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The present invention relates to a method to increase 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.
Packages impervious to liquid, e.g. for liquid foodstuffs, are manufactured
inter alia from laminated all-plastic material 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 principle 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 extraordinarily 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 manufactured 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 type, this method has proved to
give very durable seals, since comparatively 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 allow sealing of the said laminate material in T joints or
butt joints, that is to say joints of a type wherein the material sheets
concerned 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 can be 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 on the one hand difficult to
execute, so that the joint becomes impervious to liquid, and has on the
other hand 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 in connection with the joining a heat-meltable glue, a
so-called hot-melt, to the place of the joint. This solution, however, is
subject to serious disadvantages, since the application of hot-melt
constitutes an extra action which complicates the machines manufacturing
the packages.
It is an object of the present invention to provide 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 application of any kind of adhesive.
It is a further object of the present invention to provide 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.
These and other objects have been achieved in accordance with the invention
in that a method of the type described at the beginning was given the
characteristic that the said edge surface before the sealing is warmed up
by means of a jet of hot air, whose main flow has a somewhat greater
thickness than the carrier layer of the material and is directed at a
substantially right angle 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.
Preferred embodiments of the method in accordance with the invention have
been given, moreover, the characteristics which are evident from the
subsidiary claims.
By the method in accordance with the invention a new solution of the
problem of joining together laminated thermoplastic material in T joints
or butt joints is obtained. According to the invention 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 solution
shown 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.
In the following the invention will be described in more detail with
reference to the enclosed schematic drawing.
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 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.
In FIG. 1 is shown a part of a material sheet comprising a central layer 2
of porous plastic material, e.g. foamed polystyrene. The central carrier
layer 2 has a thickness of 0.8 mm and is laminated on both sides with
compact layers 3, 4 of thermoplastic material, e.g. polystyrene or
polythene. The layers 3, 4 are each approx. 0.1 mm thick. The edge 5 of
the material sheet, which on the drawing 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
number 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.
In FIG. 1a is shown the material sheet before the heating by hot air of the
material edge 5. In FIG. 1b is shown the material sheet during the heating
of the edge 5, and in FIG. 1c is shown the material sheet directly before
interruption of the heating. 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 make-up as the material sheet 1.
When the two material sheets 1, 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. 1a), whose outer layers 3, 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 flow 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. 1b, results in that the
carrier layer 2 after heating to its softening temperature draws back a
little in relation to the outer layers, owing to the fact, on the one
hand, that layer 2 melts earlier than the outer layers (which have an
appreciably higher heat capacity), and, on the other hand, the effect of
the dynamic pressure of the issuing hot air. At the same time the two
outer layers 3, 4 of the material sheet 1 will on their outer portions
situated at the edge surface 5 soften through the effect of the heat and
be bent inwards towards the centre plane of the material sheet.
On continued heating (FIG. 1c), the folding in of the edge zone 5 of the
two outer layers, 3, 4 will continue until the edges of the layers meet
and partly fuse together straight before the centre plane of the material
sheet 1. The layers will then protect the foam material lying underneath
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, a 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, 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 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 chosen so that its thickness of 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 must have a temperature of 350-450.degree.C and
preferably approx. 400.degree.C, 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. Tthe
operation may take place on a stationary or moving material web, depending
on the type of packing machine which is to be used.
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