Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
WO 2023/117604
PCT/EP2022/085747
Mondi AG
Packaging bag and use of the packaging bag
Description:
The present invention relates to a packaging bag comprising a web having a
front
wall, a back wall projecting beyond the front wall on an open top edge of the
bag,
wherein stacking openings are disposed in the section of the back wall
projecting
beyond the front wall, and wherein the web is formed at least partially of
paper.
Packaging bags of the type described above are usually also referred to as
'wicketed bags', wherein several such packaging bags can be combined into a
stack through the stacking openings and the individual packaging bags can be
removed from the stack for their filling. Such packaging bags are usually used
as
packaging for sanitary articles, e.g. for diapers or other hygiene articles.
When
diapers are packed, they are pressed before being inserted into the film bag,
so
that the diapers expand in the packaging bag and then lie tightly against the
inside
of the bag, whereby the packaging bag is formed as a package in a largely
rectangular shape.
Such packaging bags are basically known from the prior art and are described,
for example, in DE 1 291 280 B and DE 23 12 143 C2. According to such an
embodiment, the filling takes place via the open bag edge, at which the
stacking
openings in the rear wall are also provided. After the bag has been filled, it
is
sealed, whereupon the section provided with the stacking openings can be
separated and disposed of.
Such packaging bags are usually made of a heat-sealable material. In
particular,
these are thermoplastic films, such as polyethylene films, which have a
thickness
of between 40 and 80 micron, depending on the size of the bag.
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Increasingly, however, the packaging industry is striving to use packaging
materials which are largely made from renewable raw materials and which are
also easy to recycle. Against this background, paper is increasingly being
used
as a material for such wicketed bags, since the fibres required for the
production
of paper are based on renewable raw materials.
EP 3 687 914 A teaches a packaging bag of the type described above, which is
formed at least from a layer of paper and a sealing layer of polyethylene
arranged
thereon. A three-layer structure is also preferred, the outer layers being
formed
from paper and the inner layer from polyethylene. A recess is then being
provided
in the inner paper layer in the region of the heat-sealing seams, so that the
heat-
sealing seams can be formed via the sealing layer.
When incorporating paper layers to increase recyclability, the special feature
arises that the recyclability is considerably improved with an increasing
proportion
of paper, since the residual polyethylene can then be recycled together with
the
paper layers. Overall, a proportion of paper of at least 80% by weight is
aimed at
here, whereby the packaging bag is then designed to be comparatively stiff due
to the high proportion of paper. Since the corners of such packaging bags are
usually folded in at the bottom, however, the high paper content leads to high
restoring forces which impair the dimensional stability of the packaging.
Against this background, the present invention is based on the task of
providing
a packaging bag which, on the one hand, can be produced with a high proportion
of paper and which, at the same time, has a high dimensional stability.
The object of the invention and the solution to the problem is a packaging bag
according to patent claim 1 and the use of a packaging bag according to the
invention according to patent claim 14.
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Accordingly, it is important to the invention that the front wall and the rear
wall
have chamfered corners at a bottom bag edge opposite the top bag edge. In an
unfilled state of the packaging bag, this results in a substantially
trapezoidal
configuration of the packaging bag at the lower end, the chamfered corners
being
exposed after filling and forming two triangular sections of the stand-up
bottom.
The trimming of the corners ensures that no excess material is arranged in the
stand-up bottom in a filled state, whereby on the one hand the dimensional
stability of the packaging bag and on the other hand the functionality of the
stand-
up bottom is considerably improved.
Such an embodiment is of particular advantage if the front wall and the rear
wall
are connected to each other at the bottom edge of the bag via a bottom gusset.
According to such an embodiment, the packaging bag can be formed by folding
over only one web. For this purpose, the bottom gusset is first introduced
into the
web and then the front wall is joined to the rear wall. The inclusion of a
bottom
gusset ensures that the front wall and the rear wall are joined together at
the
bottom edge of the bag without a seam, resulting in a particularly high bond
strength between the front wall and the rear wall. The combination of a bottom
gusset and the integration of chamfered corners creates a bottom which is on
the
one hand dimensionally stable and at the same time stable against the loads
generated by the packaged goods. At the bottom of the bag, the front and rear
walls then merge into the bottom gusset via a crease line. In addition, a
cross
seam can also be provided at the crease line to reinforce the bottom of the
bag.
