Compression molding was developed in 1909 by Leo Bakeland to produce radio cabinets. In the compression molding process a thermoset plastic, phenolic, melamine, or polyester, is molded in an enclosed mold between two heated platens. The plastics, in powder, pellets, liquid, or performs, is introduced into the mold in a partially cured condition. A perform is the exact amount of material required to mold the part pressed into a large pellet or plug. The thermoset materials are introduce into the mold just prior to molding. The platens that hold the two halves of the mold are heated to approximately 350 degrees Farenheight. Pressure in the range of 1,500 Pounds per Square Inch is exerted on the material. The shearing action of the material being compressed together and the heat from the platens cause the plastic to become soft. The soft plastic fills the cavity and is compressed by the pressure. The additional strength of the temperature and the pressure contributes to the curing of the plastic, accelerating it by approximately four minutes. The compression molding process produces a heavy and dense product. Thermoset products have the highest electrical, heat and chemical resistance of any plastics material.

Compression molds are machined from steel. Their cavities are designed to be full positive, semi-positive, landed positive or flash molds. The fully positive mold requires that the exact amount of material be placed in the mold. The fully positive mold does not allow excess material to flow out between the parting line. Semi- positive molds allows for flash (material that flows out of the mold at the parting line) of excess materials just prior to the final closing of the mold. Landed positive molds allow for flashing in a prescribed area which forms a land or tab which can be removed and subsequently machine polished after molding. Flash molding allow for the elimination of excess materials during the final moments of the compression cycle. Products are more easily produced from flash mold since they do not require the exact amount of material. But, they must be machined and polished in post molding operations.

Handles for cooking pots, housings for hot plates, electrical receptacles, and high voltage switch housing are common products. Melamine, which enjoys lighter colors, is used in inexpensive dinner ware, kitchen utensils, and high pressure laminated counter tops. Liquid thermoset polyesters resins are molded into cafeteria trays, banquet hot food covers, and electrical boxes.

This process is not one that is easily adapted to high volume production. It is extremely difficult to fully automate a manufacturing line which uses this process.

For an incredibly detailed and accurate image of this amazing process drawn by me, click Here