HDU (high density urethane) foam is made of millions of tiny bubbles, or cells that are mostly separate from each other. Since these cells are not interconnected, it is waterproof. Also, urethane is impervious to almost all solvents. However, it does have one weakness. UltraViolet light is to HDU as Kriptonite is to Superman. UV radiation will turn HDU Foam into powder. For that reason, HDU must have a good primer and top coat of paint to block the UV rays.

It has been used commercially as exterior insulation starting with the Saturn S-II moon launch vehicle in 1964, and as exterior insulation on the Alaskan Pipeline in 1970, still in service.

As a general rule, we use 15# (15 pounds per cubit foot) density.

Advantages

The greatest advantage with HDU is how easy it is to carve and form. For this reason, HDU is often the material of choice for large prismatic letters or sculpted graphics.

It is available in extra thick sheets (1, 1 ½ , 2”, 3” and 4” thick, for example). These can easily be 3-D engraved to form large panel signs (single or double faced).

Disadvantages

Price: This stuff isn’t cheap. Compared with expanded PVC, HDU is actually more expensive for sheets of equal thickness. However, expanded PVC does not come any thicker than 1.25”.

Finishing: Filling and painting can be a challenge due to the porous nature of HDU, and as noted above, this stuff HAS to be painted. (The higher the density of the HDU, the less porous it is and the easier it is to finish. Unfortunately, as the density goes up the price goes up -a lot.) The good news is that hundreds of sign shops have been working on this. Much advice is available online and in trade magazines.