Silicone molds and polymer expanding foam


New Member
Hello everyone,
Currently I'm prepping to start a project that requires some mold making skills and was looking for input and advice from a far more knowledgeable group of souls. Nothing is set in stone but I wanted to throw my line out and make sure I was approaching the process from a logical angle.

I'm looking at making a soft casting of a few Sea Lamprey. Ideally, with some type of flexible rod core so it can be posed. I'm considering filling the mold with an expanding foam like what they use for prop weapons since it appears to be durable while still having give. On the other hand its pretty important that the mouth and dorsal fin cast well and are not easily torn or damaged. Sculpting the animal form out of clay will likely take way too long to do. However, I have the option of casting a real fresh specimen but I'm not sure i should try casting it as is or if freezing it in a resting pose would work, I'd imagine a frozen lamprey may interact poorly to being cast? I'd appreciate any knowledge, resources for casting organic stuff, material recommendations. Thanks!


Mr Mold Maker

Sr Member
That's definitely a difficult one.

What I would personally do in this situation is freeze it, and mold it in stone. Polyfoam on it's own typically doesn't leave a great skin, so a stone mold will let you skin it with latex, and also make very thin but strong fins. You'll want to do your latex skin in two layers, and the second you'll embed cotton fibers so the foam has something to grab to on the latex. I typically just tear up some cotton balls and stick them to the latex while its wet, and once it's fully dry lightly remove the fibers that aren't bonded to the latex.

I worked at a shop where we molded a 9 foot, partly taxidermy, alligator. I say partly because it hadn't gone through the whole process and was still very squishy. The smell when we cracked the silicone open was so foul it made me sick, so if you've got a fresh specimen...prepare for the horrendous smell when you open the mold. Haha.
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Active Member
Bear in mind that any foam you use will have issues with large air bubble where you don't want them. The best way to mitigate those are to create a mold with the proper venting, bu that means you will have to deal with patching the vents later. Foams are difficult to patch, though foam latex is easier to patch than polyfoams (though foam latex is far more difficult to use, and requires an oven for curing.

If you can get away with a liquid latex skin, that would likely be the easiest to deal with, though latex won't last as long as silicone skins...