Page 340 - Plastics Engineering

Processing of Plastics 323

etc in the final product. Once the moulding separates from the mould, it will
cool more slowly and will tend to be more crystalline, have greater shrinkage
and lower impact strength.
Developments in rotational moulding are continuing, with the ever increasing
use of features such as
(i) mould pressurisation (to consolidate the melt, remove pin-holes, reduce
cycle times and provide more consistent mould release),
(ii) internal heatingkooling (to increase cycle times and reduce warpage
effects).
In overall terms the disadvantages of rotational moulding are its relative
slowness and the limited choice of plastics which are commercially available
in powder form with the correct additive package. However, the advantages
of rotational moulding in terms of stress-free moulding, low mould costs, fast
lead times and easy control over wall thickness distribution (relative to blow
moulding) means that currently rotational moulding is the fastest growing sector
of the plastics processing industry. spical annual growth rates are between 10
and 12% p.a.
4.6.1 Slush Moulding
This is a method for making hollow articles using liquid plastics, particularly
PVC plastisols. A shell-like mould is heated to a pre-determined temperature
(typically 130°C for plastisols) and the liquid is then poured into the mould
to completely fill it. A period of time is allowed to elapse until the required
thickness of plastic gels. The excess liquid is then poured out and the plastic
skin remaining in the mould is cured in an oven. The moulding is then taken
from the mould.
It should be noted that when the plastisol liquid gels it has sufficient strength
to remain in position on the inside surface of the mould. However, it has
insufficient tear strength to be useful and so it has to go through the higher
temperature curing stage to provide the necessary toughness and strength in
the end-product. The mould is not rotated during slush moulding.

4.7 Compression Moulding
Compression moulding is one of the most common methods used to produce
articles from thermosetting plastics. The process can also be used for thermo-
plastics but this is less common - the most familiar example is the production
of LP records. The moulding operation as used for thermosets is illustrated in
Fig. 4.62. A pre-weighed charge of partially polymerised thermoset is placed
in the lower half of a heated mould and the upper half is then forced down.
This causes the material to be squeezed out to take the shape of the mould.
The application of the heat and pressure accelerates the polymerisation of the