Understanding rotomolding

A brief history of rotational moulding…

Rotational moulding is a plastic material transformation technique used to manufacture hollow and multi-walled polymer parts. The rotational moulding process was developed following the introduction of PVCs in the 1940s. The range of polymers expanded and with it the potential applications, resulting in rotational moulded products becoming available to an increasing number of sectors of activity, with products including trays, water recovery tanks, boats, all types of containers, automotive and agricultural fittings and equipment, etc.

The advantages of the rotational moulding process

The simplicity and upgradability of the moulds used

The moulds used for rotational moulding are simple in design since they are not subjected to any pressure, even though in mechanical terms, and where large-volume parts in particular are concerned, they have to withstand the load applied. The process also prevents the formation of any residue, thus ensuring the high quality of the rotational moulded parts produced. A high level of productivity at a competitive price – who could possibly resist?

The complexity of rotational moulded products

The rotational moulding process is perfectly conducive to the design of multilayer parts. The thickness and capacity of the parts are entirely adjustable, but it is also possible to design and develop double-walled parts, fill them with foam, incorporate inserts and even assemble or attach accessories.

Examples of rotational moulded parts:

Examples of rotational moulded parts

 

The rotational moulding process

The rotational moulding process generally comprises 5 stages:

  1. filling of the mould
  2. rotation stage
  3. fusion of the plastic material
  4. solidification of the plastic material
  5. turning out of the polymer

Filling the mould

Filling the mouldThe metal mould (steel or aluminium, depending on the application) reflects the shape of the part to be produced and is loaded with the polymer powder and colouring. The various parts of the mould are then assembled so as to make them airtight.

 
 
 

The rotation stage

The rotation stageThe mould is slowly rotated around its two perpendicular axes and placed in a kiln. The speed at which the mould is rotated, along with the difference in speed between the two axes, will affect the consistency of the finished part.

Double or triple-layer foaming operations can be performed at this stage. Once the fusion process is complete, the mould is removed.

 
 

The polymer fusion stage

The polymer fusion stageThe plastic material will soon cover the entire surface of the mould as a result of the axes rotating. The temperature rises, helping to keep the material consistent and thus eliminating any bubbles within it. Once the maximum temperature has been reached, a number of influential factors need to be monitored, including the speed at which the temperature is increased, the transfer of heat through the polymer, and viscosity, among other things

 
 

The solidification stage

The solidification stageThe mould continues to double-rotate as it is placed inside a chiller or crystallisation unit. Pulsed air and a fog of water can accelerate the process. The polymer begins to solidify, at which point the temperature drops less rapidly.
 
 
 
 

The turning out stage

The turning out stageThe part has now completely solidified and can be turned out of the mould. The tooling is unlocked and the part is removed and ready for finishing.

The cutting, conformation and finishing stages then follow and any accessories or decorative elements required are applied. A unit-by-unit quality control process is implemented in all cases. The cycle can then begin again.

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