Note: Descriptions are shown in the official language in which they were submitted.
} ! 729~n
Packing machine such as a packing table for wrapping up
goods in a plastics foil.
The present invention relates to a packing machine
5 such as a packing table for wrapping up goods in a
plastics foil taken off from a supply, and where pro-
vision is made of a heatable tear-off edge for separating
from the supply, by thermal action on the foil, a piece
of foil to be used for the wrapping.
For wrapping goods, in particular articles of
food, in a plastics foil, it is known to use packing
tables provided with a heating wire or heating rod for
cutting the foil pieces used for wrapping up the indi-
vidual articles.
The heating wire or rod which acts as a cutting
or tear-off edge is a resistance wire or a hollow rod
provided with a resistance heating element of con-
ventional type heated by passage of current, and when
the foil which for instance is drawn in the desired
2~ lengths from a supply roller is brought into contact
with the heating wire, it will be separated or cut at
$he area of contact as a result of the decomposing
action of the heat on the plastics material.
The temperature at which the heating wire or rod
25 acts on the plastics foil is, however, very critical.
If the temperature is too low, the cutting effect will
be insufficient and if it is too high, the decomposition
of the plastics material may result in a combustion
producing vapours and/or gases and also in many cases
30 ashes, which is troublesome and sometimes detrimental
to the operator of the machine and, in any case,
unacceptable when the packing machine is used for
wrapping up articles of food, e.g. meat products, which
may be cont~minated thereby. Known packing machines of
35 the type in question are therefore provided with
complicated control circuits to ensure that the correct
temperaiure of the heating wire or rod is kept
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accurately, which is however extremely difficult to
achieve with an ordinary resistance wire or resistance
heating element which is, moreover, cooled each time the
` plastics foil is cut, this taking place at varying
5 intervals, whereafter the heating wire or rod will have
to be rapidly brought back to the desired temperature
but not above this, so that in many cases, in particular
in machines for wrapping up articles of food, the control
circuits require accurate and quick-acting temperature
10 sensors or some other sensitive equipment.
These control circuits complicate the packing
machines and make them more expensive and at the same
time more vulnerable to damages and defects, so that
regular periodic controls are necessary.
The invention has for its object to provide a
packing machine such as a packing table which prevents
effectively the formation of injurious gases or vapours,
when plastics foil lengths are cut or torn off by thermal
action thereon and which therefore can also be used for
20 packing articles of food, said machine being nevertheless
of simple, cheap and robust construction, so that it can
stand a rough treatment.
The machine according to the invention is charac-
terized in that the tear-off edge is provided with one
25 or more PTC elements as a heating body.
The invention utilizes the fact that whereas the
resistance/temperature characteristic of ordinary
resistance materials is practically continuous, this
characteristic shows for PTC materials, which are ceramic
30 materials on the basis of barium titanate, an abrupt and
very steep increase at a definite temperature, the so-
called Curie temperature, the level of which can be
adjusted when preparing the material, the resistance
Yalue increasing very strongly when the temperature
35 exceeds this Curie temperature. The passage of current
is thereby limited correspondingly and the production of
heat connected therewith too, so that a temperature
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balance is established at th~ Curie temperature. ~'hen
using a PTC mate~ial whose Curie temperature has pre-
cisely the value adequàte to the purpose, or when using
a suitable number of PTC elements spaced from one
5 another at an appropriate distance which can be deter-
mined experimentally, it is thus possible to obtain at
the tear-off edge a constant temperature at which the
plastics material to be cut is not burnt but decomposes
without producing gases or vapours, no special circuits
10 or temperature sensors being required for keeping this
temperature constant. Due to the properties of the
PTC material, no special demands are made either
/ regarding feed voltage stability.
The invention will in the following be explained
15 in more detail with reference to the accompanying drawing
in which
Fig. 1 shows a packing table having a tear-off
edge in the form of a heating rod according to the
invention,
Fig.2 the heating rod dismounted from the packing
table shown in Fig.1, and
Fig.3 a sectional view of the heating rod of Fig.2.
