"mm" wrote
There's more than one kind of stryfoam maybe. What they had in the
50's was foamed plastic, hence the name. Stiff, with lots of air
bubbles.

Then theres the kind with lots of 3mm. beads crushed together. I don't
know for sure if that is styrofoam or not.

Maybe there are more.

There is only one Styrofoam. It is the blue extruded polystyrene foam board
sold by Dow under the registered trade name of Styrofoam. Owens Corning
sells a similar board (Foamular) colored pink, GP sells a yellow board.

There is also the white material that is actually expanded polystyrene board
(EPS) that is molded as a large billet and then cut into slabs by hot wire
cutters. It is often referred to wrongly as Styrofoam but the Dow lawyers
will correct you on that. The only problem with the white eps board is that
suppliers often make it as cheap as possible and do not fuse the bead as
well as it should be and you get some loose bead.