WO2014030077A2

WO2014030077A2 - Device and method for influencing liquid droplets or particles at the roller outlet of a pair of rollers

Info

Publication number: WO2014030077A2
Application number: PCT/IB2013/053331
Authority: WO
Inventors: Franz Knopf
Original assignee: Eltex-Elektrostatik Gmbh
Priority date: 2012-04-27
Filing date: 2013-04-26
Publication date: 2014-02-27
Also published as: WO2014030077A3; DE102012207148A1; DE102012207148B4

Abstract

A device and a method for influencing liquid droplets or particles (4) at the roller outlet (18) of a pair of rollers (1, 2) comprising a first rotating roller (1) and a second counter-rotating roller (2) are provided. With the aim of reducing a liquid or particle mist occurring as a result of the rotation at the roller outlet (18) by precipitation on the roller surfaces (16a, 16b) of the rollers (1, 2) or the substrate (5), the solution according to the invention envisages arranging at the roller outlet (18) a first electrode with a multiplicity of electrode tips (10a) directed towards the first roller (1), a second electrode with a multiplicity of electrode tips (10b) directed towards the second roller (2) and a DC voltage source (14) with at least one high-voltage output, each electrode tip (10a, 10b) of the multiplicity of electrode tips of the first and second electrodes being respectively connected to the at least one high-voltage output of the DC voltage source (14), and the first roller (1) being formed as a counter-electrode and interacting electrically with the multiplicity of electrode tips (10a) of the first electrode, and the second roller (2) being formed as a counter-electrode and interacting electrically with the multiplicity of electrode tips (10b) of the second electrode.

Description

DEVICE AND METHOD FOR INFLUENCING LIQUID DROPLETS OR PARTICLES AT THE ROLLER OUTLET OF A PAIR OF ROLLERS Description

The invention relates to a device and a method for influencing liquid droplets or particles at the roller outlet of a pair of rollers comprising a first rotating roller and a second counter-rotating roller, in particular for influencing a particle mist of a coating material or a printing ink of a roller inking unit of a printing press or in a roller coating machine. In roller inking units of printing presses or at the roller outlets of coating machines, an ink mist comprising printing ink or a particle mist comprising ink and/or coating material occurs where dual rollers interact, that is say where there is a pair of counter- rotating rollers, as a result of relatively high rotational speeds.

Printing and coating methods of this type in which such dual-roller systems are used are counted among the so- called contacting printing methods, in which an ink is transferred from a printing tool, that is to say the respective rollers, to another roller or to the substrate to be printed by rolling, that is to say by contact .

Depending on the type of ink or coating material that is used, the properties of the ink or particle mist occurring in the transfer of the printing ink or the coating material from an ink-carrying surface to a non- ink-carrying surface depend greatly on the type and physical properties of the ink used or the coating material used (rheological properties, viscous or free- flowing, etc . ) . In inking units of printing presses, a number of pairs of rollers arranged in series assume the tasks of so- called ink splitting, ink distribution and ink transport. Between at least two of these pairs of rollers, the corresponding substrate is passed for printing or coating. Classic paper webs, but also metal foils or metallized substrates with non-absorbent surfaces come into consideration here as substrates. Particularly when such substrates with very low or limited absorbency of the surface are used, the occurrence of an ink or particle mist is particularly pronounced .

Depending on the type of ink used or depending on the type of coating material used, particle streams with a widely varying particle size spectrum thereby occur at the roller outlet, in dependence on the rheology of the ink, its viscosity, the surface structure of the rollers and of the substrate and the rotational speed of the rollers. The particle diameter ranges from a few nanometers to a few micrometers, i.e. it is quite possible that, in dependence on the ink or coating material used, the particle diameter extends over several orders of magnitude. A larger particle diameter is accompanied by a greater particle surface area; particularly when processing low-cost inks with poor so-called mist characteristics, a low percentage of extremely high-mass particles occurs at a correspondingly high rotational speed of the rollers.

