Note: Descriptions are shown in the official language in which they were submitted.
CA 02289888 1999-11-16
CONTAINER AND LATCH WIPE THEREFOR
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of U.S. Provisional Application Serial No.
60/117,377, January 27, 1999.
BACKGROUND OF THE INVENTION
Field of the Invention
The invention generally relates to a container, and more specifically, a
molded
foam container having a cover hingedly mounted to a base and having a tab
received
within a latch wipe opening on the cover to lock the cover in the closed
position
relative to the base.
Related Art
Foam containers are commonly used for takeout food. A typical foam
container comprises an open-top base sized to receive food items, in
combination with
a cover that closes the open top of the base. The cover is usually hingedly
connected
to one side of the base and has a latch wipe opening sized to receive a tab on
the base
to hold the cover in the closed position.
The base and cover are generally molded as a single piece and can be made
from any suitable material. In applications where hot food is to be placed
within the
base, it is preferred that the container be molded from a material having
insulating
characteristics such as foamed polystyrene. One problem encountered by current
foam containers and their manufacturing processes is that the latch wipe
opening is
often deformed during manufacturing into a shape in which it no longer easily
retains
the cover tab, resulting in wasted product or a poor-performing product.
An example of a common deformity is that associated with a rectangular latch
wipe opening, disclosed in PIG. 1. The outer end of the rectangular latch wipe
opening and both edges can change shape during manufacturing from the
preferred
and initial rectangular shape to a more ovate shape, which is referred to as
"smiling."
If the degree of smiling is great enough, the locking tab cannot be retained
within the
latch wipe opening.
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SUMMARY f.~F T11E INVENTION
The invention addresses the problem of the prior art latch wipe opening
smiling effect by pre-cooling a portion of the foam sheet which will become
the
bottom edge of the latch wipe opening. The pre-cooling stabilizes the area and
reduces the contraction of the foam as it cools so a rectangular wipe can be
maintained. Smiling reduction is further accomplished by holding a portion of
the
material surrounding the latch wipe opening to prevent its movement during
thermoforming by using a textured protruding plate to cool, grip and hold the
inner
portion of the latch wipe during the wiping process.
In one embodiment, the invention relates to a method for thermoforming a
container from a heated foam sheet in a mold comprising upper and lower mold
halves that are capable of relative movement between an open position and a
closed
position. An interior form is provided on one of the mold halves for shaping
the
interior of the container and an exterior form is provided on the other mold
half for
shaping the exterior of the container. The container comprises a base having a
bottom
wall from which extends an upstanding peripheral wall. The container further
comprises a cover, which is hingedly mounted to the base, with a top wall and
a
peripheral wall depending therefrom. One of the peripheral walls has a latch
wipe
opening and the other of the peripheral walls has a locking tab that is sized
to be
received within the latch wipe opening to lock the cover in a closed position
relative
to the base. The method comprises the steps of positioning the heated foam
sheet
between the open mold halves, moving the mold half toward the closed position,
and
pre-cooling a first portion of the foam sheet adjacent to the location where
the latch
wipe opening will be formed.
The method can also include the holding of a second portion of the foam sheet
adjacent to the location where the latch wipe opening will be formed. The
holding of
the second portion can include pressing the second portion of the foam sheet
against a
textured surface on the interior form.
The pre-cooling step can include contacting the first portion of the foam
sheet
with a portion of one of the mold halves, preferably a flange extending
upwardly from
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one of the mold halves. The pre-cooling step can further include the pre-
cooling of
the foam sheet second portion in addition to the pre-cooling of the foam sheet
Frst
portion.
In another embodiment, the invention relates to an apparatus for
thermoforming a container from a heated foam sheet. The container comprises a
base
having a bottom wall and an upstanding peripheral wall along with a cover,
which is
hingedly mounted to the base, with a top wall and a depending peripheral wall.
