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Commonly used materials for the rotational moulding process . .

Majority of rotational moulding polymers are thermoplastic. Thermoplastic materials have the characteristic that they can undergo a continual cycle of heating to soften and cool to solidify just like candle wax. The added benefit is that these materials can be re-used and recycled. Some rotational moulding materials however have thermosetting characterics such as crosslinked polyethylene, once moulded these materials undergo an irreversible change.

The primary requirements are that it should flow to coat the cavity evenly as the mould is rotated and secondly, its thermal characteristics should provide stability during the heating and cooling stages of the process.

The most commonly used material is polyethylene accounting for 80-90% of consumption depending upon who you talk to! LLDPE or linear low density polyethylene may be used for tanks, pots, containers, street furniture and playground equipment, etc. and most grades are available with a UV stabiliser additive.

The higher density HDPE provides additional rigidity and there is a cross-linking polyethylene which is suitable for low temperature applications, components requiring maximum toughness or higher weather resistance.

Additives such as flame retardants and foaming or blowing agents may be added to provide a foam filled interior for insulation, floatation and/or greater mechanical stability. Chemically modified polymers permit adhesion to metals or other resins allowing metal threaded inserts to be inserted or to provide a sandwich of two dissimilar resins.

EVA copolymer provide excellent low temperature flexibility, impact resistance and may be used for soft toys etc.

Polyvinylchoride PVC compounds may be rotationally moulded to obtain semi-rigid or even flexible characteristics for toys and bellows etc.

Nylon 6 has excellent tensile strength, is rigid and has good impact strength and chemical resistance. Applications include fuel tanks, oil and solvent containers, etc.

Nylon 12 has similar characteristics to Nylon 6, but is more easily processed.

Polycarbonate has the highest rigidity of all plastics, its high impact strength making it suitable for vandal resistant items. It also has high heat resistance and may be moulded crystal clear or opaque making it suitable for street lighting and enclosures where toughness is critical.

Polypropylene is stiffer than polyethylene and is less prone to environmental stress cracking. Applications include solar panels and medical storage equipment.

Common additives include antioxidants to prevent degradation during moulding and UV stabiliser for outdoor applications. Pigments or dyestuffs may be added by the supplier or just prior to rotational moulding.

Commonly used additives enhance flow or provide anti-static characteristics. Other additives add stiffness or reinforcement and for other non-critical applications, a filler may be included to provide bulk and thus reduce the overall material cost.

The foregoing provides a basic overview of materials and is in no way exhaustive. It is vital that each project is taken on its own merit and the appropriate product design, material and process are married together. Rota Design provides unbiased and educated advice taking the overall objective of the project into account.