Such an ink or particle mist occurring in the inking units has disadvantageous effects to the extent that the servicing effort for the printing press as a whole is increased due to the additional exposure to ink. Furthermore, the particles may be precipitated on the printed products and cause a reduction in the printing quality. In particular whenever the rotational speed of the pair of rollers is chosen to be relatively high, there is also the problem that the ink mist, i.e. the particle streams that occur at the roller outlet, is/are entrained as a result of the air boundary layers obtained due to the high production speeds. As a result, the spreading of the ink or particle mist within the machine and in the surroundings is increased . On account of the increasing amount of ink mist or particles, as a result of this the printing quality is reduced still further with increasing rotational speed.

Depending on the respective particle size, i.e. depending in particular on the particle surface area or the mass, these particles leave these air boundary layers tangentially at various points of the roller circumference. As a result of the dominant kinematic viscosity of the air, smaller particles initially remain in these air boundary layers that form along the transported substrate or else along the roller surface, and are only given off to the surroundings in the next roller gap or at the next deflection of the substrate at a roller with a smaller radius, as a so-called secondary particle stream.

In other words, in particular when there is an increase in the rotational speed, and consequently the production speed, the ink or coating particles in the mist are distributed over a greater volume.

Particularly when printing or coating substrates (card; foil, etc.) for food packagings, such particles get onto the side of the substrate that later comes into contact with the food. This causes undesired effects on the food that is later to be packaged. As a result of the undesired reduction in quality, such roller printing or coating systems therefore often cannot be operated at a cost-effective production speed, i.e. a sufficiently high production speed. The distribution of the ink or coating particles over a relatively great volume of air is also accompanied by contamination of the air that is breathed by persons in the vicinity of the machine, which has adverse effects on health and may also have to be avoided for reasons of health and safety at work. Devices and methods for influencing liquid droplets or particles at the roller outlet of such pairs of rollers comprising rotating rollers are known from the prior art, the particles being electrically charged in the outlet of the dual rollers with the aid of a corona discharge device and being precipitated on the surface of the rollers.

For example, a corona electrode in the form of a wire which interacts electrically with one of the rotating rollers is known from US 3,011,435 A.

Known devices of this type for influencing liquid droplets or particles at the roller outlet of a pair of rollers thus have the disadvantage that only a relatively small amount of free charge carriers is generated, the effect of the particle charging and the precipitation thereof on the rollers by way of the electrical field forces acting not being sufficient at high rotational and production speeds as a result of this relatively low charge carrier density.

It is therefore the object of the present invention to develop a device and a method of the type mentioned at the beginning in such a way that the influencing of liquid droplets or particles at the roller outlet of a pair of rollers is improved, and consequently the undesired ink or particle mist is further reduced. This object is achieved according to the invention by a device according to independent patent claim 1 and a method according to independent patent claim 16. The device according to the invention for influencing liquid droplets or particles at the roller outlet of a pair of rollers comprising a first rotating roller and a second counter-rotating roller, in particular for influencing a particle mist of a coating material or a printing ink in a roller inking unit of a printing press or in a roller coating machine, has a first electrode with a multiplicity of electrode tips directed towards the first roller and a second electrode with a multiplicity of electrode tips directed towards the second roller and also a DC voltage source with at least one high-voltage output. According to the invention, each electrode tip of the multiplicity of electrode tips of the first and second electrodes is respectively connected to the at least one high-voltage output of the DC voltage source. Furthermore, the solution according to the invention provides that the the first roller is formed as a counter-electrode and interacts electrically with the multiplicity of electrode tips of the first electrode, and that the second roller is formed as a counter- electrode and interacts electrically with the multiplicity of electrode tips of the second electrode.

With regard to the method according to the invention for influencing liquid droplets or particles at the roller outlet of a pair of rollers comprising a first rotating roller and a second counter-rotating roller, in particular for influencing a particle mist of a coating material or a printing ink in a roller inking unit of a printing press or in a roller coating machine, a first electrode with a multiplicity of electrode tips directed towards the first roller and a second electrode with a multiplicity of electrode tips directed towards the second roller are provided, the electrode tips of each electrode being respectively arranged in a row and having in each case a dedicated current-limiting element, in particular in each case a resistor. According to the invention, the method comprises the following method steps: applying a high DC (direct) voltage to the multiplicity of electrode tips of the first electrode, so that a first gas discharge is obtained, preferably by choosing a high DC voltage of at least 1.2 times the dielectric voltage of a stable low-energy plasma discharge, between the electrode tips of the first electrode and the first roller; applying a high DC voltage to the multiplicity of electrode tips of the second electrode, so that a second gas discharge is obtained, preferably by choosing a high DC voltage of at least 1.2 times the dielectric voltage of a stable low-energy plasma discharge, between the electrode tips of the second electrode and the second roller.

