Page 286 - Plastics Engineering

Processing of Plastics 269

gallon containers have been blow-moulded. The main materials used are PVC,
polyethylene, polypropylene and PET.
The conventional extrusion blow moulding process may be continuous or
intermittent. In the former method the extruder continuously supplies molten
polymer through the annular die. In most cases the mould assembly moves
relative to the die. When the mould has closed around the parison, a hot knife
separates the latter from the extruder and the mould moves away for inflation,
cooling and ejection of the moulding. Meanwhile the next parison will have
been produced and this mould may move back to collect it or, in multi-mould
systems, this would have been picked up by another mould. Alternatively in
some machines the mould assembly is fixed and the required length of parison
is cut off and transported to the mould by a robot arm.
In the intermittent processes, single or multiple parisons are extruded using a
reciprocating screw or ram accumulator. In the former system the screw moves
forward to extrude the parisons and then screws back to prepare the charge
of molten plastic for the next shot. In the other system the screw extruder
supplies a constant output to an accumulator. A ram then pushes melt from the
accumulator to produce a parison as required.
Although it may appear straightforward, in fact the geometry of the parison
is complex. In the first place its dimensions will be greater than those of the die
due to the phenomenon of post extrusion swelling (see Chapter 5). Secondly
there may be deformities (eg curtaining) due to flow defects. Thirdly, since
most machines extrude the parison vertically downwards, during the delay
between extrusion and inflation, the weight of the parison causes sagging or
draw-down. This sagging limits the length of articles which can be produced
from a free hanging parison. The complex combination of swelling and thinning
makes it difficult to produce articles with a uniform wall thickness. This is
particularly true when the cylindrical parison is inflated into an irregularly
shaped mould because the uneven drawing causes additional thinning. In most
cases therefore to blow mould successfully it is necessary to program the output
rate or die gap to produce a controlled non-uniform distribution of thickness
in the parison which will give a uniform thickness in the inflated article.
During moulding, the inflation rate and pressure must be carefully selected
so that the parison does not burst. Inflation of the parison is generally fast but
the overall cycle time is dictated by the cooling of the melt when it touches
the mould. Various methods have been tried in order to improve the cooling
rate e.g. injection of liquid carbon dioxide, cold air or high pressure moist
air. These usually provide a significant reduction in cycle times but since the
cooling rate affects the mechanical properties and dimensional stability of the
moulding it is necessary to try to optimise the cooling in terms of production
rate and quality.
Extrusion blow moulding is continually developing to be capable of
producing even more complex shapes. These include unsymmetrical geometries
and double wall mouldings. In recent years there have also been considerable