line. © 1995 John Wiley & Sons, Inc.