Note: Descriptions are shown in the official language in which they were submitted.
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FOAM PACKAGING TRAY AND PACKAGING METHOD USING SAME
BACKGROUND OF THE INVENTION
The present invention relates generally to foam trays used for packaging
applications and, more specifically, to foam trays used in vacuum packaging
operations
for, e.g., food products.
Various forms of packaging, particularly for food products, employ a
relatively
rigid support member, such as a foam tray, in which a product is supported.
Substantially all of the air is evacuated from a predetermined space
surrounding the tray
and product, and the product is then covered by a lid, typically in the form
of a
relatively flexible, transparent film. The film is bonded to the tray around
the product,
generally by forming a heat-seal between the film and tray, to thereby enclose
the
product within the resultant package. Both the film and support member
generally
comprise materials which form a barrier to the passage of gas therethrough so
that the
entire package is substantially gas-impermeable. In this manner, the package
protects
and extends the shelf life of the product. Examples of this type of packaging
include
vacuum skin packaging (VSP) and modified-atmosphere packaging (MAP).
In vacuum skin packaging, the film is thermoformable, i.e., capable of being
formed into a desired shape upon the application of heat, and is thermoformed
about the
product on a tray by means of heat and differential pressure. Virtually all of
the air is
evacuated from a predefined space around the package so that, when the film is
attached to the tray about the product and the resultant package is
subsequently exposed
to atmospheric pressure, the film is caused to conform very closely to the
contour of the
packaged product. Generally, sufficient heat is applied to cause the film to
bond with
the tray outside the periphery of the product, either by employing a heat-
activatable
adhesive at the interface of the film and tray or by forming the film and tray
from
materials that are otherwise sealingly compatible upon the application of
heat, e.g., by
employing similar polymeric materials, such as polyethylenes, at the seal
interface that
bond to one another when heated. Alternatively, a pressure-sensitive adhesive
can be
used. Further details are described in, e.g., U.S. Pat. Nos. Re 30,009 (Purdue
et al.),
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5,346,735 (Logan et al.), and 5,770,287 (Miranda et al.), the disclosures of
which are
hereby incorporated herein by reference.
In modified-atmosphere packaging, a food product is generally packaged in a
tray having a peripheral flange to which a lidding film is secured. Prior to
securing the
film to the tray, air is generally evacuated from the interior of the tray and
replaced by a
gas which extends the shelf life of the packaged product. In one type of MAP,
a fresh
red meat or other food product is packaged in a low-oxygen environment, e.g.,
carbon
dioxide and/or nitrogen, after evacuating all or most of the air from the
package. Since
fresh red meat assumes a purple color in such a low-oxygen environment, to the
dislike
of most consumers, the lidding film contains a gas-impermeable portion that is
peelably
removable from a gas-permeable portion. At retail, the gas-impermeable portion
is
peeled from the package so that oxygen from the ambient atmosphere can enter
the
package via the remaining gas-permeable portion of the lid and cause the meat
to
"bloom," i.e., assume a bright red color that most consumers associate with
freshness.
Fresh meat remains in this state for about three days. Examples of this type
of
modified-atmosphere packaging are disclosed in U.S. Pat. Nos. 5,686,126 and
5,779,050, the disclosures of which are hereby incorporated herein by
reference.
Another type of MAP employs a high-oxygen packaging environment for fresh
red meat or poultry. This package is made by first evacuating the air from in
and
around a product-containing, gas-impermeable tray, introducing a high-oxygen
environment (i.e., higher than the concentration of oxygen found in air), and
then
attaching a gas-impermeable lidding film to the tray to enclose the product
therein. The
high-oxygen environment serves to preserve the meat (e.g., by preventing
microbial
growth), but generally for a shorter period of time relative to a low-oxygen
MAP. With
a high-oxygen MAP, however, the meat remains in a constant state of bloom, due
to the
continued exposure to oxygen, so that a peelable lid is not needed.
