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 expansion pressure.

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.



This is what urethane foam looks like on the inside. But you have to cut into a molded foam part to see this cellular structure.

Skin thickness can be held to a minimum, but there's always at least a thin film.

Skin can also be maximized. This cut section of a float shows a fairly substantial rigid skin.

Some sanding and polishing can be done on the skin without breaking through.

Deeper abrasions or cuts will expose the cellular structure beneath. Here we get a better idea of the skin's thickness from this shallow angle cross-section.


Skin is formed everywhere the expanding urethane contacts a solid surface, which of course includes core pins and inserts as well as mold cavity surfaces. Here we see the skin formed inside this mounting hole.

The skin on flexible foam is elastic, deforming along with the cellular core, and springing back with it as well.



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