Instead of forming the bottom fold integrally with the front wall and the rear
wall,
a separate bottom panel can also be provided, which is joined to the front
wall
and the rear wall by means of joining seams, in particular heat-sealing seams,
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regions of the bottom panel then forming regions of the front wall and the
rear
wall in a joining section.
Although an embodiment with a bottom gusset is a preferred embodiment of the
invention, the invention is not limited thereto. It is of course also within
the scope
of the invention that the front wall and the rear wall are joined together at
the
bottom edge by a closing seam.
According to a further embodiment the front and/or the rear front wall are
connected to the bottom gusset sections of the bottom gusset via a
transversely
extending secondary cross seam, wherein the secondary cross seam is spaced
from a lower bag end or the cross seal seam such that a compensation chamber
is formed in each case. The secondary cross seam has a number of
breakthroughs through which air can be transported outwards to the bag edges.
This greatly simplifies the packing process, since there is usually sufficient
space
available at the edges. This helps to avoid excessive pressure in the center
of the
bottom, which could cause the seams or the creased edges to burst. Equally,
however, pressure peaks can also occur on the outside, so that the air can
then
flow in the opposite direction to the center, thus relieving the corners of
the base.
According to a preferred embodiment, the proportion of paper is at least 90%
by
weight, preferably at least 95% by weight. By incorporating such a high
proportion
of paper, the recyclability can be substantially improved, whereby the special
design of the packaging bag is then of course also particularly effective,
since the
stiffness of the front wall and the rear wall also increases with increasing
proportion of paper.
According to a particularly preferred embodiment of the invention, the web
comprises a layer of paper, which is at least partially coated with a sealing
layer
made of a heat sealable material on a side adjacent to the packaging space.
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Correspondingly, the front wall and the rear wall have a corresponding coating
on an inner side. Although in principle a multilayer design with several
layers of
paper is conceivable, a preferred design is limited to a combination of only
one
layer of paper with the sealing layer of heat-sealable material arranged
thereon.
5
A heat-sealable material is a thermoplastic material which melts under the
influence of heat and then hardens when it cools down. This makes it possible
to
join the front wall and the rear wall by heat sealing. This would not be
possible
without a corresponding coating.
The sealing layer can be applied in the form of an extrusion lamination
(extrusion
coating), which can be single-layered or multilayered, in which case different
polymers can be used in the individual layers, e.g. a three-layer PE/EVOH/PE
structure.
Alternatively the sealing layer can also be applied as a dispersion coating of
a
heat-sealable lacquer particularly using a printing process. In the context of
the
invention, a lacquer is preferably understood to mean a liquid coating
material
which is applied in a thin layer and built up by chemical or physical
processes to
form a continuous, solid film.
In principle, it is sufficient within the scope of the invention if only a
partial or non-
full-surface sealing layer is provided on the front wall and/or the rear wall.
The
heat sealable material is then only applied where a corresponding formation of
heat-sealing seams is required.
Alternatively, it is also conceivable that the sealing layer covers said layer
of
paper completely on a side adjacent to the packaging space. Such an
embodiment offers the advantage that then no particularly high accuracy must
be
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placed on the positioning of the heat-sealing seams. Even small positional
deviations still lead to a firm formation of heat-sealing seams. On the other
hand,
the water vapour barrier is also considerably increased by binding a complete
layer of heat-sealable material. This is of particular advantage when used as
diaper packaging, since diapers are formed with a super absorbent material to
allow absorption of body fluids. If the packaging bag had only a low water
vapour
barrier, this would have the consequence that the super absorbent materials
would absorb the air humidity with increasing storage time. This would then
place
high demands on storage and necessitate prompt sale and consumption. A gas,
aroma or odor barrier can also be additionally incorporated above this. This
is
necessary, for example, if the diapers are perfumed or soaked with a lotion.
The
corresponding barrier layer is formed in particular from EVOH.
However, a full-surface sealing layer also leads to the problem that a
substantially
higher proportion of a polymeric portion would have to be provided on the
layer
of paper than would be the case with only a partial coating. Accordingly, the
sealing layer should be much thinner than would be necessary with only a
partial
coating.
