Note: Descriptions are shown in the official language in which they were submitted.
.. CA 02277660 1999-07-19
Method and Apparatus for Reducing Cycle Times in Traditional and Heat-Set Blow
Moulding and Thermoforming Processes
Background of the Invention
The blow moulding and thermoforming processes involve the deformation into a
mould cavity of a polymeric cylinder (preform) or a sheet through the combined
application of an air pressure gradient and a mechanical assistance (plug or
stretch
rod).
The preform or sheet is heated during what is referred to as the reheat stage,
prior to
the application of the deformation into the mould cavity. Heating times are
generally long in duration (order of minutes). Some methodologies such as
halogen
heating do exist that result in extremely rapid heating times (order of
seconds), but
these methods have a limitation in that a steep temperature gradient exists
through
the wall of the polymer. The surface of the preform or sheet will tend to
burn.
During the heating of the preform or sheet, crystallinity can be developed
that will
result in improved mechanical and permeability properties of the final part.
Furthermore a variation of the blow moulding process, known as heat-set blow
moulding results in a further increase in crystallinity. In the family of heat-
set blow
moulding processes, the blow mould is kept at an elevated temperature so as to
initiate the development of crystallinity. Once the crystallinity reaches the
desired
level, a quenching process is introduced. The quenching process results in the
crystalline spherulites maintaining a small size, and therefore maintaining an
acceptable level of clarity. The main limitation of the heat-set process is
the
relatively long residence times in the blow mould (order of minutes).
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The goal of this research activity is to develop a novel methodology that can
reduce
cycle times for both stretch blow moulding and thermoforming. The reduction of
the cycle time can be achieved either during the
~ preform reheating or
~ blow mould heat setting.
The objective of this report is to outline the developments to date as well as
our
proposed system for the reduction of the cycle times while achieving
relatively high
levels of crystallinity.
Summary of the Invention
According to the invention, there is proposed a system with coupled and
simultaneous
mechanical vibration (simultaneous longitudinal and torsional) and non-contact
heating of the prefonn/sheet and/or the formed part. This approach would
accelerate
the development of the crystallinity as well as the heat transfer. As a result
of the
above method, cycle times for fonming processes such as blow moulding and
thermoforming can be reduced. The pre-processing and post-processing steps are
effected independently of the part forming proper.
In the case of the treatment of the formed part, the application of a
differential pressure
gradient across the part may be required to ensure reasonable dimensional
stability.
The simultaneous application of a convective heat transfer mechanism to the
system,
such as blowing hot air or quenching cold gas, can also be included to improve
the
cycle time reduction.
The novel features of the proposed method of the invention are:
~ The use of vibration technology in conjunction with non-contact radiative
heating
for the preform/sheet and/or formed part heating stages.
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CA 02277660 1999-07-19
~ The optional additional use of a connective heat transfer mechanism (in
addition to
the ratiation heating) to improve the cycle time reduction
~ The vibration/radiative heating is introduced independently of the part
forming in the
mould, i.e. prior to forming during preformlsheet heating (pre-processing)
and/or
after forming in the already formed part (post processing).
~ The material is treated in the semi-solid state, which has not been
considered to date
(only the molten polymer state has been considered in extrusion and injection
moulding systems).
~ The projected reduction in heating times as a result of the coupled
vibration and
radiative heat transfer is also new for forming processes such as blow
moulding and
thermoforming and has potential for other polymer processes that employ a
radiative, connective or contact heating step.
Brief Description of the Drawings
In the drawings:
Fig. 1 is an overall view of the apparatus of the invention,
Fig. 2 is a schematic illustration showing the preform attached to a
longitudinal vibration
unit of the apparatus,
Fig. 3 is an enlarged view of the longitudinal vibration unit,
Fig. 4 illustrates the effect of vibration on preform temperature, and
Fig. 5 illustrates the effect of vibration on crystallinity of the material.
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CA 02277660 1999-07-19
Detailed Description of the Invention
The proposed set-up is shown in Figure 1. The distance between the heater
elements
11 and the surface of the preform 12 is adjusted according to the blow
moulding
machine in question. The preform 12 is subjected to simultaneous heating and
vibrational motions. The preform is suspended from a fixed or rotating collet
assembly. The collet assembly includes a clamp 6 (Fig. 2) in which the neck of
the
preform 12 is clamped and a moveable rod 4 that induces the longitudinal
oscillation (Fig. 2). The longitudinal vibration rod 4 goes through the
rotational
(torsional) vibration unit 5 and is fixed to the longitudinal vibration unit 8
and these
two are independent from the rotational vibration unit 5 (Fig. 2). A vibration
cushion 1 serves to absorb the shocks and thus minimize mechanical problems
that
could occur during the assembly or operation.
Fig. 3 shows a part of the pedestal 10 that supports the entire assembly. The
longitudinal vibration unit 8 is fastened to the pedestal 10 with an elastic
attachment
member 9. Other elastic attachment members 7 connect the longitudinal rotation
unit 8 with the rotational vibration unit S. The members 7 and 9 are made of
semi-
rigid rubber.
An interchangeable ferrule 2 of the longitudinal vibration rod 4 allows a
change of
the length of the part of the rod that is in contact with the preform,
depending on the
type and size of the form.
Both oscillation directions (longitudinal and torsional, or rotational) are
set at
desired frequencies and amplitudes. The longitudinal direction is essential in
order
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CA 02277660 1999-07-19
to maximize clarity (by promoting crystalline structure). The temperature of
the
heaters 11 is set at an optimal power and frequency to optimize the heating
process.
Preliminary experiments have been performed to validate the invention. The
results
indicate that at moderate frequencies of lSHz, the preform temperature during
reheating can be increased by approximately 10°C and the crystallinity
by a level of
10%. For a typical reheating time of 40 seconds without vibration, the heating
time
can potentially be reduced by 14 seconds, with the application of vibration.
Also
for a heat-set crystallinity specification of 20%, the heat-set time can
potentially be
reduced by 50 seconds. The reader is referred to the results of Figures 4 and
5. It
should be noted that Fig. 5 refers to density vs time., but it is known that
density of
the polymer is directly proportional to its crystallinity and therefore the
graph of
Fig. 5 reflects the change of crystallinity during the application of the
invention..
