EFFECTS OF PROCESS VARIABLES ON THROUGH-PLANE ELECTRICAL CONDUCTIVITY OF INJECTION-MOLDED CARBON FIBER/POLYPROPYLENE COMPOSITE FOMAS

摘要

Low-pressure injection foam molding of carbon fiber/polypropylene composite foams with the aid of physical blowing agent (N2) was conducted and their foaming behavior and electrical conductivity were characterized. Cell density, cell size, fiber orientation, skin layer thickness and through-plane electrical conductivity Were measured at three different sample's locations: near the gate and at the middle and end of the injected samples. The effects of void fraction, gas content, injection flow rate, melt temperature and mold temperature on the foaming behavior, electrical conductivity and their uniformity were investigated. The proper process window was identified and an optimum degree of foaming (up to 20% void fraction) was found wherein the conductivity increased up to four orders of magnitude and its uniformity along the sample was significantly improved. Quantification of fiber orientation showed that the fibers were more randomly distributed in the case of foamed samples which correlated well with the increased through-plane conductivity. Changes introduced to the fiber orientation by foaming originated from the stretching of polymeric matrix caused by either cell growth or shear stress. The results also showed that the injection flow rate and mold temperature played an important role on the foams' conductivity through then-effects on the foam morphology of the core and thickness of the skin.