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ISSUE 04-05 / 2001 |
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| Preform Production | BACK to contents |
PET BOTTLE PRODUCTION - PREFORMS
“Dressed to Fill” from PETplanet Print is
a wide ranging introduction to the whole subject of PET bottle making and
filling**. Starting this month we will be publishing abridged versions of
the principal chapters.
PET bottles are produced by a process known as Injection Stretch Blow Moulding
(ISBM) - essentially a two-stage process involving the production of an injection
moulded PET bottle “blank” or preform (stage 1). This is subsequently reheated
to soften it sufficiently so that it can be stretched and blown inside a mould
to produce a full-sized bottle (stage 2).
The fact that ISBM is a two-stage process works to the advantage of the bottler.
In stretch blow moulding two production techniques have become established,
namely the single-stage and two stage processes.
In the single-stage process the preform is injection moulded from PET granules
in the same machine which will stretch and inflate it to form a finished bottle.
There are in fact two distinct production stages incorporated in one machine,
with the preform usually being transferred from one stage to the other while
still warm.
In the two-stage process the preform is injection moulded on one machine,
and is then either transferred directly to the second stage machine, (the
stretch blow moulding machine), or is held in stock until required.
Because the stretch blow moulding part of the two stage process is much faster
than the injection moulding of the preforms it is possible to achieve very
high outputs from two stage systems, provided that sufficient preforms are
available. The output of a stretch blow moulding machine is governed by the
number of bottle moulds and the speed. It is usually between 1,200 and 1,400
bottles per hour per mould. The following table illustrates this point:
The rate of production of injection moulded preforms is governed by the number
of cavities in the mould tool, and the cycle time, which itself is governed
by the preform wall thickness and cooling time. The table illustrates the
impact of increased wall thickness on typical cycle times*:
The bottler choosing to operate the two stage process can enjoy the option
of either injection moulding his own preforms or of purchasing them from a
specialist outside supplier. Buying preforms offers the flexibility of changing
types, weights etc. without having to spend time changing over costly injection
moulds. The bottler is able to select the most suitable preform to produce
the bottle he requires. Product changeover is also simplified; a factor which
contributes to the high outputs vital to many filling operations.
The wide choice of commercially available preforms considerably simplifies
the task of the medium sized drinks bottling company where there may well
be requirements for bottles made from PET, PEN or a blended material. There
are also multi-layer preforms with one or more layers of high barrier material,
or which use some recycled PET as a cost saving measure.
The final specification of the preform to be used will be dictated by a number
of different factors in the overall production process and market being served.
These factors include not only the bottle size and contents, but how it is
to be filled, the type of closure to be used and the shelf life required.
Whether the decision is to mould preforms in house, either in the single stage
or two stage process, or to purchase preforms from an outside supplier, the
next step, for a beverage company making its own PET bottles, is stretch blow
moulding – the subject of our next chapter.
*n.b. Machinery manufacturers are constantly improving the
performance of their injection moulding equipment. These cycle times are for
illustration only and are not related to any particular machine.
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