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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