In these and other lidded-tray type vacuum packaging operations, a problem
that
frequently occurs when using a foam tray is that the tray is damaged when the
pressure
is reduced during the process of removing air from the space around the tray
and
product. During the packaging operation, the tray-containing product is placed
into a
vacuum chamber, and the pressure is reduced very rapidly, e.g., from
atmospheric
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3
pressure to less than about 300 milibars (a reduction of over 700 mbars) in a
time
period of less than about 10 seconds. Because of this rapid reduction in
pressure, a
sudden pressure differential develops between the pressure within the cells of
the foam
and the ambient atmosphere, i.e., the pressure surrounding the tray, such that
the
pressure within the foam cells is over 700 mbars higher than that of the
ambient
atmosphere. Such pressure differential causes an immediate tendency for the
gas within
the cells to expand. In brittle foams such as polystyrene, or in foams having
mechanical
defects, e.g., inconsistent cell size, this tendency towards expansion causes
the gas in
the cells to escape forcefully from the weaker cells and surge out of the
foam, thereby
rupturing cells and damaging the structural integrity of the foam.
An additional problem caused by the sudden pressure differential between the
inside and outside of the foam during vacuum packaging is the delamination of
a film
that may be adhered to one or both major surfaces of the foam. While certain
foams are
sufficiently gas-impermeable for food packaging such as, e.g., PET foams,
other foams,
e.g., polystyrene foams, are insufficiently gas-impermeable such that a gas-
impermeable
film is often adhered to a surface of the foam in order to render a tray made
from the
foam gas-impermeable. Alternatively or in addition, a supporting film may be
adhered
to a surface of the foam in order to enhance the rigidity or crack-resistance
of a tray
made from the foam. When a sufficient amount of gas within the foam cells
expands
and escapes from the foam during evacuation, any such films, particularly gas-
impermeable films, adhered to the foam tray are often caused to fully or
partially
delaminate by the escaping gas, thereby rendering the tray, and therefore the
package,
unusable.
Accordingly, there is a need in the art for an improved foam tray that is more
suitable for vacuum packaging operations, i.e., one with increased resistance
to damage
upon exposure to a reduction in ambient pressure.
SUMMARY OF THE INVENTION
That need is met by the present invention which provides a foam sheet
comprising a cellular structure having two or more interconnected cells in
fluid
communication with one another, the foam sheet being in the form of a tray
comprising:
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a. a base and four connected side-walls, each of the side-walls being
further connected to and extending from the base to define a cavity, the tray
having an
interior surface adjacent the cavity and an exterior surface external to the
cavity; and
b. one or more perforations in the exterior surface of the tray, the
perforations extending into the cellular structure of the foam sheet to
fluidly
communicate with the interconnected cells and being sufficient to permit gas
to escape
from the cellular structure in order to substantially prevent damage to the
tray upon
exposure thereof to a reduction in ambient pressure.
The invention also provides a method of packaging a product, comprising:
a. providing a tray as described above;
b. placing a product in the cavity of the tray;
exposing the tray and product to a reduction in ambient pressure, with
the perforations permitting gas to escape from the cellular structure of the
foam sheet in
order to substantially prevent damage to the tray; and
d. attaching a lid to the tray to enclose the product within the cavity.
The invention also pertains to a method of making a tray, comprising:
a. providing a foam sheet comprising a cellular structure having two or
more interconnected cells in fluid communication with one another;
b. forming the foam sheet into a tray comprising a base and four connected
side-walls, each of the side-walls being further connected to and extending
from the
base to define a cavity, the tray having an interior surface adjacent the
cavity and an
exterior surface external to the cavity; and
producing one or more perforations in the exterior surface of the tray, the
perforations extending into the cellular structure of the foam sheet to
fluidly
communicate with the interconnected cells and being sufficient to permit gas
to escape
from the cellular structure in order to substantially prevent damage to said
tray upon
exposure thereof to a reduction in ambient pressure.
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BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a modified-atmosphere package in accordance
with the present invention, as taken from above the package, including a foam
tray,
product, and lid enclosing the product within the tray;
5 FIG. 2 is a perspective view of the modified-atmosphere package of FIG. l,
taken from the underside of the package;
FIG. 3 is a partial, cross-sectional view of the foam sheet from which the
tray
shown in FIG. 1 is made;
FIG. 4 illustrates an alternative embodiment of the foam sheet shown in FIG.
