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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Step 1: Tools and Materials

To make successful polyurethane foam castings, you will need, besides the polyurethane itself, the following:

kitchen scale (but use a dedicated one or put the whole thing in a clear plastic bag in case of spills)
small containers. paper or plastic cups work well; empty pill bottles are great but for the small neck kind you must cut this off with a knife
drill with a stirring attachment
stirrers - plastic spoons, popsicle sticks, single-use chopsticks all work well
paper towels
clamps (more on this later)
scraper (not shown)
mold release. I tried a couple things and Turtle Wax worked pretty well for the ABS-like plastic molds. Different mold releases may be better for other mold materials. (When I cast into silicone molds I did not use any release as silicone rarely needs it.)
paintbrush if your release doesn't come as a spray

It is also wise to have a way to write down the polyurethane part weights for future reference. See the discussion on step 3 for more on this.

Manny B says: Dec 24, 2012. 8:07 AM

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?

stringstretcher in reply to Manny BDec 25, 2012. 4:10 AM

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.

dgateley says: Dec 23, 2012. 5:36 PM

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.)

clazman in reply to dgateleyDec 24, 2012. 8:16 PM

Why not use the material Dentists use to create impressions? It is definitely safe and it is still elastic for removal.

dgateley in reply to clazmanDec 24, 2012. 10:20 PM

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.

clazman in reply to dgateleyDec 24, 2012. 11:17 PM

Thanks for the insight!!

wblack3 says: Dec 23, 2012. 8:14 PM

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 in reply to wblack3Dec 24, 2012. 8:18 PM

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..

Edgar says: Dec 24, 2012. 11:11 AM

Maybe it can be used for prostetics on Develloping Coutries, great idea! :)
Gone to my Blog:
http://faz-voce-mesmo.blogspot.pt/2012/12/avancos-contra-o-cancro-sobrevivencia.html

jwilliamsen says: Dec 23, 2012. 11:44 AM

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!

PuffBunny in reply to jwilliamsenDec 24, 2012. 1:02 AM

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.