What Is Rotational Molding?

Rotational molding (also referred to as rotomolding or rotational casting) is a low pressure process used to manufacture hollow plastic parts.  It involves placing a powdered plastic material (resin or polymer powder) into a mold that is slowly rotated around two perpendicular axes, relative to each other, while heated in a large, very hot oven.  The powdered material slowly tumbles in the rotating mold, reaching internal temperatures of 325-375 degrees as it melts together and sticks to the inside wall of the mold.  After the plastic resin is melted and cured, the molds are moved into the cooling chamber, while still rotating, where the heated plastic material is slowly cooled as it continues to retain the shape of the inside of the mold.

What Is Different About Rotational Molding?

There are many methods for crating plastic parts.  Injection molding is the most recognizable form because we use injection molded parts everyday such as typical water pitchers, cell phone housings, plastic drinking glasses, and a variety of familiar everyday use items.  Injection molding involves melting plastic pellets and introducing the molten material by a high pressure screw injector into a mold then cooled.  Both the inside and outside surface of the part is defined by the mold; hence, hollow parts are not created. 

Other popular plastic forming processes are extrusion or thermoforming (vacuum forming).  Hollow parts are generally made with either blow molding or rotational molding.  Blow molding is a process by which a bubble of molten plastic is expanded inside a mold through air pressure to take the shape of that mold creating high volume products such as milk jugs or shampoo bottles. 

Rotational molding is fundamentally different because it is a low-pressure process that relies on gravity to distribute the plastic to form the desired shape.

What Does Rotational Molding Produce?

Rotational molding allows for production of a countless number of usable items in a variety of shapes and sizes.  We are surrounded by hollow products in everyday life including city garbage cans, fuel containers, toys, and a variety of decorative items.