I need some foam that, once cured, can resist 350 degree plastic. I'm using a 2-part bondo mold to pour the foam in. I've been experimenting with 8lb urethane foam, but once it gets much above 150 degrees it starts expanding, cracking, and much worse releasing tons of gasses. It doesn't seem to matter how long I let it cure, I just tested a piece that I poured two weeks ago and had equally bad results. Coating it with epoxy doesn't help.
I need something that can be poured into a 2-part mold, is buoyant, reasonably tough, and won't react when melted plastic is poured next to it.
Thanks for any ideas...