U.S. patent application number 11/792070 was filed with the patent office on 2008-05-15 for method and device for sealing.
This patent application is currently assigned to Tetra Laval Holdings & Finance S.A.. Invention is credited to Gert Holmstrom, Mats Qvarford, Magnus Rabe, Magnus Wijk.
Application Number | 20080110560 11/792070 |
Document ID | / |
Family ID | 33563203 |
Filed Date | 2008-05-15 |
United States Patent
Application |
20080110560 |
Kind Code |
A1 |
Wijk; Magnus ; et
al. |
May 15, 2008 |
Method and Device for Sealing
Abstract
The present invention refers to a method for sealing a first
packaging material laminate (10) to a second packaging laminate
(12), at least the first laminate (10) comprising at least one
layer of magnetizable particles and a sealable layer (34). The
method is characterized in facing the sealable layer (34) towards
the second laminate (12), providing an alternating magnetic field
to the laminates in a sealing zone, thereby generating magnetic
hysteresis losses in the laminate (10) comprising the magnetizable
particles, which losses create heat substantially melting the
sealable layer (34) in the sealing zone, and applying a sealing
pressure to the first and second laminate (10, 12), which pressure
causes the first and second laminate (10, 12) to be pressed
together in the sealing zone, thereby sealing the laminates (10,
12) to each other. The invention also relates to a device for
carrying out the method.
Inventors: |
Wijk; Magnus; (Lund, SE)
; Rabe; Magnus; (Akarp, SE) ; Holmstrom; Gert;
(Lund, SE) ; Qvarford; Mats; (Lund, SE) |
Correspondence
Address: |
BUCHANAN, INGERSOLL & ROONEY PC
POST OFFICE BOX 1404
ALEXANDRIA
VA
22313-1404
US
|
Assignee: |
Tetra Laval Holdings & Finance
S.A.
Pully
CH
|
Family ID: |
33563203 |
Appl. No.: |
11/792070 |
Filed: |
December 13, 2005 |
PCT Filed: |
December 13, 2005 |
PCT NO: |
PCT/SE05/01911 |
371 Date: |
June 1, 2007 |
Current U.S.
Class: |
156/272.4 ;
156/380.9 |
Current CPC
Class: |
B29L 2009/00 20130101;
B29C 53/38 20130101; B29C 66/8122 20130101; B29C 66/8122 20130101;
B29C 66/1122 20130101; B29C 66/4322 20130101; B29C 66/72321
20130101; B29C 66/81262 20130101; B29C 66/80 20130101; B29K
2995/0008 20130101; B29C 65/3668 20130101; B29L 2031/712 20130101;
B29C 66/43 20130101; B29C 66/8322 20130101; B29C 66/723 20130101;
B29C 65/3612 20130101; B29C 66/49 20130101; B29C 66/8122 20130101;
B29C 66/8122 20130101; B29K 2905/10 20130101; B29K 2821/00
20130101; B29C 66/81871 20130101; B29K 2909/02 20130101 |
Class at
Publication: |
156/272.4 ;
156/380.9 |
International
Class: |
B32B 37/26 20060101
B32B037/26 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 14, 2004 |
SE |
0403038-3 |
Claims
1. Method for sealing a first packaging material laminate to a
second packaging laminate, at least the first laminate comprising
at least one layer of magnetizable particles and a sealable layer,
the method comprising: providing an alternating magnetic field to
the first and second packaging laminates in a sealing zone to
generate magnetic hysteresis losses in the laminate comprising the
magnetizable particles and create heat substantially melting the
sealable layer in the sealing zone, and applying a sealing pressure
to the first and second packaging laminates to press together the
first and second packaging laminates in the sealing zone, thereby
sealing the first and second packaging laminates to each other.
2. The method according to claim 1, comprising providing the
alternating magnetic field in such a way that a main direction of
the magnetic field lines is substantially parallel with a plane
constituting the first packaging material laminate.
3. The method according to claim 1, comprising generating the
alternating magnetic field of a strength substantially large enough
to make the magnetizable particles substantially reach the magnetic
saturation level.
4. The method according to claim 1, comprising alternating the
magnetic field with a frequency in the interval 0.5-5 MHz.
5. The method according to claim 1, wherein the magnetizable
particles comprise Fe.sub.3O.sub.4 particles.
