U.S. patent application number 09/979073 was filed with the patent office on 2002-12-05 for packaging container, packaging body including the same, and packaging method.
Invention is credited to Iwasaki, Yoshio, Kawanishi, Norio, Oguri, Toshio, Ohshita, Minoru.
Application Number | 20020179607 09/979073 |
Document ID | / |
Family ID | 27343003 |
Filed Date | 2002-12-05 |
United States Patent
Application |
20020179607 |
Kind Code |
A1 |
Iwasaki, Yoshio ; et
al. |
December 5, 2002 |
Packaging container, packaging body including the same, and
packaging method
Abstract
The present invention provides a tray for which sufficient
adhesion strength can be obtained when thermal bonding between the
tray and a stretch film is conducted using a slanted heat roller.
The tray 10 is provided with a bottom panel 11, a wall panel 12,
and a flange 13. Bottom panel 11 is a panel for placing an article
to be packaged. Wall panel 12 extends upward from bottom panel 11
in such a manner that it surrounds bottom panel 11. Flange 13
extends outward from the upper end part of wall panel 12. Flange 13
further has a curved part 13a on its wall panel side and a slanted
part 13b positioned to the outside of curved part 13a. The cross
section of the upper surface of slanted part 13b is substantially a
straight line and curved part 13a is formed such that wall panel 12
and slanted part 13b have an uninterrupted shape.
Inventors: |
Iwasaki, Yoshio; (Ritto-shi,
JP) ; Ohshita, Minoru; (Ritto-shi, JP) ;
Oguri, Toshio; (Ritto-shi, JP) ; Kawanishi,
Norio; (Ritto-shi, JP) |
Correspondence
Address: |
Shinjyu Global Ip Counselors
1233 Twentieth Street NW Suite 700
Washington
DC
20036
US
|
Family ID: |
27343003 |
Appl. No.: |
09/979073 |
Filed: |
November 16, 2001 |
PCT Filed: |
March 19, 2001 |
PCT NO: |
PCT/JP01/02180 |
Current U.S.
Class: |
220/359.4 |
Current CPC
Class: |
B65D 77/2024 20130101;
B65D 1/34 20130101 |
Class at
Publication: |
220/359.4 |
International
Class: |
B65D 041/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 6, 2000 |
JP |
2000104433 |
Nov 14, 2000 |
JP |
2000346183 |
Feb 13, 2001 |
JP |
2001036065 |
Claims
What is claimed is:
1. A packaging container, comprising: a bottom panel for placing an
article to be packaged, a wall panel having an upper end part, said
wall panel extending upward from said bottom panel such that said
wall panel surrounds said bottom panel; and a flange extending
outward from said upper end part of said wall panel, said flange
having a curved part on said wall panel side and a slanted part
positioned outside of said curved part, said curved part having an
upper surface of which a cross section is substantially straight,
said curved part being formed such that said wall panel and said
slanted part define a continuous shape.
2. A packaging container as recited in claim 1, wherein said
slanted part is at a slant angle in a range from 20 to 60 degrees
with respect to said bottom panel.
3. A packaging container as recited in claim 1 or 2, wherein a
width of said slanted part is 2 mm or greater.
4. A packaging container as recited in any one of claims 1 to 3,
wherein a bonding agent is applied to a surface of said flange.
5. A packaging body, comprising: a packaging container as recited
in any one of claims 1 to 4; and a film covering an upper surface
of said packaging container, a periphery of said film being bonded
to said flange.
6. A packaging body as recited in claim 5, wherein the periphery of
said film follows said curved part of said flange without any air
gaps and is bonded to said slanted part of said flange.
7. A packaging body as recited in claim 5 or 6, wherein said flange
further has a protruding part that is positioned on an outer
peripheral side of said slanted part for cutting the film.
8. A packaging body as recited in any one of claims 5 to 7, wherein
said film is thermally bonded to said flange by pressing a slanted
hot body against said slanted part while the periphery of said film
is in close contact with said curved part and said slanted
part.
9. A packaging method, comprising: a first step in which an article
to be packaged is placed in a packaging container having a slanted
flange at its periphery, a second step in which a film is supplied
above the packaging container and tension is applied to the film, a
third step in which the packaging container is raised and the
flange is made to touch against the tensioned film; and a fourth
step in which a hot body is pressed against the flange touching the
film at a slant angle larger than the slant angle of the
flange.
10. A packaging method as recited in claim 9, wherein the hot body
is pressed against the flange such that a width of a portion where
the flange and the hot body contact each other is 2 mm or greater
in said fourth step.
Description
TECHNICAL FIELD
[0001] The present invention relates to a packaging container, a
packaging body including the same, and a packaging method. More
particularly, the present invention relates to a packaging
container for packaging an article to be packaged by bonding a film
to a flange, a packaging body including the same, and a packaging
method.
BACKGROUND
[0002] The practice of using a film to package an article contained
in a packaging container has been used for some time. Widely
practiced methods include the "overlap method," in which an article
to be packaged is placed inside a container and the entire
container is covered with a film, and the "mold fixed seal method,"
in which the packaging container is fixed in a mold and a film is
put over the upper surface of the container and thermally
welded.
[0003] However, both of the aforementioned methods have problems.
