A hollow mold is filled with a charge or shot weight of air cure resin. It is then slowly rotated (usually around two perpendicular axes) causing the softened material to disperse and stick to the walls of the mold. In order to maintain even thickness throughout the part, the mold continues to rotate at all times during the casting phase to avoid sagging or deformation. This process was applied to plastics in the 1940's, but in it's early years was rarely used due to a slow process and restriction to a small number of plastics. Over the past two decades, improvements in process control and developments with air cure epoxies and plastic powders have resulted in significant increase in usage.