Note: Descriptions are shown in the official language in which they were submitted.
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DIMENSIONALLY STABLE T~ERMOPLASTIC TUBULAR
ARTICLES
~his application is a division of Canadian Serial
Serial No. 501,674, filed February 12, 1986.
This invention rela~es to thermoplastic tubular
articles made from crystallisable polymers and dimension-
ally stable up to a specific elevated temperature, which
articles are particularly tubular bodies of saturated
linear polyester materials such as polyethylene tereph-
10 thalate, intended for processable food and beverage
containers.
It is known that biaxial drawing bf a thermoplastic
saturated linear polyester material, such as polyethylene
terephthalate, can improve its mechanical properties while
inducing a degree of biaxial orientation and crystallisation,
15 without impairing the clarity of the material. The material
will, however, shrink if heated above the temperature at which
it was drawn. It is known that the tendency of biaxially drawn
polyester film material to shrink can be decreased by annealing
the material under restraint, utilising temperatures in the
20 range 150C to 230C, the process being known as heat setting.
Biaxial drawing is also effected, for example, in the
stretch-blow moulding of polyethylene terephthalate bottles.
Another method of forming biaxially oriented tubular articles
which may be used as bodies for processable food containers is
25 disclosed in our copending U.K.Patent Application No. 8037137,
published under No. 2089276A. The forming process described
therein involves longitudinal stretching and radial expansion
under internal fluid pressure within a mould and is normally
carried out at a temperature in the range from 75C to 120C.
30 The tubular bodies thereby produced are dimensionally stable up
to the temperatures encountered in many container filling
operations, but a heat-setting treatment is necessary if the
containers are to be filled with hot product (e.g. at 80C
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to 100C) or are to be subjected to pasteurisation (at about
60C to 100C) or sterilisation (at about 120C). Without such
a heat-setting treatment, the thermoplastic linear polyester
material (e.~ polyethylene terephthalate) would shrink and
distort to an unacceptable degree during the hot filling or
processing, as a result of relaxation of strained portions of
the material back into their preferred state in which the
molecular chains are coiled rather than extended. In the
heat-setting treatment described in Specification 2089276A, the
tubular articles were held for a time at a temperature at least
equal to, and preferably somewhat greater than, the
sterilisation or pasteurisation temperature whilst restraining
them from shrinking beyond their desired circumferential and
longitudinal dimensions. The restraint was provided by internal
fluid pressure, so as to set the strained? oriented amorphous
portions of the polyester material by at least partial
crystallisation and relaxation. Internal fluid pressure cannot
be conveniently used when the tubes have been cut so as to be
open at both ends, and it is not convenient to combine the
heat-setting with the forming process of our Specification No.
2089276A because it would involve additional complication and
extended process times.
European patent application published under No. 0081451
discloses a heat-setting process and apparatus for making
closed-ended containers from PET. The process comprises
deep-drawing a pre-heated sheet of PET into a heated female
mould by means of a male plug, which is at a temperature below
the glass transition temperature lTg) of the PET. The mould is
kept at a temperature above Tg, typically 140C. Air pressure
introduced around the plug is used to blow the plug-
formed article into contact with the mould for heating it above
Tg. When it has been in contact with the mould for long
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enough to be heat-set, it is allowed to shrink back
into contact with the plug which restrains it from
further shrinkage and cools it to a temperature below
the heat-set temperature. Contact between the heat-set
article and the plug can be released by forcing air
through an axial passage in the plug. In this process
the heat-setting is essentially carried out while the
article is held against the female mould by internal
gas pressure, rather than when it is restrained from
axial shrinkage by the plug. The heat-set operation
cannot be carried out independently of the biaxial
drawing, and is clearly not applicable for heat-setting
open-ended tubes.
According to the present invention, there is provided
a food container comprising an open-ended cylindrical
body which is made of a crystallisable polyethylene tere-
phthalate material. End closure means at each end of
the body form with the body a closed container. The
body material is biaxially oriented and the container
body is heat-set, whilst restrained against radial or
axial shrinkage, at a temperature between about 180C.
and about 240C., whereby the container is able to with-
stand hot-filling, pasteurisation or sterilisation
temperatures of up to about 120C. without undergoing
shrinkage of its linear dimensions by more than 3%.
The heat-setting operation is effected independently
of the forming operation and does not cause problems there-
in. A residual linear shrinkage of up to 3% is usually
found after heat-setting, but it can be eliminated by a
further step of re-heating the article to the specific
elevated temperature without internal restraint against
shrinkage. Such residual shrinkage may be used to
eliminate or reduce head space in a container having a
body heat-set by the method of the invention.