In the overlap method, the entire packaging container is covered in
a film and the film is overlapped and thermally welded to itself at
the bottom of the container. Consequently, the interface between
the container body and the film is merely a state of physical
contact and, if the contents are liquid, there is the risk that the
liquid will leak if the container is tilted. In short, the overlap
method provides inferior sealing performance. Additionally, since
the entire container is covered, large amounts of film are used
because the film must be several times larger than the planar size
of the container; this is disadvantageous from the standpoint of
trash disposal and reducing the consumption of resources.
[0004] Meanwhile, the mold fixed seal method requires a
thermo-compression bonding mold for each container. Since this
method lacks flexibility to accommodate different sizes and shapes
of container, it is seldom used in industries requiring many types
of packaging container.
[0005] Therefore, in Japanese Patent Application H11-137025, the
present applicant proposed a method in which a film covering the
upper surface of a packaging container (tray) is touched with a
heated roller in a slanted state and the roller is rolled over the
flange of the packaging container. With this method, a mold is not
needed for each container and, furthermore, thermal bonding of the
film with the packaging container and thermal cutting can be
accomplished simultaneously using a heated roller slanted at a
prescribed angle. Consequently, the method can accommodate a wide
variety of container shapes and containers with excellent sealing
performance can be obtained.
[0006] Existing containers, however, are shaped such that the
flange is in a horizontal plane or such that the flange is curled.
Therefore, even if the beat roller is touched against the flange at
the proper angle, there are times when the two make linear contact
and sufficient bonding strength cannot be obtained. Furthermore, a
linear seal has the disadvantage of poor sealing performance,
particularly with respect to liquids. There is also the danger of
developing pinholes.
PRESENTATION OF THE INVENTION
[0007] The object of the present invention is to provide a
packaging container, a packaging body including the same, and a
packaging method, in which sufficient bonding strength can be
obtained when a packaging container and a film are thermally bonded
using a heat roller slanted at a prescribed angle.
[0008] The packaging container of claim 1 is provided with a bottom
panel, a wall panel, and a flange. The bottom panel is a panel for
placing an article to be packaged. The wall panel extends upward
from the bottom panel in such a manner that it surrounds the bottom
panel. The flange extends outward from the upper end part of the
wall panel. The flange has a curved part on its wall panel side and
a slanted part positioned to the outside of the curved part. The
cross section of the upper surface of the slanted part is
substantially a straight line and the curved part is formed such
that the wall panel and the slanted part have an uninterrupted
shape.
[0009] The flange on a conventional packaging container has a
planar shape oriented in a horizontal plane or a curled curve
shape, and linear contact results when the heated roller (hot body)
is made to contact the flange in a slanted condition. Therefore,
the packaging container and the film covering the upper surface of
the packaging container are bonded together in a linear seal-like
state. Consequently, the film is easily ripped or peeled due to the
transport and physical shock of the distribution process and it is
highly possible that the stored article will be exposed. Also,
leakage will occur if the packaged article is a liquid. Since the
bonding is linear, the line-sealed bond section sometimes develops
pinholes and the like due to friction when the hot body moves,
resulting in the loss of sealing performance.
[0010] Therefore, the packaging container of this claim has a
flange provided with a slant that is roughly aligned with the angle
at which the hot body makes contact. Also, since the upper surface
of the slanted part of the flange is a straight line (the cross
section of the upper surface is a straight line), the seat formed
between the packaging container and the film when the slanted hot
body touches the flange is a planar seal rather than a linear seal
and a stronger bond with improved sealing performance is obtained.
It is acceptable if the straight line of the slanted part of the
flange mentioned here is substantially straight. That is, it is
acceptable if there is a slight curve because the pressure
resulting when the hot body is pressed against the flange will
cause the contact surface of the flange to become a straight line,
making a planar bond possible.
[0011] There are cases where the slant angle of the straight-line
slanted part of the flange is parallel with the slant angle of the
hot body from the beginning and there are cases where the pressure
of the hot body being pressed against the flange during sealing
causes the slant angle to become roughly equal to that of the hot
body. In either case, the contact is planar and a planar seal
results. In the latter case, since the slant angle is aligned by
pressing the hot body against the flange, a seal pressure gradient
runs from the inside of the flange to the outside of the flange.
Consequently the pressure is stronger toward the outside edge of
the flange and this pressure creates a condition in which it is
easier for the film to be cut (thermally cut). It is therefore
preferred that the slant angle of the hot body and the
straight-line slanted part of the flange be aligned by the pressing
of the hot body, as in the latter case.
[0012] If a conventional packaging container is used and sealing is
performed with a slanted heat roller, a phenomenon occurs wherein
the film does not make close contact (does not make completely
close contact) with the packaging container at the portion of the
flange positioned to the inside of the seal portion. In short, a
layer of air forms between the film and the flange of the packaging
container during sealing.
[0013] This air layer insulates the heat provided by the hot body
during sealing and the heat from the hot body is concentrated on
the film, thus applying a large thermal stress. With the thermal
resistance of the films normally used in packaging, this phenomenon
causes pinholes to develop.
[0014] To prevent this occurrence of pinholes, it is necessary to
make the film touch closely against the packaging container even at
portions that will come into close proximity of the hot body--if
not in contact with the hot body--so that the heat from the hot
body is not concentrated on the film.
