EP2040512A2 - Device and method for inductive heating of an electrically conductive workpiece - Google Patents

Abstract

The device has a U-shaped magnet core (3) with two thighs (4). An electrically conducting coil (5) is arranged on one of the thigh of the U-shaped magnet core and is connected to an alternate current source. A magnet yoke (6) is arranged at distance of free standing end thigh of the U-shaped magnet core, so that a closed magnetic circuit with an air gap (7) is formed. The size of the air gap is selected in such a manner that the electrically conducting workpiece (2) is feasible contact less by the air gap. An independent claim is included for a method for inductive heating of an electrically conducting workpiece.

Description

The invention relates to an apparatus and a method for inductive heating of an electrically conductive workpiece, which is formed in the form of a closed loop, according to the preamble of the independent claims. By a workpiece formed in the form of a closed loop is meant any workpiece that includes an opening, such as an annular workpiece or an ellipsoidal, square or rectangular workpiece that encloses an opening, which may also be ellipsoidal, square or rectangular can. In particular, the workpiece may be a lid blank with an enclosed opening as used to make a lid with a tear-open film.

From the patent application WO 2006/042426 A1 For example, a method and apparatus for making covers with a film are known. The known device has a conveying device and a plurality of processing stations and a test station. The conveyor conveys lid blanks to the individual processing stations or to the testing station. In a first processing station, a lid blank with an enclosed opening is created by stamping an opening into a disk by punching. The inner edge of the lid blank is pulled down in the next processing station. In the following processing station, a tabbed film is placed over the enclosed opening of the lid blank and secured by heat sealing. For this purpose, the film is provided on its underside with a plastic layer and is punched out of a wide film web. It is then trapped above the Opening, which is also referred to as a central recess, placed and pressed under heat at the edge of the enclosed opening, so that the film is sealed to the lid blank by melting and subsequent cooling of the plastic layer. There may be provided an additional processing station for cooling. Then, the film is provided in a further processing station with an embossing and now located below the film inner edge of the lid blank is flanged. Finally, the now finished covers are subjected to a test in a test station, which includes a density test for the applied film on the lid.

As in the patent application WO 2006/053457 A2 described, the film can be sealed or glued to the cover ring by partially performed in a first step, the bonding or sealing and completed in a second step, with this second step, the connection between the film and cover blank is strengthened overall. The gluing seal is done by heat. The partial bonding and sealing is produced in that the bonding or sealing takes place only in predetermined partial areas and / or is not completely formed by the lack of heat. The temperature during bonding or sealing is preferably approximately 200 ° C. in the first step and / or in the second step.

Based on the principle of induction heating, it is known to generate heat in an electrically conductive workpiece by means of an inductor, which typically comprises a U-shaped magnetic core with two legs, with an electrically conductive coil wound around each leg to which an electrical power source connected. The wound with the coils legs of the U-shaped magnetic core represent the poles. The inductor forms an open magnetic circuit. To heat a workpiece, the poles are held close to the surface of the workpiece to be heated. With alternating current fed in, a magnetic alternating field is created under whose influence high-frequency eddy currents are generated on the surface of the workpiece, so that heat is generated in the form of eddy current losses (so-called skin effect). This is used for example for heating a can wall (see US 5,690,851 A ). Corresponding inductors are for example also from the patents US 5,101,086 A and in US Pat. No. 7,022,951 B1 known. The thinner-walled a workpiece, the greater must be the frequency of the AC power source and thus the generated alternating magnetic field, so that the eddy currents generated in the workpiece a sufficiently large electrical resistance is opposed, so that sufficient heat can be generated by eddy current losses.

From the patent US 4,740,663 A For example, an induction heating unit is known, by means of which electrical energy can be induced in the metallic periphery of a metal foil carrier of a lid for a container, so that the lid is heated and heat-bonded to the container. The induction heating unit has a magnetic core with two oppositely arranged U-shaped sections, wherein a coil is arranged within a section. By means of the opposite legs of one side of the magnetic core, the lid is clamped, while there is an air gap between the opposite legs of the other side. There is a closed magnetic circuit, with two opposing legs forming a pole. In the lying between the legs of a page lid is caused by eddy currents and Ummagnetisierungsverluste a partial heating. The cover, the two legs of the one pole or the one side of the magnetic core with each other connects, so to speak, the magnetic field between the two poles of the magnetic circuit short.