With respect to the device according to the invention, there is the particular advantage here that, with simple installation and operating conditions, the multiplicity of electrode tips of the first electrode and the multiplicity of electrode tips of the second electrode bring about good influencing, and consequently reduction, of the ink or particle mist even at high rotational and production speeds. The clear electrical assignment of the multiplicity of electrode tips of the first electrode to the first roller and the clear electrical assignment of the multiplicity of electrode tips of the second electrode to the second roller ensures here that the electrical influencing of the ink particles definitely acts against the respectively assigned counter-electrode or the substrate that may be guided along it. As a result of this, a sufficient number of free charge carriers, a sufficient charging of the ink particles, and consequently a good deposition performance of the device according to the invention, are always ensured.

A particular advantage is also produced by neither the multiplicity of electrode tips of the first electrode nor the multiplicity of electrode tips of the second electrode having to be arranged in a particular way strictly symmetrically with respect to the roller outlet gap. The clear electrical assignment of the multiplicity of electrode tips of the first electrode to the first roller and of the multiplicity of electrode tips of the second electrode to the second roller is obtained here by a suitable geometrical arrangement of the respective electrode tips to the respective roller. It goes without saying here that the distance of the electrode tips of the first electrode from the first roller is significantly less than the distance of the electrode tips of the first electrode from the second roller .

The same correspondingly applies by analogy to the distance of the electrode tips of the second electrode from the second roller, which should be chosen much smaller than the distance of the electrode tips of the second electrode from the first roller.

The term "distance of the electrode tips from the roller" should be understood here as meaning in each case the spatial distance from the ends of the electrode tips to the respective surface of the roller.

With respect to the method according to the invention, there is in turn the particular advantage that, with simple installation and operating conditions, the multiplicity of electrode tips of the first electrode and the multiplicity of electrode tips of the second electrode bring about good influencing, and consequently reduction, of the ink or particle mist even at high rotational and production speeds. The clear electrical assignment of the multiplicity of electrode tips of the first electrode to the first roller and the clear electrical assignment of the multiplicity of electrode tips of the second electrode to the second roller ensures here that the electrical influencing of the ink particles definitely acts against the respectively assigned counter-electrode or the substrate that may be guided along it. As a result of this, a sufficient number of free charge carriers, a sufficient charging of the ink particles, and consequently a good deposition performance when the method according to the invention is applied, are always ensured.

Advantageous developments of the invention are specified in the subclaims.

For example, it is provided that the multiplicity of electrode tips of the first electrode and the multiplicity of electrode tips of the second electrode are arranged at a distance from the respectively electrically interacting roller in such a way that, during the operation of the device, a first gas discharge is obtained between the multiplicity of electrode tips of the first electrode and the electrically interacting first roller, and that, during the operation of the device, a second gas discharge is obtained between the multiplicity of electrode tips of the second electrode and the electrically interacting second roller. The fact that each of these gas discharges in each case definitely interacts electrically with one of the two rollers means that a discharge path reliably directed in the direction of this respective roller is obtained at each of the rollers in the region of the roller outlet gap. Along this first or second discharge path there then takes place a further improved and highly effective influencing of the ink or coating particles, whereby they are precipitated on the substrate that may be present or on the surface of the rollers, and subsequently no longer have a disturbing effect.

According to a particularly preferred aspect of the solution according to the invention, it is provided that each electrode tip of the multiplicity of electrode tips of the first electrode is respectively connected by way of a dedicated current-limiting element to the at least one high-voltage output of the DC voltage source. By analogy, it is provided that each electrode tip of the multiplicity of electrode tips of the second electrode is respectively connected by way of a dedicated current-limiting element to the at least one high-voltage output of the DC voltage source. The current-limiting elements are preferably formed here as resistors and designed to limit the obtained current of the respective gas discharge. As a result of this, during the operation of the device, a stable low- energy plasma discharge is in each case obtained from the electrode tips of the first electrode in the direction of the first roller and from the electrode tips of the second electrode in the direction of the second roller. In other words, a DC plasma of the "nonequilibrium plasma under atmospheric pressure" type is therefore respectively produced between the electrodes in the region of the flowing air boundary layers forming as a result of the rotation. By contrast with an AC plasma in conjunction with a so-called "dielectric barrier discharge (DBD)", the efficiency of such a DC plasma is very high. As a result, a very good deposition rate is obtained.