[0015] In view of this necessity, the packaging container of this
claim has a curved part formed on the inside of the slanted part
(seal portion) of the flange so that the film makes close contact
with the curved part. The heat from the hot body thermally bonds
the film to the slanted part and also is conducted through the
curved part of the flange and radiated away. As a result, the
pinholes that occur with conventional packaging containers are held
in check and packaging with good sealing performance can be
accomplished.
[0016] The film is put into close contact with the flange of the
packaging container and, in order to maintain the close contact,
the film is pulled by the apparatus and held in a state of tension.
It is also better if the surfaces of the slanted part and curved
part of the flange of the packaging container have enough contact
surface area for the film to bond sufficiently thereto. That is, it
is better to have planar contact and not point contact (in order to
secure a sufficient static coefficient of friction) and it is
better if the surface is smooth. In general, if the surface has a
luster, sufficient close contact can be achieved between the film
and the packaging container. As mentioned earlier, putting the film
in close contact with the flange is necessary in order to prevent
the heat of the hot body from concentrating on the film during
sealing and causing pinholes and the like. Regarding the appearance
of the packaging container after packaging, having some tension in
the film improves the appearance and increases the transparency and
visibility of the film when the contents are viewed through the
film from the outside after the contents have been inserted and
packaged. It is also preferable to apply tension to the film and
contrive to improve the appearance from the standpoint of improving
the consumer's desire to purchase. Therefore, since the present
invention holds the film in a state of tension and simultaneously
seals with a hot body, it is best if the flange surface is smooth
in order to prevent the tension from acting directly on the seal
part immediately after sealing and also in order to reduce the
center-directed film tension caused by frictional forces at the
surface of the flange so that the risk of bad sealing occurring
because of film tension immediately after sealing can be reduced as
much as possible.
[0017] The packaging container of claim 2 is a packaging container
as recited in claim 1, wherein the slanted part is at a slant angle
in the range from 20 to 60 degrees with respect to the bottom
panel.
[0018] The packaging container of claim 3 is a packaging container
as recited in claim 1 or claim 2, wherein the width of the slanted
part is 2 mm or greater.
[0019] Based on test results, it was found that high bonding
strength and high sealing performance were obtained when the width
of the slanted part where the film is thermally bonded was 2 mm or
greater.
[0020] The packaging container of claim 4 is a packaging container
as recited in any one of claims 1 to 3, wherein a bonding agent is
applied to the surface of the flange.
[0021] The packaging body of claim 5 is equipped with a packaging
container as recited in any one of claims 1 to 4 and a film. The
film covers the upper surface of the packaging container and its
periphery is bonded to the flange.
[0022] The packaging body of claim 6 is a packaging body as recited
in claim 5, wherein the periphery of the film follows the curved
part of the flange without any air gaps and is bonded to the
slanted part of the flange.
[0023] The packaging body of claim 7 is a packaging body as recited
in claim 5 or 6, wherein the flange further has a protruding part.
The protruding part is positioned on the outer perimeter of the
slanted part and aids in the thermal cutting of the film.
[0024] The packaging body of claim 8 is a packaging body as recited
in any one of claims 5 to 7, wherein the film is thermally bonded
to the flange by pressing a slanted hot body against the slanted
part while the periphery of the film is in close contact with the
curved part and the slanted part.
[0025] The packaging method of claim 9 is provided with a first
step, a second step, a third step, and a fourth step. In the first
step, an article to be packaged is placed in a packaging container
having a slanted flange at its periphery. In the second step, a
film is supplied above the packaging container and tension is
applied to the film. In the third step, the packaging container is
raised and the flange is made to touch against the tensioned film.
In the fourth step, a hot body is pressed against the flange
(against which the film is touching) at a slant angle larger than
the slant angle of the flange.
[0026] Here, the hot body presses against the slanted flange of the
packaging container at a slant angle that is larger than the slant
angle of the flange. Since a typical packaging container is
elastic, the pressure of the hot body causes the flange to deform
so as to align with the hot body and, when the hot body touches the
flange, a planar seal--not a linear seal--is formed between the
packaging container and the film. As a result, a strong bond with a
high sealing performance is obtained.
[0027] The deformation of the flange when the hot body is pressed
there-against causes the pressure applied to the outer edge section
of the seal portion to be stronger than that applied to other
portions. More specifically, in the case of a packaging container,
such as a plastic tray that is highly elastic, the difference in
pressure between the inside and outside of the seal portion will be
large. Consequently, the heat of the hot body and the tension in
the film make it possible for the film to be thermally cut easily
at the outside edge section of the seal portion. For example, it is
possible to thermally cut the film automatically if, at the
completion of sealing, tension is applied between the seal portion
of the film and the portion to the outside thereof.
[0028] The thermal cutting performance will decline if the
difference between the slant angle of the flange and the slant
angle of the hot body is too small, and it is difficult to obtain a
planar seal if the same difference is to large. Therefore, it is
preferable to set this difference to a value that is appropriate
for both the cutting performance and achieving a planar seal.
[0029] In the packaging method of claim 9, there is almost no
bonding of the film at the apex of the flange; rather, a planar
seal is obtained at the slanted portion of the flange only. The
reason is that the hot body is pressed against the slanted flange
of the packaging container at a slant angle that is larger than the
slant angle of the flange. If the film were bonded to the apex of
the flange, the film would be in a softened state and in tension at
the apex bonding point when the hot body was released and the
flange returned to its original shape. As a result the film would
be thermally cut or become crinkled, degrading the product value of
the packaged article. With this method, however, the occurrence of
such trouble is suppressed because there is almost no bonding of
the film at the apex of the flange.
