Thermo set plastic parts are integral to our artificial world. Many objects made from synthetic materials, plastic and rubber most notably, are formed in automated, or semi-automated processes using molds. A part that is made from just one material may be formed in an injection mold, where molten material is injected at high pressure into a preheated mold. The mold is then chilled, the part cools to a solid within the mold, and the part is ejected into a hopper. However, some objects are composites of two or more materials that have different melting temperatures, along with all of the other thermal dynamic differences that go along with that, and must be assembled in a different way. They could be separately produced and then glued or otherwise assembled by human hands, but this is expensive and time consuming. It also does not produce the strongest component bonding. To achieve these goals transfer molding is required.
There are many household objects that can be made by transfer molding processes. One would be a dining utensil, like a fork. The body of the fork and the tines are typically stamped and formed out of one piece of sheet stainless steel. To make the handle rounder and more comfortable to hold in the hand, a thicker plastic portion can be added. Two holes can be punched out of the handle area of the steel to make mechanical bonding easier. The steel portion of the fork can be placed into the cavity of a transfer mold. The remaining cavity of the mold is in the shape of the handle to be formed. Plastic can then be injected, either by having an adjacent material cavity (a plunger) where the mold material is heated then pressed into the cavity, or by means of a continual feed screw and heating system. The mold, like an injection mold, is then chilled and the mold opens and the part is ejected. In order to control the heat changes more rapidly, metal molds are required. Now imagine the same process just described, but instead of making a fork with its single piece of metal inserted, the process is used to make a multi connector electrical plug. The industrial utility is apparent.