well my helmet is fiberglass and I have left it for a few days in my truck in Vegas heat and I didnt have a problem. why are you concerned with heat, what are you planing on doing??????
Well, I love Star Wars as much as anyone here on Mercury, but honestly the costumes just don't seem to last...
Actually, I'm thinking about using a metallic film instead of paint, and they're heat applied. I'll be experimenting with it on some plastic armor pieces soon, and if it works well, then I'll need a helmet to match.
Actually fiberglass is a kind of plastic. Fiberglass is just that(glass fibers) but the resin and gelcoat are two-part chemical plastics. It can take much more heat than vacu-form plastics. I don't know how much is too much, but I've put my fiberglass gauntlets and helmet thru quite a bit to reshape parts with no problems.