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How to Cast Polystyrene

Use polystyrene to make a variety of casting molds.
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Polystyrene is more commonly known as Styrofoam, and consists of 95 percent air. The material has good insulation qualities useful for keeping liquids warm or cold. Casting with polystyrene is used to manufacture metal molds that later house a variety of materials. The process is referred to professionally as a lost-foam process or a full mold process because the polystyrene is vaporized and replaced by the molten metal, leaving a mold in its place. The technique is popular in the automotive industry to produce parts, including cylinder heads, crankshafts and engine blocks.

Construct your mold. Smaller molds are usually constructed by cutting pieces from a larger piece of polystyrene and then assembling them. Larger molds come from an aluminum mold filled with polystyrene pieces, then heated. The pieces take the shape of the mold and the finished mold can later be removed. Insert a pouring spout into the top of either type of molds. Apply refractory paint to the finished product with a sponge applicator to prevent surface defects.

Fill a large metal flask partially with sand and place the mold so the the top of the pouring spout is level with the top of the flask. Pour in the remaining sand, periodically shaking the flask so the sand distributes evenly. When the sand completely covers the mold and is level with the top of the pouring spout, check that the vents on the flask are open.

Put on heavy work gloves and heavy goggles. Slowly pour in the molten metal until it reaches the top of the pouring spout. The heat from the metal will cause the polystyrene to vaporize, gradually letting the metal assume the shape of the mold. Any gases produced will pass through the vents in the flask. After the metal has hardened, remove mold from the sand.

Things You'll Need:

  • Polystyrene
  • Aluminum mold
  • Pouring spout
  • Refractory paint
  • Sponge applicator
  • Sand
  • Vented metal flask
  • Molten metal
  • Heavy work gloves
  • Heavy goggles


No binders are needed for the sand, so it can be reused. Applying refractory paint can be time-consuming, but is essential for a good finished product. If you need a core in your mold, secure it in the cavity of the polystyrene mold before pouring the metal.

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