[0030] The packaging method of claim 10 is a packaging method as
recited in claim 9, wherein in step 4 the hot body is pressed
against the flange such that the width of the portion where the
flange and the hot body contact each other is 2 mm or greater.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1 is a cross sectional view of the tray raw material in
the first embodiment.
[0032] FIG. 2 is a cross sectional view of the tray in the first
embodiment.
[0033] FIG. 3 is a schematic view of the packaging apparatus in the
first embodiment.
[0034] FIG. 4 is a view for explaining the packaging operation of
the first embodiment.
[0035] FIG. 5 is a view for explaining the packaging operation of
the first embodiment.
[0036] FIG. 6 is a plan view of the tray, stretch film, and heat
rollers in the first embodiment before packaging.
[0037] FIG. 7 is a plan view showing the seal operation of the heat
rollers in the first embodiment.
[0038] FIG. 8 illustrates the close contact of the stretch film Fm
with respect to the flange 13 in the first embodiment before
sealing.
[0039] FIG. 9 illustrates the condition of the heat roller, tray,
and stretch film in the first embodiment during sealing and thermal
cutting.
[0040] FIG. 10 illustrates the condition of the stretch film during
the sealing operation for a conventional tray and for a tray in
accordance with the first embodiment.
[0041] FIG. 11 illustrates the condition of the heat roller, tray,
and stretch film of the third embodiment during sealing and thermal
cutting.
[0042] FIG. 12 illustrates the condition of the heat roller, tray,
and stretch film of the fourth embodiment during sealing and
thermal cutting.
BEST MODES FOR WORKING THE INVENTION
[0043] [Embodiment 1]
[0044] <Raw Material of Tray>
[0045] FIG. 1 shows a cross sectional view of the raw material of
the tray (packaging container) that embodies the present invention.
This raw material is a plastic sheet. This plastic sheet 90 is
formed into a tray by pressure and vacuum forming.
[0046] The plastic sheet 90 shown in FIG. 1 comprises thermally
meltable plastic material layers 91, 93 joined to a foamed plastic
material layer 92.
[0047] Thermally meltable plastic material layer 91, which serves
as the upper surface of the tray, is thermally bonded to a stretch
film by a hot body (heat roller 3, discussed later) and must be
made of a material that can be thermally bonded to the stretch
film. Here, the stretch film has a three-layer structure comprising
ethylene vinyl acetate copolymer, polypropylene "Catalloy (Montell
Polyolefins)", and ethylene vinyl acetate copolymer, respectively.
Therefore, a material having similar qualities to that of the
stretch film is used for the thermally meltable plastic material
layer 91. More specifically, ethylene vinyl acetate copolymer,
polypropylene, or polyethylene is used as the thermally meltable
plastic material layer.
[0048] Polystyrene, polypropylene, or other foamed body is used for
the foamed plastic material layer 92.
[0049] For plastic material layer 93, which serves as the outer
surface of the tray, a material having roughly the same heat
shrinkage rate as plastic material layer 91 is used. Thus,
deformation of the tray after forming is suppressed because the raw
material is a material with plastic material layers 91, 93 having
similar shrinkage rates joined to both surfaces.
[0050] When it is important that the raw materials of the stretch
film and tray serve as a gas barrier, it is good to use a multiple
layer structure that includes at least one layer of a film made of
ethylene vinyl alcohol copolymer, polyvinyl alcohol, or the like.
When it is important for the raw materials to be aroma proof, it is
good to use a multiple layer structure that includes at least one
layer of a film made of a polyester (polyethylene terephthalate,
polyethylene naphthalate, or the like). Materials and the thickness
thereof should be selected based on the gas for which the material
is to serve as a barrier. It is also possible to add a gas barrier
property by depositing aluminum or a ceramic by vapor deposition.
Aluninum foil, iron foil, or other metal can also be used as a
barrier layer; in the case of the tray, such a layer can be used
for a layer other than the innermost layer, which is the bonding
layer. However, if a metal barrier layer is used in a stretch film,
the transparency will be hindered and there will be situations
where the film is difficult to use.
[0051] <Shape of the Tray>
[0052] Using a plastic sheet 90 like that described above and shown
in FIG. 1 as the raw material, a tray 10 is formed into the cross
sectional shape shown in FIG. 2.
[0053] As shown in FIG. 2, tray 10 comprises a rectangular bottom
panel 11, four wall panels 12, and a flange 13. The four wall
panels 12 extend upward from the four sides of bottom panel such
that they surround the bottom panel. Flange 13 is the portion that
extends outward from the upper end part of wall panels 12.
[0054] Flange 13 comprises a curved part 13a on the wall panel 12
side and a slanted part 13b positioned on the outside of curved
part 13a.
[0055] Curved part 13a is formed such that wall panel 12 and
slanted part 13b blend with an uninterrupted shape. In view of
making the stretch film contact the tray closely, the radius of
curvature of curved part 13a is in the range from R2 (radius 2 mm)
to R10 and preferably in the range from R3 to R8.