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Where the polymer is a saturated linear polyester, the
temperature to which the tube is heated on the mandrel is
preferably a~ least 60 higher than the specific elevated
temperature, and in particular, where polyethylene terephthalate
is used, the specific elevated temperature is preferably about
120C and the tube is heated to at least 180C on the mandrel.
The degree of dimensional stability which is achieved
depends on the temperature to which the tube is heated on the
mandrel (the "heat-setting temperature") and the time for which
the tube is maintained at this temperature, and it may be
adjusted to the particular requirements for which the tubular
articles are made. For example, if it is desired that the
linear dimensions of the article should not alter by more than
3~ when it is heated to a specific elevated temperature of
120C, the tube may be heated at 180C for a period of 1 to 5
minutes or, preferably, at 240C for 12 to 15 seconds.
The contact between the tube and mandrel may be released by
forcing fluid (e.g. air) under pressure from inside the mandrel
through small holes in its surface to form an fluid cushion
between the tube and mandrel. Alternatively it may be released
by collapsing at least a part of the mandrel surface inwards.
The tubular article may be given a shape which departs from
that of a plain cylindrical tube by providing the mandrel with a
complementary shape, which is adopted by the tube as it shrinks
on to the mandrel. For example, the tubular article may be
formed with circumferential beads by providing the mandrel with
corresponding circumferential projections. Other shapes may be
produced in this manner, such as tapered, barrelled or waisted
tubes, or tubes of rectangular, oval or other cross-sections.
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When making a tubular article which is open at both ends,
the clamping is preferably effected by circumferentially
clamping both ends of the tube to the mandrel. The tube may
then be cut into individual lengths before it is separated from
the mandrel. For example, the tube may be cut by knives which
penetrate through the tube material into circumferential grooves
in the mandrel as the mandrel is rotated.
When making a tubular article which is closed at one end,
preferably a mandrel is used which has a correspondingly shaped
closed end and clamping is effected by clamping the other, open
end of the tube to the mandrel. The open end of the tube may
be clamped axially against a radially extending flange on the
mandrel. Preferably the closed end of the tube is additionally
clamped axially against the closed end of the mandrel.
In a preferred embodiment of the invention, using a
plurality of vertical hollow mandrels of high thermal
conductivity, each mandrel is indexed past operating stations
at which:-
(a) the tube is fitted over the mandrel, whose temperature
is below the glass transition temperature of the
polymer,
(b) clamps are applied to the tube to clamp it to the
mandrel,
(c) a heating element is introduced into the interior of the
mandrel to heat it to a temperature at least 60C higher
than the specific elevated temperature,
(d) the said higher temperature is maintained for a time
sufficient to ensure the required degree of
dimensional stability,
(e) the heating element is removed, and the tube is cooled
to below the specific elevated temperature,
(f) the clamps are released,
, (g) the ends of the mandrel are sealed and air under
pressure is introduced into it and forced out through
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small holes in its surface to release contact between the
tube and the mandrel, and
(h) the tube is removed from the mandrel.
Where the tube is long enough to make several open-ended articles
(e.g. container bodies), it is convenient to arrange that, after
cooling of the tube, the mandrel is rotated and knives are brought
into contact with the tube to cut it into individual lengths.
The invention also pertains to apparatus for making a thermo-
plastic tubular article of a crystallisable polymer which is heat-set
so as to be dimensionally stable up to a specific elevated temperature,
comprising a mandrel having a plurality of apertures in its surface,
heating means for heating the mandrel internally and thereby heating
the article to a temperature higher than the specific elevated
temperature, clamping means for clamping an at least partially
biaxially oriented tube of the crystallisable polymer to restrain
axial shrinkage of the tube, and a pressurized fluid supply for
supplying fluid under pressure from inside the mandrel through
the plurality of apertures in its surface, to release contact
hetween the mandrel and a tube shrunk thereon.
Specific embodiments of the invention will now be described
in more detail by way of example and with reference to the accompanying
drawings in which:-
Figure 1 is a diagrammatic perspective view of apparatus
including a mandrel for use in heat-setting tubular bodies for making
multiple container bodies by the method of the invention,
Figure 2 is a diagrammatic sectional view of a modified form of
the mandrel, showing a heating element inserted therein,
Figure 3 is a view similar to Figure 2 of a further form of
mandrel for heat-setting single container bodies,
Figure 4 is a diagrammatic cross-sectional view showing a
modification of the mandrels of Figures 1, 2 or 3,
Figure 5 is a perspective view of a thermally processable container
with a body heat-set by the method of the invention on a mandrel as
shown in any one of Figures 1 to 4, appearing with Figure 2, and
Figure 6 is a diagrammatic sectlonal view of another form
of mandrel for use in heat-setting tubular bodies which are
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closed at one end.
