Duramold is a composite material process developed by Virginius E. Clark. Birch plies are impregnated with phenolic resin, such as Haskelite and laminated together in a mold under heat (280 °F, 138 °C) and pressure for use as a lightweight structural material. Similar to plywood, Duramold and other lightweight composite materials were considered critical during periods of material shortage in World War II, replacing scarce materials like aluminum alloys and steel.
The material has some advantages over metal in strength, construction technique, and weight. A cylinder made of duramold is 80% stronger than a cylinder made of aluminum. There are over 17 varieties of Duramold, using various quantities of birch or poplar wood, with as many as seven plies. The Duramold process has also been used to make radomes for aircraft as well as missile bodies.
The Fairchild Aircraft Corporation patented the process, designing and constructing the AT-21, (NX/NC19131) as the first aircraft made using the Duramold process. Several aircraft used Duramold in parts of their structure but the largest aircraft manufactured with the process is the Hughes H-4 Hercules, which was almost completely built with Duramold in very large sections. Hughes Aircraft had purchased rights to the process for this use.
The Duramold and Haskelite process was first developed in 1937, followed by Gene Vidal's Weldwood and later the Aeromold process produced by the Timm Aircraft Company. The aeromold process differs in that it is baked at a low 100 °F at cutting and forming, and 180 °F for fusing together sections after the resins are added.