One
of the peripheral walls has a latch wipe opening and the other of the
peripheral walls
has a locking tab that is sized to be received within the latch wipe opening
to lock the
cover in a closed position relative to the base. The apparatus comprises upper
and
lower mold halves, each of which includes an inner surface, which are in
opposing
relationship. One of the inner surfaces has a container-forming portion that
defines
the interior of a container and the other of the upper surfaces has a
container-forming
portion that defines the exterior of a container. The mold halves are movable
between
an open position permitting the insertion of a foam sheet and a closed
position where
a foam sheet positioned between the mold halves conforms to the container-
forming
portions to form a container. A flange with a cooling surface extends from one
of the
inner surfaces a sufficient distance so that a first portion of a heated sheet
adjacent to
the location where the latch wipe opening will be formed contacts the cooling
surface
prior to contacting the container-forming portion of the one of the inner
surfaces as
the mold halves are moved to the closed position to thereby pre-cool a portion
of the
heated sheet.
Preferably, the apparatus further comprises one or more pairs of inner and
outer forms, which together form the interior and exterior surfaces of the
container. A
textured surface can be provided on the container-forming portion of the one
of the
inner surfaces at a position where a second portion of a foam sheet adjacent
to the
location where a latch wipe opening will be formed. The textured surface is
preferably a knurled portion on the upper surface container-forming portion
defining a
container interior.
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A shear key can also be provided on the upper surface container-forming
portion defining a container interior and positioned below the knurled portion
whereby upon the closure of the mold halves, the shear key forms a slit in the
foam
sheet to create the latch wipe opening in the knurled portion grips a portion
of a foam
sheet above the slit to control the shape of the latch wipe opening.
Preferably, the height and position of the flange are selected such that a
portion of a foam sheet contacting the flange will lie below the shear key
upon
complete closure of the mold halves. The flange is movably mounted to the mold
half
having said one inner surface and is movable between an extended position
where the
cooling surface is positioned above said one inner surface when the mold
halves are
open and a retracted position where the cooling surface is substantially
coplanar with
said one inner surface when the mold halves are closed.
The mold halves can have an internal fluid conduit through which a coolant is
passed to maintain the flange cooling surface in a temperature less than the
1 S temperature of a foam sheet for forming the container.
In a third embodiment, the invention relates to a foam container comprising a
base having a bottom wall and a peripheral wall extending from the bottom
wall, with
a latch wipe opening formed in the peripheral wall. The container further
includes a
cover having a top wall and a peripheral wall extending from the top wall,
with a
locking tab extending from the cover peripheral wall. The cover peripheral
wall is
hingedly mounted to the base peripheral wall for movement between a closed
position
where the cover overlies the base and an open position where the cover is
retracted
from the base. The locking tab is sized to be received within the latch wipe
opening
to lock the cover in the closed position. The latch wipe opening is formed by
pre-cooling a portion of the foam sheet forming the container at a location
adjacent to
the latch wipe opening.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
FIG. 1 illustrates a prior art latch wipe opening having the smiling
characteristic;
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FIG. 2 is a perspective view of a, container made according to the invention;
FIG. 3 is a sectional view taken along line 3-3 of FIG. 2;
FIG. 4 illustrates a forming station having upper and lower mold halves that
in
combination with a sheet of foam material are used to manufacture the
container
according to the invention;
FIG. S is a perspective view of a portion of the lower mold half illustrating
an
inner form and a flange clamp in the raised position for reducing the smiling
effect on
the latch wipe;
FIG. 6 is similar to Fig. 5 but shows the flange clamp in the lowered
position;
FIG. 7 is a side-sectional view of the sheet material and its relationship to
the
upper and lower mold halves as they begin to contact the sheet during the
molding
process;
FIG. 8 is a side-sectional view similar to FIG. 7 and illustrating the
relative
positions of the sheet material, upper and lower mold halves, and the flange
clamp just
1 S as the sheet material makes contact with the flange clamp during closure
of the mold;
FIG. 9 is a side-sectional view similar to FIG. 8 and illustrating the
relative
positions of the sheet material, the upper and lower mold halves, and the
flange
clamp, when shear keys on the upper and lower mold halves engage the sheet
material
to form the latch wipe opening; and
FIG. 10 is a side-sectional view similar to FIG. 7 and illustrating the
relationship between the sheet material, the upper and lower mold halves, and
the
flange clamp when the mold halves are in the closed position.