The known devices for inductive heating, which are also referred to as inductors, are supplied with high-frequency energy and typically a separate inductor with suitable dimensions is required for each workpiece of a certain dimension. Even different materials often require a different type of inductor, as is the case, for example, with workpieces made of aluminum compared to workpieces made of a steel alloy, since this material has a particularly strong influence on the coupling. If the inductor is to be used in a device with a conveying device, then it can occur in the known inductors that the typically metallic guides come into operative connection with an inductor arranged close to the workpiece and are likewise heated.

It is an object of the present invention to provide an apparatus and method for inductive heating, with which electrically conductive workpieces, which are formed in the form of a closed loop, different materials and different dimensions or sizes can be heated uniformly.

By a workpiece formed in the form of a closed loop is meant a workpiece having an enclosed opening, ie an opening completely surrounded by a rim, such as annular or elliptical, square or rectangular workpieces an enclosed opening, which is for example circular, elliptical, square or rectangular, have. Such workpieces are, for example, as lid blanks for the production of lids, which are provided with a film, in particular a tear-open film or a release liner, ie with a film which has a tab for tearing or peeling wherein the film as described above can be applied to the lid blank by sealing or gluing.

The object is achieved by an apparatus for inductive heating with the features of claim 1 and by a method for inductive heating with the feature of claim 10. The inventive device for inductive heating can also be referred to as an inductor.

The inventive device for inductive heating comprises a U-shaped magnetic core with two legs, wherein at least one electrically conductive coil is arranged on at least one leg of the U-shaped magnetic core, which is connectable to an AC power source and which in particular surrounds the leg. The device according to the invention is characterized in that a magnetic yoke is provided, which is arranged at a distance from at least one freestanding end of a leg of the U-shaped magnet core, so that a closed magnetic circuit is formed with at least one air gap. The height of at least one air gap, preferably both air gaps, is selected such that an electrically conductive workpiece can be performed without contact through the air gap. Under a U-shaped magnetic core and a C-shaped magnetic core is understood. The magnetic core may in particular be a ferrite core.

According to a preferred embodiment of the device according to the invention, at least one electrically conductive coil is arranged on each leg of the U-shaped magnetic core, wherein the coils are wound in opposite directions and can be connected to an AC power source. Further, the magnetic yoke is spaced from the free-standing ends of both legs of the U-shaped magnetic core, so that a closed magnetic circuit is formed with two air gaps.

The area or cross-sectional area of at least the leg, to which the at least one air gap through which the electrically conductive workpiece is passable, is preferably designed such that it fits into the opening enclosed by the electrically conductive workpiece. In this way it is achieved that the workpiece can be positioned in the air gap such that it projects beyond the air gap on all sides, i. surrounds the air gap. The workpiece is thus in inductive heating so outside the air gap and thus outside a zone of high magnetic field strength.

The inventive method is characterized by the fact that in an inventive device, an electrically conductive workpiece is introduced in the form of a closed loop without contact in the air gap, so that the workpiece surrounds the air gap and an enclosed by the workpiece opening is at least partially in the air gap and that the Coils of the inventive device are supplied with an alternating current, ie be connected to an AC power source. The enumeration of the steps of the method according to the invention should in this case not represent a chronological order, so that the coils are preferably also supplied with an alternating current even before the tool is introduced into the air gap.

In a production operation, the device according to the invention is preferably operated in continuous operation, so that the coils of the device according to the invention are permanently supplied with alternating current. In this way, a difficult control and a settling delay of the device according to the invention can be avoided. The workpiece or its edges are then at least partially exposed to the prevailing in the air gap alternating field when introduced into the air gap, which is negligible at correspondingly small cycle times.

