Rotomoulding offers a unique niche in plastics manufacturing by reason of its relatively low tooling costs, economical short runs and ability to cope with large moulds. It is ideally suited to new product development and prototype work can be carried out without vast expense.
The process of rotomoulding is essentially very simple and can be broken down into four key stages.
Loading
The metal mould is filled with a predetermined quantity of polymer powder, closed, clamped and then passed into an oven chamber.
Heating
The mould is heated externally to a high temperature and rotated around both vertical and horizontal axis. As the powder inside heats up, it begins to melt and adhere to the inner surface of the mould. This continues until an even layer of molten plastic is formed over the surface of the mould.
Cooling
In the cooling chamber air is directed at the mould and in some cases water jets are used. When the plastic inside the mould has become solid, the mould can be removed from the chamber.
Unloading
The plastic component is then removed from the mould and allowed to finish the cooling process unrestricted by the mould. Cycle time can vary from 20 minutes to one hour. The plastic is formed without pressure and as a result has no moulded in stresses.
Rotational molding is a highly versatile manufacturing option that allows for unlimited design possibilities with the added benefit of low production costs.
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In Rotomolding Process the main process variables are:
- Oven temperature
- Oven residence time
- Amount of polymer in the mould
- The speeds of rotation of the mould
- Nature of the cooling medium
- Duration of the cooling periods
- De-moulding temperature
The other factors affect the quality of product; although they may not be under the direct control of moulder.
These include:
- Power particle size
- Power particle size distribution
- Melt flow behavior of the polymer
- Density of the polymer
- Mould material
- Shape of the mould
- Thickness of the mould
- Efficiency and type of oven
- Efficiency and type of cooling bay
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Recent Improvements in Rotational Molding Process
Before some time the process was largely experiential, relying on both trial and error, the experience of the operator to judge when the part should be removed from the oven, and when it was cool enough to be removed from the mold. However technology has improved in recent years which allow the internal air temperature in the mold to be monitored, removing much of the guesswork from the process.
Further current research is looking into lowering the long cycle times as well as improving part quality. The most promising area is within mold pressurization - it is well known that applying a small amount of pressure internally to the mold at the correct point in the heating phase of the cycle speeds up the coalescence of the polymer particles during the melting phase, producing a part with less bubbles in a shorter period of time than at atmospheric pressure. This pressure also delays the separation of the part from the mold wall due to shrinkage during the cooling phase, aiding removal of heat from the part and therefore speeding the cooling phase too. The main drawback to this is the danger of explosion of a pressurised parts to the operator- something that has prevented mold pressurisation becoming adopted on a large scale by roto molding manufacturers.
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