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What is a Sheet Molding Compound?

By Paul Scott
Updated May 17, 2024
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Sheet molding compound (SMC) is a ready-to-use, glass reinforced thermoset polymer resin in sheet form commonly found in compression molding processes. The product is manufactured by extruding a layer of resin compound onto a plastic sheet and applying chopped glass fiber rovings to its surface. Once the fiber strands have penetrated the resin, a top layer of plastic is applied and the completed resin sheet is rolled up and left to mature. When the resin has matured, the sheet may be cut to size and placed in the mold where heat and pressure completes the curing process. The SMC process is desirable because it may be used to mold complex shapes with little waste, requires minimal labor, and makes high production volumes possible.

Ready-to-use thermoset resins are widely used in the injection and compression molding industries and are presented in two basic formats: bulk molding compound and sheet molding compound. SMC is often the preferred choice for compression molding processes and consists of flat sheets of matured, glass fiber impregnated resin. The sheets are available in rolls weighing up to 880 lbs. (400 kg) which are cut to size prior to molding. These resin rolls are typically interleaved with plastic sheets to prevent the layers from adhering to each other.

Typical sheet molding compound production involves dispensing a layer of compound consisting of resin, catalyst, fillers, coloring, mold release, and maturation agents onto a sheet of plastic. Glass fiber rovings are then chopped into strands approximately 1 inch (25.4 mm) long and distributed onto the surface of the resin layer. These strands penetrate the layer evenly to form a reinforcing matrix at which point a second sheet of plastic is placed on top. The completed composite sheet is then rolled up and removed to a storage area where it is left to mature for approximately 48 hours. Once exposed to heat and pressure in the mold, the sheet molding compound cures completely and the finished product may be removed.

Products manufactured from sheet molding compound stock exhibit excellent mechanical strength, good surface appearance, and outstanding electrical insulation properties. These characteristics make SMC products ideal for the production of boat hulls, high demand electrical components, class A automobile body parts, and sanitary ware. Processes utilizing sheet molding compound stocks are also attractive because minimal labor is required, high production volumes and complex shapes are possible, and very little scrap is generated.

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