EP1285875B1 - Device for uniting at least two paper webs into a multilayer web - Google Patents

Abstract

The device for joining at least two paper strips (1a-1e) to form a paper web comprises two sandwich rollers (3) between which the paper strips are fed and from which they are brought together to form a web. First and second rollers are used, which have a highly conductive or semi-conductive outermost layer (34). Charging devices are available, which electrically charge the outermost layer of the first roller. The device incorporates an inductor electrode (2), arranged at a distance from the first roller. A further inductor electrode is spaced from the second roller, the outermost layer of which is charged electrically with a counterposed charge to that of the first roller. The first and/or second rollers have/has a semi-conductive outermost layer (34), beneath which is a highly conductive layer (33) and beneath which is an insulating layer (32) and a core (31).

Description

The present invention relates to an apparatus for joining at least two paper webs to a paper strand as defined in the preamble of independent claim 1.

For example, in folders of gravure and offset printing presses today, an electrostatic paper strand adhesion is used, which serves to hold together several united into a paper strand paper webs electrostatically. For uniting the paper webs are normally used two mutually spaced or almost without spacing sandwich rolls, between which the paper webs are performed and pressed together. In the paper direction of the two sandwich rolls usually a pair of knife rollers is arranged, through which the paper strand is divided into individual sheet packages. The single-sheet packets pass through a collecting cylinder into a folding cylinder and are finally folded. So that the individual paper webs remain united during the whole process, the outermost paper web on one side with positive charge and the outermost paper web on the other side with negative charge is electrically charged, so that there is a flow of current through the paper strand and to an electrostatic Liability between the individual paper webs comes, which persists until after folding.

To generate a current flow through the paper strand through various possibilities are known. In the CH-A-659 035 the charging of the two outermost paper webs by means of two inductor electrodes is known, which are arranged between a first and a second pair of sandwich rolls on both sides of the paper strand spaced therefrom. The two inductor electrodes, powered by a high-voltage DC power source, charge the two outermost paper webs directly.

A disadvantage of this charging variant is the additional space required by the inductor electrodes. Especially with older folders they can be due to the limited space often bad or not install. In addition, the directly opposite electrodes must be adjusted or covered depending on the paper strand width, so that they are always smaller than the paper strand width in the performance field, since otherwise a short circuit could occur. On the other hand, the performance field should not be too small, in order not to reduce the adhesion to the side edges, which would lead to so-called beating corners. Finally, in the paper strand between the individual paper webs in the area of the inductor electrodes, there are still often minimal air layers which can impair the adhesive effect.

In order to avoid the disadvantages of these inductor electrodes acting directly on the paper strand, paper strand adhesions have been developed which charge the paper strand via the sandwich rolls, which are in contact with the outermost paper web on both sides of the paper strand. For this purpose, the sandwich rolls have a semiconductive outermost layer which is supplied with power via the roll core. The power supply of the roller core itself via the shaft, for example, by sliding contact of the current source led from the power line via a slip ring with the shaft from the outside or, if the shaft a hollow shaft is, from the inside or by direct connection of the power line with a roller bearing shell.

A disadvantage of this indirect type of charging of the paper strand is that the bearings of the rolls must be laboriously isolated because the roll core is under tension. In addition, the power transmission via slip rings or roller bearings requires appropriate maintenance, which must be performed by specially trained personnel. Furthermore, the current distribution on the roll surface is not uniform in every case. At reduced paper strand widths, this can lead to a loss of effect, since where the two sandwich rolls directly touch without intervening strand of paper, a short circuit occurs.

Further devices for uniting paper webs into a paper strand and for loading the outer sides of webs of material are disclosed in the documents EP 0 378 350, which is to be regarded as the closest prior art, as well as in DE 2 754 179 and in DE 19713662.

In view of the disadvantages of the previously known devices described above, the invention has the following object. To create is a device for joining at least two paper webs to a paper strand of the type mentioned, which is easy and quick to install and maintain. Preferably also existing folders should be retrofitted in a simple manner.

This object is achieved by the device according to the invention, as defined in independent claim 1. Preferred embodiments will become apparent from the dependent claims.

The invention essentially consists of the following: An apparatus for uniting at least two paper webs into a paper strand comprises two sandwich rolls, between which the paper webs can be carried out and from which they can be brought together to form a paper strand. The device comprises a roller pair with a first and a second roller, each having a highly conductive or semiconducting outermost layer. These first and second rolls may be the two mentioned sandwich rolls. Charging means are provided to electrically charge at least the outermost layer of the first roller, the charging means according to the invention comprising an inductor electrode spaced from the first roller.

