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Thermoset Transfer Molding

Thermoset transfer molding falls between thermoset compression molding and thermoset injection molding, and it shares some features of each:

A preheated preform is placed in a compression chamber, called the well, or the pot.

As the transfer plunger closes, the preform is compressed and starts to flow

Now the transfer plunger acts as a ram to force the hot, flowing resin into the mold

The filled, heated mold is held shut until the resin cures …

… then the mold is opened and the finished parts are ejected, along with the runners and the cull.

What characterizes compression molding is the method of charging the mold with resin: the plastic resin is placed directly in the bottom cavity of the open, heated mold, and then the top half of the mold is closed down on the resin under tremendous pressure, causing the resin to flow throughout the cavity until it completely fills the now-closed mold and assumes the shape of the finished part.

The compression action itself generates heat, and in conjunction with the heat of the mold, raises the temperature of the resin to the point where a chemical reaction called “crosslinking” takes place. It is this crosslinking that gives thermoset molded parts their characteristic strength, hardness and rigidity.

The heated mold is held closed for sufficient time to ensure that the crosslinking reaction is complete; this is called “curing,” and the length of time required for the cure cycle is almost entirely dependent on the wall thickness of the finished part. The thicker the part at its maximum cross section, the longer the curing time required for a given mold temperature.

Thermoset molding resins typically come in a powder or granulated form. Metering the correct amount of resin (called the “shot”) into the mold is accomplished in one of two ways. One is called “preforming” and the other is called “direct charging.”

In preforming, the exact amount of raw resin required to completely fill the mold (plus a small excess percentage to allow for squeeze-out) is compressed into uniform “pills” — sometimes called “hockey pucks” in industry slang, because that’s what the larger ones look like. These preformed “pills” of resin are produced in batches prior to molding using machines called, appropriately enough, preformers. Preforms are made in various sizes, depending on the size of the mold.

Just before use, these preformed pills are warmed up in a preheater — which is essentially an industrial-strength microwave oven — which softens the pill into a pliable, dough-like consistency so that the resin will flow more easily under compression. In the molding process, one of these preformed, preheated pills of resin is placed in the bottom cavity of the mold, and the mold is closed, compressing the softened resin into the shape of the mold. (Exceptions to preheating include Bulk Molding Compound (BMC) materials, which are typically formed in logs or used in bulk and are normally molded cold.)

Thermoset transfer molding — Similarities to compression molding:

    • Preheated Preforms. Like compression molding, the entire charge for a full shot of parts is delivered in the form of a manually-loaded preheated preform of resin. Unlike compression molding, the preform is not placed directly in the mold, but rather is placed in a chamber called the well, or the pot.In the case of Bulk Molding Compound (BMC), the charge will not be the typical preform “pill,” but will more likely be a section of log (a sort of extruded preform, cut to length for the volume required) or even a measure of cold bulk powder.
    • Compression and heat are used to plasticize and cure the resin. However, unlike compression molding where plasticizing and curing both take place in the cavity of the mold, in transfer molding the two processes — plasticizing and curing — take place in different sections of the mold.
  1. Plasticization occurs in the well.
  2. The plasticized resin, now a flowing fluid, is forced out of the well through a sprue and a system of runners and gates, into the mold cavities.
  3. Curing (crosslinking) occurs in the mold cavities.

Thermoset transfer molding — Similarities to injection molding, plastic mold maker and so on

    • Closed-Mold Processing. Unlike compression molding, where the charge is placed into an open mold which then closes, in transfer molding the mold is clamped closed and a hydraulic piston pushes the plasticized preform into the mold cavities through a system of channels and entry holes (runners and gates, respectively).
    Closed-mold benefits include:

  1. Less flash at the parting line.
  2. Better control of dimensions across the parting line.
  3. The ability to support metal inserts on both ends so they won’t shift as the plasticized resin flows around them.
    • Multi-Cavity Molds. Thermoset compression molds are typically single-cavity affairs; thermoset transfer molds — like plastic molds — are more likely to be multi-cavity.
    Transfer molding techniques can be used with single cavity molds, of course; typical single-cavity situations where transfer molding can be used to advantage include:

  1. Single cavity molds where the shape or draft of the part does not allow room to place a normally-shaped preheated preform.
  2. Single cavity molds with delicate metal insert placements that cannot be properly supported in an open-mold setting (the thicker resin flow that occurs when a compression mold closes on a preheated preform can dislodge or deflect inserts).
  3. Tight tolerance situations where controlling the dimensions across the parting line is especially critical (in a well-maintained transfer mold, flash at the parting line is a consistent, predictable thickness, and pretty well non-existent anyway).