The method according to the invention and the device according to the invention can be used particularly advantageously for heating flat workpieces, wherein a flat workpiece also means a thin workpiece. The device according to the invention and the method according to the invention are advantageously suitable for penetrating a workpiece which also encloses the air gap over a wide distance.

The AC power source may also be referred to as a generator. It is preferably a low-frequency and thus inexpensive AC source used. Furthermore, the use of a low frequency has the advantage that the coil losses are smaller and long leads can be used. That a low frequency can be used as the operating frequency is made possible in particular by the fact that the method according to the invention and the device according to the invention are essentially not based on the skin effect.

The fact that the device according to the invention is preferably operated at a relatively low frequency, which is preferably between 18 and 40 kilohertz, makes it possible to position and operate the device even at a greater distance from the AC power source without using a transformer, for example at a distance of 20 meters ,

wobei U_ind die Induktionsspannung, Φ _magn der Magnetfluss, t die Zeit, B die Flussdichte, A die vom Magnetfeld durchdrungene und von dem Werkstück eingeschlossene Fläche als Konstante und f die Fläche als Variable sind. Von dem Magnetfluss Φ _magn wird angenommen, dass er sich mit der Zeit t ändert. Bei der angegebenen Gleichung ist die Wicklungszahl bzw. Windungszahl der Spulen bereits in dem Magnetfluss berücksichtigt bzw. (mit-)enthalten.When an AC power source is connected to the coils, there is a change in the magnetic field strength of the magnetic circuit formed by the U-shaped magnetic core, the magnetic yoke and the coils, resulting in a change in the flux density. As a result of this change in the flux density, an induction voltage is induced in the workpiece in accordance with the following law of induction, which is essentially independent of the dimensions of the workpiece, in particular the diameter of an annular workpiece. $${\mathit{U}}_{\mathit{ind}}=\frac{d{\mathrm{\Phi }}_{\mathit{magn}}}{dt}=-\frac{d}{dt}{\int }_A\tilde{B}\cdot \widetilde{\mathit{df}}$$

where U _{ind is} the induction voltage, Φ _{magn is} the magnetic flux, t is the time, B is the flux density, A is the area _{traversed by} the magnetic field and enclosed by the workpiece as a constant and f is the area as a variable. The magnetic flux Φ _magn is assumed to change with time t. In the given equation, the winding number or number of turns of the coils is already taken into account in the magnetic flux or included.

The electrical resistance, which may also be referred to as impedance or alternating current resistance, of the electrically conductive workpiece in the form of a loop is greater than zero, so that an induction voltage is induced. In an annular workpiece, the electrical resistance is also referred to as ring impedance. The electrical resistance results in an electric current in the workpiece, which leads to a uniform or homogeneous heating or heating of the workpiece. Advantageously, in the device according to the invention and in the method according to the invention, the heat generation in the workpiece is essentially not effected by skin effects on a surface of the workpiece or by re-magnetization losses.

By means of the device according to the invention and the method according to the invention, workpieces of different dimensions or size which surround the air gap can be heated uniformly and substantially independently of the distance between the device or its air gap and the workpiece when the workpiece surrounds the air gap. The same device according to the invention can be used for workpieces of different dimensions. The geometry of the inventive Device or of the magnetic alternating field generated by the inventive device does not have to be adapted to the workpiece, as long as the workpiece encloses the air gap.

The device according to the invention can be used for example for heating a lid blank with an enclosed opening, in particular in or in a system for producing lids provided with a foil. Such a plant typically has a conveyor / transport system or system for conveying and guiding lid blanks and various processing stations for processing the lid blanks (see WO 2006/042426 A1 as described above) and is designed as a continuous system.

