Rotational moulding is a manufacturing process for thermoplastic materials that allows hollow articles to be produced of any shape or size. It is a technology that is different from conventional injection and blow moulding processes, and is particularly suited for use in the production of small series of articles, even geometrically complex ones, particularly of large size and hollow articles which are impossible to produce otherwise.
The process began to be used typically for containers, taking advantage of the fact that hollow articles could be produced without the subsequent need to weld or assemble. Applications have multiplied, thanks to the continuing development of research applied to the polymers used in the process: polyethylene, (LLDPE, LDPE, LHDPE, HDPE), reticulated polyethylene (XLPE), polypropylene, E.V.A., nylon, polycarbonate, PVC, etc.
Following are the main advantages of the rotational moulding process:
- Maximum versatility: the possibility of obtaining articles of different size and shape from the machine simultaneously;
- An unlimited design field for the article to be produced: just think of the shape of any formed container or recipient;
- Small initial investment compared with the productive capacity available;
- Complex shaped articles can be moulded in one piece, maintaining the consistency of wall thickness;
- Articles of differing thicknesses can be produced simultaneously;
- Colours and materials can be changed rapidly;
- Doubled wall articles can be produced;
- The low pressures used and reduced speed of rotation minimise wear of the moulds and moving parts;
- Possibility of changing the sizes of articles to achieve the rigidity and impact resistance desired;
- “Automatic” inclusion of metal, plastic and rubber parts;
- Limited wastage: an almost exact correspondence between the quantity of material required and that utilised. The residues of work originate from the trimming and the removal of certain appendages;
- Low cost of moulds and reduced management costs.
The basic process is straightforward. The mould is filled with either powder, micro pellets or liquid polymer. The mould rotates on two axes simultaneously while it is heated and then cooled.
The shaping of the article occurs in different ways, according to whether the material used is powder, micro pellets or liquid.
During the heating phase, powder or micro pellets (usually polyethylene) initially forms a porous film on the internal surface of the mould. The remaining material, which has gradually become fluid, then adheres to the film and creates a uniform layer which will solidify in the subsequent cooling phase.
Liquid material (normally PVC and PLASTISOL), on the other hand, runs along the walls of the mould and is heated to the temperature at which the fluid solidifies, taking on the shape of the mould which will be cooled subsequently in a water bath or with blown air. The cooling phase is crucial for certain products because each material needs different rate of temperature, generally longer than that needed for polyethylene.
The final phase is opening the mould to extract the article and introduce fresh raw material for the subsequent cycle.
The most widespread system of rotational moulding uses a horizontal rotating unit commonly called a carousel equipped with 3, 4 or more mould carrying arms which are made to pass automatically through the various stations (loading, cooking, cooling, unloading).
Different types of heating can be used though forced hot air has proved to be the cleanest, cheapest and safest method.
The cycle is completely automated: only the loading/unloading operations are performed manually allowing for greater versatility of the process. The technology needed to automate the loading/unloading of moulds already exists, but it is hardly ever used because it reduces flexibility considerably. For the moment, only automated systems for weighing and dosing of resin are economically viable.
Although the cooling phase is usually considered the least important, research has shown that correct control of the cooling cycle can provide dimensional balance, therefore improving the physical properties of the moulded article. Depending on the chemical properties of the polymer, the cooling phase produces different effects on the physical properties of the article.
In crystalline polymers, a good control of the cooling process minimizes distortion in the crystal formation, especially if high strength is required at low temperatures.
In amorphous polymers on the other hand, cooling has a minimal effect on the physical properties of the product, therefore it is not necessary to pay as much attention to the crystal formation.