What's the difference between all these Foam Props?

That is a multifaceted question indeed!
Most foam type props are made from either Styrofoam(trademane), Expanded Polystyrene or Polyurethane foam. We'll examine the pros and cons.
Here comes some science...hold on tight...

Polystyrene is a polymer made from the monomer styrene, a liquid hydrocarbon that is commercially manufactured from petroleum. At room temperature, polystyrene is normally a solid thermoplastic, but can be melted at higher temperature for molding or extrusion, then resolidified.
Expanded Polystyrene (or EPS) is a rigid, closed-cell, white beaded polystyrene foam.

Styrofoam is a rigid, open-cell extruded polystyrene foam.


Polyurethane foam is produced by mixing two liquids, a polyether polyol and a polyfunctional isocyanate. A rigid foam is produced which is many times larger than the original volume.

OK, so here's the low down on what each is typically used for...

EPS is what everyone thinks of and calls, Styrofoam (which is actually a tradename like Kleenex). It is that nice lightweight, insulative material for coffee cups and to-go food containers etc.
It is also used for packing material (think foam peanuts), building insulation, crafts and model building stuff - the foam in Fome-Cor boards.
The Pros: It is very easy to cut with a hot-wire cutter. So it is very easy to build non-weight-bearing props and architectural models. It is also inexpensive to make and thus end products are also inexpensive for consumers to buy! (aka the "props"at large department/discount type stores)
The Cons: It's cheap, not very durable and also readily available at those large department/discount type stores.

Polyurethane Foam comes in two forms: the flexible cushion-type and the rigid-type, both come in many densities. (Here's more science stuff...)
Density is defined as the weight of 1 cubic foot of the material. In other words, a 12" x 12" x 12" cube of 1# density foam will weigh 1 pound, that same 12" cube of 5# denisty foam will weigh 5 lbs. Higher density foams are harder to the touch and more rigid, but they weigh more.
Flexible Polyurethane foam is probably what you are sitting on right now... the padding in your chair, your cushions, maybe pillows.
Rigid Polyurethane foam is "becoming increasingly important as replacement products for wood in several applications where substrate consistency and resistance to moisture and decay are important. The excellent chemical and solvent resistance of urethane compounds makes them useful where many other resins might fail."
The Pros: Basically, this stuff is tough and weather resistant!
The Cons: More expensive raw materials than beaded foams and seems more expensive to purchase in relation to other "similar" products...

Rigid Polyurethane is our foam of choice for the props we manufacture and sell.

Tune in the next few days to see stress test examples.

Poly Vs. Beaded...