In order to enable heat-sealing seams that are resistant to bonding despite
the
thinner coating, these are then preferably formed wider than in the case of a
thicker coating, whereby the bonding area is increased. Alternatively, a
combination of both forms is also possible. In this case, a full-surface
sealing
layer and an additional layer of a heat-sealable material would then be
applied to
the paper layer. Such an embodiment serves primarily to increase the water
vapour barrier. In the areas where the formation of heat-sealing seams is then
envisaged, the additional layer thickens the layer made of heat-sealable
material
in order to increase the bond strength. Furthermore, it is also conceivable to
provide an additional layer in particularly stressed areas, e.g. in corner
areas. The
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width of the heat-sealable seams is between 2 and 10 mm, in particular between
4 and 6 mm, irrespective of the positioning.
The heat-sealable material for the sealing layer can be applied via an
extrusion
lamination and the heat-sealable material for the additional layer can be
applied
via an extrusion lamination or a dispersion coating. The last variant in
particular
can be of advantage in keeping the proportion of foreign substances low, since
the dispersions can be applied very thinly, especially with a basis weight of
between 1 gsm and 3 gsm. In addition, typical barrier materials can also be
used
in such an embodiment. The barrier layer can then be formed from a material or
materials from a group comprising acrylic polymers, acrylic copolymers,
polyvinyl
acetate, polyvinyl alcohol, ethylene vinyl acetate, polyvinyl chloride,
styrene
butadiene copolymers, polyvinylidene chloride, polyacrylonitrile, polyacrylic
acid
ester, natural wax, synthetic wax, starch, chalk, talc.
Based on such an embodiment, the invention preferably provides that the front
wall and the rear wall are connected to one another at the lateral edges via
longitudinal sealing seams. Accordingly, a bottom gusset can first be formed
from
the web by folding it over and then the front wall and the rear wall can each
be
joined to one another at the lateral edges by means of a longitudinal seal
seams.
Accordingly, a heat-sealable coating must then have been applied beforehand in
the region of these.
In the bottom area it is also preferably provided that the bottom gusset
sections,
which are separated from each other by the bottom gusset, are connected in the
corners to the adjacent front wall or to the adjacent rear wall via inclined
sealing
seams. Starting from an unfilled, flat packaging bag, heat-sealing seams thus
extend along the lateral edges and in the region of the chamfered corners, no
heat-sealing seam being provided only in a bottom edge because of the bottom
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gusset provided there and at a top edge because of the bag opening provided.
The chamfered corners preferably have an angle of between 40 and 50 ,
particularly preferably 45 , so that the longitudinal sealing seams and the
adjacent inclined sealing seams accordingly enclose an angle of 130 to 140 ,
preferably 1350
.
In principle, any material can be used as a heat-sealable material for the
coating.
Polyolefins are primarily used, preferably polyethylene or polyethylene
copolymers such as HDPE, LLDPE, LDPE or EVA or mixtures thereof.
The paper used is also basically not limited to any particular. Preferably it
is a
kraft paper. Kraft paper is understood to be a packaging paper which is formed
from bleached or unbleached sulphate pulp and has high static and dynamic
strengths.
According to a preferred embodiment, the layer of paper has a basis weight
between 60 and 120 gsm, preferably between 70 and 90 gsm. The basis weight
of the heat sealable material is preferably between 1 and 16 gsm, preferably
between 3 and 10 gsm. Where a full surface coating is provided, the basis
weight
of the heat-sealable material may be between 2 and 8 gsm, preferably between
3 and 7 gsm. Where thicker areas of the coating are provided for the formation
of
heat-sealable seams in the case of a full-surface coating, the weight per unit
area
is preferably between 8 and 16 gsm, more preferably between 10 and 14 gsm.
This also applies to non-full-surface coatings.
According to a preferred embodiment, the packaging bag has ventilation
openings. These ventilation openings are of particular advantage since the
goods
to be packed, in particular diapers, are introduced into the packaging bag in
a
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tight-fitting manner, so that the air initially contained in the packaging bag
must
escape. On the one hand, this enables a rapid filling and, on the other hand
avoids excessive stress on the heat-sealing seams due to the increased
internal
pressure caused by the compressed air.
Preferably, these vents are formed by unsealed portions in the heat-sealing
seams. In particular, the ventilation openings are arranged in the diagonal
seal
seams. Accordingly, each diagonal seal seam has one, two or more unsealed
sections through which air can escape from the packaging bag. Such an
embodiment is of particular advantage since no breakthroughs have to be made
in the material of the web. This is advantageous since paper is generally very
susceptible to tearing if there are already material weaken ings in the web.
