Improved through-plane electrical conductivity in a carbon-filled thermoplastic via foaming

Flow-induced orientation of the conductive fillers in injection molding creates parts with anisotropic electrical conductivity where through-plane conductivity is several orders of magnitude lower than in-plane conductivity. This article provides insight into a novel processing method using a chemical blowing agent to manipulate carbon fiber (CF) orientation within a polymer matrix during injection molding. The study used a fractional factorial experimental design to identify the important processing factors for improving the through-plane electrical conductivity of plates molded from a carbon-filled cyclic olefin copolymer (COC) containing 10 vol% CF and 2 vol% carbon black. The molded COC plates were analyzed for fiber orientation, morphology, and electrical conductivity. With increasing porosity in the molded foam part, it was found that greater out-of-plane fiber orientation and higher electrical conductivity could be achieved. Maximum conductivity and fiber reorientation in the through-plane direction occurred at lower injection flow rate and higher melt temperature. These process conditions correspond with foam flow during filling of the mold cavity, indicating the importance of shear stress on the effectiveness of a fiber being rotated out-of-plane during injection molding.