6. The method according to claim 1, comprising providing said
alternating magnetic field by at least a sealing jaw, said sealing
jaw being an inductor comprising a conductor connected to an
alternating current supply.
7. The method according to claim 1, comprising enhancing said
magnetic field by using an electrically conducting anvil.
8. The method according to claim 7, comprising providing said anvil
opposed to the sealing jaw, the anvil being passive and inducing a
current in response to the current in the sealing jaw, thereby
generating a magnetic field enhancing the field generated by the
sealing jaw.
9. The method according to claim 7, comprising applying said
sealing pressure by said sealing jaw and said anvil.
10. Device for sealing a first packaging material laminate to a
second packaging laminate, at least the first laminate comprising
at least one layer of magnetizable particles and a sealable layer,
the device comprising: means for providing an alternating magnetic
field to the first and second packaging laminates in a sealing zone
to generate magnetic hysteresis losses in the laminate comprising
the magnetizable particles and create heat substantially melting
the sealable layer in the sealing zone, and means for applying a
sealing pressure to the first and second packaging laminates to
press together the first and second packaging laminates in the
sealing zone, thereby sealing the first and second packaging
laminates to each other.
11. The device according to claim 10, wherein said means for
providing the alternating magnetic field is a sealing jaw formed as
an inductor comprising a conductor connected to an alternating
current supply.
12. The device according to claim 11, comprising an electrically
conducting anvil to enhance said magnetic field.
13. The device according to claim 12, wherein said anvil is
provided with a conductor adapted to induce a current in response
to the current in the sealing jaw, thereby generating a magnetic
field enhancing the magnetic field generated by the sealing
jaw.
14. The device according to claim 12, wherein said anvil is adapted
to be positioned opposed to the sealing jaw, cooperating with the
sealing jaw.
15. The device according to claim 12, wherein said means for
applying said sealing pressure is said sealing jaw and said
anvil.
16. The device according to claim 12, wherein the sealing jaw
comprises an action face adapted to bear against one of the first
and second packaging material laminates in the sealing zone during
sealing, and the anvil comprising a substantially corresponding
action face adapted to bear against the other of the first and
second packaging material laminates during sealing.
17. The device according to claim 16, wherein the conductor of the
sealing jaw is substantially embedded in the action face of the
sealing jaw in such a way that the conductor is adapted to be in
contact with the one of the first and second packaging
laminates.
18. The device according to claim 17, wherein the conductor of the
anvil is substantially embedded in the action face of the anvil in
such a way that the conductor is adapted to be in contact with the
other of the packaging laminates.
19. The device according to claim 18, wherein a protective layer is
provided on the action face of the anvil so that said contact
between the conductor of the anvil and the one of the first and
second packaging laminates is indirect.
20. The method according to claim 1, comprising alternating the
magnetic field with a frequency in the interval 1-4 MHz.
Description
THE FIELD OF INVENTION
[0001] The present invention refers to a method and a device for
magnetic hysteresis sealing of a packaging material laminate
comprising at least one layer comprising magnetizable
particles.
BACKGROUND OF THE INVENTION
[0002] In the international patent publication WO 03/095198, which
is hereby incorporated by reference, a packaging material laminate
is described which comprises at least one layer comprising
magnetizable particles. The laminate is of the type used for
manufacturing of for example liquid food packages, and generally
comprises a layer of paper or carton, layers of plastic and
barriers, such as for example oxygen barriers. One of the outer
layers is normally a sealable layer of a thermoplastic material
which is used when sealing one laminate to another. Using
thermoplastic layers is known in the art and will not be further
described herein.
[0003] The magnetizable particles can for example be magnetite,
Fe.sub.3O.sub.4 and have a mean size of about 0.5 .mu.m. Other
materials and particle sizes can of course also be used. There
exist other materials, such as for example magnetite
Fe.sub.2O.sub.3, as well as other particles sizes (larger and
smaller). Some may give higher seal heating power. However, care
should be taken when choosing particles. Some particles can not to
be used in food packaging due to legislation; others involve higher
costs due to their manufacturing. At present, smaller particles
than 0.5 .mu.m need an expensive chemical manufacturing process,
whereas larger particles can be manufactured mechanically by
sifting.
