The electrical conductivity and percolation threshold of single and hybrid carbon filled composites are experimentally investigated. Polystyrene, carbon fiber (CF) and carbon black (CB) at three CF/CB ratios of 1.67, 3.33, 6.67 were compounded in a twin screw extruder micro‐compounder and compression molded into sheets. The through‐plane and in‐plane electrical conductivity of the composites are measured by 2 and 4 probe techniques. The percolation threshold of the single filler and hybrid composites are determined from the experimental results using a percolation model. The hybrid composites have a higher value of electrical conductivity and lower percolation threshold than the single CF filler composite except for the CF/CB ratio of 6.67. The percolation threshold for the cases of single filler and hybrid composites are modeled. The hard core / soft shell model is used and it is assumed that the percolation in a particle filled system depends on the ratio of tunneling distance to particle diameter. This ratio is determined by modeling single filler composites using the experimental data and kept constant in the modeling of the hybrid system. Finite size scaling is used to determine the percolation threshold for the infinite size hybrid system containing (nanosize) particles and micron size fibers for three CF/CB ratios. The simulation results show that the percolations of hybrid composites have the same trends observed in the experimental results. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41744.

The microstructure of fiber glass bundles used in the direct sheet molding compound (D‐SMC) was investigated in a flat plaque mold. Analyses of the bundle deformation, including bending and deformation of the tow shape, were done for the charge and specific sites within a square plaque part for 30% and 62% of initial charge mold area coverages. Microscopy and micro‐computed tomography (micro‐CT) of the samples clarify the microstructure of the bundles after flow. Samples were taken at different flow locations within the plaque corresponding to increasing flow length from the center. The bundles at the part edge and corner deform more than the bundles close to center of the mold in both initial charge cases. The bundles flattened at all positions and bundle bending was mainly observed at the corner. The tow width changes and tow deflections were higher in the samples of 30% mold area coverage. The micro‐CT images showed that the bundles keep their cohesion and stay straight within the middle of the flow path position, but bend at the edge of the mold. Mold filling simulation using Moldflow™ (Autodesk) predicted fiber tow orientation through the thickness using the reduced‐strain closure (RSC) models for fiber distribution for 30% and 62% initial mold area coverage. The measured value of orientation from micro‐CT images confirms the random orientation through the thickness, consistent with the RSC model. POLYM. COMPOS., 40:E69–E77, 2019. © 2017 Society of Plastics Engineers