A heated hollow mold is filled with a charge or shot weight of material, it is then slowly rotated (usually around two perpendicular axes) causing the softened material to disperse and stick to the walls of the mold. In order to maintain even thickness throughout the part, the mold continues to rotate at all times during the heating phase and to avoid sagging or deformation also during the cooling phase. The process was applied to plastics in the 1940s but in the early years was little used 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.
Hint: click on the image to the right to view the "Rotational Moulding 101" video.
The rotational molding process is a high-temperature, low-pressure plastic-forming process that uses heat and biaxial rotation (i.e., angular rotation on two axes) to produce hollow, one-piece parts. Critics of the process point to its long cycle times—only one or two cycles an hour can typically occur, as opposed to other processes such as injection molding, where parts can be made in a few seconds. The process does have distinct advantages. Manufacturing large, hollow parts such as oil tanks is much easier by rotational molding than any other method. Rotational molds are significantly cheaper than other types of mold. Very little material is wasted using this process, and excess material can often be re-used, making it a very economically and environmentally viable manufacturing process. (From Wikipedia, the free encyclopedia)
The concept of rotational moulding is fairly simple and consists of 4 steps:
- Mould charging: The process begins with filling a hollow mould with a pre-determined quantity of polymer powder. This powder can be pre-compounded to the desired color. The oven is preheated by convection, conduction, (or in some cases radiation) to temperature ranges around 260 °C – 370 °C, depending on the polymer used. When the powder is loaded into the mould it is closed, locked, and loaded into the oven.
- Mould rotation and heating: Inside the oven the mould is bi-axially rotated (i.e., rotated around two axes) as the polymer melts and coats the inside of the mould. The rotation speed is slow, less than 20 rotations/minute; the process is not centrifugal. During this phase of the rotational moulding process timing is critical. If the mould spends too much time inside the oven the polymer will degrade – this will reduce its impact strength. If it spends too little time inside the oven melting of the polymer will be incomplete and it will not fully coalesce on the mould wall, creating large bubbles in the item.
- Mould rotation and cooling: After the melting has been consolidated to the desired level and the timing is right, the mould is removed from the oven and cooled. Cooling of the mould is typically done with air (by fan), water or sometimes a combination of both. Cooling allows the polymer to solidify to the desired shape and shrink slightly so that it can then be handled by the operator and removed from the mould. The cooling time can typically be measured in tens of minutes. It is important that the cooling rate be carefully measured because rapid cooling causes the polymer to shrink too fast and warps the part.
- De-moulding of the final product: When it has cooled sufficiently to be handled and the polymer can retain its shape, the mould is opened and the part is removed. The moulding process can then be repeated by adding the polymer powder to the mould.