The embodiments of the invention illustrated in Figures 1
to 4 are particularly concerned with the production of tubular
bodies of biaxially oriented polyethylene terephthalate (PET)
for thermally processable food containers as illustrated
in Figure 5, which have end closures seamed to the ends of the
body. The tubular bodies, open at both ends, are made from
tubes of at least partly biaxially oriented PET produced by the
process disclosed in our prior British Patent Application No.
8037137 (Publication No. 2089276A). As explained above, it
is necessary to carry out a heat-setting treatment on the
oriented PET tube to ensure that the containers will be able to
withstand hot-filling, pasteurisation or sterilisation processes
at temperatures of from 60C to 120C, without unacceptable
shrinkage. In practice, it is a requirement that the container
bodies have sufficient dimensional stability to match the end
closures in a~precise manner for the production of gas-tight
seams and to avoid stressing the seams excessively through
subsequent shrinkage.
Figure 1 illustrates apparatus including a mandrel on which
the heat-setting process can be carried out in accordance with
the invention. The cylindrical mandrel body 10 is hollow and
its ends are closed by end plugs 11 and 1`2 carried by rotatable
shafts 111 and 121 which can be shifted axially to enable the
mandrel to be removed. Plug 12 and shaft 121 have an inlet
passage 14 and control valve 141 for compressed air. The
mandrel body 10 is provided with a plurality of small holes 16
in its surface. At each end, it is provided with a
circumferential clamp member, e.g. in the form of a snap-action
clip or clamping band 20. Circumferential grooves 22,
approximately 1 mm. in width, are formed in the surface of the
mandrel body 10 at intervals corresponding to the length of the
container bodies to be produced. A shaft 15 carrying rotary
knives 17 is mounted parallel to the mandrel 10 on pivotal
brackets 19 so as to be capable of being shifted towards the
mandrel so thst the kni~e~ an ~:-er i~to the groov 5 22.
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Figure 2 illustrates an alternative fonm of mandrel body
101 in the vertical position. The foot of the body 101 has a
flange 24 to support a tube 26 of at least partially biaxially
oriented PET resin. The outer diameter of the mandrel body 101
is clearly shown to be only very slightly less than that of the
PET resin tube 261 so that the latter can be fitted thereover
without excessive friction but also without substantial play.
In this embodiment, the mandrel body 101 is made of a material
of high thermal conductivity but is sufficiently thin in section
to have a small thermal capacity. The interior of the mandrel
body 101 is slightly tapered towards the top so as to make
contact with a correspondingly shaped heating element 28 when
the latter is inserted from below, as shown in Figure 2, to fill
the interior of the mandrel.
Figure 3 illustrates a mandrel body 102 which is similar to
that of Figure 2 but of lesser height, for production of single
container bodies. In this case, a circumferential groove 18 is
provided at the upper end of the mandrel body 102 to co-operate
with clamping members 201 in the form of laterally movable
2 0 half-rings.
In use of the mandrels of Figures 1 to 3, when a PET tube
26 has been fitted over the mandrel body 10, 101 or 102, and the
ends of the tube 26 have been clamped by means of clamp members
20, the temperature of the mandrel and tube is raised, e.g. by
introduction of the heating member 28 of Figures 2 and 3, from
an initial temperature below the glass transition temperature of
the resin to a temperature which is at least 60C above the
specific elevated temperature, up to which the can bodies are to
be dimensionally stable. The temperature to which the mandrel
is heated may range from 125C to 240C, for heat-setting the
PET tube up to specific elevated temperatures ranging from 65C
to 180C, but it is preferably from 18QC to 240C, at which the
r,ate of annealing of the PET resin is sufficiently high to
obtain rapid ,heat-setting. Using a temperature of 240C, it is
only necessary to ensure that the whole of the PET resin attains
this temperature ~or about 12 to 15 seconds. Temperatures above
240C should preferably not be used because the material can
become cloudy and embrittled. The PET tube 26 shrinks radially
s into contact with the mandre1 10, 101, or 102 but is restrained
from any substantial radial or axial shrinkage by the mandrel
and the clamps 20, 201. The supply of heat is then stopped,
e.g. by withdrawing the heating member 28, and the tube and
mandrel are then allowed to cool to a temperature below the said
specific elevated temperature (i.e. the temperature to which the
tube has been heat-set).