[0004] The magnetizable particles are dispersed in any of the
layers of the packaging material laminate, preferably in one of the
plastic layers. Alternatively, they can be applied in a printing
ink or a hot melt, which in turn is applied to the packaging
material in for example a sealing zone, as described in the Swedish
applications No. 0501409-7 and No. 0501408-9.
[0005] A packaging material laminate that comprises magnetizable
particles can be sealed to another packaging material laminate
using heat generated by magnetic hysteresis losses. By applying an
alternating magnetic field near a sealing zone of the first and
second laminates the magnetic material will be magnetized according
to the hysteresis loop in FIG. 1a. The vertical axis represents the
magnetic moment B in the material and the horizontal axis
represents the applied magnetic field H. The area enclosed by the
loop represents the energy which is generated in the material due
to the magnetizable particles. Since this is the energy that will
be used to melt the outer sealable layers of the laminates to
thereby create the seal, it is understood maximizing the hysteresis
loop area will optimize the sealing process.
[0006] Generally, energy and sealing time are two parameters for
controlling a sealing process. If the sealing energy is reduced,
the sealing time will have to be increased, and vice versa. The
same applies for magnetic hysteresis sealing. The larger the
hysteresis loop area can be made, the shorter sealing time is
needed (provided that the amount of particles in the laminates is
the same). In a high speed packaging machine the sealing time is
crucial. Then, if magnetic hysteresis sealing should be considered
as a possible alternative to other sealing techniques, such as for
instance induction sealing or ultrasonic sealing, the sealing time
needed ought not to exceed the time required by the other
techniques.
[0007] The hysteresis energy generation can be controlled in
substantially two ways.
[0008] One way is to increase the hysteresis loop area. It can be
accomplished by increasing the applied magnetic field H until a
level of magnetic saturation is reached in the particles of the
laminates. The magnetic saturation level S is shown in FIG. 1b.
However, increasing the magnetic field H above the saturation level
S will not extend the hysteresis loop area.
[0009] Another way is to increase the frequency of the applied
alternating magnetic field. Each cycle gives rise to energy
generation corresponding to the hysteresis loop area, and by
increasing the number of cycles per time unit the total amount of
energy generated is increased. Hence, as an example, 1 Hz gives an
energy contribution of one loop area per second, whereas 2 Hz will
give a double energy contribution per second.
[0010] In this context it should be mentioned that authorities
regulate the amount of electromagnetic radiation that can be
emitted and which frequency bands are open for public use. In some
frequency band intervals the use is restricted. In Europe this is
presently controlled by the EMC Directive.
[0011] As mentioned above there are regulations on the amount of
radiation that can be emitted. If exceeding the allowed values the
device or machine needs to be shielded off from the surrounding
environment. Such shielding is generally accomplished by
mechanically encapsulating the device or machine in which the high
frequency is used. However, it is known that emissions from high
frequency devices are more difficult to shield off since they are
more likely to slip out through any tiny opening in the shield.
Thus, for practical and economic reasons it is preferred to use low
frequencies. At present, an interval between 0.5-5 MHz is
preferred. Hence, it is important to be able to optimize the
hysteresis loop area not to be forced to increase the
frequency.
SUMMARY OF THE INVENTION
[0012] An object of the invention has been to find an efficient and
practical way of using magnetic hysteresis for sealing packaging
material laminates in a high speed packaging machine. Another
object is to achieve a sealing technique by which overheating of
the packaging material laminates is prevented.
[0013] These objects have been achieved by a method comprising
providing an alternating magnetic field to the laminates in a
sealing zone, thereby generating magnetic hysteresis losses in the
laminate comprising the magnetizable particles, which losses create
heat substantially melting the sealable layer in the sealing zone,
and applying a sealing pressure to the first and second laminate,
which pressure causes the first and second laminate to be pressed
together in the sealing zone, thereby sealing the laminates to each
other.
[0014] Applying a magnetic field to packaging material laminates
comprising magnetizable particles is effective and has been found
to be an equally good sealing technique compared to the techniques
normally used in packaging machines. Further, the use of magnetic
hysteresis sealing prevents overheating of the packaging material
laminates. This is due to the fact that the ferromagnetic
properties of the magnetizable particles slowly start to cease when
the laminates are heated. Thus, during temperature raise the
hysteresis loop area will decrease. Consequently, the energy
generated in the material will also decrease. Such energy decrease
will in turn also decrease the heat generated in the material.