[0056] As shown in FIG. 2, the cross section of the upper surface
of slanted part 13b is substantially a straight line and extends in
a slanted direction from the outside edge of curved part 13a. The
slant angle .alpha. (see FIG. 2) between the slanted part 13b and
the bottom panel 11 is set to an angle in the range from 20 to 60
degrees so that the heat rollers (hot bodies) 3 (discussed later)
achieve planar contact when they contact the slanted part of the
flange. Although slant angles a can be between 5 and 90 degrees,
the range from 20 to 60 degrees is preferred in view of the slant
of the heat rollers 3 and other factors. As mentioned earlier, it
is not necessary for the slant angle of heat rollers 3 to be
identical to the slant angle a of slanted part 13b of flange 13. In
fact, it is preferable to set the slant angle .alpha. of slanted
part 13b and the slant angle of heat rollers 3 such that these
angles are only roughly aligned when heat rollers 3 are pressed
against the flange during sealing.
[0057] Flange 13 is provided with width dimension D (see FIG. 2) in
order to secure a prescribed seal width. Width dimension D of
flange 13 is assumed to be in the range from 1 to 15 mm and
preferably in the range from 3 to 10 mm. It is also preferred that
the width dimension of slanted part 13b of flange 13 be 2 mm or
greater.
[0058] <Thermal Bonding Operation of the Tray and Stretch
Film>
[0059] Next the operation of the packaging apparatus that thermally
bonds the stretch film to the tray is explained. The packaging
apparatus 1 is the same as the packaging apparatus disclosed in
Japanese Patent Application H11-137025.
[0060] As shown in FIG. 3, tray 10 is placed on holding member 2
and then stretch film Fm is fed from delivery section 51 of film
feeding mechanism 5. The fed stretch film Fm is held between two
belts B that traverse transport rollers 52a, 52b and two belts B
that traverse presser rollers 52c, 52d; these belts are moved in a
direction perpendicular to the transport direction (horizontal
direction in FIG. 6) so that the stretch film Fm is stretched in
the widthwise direction (vertical direction in FIG. 6). As a
result, tension is generated in the stretch film Fm in the
widthwise direction.
[0061] Next, moving section 63 of lift mechanism 6 rises, pushes
tray 10 up against the stretch film Fm, and stops (see FIG. 4).
Here, the upward pushing of tray 10 produces tension in the stretch
film Fm in the transport direction. As shown in FIG. 8 (enlarged
view), the stretch film Fm is in close contact with the slanted
part 13b and outside portion of the curved part 13a of flange 13 of
tray 10.
[0062] Then, the downward force applied by lever mechanism 74
against presser plate 73 is released and link mechanism 72 is
released so that heat rollers 3 move into contact with tray 10 due
to their own weight (see FIG. 5). Since heat rollers 3 are
independent of one another, each heat roller 3 contacts tray 10
with roughly the same pressure.
[0063] Next, motor 82 rotates shaft 81 through a prescribed angle,
thus causing heat rollers 3 to rotate as shown in FIG. 7. Heat
rollers 3 move along the periphery (flange 13) of tray 10 and
thermally weld stretch film Fm to tray 10, thus forming a seal.
Here, heat rollers 3 roll along flange 13 as they move.
[0064] FIG. 9 shows an enlarged view of the contact area between
heat rollers 3 and tray 10 when the former move along the periphery
of the latter. The upper opening of tray T is covered by stretch
film Fm, which is tensioned in both the lengthwise and widthwise
directions, and stretch film Fm is pressed firmly against slanted
part 13b and the outside portion of curved part 13a of flange 13 of
tray 10. Heat rollers 3 touch diagonally against the portion where
stretch film Fm contacts flange 13 and apply both heat and
pressure. This heat and pressure cause stretch film Fm and flange
13 to fuse together thermally.
[0065] At the same time, stretch film Fm melts and is cut away at
the edge portion (which is, in this case, the outside edge part of
slanted part 13b of flange 13 where heat rollers 3 are touching) of
tray 10 because the heat and pressure are concentrated on stretch
film Fm and stretch film Fm is in tension at the edge portion of
tray 10 (see FIG. 9). More specifically, the angle of the heat
rollers 3 with respect to the horizontal plane is set to be
slightly larger than the angle of the slanted part 13b of flange
13, as shown in FIG. 8. As shown in FIG. 9, when heat rollers 3
touch against flange 13, the pressure of the heat rollers is
largest at the outside edge part of flange 13 and, consequently,
the tension in stretch film Fm causes stretch film Fm to be
thermally cut at the outside edge part of flange 13. Since the
pressure is largest at the outside edge part of flange 13, a
pressure gradient exists across flange 13 from curved part 13a to
the outside edge part. Therefore, the pressure on film Fm is
smaller in the vicinity of flange curved part 13a than at the
outside edge part and the load of pressure and beat are reduced at
the boundary of the inner edge part of flange 13, which is to be
sealed. Thus sealing is accomplished without placing a load on the
stretch film Fm. Upon viewing a cross sectional photograph of the
flange 13 after sealing tray 10 with stretch film Fm, it was found
that at the inside seal end part of tray 10 there were no cracks in
the film caused by heat and pressure and there was no degradation
of the film. This fact, too, results in suppressing the occurrence
of pinholes and cracks.
[0066] After heat rollers 3 have finished sealing stretch film Fm
to tray 10, heat rollers 3 are raised and lift mechanism 6 lowers
tray 10. Then the sealed tray 10 is removed from holding member 2.
When the next cycle begins, stretch film Fm--of which a portion has
been thermally cut away in the process of sealing tray 10--is taken
up by driving take-up section 53.
