Moldmaking Mold core?

I use crumpled up aluminum foil, not too tightly crumpled.

Then depending on what materials I'm using, either cling film or a masking tape layer.

 

It depends on what medium I am working in, but I usually use tin foil or a combination of balled up tissu paper and tinfoil.

If it's a part I am fairly confident about the shape of I will try to make a tightly packed tin foil core in the shape I want the actual inside of the piece to be. Keeping in mind that the mold needs to be able to be removed from the inside of the finished piece I use the core as a test for that. If it's easy to remove I did it right and the mold will very likely be easy to remove as well. And if not, well, lucky I found out when I did, right?
Even if silione is soft and can stretch and compress a bit to get past overhang, when the polyurethane is freshly cast it will be slightly soft and can deform if it needs force to get it out of the mold, so it's always good to plan ahead.

 

I did this for my doll, she was made with paper clay and epoxy, some of her parts were made with paper balls and put the shape together with a layer of masking tape, it worked perfectly! ^_^

 

crumpled up tin foil

 

This seemed like a decent place to ask- does anybody know how spray-on insulation foam (like the canned Great Stuff) might work for a core?

 

I used to use it as the core for paperclay cosplay horns! it's pretty similar to styrofoam but doesn't crumble as easily and is very light and sturdy, so it could probably work! You'll just likely need to test and see if acetone or similar materials will dissolve it.

 

Awesome, thank you! Guess I need to get to experimentin'!

 

For airdry, stoneclay and paperclay I used plasticine covered with gauze diped in plaster of paris. I hated stirofoam, it was hard for me to cut and messy. I choped so hard my finger I nearly need to go on stitching by cutung stirofoam. So I invented core.
Idea was to create easy core- with plasticine and make insulation which is in same time
a)good for adhesion of stoneclay, b) hard and keeps shape, c)keeps away grass from plasticine.
I made it on plasticine with same process as doctors put on broken hand. So I made core from plasticine, mummy it with bandages and dip it in plaster of paris.

 

It depends on what medium you are working in. If you are working with polymer clay, wax or anything else that needs heat while working you need to avoid to trap air in the core, as it will expand from the heat. The foam you mentioned would be a type of soft polyurethane, right? That should probabaly not be heated at all to avoid toxic fumes.
For air dry clays it should probably work fine, but you how du you control the shape of the core?

 

OH, that's an excellent point! I was thinking for air dry clay.

As for shaping, I find it a little easier to carve than standard styrofoam.