This process is used principally to form hollow articles, although imaginative designers have moulded products such as tool boxes and pallets which are a major departure from the more famililar footballs and tanks. In the method, a measured quantity of liquid (plastisol) or powdered plastic is mixed with catalyst and fed into the open mold as quickly as possible which is then rotated relatively slowly about two axis in an environment above the melt temperature of the plastic (usually 230°C to 400°C). The speeds of rotation are usually less than 30 rpm. The speed ratio between the two axis of rotation (major or minor) is important and is selected to suit particular product shapes. The ratio can vary from 10:1 to 1:10.
When the plastic has coated the inside of the mould because of gravitational & centrifugal forces, from the rotation of the mould, the latter is cooled, while it is still rotating to solidify the plastic. The mould halves are then opened and the moulding is removed. Cycle times are relatively slow (3-20 mins), but as this is a low pressure process, the equipment is simple and the moulds are very inexpensive because, no cooling channels are required inside the mould, when compared with, say, an injection mould for a similar size of moulding.
To help increase production rates, multi-station machines are available. For example a three mould system can have one mould being charged with material, one mould rotating in the oven and one mould in the cooling zone. Mould heating may be achieved by one of a variety of methods which include direct gas flamed, hot air, and infra-red. Some newer types of machines have the mould heated by synthetic oil flowing in a jacket around the mould. Cooling is normally achieved by the air or a water spray, but in the liquid heated machine the circulation oil is also used for cooling.
The wall thickness of the product is controlled by the amount of powder put into the mould. There is no waste in the form of sprues,runners, and so on, and therefore no trimming is required. But in some cases part of the moulding may be cut off and discarded, for example to produce an open water tank. In many cases the mould is designed so that the container and its lid are moulded in one piece and when they are cut apart the lid fits over the rim of the container. Dust bins complete with their lids can be moulded in this way.
Not all plastics are suitable for rotational moulding. The most commonly used materials are PVC plastisol and powdered polyethylene HDPE, LLDPE but acetal, nylon (types 6,11, and 12), cellulosics, ionomers, polycarbonate, and EVA have also been used. Nylon and polycarbonate usually require an inert atmosphere in the mould to prevent embrittlement. The MFI and material density characteristics are taken into account for the selection of raw materials.
Although rotational moulding is labour intensive and has slow cycle times, its simplicity and inexpensive moulds are making it one of the expanding sectors of polymer processing. The moulds are usually made from aluminium or sheet metal, so they can be fabricated quickly and alterations are not a problem. The process also lends itself to short runs and, in general, it is competitive with blow moulding and injection moulding in many areas-particularly for larger products. The unique feature of the process that one can manufacture a product from different colours at a time rather in a single cycle in multicavity.
Application areas for rotational mouldilng vary from very large storage tanks (12,000 gallons in one case) to missile nose cones. Typical products include boats, chairs, traffic cones, petrol tanks, barrels, and toys.
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