3,
in which an interior film and an exterior film are adhered to both major
surfaces of the
foam sheet;
FIG. 5 is a schematic, cross-sectional view of a vacuum packaging process in
accordance with the present invention; and
FIG. 6 is a schematic view of a process for making trays in accordance with
the
present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIGS. 1 and 2 illustrate a modified-atmosphere package 10 in accordance with
the present invention, the package including a tray 12, product 14, and lid 16
attached
to the tray such that the product is enclosed between the tray and lid. It
should be
understood that the present invention is equally applicable to other types of
packaging,
such as vacuum-skin packaging, and that a MAP is shown for illustrative
purposes only
without any intent to limit the invention.
Tray 12 is formed from a foam sheet, and comprises a base 18 and four
connected side-walls 20a-d, with each of the side-walls being further
connected to and
extending from the base 18 to define cavity 22 in which product 14 is
disposed. Tray
12 has an interior surface 24 adjacent cavity 22 and an exterior surface 26
external to
the cavity.
As shown in FIG. 3, the foam sheet 28 from which tray 12 is formed comprises
a cellular structure having two or more interconnected cells 30 in fluid
communication
with one another. Generally, such interconnected cells are adjacent cells that
have
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6
opened during the formation of the foam such that the cell wall separating the
two cells
has developed an opening therein to permit communication between the two
cells. This
interconnection may also occur as a series of three or more cells in fluid
communication. Depending on the foaming conditions and polymer forming the
cell
walls, there may be a plurality of such interconnected cell groupings randomly
distributed throughout the foam. Some of the cells may also be closed cells
interspersed among the interconnected/open cells as shown. A closed cell is
one having
no opening in the wall that forms the cell. Foams in accordance with the
present
invention preferably have between 20-90 percent by volume (based on the total
volume
of the foam) interconnected (open) cells and 10-80 vol. % closed cells. More
preferably, such foams have between 30-80 vol. % interconected cells and 20-
70%
closed cells and, most preferably, between 30-60 vol. % interconnected cells
and
between 40-70 vol. % closed cells.
Significantly, in accordance with the present invention, one or more
perforations
32 are provided in the exterior surface 26 of tray 12, with such perforations
extending
into the cellular structure of the foam sheet 28 to fluidly communicate with
the
interconnected cells 30. The perforations 32 are sufficient, in number and
size, to
permit gas to escape from the cellular structure of foam sheet 28 in order to
substantially prevent damage to the tray upon exposure thereof to a reduction
in
ambient pressure, e.g., as would occur during vacuum packaging as explained
above.
The number and size of the perforations that are sufficient to permit enough
gas to
escape during exposure of the tray to a reduction in pressure is dependent on
a number
factors, including:
1) the type of foam, with more resilient or higher strength foams such as PET
or
PP foams needing fewer/smaller perforations than more brittle foams such as
polystyrene;
2) the ratio of interconnected v.s. closed cells in the foam, with the number
and
size requirements of the perforations increasing with a higher ratio of
closed/interconnected cells (unless the increased ratio of closed cells
results in a
stronger foam);
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3) the extent to which the ambient pressure surrounding the tray is reduced,
with
greater pressure decreases necessitating more and/or larger perforations; and
4) the speed at which the pressure is reduced, with greater speed
necessitating
more and/or larger perforations.
In general, it is preferred that the number and size of the perforations be
kept to
a minimum, inasmuch as the rigidity and strength of the foam tray decreases
with
increasing perforation size and number. It has been found, however, that a
suitable
balance between sufficient gas flow during pressure reduction and minimal
decrease in
strength and rigidity can be achieved. For example, for trays formed from
polystyrene
foam sheet having a thickness of approximately 150 mils (1 mil = 1/1000 inch),
approximately 1 perforation/in2, with each perforation having a diameter of
approximately 30 mils and penetrating into the exterior surface of the tray to
a depth
ranging from about 50 to 100 mils, was sufficient to substantially reduce and
essentially
eliminate damage to the trays when subjected to a reduction in ambient
pressure from
atmospheric pressure (approx. 1000 mbars) to about 5 mbars over a period of
less than
2 seconds. Such conditions approximate the prevalent conditions to which foam
trays
are subjected in a typical low-oxygen type MAP operation.
