Casting Flexible Polyurethane Foam




Introduction: Casting Flexible Polyurethane Foam

About: I run Neal's CNC in Hayward, CA, an expert CNC cutting and fabrication service. Check out what we do at I'm a founding member of Noisebridge, a hackerspace in San Francisco, and Ac...

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.

Step 2: Preparation

Polyurethane foam has a pot life of approximately 30 seconds until it starts expanding noticeably.  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.  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.

Mold release can go on before or after you're all set up, but don't let it dry out too long or it loses some of its effectiveness.  If you are using a commercial, specially formulated mold release, it may come in a spray can in which case spraying it on is pretty straightforward.  I found car wax to work pretty well for the combination of polyurethane and the Objet Connex printers' ABS-like plastic that my molds are made from (for some reason a spray silicone release, which I've used successfully on many other projects, didn't do so well in this case).  I brush it on with a paintbrush, being careful to get it well into any overhangs in the mold.  It is also a good idea to put mold release on the areas of the mold that fit together, and in the registration keys.  If the foam overflows, it will touch these areas, and it's nice if it doesn't stick.

Step 3: Mix and Pour

The big question here is, how much to mix?  The polyurethane I use (from Douglas & Sturgess) is said to expand about 10x.  However it is mixed by weight, 3 parts of part B to 1 part of part A.  So how much do I mix to fill the mold up?  I don't want it to not fill out the mold completely, nor do I want huge overflow, or (if I've clamped super tightly) a foam that is too dense.  The expansion is pretty strong but it is not an irresistible force; if there's an immovable mold it will simply not make holes as large as it wants to, resulting in a denser, more rigid foam.

I haven't found a better answer than trial and error for this; however here's how I came up with a starting point.  I filled one of my molds (which had one half with the bulk of the hollow part and one that was basically a lid) with water, and measured that.  Then I took a tenth of that and poured it into one of the cups I use to mix.  I filled another identical cup to 3/4 of the water height with part B of the polyurethane and weighed it.  I then used the closest multiple of 3 (in grams) for the part B, since I only have a gram precision scale.  So I can have increments of 4 grams, which so far has proved to be sufficient.  I usually prefer to overfill a bit, and let the foam squeeze out the sides of the mold, because I can easily cut it off along the parting line with scissors.

The expansion is affected by a number of factors, including ambient temperature, ambient humidity, exact mix proportion (i.e. is it actually 302:100 or 293:100 rather than exactly 300:100), age of the polyurethane parts, and probably others.  Higher temperature increases expansion; higher humidity and age of the parts decreases it.  Basically it is nearly impossible to get EXACTLY right, but it's flexible, so that's OK.

Anyway on to the process!  Pour a cupful of part B to the right weight, not forgetting to tare the scale to your container.  Pour a third of that weight of part A into a different container.  With the mixer, molds and clamps right there to hand, pour the part B into the part A cup.  Hold the bottom and mix with your drill for ten or 15 seconds, scraping the bottom edges and sides as much as you can.  Set the drill down on your drop cloth and immediately pour the mixture into the bottom part of your mold, using a stick or spoon to help it out.  Place the top half of the mold on, and clamp.

I have been using string to clamp my molds together, as I didn't have any large enough clamps around.  It works OK.  The advantage is that it doesn't matter what the shape of the mold is; I can always wrap string around it.  Clamps require two parallel surfaces, which my molds often have, but not always.  It is also possible (but again requires parallel surfaces) to weight the mold with a heavy stack of books or something like that, but the foam is remarkably strong when expanding and you really need a HEAVY stack.

Step 4: Demolding

Quite quickly the foam will expand to its final size.  My experience was that this happened within about 5 minutes tops.  The foam requires another 20 or 30 minutes to cure before it can be safely demolded but you'll see it start to overflow, if it's going to, within a minute or two.  You can track its curing process by the reduction in stickiness of the overflow (if any) but 30-40 minutes from mix should be plenty.

To demold, first remove the clamps (or string) and scrape off any overflow with a scraper.  Then work the top half of the mold off as gently as you can.  You may need to pry with the scraper or a screwdriver.  Go gently, you don't want to tear the foam.  Generally you can hear it peeling off the mold, and as long as you hear that sound, don't add any more pressure, just keep pulling gently and consistently.

Once the top of the mold is off, you can grab the casting with your fingers and work it loose from the bottom half of the mold.  Same deal: gentle but consistent.  You may be able to work your fingers in between the foam and the mold to loosen it.  It'll be somewhat slippery from the mold release but still grippable.

Once your castings are out, trim them at the parting line (if necessary) and wash off the mold release with soap and water.  You probably don't need to wash the mold itself, but wipe off any remaining mold release so you can add fresh for your next cast.



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    23 Discussions

    Can we make 2 mm thickness sheets by Polyol felexable and Isonate
    In epoxy mold

    I’d like to make a casted fursuit Head out of this
    Would I be able to make a clay base then just pour this over the base?
    It’d be too much clay to make a cover to go over it, so would that work?

    Awesome ible!

    I hope to use this foam
    product someday, with a wet-suit, and a pair of neoprene rubber gloves
    to make a life-sized model of the robot Jet Jaguar (from the 1973 movie
    "Godzilla vs Megalon")


    Hello, I'm trying to find the stuff to make the actual foam, could anyone tell me what it's called and suggest some companies that make it and places that I could get it?

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


    anilgb @

    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?

    1 reply

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

    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?

    1 reply

    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.

    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.)

    3 replies

    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.

    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?

    1 reply

    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..

    Maybe it can be used for prostetics on Develloping Coutries, great idea! :)
    Gone to my Blog:

    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!

    1 reply

    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.

    How long will this material last? Thinking of making fingers for a robotic hand, but it needs to be long lasting. Very cool stuff!

    Filed away for a dozen uses. Many thanks.

    What stirring attachment do you use?