DETAILED DESCRIPTION
FIG. 2 illustrates a container 10 according to the invention, and comprising a
cover 12 and a base 14. The cover 12 has a top wall 16 from which extend a
pair of
side walls 18, 20, a back wall 22, and a front wall 24. The side walls 18, 20,
back
wall 22, and front wall 24 all terminate at a peripheral lip 26. A latch wipe
opening
28 is provided in the front wall 24 adjacent to the peripheral lip 26.
The base 14 has a structure similar to the cover 12 and comprises a bottom
wall 36 from which extend a pair of side walls 38, 40, a back wall 42, and a
front wall
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44, all of which terminate in a peripheral lip 46. A locking tab 38 extends
from the
base front wall 44 near the peripheral lip 46. The locking tab 48 can be
formed as a
portion or part of the peripheral lip 46 or can be completely independent of
the
peripheral lip 46.
Preferably, the cover 12 and base 14 are hingedly connected by joining the
peripheral lips 26 and 46 along the back walls 22, 42 to form a hinge 50. By
hingedly
joining the cover 12 and base 14 in this manner, the cover 12 can rotate about
the
hinge 50 into a position where it overlies the base 14 and the locking tab 48
is
received within the latch wipe opening 28 to lock the cover with respect to
the base.
Looking at the cover 12 and base 14 in greater detail, the cover peripheral
lip
26 is formed with a rib 54 sized to be received within a channel 56 in the
base
peripheral lip 46. The rib 54 and channel 56 are generally U-shaped and extend
along
their respective front walls and side walls. The rib 54 and channel 56 form a
seal to
help retain heat and fluid within the container when closed.
The latch wipe opening 28 will now be described in greater detail with general
references to FIG. 2 and with specific reference to FIG. 3, which illustrates
the latch
wipe opening 28 from inside the cover 12. The latch wipe opening 28 is formed
in an
inset portion 60 of the cover front wall 24 and has a rectangular shape
defined by an
upper edge 62, lower edge 64, and side edges 66. The lower edge 64 is
generally
adjacent to the peripheral lip 26. The front wall is formed with a textured or
knurled
portion 68 provided above the upper edge 62 of the latch wipe opening 28.
The manufacturing process of the container 10 is illustrated in FIGS. 4-10.
FIG. 4 illustrates a typical forming station 70 at which multiple containers
10 are
formed. The forming station 70 comprises an upper mold half 72 and a lower
mold
half 74 between which is fed a sheet 76 (shown in phantom) of material from
which
the containers 10 are made. The sheet material 76 is preferably foamed
polystyrene
and is a continuous, indexed sheet. The upper and lower mold halves 72, 74
both
have multiple complementary interior form 78 and exterior form 80 (FIGS. 7-10)
mold forms that in combination form the individual containers 10.
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Referring to FIGS. 4-6, the interior form 78 comprises a cover portion 84 and
a base portion 86, which are complementary in shape to the interior of the
finished
cover 12 and base 14 of the container 10. The cover portion 84 includes top
wall 96,
side walls 98, 100, back wall 102, and front wall 104, all of which correspond
to the
similar walls on the finished container 10. The cover portion 84 further
includes a
channel 106 that forms the rib 54 on the finished container 10. An inset
portion 108 is
formed in the front wall 104 of the interior form 78 to form the inset portion
60 on the
finished container 10.
The inset portion 108 of the interior form 78 includes a shear key 112 and a
knurled portion 114. The knurled portion 114 is positioned above the shear key
112.
