SMC, TMC and BMC thermosetting compounds can be molded in any of the three higher volume molding processes: Compression, Injection or Transfer molding. The difference between the processes is the delivery method of the material into the mold cavities. For all processes, a mold constructed of mild or hardened tool steel is heated to 300°F (150°C) by way of oil, steam or electric cartridge elements. The mold is chrome plated to reduce wear on the mold and improve part release. Part ejection is accomplished with hydraulically actuated ejector pins and air poppets. Part removal can either be manual or automatic.
This molding process utilizes a pre-staged charge of material that is created by manually or automatically cutting, stacking, arranging and weighing the individual SMC or TMC pieces. This charge is either manually or automatically loaded into the open mold. Once the charge is placed, the mold is closed with a fast closing speed followed by a controlled final closure speed once the thermoset compound begins to flow. As the compound flows in the mold, it reaches a telescoping shear edge zone at the perimeter of the cavity that enables air to escape. The major advantages of compression molding:
The presses used for injection molding are horizontally operated with a unique material delivery system. The thermoset compound is fed and injected with a high speed reciprocating screw, or a plunger system that reduces glass degradation for better part strength. The major advantages of injection molding:
The presses used for transfer molding are vertically operated and equipped with a hydraulic transfer system and controls. The thermoset compound is loaded into a chamber known as the transfer pot. The mold is then closed and a transfer plunger forces the compound from the pot through the runner system and gates into the mold cavities. The major advantages of transfer molding: