Note: Descriptions are shown in the official language in which they were submitted.
CA 02569303 2006-11-29
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SELF-TENTING COTTER F~R EATABLE F~4~ PACKAGE
This iilvention relates to a self venthlg cover for a heatable food paclcage,
and in the
preferred embodiment provides cover particularly suitable for a food package
intended for
mice owave heatilzg.
Food products, pax-ticularly pre-coolced food products, are conunonly sold hl
packages
which are intended for re-heathlg either in a conventional oven or in a
microwave oven. Such
paclcages commonly comprise a relatively rigid tray which contains the food
product in
question and a film cover which is secured to the tray around the top edge
thereof.
Heating the paclcage results hl the generation of steam within the paclcage
and if no
steps are taken to vent the steam, pressur a generated in the pack is liable
to "explode". This is
highly undesirable. The traditional solution to this problem was to instruct
users of the pacl~
to pierce the film cover prior to heating of the paclL. This proceduare is
effective provided that
the customer does as instructed and pierces the film. However, if the user
fails to pierce the
film the problem of pack explosion remains.
With a view to alleviating the problem outlined above packages have been
proposed
which include vent systems which automatically vent the package in the event
of elevated
temperature and/or elevated pressure within the pack. Such systems are shown,
for example,
in EP-A-0174188 and US-A-5989608. Whilst the prior art proposals appear to
offer solutions
to the problem outlined above, they are in themselves deficient in various
respects. In
particular, the automatic venting systems of the prior art may iulder certain
circiunstances not
be effective to provide the required venting with the result that excessive
pressure builds up
within the pack, and also suffer from the disadvantage that the venting
aperture will be re-
sealed in the event that pressure within the pack falls below the pressure on
the exterior of
the pack. This can result in implosion of the pack.
In addition to the problems with prior art arrangements as described above, it
is now
perceived that the presence of some excess pressure withilz a pack is
desirable during the
heating process. Accordingly, whilst venting of a package to prevent explosion
is desirable,
that ventiilg should ideally be canied out in a controlled manner, at least
during the early
phases of heating, so as to derive benefit from a super-ambient steam pressure
generated
within the package.
With a view to obviating the disadvantages outlizied above and providhig a
controlled
and reliable venting arr angement there is provided, in accordance with the
present invention,
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2
a self venting cover for a heatable food package comprising: a sheet of fihn
having at least
one opening formed therein to permit the passage of steam therethrough; and a
label secured
to the fihn for controlling the flow of steam through the opening, the label
comprising a sheet
of material having a first region secured to the film in an area remote from
the opel~ing and
having a second elongated region covering the opening, wherein the elongated
region is ol~ly
pal-tially secured to the film and a vent passage is defined between the
unsecured portion of
the elongated region and the film itself, the vent passage extending from the
opening towards
the periphery of the label to permit controlled venting of steam from the
openitag.
Preferably, the label includes a strip which is 11011-adheSlVe alld which, in
one
embodiment, extends the full length of the label, in particular all the way to
the opposing
edges of the label to provide two vent passages and which, in total, extends
the full length of
the cover to pr ovide two vent passages from the aperture. Such an arrangement
is particularly
easy to mass-produce because the non-adhesive strip and adjacent adhesive
areas can be
produced in the machine direction of the machine which is fabricating the
cover.
In an alternative embodiment, however, the non-adhesive portion of the label
is
surrounded by adhesive, at least one side of which is relatively narrow
compared with the
adhesive bounding at least one other side of the non-adhesive portion. IN this
embodiment,
then, the passage does not extend all the way to the edge of the label and
instead the
relatively narr ow region of adhesive bordering the non-adhesive portion will
lift as pressure
within the passage increases to a pre-defined level so as to allow venting to
atmosphere.
Preferably, the label includes a relatively narrow region adjacent the
aperture and a
relatively wide region located spaced from the aperture. The r elatively wide
region provides
a large area for adhesive bonding or welding of the cover to the film. By this
means,
complete detachment of the cover from the film is avoided. The relatively
narrow region
allows the portion of the label which overlies the aperttu a to become
detached from the film
during heating, then eby providing a free and unrestrictive passage through
the opening in the
later stages of package heating.
Preferably, at least the portion of the label which overlies the opening is of
a material
which tends to shrink when heated. Such shrilllcage assists in detaclW g the
label from the
film in the region of the opelung and ensures that the label will not re-close
the opening ill
the event that pressure within the package temporarily falls to a sub-ambient
level.
In one embodiment, the opening is in the form of a slit cut in the material of
the film.
The slit may be a simple linear slit running in the machine direction of the
machine used for
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3
forming the cover. Such an anallgement is particularly easy ~to implement in
practice.
In an altenlative embodiment, however, the opening is iu the form of a hole
punched
in the material of the film. More particularly, the two concentric,
circtunferentially spcaed
circular ar cs axe cut iii the film so as to form two selmi-circular flaps
which are comlected to
the f1h11 by a pair of common, radial tabs which separate the arcuate cuts. It
will, of course,
be uilderstood that other hole shapes inay be used, such as square, but
appropriate shaping of
the slits which form the two flaps.
The invention inay be embodied as a cover which is secur ed by appropriate
means to a
relatively rigid tray. In the preferred embodnnent, the cover is securable to
a tray by way of a
peelable bond facilitating easy removal of the cover in order to seine the
product contained
within the tray. In the alternative, the cover may be used in association with
other film
material to form a paclcage i11 another form such as a flexible bag or pouch.
In practice, the material of the label can be relatively thin material, for
example 50
microns thiclc PVC. Such an arrangement is particularly desirable because the
covers cail be
pre-formed in a packaging factory environment and reeled for delivery to a
food packaging
factory for ultimate use. This is a considerable advantage over relatively
complex
arrangements using multi-layer materials which are not susceptible to reel
delivery.
The above and further features and advantages of the invention will become
clear
fiom the following description of embodiments thereof, reference being made to
the
accompanying drawings, in which
Figure 1 is a top plan view of a food paclcage provided with a cover in
accordaalce
with a first embodiment of the present invention; and
Figure 2 is a top plan view of a label in accordance with a second
einbodilnent of the
ll1Ve11t10I1.
Referring first to Figure l, the package 1 illustrated comprises a cover 2
which is
bonded to the upper edge of a tray contaiiling a food product. The cover 2
comprises a sheet
of fihn material 3 and a label 4 secured to the film material by adhesive. The
film 3 is formed
with a slit 5 rui111ii1g ll1 the machine direction A of the machirla which was
used to form the
cover. The slit 5 may be a through slit or maybe a line of weakness which,
when pressure
begins to build withizl the package, will open to provide an opening. The
position ofthe slit 5
is such that it is covered by an elongate portion 6 of the label 4.
The label 4 includes, in addition to the elongate portion 6, a generally
circular pol-tion
7. The label 4 is secured to the film 3 by two areas of adhesive 8, 9. The ar
eas of adhesive are
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4
separated by a central strip 10 which is not adhesively secured to the fihln.
As illustrated, the
strip 10 runs the entire length of the label 4 in the machine direction A.
In this embodiment of the invention, the strips 8, 9 and 10 are formed by
laying down
two spaced apart strips of adhesive on the material of the label 4 before the
label is secured to
the fihn. It is to be noted, however, that other arrangements are possible
such as other pattern
coatings, or in particular if desired the entire surface of the label 4 facing
the film may be
coated with adhesive, and a central strip of that adhesive may be treated in
some way to
prevent adhesion to the underlying film 3. Alternatively, it may be possible
to secure tlne
label 4 to the film by means other than adhesive, for example a welding
process.