The perforations 32 preferably extend no more than partially into the foam
sheet
28. In this manner, the packaging environment in which the product 14 is
packaged
will be preserved. In addition, complete perforations would allow juices from
packaged
meat or poultry to leak from the package. Preferably, at least one perforation
32 is
disposed in the base 18 and at least one perforation 32 is disposed in each of
the side-
walk 20a-d. Any suitable number of perforations 32 may be employed as noted
above,
ranging, e.g., from about 2 perforations/in2 to about 1 perforation/2 in2. The
perforation
diameter may range, e.g., from about 15 to about 60 mils, but preferably
ranges from
about 20 to about 40 mils. The perforation depth may range from about 5% of
the foam
sheet thickness, as measured from the exterior surface 26, to about 75% of the
foam
sheet thickness. Preferably, the perforation depth ranges from about 10% to
about 50%
of the foam sheet thickness.
Foam sheet 28 may be formed from any suitable or desired material and may
comprise, e.g., at least one material selected from the group consisting of
polyolefin
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(e.g., polyethylene or polypropylene), polystyrene, polyurethane, and
polyester (e.g.,
PET), including blends of the foregoing materials. Such blends may include
recycled
scrap foam sheet blended with virgin polymer. The recycled foam sheet may
include
the foam itself plus any films) adhered thereto. A preferred polymer for meat
packaging is polystyrene.
In an alternative embodiment, the foam sheet 28 may include a film adhered to
one or both major surfaces thereof, i.e., to interior surface 24 and/or to
exterior surface
26 of tray 12. Such alternative embodiment is illustrated in FIG. 4, wherein
tray 12'
includes an exterior film 34 in adherence with exterior surface 26. Such
exterior film
may be useful or necessary to enhance the strength, gas-impermeability,
rigidity, and/or
shatter resistance of the foam tray. When an exterior film 34 is included, it
is preferred
that such film has one or more perforations 32' therein aligned with
perforations 32 in
foam sheet 28. In this manner, the aforementioned function of perforations 32
will not
be impeded by the inclusion of exterior film 34. Exterior film 34 may include
any
suitable material, such as, e.g., at least one material selected from the
group consisting
of polyolefm, polystyrene, polyester, polyamide, and blends of the foregoing
materials.
For example, when foam sheet 28 is formed from polystyrene, it is preferred
that an
exterior film 34 be included to enhance the rigidity and shatter resistance of
tray 12',
such film preferably comprising oriented polystyrene, high impact polystyrene,
styrene/butadiene copolymer, or blends thereof.
Alternatively or in addition to an exterior film 34, tray 12' may include an
interior film 36 in adherence with interior surface 24. Such interior film 36
may be
desirable in order to increase the gas-impermeability of tray 12', enhance the
ability of a
lid to be sealed to the tray, provide a liquid barrier to any juices that may
emanate from
product 14, etc. In the case where it desirable to package product 14 under
vacuum or
in a modified-atmosphere, e.g., where product 14 is perishable in the presence
of
atmospheric concentrations of oxygen such as, e.g. meat, poultry, produce,
etc., and
foam sheet 28 is not sufficiently gas-impermeable, it is preferred that
interior film 36 is
substantially gas-impermeable. The interior film 36 may thus comprise at least
one
material selected from the group consisting of ethylene/vinyl alcohol
copolymer,
vinylidene chloride and copolymers thereof, acrylonitrile, polyester and
copolymers
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thereof, polyamide, and blends of such materials. Suitable interior films for
use in
accordance with the present are described in U.S. Patent Nos. 4,847,148 and
4,935,089,
and in U.S. Serial No. 08/326,176, filed October 19, 1994 and entitled
"Film/Substrate
Composite Material" (published as EP 0 707 955 A1 on April 24, 1996), the
disclosures
of which are hereby incorporated herein by reference.
Referring back to FIGS. l and 2, another preferred feature of tray 12 is a
continuous flange 38 connected to and extending from side-walls 20a-d to
define a
surface 40 to which lid 16 may be attached in order to enclose product 14
within cavity
22. Tray 12 can have any desired configuration or shape, e.g., rectangular,
square,
round, oval, etc., with the depth of cavity 22 being of any desired dimension.