Such a DC plasma is distinguished by the fact that it has electron densities of up to 10 electrons per cm . These electrons have electron temperatures (electron energies) of up to about 50 000° Celsius. By contrast with this, the ions and the neutral gas particles, that is to say all the heavy particles within this DC plasma, have a much lower temperature in relation to the electron temperatures, of approximately 100° Celsius or below. Furthermore, the number of negative electrons and positive gas ions are not in equilibrium.

In order to produce the DC plasma (direct current plasma) , at the multiplicity of electric tips of the first electrode and at the multiplicity of electric tips of the second electrode there is a high electrical voltage, the value of which lies above the dielectric voltage for the given electrode geometry. The dielectric voltage is obtained as a result of the form of the electrodes, the geometrical arrangement of the electrodes in relation to the respective counter- electrode, that is to say the respectively assigned roller, and the disruptive field strength of the medium guided past along this low-energy plasma discharge path within the air boundary layer.

The fact that the current-voltage characteristic of such a DC plasma has a negative progression means that a dedicated current-limiting element in each case limits the current flow. These current-limiting elements that are respectively assigned specifically to the electrode tips, for example high-impedance resistors, then limit the current flow through the plasma, so that no arc discharge occurs in spite of the high voltage that is present, exceeding the dielectric voltage . Compared with a corona gas discharge, a much higher charge carrier density is obtained within such a stable low-energy plasma, so that even very great particle surface areas, which may occur when low-cost inks or coating materials are used, are sufficiently charged.

The much higher charge carrier densities therefore ensure that even the greatest particle surface area that occurs can assume the natural maximum-possible limiting charge in a very short time. Here it should be noted that the particle charging takes place primarily by way of the mechanisms of so-called field charging. The strong electrical field that is necessary for such field charging is obtained within the DC plasma discharge path. By contrast with this, smaller particles, i.e. particles smaller than about 200 nm, predominantly follow the laws of diffusion charging.

The fact that two electrodes with in each case a multiplicity of electrode tips are provided for producing such a DC plasma, and are respectively clearly assigned to only one of the roller surfaces, produces the particular advantage here that substantially the complete ink mist is precipitated within the air boundary layers. Therefore, two plasma cones form, the clear assignment by way of the distance from the respective roller surface having the effect that each plasma cone definitely acts against this roller surface or against a substrate possibly transported along there, the rollers acting as counter- electrodes. The DC plasma can in this way form individually on the shortest path through the particle- transporting air boundary layer against the counter- electrodes . The provision of individual current-limiting elements that are isolated from one another for each individual one of the multiplicity of electrode tips also ensures that no undesired electrical spark discharge takes place even if soiling or the like should happen to occur, influencing the electrical resistance of an electrode tip. The quality of the ink mist suppression around this individual tip consequently only decreases by a small amount in the time up to the next cleaning of this individual tip.

According to a further aspect of the invention, it is provided that the multiplicity of electrode tips of the first electrode are arranged in a first row, which runs largely parallel to the axial direction of the first roller, so that the electrode tips are arranged substantially at a first distance from the surface of the first roller. In an analogous way, it is provided that the multiplicity of electrode tips of the second electrode are arranged in a second row, which runs largely parallel to the axial direction of the second roller, so that the electrode tips are arranged substantially at a second distance from the surface of the second roller.

Such a double emission tip array arranged along the roller gap ensures that a relatively extended gas volume can be transformed into the plasma state along the entire distance of the roller gap. Arranging the electrode tips in rows consequently has the effect of producing a relatively homogeneous stable low-energy plasma respectively between the first row of electrode tips and the first roller and between the second row of electrode tips and the second roller. As a result, the ink mist or the mist of coating material particles is effectively precipitated substantially without interruption along the entire extent of the roller gap on the rollers or on a substrate possibly transported along with them, and is consequently reduced substantially completely and over the full working width of the roller outlet.

