Compression moulding Compression moulding is a manufacturing process where a plastic sheet is heated until pliable, formed to a specific shape in a mold, and trimmed to create a usable product. The sheet (or “film” when referring to thinner gauges and certain material types) is heated in an oven until it can be stretched into or onto a mold and cooled to a finished shape. Compression molding is the only plastic molding process that can mold thermoset plastics. Because of their structure, preheating as used in other molding processes would make these products assume the wrong final shape by curing too early in the process. The plastics that are used in compression molding include: polypropylene ( PP ), ethylene-vinyl acetate (EVA), urethane systems and a variety of foam systems.