The knurled portion 114 extends beyond the surface of the inset portion 108,
preferably by approximately half the container wall thickness.
The base portion 86 includes a bottom wall 116, side walls 118, 120, back wall
122, and front wall 124, all of which respectively form the corresponding
parts of the
1 S base 14 of the finished container 10. A rib extends from the base portion
86 of the
interior form 78 and forms the channel 56 on the container 10.
The exterior form 80 generally conforms to the shape of the interior form 78.
Therefore, wherever the interior form 78 has a projection or recess, the lower
form
will have a complementary recess or projection. Therefore, the upper form will
not be
described in detail.
The interior form 78 further includes a flange clamp 130 that is movable
between a raised position (FIG. 5, 7) and a lowered position (FIG. 6, 10),
flush with
the upper surface of the interior form 78. The flange clamp 130 has an upper
surface
132, which generally conforms to the upper surface of the interior form 78.
The
flange clamp 130 is positioned in front of the inset portion 108 of the
interior form 78
and cools the portion of the sheet material immediately below the latch wipe
opening
28 prior to that portion of the sheet material being formed against the front
wall 104 of
the cover portion 84.
The thermoforming process of the container 10 according to the invention will
now be described with respect to FIGS. 4-10. During the thermoforming process,
the
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upper and lower mold halves 72, 74 are separated and the sheet material 76 is
moved
between the open mold halves 72, 74. The sheet material is initially preheated
to a
temperature of approximately 260°F to 300°F. The preheating of
the sheet material
aids in its subsequent forming by permitting the material to stretch or flow
without
tearing.
Referring to FIG. 7, the mold halves 72, 74 are then simultaneously moved
toward each other. As the mold halves are closed, they make contact with the
sheet
material 76. At the point where the mold halves 72, 74 initially make contact
with the
sheet material, the upper surface 132 of the flange clamp 130 is still
slightly disposed
below and not contacting the sheet material 76.
Referring to FIG. 8, upon the continued closure of the mold halves 72, 74, the
sheet material begins to follow the contour of the upper and lower mold halves
and
the upper surface 132 of the flange clamp 130 makes contact with the lower
surface of
the sheet material 76. The contact between the sheet material and the flange
clamp
upper surface occurs approximately '/2 inch from complete mold closure for the
particular container shown in the drawings. Since the mold halves 72, 74 are
generally kept at a temperature lower than the sheet material 76, the
contacting of the
sheet material with the upper surface 132 of the flange clamp 132 cools that
portion of
the sheet material faster than the portions of the sheet material that are not
in contact
with the mold halves 72, 74. The upper surface 132 of the flange clamp 130 is
strategically located to contact and cool the sheet immediately below the
portion of
the sheet material in which the latch wipe opening 28 is ultimately formed.
The time
that the sheet material remains in contact with the flange clamp prior to
formation of
the latch wipe controls the pre-cooling effect that the flange clamp has on
the sheet
material for a given flange clamp temperature. The contact time is generally a
function of the machine variables including the speed at which the mold halves
close
and the time they remain closed.
Referring to FIG. 9, as the mold halves 72, 74 continue closing, the flange
clamp 130 is urged downwardly. The mold halves 72, 74 ultimately close a
sufficient
distance, approximately '/4 inch, where the shear key 112 and a corresponding
shear
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key 112' on the exterior form begin contacting opposite sides of the sheet
material 76
at the point where the latch wipe opening 28 is to be formed. The shear keys
112,
112' displace the sheet material 76, which is still warm enough to move, to
form the
latch wipe opening 28.