Accordingly, since the heat is decreasing, the magnetization of the
particles can increase again, and can increase until the heat
generated in the material makes it start decreasing again. Thus, a
system has been formed in which the temperature will fluctuate
within a certain range, but will never raise above it. By a
suitable choice of magnetizable particles, particle amounts and
packaging material laminate structures the risk of overheating is
eliminated.
[0015] In a presently preferred embodiment the method comprises the
step of providing the alternating magnetic field in such a way that
the main direction of the magnetic field lines is substantially
parallel with a plane constituting the first packaging material
laminate. In this way a magnetic field is generated in the sealing
zone which magnetic field is sufficient to achieve a sealing time
and a frequency that are industrially applicable in a high speed
packaging machine. It has been found that the area of a hysteresis
loop where the main direction of the magnetic field lines is
substantially parallel with the laminate plane is substantially
equal to the area of a hysteresis loop where the main direction of
the magnetic field lines is substantially perpendicular to the
laminate plane. The difference is that the magnetic field needed to
obtain the area in the perpendicular case is higher, in fact almost
twice as high. Thus, the parallel case would seem more
efficient.
[0016] In another presently preferred embodiment the method
comprises the step of generating an alternating magnetic field of a
strength substantially large enough to make the magnetizable
particles substantially reach the magnetic saturation level. As
described above, the larger the hysteresis loop area can be made,
the more energy is produced in the material. Since the area is
increasing up to the magnetic saturation level of the material, it
is preferable to apply a magnetic field large enough to make the
material reach that level. However, above the saturation level the
area will not increase, and therefore there is no value of applying
a stronger magnetic field.
[0017] In yet another presently preferred embodiment the method
according to the invention comprises the step of providing said
alternating magnetic field by at least a sealing jaw, the sealing
jaw being an inductor comprising a conductor connected to an
alternating current supply. This is good from an economic point of
view since the basis of conventional inductors used for induction
sealing can be used.
[0018] In a further presently preferred embodiment the method
comprises the step of enhancing said magnetic field by using an
electrically conducting anvil. In yet a further embodiment the
method comprises the step of providing said anvil opposed to the
sealing jaw, the anvil being able to induce a current in response
to the current in the sealing jaw, thereby generating a magnetic
field enhancing the field generated by the sealing jaw. The
parallelism of the magnetic field lines is increased and a stronger
magnetic field can be obtained without having to increase the
current supplied to the inductor of the sealing jaw.
[0019] Further presently preferred embodiments are described in the
additional appending dependent method claims.
[0020] The present invention also comprises a device which is
characterized in that it comprises means for providing an
alternating magnetic field to the laminates in a sealing zone,
thereby generating magnetic hysteresis losses in the laminate
comprising the magnetizable particles, which losses create heat
substantially melting the sealable layer in the sealing zone, and
means for applying a sealing pressure to the first and second
laminate, which pressure causes the first and second laminate to be
pressed together in the sealing zone, thereby sealing the laminates
to each other.
[0021] In at least one of the presently preferred embodiments of
the device said means adapted to provide the alternating magnetic
field is a sealing jaw in the form of an inductor comprising a
conductor connected to an alternating current supply and that the
means for applying said sealing pressure is said sealing jaw and an
anvil. In this way the magnetic field and the pressure is applied
by the same means, and the means are substantially conventional
equipment used for induction sealing. This is advantageous from an
economic point of view.
[0022] In another presently preferred embodiment the anvil is
electrically conducting anvil and provided to enhance said magnetic
field. Said anvil is provided with a conductor adapted to induce a
current in response to the current in the sealing jaw, thereby
generating a magnetic field enhancing the magnetic field generated
by the sealing jaw.
