Note: Descriptions are shown in the official language in which they were submitted.
1o77666
The present invention relates to a method to improve in
the T or butt sealing of a laminated thermoplastic material,
of the type which comprises a carrier layer of porous plastic
material and outer layers of compact plastic material, the
strength of the seal by heat treatment of the edge surface at
whichthe sealing is to be carried out.
Packages impervious to liquid, e.g. for liquid foodstuffs,
are manufactured inter alia from laminated all-plastic material
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which comprises a central layer of foamed, porous plastic material
and on either side of this, laminated layers of compact plastic
material. The central layer is appreciably thicker than the outer
layers and may e.g. have a thickness of 0.8 mm, whilst each of
the outer layers has a thickness of approx. 0.1 mm. This
laminated material may also comprise further layers of plastic
material or e.g. aluminium foil, but the main concept of the
material is the joist-like construction with a comparatively
thick, porous central layer and two laminated layers on either
side of this which impart a very good rigidity to the material.
This material has been the object of increasing popularity in
recent times, which is due primarily to the fact that the material
is extra-ordinarily light and furthermore cheaper than the types
of packing laminate used previously with a central layer of paper.
The different types of known packages which are manu-
factured from the said laminated plastic material are of various
different designs and consequently also have joints or seams of
many different types. The different types have in common,
however, that in the joining and sealing the thermoplastic
properties of the material are utilized, that is to say, the
joining is generally carried out so that the area intended for
joining is heated to such a temperature that the thermoplastic
material softens, whereupon a direct joining and pressing together
takes place. In overlap joints, or joints of the inside-to-inside
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type, this method has given very durable seals, since comparative-
ly large portions of the surface layers of the material are
joined to one another. To offer greater possibilities of
shaping and design of the types of packages it is also desirable
to provide for sealing of the said laminate material in T joints
or butt joints, that is to say joints of a type wherein the
material sheets are joined in T shape or edge to edge. In joints
or seams of this type, at least as far as the one material sheet
is concerned, only the edge surface of the material sheet is
used for the seal. Since the central carrier layer of the foamed
plastic material in itself has very low strength, it is in effect
only the edge surfaces of the two homogeneous plastic layers
which are utilized in the joining, and as a consequence thereof
this type of joint is difficult to effect, so that the joint
becomes impervious to liquid, and has unsatisfactory strength.
It has been suggested to overcome this problem by not
utilizing the thermoplastic properties of the material in the
joining process, but instead to apply, for joining, a heat-
meltable glue, so-called "hot-melt", to the location of the joint.
This however, is subject to serious disadvantages, since the
application of hot-melt constitutes an extra operation which
complicates the machines manufacturing the packages.
The present invention provides a method which makes it
possible to join together thermoplastic material of the said type
in T joints or butt joints whilst utilizing the thermoplastic
properties of the material and without the necessity of application
of any kind of adhesive.
The present invention also provides a method for
achieving tight and strong T or butt joint seals of the said
laminated thermoplastic material, which method is not subject
to the aforementioned disadvantages.
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According to the present invention there is provided
a method of improving, in the T or butt sealing of a laminated
thermoplastic material of the type which comprises a carrier
layer of porous plastic material and outer layers of compact
plastic material, the strength of the seal by heat treatment of
the edge surface at which the sealing is to be carried out,
the said edge surface before the sealing being heated by a jet
of hot air, whose main stream has a greater thickness than the
carrier layer of the material and is directed substantially at
right angles against the edge surface so as to provide before
sealing, melting of the edge surface such that the carrier layer
draws back and the two outer layers are melted towards each
other and meet substantially in the centre plane of the material.
In accordance with the invention, the edge surface
before the sealing is warmed up by means of a jet of hot air,
whose main stream has a somewhat greater thickness than the
carrier layer of the material and is directed at substantially
right angles against the edge surface, so as to provide before
the sealing operation such a melting of the edge surface that
the carrier layer draws back and the two outer layers are melted
towards each other, and meet substantially in the centre plane
of the material.
By the method in accordance with the invention a
solution of the problem of joining together laminated thermoplastic
material in T joints or butt joints is obtained. No additive
material of any kind is used, but instead the material edge
itself is transformed, so that it becomes more suitable for
sealing. The method may be carried out in conjunction with the
normal, necessary heating up of the edge in conjunction with the
actual sealing and does not require any complicated auxiliary
equipment but merely a more accurate and specially oriented heating
of the material edges which are to be joined together.
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The present invention will be further illustrated
in more detail with reference to the accompanying schematic
drawings in which:
Fig. 1 a-c show partly in section and on an enlarged
scale the successive heating of a material edge by using the
method in accordance with one embodiment of the invention.
Fig. 2 shows in section and further enlarged two
material sheets after heating by the method in accordance
with the invention and subsequent joining together.
Referring to Fig. 1, a material sheet comprises a
central layer 2 of porous plastic material~ e.g. foamed poly-
styrene. The central carrier layer 2 has a thickness of 0.8 mm
and is laminated on both sides with compact layers 3 and 4 of
thermoplastic material, e.g. polystyrene or polythene. The
layers 3 and 4 are each approx. 0.1 mm thick. The edge 5 of the
material sheet, which in Fig. 1 is on the righthand side, is
cut off, substantially at a right angle to the plane of the
material sheet 1. To the right of this sheet edge 5 is a nozzle
6 which is provided with a plurality of air exit holes 7 which
are cylindrical or oblong and are arranged close to one another
in a row, the length of which corresponds to the width of the
material sheet or to the length of edge surface 5 which is to
be heated by means of a jet of hot air issuing through the ducts
7 of the nozzle 6. The duct 7 has a thickness which corresponds
to approx. 1-3 times the thickness of the carrier layer 2.
