Moulding Process: Centrifugal Force

The rotational moulding 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 moulding, 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 moulding than any other method. Rotational moulds are significantly cheaper than other types of mould. 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.

Rotational Moulding Process

The rotational moulding process consists of four distinct phases

  1. Loading a measured quantity of polymer (usually in powder form) into the mould.
  2. Heating the mould in an oven while it rotates, until all the polymer has melted and adhered to the mould wall. The hollow part should be rotated through two or more axes, rotating at different speeds, in order to avoid the accumulation of polymer powder. The length of time the mould spends in the oven is critical: too long and the polymer will degrade, reducing impact strength. If the mould spends too little time in the oven, the polymer melt may be incomplete. The polymer grains will not have time to fully melt and coalesce on the mould wall, resulting in large bubbles in the polymer. This has an adverse effect on the mechanical properties of the finished product.
  3. Cooling the mould, usually by fan. This stage of the cycle can be quite lengthy. The polymer must be cooled so that it solidifies and can be handled safely by the operator. This typically takes tens of minutes. The part will shrink on cooling, coming away from the mould, and facilitating easy removal of the part. The cooling rate must be kept within a certain range. Very rapid cooling (for example, water spray) would result in cooling and shrinking at an uncontrolled rate, producing a warped part.
  4. Removal of the part.

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