[0023] Additional presently preferred embodiments are described in
the appended dependent device claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] In the following, a presently preferred embodiment of the
invention will be described in greater detail, with reference to
the enclosed drawings, in which:
[0025] FIG. 1a schematically shows a hysteresis loop,
[0026] FIG. 1b schematically shows a hysteresis loop where the
applied magnetic field is higher than the magnetic saturation level
of the magnetizable particles,
[0027] FIG. 2 schematically shows two packaging material laminates
to be sealed together in a begging joint by means of a sealing jaw
and an anvil,
[0028] FIG. 3a schematically shows a view of the action face of the
sealing jaw shown in FIG. 2,
[0029] FIG. 3b schematically shows a cross section through said
sealing jaw,
[0030] FIG. 4 schematically shows a view of the action face of the
anvil shown in FIG. 2,
[0031] FIG. 5 schematically shows a cross section of the sealing
jaw, the packaging material laminates, the anvil and the magnetic
fields in the sealing zone,
[0032] FIG. 6a shows a hysteresis loop where the magnetic field is
applied substantially parallel to the plane of the packaging
material laminate,
[0033] FIG. 6b shows a hysteresis loop where the magnetic field is
applied substantially perpendicular to the plane of the packaging
material laminate, and
[0034] FIG. 7 schematically shows a blank formed as a sleeve by
means of a longitudinal seal.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0035] FIG. 2 shows a presently preferred embodiment of the
invention. A first and a second packaging material laminate 10, 12
to be sealed together in a joint by means of a sealing jaw 14 and
an anvil 16. In the joint shown the two laminates are abutting each
other with their inside surfaces facing each other. In this
presently preferred embodiment the sealing jaw 14 is an inductor
similar to the ones used for induction sealing (where the laminate
comprises aluminum foil that generate heat). The inductor 14 is
here coupled to an alternating current supply 18. The alternating
current is preferably in the range of 75-300 A and the power needed
from the power supply is a few kW. A preferred interval is 2-10 kW.
The frequency is preferably in the MHz range, and a preferred
frequency interval is 0.5-5 MHz. A most preferred interval is 1-4
MHz. The frequencies that are prohibited for common use due to
authority regulations are of course, in practice, excluded from
said intervals.
[0036] The inductor 14 comprises an insulator 20 having an action
face 22 which will abut the laminate in the sealing zone during
sealing, see FIG. 3a. In the action face 22 a conductor 24 is
embedded, said conductor 24 being provided to be in contact with
the laminate during sealing, see FIG. 3b. The conductor 24 is
manufactured of an electrically conducting material, i.e. a
material with low resistivity and is preferably provided with
cooling channels. Preferably, the conductor 24 can be manufactured
from copper. Further, the conductor 24 has the form of an open loop
where each respective end is connected to the alternating current
supply 18, see FIG. 3a. The entire loop is adapted to be in contact
with the packaging material laminate, i.e. the plane of the loop is
substantially parallel to the plane of the packaging material
laminate. Further, the loop is elongated and extends along the
longitudinal extension of the sealing jaw 14. The opening in the
loop where the current connections are placed is positioned in one
end of the elongated sealing jaw 14.
[0037] The insulator 20 on the other hand is manufactured from an
insulating, non-conducting material with or without magnetic
permeability characteristics. Preferably, plastic materials or
ceramics can be used. To enhance the intensity of the magnetic
field and to direct the field lines a magnetic permeable material
can be provided in the insulator 20. One way is to provide the
insulator 20 with ferrite powder. The powder can be added during
moulding of the plastic or ceramic insulator. Another way is to use
inserts 26 of a material with a permeability value within the
following range: .mu.=10-2500. Preferably, inserts 26 of for
example Ferrotron.TM., or materials with values in the upper part
of the range, can be provided in the insulator 20 near the
conductor 24. The technique is similar to that within the induction
sealing technology.
[0038] During sealing the sealing jaw 14 is cooperating with an
anvil 16, see FIG. 2. In the example the anvil 14 is electrically
conducting, but passive, i.e. not connected to a power supply. An
embodiment of the anvil 16 will be described with reference to FIG.
2 and FIG. 4. The anvil 16 has a corresponding action face 28
adapted to be facing the action face 22 of the sealing jaw 14
during sealing, compare FIG. 3b. Further, said anvil 16 is made
from an insulator 30, which insulator 30 is provided with a
conductor 32. Said conductor 32 is made of an electrically
conducting material, i.e. a material with low resistivity,
preferably copper. It is embedded in the action face 28 of the
insulator 30 in such a way that the conductor 32 is adapted to be
in contact with the packaging material laminate. Alternatively, the
conductor 32 can be covered by for example a layer of rubber, at
least in the action face 22, to protect the packaging material
laminate from direct contact with the conductor. Further, the
conductor 32 has the form of a closed loop. The entire loop is
adapted to be in contact with the packaging material laminate, i.e.
