Rotomolding is the simplest method of forming a closed, hollow part and the only cost-effective way to form very large hollow parts such as tanks holding thousands of gallons. It distributes resin inside a hot, closed mold. As the mold rotates, the plastic sticks to the inner wall.
The seeds of rotomolding sprang from metal forming. A British patent granted in 1855 to R. Peters is the first documented use of heat and biaxial rotation for forming of hollow parts—in this case, of metal. Peters used centrifugal force to push the material against the mold cavity for making artillery shells.
The momentum behind rotomolding started to pick up in the late 1920s. In 1926, Waldo Semon at B.F. Goodrich discovered that adding a plasticizer could soften rigid PVC, which before then was considered to have no commercial value. Softening the vinyl made it suitable for rotomolding. The first liquid plastisols were introduced by Union Carbide in 1946.
Then in the 1930s, a rotational slush-casting technique, in which a mold was charged by pouring molten metal through an orifice, was adapted by the rubber industry to process PVC plastisols into parts such as gloves, doll parts, and boots. The doll industry gave slush molding its first real impetus in the 1940s.
Tooling advanced with pioneering efforts into cast aluminum by Allen Kelch of Kelch Aluminum Molds and Bud LaMont of Plastic-Cast around 1949-50. Electroformed tooling had been used since the ’40s.
Until 1951, all rotomolding equipment was custom built. Then Vinl-Cast Inc., of Akron, Ohio, offered a standard machine—a four-station carousel unit with four horizontal arms extending from a central pillar. The four-step process began with mold charging, then steam-heating the mold in oven, followed by submersion of mold in a bath of hot paraffin or glycerin to promote fusion of the plastic, and last, a cooling water bath. In 1952, Sun Rubber Co. also developed a machine for rotomolding plastisols.
Batch-type molding machines made way for newer continuous models like the shuttle or straight-line unit introduced in 1966 by Brooklyn Blower & Pipe. One mold was heated while another was being cooled and unloaded. Machines become bigger and more complex, adding arms and spindles that handled up to 5000 lb.
In 1953, LaMont abraded PE pellets into powder using a rotary wire brush. That made it possible to rotomold the first PE part, a Mickey Mouse figurine. In 1955, Pallmann created a mill to pulverize PE into rotomoldable powder. The first PE resin sold in powder form came from U.S. Industrial Chemical (now Equistar) in 1961. Special formulations of HDPE and LDPE were created for the new process. Other important material entries for rotomolders were polycarbonate in 1968, crosslinked PE from Phillips Chemical in 1970, nylon in 1978, LLDPE in 1980, and PP in 1995.