Similarly,
flange 38 may have any desired shape or design, including a simple,
substantially flat
design which presents a single sealing surface as shown, or a more elaborate
design
which presents two or more sealing surfaces, such as, e.g., the flange
configurations
disclosed in U.S. Patent Nos. 5,348,752 and 5,439,132.
Although product 14 is illustrated as having a maximum height that is below
the
maximum height of tray 12, i.e., below the level at which flange 38 is
located, the
invention is not limited to such "low profile" products. Rather, "high
profile" products
may also be packaged in accordance with the present invention, i.e., those
having a
maximum height which is above the level at which flange 38 is located so that
the
portion of the product which extends above the level of flange 38 will be in
direct
contact with lid 16.
Referring now to FIG. 5, a packaging method in accordance with the present
invention will be described. The method includes placing product 14 into
cavity 22 of
tray 12. Alternatively, tray 12' may be used (as described above with regard
to FIG. 4),
or variations thereof with different types or combinations of films or with
only one film
may be used as desired. Tray 12 and product 14 are exposed to a reduction in
ambient
pressure in vacuum chamber 42, wherein upper and lower chamber halves 44 and
46
converge in the direction of the arrows and close around tray 12 and product
14 to form
a sealed enclosure 48. Valves (not shown) in communication with exhaust ports
50 and
52 are then opened to a vacuum source (not shown), such as a single or
separate
vacuum pumps, which reduces the ambient pressure surrounding tray 12 and
product 14
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by removing substantially all of the air from sealed enclosure 48 as indicated
by the
arrows.
In general, when packaging fresh meat (e.g., beef, veal, lamb, pork, etc.),
poultry
or fish, the ambient pressure in vacuum chamber 42 is preferably reduced by at
least
5 about 700 milibars, i.e., to less than about 300 mbars, from atmospheric
pressure
(approximately 1000 mbars). For high-oxygen MAP, this pressure reduction
preferably
results in a final pressure in chamber 42 ranging from about 200 to about 50
mbars and,
more preferably, from about 150 mbar to about 75 mbars, such pressure
reduction
taking place in less than about 10 seconds, more preferably in less than about
5 seconds
10 and, most preferably, in less than about 2 seconds. Such speed is important
to meeting
the needs of modern, high-speed packaging requirements. For low-oxygen MAP and
VSP packaging, the pressure reduction in vacuum chamber 42 preferably results
in a
final pressure of less than about 50 mbars, more preferably less than about 30
mbars,
more preferably still less than about 20 mbars and, most preferably, less than
about 10
mbars, again preferably in less than about 10 seconds, more preferably in less
than
about 5 seconds and, most preferably, in less than about 2 seconds.
As noted above, with conventional foam trays, such vacuum packaging process
has been found to result in damage to the cellular structure of the tray and
delamination
of any films) that may be adhered thereto. In accordance with the present
invention,
such tray damage is avoided by providing one or more perforations 32 in the
exterior
surface of the tray (see FIGS. 1-4), the perforations permitting gas to escape
from the
cellular structure of foam sheet 28 during pressure reduction in vacuum
chamber 42.
After the desired pressure has been achieved in sealed enclosure 48, the final
step in the packaging process is to attach lid 16 to tray 12 in order to
enclose product 14
within cavity 22. This may be accomplished as shown by suspending web 54 of
lidding
film above tray 12 and product 14 in vacuum chamber 42 as shown. After the
pressure
has been sufficiently reduced, web 54 is moved into contact with flange 38 of
tray 12
and attached thereto by any suitable means. This is preferably carried out by
translating
seal bar 56 downwards as indicated by the arrow until the ends 58 contact web
54, and
further press web 54 into contact with the upper surface 40 of flange 38. Seal
bar 56
then applies heat and continued pressure to web 54 at flange 38 in a downward
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direction to thereby effect a heat-seal between the web and flange. The heat
and
pressure applied by seal bar 56 are preferably sufficient to cause the web 54
and tray 12
at flange 38 to fuse together, thereby creating a hermetic enclosure for
product 14.
Thereafter, the seal bar 56 is raised to the starting position shown in FIG.