According to a further aspect of the invention, it is provided that the high voltage delivered to the respective high-voltage output of the DC voltage source is at least 1.2 times, preferably at least 1.5 times, the dielectric voltage for the respective electrode geometry arrangement and the distance of the roller assigned to the respective electrode tip. This ensures that, in spite of a possible geometrical asymmetry with respect to the rollers acting as a counter-electrode and in spite of possible material-induced different electrical path resistances, each individual tip can individually build up its plasma discharge reliably. Making the high voltage at the respective high-voltage output of the DC source higher than the dielectric voltage at least by a factor of 1.2, preferably a factor of 1.5, then ensures particularly in non- homogeneous situations of the path resistance of the ink mist transported past that there is always a sufficient reduction of this ink or particle mist occurring.

According to a further aspect of the invention, it is provided that the first roller and/or the second roller is or are connected to earth potential.

In this way it is ensured that the greatest possible amount of ink or particle mist is precipitated on the respective roller or a substrate possibly transported with it, which is guided along this roller.

According to an advantageous development of the invention, it is provided that the at least one high- voltage output of the DC voltage source delivers a negative high voltage to the electrode tips of the first electrode and/or the second electrode, so that, during the operation of the device, a stable, largely homogeneous low-energy plasma is in each case obtained, emanating from the respective electrode tips.

In particular whenever the respectively assigned roller is at earth potential, this results in a homogeneously acting plasma emanating from the respective negative tip, showing no local inhomogeneities that potentially bring about undesired vortices of the ink mist. By contrast with this, a positive plasma, i.e. emanating from a positive tip to earth potential, tends to form so-called streamers, which are accompanied by undesired inhomogeneities as a result of the local lightning-like constrictions occurring.

According to a further aspect of the invention, it is provided that the distance of the multiplicity of electrode tips of the first electrode and/or the second electrode from the respective surface of the respectively electrically interacting roller lies in the range between 5 and 15 mm, preferably between 8 and 12 mm. In this range it is ensured that a level of the high voltage that can be produced in a technically relatively undemanding way is already sufficient to exceed the disruptive field strength.

According to a further aspect of the solution according to the invention, it is provided that the distance of the electrode tips within a row of electrode tips lies in the range between 4 and 8 mm and is preferably approximately 5 mm. Such a regular tip spacing in this spacing range of approximately 5 mm likewise contributes to a good reduction of the ink mist, with at the same time largely uninfluenced flow conditions. According to a further aspect of the invention, the influencing of the particles or ink droplets takes the form of charging the same and subsequently changing the direction of these charged particles of the particle mist. Here, the first gas discharge, i.e. for example the first DC plasma discharge, particularly causes a precipitation of particles on the first roller. By analogy, the second gas discharge, i.e. particularly the second DC plasma discharge, causes a precipitation of particles on the second roller. It goes without saying that it is also possible that a substrate, for example a paper or foil web or the like to be printed, is transported along on one of the two rollers, so that the particles are precipitated on the substrate then located along the gas discharge path, between the electrode tip and the respective roller acting as a counter-electrode .

According to a further aspect of the invention, it is provided that the first roller, the second roller or else both rollers respectively has/have a surface which has a restrictedly conductive coating. The restrictedly conductive coating is preferably formed from rubber, NBR or Rilsan, and has a resistivity of between 10⁶ and 10⁷ Ω x m. However, it goes without saying that it is also possible to provide coatings with lower-impedance resistivities.

Such coatings may also be advantageous for technical printing reasons, for example if a changed ink transfer is necessary, or an elasticity is necessary on account of a specific pressing pressure between the rollers of the pair of rollers. The fact that the respective coating is formed with sufficiently low impedance means that a very good reduction of the ink or particle mist continues to be obtained. According to a further aspect of the invention, it is provided that the first electrode and the second electrode are arranged together in a housing. The arrangement of the electrodes is such that the first row and the second row of electrode tips run at a distance from one another. The electrode tips of the first row may preferably run here with respect to the electrode tips of the second row laterally offset from one another .