Prior to the shear keys 112, 112' forming the latch wipe opening 28, the sheet
material 76 makes contact with the knurled portion 114. The knurled portion
114,
like the rest of the form, is kept at a lower temperature than the sheet
material. Thus,
as the sheet material makes contact with the knurled portion, this portion of
the sheet
material is cooled. While this portion of the sheet material is pre-cooling
because of
its contact with the knurled portion of the form, the material is still warm
enough to
easily stretch or flow about the ridges and valleys in the knurled portion,
which
effectively grip the sheet material. Since the knurled portion extends out
from the
surface of the interior form 78, the portion of the sheet material contacting
the knurled
portion 114 is slightly compressed as the mold halves 72, 74 close to thereby
hold the
sheet material in the gripped position. The portion of the sheet material
above the
latch wipe opening 28 is thus pre-cooled, gripped, and held during the
thermoforming
process to retard the tendency of the sheet material to smile during the
thermoforming
process.
Referring to FIG. 10, the mold halves 72, 74 are completely closed and the
flange clamp 130 is flush with the upper surface of the interior form 78. In
this
position, the portion of the sheet material that was in contact with the
flange clamp is
slightly compressed between the mold forms. A vacuum is now applied to
increase
the surface contact between the sheet material and the lower mold half 74.
Liquid
coolant circulating through the mold halves then results in cooling of the
sheet
material.
To complete the thermal molding process, the mold halves 72, 74 are opened
and the formed sheet is removed and cut into individual containers 10.
The container made according to the invention is superior relative to the
prior
art because the latch wipe opening is kept rectangular, permitting the proper
functioning of the locking tab. The rectangular latch wipe opening is obtained
by
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pre-cooling the portions of the sheet material above and below the area where
the
latch wipe opening is to be formed. Specifically, the sheet material above the
latch
wipe opening, in other words, opposite the mold closure direction, is cooled,
gripped
and held in place during the forming of the latch wipe opening, by the
protruding
knurled plate which tends to keep the sheet material above the opening from
deforming once the sheet material is slit to form the latch wipe opening.
Although the invention is described with the portion of the sheet material
below the latch wipe opening being pre-cooled by contact with the flange clamp
and a
portion of the sheet material above the latch wipe opening is pre-cooled by
contacting
the form, gripped by the knurled portion of the form, and held by compression
between the upper form and the knurled portion, it is within the scope of the
invention
for any of these methods to be used alone or in combination to limit the
smiling effect
associated with the creation of the latch wipe opening. The particular
combination of
pre-cooling, gripping, and holding will be selected depending on the type of
container
1 S being molded and the location of the latch wipe opening.
For example, in the preferred embodiment described above, the amount of
pre-cooling is different for the material above and below the latch wipe
opening
because the material above the latch wipe opening has a shorter contact time
with the
knurled portion than the contact time between the lower portion of the
material and
the flange clamp. The knurled portion is added to provide a mechanical holding
to the
latch wipe upper edge in addition to the material property effect obtain by
pre-cooling.
Depending on the location of the latch wipe opening and the shape of the
container, it
may be desirable to have a knurled portion both above and below the latch wipe
opening, which can optionally include a flange clamp for pre-cooling the
portion of
the sheet material below the latch wipe opening. Alternatively, it is within
the scope
of the invention to exclude the use of the knurled portion above the latch
wipe
opening. In almost all configurations, there will inherently be some degree of
pre-cooling and compressive holding because the forms are maintained at a
temperature below that of the sheet material and the sheet material is
compressed
between the forms as they close.
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Although the pre-cooling is illustrated as being accomplished by the use of a
moveable flange clamp, it is within the scope of the invention for other
suitable
mechanisms and devices to achieve the pre-cooling. For example, additional
pre-cooling can be created by cooling a portion of the form above and below
the latch
wipe opening to a cooler temperature than the rest of the form. However, a
discrete
cooling solution is substantially more expensive than the preferred
embodiment.
Also, the gripping function can be accomplished by mechanical devices other
than
forming a knurled portion in the form.
While the invention has been specifically described in connection with certain
specific embodiments thereof, it is to be understood that this is by way of
illustration
and not of limitation, and the scope of the appended claims should be
construed as
broadly as the prior art will permit.