5, the
chamber halves 44, 46 are opened, and lid 16 is severed from web 54 by a
suitable
cutting device (not shown).
Suitable equipment for carrying out the aforedescribed packaging method is
commercially available by, e.g., Ross Industries, Inc. or Multivac, Inc., such
as the Ross
Inpack~ 3320 or the Multivac~ T500 packaging machines. Further details
concerning
the packaging process are described in the above-referenced U.S. Pat. No.
5,779,050.
The foregoing packaging method is ideally suited for products that are
perishable in the presence if air such as, e.g., fresh red meat products
(e.g., beef, veal,
lamb, pork, etc.), poultry (chicken, turkey, etc.), fish, cheese, produce
(fruits and
vegetables), etc. When such products are to be packaged, it is preferred that
both tray
12 and lid 16 are substantially gas-impermeable, and that they are attached
with a
hermetic seal, e.g., a heat seal as described above, in order to form a
substantially gas-
impermeable enclosure. As used herein, the phrase "substantially gas-
impermeable"
refers to a film or tray that admits less than 1000 cc of gas, such as oxygen,
per square
meter of film per 24 hour period at 1 atmosphere and at a temperature of 73 ~
F (at 0%
relative humidity). More preferably, a substantially gas-impermeable film or
tray
admits less than about 500, such as less than 300, and less than 100 cc of
gas; more
preferably still less than about 50 cc, and most preferably less than 25 cc,
such as less
than 20, less than 15, and less than 10 cc of gas per square meter per 24 hour
period at 1
atmosphere and at a temperature of 73 D F (at 0% relative humidity).
Any desired amount of air may be removed from the enclosure 48 of vacuum
chamber 42 during the evacuation step, e.g., ranging from 1 % to 99.999% by
volume.
In the case where a fresh red meat or poultry product is to be packaged, the
amount of
air removed preferably ranges from about 99% to about 99.999%, and more
preferably
from about 99.5% to about 99.999% by volume. As a result, the cavity 22 of
package
10 will preferably contain less than 1 % oxygen by volume, more preferably
less than
0.5% oxygen, even more preferably less than 0.1 % oxygen, and most preferably,
less
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than 0.05% oxygen by volume. If package 10 is to be a vacuum-skin package
(VSP),
substantially no gasses will be present in cavity 22. Alternatively, when
package 10 is a
modified-atmosphere package, following the reduction in ambient pressure in
vacuum
chamber 42, a gas that extends the shelf life of the product 14 is introduced
to into
enclosure 48, e.g., via port 50 and/or 52, prior to the attachment of lid 16.
By thus
introducing such a gas to cavity 22 and product 14 prior to securing lid 16 to
tray 12,
the gas is contained within package 10 and surrounds product 14, e.g., to
extend the
shelf life thereof or to provide other intended benefits as desired. Preferred
gases to
replace the evacuated air include, e.g., carbon dioxide, nitrogen, argon,
etc., and
mixtures of such gases, such as a mixture of carbon dioxide and nitrogen.
Lid 16 is preferably a substantially transparent, flexible film, which may be
supplied as a web 54 as shown in FIG. 5, e.g., from a storage roll (not
shown). When
package 10 is to be a non-peelable VSP or MAP package, such as a high-oxygen
package as described earlier herein, the only additional requirements that are
preferred
for lid 16 is that it be sealable to tray 12 and, for packaging perishable
foods such as
fresh meat or poultry products, that it be substantially gas-impermeable. When
package
10 is a peelable VSP or MAP package, a further requirement is that film 16 be
peelably
delaminatable into gas-permeable and gas-impermeable portions, with the gas-
permeable portion remaining attached to the package and the gas-impermeable
portion
being peelably removable at retail so that the packaged fresh meat or poultry
product is
allowed to bloom to a red or, in the case of fresh poultry, pink color that
consumers
associate with freshness, while the remaining gas-permeable lid portion
continues to
protect the product by preventing contact with dust, dirt, moisture, and other
contaminates while the package is displayed for sale at retail. A preferred
peelable film
for peelable VSP packaging is described in the above-referenced U.S. Pat. No.
