What is thermoset compression and transfer molding?

Thermoset compression molding is typically a manual process that transforms granular material into molded shapes by adding material directly into an open mold cavity and then closing the mold. Thermoset transfer molding is a similar process that involves the extra step of pre-forming a “pill” of material before it is injected into a closed mold through the mold’s runner and gate system. Rebling Plastics has been using both processes since the company’s inception.

Thermosets are materials that cure via an irreversible chemical reaction under heat and pressure, rather than cooling to a rigid state as thermoplastic injection molding materials do. This chemical reaction results in a strong highly cross-linked molecular structure and is the reason that thermoset materials have the ability to retain their properties at elevated temperatures.  Typical applications are electrical connector housings, automotive ashtrays, cookware appliance handles and knobs.

The thermoset compression molding process begins by adding granular material directly into the cavity(s) of the mold. Closing the mold provides the force necessary to flow the material into the cavity voids; the heated mold (300° F and higher) initiates the chemical curing. Since there is no runner and gate system in a compression mold, the mechanical properties of compression molded parts are somewhat better than identical parts molded by transfer or thermoset injection molding.

Thermoset transfer molding typically requires placing a preheated preform of material into the mold “pot” before a plunger applies pressure to the material, forcing the material through a runner and gate and into the mold cavity. Thermoset transfer molding is generally considered more precise and creates less flash than compression molding. Transfer molding is ideal for insert molding since the molding machine clamp opens and closes vertically and retention of the insert is more readily accomplished through a closed mold. 

Unlike thermoplastic injection molding, the extra material left behind in the mold must be disposed of, since cured thermoset material from sprues and runners cannot be reground and reprocessed. When the mold cavities are filled, the parts must cure to a solid form. The mold opens for part removal, and parts are ejected and removed by hand or automated equipment.

Thermoset Transfer and Compression Molding Pros and Cons

The main advantage of thermoset transfer and compression molding is the use of certain very high temperature thermoset materials. Compared to thermoplastics, these materials are more dimensionally stable- meaning the shrinkage doesn’t change with processing or environmental conditions and is less sensitive to wall thickness variations. Many of these materials are not available in an injectable grade, and tooling can be less expensive than thermoset injection molding.

Like thermoset injection molding, transfer and compression methods generate flash. In general, thermoset materials have a low viscosity. So, when the material fills the mold cavity under pressure, some of the material will leak between the two halves of the mold. This is called flash. While this problem can normally be prevented in thermoplastic injection molding by proper mold construction and processing parameters, flashing often cannot be avoided in thermoset molding and in many cases is desirable. Therefore, we must remove the flash as a secondary operation. When possible, the parts are de-flashed in an automatic tumbling operation rather than hand de-flashing to save costs.

Also similar to thermoset injection molding is the amount of material waste in thermoset transfer molding. The cured runners and flash material cannot be melted and reused, so it is disposed as waste. Thermoset materials also tend to wear the mold more quickly, so tooling maintenance and replacement costs are usually higher than that of thermoplastics.

Thermoset compression and transfer molding equipment

A current list of this equipment can be found at our facilities list page. We have thermoset compression and transfer molding presses ranging from 12 to 80 ton clamping pressure. Having these types of presses allows us to make different sized parts with the option of making multi-cavity molds. We carry excess capacity on these machines, so lead times are usually short depending on material availability. In addition to the presses, we also use auxiliary equipment for these processes including pre-forming equipment and pre-heaters for the pills used in transfer molding. Automatic sandblasting and tumbling machines are also used to save costs of flash removal.