Foaming, or blowing, offers several
potential advantages for polyurethane parts.
You can "fill" the same volume
with less material, so the part is lighter and it costs less, while
still maintaining most of its structural integrity.
You can make parts light in the
first place, if that's what you need, and even control the overall
weight to close tolerances. (Ideal for floats.)
In the case of flexible (elastomeric)
urethane, you can make parts softer, or "cushier."
"But what about the coarse, porous
cellular structure? That doesn't look very finished." Not to
worry. You never see the cellular structure in a molded foam part.
This is because the urethane forms a solid skin everywhere it touches
the mold surface. The foaming bubbles, or cells, either compress,
deflate, or break as they're pushed against the mold surfaces by the
The thickness of the skin can be controlled in a number of ways. But
there is always some skin, even if it's only one cell membrane thick.
The skin on rigid foam acts like a shell, making the part feel solid,
permitting a smooth cosmetic surface, and adding strength and
durability. On the other hand, flexible foam skin is elastic, so it
flexes and rebounds with the foam. It can seem like a vinyl covering
or post-applied coating, but integral urethane skin is stronger and
tougher than equivalent-thickness post-applications.