[0067] <Comparison with Seal Operation of Conventional
Tray>
[0068] FIG. 10 shows a case where a stretch film Fm is sealed to a
conventional tray 100 and a case where a stretch film Fm is sealed
to a tray 10 in accordance with the embodiment.
[0069] (Conventional Tray)
[0070] FIG. 10(a) is an expanded view of the vicinity of the flange
113 when heat rollers 3 thermally weld stretch film Fm to
conventional tray 100. Flange 113 of tray 100 comprises a first
curved part 113a, a horizontal part 113b, a second curved part
113c, and a vertical part 113d. As is clear from the figure, the
contact of heat roller 3 with second curved part 113c of flange 113
causes stretch film Fm to be sealed to tray 100. An air layer S1 is
formed between flange 113 and stretch film Fm in the space to the
outside of vertical part 113d near second curved part 113c. Because
air layer S1 serves as thermal insulation, the heat from heat
rollers 3 concentrates on the stretch film Fm at the boundary line
between second curved part 113c and vertical part 113d and the
stretch film Fm is thermally cut.
[0071] However, with conventional tray 100, air layer S2 tends to
form in a similar manner between stretch film Fm and the horizontal
part 113b of flange 113. Air layer S2 serves as thermal insulation
with respect to the heat from heat rollers 3 and causes a high risk
of pinholes developing on the inside of the seal portion due to
heat. If such pinholes develop, the inside of tray 100 cannot be
kept in a sealed state. Furthermore, if the pinholes are large, the
packaging itself cannot be accomplished.
[0072] (Tray of This Embodiment)
[0073] FIG. 10(b) illustrates thermal sealing of a stretch film Fm
to a tray 10 in accordance with this embodiment. Since tray 10 has
slanted part 13b disposed to the outside of curved part 13a,
stretch film Fm makes close contact with slanted part 13b and the
outside portion of curved part 13a of flange 13 when stretch film
Fm covers curved part 13a. Heat rollers 3 touch chiefly against
slanted part 13b and hardly touch curved part 13a at all.
Therefore, the air layers S1, S2 shown in FIG. 10(a) do not form
and stable sealing is accomplished without the development of
pinholes.
[0074] When heat rollers 3 thermally weld stretch film Fm to tray
10, which has a flange 13 as just described, curved part 13a
deforms slightly so that the slant angle of heat rollers 3 and the
angle of flange 13 are roughly the same and heat rollers 3 rotate
as they move. Since the thermal welding is conducted in this
manner, the seal is formed over roughly the entire width of flange
13. Also, in addition to thermally welding stretch film Fm to
slanted part 13b, the heat from heat rollers 3 travels through
curved part 13a and radiates away. As a result, the risk of
pinholes developing is avoided.
[0075] <Comparison of Test Results with Conventional
Tray>
[0076] The results of tests comparing an operative example of a
tray 10 in accordance with this embodiment and an operative example
(comparative example) of a conventional tray 100 are discussed.
[0077] Regarding the raw material of tray 10, undrawn polypropylene
films (CCP) of thickness of 40 .mu.m were used for both surface
layers. A foamed polystyrene sheet (thickness 1.5 mm, weight 260
g/m.sup.2) was used for the base material sandwiched between the
surface layers. Both materials were formed by coextrusion into a
sheet comprising three layers made of CCP (40 .mu.m), foamed
polystyrene (1.5 mm), and CPP (40 .mu.m), respectively. The sheet
was then set into a mold and made into food tray 10 using a vacuum
pressure forming machine.
[0078] The dimensions of the tray were as follows: external
dimensions 130 mm.times.180 mm, depth 30 mm, flange angle .alpha.
35 degrees, flange width 5 mm, and radius of curvature of curved
part 5 mm (R5).
[0079] Tray 10 was placed in a heat roller type stretch film
packaging apparatus like that described previously and a stretch
film Fm was heat sealed to the opening of the tray. For the stretch
film Fm, a stretch film Fm having a thickness of 15 .mu.m and three
layers made of the following materials, respectively, was used:
linear low-density polyethylene (LLDPE), polypropylene "Catalloy",
and linear low-density polyethylene (LLDPE).
[0080] The sealing temperature (hot roller temperature) was set to
190.degree. C.
[0081] Tray 100 of the comparative example used the same material
as was used for tray 10 and was given similar dimensions and a
flange 113 shaped like that shown in FIG. 10(a).
[0082] The seal strength and pinhole development of both trays were
checked and the results are shown in Table 1.
1 TABLE 1 Seal strength Sample (gf/15 mm) Pinhole development Tray
10 578.7 None Tray 100 361.1 Two or more in all samples *The seal
strength was measured using a Strograph V1-C universal tester made
by Toyo Seiki Seisakusho, Ltd. *The number of samples N was 10.
After cutting the sample into 15-mm wide portions, the seal
strength was measured using a tensile tester. *The seal strength is
the average value for the entire perimeter of the tray.
[0083] As the results indicate, tray 10 (the operative example of
the present invention) had a stable seal strength of at least 500
gf/15 mm.
[0084] Conversely, in the case of tray 100 (comparative example),
almost every tray showed pinholes and there were no samples that
did not develop pinholes.
[0085] The following statements are clear based on these results.
