Rotational molding is a process that uses centrifugal force to create one-piece hollow items, such as traffic cones and water tanks. The process involves filling a preheated mold with raw materials, rotating it to distribute the material, and then cooling it. It is ideal for short-run parts but not suitable for high volume production. The process was first used in Great Britain in 1855 and has since evolved to include a wide range of materials and additives. Recent developments have seen natural materials added to the range of regularly rotationally molded materials.
Rotational molding is a specialized molding process for producing one-piece hollow items. The process uses the principles of centrifugal force to move the molten material against the inside surfaces of a rotating die and leave the core hollow. Rotational molding produces items such as traffic cones, oil and water tanks, plaster statues and chocolates. There are several types of general purpose rotational molding machines, including clamshell, shuttle and carousel varieties which, while different in their specific design features, all work on the same principle. Rotational molding, as it is also known, is not suitable for high volume production due to the long process cycle, but it is ideal for the production of short run parts.
The rotational molding process was first used in 1855 in Great Britain as a metal stamping process to produce items such as artillery shells. The process molded various materials including plaster of paris and wax until the early to mid 1900s when plastics were first molded by rotary molding in the early 1950s. The basic principle behind rotational molding can be demonstrated by the old experiment of swinging a bucket full of water in an arc at arm’s length. Even if the bucket is swung overhead, no water will spill out due to centrifugal force pushing the water from the center of the arc to the bottom of the bucket. As the rotoformer die assembly rotates, the same phenomenon pushes the molten material out of the die cavity, thus forming a hollow shape.
The process typically involves filling a preheated mold with the applicable raw materials. The mold is then rotated, thus migrating the molten material adhering to the inner walls of the mold cavity. The mold is rotated constantly during the melting and cooling period to avoid sagging and warping and to maintain a uniform thickness in the molded part. The rotation of the mold generally takes place along two perpendicular axes which impart the necessary centrifugal force without causing the mold to rotate too rapidly. Once the mold has cooled, it can be split and the part removed for finishing.
This molding process is ideal for producing one-piece hollow objects such as kayaks, water and oil tanks, trash cans, toys, traffic cones, balls, and even the ubiquitous plastic flamingo lawn ornament. Rotational molding offers most of the flexibility of injection molding as a wide range of raw materials can be used, additives such as flame retardants and UV protectors can be added, and the stock colored before molding. There are different machines used for rotational molding such as clamshell, up and over, carousel and shuttle variants. While they differ in terms of specific design features, they all work on the same principles.
Rotomolding is a slow process with extended cycle times and is generally not considered viable for high production volumes. It is, however, ideal for producing short-term items such as storage tanks, boat and kayak hulls, and waste containers. Recent developments in rotational molding have seen natural materials such as stone chips and sandstone composites added to the range of regularly rotationally molded materials. Other well-known items that come out of rotational molds include plaster figurines and hollow chocolates.
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