Hi
So this has happened to me a couple of times with a couple of different moulds and it would be helpful to know what I'm doing wrong. Anyone's contribution would be helpful as it's something I really need to get to the bottom of for my work.
I made a RTV silicone one piece mould from the attached cut glass butterfly. The original was secured to the base with sulphur free plasticine in a home made foam board box. The silicone was degassed and the mould was cured in a pressure pot. The base of silicone was quite thin (probably only a few mm) as I was running out. 
I poured some epoxy resin into the front part of the mould and left it overnight (this epoxy normally hardens enough after 6-8 hours)  in the pressure pot. I removed it the next day to add another layer. While it was sat there the base expanded as obviously some trapped air has expanded and created a bulge. The seal wasn't broken so the cast was not lost but I need to know what (if anything) I am doing wrong? It also happened to a curled up dog that had no undercuts or obvious places for air to get trapped. I can't remember what the base or mould box was in that instance.
Some suggestions I have been given:
- Silicone base was too thin
- never use paper backed foam base board in a vacuum chamber or pressure pot
- you don't need to cure the mould under pressure

What are your thoughts on the suggestions? I need to re-make these moulds and need to know if I shouldn't be using foam board, not putting them in the pot to cure and what should be the minimum thickness of silicone?

On another note. I de-gassed some RTV silicone (not a bubble left) and poured into 2 separate moulds. Both were in boxes made from foam board. One was a granirte horses head (with a wooden cocktail stick sprue) and the other was a shiny ornament (Not really sure what it's made from, maybe resin, maybe stone). I then placed the moulds with silicone back into the vac chamber (separately) and they bubbled like a witches cauldron?! Was it the foam board do you think? It has to be doesn't it?

Really looking forward to any wisdom on this.
Many thanks - wendy