Since flange 13 of tray 10 is formed with a slanted part 13b that
has a slant angle corresponding to the slant angle of beat rollers
3 and a curved part 13a that allows stretch film Fm stay in contact
with the surface of tray 10 (i.e., surface of curved part 13a) up
to the place in the inward direction of the tray where heat rollers
3 no longer touch, the heat from heat rollers 3 never concentrates
on the stretch film Fm only and is always transmitted to tray 10 as
well. Thus, the thermal stress on stretch film Fm is reduced and
the development of pinholes is suppressed.
[0086] [Second Embodiment]
[0087] <Raw Material and Shape of Tray, Application of Bonding
Agent, and Pinholes>
[0088] For this embodiment, the tray was made by vacuum pressure
forming a 0.8-mm thick polypropylene sheet.
[0089] The tray dimensions were as follows: external dimensions 140
mm.times.210 mm, depth 25 mm, flange angle .alpha. 35 degrees, and
flange width 10 mm. A bonding agent called Hot Lacquer Heat Sealing
Agent AD-1790-15 made by Toyo-Morton, Ltd., was applied to the
flange.
[0090] Because this was a test, a prescribed amount of the bonding
agent was applied to the flange in a non-automatic manner and dried
thoroughly.
[0091] After the bonding agent was completely dry, the tray was
placed in the heat roller type stretch film packaging machine and
the stretch film was heat sealed to the tray.
[0092] The stretch film used for the sealing had a thickness of 15
.mu.m and a three-layer structure comprising layers made of
ethylene vinyl acetate copolymer, polypropylene "Catalloy", and
ethylene vinyl acetate copolymer, respectively.
[0093] <Seal Strength Measurement and Check for Pinhole
Development>
[0094] Similarly to the first embodiment, the heat seal strength
was measured for samples comprising a tray to which a stretch film
had been heat sealed.
[0095] The measurement results indicate that a completely secure
bond was obtained. The number of samples N was 10 (i.e., there were
10 test trays) and the measurement results showed that the strength
was at least 500 gf/15 mmn for all samples. It was also observed
that no pinholes developed.
[0096] These results indicate that the bonding agent melted upon
absorbing heat from the heat rollers and welded the stretch film
thoroughly to the flange. The results also indicate that the
concentration of heat from the heat roller on the stretch film is
suppressed and thus the development of pinholes is suppressed.
[0097] In both of the embodiments discussed thus far, the flange
comprises a slanted part whose slant angle is roughly aligned with
the slant angle of the heat rollers and a curved part that enables
something other than an air layer, e.g., the tray itself or a
bonding agent, to exist in addition to the stretch film in the
vicinity of the heat rollers. As the seal strength measurements
indicate, these embodiments suppress the problem of heat from the
heat rollers being concentrated on the stretch film and thus reduce
the occurrence of pinholes and other defects. These embodiments
also make it possible to achieve a more safe and reliable seal.
[0098] [Third Embodiment]
[0099] In the previous embodiments, the stretch film Fm was
thermally cut by concentrating heat and force on the stretch film
Fm at the outer edge part of slanted part 13b of flange 13. It is
also acceptable to shape the flange 13 as shown in FIG. 11 in order
to apply more pressure and cut the film more reliably.
[0100] In the flange 13 of tray 10 in FIG. 11, a protruding part
13c is formed on the outer perimeter of slanted part 13b.
Protruding part 13c protrudes toward heat rollers 3 and serves in
the thermal cutting of stretch film Fm. The tip of protruding part
13c is pointed so that high pressure acts on the stretch film Fm
where it is pinched between protruding part 13c and heat rollers 3.
As a result, stretch film Fm is reliably cut at this portion.
[0101] It is also possible to use a foamed hot melt type bonding
agent as the bonding agent. When bonding is conducted by melting
the surface of the packaging container and the inside surface of
the film, the heat supplied from the heat rollers is radiated into
the air and the thermal efficiency declines. If a foamed hot melt
type bonding agent is applied to the flange, the foamed boding
agent will absorb the heat from the heat rollers and melt, thus
serving its function as a bonding agent.
[0102] [Fourth Embodiment]
[0103] FIG. 2 shows a case where the slant angle of the heat roller
3 is roughly the same as the slant angle .alpha. of the slanted
part 13b of flange 13. However, it is also preferable to take
advantage of a structure like that shown in FIG. 12, in which the
slant angle .beta. of heat roller 3 is slightly larger than the
slant angle .alpha. of flange slanted part 13b of tray 10.
[0104] <Relationship between the Tray and Heat Roller>
[0105] FIG. 12 illustrates stretch film Fm being tensioned and set
on the upper surface of tray 10 and then sealing being conducted by
heat roller 3. As shown in FIG. 12 (a), at this stage heat roller 3
has a slant angle, that is slightly larger than the slant angle a
of slanted part 13b of flange 13.
[0106] <Sealing Operation>
[0107] When heat roller 3 (having slant angle .beta.) seals the
stretch film to slanted part 13b of flange 13 (having slant angle
.alpha.), the sealing is accomplished as shown in FIG. 12 (b). The
pressure applied by heat roller 3 (having slant angle .beta.)
causes flange 13 of tray 10 to deform and the slant angle of
slanted part 13b to shift from a toward the slant angle .beta. of
heat roller 3. When this occurs, the vertical centerline of curved
part 13a of flange 13 tilts through angle .gamma. and the slant
angles of heat roller 3 and flange 13 become roughly equal. Flange
13 of tray 10 possesses elasticity; slanted part 13b tilts and its
slant angle changes when heat roller 3 applies pressure.