Accordingly, the method according to the invention can be used for heating a lid blank having an enclosed opening, in particular in or in a system for producing covers with foils. Since the device according to the invention can be formed essentially independently of the workpiece shape and size-apart from the size and the dimensions of the enclosed opening-the device according to the invention can be designed such that it does not interfere with the conveying device and / or the processing stations of a construction space Plant for the production of foil-providing lids comes into conflict. Since a closed magnetic circuit with two air gaps is formed by the device according to the invention, resulting in a substantially closed leadership of the magnetic field, spurious emissions are minimized and the conveyor is only exposed to a very weak magnetic field, so that heating of the conveyor practically avoided or can be neglected.

In the production of lids with a film, a blank lid before sealing or Bonding with a film advantageously by means of the inventive device and the inventive method heated in the context of preheating. After preheating, the film is then applied to the workpiece such that the enclosed opening of the workpiece is sealed by the film, preferably a pre-seal / pre-bond step and the pre-seal / pre-bond step, a master seal / bond step follow (cf. WO 2006/053457 A2 ). In the preheating by means of inductive heating, for example, annular or rectangular workpieces with an enclosed opening made of a steel alloy or aluminum in a system for producing lids with films, which has a conveyor, are heated in a few hundred milliseconds contactless by about 80 ° C uniformly. The sealing / gluing, both the pre-seal / -verklebung and the main sealing / gluing then preferably carried out at about 200 ° C. By inductive preheating the quality of the subsequent seal / bond can be increased and depending on the material of the film, the production rate can be increased because the additional preheating has a more efficient energy supply.

It is advisable in such a system for producing lids with films, in or in which the inventive device and the inventive method to be used to isolate the environment of the workpiece, in particular a conveyor, for example by the conveyor belts are made of plastic.

• Figur 1 zeigt eine schematische Schnittdarstellung einer erfindungsgemässen Vorrichtung mit einem Werkstück und
• Figur 2 zeigt einen Querschnitt durch die in Figur 1 dargestellte erfindungsgemässe Vorrichtung und das Werkstück an der Position des Werkstücks.

Further advantageous embodiments of the invention will become apparent from the dependent claims and the embodiments illustrated in particular with reference to the drawings. Show it:

• FIG. 1 shows a schematic sectional view of an inventive device with a workpiece and
• FIG. 2 shows a cross section through the in FIG. 1 illustrated inventive device and the workpiece at the position of the workpiece.

In the figures, like reference numerals designate structurally or functionally identical or equivalent components.

The FIGS. 1 and 2 show an inventive device 1, in which an electrically conductive, exemplary annular workpiece 2 is introduced. Of course, the workpiece 2 may also have another shape, for example a rectangular shape with an enclosed opening.

The device 1 comprises a U-shaped magnetic core 3. A U-shaped magnetic core 3 is also understood to mean a C-shaped magnetic core. The magnetic core 3 may comprise as material N87. Preferably, the magnetic core 3 is made of the material N87. On the legs 4 of the magnetic core 3, a coil 5 is applied in each case, wherein the coils 5 are wound in opposite directions. This is in the FIG. 1 clearly symbolized by the cross symbols and the dot symbols in the coils 5, wherein the point symbol symbolizes in a current flowing through a coil 5 out of the viewing plane, on the viewer stream and the cross symbol flowing into the viewing plane stream.

Each coil 5 is preferably formed by a high-frequency strand with a predetermined single strand arrangement comprising at least 500 individual strands, more preferably 1000 individual strands. Each coil 5 preferably has 14 turns.

The device 1 further comprises a magnetic yoke 6 (not shown in FIG FIG. 2 ), with the formation of air gaps 7 at a distance from the not closer designated, free-standing ends of the legs 4 of the U-shaped magnetic core 3 is arranged. The height of the air gaps 7 is selected such that the workpiece 2 can be positioned through an air gap 7 and thus can be positioned in the vertical direction between the magnetic core 3 and the magnet yoke 6. The air gaps 7 preferably have a height of 6.5 millimeters. The device 1 according to the invention thus forms a closed magnetic circuit with two air gaps 7.