Particularly preferably, in particular in the case of a regular tip spacing of approximately 5 mm, the rows of electrode tips are arranged laterally offset approximately 2.5 mm from one another. Such an offset arrangement improves the air flow conditions along the electrode tips.

An arrangement within a common housing also has the structural advantage that the electrodes are arranged and accommodated within the housing in an expedient way, only the electrode tips protruding out of the housing. The clear assignment of each row of electrodes to the respective counter-electrode, i.e. to the respective roller, means that the arrangement within a housing produces virtually constant conditions with respect to the DC plasma within the entire device.

According to a further aspect of the invention, the housing is arranged in such a way that the electrode tips of the first row and of the second row are arranged parallel to the roller outlet. The distance of the rows of electrode tips from the roller outlet is approximately 0.4 to 0.6 times, preferably 0.5 times, the roller radius of the first roller, the second roller or - in the case of rollers of the same diameter - also of both rollers with respect to a gauge axis. The gauge axis runs perpendicular to the roller axes and parallel to the transporting direction in the tangent line in the roller outlet.

Such a geometrically adapted installation of such a double-row DC plasma electrode arrangement within a housing ensures that the negative pressure occurring in the outlet of the dual-roller system is not influenced by an electrode body along which the air flowing into the outlet of the dual-roller system has an excessive air resistance. An appropriate choice of the distance then results in an acceptable precipitation performance of the ink mist on the respective roller acting as a counter-electrode or on the substrate transported past there .

However, it goes without saying that it is also possible to determine an optimum for the arrangement of a device according to the invention in a one-off experiment, in order to obtain a minimum-possible power output of the DC source.

In any event, in the case of such a structure, the ion wind of the DC plasma supports the air flowing in the region of the roller gap. Here it should be noted that, whenever the electrode housing is brought too close to the roller outlet, i.e. the roller gap, the deposition performance or precipitation performance of the ink mist or particle mist is reduced, since in such a case air flows in from the side in an undesired way and the ink mist is precipitated on the electrodes or on the electrode tips as a result of the lateral flow.

Correspondingly, whenever the electrode is removed too far from the roller, there is no longer a sufficiently high current density, since in this case the dielectric voltage is not reached and a plasma no longer forms along the entire path, at most in the submillimeter range of the respective electrode tips. In other words, a reduced reduction of the ink or particle mist is obtained both when the air streams are influenced by arrangement too close to the roller gap and when the so-called plasma distance criterion is infringed (distance too great) .

According to a further aspect of the invention, it is provided that the housing additionally has at least one air duct, at least one outlet opening and preferably a compressed-air connection. The at least one outlet opening serves for feeding air in centrally between the rows of electrode tips and is correspondingly arranged. By applying compressed air to the air duct by way of the compressed-air connection, it is then possible to introduce a central, relatively small additional air stream into the region between the rows of electrodes.

Such an arrangement is particularly advantageous if, for example, a high percentage of very large particles in the micrometer range have to be deposited or precipitated as a result of low-quality or very low- cost inks. Here it may be envisaged in particular to provide a multiplicity of outlet openings with a diameter of in each case approximately 1 mm and a distance from one another in the range between approximately 5 and 15 mm, preferably approximately 10 mm.

An exemplary embodiment of the invention is explained in more detail below on the basis of the schematically represented drawing, in which:

Figure 1 shows a sectional view of the device according to the invention for influencing liquid droplets or particles at the roller outlet of a pair of rollers. In the single figure (Figure 1), a dual-roller system is shown, consisting of a first roller 1 and a second roller 2, which in Figure 1 are only represented incompletely in a sectional view. An arrow 3a indicates the direction of rotation of the first roller 1 during operation; in a corresponding way, the direction of rotation counter to the direction of rotation 3a in the first roller 1 is indicated by means of an arrow 3b. In the tangential contact region of the rollers 1, 2, a roller outlet 18, sometimes also referred to as a roller gap or nip, is formed. In dependence on the respective use of the rollers 1, 2, within a printing inking unit for example, it may be provided that a substrate 5 is transported along between the first roller 1 and the second roller 2.