[0108] When heat roller 3 applies pressure and the slant angle of
slanted part 13b of flange 13 roughly aligns with that of heat
roller 3, the straight-line portion of the slanted part is
planar-sealed with stretch film Fm. The width of the seal is at
least 2 mm and corresponds to the width of the portion where
stretch film Fm and heat roller 3 contact the flange. Meanwhile,
the outside edge part of flange 13 is positioned at the top of the
pressure gradient that develops along the portion where heat roller
3 is touching. In addition to the pressure applied by heat roller
3, the heat from heat roller 3 and the tension of stretch film Fm
directed outward from the tray cause stretch film Fm to be
thermally cut at the outside edge part of flange 13.
[0109] On the inside portion of flange 13, there exists a portion
of length r (see FIG. 12 (b)) where, stretch film Fm contacts the
flange but heat roller 3 does not.
[0110] <Features>
[0111] (1)
[0112] After seat sealing is completed and heat rollers 3 move away
from flange 13, flange 13 returns from its elastically deformed
state to its original slant angle as shown in FIG. 12 (c). At this
stage, there exists on the outside portion of curved part 13a of
flange 13a section of length r (see FIG. 12 (c)) that is not
thermally bonded to the flange but is in close contact with stretch
film Fm. This section of length r maintains the tension of the
stretch film Fm covering tray 10 and reduces the load on the inside
end face of heat sealed section d, thus serving to prevent poor
sealing.
[0113] If the seal is formed up to a position that was further
inward than the vertical centerline of curved part 13a of flange 13
before sealing, the seal portion will reach the inside edge part of
the portion where stretch film Fm is in close contact with flange
13 and be directly heated by heat rollers 3. Therefore, the tension
in stretch film Fm will act directly on the portion of stretch film
Fm at the inside end face of the seal portion, i.e., a portion of
stretch film Fm that has been bonded but has not completely cooled.
Before stretch film Fm hardens, the portion of stretch film Fm at
the inside end face of the seal portion (i.e., the inside edge part
of the closely contacting portion) will develop such defects as
tears or pinholes.
[0114] Conversely, this embodiment suppresses the development of
such defects as tears or pinholes in stretch film Fm because the
slant angle .beta. of heat rollers 3 is set such that, during
sealing, the area to the outside of the vertical centerline of
curved part 13a of flange 13 is sealed and the area to the inside
is not sealed.
[0115] (2)
[0116] Tray 10 is made by molding a sheet into a specified shape
using vacuum air-pressure forming and then cutting off the edges to
obtain a specified flange dimension. During this molding process, a
mold is set on the outside of tray 10 and the outside dimensions
can be molded accurately. The inside dimensions, however, are quite
difficult to mold accurately and precisely. It is also extremely
difficult for the cutting blade to cut all four edges of flange 13
uniformly; a portion of the formed sheet may remain on flange 13
such that a flat part is formed on the end face of flange 13. Thus
there are cases where slant angle error occurs during molding and
cutting error occurs during cutting of the edges.
[0117] Therefore, even in the same tray, all four flanges 13 will
not have slanted parts 13b with completely identical slant angles
ox and cutting error will result in a horizontal flat part on some
of the edges.
[0118] In order to prevent poor seal sealing from occurring because
of such errors during molding, it is necessary to have a sealing
mechanism that allows a certain degree of error. In view of this
necessity, it is preferable to adopt a structure, such as that of
this embodiment, in which the slant angle .beta. of heat rollers 3
is slightly larger than the slant angle .alpha. of flange 13.
[0119] (3)
[0120] If molding error were to cause the slant angle .alpha. of
slanted part 13b of flange 13 to be larger than the slant angle
.beta. of heat rollers 3, it is feasible that heat rollers 3 might
not contact the outer edge part of flange 13 and thermal cutting
could not be accomplished. However, if the slant angle .beta. of
heat rollers 3 is slightly larger than the slant angle .alpha. of
flange slanted part 13b as in this embodiment, then molding error
will almost never cause the slant angle .alpha. of slanted part 13b
of flange 13 to be larger than the slant angle .beta. of heat
rollers 3.
[0121] (Industrial Applicability)
[0122] If this invention is used, a slanted part will be formed on
the flange and, since the upper surface of the slanted part is
straight, a planar seal will be formed between the film and the
packaging container when the slanted hot body is touched against
the flange. Consequently, a stronger bond with higher sealing
performance is obtained.
[0123] Furthermore, since a curved part is formed to the inside of
the slanted part, the film makes close contact with the curved part
and the heat from the hot body travels through the curved part and
is radiated away. As a result, the pinholes that occur in
conventional packaging containers are suppressed and packaging with
good sealing performance is possible.
[0124] In another mode of the present invention, the hot body is
pressed against the slanted flange of the packaging container at a
slant angle that is slightly larger than the slant angle of the
flange. The deformation of the flange allows a planar seal to be
made between the packaging container and the film. Meanwhile, the
pressure applied at the outside edge part of the seal portion is
larger than the pressure in other sections and the film can be
thermally cut easily at the outside edge part of the seal portion.
Since there is almost no bonding of the stretch film at the apex of
the flange, such trouble as holes developing in the film is
suppressed.
* * * * *