the plane of the loop is substantially parallel to the plane of the
packaging material laminate. Further, the loop is elongated and
extends along the longitudinal extension of the anvil. The
insulator 30 of the anvil 16 can be similar to the insulator 20 of
the sealing jaw 14. It is manufactured from an insulating,
non-conducting material with or without magnetic permeability
characteristics. Preferably, plastic materials or ceramics can be
used. A magnetic permeable material can be provided in the
insulator 30. Either the insulator is provided with ferrite powder
(added during moulding of the plastic or ceramic insulator), or
inserts (not shown) of a material with a permeability value are
used. The insert material can have a permeability value in the
range of .mu.=10-2500. Preferably, inserts of for example
Ferrotron.TM. can be used.
[0039] The sealing jaw 16 and the anvil 14 are provided to apply a
sealing pressure to the packaging material laminates pressing them
together in a sealing zone. The way of applying the pressure is
known in the art and will not be described further herein.
[0040] When sealing together two packaging material laminates 10,
12 a sealable layer 34 of the first laminate 10 is placed facing
the other laminate 12. Then, the sealing jaw 14 and the anvil 16
press the laminates 10, 12 towards each other. An alternating
current is thereafter supplied to the conductor 24 of the sealing
jaw 14. The current generates a magnetic field in the sealing zone
of the laminates 10, 12. The magnetic field lines will be
substantially parallel to a plane 36 of the laminates 10, 12. The
direction of the field lines will be further described below. Said
magnetic field affects the magnetizable particles in the laminate,
which has been described in the introduction, and the energy from
the hysteresis losses melts the sealable layer 34. The energy from
the hysteresis losses will be in the range of 5-50 Joule, probably
around 10 Joule.
[0041] During operation a current is induced in the anvil 16
because of the inductor in the sealing jaw 14 on the other side of
the packaging material laminates 10, 12. The current that is
induced generates a magnetic field which will enhance the strength
and direction of the magnetic field applied by the inductor of the
sealing jaw 14.
[0042] In this embodiment the sealing pressure is applied
substantially simultaneously as the magnetic field is applied. This
means that the sealing pressure may be applied at the same time as
the magnetic field, or slightly afterwards or slightly before.
Preferably, the sealing pressure is applied before the magnetic
field is applied. Alternatively, in some applications, the magnetic
field and the pressure may be applied separately (by separate
means) and in sequence. In a first step the magnetic field may be
applied and when it has heated the laminate, the sealing pressure
is applied in a second step. Different means can be used to apply
the magnetic field and the sealing pressure.
[0043] When the sealing zone has been sealed, i.e. the sealable
layer 34 has been melted, the application of magnetic field is
ceased. Preferably, the sealing pressure is maintained for a short
period of time for cooling purposes. This period of time may be in
the range of 100-200 ms. The cooling procedure is known from other
sealing technologies.
[0044] It has been found that the dissipated energy from the
hysteresis losses is directly proportional to the frequency.
Further, it has been found that the relation between the frequency
and the sealing time is substantially linear. Thus, the sealing
time will have to be increased if the frequency is decreased. In
addition, it has been found that the relation between the particle
concentration and the sealing time is substantially linear. Hence,
a larger amount of particles in the laminate will reduce the
sealing time and vice versa.
[0045] Magnetizable particles that can be used are magnetite,
Fe.sub.3O.sub.4. Trials have been made with a mean size of the
particles of about 0.5 .mu.m (particles from Hoganas, X-MP4). The
results are positive. Other materials and particle sizes can of
course also be used. However, care should be taken when choosing
particles. Some kinds are can not to be used in food packaging due
to legislation; others involve higher costs due to their
manufacturing. At present, manufacturing of smaller particles than
0.5 .mu.m need a more complicated manufacturing process.
[0046] A sample has been made in which magnetic particles of the
described type (Fe.sub.3O.sub.4, particle size about 0.5 .mu.m) are
comprised in a polyethylene (PE) layer in each respective laminate.
The amount of the magnetic particles in the PE layer is about 17
g/m.sup.2. Using a frequency of 2 MHz and applying a magnetic field
strength close to the magnetic saturation level, the sealing time
for properly sealing together the two laminates 10, 12 will be
approximately 100 ms. It should be understood that other amounts of
the magnetic particles, as well as other sizes of the particles and
other packaging material laminates will require different sealing
times and/or different frequencies.
