Definition: Foam became the go to material for surfboard cores some years after Bob Simmons first used polystyrene foam back in the late 1940's. The use of foam was monumental in making surfboards lighter and more maneuverable. The next step in foam, polyurethane, was chemical based and didn't melt like Styrofoam under the warmth of polyester resin. It was more plentiful than balsa as well as light and relatively cheap.

By the 1960's, balsa was out and foam was in. Since then, the basic polyurethane blank has become the standard by which surfboards are built as evidenced by the rise of industry behemoth Clark Foam. While Clark's eventual (and surprising) folding saw a scramble back to other materials like polystyrene (for epoxy boards), polyurethane has remained the cheap and easy way for build a surfboard.