In the expanded polystyrene casting process, a sand mold is packed around a polystyrene pattern representing the metal casting to be manufactured. The pattern is not removed, and the molten metal is poured into the pattern which is vaporized from the heat of the metal. The liquid metal takes the place of the vaporized polystyrene and the casting solidifies in the sand mold.
In metal casting industry this process is known as the lost-foam process, evaporative pattern casting, or the full mold process. A large variety of castings of different sizes and materials can be manufactured using this technique. Parts produced in manufacturing industry using this process include crankshafts, cylinder heads, machine bases, manifolds, and engine blocks.
The first step in the evaporative casting process is to manufacture the polystyrene pattern. For small production runs, a pattern may be cut from larger sections of polystyrene material and assembled together. For large industrial manufacturing processes, the pattern will be molded. A die, often made from aluminum, is used for this process. Polystyrene beads are placed in the die and heated, they expand from the heat and the foam material takes the shape of the die.
Depending upon the complexity of the casting, several of these polystyrene sections may have to be adhered together to form the pattern. In most cases the pattern is coated with a refractory compound, this will help create a good surface finish on the metal casting. In addition to the casting itself, the foam pattern will also include the pouring cup and gating system.
The pattern is then placed in a flask and molding sand is packed around it. The sand may or may not contain bonding agents, dependent upon the particular manufacturing procedure.
The molten metal is then poured into the mold without removing the pattern. The liquid metal vaporizes the polystyrene pattern, as it flows through the mold cavity. Any left over product from the vaporized polystyrene material is absorbed into the molding sand.
The molten metal is then allowed to harden within the sand mold. Once solidified, the casting is removed.