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What is Rotational Molding?

By Paul Scott
Updated May 17, 2024
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Rotational molding is a specialized molding process for manufacturing one-piece, hollow items. The process uses the principles of centrifugal force to move molten stock up against the inside surfaces of a rotating mold and leaving the central part hollow. Rotational molding produces items such as traffic cones, oil and water tanks, plaster statues, and chocolates. There are several types of rotational molding machines in general use including clamshell, shuttle, and carousel varieties which, although different in their specific design features, all work on the same principle. Roto molding, as it is also known, is not suitable for high volume production runs due to the long process cycle, but is ideally suited to the manufacture of short-run parts.

The rotational molding process was first used in 1855 in Britain as a metal molding process to produce items such as artillery shells. The process molded various materials including plaster of Paris and wax through the first half of the 1900s when plastics were first molded with roto molding in the early 1950s. The basic principle which underpins rotational molding may be demonstrated by the old experiment of swinging a water filled bucket in an arc at arms length. Even if the bucket is swung overhead, the water does not spill out due to centrifugal force pushing the water away from the center of the arc towards the bottom of the bucket. When the roto molding machine's mold unit rotates, the same phenomenon forces the molten material toward the outside of the mold cavity, thereby forming a hollow molding.

The process typically involves filling a preheated mold with the applicable raw materials. The mold is then rotated, thus causing the melting material to migrate and adhere to the inside walls of the mold cavity. The mold is rotated constantly during the melt and cool-off period to prevent sagging and deformation and to maintain an even thickness in the molded part. The rotation of the mold is generally along two perpendicular axes which impart the necessary centrifugal force without having the spin the mold too rapidly. Once the mold has cooled, it may then be split and the part removed for finishing.

This molding process is ideally suited to producing one-piece, hollow items such as kayaks, water and oil tanks, refuse bins, toys, traffic cones, balls and even the ubiquitous plastic flamingo lawn ornament. Rotational molding features most of the flexibility of injection molding in that a wide range of raw materials can be used, additives such as flame retardants and UV protectors can be added, and the stock colored prior to molding. There are several different machines used for rotational molding such as the clamshell, up and over, carousel, and shuttle variants. Although they differ in terms of specific design features, all work on the same principles.

Roto molding is a slow process with extended cycle times and is generally not considered viable for high production volumes. It is, however, ideal for the manufacture of short run items such as storage tanks, boat and kayak hulls, and refuse containers. Recent developments in rotational molding have seen natural materials such as stone chip and sandstone composites added to the range of material regularly roto molded. Other well known items which come out of rotational molds include plaster statuettes and hollow chocolates.

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