Compression molding uses sheet molding compound (SMC), bulk molding compound (BMC), or carbon fiber in a thermoset process to produce a greater strength-to-weight ratio over steel and aluminum parts. Compression molding is used for high volume production, is capable of detailed/complex shapes, and eliminates the need to use secondary parts in the assembly process (while still maintaining Class A surface finishes). On the other hand, open molding (hand lay-up/spray-up molding) is a cost-effective solution for lower volume/larger structural parts.
A method of molding in which the molding material, which is usually preheated, is first placed in an open, heated mold cavity. The mold is closed with a top force or plug member, pressure is applied to force the material into contact with all mold areas, and heat and pressure are maintained until the molding material has cured. The process employs thermosetting resins in a partially cured stage, either in the form of granules, putty-like masses, or preforms. Compression molding is a high-volume, high-pressure method that is suitable for molding complex, high-strength fiberglass reinforcements. Advanced composite thermoplastics can also be compression molded with unidirectional tapes, woven fabrics, randomly orientated fiber mat or chopped strand. The advantage of compression molding is its ability to mold large, fairly complex parts. Compression molding produces fewer knit lines and less fiber-length degradation than injection molding.