The yoke 6 is preferably formed I-shaped. However, it may also be U-shaped, this also a C-shaped configuration is understood. In a U-shaped configuration, not shown, the free-standing ends of the legs of the magnetic yoke 6 in the direction of the free-standing ends of the legs 4 of the magnetic core 3, wherein the legs of the magnetic yoke 6 spaced to form the air gaps 7 opposite to the free-standing ends of the legs 4 of the magnetic core 3 are arranged. In particular, if only one air gap 7 is provided, the magnetic yoke 7 may also be L-shaped.

The magnetic core 3 and the coils 5 are preferably encapsulated with an electrically insulating potting compound to mechanically and chemically protect the magnetic core 3 with the coils 5. The potting compound used is particularly heat-conducting, preferably having a minimum thermal conductivity of 0.6 W / (m · K).

In order to increase the performance, the magnetic core 3 may be provided with a cooling, in particular with a water cooling (not shown).

If the coils 5 are connected to an unillustrated AC source or a generator, then in the inventive device 1, a magnetic alternating field 8. Each of the legs 4, around which a coil 5 is wound, represents a pole of a magnetic circuit, wherein the polarity of the respective pole varies depending on the polarity of the alternating current. In the FIG. 1 are exemplary field lines 8 indicated for the magnetic alternating field. In and near the air gaps 7 regions of high magnetic field intensity 9 arise.

The workpiece 2 is carried by an air gap 7 and positioned so that its enclosed opening 10 at least partially lies in the air gap 7 and the workpiece 2 itself surrounds the air gap 7, i. outside the air gap 7 is located. In this case, a region 11 of the workpiece lies outside the device 1, while a further region 12 lies in the horizontal direction between the two poles formed by the legs 4 wrapped with the coils 5 and in the vertical direction in the air gaps 7 between the magnetic core 3 and the magnet yoke 6 lies. Thus, the further region 12 lies in an area of low field strength between the two poles of the magnetic core 3. The workpiece 2 thus surrounds the alternating field guided in the magnetic core 3 and in the magnetic yoke 6 and a voltage is induced in the workpiece 2 by the alternating field, as described in the introduction, which is substantially independent of the dimensions of the enclosed opening 10 of the workpiece 2.

Thus, the field strength between the poles of the magnetic core 3 is as low as possible and outside the air gaps 7 at most low interference emissions occur, the air gaps 7 with the lowest possible height, in particular with a height of 6.5 millimeters, executed. Accordingly, the device according to the invention is particularly suitable for flat workpieces 2, i. suitable for workpieces 2 with a small vertical extent. As a flat workpiece 2 and a thin workpiece is considered.

The inventive device 1 and the workpiece 2 basically correspond to a transformer with a very large air gap, wherein the device. 1 the primary winding and the workpiece 2 represent a short-circuited secondary winding of the transformer.

The alternating current source or the generator for supplying the coils 5 preferably generates a current with a low frequency, in particular with a frequency in the range of 18 to 40 kilohertz, wherein the alternating current source or the generator may have a switched resonant circuit. The current generated by the AC source or generator preferably has a root mean square (rms) of about 10 to 25 amperes). If no workpiece 2 is introduced into the device 1, so that it is operated idle, this results in a decrease in the resonant frequency of the device 1, which is preferably connected to a capacitive AC power source, and the system of device 1 and not shown AC power source is automatically relieved.

According to another, not shown embodiment of the inventive device may be provided in addition to the first U-shaped magnetic core 3, a second U-shaped magnetic core, wherein the legs 4 of the first U-shaped magnetic core 3 associated coils 5 now also the legs of the other , second U-shaped magnetic core are assigned. The coils 5 surround in this embodiment, the legs of both U-shaped magnetic cores. The further, second U-shaped magnetic core preferably has the same dimensions as the first U-shaped magnetic core, in the in connection with the FIGS. 1 and 2 described embodiment 1 of the inventive device is provided. The other, second U-shaped magnetic core is also a magnetic yoke, which may be configured I-shaped or U-shaped, assigned, said second magnetic yoke in addition to the first magnetic yoke 6 in the FIGS. 1 and 2 1 embodiment of the inventive device is arranged, so that air gaps with a larger cross-section than in the inventive Device 1 with only one magnetic core 3 and only one magnetic yoke 6. Of course, the first magnetic yoke 6 may also be formed so broad that it fits over both magnetic cores or forms the air gaps with two magnetic cores.

