Compression Molding is the first technique developed in the early 1900s to mold Bakelite (phenolic) molding compounds. It is used today to process many varieties of thermoset plastics and composites, including phenolic, epoxy, melamine, and diallyl phthalate.
The compression molding press is simply a vertically oriented hydraulic press. The compression mold is clamped into the compression molding press which consists of two halves: the cavity and the force or core on the opposite side. The mold is heated to the temperature as specified by the material manufacturer, which is usually around 300° F, and the material is placed directly in the cavity. The mold is then closed under pressure, compressing the material and causing it to flow into the void between the cavity and the core. The mold is held closed for several minutes (depending on the material used and the wall thickness of the part), while the material is curing or "setting" into the form of the mold surface. The mold is then opened and the part ejected from the mold cavity.
Compression molding lends itself to relatively simple parts, but since there are no sprues, runners or gates through which the material must flow, the material can be heavily loaded with a composite of fillers and fibrous reinforcements that remain in a random orientation in the molded part resulting in superior strength properties.