The hardener itself will tear through that styrofoam, it's brutal stuff.
Epoxy resin won't eat polystyrene.
Your best bet is Polyurethane foam. You can buy it in 4 x 8 sheets in varying densities.
alright, so I decided to do a little experiment to end conjecture (including my own)
here are my two test cups. both polyester resin, I didnt try vinyl ester nor epoxy, I have neither here right at this second. the left is home depot brand, cheapest money can buy. the right side is surfboard resin, the same stuff but clear and UV protected. also double the cost.
below you can see the two test pieces. One is the polystyrene expanding balls. you see this stuff in like computer monitor boxes and stuff, its made in molds. also, when it overexpands and cant be used for packaging, its the beans inside beanbag chairs. the second stuff I tried is packing peanuts, three of them. neither of my test pieces are the foam I used in my successful glassing attempt. I dont have any of that lying around.
just added to each cup, no catalyst. so far, so good.
styrofoam is still all there, so far so good
there go the peanuts, they dissolved. no catalyst yet.
you cant see it here, but the tip of the PS is gone. I had jsut added teh catalyst, but it had been slowly eating up. the bottom of the PS is nearly gone in teh surf resin cup, it was far thinner. so without catalyst even, resin eats packaging styrofoams.
you can see the peanut disolving without even disturbing it.
final pic, you can see everything disolving more, catalyst has been added, not that it makes a difference.
resin eats polystyrene.
Just Rap it in plastic, or Duct tape.
^ or like i said good ol aluminum foil
i use paper mache to get a mold. let it dry completely. then line the inside with foil and masking tape.
Wax paper, saran (plastic) wrap also work real nice like. Just tape it in place where you don't want the resin.
The only problem with aluminum foil is that it tears easily... but as said above, it will also work.
Yeah the foil does tear. Usually it doesnt matter since it could be sanded off. But if you want to be thorough you can spray the mold with wax.
I have a wrecked RV that I wanted to form some foam and then fiberglass over it when I ran into the melting prob (thankfully in a pre test) so I found this site. I still don't know where to purchase urethane foam in sheets yet tho.
Urethane Foam Is The Answer
urethane foam can be had from www.freemansupply.com
I personally use RenShape 5025 from them because it's dense enough to hold a very sharp edge, yet still hand-workable.
it is resistant to most every kind of resin and is a good choice for making a pattern for FRP projects and can be finished to a mirror shine with the right paints if you're just making cosmetic parts.
if you're interested in using cheap foam like EPS (blue/pink boards from home depot) then i would suggest either using an epoxy resin (no styrene, thus no eating foam) or painting your shape with a thick layer of latex paint. This will provide a barrier between the resin and EPS. Should help preserve the shape better than aluminum foil (just because foil wrinkles and paint goes on smooth)