Note: Descriptions are shown in the official language in which they were submitted.
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HOT FILL THERMOPLASTIC CONTArNER
This invention relates to a method of hot filling a thermo-
plastic container. More particularly, this invention relates to a
method of hot filling a thermoplastic container so as to avoid
deformation and collapse of the container under internal vacuum
caused by subsequent cooling of the container and contents.
When foods are packaged in rigid containers, they are fre-
quently filled with liquid food stuffs at an elevated temperature
of about 190 or 200 F to destroy bacteria. This is referred to as
"hot filling". After the filling, the head space is often purged
with inert gas to reduce the oxygen content and the lid or closure
is sealed while the contents are still hot and the sealed container
is then cooled to roam temperature. At the hot fill temperature,
the vapour pressure of water is about 10 psi or 500 mm of Hg.
Sealing takes place at atmospheric pressure of 15 psi so that the
head space in the can may very well be occupied more by water vapour
(by volume) than by air or inert gas. When the can is cooled, the
water vapour pressure is reduced to a level of only about .5 psi and
the water vapour in the head space condenses, thus creating a
vacuum. This vacuum could be as much as 9 psi if the air space has
been saturated with water vapour, but usually the air space does not
approach the saturation point so that the vacuum created is typi-
cally in the region of one to three psi. However, this is still
enough to cause vacuum collapse of many plastic containers.
Plastic containers having flexible bottom walls to accommodate
the development of a vacuum within are well known and commercially
available. U.S. Patent No. 4,2S5,457 and U.S. Patent No. 4,219,578
are both concerned with an anti-buckling device for beer cans. m ey
disclose the use of an anti-buckle ring which braces the can's
interbase wall and first and second radius portions fm m substan-
tially interradial displacement.
U.S. Patent No. 4,125,632 discloses a method for the produc-
tion of a container body which is originally manufactured with
a convex base. me material in the convex base is thinner than that
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in the side wall.
U.S. Patent No. 3,409,167 discloses a flexible bottom which
accommodates an intern 1 vacuum.
Reissue U.S. Patent No. 31,762 discloses a container which, as
originally manufactured, contains a side wall convex "bulge".
U.S. Patent No. 4,459,793 mvolves the use of an interplastic
liner within a metal can to absorb the vacuum shrink, but without
any controlled volume change of the primary metal can.
U.S. Patent No. 4,125,632 provides a weak bottom for collapse
in preference to the side walls but without any controlled volume
change or use of vacuum draw dcwn.
One solution has been to make the container walls and base
very thick so that they can resist collapse, but this is econami-
cally unattractive and sametimes difficult to accomplish. Appli-
cant's idea is aimed at artificially increasing the internal
pressure within the container, thus reducing the vacuum and the
tendency of the side walls to collapse.
Applicant's invention comprises a method of hot filling a
thermoplastic container so as to avoid deformation and collapse of
the container, resulting in a container with a concave base which
comprises the steps of continuously applying a vacuum means to the
base of a thermoplastic container so as to draw down the base of
the container into a convex shape, filling the container with
desired contents during continuous application of the vacuum means,
sealing the container by an appropriate sealing means during
continuous application of the vacuum means, discontinuing the
vacuum means, and cooling the container, whose base, in the absence
of the external vacuum, inverts to a concave shape, which was its
original formed shape.
Figure 1 illustrates the thermoplastic container as originally
manufactured, which has a concave base.
Figure 2 illustrates the continuous application of vacuum
means to the base of the container to draw the base into a convex
shape.
Figure 3 illustrates the sealing of ~he filled container during
continuous application of the vacuum means.
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Figure 4 illustrates the cooling of the container after the
vacuum has been discontinued where the base of the container has
inverted to its original concave shape or something approaching
same.
Figure 5 illustrates the use of a positive stop device during
draw down of the base of the container by vacuum means.
Containers are typically hot filled and packaged with enough
air space so that after cooling 10% or more (sometimes much more)
of the volume of the container will remain filled with air of some
inert gas used to reduce the oxygen content. This is to prevent
spillage upon opening the container by the consumer. me air space
at equilibrium before cooling contains about two thirds by volume
water vapour, and after cooling only about 3% water vapour. If the
container remains rigid, as in the case of a metal can, a vacuum of
about lO psi can result. Plastic containers do not possess the
rigidity of metal cans and thus collapse under very slight vacuum
often as little as l psi. This means that a volume change equal to
more than half of the initial air volume is required to eliminate
the vacuum.