Referring to Fig. la the material sheet before the
heating by hot air of the material edge 5 is shown. In Fig. lb
the material sheet during the heating of the edge 5 is shown and -
in Fig. lc the material sheet directly before interruption of the
heating is shown. Fig. 2 shows the material sheet 1 after
being joined together in a T joint with a second material sheet
8, which may be of the same type and composition as the material
,
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sheet 1.
When the two material sheets 1 and 8 are to be joined
together by means of a T-joint, the procedure according to the
invention is as follows: The one material sheet 1 (Fig. la),
whose outer layers 3 and 4 are of identical thickness, is placed
so in relation to the opening 7 of the nozzle 6, that the
centre-line of the said opening coincides with the centre plane
of the material sheet 1. The distance between the edge surface
5 of the material sheet 1 and the opening of the air duct 7 is
appropriately chosen so that the distance is 1-1.5 times the
thickness of the material sheet 1. The alignment and the
adjustment of the distance between the material sheet 1 and the
nozzle 6 can be carried out, depending on the type of packing
machine in which the operation is taking place, either by moving
of the material sheet 1, by moving of the nozzle 6 or by moving
of both parts.
After the relative alignment of the material sheet 1
and the nozzle 6, a hot air stream is blown via the duct 7 of the
nozzle against the edge surface 5 of the material sheet 1. By
virtue of the alignment of the nozzle 6 the main stream of the
; air jet, which may be assumed to have almost the same thickness
or diameter as the duct 7, will strike the edge surface 5
symmetrically which, as can be seen from Fig. lb, causes the
carrier layer 2 after heating to its softening temperature to
draw back a little in relation to the outer layers 3 and 4, as
layer 2 melts earlier than the outer layers 3 and 4(which have
an appreciably higher heat capacity), and due to the effect of
the dynamic pressure of the issuing hot air. At the same time
the two outer layers 3 and 4 of the material sheet 1 soften on
their outer portions situated at the edge surface 5 due to the
effect of the heat and are bent inwards towards the centre plane
of the material sheet.
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On continued heating (Fig. lc), the folding in of the
edge zone 5 of the two outer layers 3 and 4 will continue until
the edges of the layers meet andpartly fuse directly together
over the centre plane of the material sheet 1. The layers will
then protect the foam material under lying from further melting.
Directly after this softening and transformation of
the edge surface 5 of the material sheet 1, the nozzle 6 is
removed and the edge surface 5 of the material sheet 1 is joined
together with the second material sheet 8 to which it is to
be joined in a T-joint. Parallel with the described heating of
the edge surface 5 of the material sheet, heating is carried
out appropriately also of the surface layer of the other material
sheet 8 which is to be used for the assembling.
In Fig. 2 is shown on a larger scale a section
through the two material sheets 1, 8 joined together. It is ~ -
evident from the Figure that the joint has been carried out -
wholly between the two folded-in outer layers 3 and 4 of the
material sheet 1 and the one outer layer of the other material
sheet 8, so that a very strong and durable attachment has been
achieved.
In the case described above the two outer layers of
the laminated thermoplastic material are thus of the same type
and thickness, which means that in order to ensure a regular,
symmetrical folding-in of the outer layers, the hot air jet
should be directed towards the central part of the carrier
layer 2 exposed on the edge of the laminate. In other words,
the nozzle is situated in the prolongation of the centre plane
of the material sheet.
If the material sheet whose edge surface is to be
utilized for sealing is covered with outer layers of different
thickness or of different type, it may become necessary to shift
the nozzle parallel in relation to the centre plane of the
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1077666
material sheet, so that a symmetrical folding-in of the outer
layers can be achieved. The nozzle is moved in the direction
towards the thicker surface layer, and the necessary distance
will best be ascertained in each individual case. Such a shifting
of the nozzle may also be necessary for other reasons, e.g. if
the packing machine in which the process is carried out causes
an unequal heat discharge to take place from the outer layers
of the laminate.
The thickness of the hot air jet is adapted to the
thickness of the material sheet which is to be treated. The main
flow of the hot air jet, whose thickness can be regarded as
corresponding substantially to the thickness of the nozzle hole 7,
is selected so that its thickness is 1-3 times the thickness of
; the carrier layer of the laminated material. The hot air jet
has a total thickness which is 1.5 to 4 times the total thickness
of the material. For the material thicknesses in actual use
for the manufacture of packing containers for beverages and the
like this means that the hot air jet has a total thickness of
between 1-5 mm. For thermoplastic material of the said types,
that is to say polystyrene or polythene, the hot air jet should
have a temperature of 350-450C and preferably approx. 400C,
and a speed of 6-12 m/sec, preferably approx. 7.5 m~sec. At
lower temperatures and speeds the required heating of the
material is not obtained within a reasonable period, and higher
temperatures and air speeds produce a rather irregular deformation
of the material edge.
The folding down or in of the outer layers of the
thermoplastic material can take place by means of the method
in accordance with the invention, independently of a possible
orientation in the laminate layers. The operation may take
place on a stationary or moving material web, depending on the
type of packing machine which is to be used. ;~