With this, not shown embodiment of the inventive device workpieces with very large diameters and / or workpieces, which consist of a material with a low resistivity or include such material, are heated. Again, it is assumed that the workpieces have an enclosed opening which lies at least partially in an enlarged air gap in this embodiment and the workpiece surrounds the air gap. The available heating power is in this embodiment by the two magnetic cores, which are placed side by side and are wrapped by the same two coils, with respect to in the FIGS. 1 and 2 illustrated embodiment 1 of the inventive device increases. The necessary number of turns or number of turns of the coils can be reduced by the factor √2, so that instead of 14 turns / windings at the in the FIGS. 1 and 2 illustrated embodiment of the inventive device results in a number of turns / winding number of 9 or 10 per coil. By reducing the number of turns / number of turns required, copper losses are advantageously reduced (the coils typically comprise copper as the material), and the saturation effect of the magnetic core can be greatly reduced, so that the device according to the invention has lower heat losses. Because of the lower losses in the magnetic cores can be worked with a higher field strength of the alternating magnetic field.

Claims (12)

Apparatus for inductively heating an electrically conductive workpiece (2) formed in the form of a closed loop, comprising a U-shaped magnetic core (3) having two legs (4), on at least one leg (4) of the U-shaped one Magnet core (3) at least one electrically conductive coil (5) is arranged and connectable to an AC power source, characterized in that a yoke (6) is provided by at least one free-standing end of a leg (4) of the U-shaped magnetic core (3 ), so that a closed magnetic circuit with at least one air gap (7) is formed, wherein the height of the air gap (7), is selected such that the electrically conductive workpiece (2) by the air gap (7) is feasible without contact. Apparatus according to claim 1, characterized in that on each leg (4) of the U-shaped magnetic core (3) at least one electrically conductive coil (5) is arranged, wherein the coils (5) are wound in opposite directions and can be connected to an AC power source, and in that the magnetic yoke (6) is arranged at a distance from the freestanding ends of both legs (4) of the U-shaped magnet core, so that a closed magnetic circuit with two air gaps (7) is formed. Apparatus according to claim 1 or 2, characterized in that the surface of at least the leg (4), which is associated with the at least one air gap (7) through which the electrically conductive workpiece (2) is feasible, is formed such that in the opening (10) enclosed by the electrically conductive workpiece (2) fits. Device according to one of the preceding claims, characterized in that the magnetic yoke (6) is I-shaped or U-shaped. Device according to one of the preceding claims, characterized in that the at least one coil (5) is formed from at least one stranded wire. Device according to one of the preceding claims, characterized in that the U-shaped magnetic core (3) and the at least one coil (5) are encapsulated with an electrically insulating, in particular heat-conducting, potting compound. Device according to one of the preceding claims, characterized in that the U-shaped magnetic core (3) is provided with a cooling, in particular a water cooling. Device according to one of the preceding claims, characterized in that in addition to the U-shaped magnetic core (3), a further U-shaped magnetic core is provided, wherein the at least one at least one leg (4) of the U-shaped magnetic core (3) associated coil also associated with a leg of the further U-shaped magnetic core. Use of the device according to one of the preceding claims for heating a lid blank with an enclosed opening, in particular in a system for producing lids with a film. Method for inductive heating of an electrically conductive workpiece (2), which is in the form of a closed loop, with a device (1) according to one of Claims 1 to 8, characterized by the following steps: - Contactless introduction of the workpiece (2) in an air gap (7), so that the workpiece (2) surrounds the air gap (7) and one of the workpiece (2) enclosed opening (10) at least partially in the air gap (7) and - supplying the coils (5) of the device (1) with an alternating current. A method according to claim 10, characterized in that the method is used for heating a lid blank with an enclosed opening. A method according to claim 11, characterized in that the cover blank is heated prior to sealing or gluing with a film.

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