5,770,287. Preferred films for peelable MAP applications are disclosed in the
above-
referenced U.S. Pat. No. 5,686,126, and also in U.S. Serial No. 08/764,405
filed 11-
Dec-1996 and entitled LAMINATE HAVING A COEXTRUDED, MULTILAYER
FILM WHICH DELAMINATES AND PACKAGE MADE THEREFROM, the
disclosure of which is hereby incorporated herein by reference.
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Referring now to FIG. 6, a preferred method for making a tray in accordance
with the present invention will be described. Essentially, the method includes
the steps
of
a. providing a foam sheet 28 as described above;
b. forming foam sheet 28 into a tray 12 (or 12') as described above; and
c. producing one or more perforations 32 in the exterior surface 26 of the
tray as described above. The step of producing the perforations may occur
before or
after the forming step as desired. In the process illustrated in FIG. 6 and
described
below, the perforations are produced prior to forming the foam sheet into a
tray.
However, the perforations can also be formed in the side-walls and/or base of
the tray
after the tray has been formed.
Foam sheet 28 is preferably provided as a continuous web from storage roll 60,
from which the sheet is unwound via nip rolls 62a and 62b. Nip rolls 62a-b
rotate in
opposite directions to move the foam sheet in the direction of the arrow. In
producing
trays 12' as shown, exterior film 34 and interior film 36 may be similarly
provided as
webs from respective storage rolls 61 and 63, and are laminated to opposite
surfaces of
foam sheet 28 in nip rolls 62a-b to form composite web 59. The nip rolls can
be heated
if desired to facilitate the lamination process. Alternatively, films 34
and/or 36 can be
adhered to foam sheet 28 via coextrusion, extrusion-coating, coextrusion-
coating, vapor
deposition, or any suitable process for adhering the films to the foam sheet.
Perforator roll 64 rotates against backer roll 66, and composite web 59 is
passed
between rolls 64 and 66 to simultaneously produce perforations 32' in the
exterior film
34 and perforations 32 in the exterior surface 26 of foam sheet 28 as shown in
FIG. 4.
Forming the perforations 32, 32' simultaneously facilitates their alignment
with one
another. Perforator roll 64 includes an array of perforation needles 69
arranged to
provide a desired perforation pattern on web 59 and on the resultant trays
12'. The
diameter and length of the perforation needles is selected to provide a
desired diameter
and depth of perforations 32 in the resultant tray. The needles 69 may
optionally be
heated to facilitate the perforation process.
Forming station 68 is preferably a thermoforming apparatus including a
stamping device 70, which reciprocates against correspondingly shaped and
stationary
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14
receiving mold 72. Trays are thus formed in the shape of stamping device 70
and
receiving mold 72 when the stamping device 70 moves against composite web 59
and
presses it into receiving mold 72 with sufficient heat and pressure to cause
the
foam/film composite web to assume the shape of the stamp and receiving mold.
Stamping device 70 and/or receiving mold 72 are preferably heated to
facilitate the
thermoforming process. If desired, vacuum ports (not shown) in communication
with a
vacuum source may be provided in receiving mold 72 in order to further
facilitate the
thermoforming process by pulling the web 59 against the mold 72.
After the trays have been formed in station 68, the web 59 is indexed to
cutting
station 74, wherein cutting device 76 severs trays 12' from the web. The
severed trays
then drop out of the web and into stack 78 for subsequent removal and use. The
remainder of the composite web may be processed as scrap, preferably by
grinding and
pelletizing the scrap and recycling it, e.g., by blending it with virgin
polymer to make
foam sheet 28 as disclosed, e.g., in U.S. Pat. No. 5,118,561.
Although the foregoing process has been described in connection with the
production of tray 12' as shown and described with reference to FIG. 4, it is
to be
understood that the process is equally applicable to the production of tray 12
as shown
and described with reference to FIG. 3, by simply omitting films 34 and 36.
EXAMPLES
For each of the following examples, composite foam trays were produced in
accordance with the above-referenced U.S. Serial No. 08/326,176 and as shown
in FIG.
