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What it's all about.
Blow molding is another common type of plastic molding.
In this process a plastic tubular form, produced by extrusion
or injection molding, is used to form the part. This form,
called a parison, is softened inside a mold and then injected
with air orother compressed gas. This expands the parison
against the sides of the moldcavity, forming a hollow object
the size and shape of the mold. Blow molding isoften used
to produce plastic vessels and containers.
Injection
Blow Molding
In the injection blow molding process, the material is injection
molded. The hot material, still on the core pin, is then
indexed to the blow molding station where it is blown into
a bottle and allowed to cool. For processing PET it is critical
that this core pin be cooled. The bottle is then indexed
to the next station and ejected. Injection blow molding
allows more precise detail in the neck and finish (threaded)
area than extrusion blow molding. Little, if any, improvement
in physical properties is realized in the injection blow
molding process since very little orientation occurs. Further,
the injection blow molding process is normally limited to
the production of relatively small bottles, i.e., 180 mL
(6 fluid oz) or less. Eastapak polymer 9921 has been successfully
used in the injection stretch blow molded process.
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Extrusion
Blow Molding
The
extrusion blow molding process begins with the conventional
extrusion of a parison or tube, using a die similar to that
used for making plastic pipe.
The parison is commonly extruded downward between the two
halves of an open blow mold. When the parison reaches the
proper length, the mold closes, catching and holding the neck
end open and pinching the bottom end closed. A rod-like blow
pin is inserted into the neck end of the hot parison to simultaneously
form the threaded opening and to inflate the parison inside
the mold cavity. After the bottle cools, the mold opens to
eject the bottle. The excess plastic is trimmed from the neck
and bottom pinch-off areas.
Extrusion blow molding:
*Can
be used to process many different plastics, including HDPE,
PVC, PC, PP, and PETG.
*Requires
relatively small capital investment in equipment.
*
Is suitable for small production runs.
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Stretch
Blow Molding
This
process involves the production of hollow objects, such as
bottles, having biaxial molecular orientation. Biaxial orientation
provides enhanced physical properties, clarity, and gas barrier
properties, which are all important in products such as bottles
for carbonated beverages.
There are two distinct stretch blow molding techniques. In
the one-stage process, preforms are injection molded, conditioned
to the proper temperature, and blown into containers—all in
one continuous process. This technique is most effective in
specialty applications, such as widemouthed jars, where very
high production rates are not a requirement.
In the two-stage process, preforms are injection molded, stored
for a short period of time (typically 1 to 4 days), and blown
into containers using a reheat-blow (RHB) machine. Because
of the relatively high cost of molding and RHB equipment,
this is the best technique for producing high volume items
such as carbonated beverage bottles.
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