The fact that the charging of the first roller and possibly the second roller by means of one or two Induktorelektroden, the installation and maintenance of the device is substantially simplified and shortened, since an inductor without large intervention can be mounted and replaced, for which not even trained Staff is needed. Because no sliding contact for power transmission is present, the maintenance requirements of the device is also much smaller. Furthermore, depending on the embodiment, an electrical insulation of the bearings of one or both rollers can be dispensed with. Finally, a more uniform current distribution can be achieved with an inductor electrode acting directly on the roll surface.

In an advantageous embodiment variant, the device according to the invention comprises a further inductor electrode which is arranged at a distance from the second roller in order to electrically charge its outermost layer with a charge of opposite magnitude to the outermost layer of the first roller. By charging both rollers, the tension on each of the two rollers can be kept significantly lower compared to a variant in which only the first roller is charged. In contrast, the latter embodiment has the advantage that it has only one inductor electrode gets along and is therefore cheaper.

In a preferred embodiment, the first and / or the second roller have a semiconductive outermost layer, an underlying highly conductive layer, an underlying insulating layer and a core. The highly conductive layer arranged under the semiconducting outermost layer, which transfers the current through the paper strand to the counter-roller, enables a uniform current distribution even when a short electrode is used as the inductor electrode, which extends over only a part of the width of the paper strand. The insulating layer prevents the charge from flowing into the core, which is usually connected to the machine earth via the bearings.

In another advantageous embodiment variant, the first and / or the second roller have a semiconducting outermost layer, an underlying insulating layer and a core. Although the roll construction is simpler here, the power field of the associated inductor electrode must extend over approximately the entire roll width in order to ensure optimum current distribution.

A further alternative embodiment variant is characterized in that the first and / or the second roller have an upwardly or semiconducting outermost layer and a core directly underneath. Although the roll construction is even simpler here, depending on the arrangement of the roll, the roll bearings must be isolated.

In another preferred embodiment, the second roll is a steel roll and the highly conductive outermost layer is formed by the steel roll itself.

Via this second roller, whose bearings are not insulated, the current flows into the machine earth. It is not charged by means of an inductor electrode.

Advantageously, the bearings of the first and / or the second roller are electrically isolated in certain embodiments. It can then be dispensed with an insulating layer around the core.

Preferably, the first and second rolls are the two sandwich rolls. Immediately after merging the paper webs to the paper strand they are charged so that the electrostatic adhesion begins to act immediately. In this way, the adhesion of deteriorating inclusion of air layers can be avoided.

In other, depending on the available space advantageous embodiments, the first and the second roller in the direction of the paper strand after the two sandwich rolls are arranged so that the paper strand between them is feasible and abuts both, preferably the first and the second roller in the direction of Paper strand are arranged offset.

Advantageously, the inductor electrode or electrodes are arranged by the first and optionally the second roller at an effective distance of between 1 mm and 50 mm, wherein the voltage applied to the inductor electrode or electrodes is preferably between 10 kV and 60 kV amounts to. It can be achieved as optimal adhesion.

Fig. 1 -

eine Seitenansicht eines Falzapparats mit einem ersten Ausführungsbeispiel der erfindungsgemässen Vorrichtung zum Vereinigen von mindestens zwei Papierbahnen zu einem Papierstrang mit 3-Schicht-Walzen als erster und zweiter Walze;

Fig. 2 -

eine Ansicht von vorne der ersten und zweiten Walze und der Induktorelektroden von Fig. 1;

Fig. 3 -

eine Perspektivansicht einer länglichen Induktorelektrode mit einer Reihe von Emissionsnadeln;

Fig. 4 -

das elektrische Schaltbild der Induktorelektrode von Fig. 3;

Fig. 5 -

eine Perspektivansicht einer halbbogenförmigen Induktorelektrode;

Fig. 6 -

die Induktorelektrode von Fig. 5 angesetzt an eine Walze;

Fig. 7 -

ein zweites Ausführungsbeispiel der erfindungsgemässen Vorrichtung mit einer 3-Schicht-Walze als erster Walze und einer 1-Schicht-Walze als zweiter Walze und mit zwei Induktorelektroden;

Fig. 8 -

eine Seitenansicht der Vorrichtung von Fig. 7;