In the cases of Figures 1 and 2, the mandrel bodies 10, 101
are now rotated and the knives 17 (Figure 1) are brought into
contact with the rotating PET tube 26 so as to penetrate through
lS the tube material into the circumferential grooves 22 and
thereby cut the tube into individual can body lengths. In the
case of Figure 3 this step is unnecessary, but the groove 18 and
clamp members 201 form a circumferential groove in the top of
the body to facilitate subsequent seaming of an end closure to
it.
Finally the clamp members 20, 201 are released, the ends of
the mandrel body 10, 101, 102 are sealed, e.g. by the end plugs
11, 12 of Figure 1, and compressed air is admitted into the
mandrel body, e.g. through the valve 141 and inlet 14, being
then forced out of the holes 16 so as to release contact between
the tube and mandrel by an air cushion effect. The can body or
body sections are then removed from the mandrel.
Instead of using compressed air to release the contact
between the tube and mandrel, a part of the mandrel surface may
be collapsed inwards as shown in Figure 4. The mandrel body 103
is here provided with one or more sections 32 extending the
whole length of the mandrel and movable radially from the
position shown in chain-dotted lines to that shown in full lines
to collapse a part of the mandrel surface, so reducing the
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effective circumference and releasing contact with a PET tube
which has been heat-shrunk thereon. An alternative type of
collapsible mandrel which could be used is a hydraulically or
pneumatically inflated tube, e.g. of silicon rubber.
As shown in Figure 5, can ends 30, 31, e.g. of metal, can
conveniently be seamed to the ends of the heat-set tubular can
body 26 to form a processable container.
In production, a plurality of mandrel bodies 101 or 102 are
arranged vertically on an indexing platform so as to be indexed
past stations at which the various operations are carried out,
as follows:-
ta) a PET tube (26) is fitted over a mandrel body 101 or 102
while the temperature of the mandrel is below the
glass transition temperature of the resin,
(b) the circumferential clamps 20, 201 are applied to each
end of the PET tube 26,
(c) the heating element 28 is introduced from below into
the interior of the mandrel body so as to make contact
with it and to heat it rapidly by conduction to a
temperature of 180C to 240C,
(d) this temperature is maintained for long enough to ensure
the required heat-setting,
(e) the heating element 28 is removed and the PET tube 26 is
allowed to cool to below 140C,
(f) where a multiple-length mandrel body 101 is used, the
mandrel body is rotated and knives are brought
into contact with the tube 26 to cut it into individual
lengths,
(g~ the ends of the mandrel body 101 or 102 are sealed and
compressed air is introduced to release contact between
the tube 26 and mandrel
body,
, and
(h) the can body sections are removed from the mandrel body.
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The method of the inven~ion can also be employed for heat-
setting thermoplastic tubular articles which are closed at one
end. Figure 6 illustrates the heat-setting of a PET container
34 which is formed with a recessed bottom 36. The container 34
is shown inverted on a tubular mandrel 104 which has a closed
end 38 whose shape corresponds with that of the container bottom
36. The clamping means in this case comprise a clamp ring 202
which is movable axially of the mandrel to clamp a flange 40 on
the end of the container 34 against a base plate 42, and a
pressure member 203 with an end shaped to correspond with the
shape of the container bottom 36, which is movable axially to
clamp the container bottom against the closed end 38 of the
mandrel body.
The mandrel body 34 is formed with air holes 16 as in the
embodiments of Figures 1, 2 and 3 for removal of the heat-set
container. Heating may be effected by a heating element 28 as
before.
Alternative means of heating the PET tube may be used in
any of the embodiments described above, such as inductive
heating of a metal mandrel, using an induction coil within the
mandrel. ~ielectric radio-frequency heating of the PET tube may
be employed, using inner and outer electrodes, the mandrel then
forming a heat sink to receive heat from the PET tube as it
cools down. Radiant heat, e.g. in the infra-red region, may be
~5 used, or the mandrel and tube may be introduced into a fluidised
bed maintained at the desired temperature.
Other alternatives are heating of the interior of the
mandrel by flame, and interior or exterior heating by hot air or
by live steam.
The mandrel need not be cylindrical but could for example
be oval or rectangular in section, so as to use the heat-setting
shrinkage for accurate dimensioning of shaped tubes.
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