The rollers 1, 2 are respectively provided with a conductive coating that is not represented in Figure 1, so that conductive surfaces 16a, 16b of the rollers 1, 2 are obtained. By means of earthing connections 15a, 15b, schematically represented in Figure 1, both rollers are also connected to earth potential, so that the surfaces 16a, 16b of the rollers 1, 2 are also at earth potential on account of the limitedly conductively formed coatings.

A particle stream 4 forms in the roller outlet 18 and, as a result of the relatively high rotational speeds of the rollers 1, 2, is transported to a greater or lesser extent, depending on the respective particle size of the individual particles within the particle stream 4, along a first air boundary layer, the direction of flow of which is schematically indicated by an arrow 6a, or along a second air boundary layer, the direction of flow of which is indicated by an arrow 6b. In an electrode housing 9, a multiplicity of electrode tips 10a of a first electrode and a multiplicity of electrode tips 10b of a second electrode are respectively arranged in a first row of electrode tips 20a and in a second row of electrode tips 20b, which extend along the roller outlet 18 into the plane of the drawing .

Each individual one of the electrode tips 10a, 10b is provided with a dedicated current-limiting element 12a, 12b, preferably formed as a resistor. By way of these current-limiting elements 12a, 12b, the electrode tips 10a, 10b are connected in an isolated way to an output of a DC voltage source. It goes without saying that it is also conceivable here, if need be, to provide different outputs for the first row of electrode tips 20a and the second row of electrode tips 20b with different voltage parameters. It is correspondingly possible to use current-limiting elements 12a with different electrical characteristics from current-limiting elements 12b. As a result of applying a high voltage, which lies well above the dielectric voltage for the electrode geometry, a first gas discharge 11a and a separate second gas discharge lib form, in each case in the form of a DC voltage plasma cone. These plasma cones 11a, lib respectively extend in the direction of the first or second roller 1 or 2 assigned to the respective row of electrode tips 20a or 20b.

The dielectric voltage is obtained as a result of a distance x of the first row of electrode tips from the surface of the first roller 1 or a distance y of the second row of electrode tips 20b from the surface of the second roller 2. In order to achieve a clear electrical assignment of the first row of electrode tips 20a to the first roller 1 and analogously a clear electrical assignment of the second row of electrode tips 20b to the second roller 2, the rows of electrode tips 20a, 20b are arranged at a distance z from one another.

The distances x, y and z are chosen such that the flow paths of the inflowing air 7a, 7b are not adversely influenced by the geometry of the electrode housing 9 in such a way that, for example, laterally inflowing air in the region of the roller outlet causes the formation of an undesired flow, which could lead to the ink or particle mist being precipitated on the rows of electrode tips 20a, 20b.

The electrode housing 9, and consequently the first row of electrode tips 20a and the second row of electrode tips 20b, is/are arranged in such a way that both the distance x of the first row of electrode tips from the surface 16a of the first roller 1 and the distance y of the second row of electrode tips from the surface 16b of the second roller 2 are small enough with respect to the high voltage made available by the DC voltage source 14 and respectively present at the rows of electrode tips 20a, 20b that in each case a dedicated, stable low-energy plasma discharge 11a, lib can form.

The plasma cones of these stable low-energy plasma discharges 11a, lib are respectively directed at the roller 1 or 2 interacting electrically with the respective row of electrode tips 20a, 20b as a counter- electrode. As a result of the simultaneous charging and deflecting effect of the low-energy plasma discharges 11a, lib, i.e. due to the initially occurring particle charging within the plasma cone 11a, lib as a result of field charging or - in the case of smaller particles - also due to diffusion charging and the subsequent deflection of a large part of the particles within the particle stream 4 as a result of the electrical field forces along the discharging paths 11a, lib, a large part of the particles within the particle stream 4 is consequently precipitated on the roller surfaces 16a, 16b or on the substrate 15 guided along the first roller 1.

This produces a considerable reduction in the undesired ink mist or particle mist in coating methods, so that a high-quality printed product or coated product can be obtained with at the same time a high rotational speed of the rollers 1, 2, and consequently a high production speed of the printed or coated substrate. Furthermore, the undesired contamination of the surrounding breathing air with aerosol mist is reduced considerably .