6, resulting in the three-layer structure shown in FIG. 4. The exterior film
(34) was
oriented polystyrene film, the foam sheet (28) was foamed polystyrene (foamed
with a
blend of pentane and carbon dioxide as the blowing agent) blended with
recycled scrap
from prior production of trays having the same composite structure, and the
interior
film (36) had the following structure:
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LLDPE/EVA/TIE/EVOH/TIE/EVA/SBC
where:
"LLDPE" is a layer comprising DOWLEX 2244A (T'M) heterogeneous ethylene/octene
copolymer having a melt index of 3.3 and a density of 0.916 g/cc; obtained
from The Dow
5 Chemical Company, of Midland, Michigan.
"EVA" is a layer comprising ELVAX 3165 (TM) ethylene/vinyl acetate copolymer
having
18 percent by weight of vinyl acetate, a melt index of 0.7 and a density of
0.94 g/cc;
obtained from E.I. Dupont de Nemours, of Wilmington, Delaware (both "EVA"
layers in
10 the above film structure are the same).
"'TIE" is a layer comprising TYMOR 1203 (TM) anhydride-grafted linear low
density
polyethylene having a melt index of 1.6 and a density of 0.910 g/cc; obtained
from Morton
International of Chicago, Illinois (both "TIE" layers in the above film
structure are the
15 same).
"EVOH" is a layer comprising LC-H101BD (TM) ethylene/vinyl alcohol copolymer
having
38mole percent of ethylene, a melt index of 1.5 and a melt point of 175
°C; obtained from
EVAL Co. of America, of Lisle, Illinois; and
"SBC" is a layer comprising KK36 (TM) styrene/butadiene copolymer having 75
percent by
weight of styrene, a melt index of 8.0 (Condition G of ASTM D-1238) and a
density of 1.01
g/cc, obtained from Phillips 66, of Pasadena, Texas.
The interior and exterior films had a thickness of approximately 1 mil and the
foam portion had a thickness ranging from about 110 to about 170 mils.
Example 1
Approximately 70 composite foam trays as described above, having the
dimensions (LxWxD) 6"x6"x2", were tested in a vacuum chamber that reduced the
ambient pressure around the trays from about 1000 mbars to about 5 mbars in
less than
2 seconds. The trays were of marginal quality, with uneven cell size
distribution. Half
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16
of the trays were perforated in accordance with the present invention on the
outer
surface thereof with perforation needles having a diameter of 30 mils, to a
depth of
approximately 40% of the tray wall thickness, and spaced at about 1
perforation/in2.
Approximately 40% of the non-perforated trays failed, i.e., were structurally
damaged
due to rupture of the foam cell walls and/or exhibited delamination of the
interior film.
On the other hand, only about 4% of the perforated trays exhibited such
damage.
Example 2
Approximately 100 composite trays as described above, judged to be of good
quality (even cell size distribution), were tested in a vacuum chamber as in
Example 1.
The trays had the dimensions (LxWxD) 9"x7"x2". Half of the trays were
perforated in
accordance with the invention on the outer surface with perforation needles
having a
diameter of 30 mils, to a depth of approximately 40% of the tray wall
thickness, and
spaced at approx. 1 perforation/in2. Perforation occurred prior to
thermoforming the
laminated film/foam/film web into trays. Approximately 6% of the non-
perforated
trays failed, i.e., were structurally damaged due to rupture of the foam cell
walls and/or
exhibited delamination of the interior film. On the other hand, none of the
perforated
trays exhibited such damage.
Example 3
Approximately 250 composite trays as described above, judged to be of good
quality (even cell size distribution), were tested in a vacuum chamber as in
Example 1.
The trays had the dimensions (LxWxD) 6"x6"x2.5". Half of the trays were
perforated
in accordance with the present invention on the outer surface with perforation
needles
having a diameter of 15 mils, to a depth of approximately 58% of the tray wall
thickness, and spaced at about 1 perforation/in2. Perforation occurred prior
to
thermoforming the laminated film/foam/film web into trays. Approximately 6% of
the
non-perforated trays failed, i.e., were structurally damaged due to rupture of
the foam
cell walls and/or exhibited delamination of the interior film. On the other
hand, none of
the perforated trays exhibited such damage.
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17
While the invention has been described with reference to illustrative
examples,
those skilled in the art will understand that various modifications may be
made to the
invention as described without departing from the scope of the claims which
follow.