[0047] FIG. 5 schematically shows a cross section of the sealing
jaw 14, the packaging material laminates 10, 12, the anvil 16 and
the magnetic fields in the sealing zone. Due to the cross section
the conductor 24 of the sealing jaw 14 and the conductor 32 of the
anvil 16 are shown as two circles each. In the circles the
momentary direction of the current is shown. The arrows represent
magnetic field lines and it can be seen that the main field lines
near the packaging material laminates 10, 12 are substantially
parallel with the plane 36 of the packaging material laminates 10,
12. Further, it can be seen that the contribution from the anvil 16
is enhancing the magnetic field generated by the sealing jaw 14.
The arrows representing the magnetic field from the anvil 16 are
also substantially parallel with the plane 36 of the packaging
material laminates 10, 12 and are directed in the same direction as
the magnetic field lines of the sealing jaw 14.
[0048] In FIG. 6a and FIG. 6b hysteresis loops are shown. FIG. 6a
describes the case above, i.e. where the magnetic field is applied
in parallel with the plane of the packaging material laminate,
whereas FIG. 6b describes a case where the magnetic field is
applied perpendicular to the plane of the packaging material
laminate. It can be seen that the two areas are substantially
similar in size, but that the magnetic field needed two obtain the
area in FIG. 6b is higher, in fact almost twice as high. Thus, it
can be concluded that it is more efficient to apply the magnetic
field substantially parallel to the packaging material
laminate.
[0049] The wording "parallel with the plane of the packaging
material laminate" should include also the case where the packaging
material is curved in the sealing zone. The magnetic field lines
should then be directed to follow the curve, i.e. be substantially
parallel to the respective tangent of the points in the curve.
[0050] The invention has now been described according to a
presently preferred embodiment of the invention. However, it should
be understood that the invention is not limited to this embodiment,
but could be modified in any way within the scope of the enclosed
claims.
[0051] For example, in the embodiment a first and a second laminate
10, 12 have been described. However, it should be understood that
the first and second laminate can be a first and second portion 10,
12 of the same laminate. For instance, a rectangular blank or a web
is to be formed into a sleeve or a tube and sealed along two
longitudinal edges in an overlapping joint area 38. The first
laminate 10 will then constitute a portion of the blank along the
first edge, whereas the second laminate 12 will constitute a
portion of the blank along the second edge. FIG. 7 is showing a
blank being formed into a tubular sleeve. The areas 38 will later
create an overlap which will be sealed.
[0052] The sealing method can be used when sealing joints like the
one shown, i.e. joints where the two laminates are abutting each
other with their inside (or outside) surfaces facing each other. It
can also be used for sealing overlapping joints where an outside
surface of one of the laminates are abutting an inside surface of
the other laminate forming an overlap.
[0053] In this description the sealing pressure and the magnetic
field are applied by one and the same device, i.e. the pair of
sealing jaw 14 and anvil 16. However, it should be understood that
the pressure and the field could be applied by different devices,
i.e. the field and pressure being applied separately.
[0054] In the described embodiment the sealing jaw 14 is an
inductor comprising a conductor 24 connected to an alternating
current supply 18. The described anvil 16 is electrically
conducting and passive, i.e. is not connected to any power supply,
but comprises a conductor 32 that is arranged to be able to induce
a current in response to the current in the conductor 24 of the
sealing jaw 14. However, it should be understood that the anvil 16
could be of the electrically conducting type, but instead be
active. It will then be of the same type as the sealing jaw 14,
i.e. connected to an alternating current supply. The sealing jaw 14
and the anvil 16 could be connected to the same control and power
to supply system, or be connected to separate systems.
[0055] In another embodiment the anvil could be constituted as a
conductor in the form of a copper plate. The copper plate could
have direct contact with the packaging material laminate or be
covered by for example a layer of protective rubber. The rubber
will be provided at least in between the conductive copper plate
and packaging material laminate, i.e. the action face of the anvil
will be in rubber, and the packaging material will have indirect
contact with the conductive plate.
[0056] Alternatively, the anvil 16 could be manufactured without
any conducting capabilities, i.e. the anvil 16 would be an
insulator and made of for example rubber. However, then it would of
course not be able to enhance the magnetic field.
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