Rotational molding is a three station process. An enclosed mold rotates as it is moved from a loading, heating, and cooling station. The mold is attached to the end of a three arm turn table. The device at the end of each arm is capable of holding the mold and rotating it throughout the process. A pre measured amount of powder of liquid plastic is placed inside the mold at the first station. At the same time, a second arm is located at the second stage, inside an oven, one third the distance around the circumstance. As the mold rotates in its second stage, the oven heats the mold and the plastic. As the mold rotates, the plastic falls against the wall, melting and fusing plastic together. Once all the plastic has coated, melted, and fused together, the mold is moved to the cooling stage. In the cooling phase, the mold continues to rotate as water is sprayed over the mold surface. The mold and plastic cool and then are rotated for removal. The mold is continuously rotated through out the second and third stage. When it returns to its initial position, the mold stops rotating to remove the part and to recharge the mold.

Rotational molding is a slow and low pressure process. Plastic parts that are enclosed, such as children toys, barrels, surf boards, tanks, and watering pitchers, are rotationally molded. Molds can be easily made from aluminum sheet or castings. Molds are thin walled for good heat transfer. The wall thickness of the part is controlled by the amount of material placed in the mold. The interior surfaces of all rotationally molded parts are smooth with a high gloss finish. The exterior appearance of the part duplicates the mold surface finish. Generally, these products have thick walls and are heavy. Small rotational molding machines have multiple cavities for the production of enclosed hollow toys and balls. Large 40,000 gallon acid storage tanks have been successfully molded with this process. Liquid storage tanks with complex shapes are easily and economically produced by rotational molding.

For an illustration of this process, click here.