The steel frames provide strength for the epoxy insert as well as providing mounting fixtures for installation on production injection molding machines. The epoxy is cast around a pattern of clay or plaster. The epoxy is held within a steel box. The cast epoxy mold can produce 5-100 parts depending on the injection pressure and material used. Epoxy molds are especially appropriate when low temperature, low viscosity plastic materials, polystyrenes, polyethylene, cellulose, and polypropylenes, are used.
Epoxy molds are good insulators but the molded part cools slowly.
This extends cycle times, increases shrinkage, decreases internal stress and warp of the part. The cavities in a epoxy mold are designed larger than the cavities produced in a production steel mold to compensate for the higher shrinkage of plastic molded in these types of molds. The use of conventional machining techniques to shape plastic solids and foams stock is often the fastest and most economical way to make plastic prototypes. A special grade of high density polyurethane foam is often used to simulate the external appearance of the prototypes. The large foamed blocks have the density of common hardwood.