Don’t get confused by the alphabet soup of RC model airplane foams.
Foam has revolutionized RC model airplane building. It is inexpensive to buy, costing a lot less than an equivalent block of balsa. It is of a consistently high quality and widely available. It can be easily cut, sanded, and carved.
The stiffness of foam, when looked at by weight, is comparable to balsa. Foams do not have balsa’s strength, and are generally lower in density than woods. All of these differences require a new set of building techniques. But when used properly, foam is an amazing building material.
Making Foam from Plastic
Most plastics are made from petroleum. The foams that we use in our model airplanes are normally created through a two step process. First, tiny beads (about 1 mm diameter) are manufactured containing a small amount of a so-called blowing agent. This is a liquid that turns into a gas when heated. The second step is the actual expansion of the beads in high temperature molds. Depending on the plastic, an extrusion process may be preferable over an expansion process.
Besides creating foams from pure types of plastics, a popular trend nowadays is mixing two different plastics to try and create a foam that minimizes their disadvantages and enhances their strengths.
The density of foam is more a factor of the foaming process than the type of plastic that was used. Do not use it alone to identify the type of foam.
Polystyrenes (XPS and EPS)
Polystyrene foams have a long history of use in model airplanes. We prefer the extruded varieties (XPS) because of their superior strength. XPS foams include Depron, Midwest Cellfoam 88, Dow BlueCor, foamboard foam, and Owens Corning Foamular. They have good stiffness, but are prone to snapping in a crash.
Expanded polystyrene (EPS) foams do not seem to be getting much use anymore in model airplanes. Compared to the XPS foams, they are relatively heavy and weak. Dow Styrofoam, foam coolers, and disposable foam cups are made out of EPS.
Expanded Polypropylene (EPP)
EPP foam is less stiff than XPS, but has great resilience (ability to spring back when deformed) and tear resistance. In other words, EPP foams survive crashes much better than XPS foams.
Most foams that we use are rated up to 160 F (75 C). This means that their physical properties are guaranteed to be consistent up to at least that temperature. EPP, however, is good up to 212 F (100 C). I can imagine some RC applications where the difference might be important.
Expanded Polyethylene (EPE)
EPE foams are similar to EPP foams. The best way to tell them apart is by feeling them. EPP foams feel rough to the touch compared to EPE foams. EPE is almost indestructible in a crash, at the cost of some more stiffness (when compared to EPP).
Pool noodles are normally made out of EPP, but I have come across some made out of EPE.
There are many proprietary foam blends on the market today. Most are very similar to EPP.
One reason why the blends were created is because it is not easy to create accurate molded parts out of EPP. You see, EPP has a tendency to expand a little bit after it is removed from the mold.
I’m sure marketing played a role in the creation of these foam blends, too. Would you rather say that your molded foam airplane is made out of EPP or of HyperXYZ Super Duper foam?
Sometimes you see the name expanded polyolefin (EPO) used. Well, guess what? EPP and EPE are both polyolefins. It is just sleight of hand naming from the manufacturers.
Some of the foams in this general category include Z-Foam, Aerocell, Elapor, EPO, and Arcel.