Effect of shear heating during injection molding on the morphology of PC/LCP blends

Fiber relaxation of liquid crystalline polymer (LCP) in the mold during injection molding was investigated. A blend of LCP and polycarbonate was used. The LCP used, namely LC5000, is a thermotropic LCP consisting of 80% and 20% of hydroxybenzoic acid and ethylene terephthalate, respectively. The filling of the mold and the temperature profile of the melt in the mold, after the mold has been completely filled, were computed using the finite element/finite difference method (FE/FDM). The morphology of the fibers was greatly influenced by the temperature of the different layers in the sample. This was confirmed by scanning electron microscopy (SEM) examination of the injection-molded specimen. When shear heating caused the temperature of the melt to increase above 280 °C, relaxation of the fibers was rapid. This resulted in a final morphology where the LCP existed in short fibers or ellipsoids. It was concluded that the high shear rate, which is needed for fiber deformation, must be accompanied by fast cooling to minimize the effects of shear heating, so that the fibers formed could be retained.