Determination of the orientation of short glass fibers in sheet molding compound (SMC)

Abstract A computer based semiautomatic procedure has been developed to measure the fiber orientation on the surface of Sheet Molding Compound (SMC) plaques. The principal orientation direction and the Hermans' orientation descriptor were used to summarize the fiber orientation distribution. The results show that only slight orientation was produced in the SMC plaque with a centrally placed charge occupying 40 percent of the area of the mold. Plaque to plaque, top surface to bottom surface, and region to region variability in the fiber orientation were observed. With a flow length of less than 3 fiber lengths in these plaques, the Hermans' orientation descriptor was in the range of 0.1 to 0.5. The descriptor varied appreciably with increasing sampling area and consistently displayed a sharp transition across the boundary of the original charge area, indicating distinct flow behaviors inside and outside the charge area. Likewise, the principal orientation direction varied with increasing sample area, albeit with some degree of coordination between the top and the bottom surfaces. Finally, the orientation of fibers in the four quadrants of the same plaque differed significantly. Based on the descriptor for each quadrant, the flow could be radial from the center of the plaque, rotational with the top and bottom surfaces in similar angular displacements, or twisted with the two surfaces moving in opposite directions, all indicating local nonuniformity of the flow.