Hey Bubbels, if you're using a one part polyfoam, (like Great Stuff) it needs a small amount of moisture to cure. You say caved in ones... that makes me believe the foam didn't cure in the middle. Well, cured at a much slower rate at least.
When used to fill a large void, such as a skull mold, very lightly mist the mold with water before adding a layer of foam. Mist again and add another layer, repeating until you have enough foam to fill the mold. Don't wait between layers, you want the water entrapped in the foam. Be careful not to create air pockets at the surface of the mold when doing the layers.
Or... Go for a hollow cast by misting, then covering the surface of the mold with foam. Another light mist wouldn't hurt at the end.
Scourge, I'm a little surprised you had such troubles with a two-part foam. It looks to me that you used a rigid non-skinning foam. If so, I think the problem may have been with pressure building up in the mold, restricting the expansion. I believe that if your mold had a vent hole, you would have found better results. If you want a hollow skull, a straw through the vent hole into the center of the mold works well.
Even though the skulls are not exactly what you were shooting for, I say they still have potential for some really cool looking props! Even just a fragment of a skull is useful!
Don't give up on the foam skulls you two, I think you both will be happy once you get the kinks out.