The
inclusion and connected sections in the heat seal seams allows the formation
of
ventilation openings without weakening the material at the front wall or the
rear
wall. In addition, the openings close as soon as the bag has been filled and
assumes an essentially cuboid shape.
Accordingly, the web has a heat-sealable area not only on an inner side but
also
on an outer side. In contrast to the inner side, however, this area is not
arranged
over the entire surface of the outer side and serves in particular to form a
flat
head area. In the course of sealing, the front walls are usually joined
together by
a transverse sealing seam, whereby a corresponding fold must be made in order
to form a cuboid shape, in which sections of the front wall and the rear wall
are
folded onto themselves. The heat-sealable areas on the outer side can also be
applied via a dispersion lamination with a heat-sealable lacquer.
The areas with the heat sealable material are arranged to be folded over on
themselves and then sealed together by the application of heat. The sealed
areas
provide additional bond strength and dimensional stability to the top of the
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packaging bag when filled.
The packaging bag according to the invention preferably has a width of between
150 and 500 mm and a length of between 200 and 700 mm.
5
According to another aspect having independent meaning, it is also possible
that
instead of chamfered corners, the bottom gusset parts are fixed together at
the
side edges. For example, the bottom gusset sections may also be formed with an
at least partial coating of a heat-sealable material so that they can be
joined
10 together by a heat-sealable seam.
Alternatively, it is also possible for one or more openings to be arranged in
the
bottom gusset, in which case the front wall and the rear wall can be joined
together through the bottom gusset. This then also fixes the bottom portions
together. There is no need for additional coating of the bottom gusset with
the
heat-sealable material.
In addition, substantially all of the previously mentioned features can be
related
to such an embodiment.
It is further an object of the invention to use a packaging bag according to
any
one of claims 1 to 13 for packaging diapers. Typically, for this purpose, a
plurality
of diapers are placed one on top of the other in a folded state and are
inserted
together into the film packaging
According to a further development, the packaging bag is then sealed at a head
region with a transverse sealing seam. According to the previous description,
the
areas provided with a heat-sealing lacquer can then also be placed and sealed
to one another.
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Since the protruding section of the back wall is intended solely for securing
the
unfilled packaging bags, it is usually detached from the packaging bag after a
transverse sealing seam has inserted, since it is of no further use.
According to a particularly preferred embodiment of the invention, the
packaging
bag filled with the diapers has a substantially cuboid shape after closing.
In the following, the invention is explained in more detail with reference to
an
example of an embodiment. It shows:
Fig. 1A, 1B a packaging bag according to the invention
in an unfilled
state
Fig. 2A, 2B, 2C the essential steps for producing a packaging bag according
to Figure 1
Fig. 3A, 36 a sectional view through a packaging bag
according to the
invention in the region of a longitudinal seam
Fig. 4 the packaging bag according to the invention
in a filled state
with a view of the base of the stand
Fig. 5, 6 alternative designs of the packaging bag
Fig. 7A, 7B designs of the bottom with a separate bottom
panel
Fig. 8 a further design of the bottom
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Fig. 1A, 1B show a packaging bag according to the invention in a laid-flat and
unfilled state with a front wall 1 and a rear wall 2, which protrudes with a
section
3 opposite the front wall 1 and wherein stacking openings 4 are arranged in
the
protruding section 3.
The stacking openings 4 are provided so that several packaging bags arranged
one above the other can be gripped by bars in order to be able to simplify the
packaging process considerably.
The front wall 1 and the rear wall 2 are connected to each other at the
lateral
edges by longitudinal sealing seams 5, which merge into inclined sealing seams
7 in a bottom bag edge 6. The special feature here is that the inclined
sealing
seams 7 do not connect the front wall 1 to the rear wall 2. Rather, a bottom
gusset
8 with bottom gusset sections 9, 10 is provided, the bottom gusset section 9
being
connected to the front wall 1 via the inclined sealing seam 7 and the bottom
gusset section 10, which is not shown in greater detail in Fig. 1A and 1B,
being
connected to the rear wall 2. In contrast to Fig. 1A, Fig. 1B shows an
embodiment
in which the front wall 1 and the rear wall 2 are connected not only by the
inclined
sealing seam 7 but also by a cross seam 19 to the bottom gusset section 10.
