SMC AND BMC

BULK AND SHEET MOLDING COMPOUNDS

Bulk molding compounds (BMC) and sheet molding compounds (SMC) are fiber-reinforced polyester (FRP) composites that can be molded into parts for applications in the automotive, appliance, electrical and construction industries.

BMC and SMC, although similar in composition, are processed differently. However, in both cases, production of a molded part from BMC or SMC is a twopart process: blending of the resin mix and compression molding of the final part. With BMC materials, the polyester resin and additives are first blended to form a resin mix. Filler is then added. Next, the mixing speed is decreased in the “premix” to incorporate the glass fibers thoroughly without breakage. The resulting material is a thick paste with the consistency of wet clay. If the material is to be concentrated further, thickening agents are added at this time.

The BMC is now ready to be compression molded. A predetermined amount of BMC is placed in a mold consisting of matched dies. The mold is closed at a pressure of 350 psi to 2,000 psi and kept at a temperature of 250° to 350°C (480° to 662°F). Generally, the material is molded for one second for every mil of thickness in the finished part; however, actual operating conditions vary and are regarded as proprietary information throughout the industry. The blending of SMC materials is very similar, except the inert filler is added gradually throughout the mixing operation instead of all at one time.

SMC has the consistency of a liquid paste or pancake batter. The SMC is then transferred to an impregnator where the paste is compressed into two sheets. Fiberglass roving or mat is then laid on one sheet. The other sheet is then laid over the roving to create a three-layer sandwich structure. This composite structure is further compressed in compaction belts. The final compacted SMC is cut to appropriate lengths for its final molding application. These sheets are then stored until the proper viscosity for molding is reached. From this point, the SMC molding process is similar to that of BMC. The sheet is placed between matched dies that are compressed and cured under the same general conditions as BMC.

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ALICE CHEN

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