Transfer molding

Transfer molding (BrE moulding) is a manufacturing process where casting material is forced into a mold. Transfer molding is different from compression molding in that the mold is enclosed [Hayward] rather than open to the fill plunger resulting in higher dimensional tolerances and less environmental impact. Compared to injection molding, transfer molding uses higher pressures to uniformly fill the mold cavity. This allows thicker reinforcing fiber matrices to be more completely saturated by resin. Furthermore, unlike injection molding the transfer mold casting material may start the process as a solid. This can reduce equipment costs and time dependency. The transfer process may have a slower fill rate than an equivalent injection molding processes.

The mold interior surfaces may be gel-coated. If desired the mold is first pre-loaded with a reinforcing fiber matrix or perform. Fiber content of a transfer molded composite can be as high as 60% by volume. The fill material may be a preheated solid or a liquid. It is loaded into a chamber known as the pot. A ram or plunger forces material from the pot into the heated mold cavity. If feed-stock is initially solid, the forcing pressure and mold temperature melt it. Standard mold features such as sprue channels, a flow gate and ejector pins may be used. The heated mold ensures that the flow remains liquid for complete filling. Once filled the mold can be cooled at a controlled rate for optimal thermoset curing.

The industry identifies a variety of processes within the transfer molding category. There are areas of overlap and the distinctions between each method may not be clearly defined.

Resin transfer molding (RTM) uses a liquid thermoset resin to saturate a fiber preform placed in a closed mold. The process is versatile and can fabricate products with embedded objects such as foam cores or other components in addition to the fiber preform.

Vacuum assisted transfer molding (VARTM) uses a partial vacuum on one side of a fiber mat to pull the resin in for complete saturation. VARTM uses lower plunger forces which allows molding to be carried out with cheaper equipment. The use of a vacuum may allow the resin to adequately flow and or cure without heating. This temperature independence allows thicker fiber preforms and larger product geometries to be economical. VARTM can produce parts with less pourosity than regular transfer molding with a proportional increase in casting strength.

Also called transfer micromolding, this is a process that uses a mold to form then transfer structures as small as 30 nm onto thin films and microcircuitry. Unlike normal scale transfer molding, the micro form can and is used with metals as well as non metals.

Limiting defects is key when commercial producing any sort of material. Transfer molding is no exception. For example, voids in a transfer molded parts significantly reduce strength and modulus. There can also be defects when fibers are used around sharp corners. The resin flow can create resin rich zones on the outside of these corners. I will focus in on the causes of these defects for the rest of the section.

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