Fig. 9 -

ein drittes Ausführungsbeispiel der erfindungsgemässen Vorrichtung mit einer 3-Schicht-Walze als erster Walze und einer 1-Schicht-Walze als zweiter Walze und mit nur einer Induktorelektrode;

Fig. 10 -

eine Seitenansicht der Vorrichtung von Fig. 9;

Fig. 11 -

ein viertes Ausführungsbeispiel der erfindungsgemässen Vorrichtung mit 1-Schicht-Walzen als erster und zweiter Walze;

Fig. 12 -

eine Seitenansicht der Vorrichtung von Fig. 11;

Fig. 13 -

ein fünftes Ausführungsbeispiel der erfindungsgemässen Vorrichtung mit 2-Schicht-Walzen als erster und zweiter Walze;

Fig. 14 -

eine Seitenansicht der Vorrichtung von Fig. 13;

Fig. 15 -

ein sechstes Ausführungsbeispiel der erfindungsgemässen Vorrichtung mit einer 2-Schicht-Walze als erster Walze und einer 1-Schicht-Walze als zweiter Walze und mit zwei Induktorelektroden;

Fig. 16 -

eine Seitenansicht der Vorrichtung von Fig. 15;

Fig. 17 -

ein siebtes Ausführungsbeispiel der erfindungsgemässen Vorrichtung mit einer 2-Schicht-Walze als erster Walze und einer 1-Schicht-Walze als zweiter Walze und mit nur einer Induktorelektrode;

Fig. 18 -

eine Seitenansicht der Vorrichtung von Fig. 17;

Fig. 19 -

ein achtes Ausführungsbeispiel der erfindungsgemässen Vorrichtung mit in Laufrichtung des Papierstrangs zueinander versetzten 3-Schicht-Walzen als erster und zweiter Walze;

Fig. 20 -

ein neuntes Ausführungsbeispiel der erfindungsge- mässen Vorrichtung mit im Vergleich zum achten Ausführungsbeispiel noch weiter versetzten 3-Schicht-Walzen als erster und zweiter Walze; und

Fig. 21 -

ein zehntes Ausführungsbeispiel der erfindungsgemässen Vorrichtung mit einer 3-Schicht-Walze als erster Walze und einer Stahlwalze als zweiter Walze, wobei die beiden Walzen in Laufrichtung des Papierstrangs zueinander versetzt sind.

In the following, the device according to the invention for joining at least two paper webs into a paper strand will be described with reference to the attached drawings described in more detail with reference to ten exemplary embodiments. Show it:

Fig. 1 -

a side view of a folding apparatus with a first embodiment of the inventive device for uniting at least two paper webs to a paper strand with 3-layer rolls as the first and second rolls;

Fig. 2 -

a front view of the first and second rollers and the inductor electrodes of Fig. 1;

Fig. 3 -

a perspective view of an elongated inductor electrode with a series of emission needles;

Fig. 4 -

the electrical diagram of the inductor of Fig. 3;

Fig. 5 -

a perspective view of a half-arc-shaped inductor electrode;

Fig. 6 -

the inductor electrode of Figure 5 attached to a roller;

Fig. 7 -

a second embodiment of the inventive device with a 3-layer roller as a first roller and a 1-layer roller as a second roller and with two inductor electrodes;

Fig. 8 -

a side view of the device of Fig. 7;

Fig. 9 -

a third embodiment of the inventive device with a 3-layer roller as a first roller and a 1-layer roller as a second roller and with only one inductor electrode;

Fig. 10 -

a side view of the device of Fig. 9;

Fig. 11 -

A fourth embodiment of the inventive device with 1-layer rollers as the first and second roller;

Fig. 12 -

a side view of the device of Fig. 11;

Fig. 13 -

a fifth embodiment of the inventive device with 2-layer rollers as the first and second rollers;

Fig. 14 -

a side view of the device of Fig. 13;

Fig. 15 -

a sixth embodiment of the inventive device with a 2-layer roller as a first roller and a 1-layer roller as a second roller and with two inductor electrodes;

Fig. 16 -

a side view of the device of Fig. 15;

Fig. 17 -

a seventh embodiment of the inventive device with a 2-layer roller as a first roller and a 1-layer roller as a second roller and with only one inductor electrode;

Fig. 18 -

a side view of the device of Fig. 17;

Fig. 19 -

an eighth embodiment of the inventive device with offset in the direction of the paper strand 3-layer rollers as the first and second rollers;

Fig. 20 -

A ninth embodiment of the invention meassen device with compared to the eighth embodiment further offset 3-layer rollers as first and second roller; and

Fig. 21 -

A tenth embodiment of the inventive device with a 3-layer roller as a first roller and a steel roller as a second roller, wherein the two rollers are offset from each other in the direction of the paper strand.

