Craft Foam could be used for the chest armor, but to get the right look to it it will take a lot of work. I did a web search on Excite and found a web page that discusses the use of craft foam in the use of makeing Elven Armor from the Lord of the rings movies and should give enough information to do Zam parts if you wish. Between Craft Foam and Sintra I got two new friends in materials for armor making. Also I would pose this question on one of the fett parts of the board as I know one or two of the Fett guys knows the lady who made the Elven Armor out of the Craft foam. I met her at Dragon Con and even sat through a panel on panel she was on that discussed things on armor making.
You can vacuum form both craft foam and Sintra. Most people don't realize that.
Craft foam is easy, but there's a couple of tricks: (1) heat it low and slow, and (2) don't expect it to sag.
For Sintra, you also want to heat it low and slow, and it's good to have a high vacuum system. (Which can be easier & cheaper to make than most people realize.)
When vacuum forming craft foam (EVA foam), you want to get it to about 250 degrees. Since it doesn't sag---it just disintegrates if you heat it too much---you either want to use a touchless infrared thermometer (about $20 on sale pretty regularly at Harbor Freight) or watch for a change in the surface texture... it gets a little smoother and glossier, not so matte.
You can get EVA foam in other thicknesses and densities, and it vacuum forms very well. (I've had good luck vacuum forming pieces of a 3/8" thick TexSport brand camping mat... that's very light stuff, though. For most purposes you want something denser.)
3mm craft foam makes wonderful vacuum-formed masks... flexible, but they hold their shape well. Hobby Lobby has good prices on craft foam.
do you have pics of vac formed craft foam? just wondering. One thing to consider, craft foam although you say smoother and glossier, is it as smooth as sintra when vacc'd? The end product of rubnbuff/paint on the chestplating is really important for end product to look "just right." I would be afraid that the foam, if porous, would be too matte in comparison to the sintra. From chestplating on down through the resin lower armour, things have to match on the Zam costume; with the color of flat aluminum, thus would pourous craft foam actually be smooth enough to hold the colors well to compare with resin?
Durability - if you don't use a thick enough grade of sintra/styrene for the chest plating, you may suffer from cracking your chest plating screwing in the center hinge and/or the lower armor not being able to be supported. Thus, I would need to also really question the durability of craft foam~ does it hold the weight? And, would it support a center hinge?
I tried using craft foam to make various pieces of Zam's armor, but wasn't pleased with the durability or the look. It doesn't ~appear~ to rip easily, but when attached to heavy stuff it just didn't hold up. And I don't remember what I used for coloring, but I never got the texture and color to look good. I'm still using it for pieces that I need to WEAR Zam in public, but have no good alternatives yet (currently just the shin guards, with the bottom section over the foot cut from a roll of cork).
Craft foam is not very durable---it's not a substitute for hard/tough plastic armor. It's very good for some other things, though, and it's easy to make a new one if you're vacuum forming. It only takes a few minutes.
Here are some examples...
I whipped out this mask for my wife one afternoon, starting from a cast I'd made of her face, for a party that night:
The one on the left was my first try at 6mm Foamies, without a high vacuum system and probably without cooking it long and slow enough. It came out disturbing-looking; I like that.
The one on the right is 2mm Foamies (or Funky Foam from Hobby Lobby; they're pretty much the same), also with just a vacuum cleaner for suckage.
You can smooth out vacuum-forming bucks by vacuum-forming thick plastic over them and recasting them from that. (You may need to redo any edges that need to be sharp, of course.) I intend to use craft foam for that in the future. (I hate sanding.)