PVC foam board is plastic. Sintra is a brand name of it. Think of PVC pipes at the hardware store, that is the kind of PVC that you are talking about. There are also different densities of PVC so there is a lot to learn about it.
As far as armor goes I have been using PVC pipe and picking up scraps and trash here and there, I have spent no money on it. I have had pretty good luck making armor out of it and the price is right. Here is a pic of a custom I have been working on. It was made from 4 and 6 inch PVC pipe
I know this may not have answered your question completely but I think the "foam board" at Michaels is not anywhere near as strong as PVC pipe
I picked up some scraps from a construction site. Ther was quite a few short pieces of PVC that had been cut off and thrown away. The 6 inch pipe can be cut and made into just about all of the armor you need, you just cut it out using the WOF templates and then heat them up to flatten them out. The back plate is about the only thing that is too big to get out of a 6 inch pipe. If you find some 10 inch pipe being thrown out you can cut out a back plate, I think a 10 inch pipe has like a 22 inch circumfrence.
Some times I have had to clean the mud and rocks off of the pipes before they are ready for use, but I have had good luck with it. The other thing that I have done is to go by sign shops and ask for scrap sintra I have had good luck with that too.
thanks very much for the information it has been quite handy, And also i have a sign-a-rama by my house i was wondering do you just say "i was wondering if you had scrap sintra i could have/buy?" or are you more vague and say scrap plastic?
Expanded cell PVC foam board has got jillions of little air pockets inside of it, so it is much lighter than solid PVC pipe or sheet. It's very workable, bendable with heat, and it's pretty cheap. It's much cheaper than solid PVC because there's airspace in it. I go to a local plastic supply place, the sort that would provide materials for a sign store, and buy it by the sheet. I haven't made Fett armor, but from the R2D2 stuff I have built, I'd feel totally comfortable shaping some armor from it. Comes in all thicknesses. I go to Interstate Plastic Supply here in Sacramento. Interstate Plastics