Molding compounds are plastic materials in varying stages of pellets or granulation that consist of resin, filler, pigments, reinforcement, plasticizers, and other ingredients ready for use in a molding operation. This article describes the material components and physical properties of sheet molding compounds (SMC). The three types of resin paste mixing techniques, such as batch, batch/continuous, and continuous, for an SMC operation are reviewed. The article discusses the design features and functional operations of the two types of SMC machines, namely, continuous-belt and beltless machines. It explains the formulation and processing of bulk molding compounds and reviews molding methods for bulk molding compounds, including compression, transfer, and injection molding. The effects of the fiber type and length and the matrix type on thermoset bulk molding compounds are discussed. It describes the four injection molding processes of injection molding compounds such as feeding, transporting, injecting, and flowing.

Bulk molding compounds can be molded into a variety of complex shapes by methods that can be readily automated for high volume production. This article describes the formulation and processing (compound formation, and molding methods) of bulk molding compounds. It discusses the effects of fiber type, fiber length, and matrix type on thermoset bulk molding compounds. The markets for long-fiber-reinforced bulk molding compounds are electrical, ordnance, aerospace, industrial, sporting goods, and automotive applications.

Sheet molding compounds (SMCs) refers to both material and process for producing glass-fiber-reinforced polyester resin items. This article discusses the material components incorporated into the resin paste for desirable processing and molding characteristics and optimum physical and mechanical properties, including catalyst, fillers, thickeners, pigments, thermoplastic polymers, flame retardants, and ultraviolet absorbers. It talks about the mixing techniques available for SMC resin pastes, including batch, batch/continuous, and continuous mixing. The article also outlines the design features and the operations of continuous-belt and beltless machine type SMCs.