By means of the inclined sealing seams 7 it is possible for the packaging bag
to
be formed with chamfered corners 11 at the bottom bag edge 6. Its special
feature
becomes clear in particular from the further figures.
With reference to Fig. 1A and 1B, it is further possible to identify regions
12 with
a heat seal lacquer, which are applied to the outer surface of the front wall
1 and
also to the outer surface of the rear wall 2. Furthermore, ventilation
openings 13
are provided which are arranged in the inclined sealing seams 7 according to
Fig.
1A and in the cross seams 19 according to Fig. 1B.
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Figs. 2A, 2B, 2C show the manufacturing process for forming a packaging bag
according to Fig. 1A. For this purpose, an endless web 14 is provided, which
extends in machine direction MD and is already provided with the bottom gusset
sections 9, 10 for the formation of the front wall 1, the rear wall 2 and the
bottom
gusset 8. In addition, the stacking openings 4 are already located in the web
14.
Figures 2A, 2B, 2C describe the transport and the production of the packaging
bag in machine direction MD. According to Fig. 2B, the web 14 is folded over,
whereby the bottom gusset 8 is formed. In addition, the areas 12 with the heat-
seal lacquer are now also shown at least on the front wall 1. Then, according
to
Fig. 2C, the longitudinal sealing seams 5 and the diagonal sealing seams 7 are
heat-sealed and then individual packaging bags are separated from the web 14.
In a final step, the corners of the bottom bag edge 6 can then be chamfered.
The packaging bag according to the invention is a packaging bag which is at
least
partially formed from paper. In particular, the aim is to create a packaging
bag
which is formed from at least 80% by weight, particularly preferably 90% by
weight, very particularly preferably 95% by weight, of paper. Paper is a
renewable
raw material. In addition, a good recyclability can be ensured by the highest
possible proportion of paper. Figs. 3A, 3B show a section through the
packaging
bag in the region of the longitudinal sealing seams 5, whereby a closer
representation of the regions 12 with the heat-sealing lacquer on the outer
sides
has been omitted. The bag consists in particular of a layer 15 of paper.
Although
multilayer designs are also conceivable, only one layer of paper 15 is
provided in
the example shown.
Since paper itself has no heat-sealing capability, it is necessary to apply a
heat-
sealable material in the form of a sealing layer 16 to the layer of paper 15.
There
are various possibilities for this. Polyethylene in particular is used as the
heat-
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sealable material for the sealing layer 16, whereby the proportion of this
polymer
should be kept as low as possible due to the previously mentioned aim of a
high
paper content.
According to Fig. 3A, the coating 16 is therefore only provided in areas in
which
heat-sealing seams, e.g. longitudinal sealing seams 5 or inclined sealing
seams
7, are subsequently to be made. Since the packaging bag is also to be finally
closed with a transverse sealing seam, the application of a sealing layer 16
is
also necessary there. In areas where no heat sealing is to be carried out, a
sealing layer 16 can generally be dispensed with.
However, paper has only a very poor water vapor barrier, so it may be useful
to
provide a full-surface sealing layer 16 as shown in Fig. 3B. It should be
noted
here that such packaging bags are frequently used for packaging diapers. This
use is also within the scope of the present invention. Diapers in particular
but also
other hygiene articles have a high proportion of super absorbent material,
which
tends to absorb moisture from the environment due to its hydrophilic property.
Accordingly, by applying a thin sealing layer 16, the water vapor barrier can
be
considerably.
Due to the full-surface arrangement, however, a comparatively thin sealing
layer
16 is aimed for in order to keep the proportion of polymeric materials as low
as
possible. At the same time, however, this also impairs the heat-sealability or
the
stability of the heat-sealing seams to be applied. In principle, this can be
compensated for by wider heat-sealing seams. The embodiment according to Fig.
3B, however, provides for an embodiment in which an additional layer 20 made
of heat-sealable material is coated on the sealing layer only in sections.
This
leads to regions with a thicker layer of heat-sealable material, which are
designed
for a heat-sealing seam, and regions with a thinner layer of heat-sealable
material
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in which no heat-sealing seam is to be introduced. According to Fig. 3B, this
is
the case, for example, in the protruding section 3.
