Rotational moulding & how is it made?
What is rotational moulding?
Rotational Moulding (also known as rotomoulding) is a manufacturing process used across industries to create hollow but immensely durable plastic products; such as protective cases amongst others. Developed throughout the 20th century after rising to prominence due to the demands of wartime production. Rotational moulding today is a technique with virtually no design limitations.
What is the rotational moulding process?
The moulds are made of aluminium for good thermal conductivity and low mass.
Good thermal conductivity allows even and rapid heating of the mould to melt the thermoplastic granules placed inside. Low mass allows the mould to be rotated easily in all directions so that the melted thermoplastic coats all inside surfaces evenly. To maintain the even coating, the mould is cooled whilst still rotating. Once sufficiently cool that the thermoplastic has solidified the moulding is removed and fresh granules are placed inside for a new cycle.
Wall thickness can be easily modified by increasing or decreasing the measured quantity of granules placed in the mould. Consistency of wall thickness is obtained by controlling the orientation and rotational speed.
What materials are used in rotational moulding?
Although there are many thermoplastics available the most commonly used for rotational moulding is polyethylene. This is because it has good processing characteristics, is relatively inexpensive and has satisfactory engineering performance combined with resistance to environmental conditions. It can be coloured for corporate requirements (even mottled!) and to improve UV resistance.
There is essentially no waste material in this process because it is readily recycled, as is the end product.
Advantages of rotational moulding?
The relatively low cost of moulds and material lends itself to making large mouldings. Then the structural limitations of polyethylene on its own become apparent and reinforcing can be added to suit the particular need. This can be quite localised, such as for latch and hinge mountings on a case, or more extensive, such as a full steel chassis that is enveloped in the moulding.
If the requirement is for an enclosure even larger than the processing equipment allows then mouldings can be joined. This can be done by fabricating a steel chassis to the size required. Together with hot air welding mouldings to make an airtight whole. This technique can also help where a custom solution is needed. Larger cases currently available are over 2,5m long with height and depth over 0,5m, and further bespoke design options can be incorporated as needed.
How are rotational moulds made?
Where the molten thermoplastic can flow, there can a shape be made. The basic requirement to extract the component from its mould is the main limitation. But moulds can be made inseparable pieces where shape complexity is essential. Fine detail and surface texture can be readily reproduced. Larger scale details provide stacking locations and feet.
An attractive feature of the process is the ease of producing double wall cases and lids, the range of product options available via this method is virtually limitless!