Rotational Molding

Rotational Molding

Rotational molding is an economical way to produce hollow parts of all sizes.

Rotational Molding
(Copyright STP Rotomachinery, Cerano, Italy)

Polymer Product Manufacturing Steps

Rotational molding is one of the ways you can process the polymer in the manufacture of polymer products:

General Information

In rotational molding, powder or liquid polymer is loaded into an empty mold. The mold closes and enters a furnace where it is heated, melting the polymer. The mold rotates on two perpendicular axes to ensure that the mold is evenly coated by the polymer. The mold continues to rotate during cooling outside the furnace to maintain constant wall thickness. After cooling, the mold opens and the part is removed.

Equipment Design

In fixed-arm rotational molding machines, such as the ones shown below there are typically three arms connected to a central turret. When the turret rotates, all the arms move to the next station. In the case of long cooling times, a fourth arm and a pre-cooling station are added between the oven and cooler.

Equipment Design
(Copyright STP Rotomachinery, Cerano, Italy)

The second type of rotational molder is an independent-arm machine. These machines have up to four arms and five stations. Since each arm moves independently of the others, heating and cooling times need not be the same. This flexibility results in increased efficiency and higher quality parts.

Other types of rotational molders include the carousel and rock & rolling oven machines shown below. The carousel molder shown on the left is used for the production of very large volume items such as kayaks. These molders can be equipped with 3-4 arms that all share the same oven and cooler resulting in reduced production times. The rock & rolling oven, also known as the rocking oven, is shown on the right. The rocking oven typically has a mold that is surrounded by an oven and heated by hot air. The design concept of the rock & rolling oven machine is that it will have a rocking action about one axis, giving the “rock” aspect to its name, but it will also feature a “roll” where the oven will rotate a full 360 around the perpendicular axis.

carousel molder
rock & rolling oven

(Copyright STP Rotomachinery, Cerano, Italy)

Usage Examples

Rotational molding is used in the automotive, toy, lawn and garden, watersport, and industrial container industries to make anything from a small child’s toy to a 6,000-gallon tank. One interesting use of rotational molding is the manufacturing of medical spineboards. After rotational molding, the medical spineboards are injected with poly-foam that dramatically increases their structural integrity. Common materials used in the process are polyethylene, polypropylene, PVC, and nylon.

Rotational molding
(Copyright Sterling Tech., Lake City, PA)
Rotational molding
(Copyright R&R Tech, Edinburgh, IN)


  • Results in parts with constant wall thickness.
  • Results in parts with strong, stress-free outside corners.
  • Complex hollow or foam-filled parts are possible.
  • Molds are inexpensive compared to injection and blow molding because they do not need to withstand high pressure.
  • Metal parts may be molded directly to plastic, eliminating additional assembly.


  • Can’t yield large production volumes as quickly as extrusion and injection blow molding.



  • Leaversuch, Robert D. “Process Gains Cited For Rotomold Unit With IR Heat, Tumbling Oven.” Modern Plastics March 1993: 28-9.
  • Leaversuch, Robert D. “Rotomolding Developments Are Keyed To Extending Process Capabilities.” Modern Plastics July 1993: 46.
  • Rosen, Stephen L. Fundamental Principles of Polymeric Materials. New York: John Wiley & Sons, 1982.
  • Crawford, R.J., and James L. Throne. Rotational Molding Technology. Norwich, NY: Plastics Design Library/William Andrew Pub., 2002


  • Amber Ratliff
  • Joseph Palazzolo
  • Henry Chen
  • Jackie Priestley