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There are three types of blow molding processes
--extrusion, injection and stretch blow molding-- that
vary widely in applications and thermoplastic resins used.
Continuous Extrusion:
In this method, a separate molding and cooling station
on the equipment allows the parison to be continuously
formed. This technique is used mainly for small
thin-walled parts ranging up to containers with five
gallon capacities. Parison programming can be used
to vary the wall thickness. Continuous extrusion
also allows the use of heat-sensitive materials due to
streamlined flow areas and die designs.
Intermittent
Parison Extrusion:
This technique is performed in three basic ways
--reciprocating, ram accumulator, and accumulator head
systems. All three vary in machine design and the
flow of molten resin through the die for parison
forming. However, each system is designed to produce
larger, heavier, and thicker parts than continuous
extrusion.
Stretch Blow Molding:
Blow
moldable grades of material are initially
injection molded into preform shapes. These preforms
are then thermally conditioned and then stretched (utilizing pneumatically operated stretch
rods) low
pressure air, followed by high pressure air up to 40 bar
to form axially oriented parts with molded in necks.
The process is used to manufacture PET bottles.
Basic Blow Molding Cycle:
This process utilizes various thermoplastic materials
in a solid pelletized state and converts these materials
by way of heat, pressure and compressed air into a
finished good stat.
The pellitized raw material is conveyed to the feed
section of a plasticating extruder by way of a vacuum
loader or auger screw. The raw material is then
conveyed forward through the extruder and is plastisized
to a molten state of between 350 degrees and 500 degrees F. by way of a feed screw and external heating elements.
The material in a melt state is then reshaped into a
round hollow geometry termed a parison. This parison
is then extruded vertically from the head section of the
machine through a round die at various outside and inside
diameters.
After extrusion of the parison between the two halves
of a mold the press section closes encapsulating the
parison inside the mold halves. Upon mold close
compressed air is entered into the the parison by way of a
centrally located air pipe or by piercing air needles.
The molds are chilled with cooled water which transfers
the hear form the now formed part inside the mold.
Upon complete part cooling the press section opens and the
finished product is removed. The material which is
pinched off outside the mold cavity, or the flash, is then
fed into a granulator which cops the flash into a granule
size which can be fed back to the feed section of the
extruder.
Some of the raw materials used absorb moisture from the
atmosphere and therefore require drying by use of a hot
air desicant dryer prior to being conveyed to the
extruder.
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