Compression Molding of Thermoset/Fiber Composites

Glass-fiber-reinforced thermoset composites have long been used by the plastics industry. Two primary reasons for using glass fibers as reinforcement of thermosets are: (1) to improve the mechanical/physical properties (e.g., tensile modulus, dimensional stability, fatigue endurance, deformation under load, hardness, or abrasion resistance) of the thermosets, and (2) to reduce the cost of production by replacing expensive resins with inexpensive glass fibers. In place of metals, the automotive industry uses glassfiber- reinforced unsaturated polyester composites. One reason for this substitution is that the weight per unit volume of composite materials is quite low compared with that of metals. This has allowed for considerable reductions in the fuel consumption of automobiles. Another reason is that composite materials are less expensive than metals. The unsaturated polyester premix molding compounds in commercial use are supplied as sheet molding compound (SMC), bulk molding compound (BMC), or thick molding compound (TMC) (Bruins 1976; Parkyn et al. 1967). These molding compounds can be molded in standard compression or transfer molds. The basic challenge in molding unsaturated polyester premix compounds is to get a uniform layer of glass reinforcement in place in the die cavity while the resin fills the cavity and reaches its gel stage during cure. Temperature, mold closing speed, pressure, and cure time are all functions of the design of the part being produced. The flow of the mixture through the gate(s) can result in variations in strength across the part due to fiber orientation during the flow. The precise end-use properties depend on the fiber orientation, fiber distribution, and fiber content in the premix compounds, which are greatly influenced by the processing conditions. Since the mechanical properties of the molded articles depend strongly upon the orientation of the glass fibers, it is important to understand how to control fiber orientation during molding. Unsaturated polyester accounts for the greater part of all thermosets used in glass-fiber-reinforced plastics. Glass-fiber-reinforced unsaturated polyesters offer the advantages of a balance of good mechanical, chemical, and electrical properties. Depending upon the application, a number of additives are employed to provide specific products or end-use properties.