First embodiment - Figures 1 and 2

In this first embodiment of the inventive device, this is installed in a folder. The first and second rolls 3 are both formed as 3-layer rolls and serve as sandwich rolls, between which five paper webs 1a-1e performed and merged into a paper strand 1 and-pressed. The paper strand 1 then passes to a provided with a transverse knife blade roller 4 and a counter-roller 5, between which it is divided into individual sheet packages. The single-sheet packets are then fed by a pair of draw rollers 6, first a collecting cylinder 7 and then a folding cylinder 8 and are finally folded.

The first and second rollers 3 each have a semiconducting outermost layer 34, an underlying highly conductive layer 33, an underlying insulating layer 32, and a core 31. The core 31 comprises a shaft 311, via which the roller 3 is rotatably mounted in bearings 35 in the machine frame 9. The bearings of the rollers 3 are not electrically insulated, which is not necessary due to the insulating layer 32. The core 31 is normally made of steel, while the semiconductive outermost layer 34 is preferably made of rubber. The highly conductive layer 33 and the insulating layer 32 consist of conventional conductive or insulating materials, for example metal, rubber or synthetic resin.

To create adhesion between the individual paper webs 1a-1e, they are electrically charged. Serve for this purpose, two inductor electrodes 2, which are arranged in approximately 1-50 mm distance from each one of the two rollers 3. The two inductor electrodes 2 are placed under a high voltage opposite sign, which can be up to 60 kV or -60 kV. This results in a flow of current from one inductor electrode 2 across the air gap to the semiconductive outermost layer 34 and the highly conductive layer 33 of the first roller 3, and from there due to direct contact with the paper strand 1 therethrough to the semiconductive outermost one Layer 34 and the highly conductive layer 33 of the second roller 3. As soon as the paper strand 1 comes out during the forward movement between the two rollers 3, the flow of current in this section comes to a standstill and the outermost paper webs la, le remain due to the relatively high electrical resistance of paper provided with unevenly polarized charges. Thereby, a pressure effect is applied to the intermediate paper webs 1b-1d, which stops for a certain time.

The highly conductive layer 33 under the semiconductive outermost layer 34 serves to uniformly distribute the current over the entire width of the roll, even if, as shown, inductor electrodes 2 are used which extend only over part of the roll width. Thanks to the highly conductive layer 33, it is also possible to use short electrodes whose length is only one fraction of the roll width, e.g. 1/6, is.

The following definition applies to the entire further description. If reference signs are included in a figure for the purpose of clarity of the drawing, but are not explained in the directly associated description text, reference is made to their mention in the preceding description of the figures.

FIGS. 3 and 4

The illustrated rod-shaped, elongated inductor electrode 2 is known per se in its construction and has been marketed by the Applicant for years. In an elongate insulating body 21, a plurality of emission needles 22 are arranged at regular intervals in a row. Behind each emission needle 22, a protective resistor 23 is connected. The emission needles 22 and protective resistors 23 are advantageously positioned on a printed circuit board, which is inserted into the insulating body 21 and, for example, cast with synthetic resin. The terminal contact 24 of the inductor electrode 2 is connected to a high voltage source.

FIGS. 5 and 6

The inductor electrodes may have various shapes. In the example shown here, an inductor electrode 102 has a semi-arcuate shape and surrounds a roller 103 at a distance of between about 1 and 50 mm. The insulating body 121 of the inductor electrode 102, in which a series of emission needles 122 are arranged, extends in arc over 180 °. For ease of access during servicing, the inductor electrode 102 is preferably placed near one end of the roller 103.

Second Embodiment - FIGS. 7 and 8

In this second embodiment, the first roller 3 is again a 3-layer roller according to the first embodiment. The second roller 103 is a 1-layer roller with a high or semiconductive outermost layer 132 around a core 131 and is rotatably supported in bearings 135 in the machine frame 9. Both rollers 3, 103 are associated with an inductor electrode 2 for charging, which makes the attachment of a bearing insulation 136 necessary due to the lack of insulating layer in the second roller 103.

The flow of current is similar to the first embodiment, which results in the simpler constructed here second roller 103 at best, a less optimal current distribution.

