Jayseas,

Most foam used for models is cut by hot wire from larger blocks, usually insulating polysterene stuff. Chain hardware stores may carry that in 1", 2" or thicker slabs up to about 4' by 8'. There are a few special foam materials that can be cut with knife or saw - and the smooth blue insulating foam often seen in house construction cuts and sands nicely (and can also be hot-wire cut.) More likely to find the blue foam at a homebuilding supplier than a hardware chain store.

The foam block materials come in different weights: Common styrofoam is around 1 lb per cu ft. The blue foam is around 3 lb/cu ft. The specialty hobby foam materials? Not sure... Styrofoam is about the least strong, but the shaped foam pieces are almost always sheathed in thin balsa sheet, other thin veneers or occasionally in fibreglas and carbon fiber "layups."

There are a few other plastic foam materials - like those used in meat trays - which you can see a lot about in RC Park Flyer topics in model magazines. I've never used it, but, for instance, AMA's current Model Aviation mag has an article on suitable adhesives for this stuff. It cuts with a sharp hobby knife, and possibly also with scissors...

Larger RC ARF models may include part of their structure (e.g., wings, tails) built around hot-wire cut foam...

You'd need some sort of a controllable transformer to get a length of wire to the just-right temperature to cut block styrofoam cleanly. That, and a few special tools, and good templates to follow, and some useful experience, and good ventilation (heated styrofoam fumes can be toxic), and some useful experience. The wire is usually held by a frame that keeps it stretched tight, and allow connecting to the transformer.

Hot-wire foam cutting is not something to dabble in for one use; it requires a bit of involvement and persistence to get best use of.

For CL, most foam has been used as a form to have thin sheet balsa glued to, to build wings and tails. A month or so ago, Flying Models mag had a very innovative article explaining how to shape the wing surface by gluing it onto foam, in a method strong enough that it didn't need the foam. So, the guy used the hot-wire INSIDE the sheeting, to CUT OUT most of the foam! The model in the example was a large stunter, so BELIEVE it had plenty of strength!

Are you starting to regret asking?