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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