A heated mold causes the material within to melt and form a puddle at the bottom of the mold cavity. The mold is then slowly rotated (usually around two perpendicular axes) causing the melted material to flow and stick to the walls of the mold. In order to maintain even thickness throughout the part, the mold continues to rotate during the cooling phase. The process was applied to plastics in the 1940s but in the early years was little used because it was a slow process restricted to a small number of plastics. Over the past two decades, improvements in process control and developments with plastic powders have resulted in a significant increase in usage.
In 1855 R. Peters of Britain documented the first use of biaxial rotation and heat. This rotational molding process was used to create metal artillery shells and other hollow vessels. The main purpose of using rotational molding was to create consistency in wall thickness and density. In 1905 in the United States F.A. Voelke used this method for the hollowing of wax objects. This led to G.S. Baker's and G.W. Perks's process of making hollow chocolate eggs in 1910. Rotational molding developed further and R.J. Powell used this process for molding plaster of Paris in the 1920's. These early methods using different materials directed the advancements in the way rotational molding is used today with plastics.
Plastics were introduced to the rotational molding process in the early 1950s. One of the first applications was to manufacture doll heads. The machinery was made of an E Blue box-oven machine, inspired by a General Motors rear axle, powered by an external electric motor and heated by floor-mounted gas burners. The mold was made out of electroformed nickel-copper, and the plastic was a liquid PVC plastisol. The cooling method consisted of placing the mold into cold water. This process of rotational molding led to the creation of other plastic toys. As demand and popularity of this process increased, it was used to create other products such as road cones, marine buoys, and car armrests. This popularity led to the development of larger machinery. A new system of heating was also created, going from the original direct gas jets to the current indirect high velocity air system. In Europe during the 1960s the Engel process was developed. This allowed the creation of large hollow containers to be created in low-density polyethylene. The cooling method consisted of turning off the burners and allowing the plastic to harden while still rocking in the mold.