Process Encyclopaedia

Overview

• Think of a large polymer product, and the chances are it is made by rotational moulding.
• This versatile process is surprisingly inexpensive and is used to make a wide range of everyday products.
• The main disadvantage is the low production rate which usually limits it to smaller batches.

Process details

Stage 1: Plastic is introduced to a mould in powder form up to the mass required for the required product.

Stage 2: The mould is then closed and passed into an oven chamber. The mould is then heated externally to a temperature typically between 220°C and 400°C and is rotated around both vertical and horizontal axes.

Stage 3: As the mould rotates, the inner surface passes through the mass of powder at the bottom of the mould. As the mould heats up, the powder begins to melt and adhere to the inner surface of the mould. The mould continues to rotate in the presence of heat and more plastic melts and builds up to produce an even layer over the surface of the mould. The mould is then withdrawn from the oven whilst still rotating and moved into a cooling chamber.

Stage 4: Cool air is directed at the mould and in some cases water jets are used to cool the mould. When the plastic inside the mould has become solid, the mould can be removed from the cooling chamber. The plastic component is then removed from the mould and allowed to finish the cooling process unrestricted by the mould.

Materials and shapes

• Mainly for thermoplastics (especially polyethylene), but some thermosets can be used.
• Used to produce containers and similar hollow products with uniform thin sections.
• Tanks up to 4m across can be made this way; wall thicknesses as low as 0.4 mm are possible.
• Products are near-net-shape and rarely need further finishing.
• Parts do not have to have circular cross-section.
• The surface finish depends on the quality of the die surface; it is possible to include surface detail such as logos.
• Metal or polymer inserts can be moulded-in during processing.
• All material is used in the product so there is no scrap.
• Parts with large openings may be produced in pairs in a single mould and separated after removal, or through use of insulation in mould.
• The plastic is formed without pressure or centrifugal force and as such has no moulded in stresses.

Economics

• Cycle time is limited by heat conduction out of the mould, so increases dramatically for larger wall thicknesses.
• Thin walled products can be produced at almost 1 a minute, whereas thick walled products might be as few as 3 per hour.
• Although the tooling is dedicated, the moulds are usually quite cheap.
• Equipment is relatively cheap - between 1 & 10 thousand pounds.
• The long cycle times usually limit economic batch sizes to between 500 and 10,000.

Typical products

• buckets
• plastic footballs
• dustbins
• oil drums
• storage tanks
• traffic cones

Links

• A guide from a manufacturer.