The present study investigates the origin of voids in sheet molding compounds (SMCs) sheets and their transport during the manufacturing steps. Paste mixing, impregnation, thickening, and molding have been performed with five different SMC sheets, one without fibers and the others with four different types of glass fiber bundles. The fiber surface energy and bending rigidity were quantified, together with the paste surface energy. Void content was then measured at each manufacturing step. Two main void origins in SMC sheets have been observed: (i) air entrapment during paste mixing and (ii) poor bundle impregnation. These voids may be largely removed during SMC manufacturing. The quality of impregnation and void elimination during the flow is found to depend on the bundle characteristics (rigidity and surface energy) conferred by their sizing and over sizing. POLYM. COMPOS. 27:289–298, 2006. © 2006 Society of Plastics Engineers

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The correlation between sizing formulation, bundle mechanical characteristics, and bundle-matrix static and dynamic interactions are investigated. Two glass-fiber sizing formulations are considered, one containing polyvinyl acetate (PVAc) and the other polyester/PVAc, as conventionally used in sheet molding compounds (SMC). Axial compression tests are conducted on dry two-dimensional (2D) random suspensions. The forced packing is governed by the bending of fiber bundle segments between bundle-bundle contact points. Benchmarking of the experimental curves with a modified theoretical model provides an estimation of the fiber bundle bending rigidity under forced packing conditions. This value is found to depend on the bundle sizing as well as on the interaction with solvents present in the matrix as is the case for SMC. Free flow and molding experiments are performed on planar SMC sheets using the two different fiber bundles as reinforcements. The results confirm the dependence of the molding energy and the SMC rheology on the bundles chemical and mechanical characteristics. POLYM. COMPOS., 26:370 -376, 2005.