Picture of Casting Flexible Polyurethane Foam
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.  The molds are for animal ear shapes (it's a long story) and they need to be squishy and light weight.  The best material for this is polyurethane foam.  It comes in a two-part kit, similar to most RTV silicones and casting resins.  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.

I'll show pictures of the 3D printed molds I've used but you can also cast into molds made from many other materials.  I have had good results casting polyurethane foam into silicone molds as well.  And I would expect that molds of plaster, wood, or most other nonreactive materials would also work, with proper mold release.

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.  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.  It is common to use plaster of paris for the outer mold, and I have also successfully used thermoplastic in sheet form.

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AnilG21 month ago

will flexicast made of silicon rubber which are washable, breathable and castable be called as innovation, thanks for advice


anilgb @

kyle.kepley2 months ago

Your molds seem exceptionally smooth for 3D printed ABS. Did you smooth them out using acetone vapor or something? If not, what printer did you use?

rachel (author)  kyle.kepley2 months ago

I used an Objet Connex printer, which has an extremely fine grain. I wrote about making the molds here:

Manny B2 years ago
I'm new to casting. As in started looking into it last week. This is my newbie question: Don't you need a breather hole for the excess foam to push out the air?
You are thinking! Here you can see the air escaping out the seam on the lid. I would build in an air hole on the top-most part of the mole, allowing the largest mass of air to go where you want it, filling the mold more completely.
dgateley2 years ago
Oh, and are there precautions against using the polyurethane on skin? I'm still looking for something to fully cast my ear from the drum through outer ear. Hard substances won't work because of the danger of damaging the drum from suction, among other possibilities, and the use of foams had not occurred to me (however obvious.)
Why not use the material Dentists use to create impressions? It is definitely safe and it is still elastic for removal.
That is usually alginate which I've done some test casting with. It is very elastic, it's true, but it has insufficient (negligable) tensile strength for removal with any integrity after it sets. Pull on it and most of it remains inside of whatever you cast it in.
Thanks for the insight!!
wblack32 years ago
Nice work! Would putting a small drill hole at the top of the mould help divert the overflow that would otherwise push apart the mould pieces and make the final cast not quite right?
clazman wblack32 years ago
Should work. It is like using a :riser: in a metal casting for venting air. Problem is it needs to be in the correct spot. In your case at the last point to fill..
Edgar2 years ago
Maybe it can be used for prostetics on Develloping Coutries, great idea! :)
Gone to my Blog:
jwilliamsen2 years ago
Two minor ideas to try: using petroleum jelly as a parting compound (you can mix it with naptha to thin it out if you need to), and secondly, if you want to increase the pot life of your foam a bit more, you could put the measured components in a fridge for a while to get them cold. I notice a pretty big difference in pot life when I make castings in cooler weather, or my molds are cold.... Cool instructable!
I second the petroleum jelly - but you don't need to mix it with naptha - just warm it gently and brush on.

Good tip on chilling the foam components before mixing.
n2citrus2 years ago
How long will this material last? Thinking of making fingers for a robotic hand, but it needs to be long lasting. Very cool stuff!
dgateley2 years ago
Filed away for a dozen uses. Many thanks.

What stirring attachment do you use?
This is awesome! Favoriting!
sleeping2 years ago
bertus52x112 years ago
Thanks for sharing. This is very useful!