Third Embodiment - FIGS. 9 and 10

In this third embodiment, the two rollers 3 and 103 are constructed as in the second embodiment, except that the bearings 135 of the second roller 103 are not electrically insulated here. In addition, only the first roller 3 is charged by means of an inductor electrode 2. The flow of current takes place from the first roller 3 through the paper strand 1 to the second roller 103 and via the steel core 131 to the machine ground.

This device is relatively inexpensive because it requires only one inductor electrode 2 and one high voltage power supply.

Fourth Embodiment - FIGS. 11 and 12.

In this fourth embodiment, both rolls 103 are as single-layer rolls with a semiconductive outermost one Layer 132 formed around a core 131 around and rotatably supported in bearings 135 in the machine frame 9. Both rollers 103 is associated with an inductor 2 for charging and the bearings 135 of both rollers 103 are provided with a bearing insulation 136 to prevent current drainage to the machine ground. The current flow is similar to the first embodiment.

Fifth Embodiment - FIGS. 13 and 14

In this fifth embodiment, the first and second rollers 203 are formed as 2-layer rollers having a semiconductive outermost layer 233, an underlying insulating layer 232, and a core 231 and rotatably supported in non-electrically isolated bearings 235 in the machine frame 9. Both rollers 203 are associated with an inductor electrode 2 for charging, whose length should correspond due to the lack of highly conductive layer between the semiconductive outermost layer 233 and the insulating layer 232 respectively approximately the roll width and the paper strand width, respectively, in order to achieve an optimal current distribution. The current flow is similar to the first embodiment.

Sixth Embodiment - FIGS. 15 and 16

In this sixth embodiment, the first roller 203 is again a 2-layer roller according to the fifth embodiment. The second roller 103 is a 1-layer roller having a high or semiconducting outermost layer 132 around a core 131. The bearings 135 of the second roller 103 are provided with a bearing insulation 136 to prevent current drainage to the machine earth. Both rollers 203, 103 is associated with an inductor 2 for charging. The current flow is similar to the first embodiment.

Seventh Embodiment - FIGS. 17 and 18

In this seventh embodiment, the two rollers 203 and 103 are constructed as in the sixth embodiment, except that the bearings 135 of the second roller 103 are not electrically insulated here. In addition, only the first roller 203 is charged by means of an inductor electrode 2. The flow of current takes place from the first roller 203 through the paper strand 1 to the second roller 103 and via its steel core 131 to the machine earth.

This device is also relatively inexpensive, since it requires only one inductor electrode 2 and one high-voltage power supply.

Eighth embodiment - Figure 19

In this eighth embodiment, both rollers 3 are constructed as 3-layer rolls according to the first embodiment. Both rollers 3 is associated with an inductor 2 for charging. In contrast to the first exemplary embodiment, however, the two rollers 3 are not the sandwich rollers which merge the paper webs, which are not shown here, but rather rollers arranged further back in the running direction of the paper strand 1. The first roller 3 and the second roller 3 are also slightly offset from one another in the running direction of the paper strand 1. The current flow takes place from the first roller 3 via the paper strand 1 to the second roller 3.

Ninth Embodiment - FIG. 20

This ninth embodiment corresponds largely to the eighth embodiment. The only difference is that the first roller 3 and the second roller 3 are offset even further in the direction of the paper strand 1.

Tenth embodiment - Figure 21

In this tenth embodiment, the first roller 3 is again a 3-layer roller according to the ninth embodiment. The second roller 303 is a steel roller, which accordingly has a highly conductive, not separately formed outermost layer 331. The first roller 3 and the second roller 303 are offset from each other in the running direction of the paper strand 1. Only the first roller 3 is associated with an inductor 2 for charging. The flow of current takes place from the first roller 3 via the paper strand 1 to the second roller 3 and from this to the machine earth.

• Die Induktorelektroden können verschiedenartig aufgebaut sein und müssen nicht unbedingt Emissionsnadeln aufweisen. Denkbar sind beispielsweise auch segmentierte Elektroden mit etwa 20 mm² grossen Emissionsflächen anstatt - nadeln oder Fadenelektroden mit einem in einem Isolationskörper montierten gespannten Draht.
• Die oben beschriebenen Ausführungsbeispiele stellen nur ausgewählte Varianten von Walzen- und Induktorelektrodenkonstruktionen und -anordnungen dar. Prinzipiell sind im Rahmen der Patentansprüche noch viele andere Konstruktionen und Kombinationen möglich.

