How to Make a Urethane Foam Cast

Written by jennifer meyer
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How to Make a Urethane Foam Cast
Polyurethane foam expands because it is largely composed of air. (pink foam image by laurent dambies from

Polyurethane foam has several advantages over other casting materials. Foremost among these is its expansion rate. Many polyurethane foams expand to be many time larger than their liquid components, so it takes very little material to make a casting. However, this also makes it difficult to gauge just how much foam to use, as the expansion rate varies from product to product. Always consult the manual that comes with your polyurethane kit for precise measurements.

Skill level:

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Things you need

  • Newspaper
  • Mold
  • Mold release
  • Gloves
  • Respirator
  • Polyurethane foam kit
  • Mixing cups
  • Mixing rod
  • Large paintbrush
  • Wood plank
  • 2 clamps

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  1. 1

    Lay three layers of newspaper down over your workspace. Make sure your workspace is in a well-ventilated area. Put your mould in the centre of the newspaper. Don a pair of gloves and a respirator.

  2. 2

    Spray the inside of your mould with mould release. Mold release is a liquid that keeps your cast from sticking to the inside of your mould. Use a mould release manufactured by the same company that made your polyurethane foam and which is intended to be used with the foam. Mold release often comes in a spray bottle, although sometimes it comes in a brush-on form. Spray or brush the mould release on one side of a wooden board, as well. This board should be large enough to cover your mould, and it should have several small air holes drilled into it.

  3. 3

    Open the pair of bottles that come with your polyurethane kit for speedy access. These bottles contain the liquid polyurethane and its catalyst. Pour equal amounts of each liquids into two separate cups. Consult your manual when measuring these liquids. Some foams expand dramatically when the two liquids are mixed, so you so how much you use depends on your particular product.

  4. 4

    Pour the two liquids together into a third cup and stir it for 30 seconds. Quickly pour the mixture into your mould, using a paintbrush to spread it into hard-to-reach crevasses.

  5. 5

    Place a wooden board on top of your mould with the sprayed side facing down. Use clamps to attach it to the mould on either side. This board will keep the foam from overflowing the mould. Gases formed by the chemical reaction in the foam will vent through the air holes drilled in the board.

  6. 6

    Allow the foam to cure. This will likely take 15 minutes to half an hour, but the exact time depends on the kind and brand of foam you have.

  7. 7

    Remove the clamps and break off any excess foam that bubbled up through the holes in the wooden board. Remove the wooden board and pop your foam piece out of the mould. Rinse it with water to clear away any remaining mould release.

Tips and warnings

  • Work in a ventilated area and keep children and pets away from uncured polyurethane foam.

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