In the illustrated preferred embodinents of the invention the fihn 3 is a
polyester/polypropylene lamilate comprising a layer of approximately 12
microns of
polyester laminated to a layer of 50-70 microns of cast polypropylene. The
label is formed
from a shrinlc grade PVC fihn, for example 50 IlllCIOIIS thick and is secured
to the film by a
suitable food grade adhesive, for example an acrylic based adhesive.
The effect of the arrangement described above is that vent passages 11,12 are
defined
between the material of the film 3 and the material of the label 4, the vent
passages
connecting the opening formed by the slit 5 to the exterior of the package.
VJlnen a paclcage fonned as described above is heated, for example in a
microwave
oven, steam will be liberated from the food product contained in the package
and this will
elevate the pressure within the package. Initially, there will be little
leakage of steam from
the package. However, as the pressure and temperature of the paclcage
ilcreases the slit 5
will eventually open to define an opemilg which connects the interior of the
package to the
passages 11, 12. Steam may thereafter vent in a contr olled manner through the
passages 1 l,
12 to prevent explosion of the pack but to maintain a super-ambient steam
pressure withil
the package.
Preferably, the material of the label will tend to shrink under the influence
of heat.
Accordingly, as heating of the package continues the elongate region 6 of the
label will tend
to detach itself from the underlying film and shrink baclc towards the round
portion 7. The
round portion 7 will be less liable to detach from the fihn 3 because of the
larger area of
contact of adhesive between the round portion 7 and the film 3. The sluinl~ing
of the label
material means that after a certain period of heating the label will not be
able to re-adhere to
the film material in the region of the slot and accordingly implosion of the
paclcage will be
prevented.
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The relatively large area of adhesive provided in the rounded pol-tion 7 will
ensure
that the label does not become completely detached from the fihn 3.
Refen-ing now to Figure 2, there is shown a modified label 14 which is of
sinnilar size
and shape to the label of Figure 1 and is used in conjunction with a paclaghig
cover as
described above in connection with Figure 1. In contrast, however, the the
label of the first
embodiment, label 14 is secured to the film by a single area of adhesive 18
which in
particular extends around the entire periphery of alz adhesive free region 15
which is formed
on the elongated portion 16 of the label 14 and which, when applied to the
cover material,
overlies a venting aperture in the film so as to form a venting passage. Ill
contract with the
Velltlllg slit 5 of the embodiment of Figure l, iu Figure 2, the venting
aperture is formed by a
pair of arcuate slits , the ends of which ar a separated so as to form a pair
of substantially semi.
circular flaps 20a, 20b which remain attached to the film, and through which
steam from
within the packaging can vent it will, of course, be understood that a linear
slit may be used
with the embodiment of Figure 2, as lnay arcuate slits with the embodiment of
Figure 1.
The relatively narrow width of the adhesive 18a which bounds the adhesive free
region 15 on three sides is of lower strength than that of the adhesive 18b
provided on the
generally circular portion 17 of the label 14, due to the difference in the
contact area of the
two adhesive regions 18a, 18b. Rather than a slit 5 as in the embodiment of
Figure l, a pair
of substantially semi-circular and hence, as steam from the paclcaging vents
through the slit
5 into the venting passage formed by the adhesive free region 15, the pressure
in that region
will izlcrease until it is sufficient to overcome the bonding strength of the
adhesive 18a,
and the elongated portion 16 of the label 14 lifts, allowing venting of the
steam to
atmospher e.
AS S110Wn 11 Figure 2, the edge of the label 14 also has all adheslVe free
region 21
which extends around the entize periphery so as to form a margin around the
label which is
not bonded to the underlyhlg cover. Such a margnl may also be provided on the
label 4
according to Figure 1. Preferably, the margin is l,Slnln wide and the region
of adhesive 18a
sun oundhlg the adhesive free region 15 is 2 lnln wide.
In all other respects, the malmer of operation of the label 14 of Figur a 2,
includhig the
preferable materials and material propel-ties such as shri.llkizlg.
