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This is the most common method of blow moulding. There are two
possible methods which may be used intermittent extrusion or continuous
extrusion. For extrusion blow moulding materials having high hot
strength and hot elongation are preferred.
The main method of extrusion blow moulding utilizes continuous
extrusion. Since the parison is continuously being extruded it is
necessary to devise methods of gripping and inflating the parison
without interfering with the extrusion process. There is a variety
of ways in which this can be achieved. In one case the parison is
cut off and transferred to the mould which is remote from the extruder
die. In another method the mould moves with the extrudate whilst
gripping and inflating it. In most cases, several moulds are operating
simultaneously on horizontal or vertical rotory table to utilise
the capacity of the extruder. Another variation of the continuous
extrusion method is to extrude downwards two parallel sheets, rather
than a tube. The mould closes on the sheets trapping the inflation
nozzle between them so that desired shape may be formed in the mould.
A possible problem with this method is non-uniform wall thickness
distribution in the moulding. Some modern machines have overcome
this by programming the thickness profile of the parison as it is
extruded. This is done by adjusting the die gap automatically during
the extrusion process. Another problem is that as the parison is
usually formed vertically downwards, the self-weight of the parison
needed for large mouldings causes draw-down and thinning of the
parison. As a result the wall thickness of the moulding is difficult
to control. A method of overcoming this is to use an accumulator
cylinder. The extruder screw is used to plasticize the melt and
supply it into a cylinder containing a piston. When sufficient material
is available in the accumulator, the plunger pushes forward quickly
to produce the large parison at a rate that minimizes sagging under
its own weight. The blow moulding process then proceeds in the normal
way.
The main materials used for extrusion blow moulding are PVC, HDPE,
HMHDPE, polystyrene, polypropylene, and polythylene. The biggest
application area is for all types of bottles, but products range
from small phials to large tanks and barrels, and can also include
products such as crates and typewriter cases.
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