The packing table shown in Fig.l comprises a
packing surface A provided, on its side turning away
25 from the operator, with a supply roll B with the plastics
foil to be used for wrapping up goods such as articles
of food. The supply roll B rests on two rollers E
extending across the width of the packing table, and the
free end of the foil is led down in front of the
30 rollers E or between these and passes underneath the
packing surface A until it reaches the front of said
surface, where provision is made of an additional
roller F over which the foil can be drawn up to the top
side of the packing surface. The operator places the
35 article to be wrapped up on the packing surface A and
then draws a suitable length of foil towards the front
of the packing surface, over the roller F and over the
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artlcle which is then taken from above and moved towards
the operator onto a surface C which may possibly be a
heat plate, if the foil is of the shrinking type. During
- this last-mentioned movement of the article now
5 surrounded by the foil, the plastics foil is brought
into contact with a heating rod D extending transversely
to the table in front of the roller F, the temperature
of said heating rod D being such that the plastics
material is decomposed in the area of contact with the
10 rod, whereby the wrapped article is separated from the
supply of wrapping material.
The heating rod D is shown dismounted and with
a current supply wire in Fig. 2, and a longitudinal
section of a central part of the heating rod is shown
15 in Fig.3.
In the embodiment shown, the heating rod D
comprises a tubular envelope 1 which may be of aluminium
or any other heat resistant material and may have a
triangular, circular or rectangular profile. On the
20 inside of the envelope 1, provision is made of an
insulating layer 2 which may be an extruded silicone
rubber profile inserted in the envelope 1.
On opposite sides, electric conductors 3 are
inserted in the rubber profile, which conductors may
25 be aluminium or brass foil strips extending along the
whole length of the heating rod and being insulated from
each other. These conductors 3 are connected each to
one pole of the electric feed voltage source (not shown),
e.g. via a wire with a plug as shown in Fig.2.
Between the electric conductors 3 and in contact
therewith, heating elements 5 of PTC material are
inserted in the heating rod, said elements being, in the
embodiment shown, kept at a distance from each other by
means of insulating spacers 4 which may be made of
35 silicone rubber and are practically filling up the
intervals between the PTC elements. In principle, the
heating rod could also be filled up with PT~ material,
~ i 72~5~3
the electric conductors 3 and the insulating spaccrs 4
being then superfluous. The number of PTC heating
elements 5 per unit of length or the degree of filling
` of the heating rod depends on the Curie temperature of
5 the PTC material and of the desired surface temperature
of the heating rod. If the Curie temperature of the
PTC material corresponds practically to the desired
cutting temperature determined by the plastics foil
material, the heating rod can be filled up with this
10 PTC material, and if the Curie temperature is higher
than the desired cutting temperature, the degree of
filling of the heating rod is chosen proportionally
lower, which can take place by inserting two or more
smaller PTC elements at a suitable distance from each
~ 15 other in the heating rod, for instance as shown in Fig.3.
When the packing table whereon the heating rod is
mounted, for instance as shown in Fig.1, is to be used,
the heating rod D is connected to the feed voltage, e.g.
via an electric wire as shown in Fig.2, and the rod will
20 very quickly reach the desired surface temperature at
which the plastics used as wrapping material is just
decomposed but not burnt. By choosing the PTC material
and the degree of filling of the heating rod with said
material as explained in the previous paragraph, the
25 heating rod is so dimensioned as to produce this surface
temperature depending on the wrapping material used.
As a consequence of the above-mentioned special
properties of the PTC material, the desired surface
temperature of the heating rod will be reached but not
30 exceeded to an appreciable extent, so that even after
an operative break where no cutting and thus nG cooling
of the heating rod has taken place, the wrapping foil
will be cut solely by decomposition of the plastics
material but without combustion of said material and
35 consequently, without formation of unpleasant and
possibly injurious vapours and gases, and without
production of ashes. A packing table according to the
t I 72~5~
invention is thus extremely cleanly in use, because the
cutting of the plastics foil produces no waste at all,
and even very severe sanitary regulations as those
- imposed in particular by the veterinary services for the
5 packing of articles of food may be met safely and without
difficulty. Further adjustment or control of the
temperature beyond that resulting from the self-adjust-
ment o~ the PTC material at the Curie temperature is not
necessary, and consequently, the invention provides a
10 tear-off edge of very simple construction, working in
a safe and reliable manner and requiring practically no
maintenance.
The abbreviation PTC as used hereln means Positive
Temperature Co-efficient.
