Depending on the containers your polyurethane comes in, you might also like to pour a fair bit of the polyurethane parts into separate cups to use as a reservoir, if it's annoying to pour from the main container. You want to have everything you need- scale, cups, stirrers, drill with stirring attachment, clamps - laid out ready to use on newspaper or a plastic sheet before you start mixing. Polyurethane foam has a pot life of approximately 30 seconds until it starts expanding noticeably. "Submitted by Ace Monster Toys for the Instructables Sponsorship Program" It is common to use plaster of paris for the outer mold, and I have also successfully used thermoplastic in sheet form. Expanding foam does just that, it expands, and it will try to push the mold pieces apart, or deform the mold if it is not rigid enough.
Note that if you are going to use a silicone, or other rubber mold, you will need to be sure to use a "mother" mold, which is an outer mold that is stiff, unlike the stretchy rubber. And I would expect that molds of plaster, wood, or most other nonreactive materials would also work, with proper mold release. I have had good results casting polyurethane foam into silicone molds as well. I'll show pictures of the 3D printed molds I've used but you can also cast into molds made from many other materials. It's slightly harder to work with than those other materials due to its very short pot life, but with a bit of practice you can achieve good results. It comes in a two-part kit, similar to most RTV silicones and casting resins. The best material for this is polyurethane foam. The molds are for animal ear shapes (it's a long story) and they need to be squishy and light weight.
#Casting plaster in silicone mold series#
I have been making a series of 3D printed molds recently (see Making a 3D Printed Mold) and figured I should explain how I'm doing the casting.
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