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This method is less common in practice but has a number of distinct
advantages. In this case the parison is injection moulded onto a
core using the normal moulding process equipment described earliler.
However, before the moulding has a chance to solidify the mould
opens and the core with the plastic parison on it is transferred
to the blow mould. When this mould closes the core provides its
secondary function of inflating the parison to fill the mould. The
advantages of this method include the fact that no trimming is necessary
since the mould completely surrounds the parison and this produce
no flash. This also means that additional intricate detail can be
produced in the bottle neck area. Finally, whereas extrusion blow
moulding is limited to materials which have sufficient melt strength
to sustain their own weight during parison formation, injection
blow moulding can be used wiath almost all thermoplastics.
The main developments in blow moulding are in relation to improved
heat transfer from the shaped parison, and a considerable amount
of research has been devoted to cooling the inside of the moulding
by injecting cold fluids. This has demonstrated that considerable
improvements in the production rates can be achieved by these methods.
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