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extruder screw can be stopped and started as required to fill the accumulator in time
for the next push-out and moulding cycle, thus saving time and improve part quality.
Extrusion blow moulding process allows weight adjustment of parts through wide
range during process and the flash (wastage) generated is normally reused after
grinding. Hollow products like bottles, jars, cans, drums, carboys, others from 1 ml
to 2000 litres in wide variety of shapes (including handles) and necks are possible.
Plastic materials processed are HDPE, LDPE, PP, PVC, P.E.T., K Resin, PET.G,
Polystyrene, Nylon, TPE, Polycarbonate, others.
Parison Programming:
Parison while being extruded tends to thin at top due to self weight and also the
complex geometries of the product E.g. Jerry cans with handles necessitate varying
the parison thickness at each point along its length to get more uniform product wall
thickness.
This is achieved with Multi-point “parison programmer” which operates via
hydraulics and allows efficient control of parison wall thickness by opening or
closing the die gap according to a pre-set programme. Changing gap during
extrusion changes parison thickness and diameter. Benefits are part with reduced
weight thereby material savings, lesser inbuilt stresses and improved wall thickness
distribution which reduces cooling time and hence higher production.
In continuous extrusion machines Time based control systems are used where the
parison thickness is controlled on a definite interval of time. The total time for one
parison cycle is divided by the number of points to calculate the time interval. In
accumulator type extrusion blow Position-based system is used where a
potentiometer senses the position of the accumulator cylinder stroke (material push
out) and programs the profile points with respect to its position.

Co-extrusion (Multilayer) Blow Moulding:
Co-extrusion blow moulding involves the simultaneous extrusion of two or more
compatible resins in layers through the parison wall. This allows, for example, the
incorporation of special layers for permeation barrier, or the use of a layer of
"regrind" material in the part wall. Multiple extruders are therefore needed to feed
each material to the special co-extrusion head, which can be designed either for
continuous extrusion or accumulator head operation. Used in producing
squeezable ketchup bottles, gasoline containers others.
SEQUENTIAL BLOW MOULDING:
can be considered as a development of co-extrusion blow moulding where the
layers are “switched on and off” in a programmed way. Defined article sections are
blow moulded with specific properties by choosing a combination of different
material layers and layer thicknesses. A single article can be blow moulded with
varying hardness, elasticity or strength in defined sections. This allows production
of parts which combine sections made from two or more resins, for example hard,
rigid sections in one material and soft flexible bellows in different material. For
example, a pipe with tough middle section to withstand pressure and having soft
flexible ends for assembly.

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