EPO foam is the heaviest of the foams, but it is still stiff enough to produce details and durable enough to survive
EPP foam can spring back when it gets bent out of shape.
XPS foam is the stronger of the two polystyrene foams, but it might snap apart in even a minor crash.
Why has foam taken over in the RC world?
First, it is inexpensive to buy; in fact, it costs a
lot less than the equivalent block of balsa. It is
widely available; manmade; and easy to cut,
carve, and sand. But the one thing it is lacking
is strength, and as a rule, it is lower in density
than wood. Some types of foam are:
Polystyrenes (EPS or XPS). Polystyrene
foams have been used in RC planes for some
time, and as modelers, most of us would prefer
the extruded varieties of polystyrenes (XPS)
because it is the stronger of the two. These
include brands like Depron, Midwest Celfoam
88, and foamboard. While these foam products
do have good stiffness, they tend to snap apart
in a crash, even a minor one.
Expanded polypropylene (EPP). EPP
foam does not have the same stiffness when
compared to XPS, but it does have the ability
to spring back when it gets bent out of shape.
It is resilient and much more tear resistant. This
gives EPP foam higher marks for surviving
crashes. But because of the flexibility of the
EPP, you don’t often see it in precision-flying planes.
Expanded polyethylene (EPE). This foam is
very similar to the EPP foam, but it is almost
indestructible in a crash. To have this property,
however, the foam is very flexible.
Expanded (EPO). This a very stiff foam that is
a little heavier than EPS. It is usually a molded
foam that is kind of rubbery and durable.
In addition to the ones listed above, there
are many foam blends out on the market.
Some of them are changed slightly to allow
manufacturers to label them with their own
brand name, but many of them are similar
to the EPP foam family. The blends can be
created to enhance the properties of the base
foam by adding assets that are needed by the
manufacturer. Another reason for the blends
is that EPP foam tends to expand some when
pulled from the mold. As a result, the parts
might not match up as well as they should.
Creating a blend that does not expand allows
the parts to match up better. Some of these
foams include names that you might have
heard of, such as Z-foam, Aerocell, and Elapro.