The rotational molding process begins by carefully weighing a pre-determined amount of finely ground plastic resin powder. This material is then placed into a hollow metal mold, usually made of aluminum or sheet steel. The mold is sealed, and the roto machine arm moves into the heating oven (all the time turning bi-axially) where the mold is heated slowly to a temperature where the resin will melt. This soon-to-be-liquid material will flow and fill all of the cavities of the mold as it is turning. Following completion of this heating process, the back door of the oven chamber will open, and the arm will take the heated-mold arm toward a cooling chamber, where fans and occasionally a water spray will cool the mold, thus beginning the hardening process of the part. The mold and arm-end continues to turn. While this part is cooling, another arm is beginning its travel into the oven (average roto machines have three arms that usually move in tandem.) After a computer-controlled length of time, the cooled part leaves the cooling chamber and returns to the operator’s station where it is unloaded, and the mold is prepped and filled again for another part to be made.

An easy way to look at the process is that while one arm is heating and another arm is cooling, the third arm is unloading and reloading for another trip. Usually there are multiple molds on the ends of each arm, grouped by finished wall thickness or part size, to maximize the efficiency of the machine. But the arm-ends, where the molds are mounted, are always turning until they come out of the cooling chamber so that the part in the hollow mold does not deform.