The container 8 of the present invention is constructed with a
base lO which may be bowed upward into a concave shape as illustra-
ted in Figure l. This, of course, is a normal configuration of the
container so the container will sit flat on a table and be easily
stacked for transportation and storage. At the point of hot filling,
as seen in Figure 2, vacuum is applied continuously to the under-
side of base lO of thermoplastic container 8 so as to draw down
base lO of container 8 into a convex shape. This incxeases the
volume of container 8. Container 8 is now filled with desired
cQntents 12 during continuous application of the vacuum means ll.
As seen in Figure 3, the cover seal 13 of the container 8 is
attached by an appropriate sealing means still during continuous
application of vacuum.
In Figure 4, vacuum is discontinued and container 8 is cooled
so that base lO of container 8 inverts to a concave shape. By
reducing the volume of container 8 after sealing, the head space is
reduced thus compressing the air therein and creating a positive
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internal pressure which will reduce the vacuum created upon cool-
ing. The amourlt of positive pressure created will be proportional
to the ratio of the head space before and after the suck dcwn
vacuum is released. In most cases, inversion of base 10 will take
place spontaneously. If not, some typ of mechanical assist may be
used to achieve inversion.
Fig~re 5 illustrates an optional use of a positive stop device
14 which is adjustable to allow regulation of the amount of draw
down desired for container 8. Positive stop device 14 allows a
uniformity of draw dcwn of the containers to be achieved. The
container may be placed upon a structure 16 with an aperture 18
therewithin corresponding to the area of base 10 of container 8.
Vacuum is pulled under structure 16 to suck or draw down base 10
from an upward concave shape to a downward convex shape. The use of
lS the positive stop device 14 is only one of many methods which may
be used to achieve the desired draw down of base 10. me original
base 10 of the originally manufactured container may be of a
concave shape or may be flat. me original base 10 is preferably in
a shape other than a concave shape. Concave is defined as "a
rounding or curving inward". Convex is defined as "a rounding or
curving outward".
E~MPIE 1
A 500 mil margarine tub which normally holds 500 mils of
product with 50 mils of head space may be used as an example. m e
full overflow volume would be 550 mils. mis container may be
sucked dcwn or drawn dcwn until the volume becomes 600 mils. m e
container is filled with 500 mils of liquid at 200 F and sealed.
(This results in 100 mils of head space which contains 100 mils of
air and water vapour at 15 psia. Depending on the degree of satura-
tion, the water vapour can be anything fran a negligible amount to65 mol~ of the vapour. For example, the water vapour can represents
7.5 psi of the total 15 psi and air can represent the other 7.5
psi). Upon releasing the vacu~n, the base snaps up and reduces the
head space to 50 mil, thus compressing the air by a factor of two
and raising its partial pressure to 15 pounds. When the contents
are cooled, the water vapour condenses out and drops to a partial
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pressure of about .5 pounds. It is possible to vary the volume
change by varying the draw down vacuum of course. One means of
achieving this of course, is by use of a positive stop device which
may be adjusted for varying draw dcwn ratios. The container which
results is significantly free of deformation and collapse of the
side walls.
EXAMPLE 2
A fluted cranberry cup of 500 ml product capacity designed to
hold 500 ml of product with 100 ml of vapour space. The base is
drawn down into a convex shape by vacuum so that the volume becomes
700 mils. Upon filling with 500 mils of product at 200 F and
sealing with vacuum on, it results in 200 mils of head space
consisting of air and water vapour at 15 psi. If we assume the
pressure of water vapour in the head space is 7.5 psi, then the air
or nitrogen pressure is also 7.5 psi. Upon release of the vacuum,
the base snaps up and reduces the head space to 100 mlls, thus
compressing the vapour volume by a factor of two and raising the
total pressure to about 30 psi. When the contents are cooled, the
water vapour condenses out and drops to a water partial pressure of
about .5 psi. The pressure of nitrogen is 15 psi, thus the total
internal pressure is about 15.5 psi. The container which results is
significantly free of deformation and collapse of the side walls.
The sealing of the containers may be by use of any standard
sealing mechanism which may include paper, foil or other types of
covers which may be attached by adhesives, heat welding, sonic
welding, or double seaming or any other method.
It will become apparent that it is much easier for the base of
the container to snap back to a more original, familiar position
than it would be to snap into a totally new position, such as is
the case in some of the patents which provide for manufacture of a
container which originally contained a convex base. In a~;tion,
the base of applicant's invention does not have to be made of
thicker-walled material. Likewise, with use of an external vacuum,
the amount of draw down and pressure necessary for producing the
convex base can be controlled depending on the level to which the
contents are filled within the container.
