How to Make a Mold for Pour Foam

Written by scott knickelbine
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How to Make a Mold for Pour Foam
Poured urethane foam is a common material for casting floating objects like duck decoys. (duck decoy image by Julianna Olah from Fotolia.com)

Pourable urethane foam is an excellent choice when you want to cast lightweight shapes. It is particularly good for casting objects that must float, such as duck decoys. One important property of urethane foam is that it expands after it is mixed. For this reason, you'll need to use pourable urethane with a two-part, rigid mould that can withstand the pressure of the expanding foam. Two-part moulds take a little more doing, but the resultant mould will allow you to make dozens of urethane castings.

Skill level:
Moderate

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Things you need

  • Model for duplication
  • Scrap wood
  • Nails
  • Hammer
  • Modelling clay
  • Petroleum jelly
  • High-strength gypsum cement
  • Clean plastic bucket
  • Stirring stick or mechanical mixer
  • Spray polyurethane sealer

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Instructions

  1. 1

    Build an open-top box with scrap wood. The box should be just large enough to hold the model, with about an inch of space around all sides. The box should also be deep enough so you can cover the model with about an inch of plaster.

  2. 2

    Fill half the box with modelling clay, pressing it down into the box and smoothing the surface.

  3. 3

    Lay the model on its side in the box and press it into the clay so that half the model is above the clay and half is in it.

  4. 4

    Cover the exposed surface of the clay and the model with a thin layer of petroleum jelly.

  5. 5

    Mix five parts of the gypsum cement with two parts of water, or according to manufacturer's instructions. Allow the water to soak into the cement for five minutes, and then mix for five minutes with a stirring stick or mechanical mixer.

  6. 6

    Pour the cement into the box, up to just under the upper edge. Tap the box to make sure any trapped air bubbles escape.

  7. 7

    Allow the cement to thoroughly harden (24 hours, or according to manufacturer's directions). Invert the box and remove the model from the hardened mould.

  8. 8

    Place the hardened mould back into the box, with the moulded side up, and put the model back into the mould. Cover the exposed area of the mould and the model with a thin coat of petroleum jelly. Mix another batch of gypsum cement and pour it over the model and mould in the box.

  9. 9

    Allow the gypsum cement to completely harden, then invert the box and remove the mould. The two halves of the mould should come cleanly away from the model.

  10. 10

    Spray the inside of both halves of the mould with a polyurethane sealer to seal any pores in the cement.

Tips and warnings

  • To use the mould, apply a mould release compound to the inside of the mould, then mix two-part pourable urethane and pour it into one half of the mould. Immediately place the other half on top of the first and clamp the two halves together with a C-clamp. Separate the two mould halves after the recommended cure time for the urethane foam has elapsed.
  • Some duck decoy makers like to use separate moulds for the head and the body; this way they can assemble the decoys so that the ducks are looking in different directions.
  • Gypsum cement can get very hot as it is curing; never use this product to do life casting of body parts or for casting of any heat-sensitive models (such as candles). Remove any cement from exposed skin immediately.
  • Cover all exposed areas of the model and the clay or mould with petroleum jelly before pouring each batch of the gypsum cement. This cement will stick readily to uncoated surfaces.

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