Accordingly, a packaging bag is formed which can nevertheless be closed to a
5 large extent by heat-sealing seams. Only at a bottom edge 6
are the front wall 1
and the rear wall 2 joined by a bottom gusset 8. The advantage of the
chamfered
corners 11 with the inclined sealing seams 7 can be seen in particular in Fig.
4.
Fig. 4 shows an embodiment of the packaging bag in accordance with Fig. 1B.
The omission of the excess material results in the formation of a bottom 17
which,
10 despite the use of a relatively stiff paper, permits an essentially cuboid
configuration. The packaging bag according to Fig. 4 has been filled with
diapers
stacked one on top of the other and closed in the top area 18 by a transverse
seal seam.
15 The following table shows a comparison between a conventional
packaging bag
formed entirely of polyethylene and a packaging bag according to the invention
formed of a kraft paper having a thickness of 80 gsm and provided with a
sealing
layer 16 of polyethylene having a basis weight of 4 gsm. The sealing layer 16
extends over the entire layer of kraft paper. In the areas of the heat-sealing
seams, no additional material was applied for the sealing layer 16. Instead,
correspondingly wide heat-sealing seams were formed.
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Paper-
Method PE bag
Bag
ASTM E
Thickness micron 75
109
252
ASTM D
Base weight gsm 80
84
1505
ASTM D
Tensile strength (MD) N/inch 68
224
882
ASTM D
Tensile strength (CD) N/inch 882 66
107
Elongation at break ASTM D
758
3
(MD) 882
Elongation at break ASTM D
ok 817
7
(CD) 882
Gloss at 60 GU ISO 2813 60
13
Bag seal strength 2 ply N/25mm ASTM F 88 44
8
Seam width mm 1
5
Table
On the basis of the comparative values, it is clear that the bags have a
similar
material thickness, but differ substantially from one another, particularly
with
regard to strength, toughness and also with regard to the sealing seam
strength.
Despite the advantages of a bag made entirely of polyethylene, it was possible
within the scope of the invention to provide a packaging bag which has
sufficiently
good material properties and, in contrast to a bag made of polyethylene, was
produced to a substantial extent from renewable raw materials. The
disadvantages of using paper, in particular the high stiffness has been
eliminated
with respect to the desired dimensional stability by the use of chamfered
corners
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11. The regions 12 with the heat-seal lacquer also contribute in an
advantageous
manner to the fact that the head area 18 can be closed securely and with a
constant shape.
According to Figs. 5 and 6, the packaging bags do not have chamfered corners
7. Rather, the bottom gusset 8 is reinforced by connecting the bottom gusset 8
portions together. According to Fig. 5, openings 21 are provided in the bottom
gusset 8 portions so that the front wall 1 and the rear wall 2 are sealed
together
through the bottom gusset. According to Fig. 6, the bottom gusset sections 9,
10
have a coating of a heat-sealable material and can thus be joined to one
another
via sealing seams 22.
Figs. 7A, 7B show an alternative embodiment of the bottom gusset 6, which is
not integrally formed with the front wall 1 and the rear wall 2 but rather is
formed
via a separate bottom panel 25. The connection is made via joining seams 23,
wherein according to Fig. 7A the front wall 1 and the rear wall 2 are formed
straight at a lower joining portion 24, while according to Fig. 7B the front
wall 1
and the rear wall 2 are folded over at the lower joining portion 24.
According to Fig. 7A, the joining seams 23 are stressed in shear, and
according
to Fig. 7B, the joining seams 23 are stressed in peel. Stressing in shear is
particularly advantageous for the joining seams 23, since higher loads can be
absorbed. In addition, the design according to Fig. 7A also produces a rounder
bottom shape_
According to Fig. 8 a different embodiment of the bottom gusset is provided,
wherein the front and/or the rear walls 1, 2 are connected to the bottom
gusset
sections 9, 10 via a transversely extending secondary cross seam 26, wherein
the secondary cross seam 26 is spaced from the cross seam 19 such that a
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compensation chamber 27 is formed in each case. The secondary cross seam
26 has a number of breakthroughs through which air can be transported outwards
to the bag edges. This greatly simplifies the packing process, since there is
usually sufficient space available at the edges. This helps to avoid excessive
pressure in the center of the bottom, which could cause the seams or the
creased
edges to burst. Equally, however, pressure peaks can also occur on the
outside,
so that the air can then flow in the opposite direction to the center, thus
relieving
the corners of the base.
CA 03235249 2024-4- 16
