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Process of inflating a hot, hollow, thermoplastic preform or
parison inside a closed mold so its shape conforms to that of
the mold cavity. A wide variety of hollow parts, including
plastic bottles, can be produced from many different plastics
using this process.
Types of Blow molding
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Extrusion blow molding
In extrusion blow molding the parison is formed by forcing
molten plastic through an annular orifice in a die that is part
of the die head assembly. The orifice is formed by the space
between the mandrel and the die. Extrusion may be directly from
an extruder, or for large parts for which more material is
needed than the extruder can continuously provide an accululator
is used.
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Injection blow molding
Injection blow molding is a two stage process since the
parison is produced in a separate operation. In the first
process molten plastic is injected into a heated preform mold
around a hollow mandrel blow tube or core rod. This is similar
to insert injection molding. The workpiece for the second, blow
molding, process is the preform-mandrel assembly. The preformed
parison is placed in a larger mold cavity for blow molding.
Between the preform production and blow blow molding processes a
heated preform may be held in a temperature conditioning stage
or a cooled preform re-heated. After blow molding the part is
stripped from the core rod at an ejection station.
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Stretch blow molding
Stretch blow molding produces a part with biaxial molecular
alignment. In the process a preform, or parison, elongated
mechanically in the mold and than expanded radially in a blowing
process. A desirable resulting molecular orientation yields a
material with increased strength. This means that products that
are strength-based designs can be produced using less material
than if they were to be produced using simpler blow molding
techniques.
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