Direct charging is used in situations where the mold cavity is too small, too shallow, or too convoluted, to conveniently accept a preform. In these cases, a measured scoop of cold resin is distributed directly into the open mold cavity.

Metal inserts — electrical contacts, female threaded holes, male threaded studs, bushings, and the like — can be placed in the mold cavities, using holes or locator pins, prior to molding. Under compression, the plastic resin flows around these inserts, molding them into the finished piece. Even though thermoset plastics are machinable to a degree, the judicious use of molded-in metal inserts can often eliminate costly secondary operations and add value to the finished parts.

Metal inserts can be molded into the bottom surface, the top surface, or both surfaces of the finished parts. They can even extend completely through the piece. In cases where the inserts are especially small, or cannot be properly supported within the mold, they can be epoxied in as a secondary operation.


In general, thermoset compression molding is well suited for larger pieces of relatively simple design with large, well-anchored inserts. A properly designed and well maintained compression mold will produce very little scrap, and through the use of preheated preforms, total cycle times can rival those of custom injection molding.

On the PLUS side …

  • Less expensive molds. Compression molds are generally less expensive than other types, both in initial cost and in maintenance.
  • Tighter tolerances, made possible by relatively small shrinkage. Because of the way direct compression packs the preheated preform into the mold, the result is a denser piece that tends to cure solid to the exact dimensions of the mold cavity, without “relaxing back” to create shrinkage. (Exception: dimensions across the parting line are harder to control.)
  • Less scrap can lower material costs.
  • Low-volume jobs are more economical, due to the simplier process and reduced start-up times. Compression jobs can be cycled in and out of the workflow more quickly, and on shorter notice.

On the MINUS side …

  • Heavier flash than other methods is possible. Deflashing is a secondary operation that may add to both labor costs and scrap loss.
  • Labor costs vs. cure time trade-offs. Labor costs can be reduced by using BMC materials and other cold molding techniques which forego preheated preforms, but the cure cycles will be longer.
  • Parting line thicknesses can vary making dimensions across the parting line more difficult to control tightly.
  • Metal inserts may be flashed, leading to more labor-intensive deflashing and clean up.
  • Deep, small diameter holes are difficult to mold reliably; in general, cored holes should be limited to 2.5 times their diameter in depth.

    Typical Applications

    Thermoset compression moldings are all around you. From the knobs on your stove to the electrical boxes in a jumbo jet — if it’s plastic, hard, rigid, heat-scratch-and-chemical-resistant, an electrical insulator, and it’s got molded-in metal inserts, and it’s black (or shades of reddish brown) — chances are it’s a thermoset molded product.

    • Aerospace — an aircraft terminal housing is just one example where thermoset moldings provide the excellent dielectric properties, lightweight rigidity, and dimensional stability required by precision in-flight systems.
    • Household Appliances — a blower fan blade for a residential clothes dryer illustrates the ability of thermoset compression molded products to meet critical density and dimensional stability requirements.
    • Industrial Machinery — an adjustment wheel for an industrial milling machine displays the rugged workhorse qualities of a typical thermoset compression molding.

    Other Molding Methods

    Compare with Thermoset Transfer Molding, which is similar to compression molding, but with important differences.

    Compare also with Thermoset Injection Molding, a familiar process in thermoplastics, now equally applicable to many thermoset materials.

    Informative Links:

    For an excellent primer on thermoset molding techniques, we recommend Plenco’s Plenco Processing Guide. It covers compression, transfer, and plastic injection molding china company, with many informative articles on plastic mold, mold design, and molding processes. In fact, the entire Plenco site is just awash with useful thermoset information — some specific to their own line of resins, of course, but chock full of general interest information, too.