To the above-described inventive devices further constructive variations can be realized. Here are expressly mentioned:

• The inductor electrodes may be constructed in various ways and may not necessarily have emission needles. Also conceivable, for example, are segmented electrodes with emission surfaces of about 20 mm ² instead of needles or filament electrodes with a tensioned wire mounted in an insulation body.
• The exemplary embodiments described above represent only selected variants of roller and inductor electrode constructions and arrangements. In principle, many other designs and combinations are possible within the scope of the patent claims.

Claims (16)

1. An apparatus for combining at least two paper webs (1a-1e) into one paper stream (1), having two sandwich rolls (3, 103, 203, 303), between which the paper webs (1a-1e) can be led and by which these can be led together to form one paper stream (1), the apparatus comprising a pair of rolls with a first and a second roll (3, 103, 203, 303), each of which has a highly conductive or semiconducting outermost layer (34, 132, 233, 331), and there being charging means to charge up at least the outermost layer (34, 132, 233, 331) of the first roll (3, 103, 203, 303) electrically, characterized in that the charging means comprise an induction electrode (2; 102) which is arranged at a distance from the first roll (3, 103, 203, 303).
2. The apparatus as claimed in claim 1, characterized in that it comprises a further induction electrode (2; 102), which is arranged at a distance from the second roll (3, 103, 203, 303), to charge up its outermost layer (34, 132, 233, 331) electrically with a charge with a sign opposite to that of the charge of the outermost layer (34, 132, 233, 331) of the first roll (3, 103, 203, 303).
3. The apparatus as claimed in claim 1 or 2, characterized in that the first and/or the second roll (3) have/has a semiconducting outermost layer (34), a highly conductive layer (33) underneath the first, an insulating layer (32) underneath that, and a core (31).
4. The apparatus as claimed in one of claims 1 to 3, characterized in that the first and/or the second roll (203) have/has a semiconducting outermost layer (233), an insulating layer (232) directly underneath the first, and a core (231).
5. The apparatus as claimed in one of claims 1 to 4, characterized in that the first and/or the second roll (103) have/has a semiconducting outermost layer (132) and a core (131) directly underneath it.
6. The apparatus as claimed in one of claims 1 to 4, characterized in that the first and/or the second roll (103) have/has a highly conductive outermost layer (132) and a core (131) directly underneath it.
7. The apparatus as claimed in one of claims 1 to 6, characterized in that the second roll is a steel roll (303), and the highly conductive outermost layer (331) is formed by the steel roll (303) itself.
8. The apparatus as claimed in one of claims 1 to 7, characterized in that the bearings (35, 135) of the first and/or the second roll (3, 103, 203, 303) are electrically insulated.
9. The apparatus as claimed in one of claims 1 to 8, characterized in that the first and the second roll (3, 103, 203, 303) are the two sandwich rolls.
10. The apparatus as claimed in one of claims 1 to 9, characterized in that, in the running direction of the paper stream (1), the first and the second roll (3, 103, 203, 303) are arranged downstream of the two sandwich rolls in such a way that the paper stream (1) can be led between them and rests on both, and the first and the second roll (3, 103, 203, 303) are arranged offset in the running direction of the paper stream (1).
11. The apparatus as claimed in one of claims 1 to 10, characterized in that the induction electrode or electrodes (2; 102) is/are arranged at an effective distance of between 1 mm and 50 mm from the first and, if appropriate, the second roll (3, 103, 203, 303), and the voltage applied to the induction electrode or electrodes is preferably between 10 kV and 60 kV.
12. The apparatus as claimed in one of claims 3 to 11, characterized in that the highly conductive layer (33) underneath the semiconducting outermost layer (34) consists of rubber.
13. The apparatus as claimed in one of claims 3 to 11, characterized in that the highly conductive layer (33) underneath the semiconducting outermost layer (34) consists of synthetic resin.
14. The apparatus as claimed in one of claims 3 to 13, characterized in that the insulating layer (32) underneath the highly conductive layer (33) consists of rubber or synthetic resin.
15. The apparatus as claimed in one of claims 1 to 14, characterized in that the induction electrode is a short electrode (2; 102), which extends over only a portion of the width of the first or second roll (3, 103, 203, 303).
16. The apparatus as claimed in one of claims 1 to 14, characterized in that the length of the induction electrode is approximately equal to the width of the first or second roll (3, 103, 203, 303).

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