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.