I spent this week trying some molding on my own. I finished getting rubber on the jawless skull. I tried to create a plaster of paris mother mold by placing it on by hand. I ended up cracking both halves. I ended up cutting out a profile of the head out of thin wood and placing in the middle of the rubber mold and then creating a 1 inch thick mold of plaster. I then removed the wood divider, covered the seam in valsaline and then poured the other half. This worked GREAT! I quickly poured a rigid foam mix in the mold and gave it a try. It turned out great except the teeth broke off and the head didn't fill completly.
I finished the rubber on the jaw. I poured a rigid foam mix right into the rubber mold. The jaw turned out a little puffy because the foam pushed the mold appart. So I needed a plaster of paris mold for it. I covered the entire jaw mold in plaster of paris. Big mistake. I couldn't get the rubber mold out. I had to chip away at the top of the mother mold enough to get the rubber mold out. I cast another jaw which looks much better but still had some issues. I think I am going to have to make a 2 part mother mold for the jaw too.
I created a rubber mold of my fancy carved skull. I then used the same technique and made the plaster mother mold in 2 halves. I then cast it in rigid foam. I had to pour some extra foam to fill it in. Which worked great (no seam).