SHEET MOULDING COMPOUND (SMC):
This class of material, made in sheet form upto ¼” thickness, consists basically 1-2” long, chopped strand glass fiber roving as the reinforcing material, a polyester resin and a number of additives. The principal additives here (besides the catalysts) are the thickeners, fillers, thermoplastic polymers and pigments.
The function of the thickener (MgO) is to cause a controlled increase in viscosity of the resin, after thorough fiber wetting has taken place. The thickening of the resin then insures a uniform flow of reinforcement and resin to allparts of the mould when heat and pressures are applied. This minimizes the problem of resin-rich or resin-starved areas found in B-staged material when heated and pressed.
The function of approximately 30% by weight of powdered thermoplastic resin is to decrease the shrinkage of the polyester resin. Such resin formulation are called low shrinkage or low-profile resin. SMC containing thermoplastic resins (acrylics, PVC, styrene copolymers, PE, etc.) can be compression moulded to extremely smooth, high gloss surfaces. Gel coats are not used, but the moulded parts are usually finished by painting with automotive primers and finished coats.
Process:Here the SMC fibers are saturated with the resin by two doctor blades which meter the polymer on the PE films. The resin must be very accurately controlled to obtain uniform impregnation and because of the method of saturation, the fiber layer cannot be too thick up to ¼” approximately.