Advances
in
Rotomoulding Technology& Applications
Over
the past two decades, improvements in process, moulds and materials
have resulted in a very significant increase in usage of the process.
Rotational
moulding is a method for producing hollow plastic articles. The process
was developed in 1940, but in the early years it attracted little
attention because it was regarded as slow process, which was restricted
to a small number of plastics. However, over the past two decades,
improvements in the process, moulds and materials have resulted in
a very significant increase in usage of the process. Nowa- days, the
advantages which it has to offer in terms of the economic production
of quite complex stress free articles has made it a very competitive
alternate to injection moulding and blow moulding. The rotational
moulding industry in India as well as worldwide is the fastest growing
industry. The demand of rotomoulding products are increasing in all
sectors. Introducing the Process Rotational moulding is a process
primarily utilised in the manufacture of hollow objects from thermoplastics.
The solid (powder) or liquid polymer material is placed in a mould.
The closed mould is then heated and cooled while being rotated about
two perpendicular axes (Biaxial Rotation). The powdered material partially
fuses together and forms a porous skin on the mould surface. This
gradually melts together to form a homogenous layer of uniform thickness,
whereas when moulding liquid material , it tends to flow and coat
the mould surface until the gel temperature of the resin is reached,
at which time all flow ceases. Sufficient time must be allowed in
the oven for the material to completely distribute and fuse. The mould
is then indexed to the cooling station where rotation is maintained.
Cooling is accomplished by forced air, water spray or combination
of both. When the mould is cooled, it is indexed to unloading /loading
zone where the mould is opened, the finished part is removed and the
mould recharged for the next cycle. Some of the basic differences
that characterize rotational moulding from the other moulding process
include:
l The use of polymer fine powder
or liquid plastisols instead of pellets, also the thermally cross
linkable polyethylene
2 The melting of the moulding
material inside the closed mould is at atmospheric pressure whereas
in other plastic moulding it is under pressure
3 The use of biaxial mould rotation
l Part design freedom unknown in other moulding fields
4 The use of moulds that are comparatively
inexpensive due to the low pressure processing
5 Finished parts are removed from
the mould potentially free of re-
....contd.
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