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Rotational molding or moulding is a versatile process for creating many kinds of mostly hollow plastic parts. The phrase is often shortened to rotomolding or rotomoulding.
A heated mold causes the plastic resin within to melt and form a puddle at the bottom of the mold cavity. The mold is then slowly rotated (usually around two perpendicular axes) causing the melted plastic to flow into to the mold and stick to its walls. In order to maintain even thickness throughout the part, the mold continues to rotate during the cooling phase. The process was developed in the 1940s but in early years was used little because it was a slow process restricted to a small number of plastics. Over the past two decades, improvements in process control and developments with plastic powders have resulted in a significant increase in usage.
Contents
1 History
2 Equipment and Tooling
2.1 Rotational Molding Machines
2.1.1 Rock and Roll Machine
2.1.2 Clamshell Machine
2.1.3 Vertical or Up & Over Rotational Machine
2.1.4 Shuttle or Swing Arm Machine
2.1.5 Carousel Machine
3 Production Process
3.1 Recent improvements
4 Mold Release Agents
5 Materials
5.1 Natural Materials
6 Products
7 Design Considerations
7.1 Product Design
7.2 Designing for Rotational Molding
7.3 Material Limitations and Considerations
7.4 Wall Thickness
8 Process: Advantages, Limitations, and Material Requirements
8.1 Advantages
8.2 Limitations
8.3 Material Requirements
9 Bibliography
10 Notes
11 External links
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History
In 1855 R. Peters, of Britain, documented the first use of biaxial rotation and heat. This rotational molding process was used to create metal artillery shells and other hollow vessels. The main purpose of using rotational molding was to create consistency in wall thickness and density. In 1905 F.A. Voelke, used this method for the hollowing of wax objects in the US. This lead to G.S. Baker and G.W. Perks process of making hollow chocolate eggs in 1910. Rotational molding developed further and R.J. Powell used this process for molding plaster of Paris in the 1920. These early methods using different materials directed the advancements in the way rotational molding is used today with plastics. [1]
Plastics where introduced to the rotational molding process in the early 1950. One of the first applications was to manufacture doll heads. The machinery was made up of an E Blue box-oven machine, inspired by General Motors back axle, powered by an external electric motor and heated by floor-mounted gas burners. The mold was made out of electroformed nickel-copper, and the plastic was a liquid PVC plastisol. The cooling method consisted of placing the mold into cold water. This process of rotational molding led to the creation of other plastic toys. As demand and popularity of this process increased, it was used to create other products such as road cones, marine buoys, and car armrests. This popularity lead to the development of larger machinery. A new system of heating was also created, going from the original direct gas jets to the current indirect high velocity air system. In Europe during the 1960 the Engel process was developed. This allowed the creation of large hollow containers to be created in low-density polyethylene. The cooling method consisted of turning off the burners and allowing the plastic to harden while still rocking in the mold. [2]
In 1976, the Association of Rotational Moulders (ARM) was started in Chicago as a worldwide trade association. The main objective of this association is to increase awareness of the rotational molding technology and process. [3]
In the 1980, new plastics, (polycarbonate, polyester, nylon, etc) were introduced to rotational molding. This has lead to new uses for this process, such as the creation of fuel tanks and industrial moldings. The research that has been done since the 1980 at Queen University has lead to the development of more precise monitoring and control of the cooling processes based on their development of the otolog system. [4]
Equipment and Tooling
Rotational molding machines are made in a wide range of sizes. They normally consist of molds, an oven, a cooling chamber, and mold spindles. the spindles are mounted on a rotating axis, which provides a uniform coating of the plastic inside each mold.[5]
Molds (or tooling) are either fabricated from welded sheet steel or cast. The fabrication method is often driven by part size and complexity; Most intricate parts are likely made out of cast tooling. Molds are typically manufactured from stainless steel or aluminum. Aluminum molds are usually much thicker than an equivalent steel mold, as it is a softer metal. This thickness doesn't affect cycle times significantly since aluminum's...(and so on)