It should be noted here that the clear geometrical and electrical assignment of each of the rows of electrode tips 20a, 20b to only one of the printing rollers 1 or 2 produces plasma cones 11a, lib that respectively act definitely against the one roller surface 16a, or 16b, or the substrate 15 guided along on this roller surface. As a result, sufficient influencing effects on the ink mist are obtained along the entire printing gap or roller outlet 18.

In the exemplary embodiment represented in Figure 1, the electrode housing 9 is arranged with respect to a gauge axis 17, which runs parallel to the transporting direction in the tangent line of the roller outlet 18, in such a way that the electrode tips 10a, 10b of the first and second rows of electrode tips 20a, 20b are parallel to the roller outlet 18 at a distance that is approximately 0.6 times the roller radius R, which in the exemplary embodiment shown is the same in the case of both rollers 1, 2. This ensures that the negative pressure occurring in the outlet of the dual-roller system is not adversely influenced by the electrode housing 9 and the rows of electrode tips 20a, 20b, so that in particular the flowing air boundary layers 6a, 6b do not go over into a turbulent flow that is conducive to vortexing. In such a case, air would flow in laterally due to the infringed distance rule, whereby the device could only continue to perform its function to a limited extent, and would soil relatively quickly as a result of the vortexed ink or particle mist.

On the other hand, this distance is chosen such that the rows of electrode tips 20a, 20b are also not installed too far away from the outlet, such that the flow density along the gas discharge paths would drop too much because the disruptive voltage were not reached .

In addition, the electrode housing 9 shown in the exemplary embodiment according to the figure has a central air duct 8a, which opens out into a multiplicity of outlet openings 8b, which are arranged symmetrically between the rows of electrode tips 20a, 20b in the form of a multiplicity of bores that are formed with a respective diameter of about 1 mm at the regular spacing of approximately 10 mm. By way of a compressed-air connection 13, compressed air can be applied as and when required to the air duct 8a, leaving through the multiplicity of outlet openings 8b between the rows of electrode tips 20a, 20b and providing an additional air stream there. This additional air stream may - depending on ambient conditions during the operation of the device - turn out to be relatively small, and serves the purpose of supporting the deposition, i.e. the precipitation, of very high-mass ink or coating particles on the substrate or on the roller surfaces 16a, 16b. Such extremely high-mass particles will occur in particular at high production speeds combined with the processing of low-cost inks with relatively poor mist characteristics .

With the device described above, liquid droplets or particles within a particle stream 4 that are transported along air boundary layers 6a, 6b along the surfaces 16a, 16b of rotating rollers 1, 2 in a roller outlet 18, and consequently lead to undesired ink or particle mist, can be electrically influenced by electrical interaction between rows of electrode tips 20a, 20b, to which a high DC voltage is applied, and a roller 1, 2, which is assigned to the respective rows of electrode tips 20a, 20b, in such a way that the ink or particle mist is reduced decisively, i.e. by almost 100%.

The use of such a device is particularly advantageous in roller printing units of printing presses or at the roller outlets of coating machines. At this stage it should be pointed out that all of the parts described above are claimed as essential to the invention in themselves alone and in any combination, in particular the details that are represented in the drawing. Modifications thereof are familiar to a person skilled in the art. List of designations

1 first roller

2 second roller

3a direction of rotation of the first roller

3b direction of rotation of the second roller

4 particle stream

5 substrate

6a direction of flow of the first air boundary layer

6b direction of flow of the second air boundary layer

7a, 7b flow path of the inflowing air

8a air duct

8b outlet opening

9 electrode housing

10a electrode tip of the first electrode

10b electrode tip of the second electrode

11a first gas discharge

lib second gas discharge

12a, 12b flow-limiting element

13 compressed-air connection

14 DC voltage source

15a, 15b earthing connection

16a surface of the first roller

16b surface of the second roller

17 gauge axis

18 roller outlet

20a first row of electrode tips

20b second row of electrode tips

x distance of the first row of electrode tips from the surface of the first roller

y distance of the second row of electrode tips from the surface of the second roller z distance of the first row of electrode tips from the second row of electrode tips

R roller radius

Claims

Patent claims

A device for influencing liquid droplets or particles (4) at the roller outlet (18) of a pair of rollers (1, 2) comprising a first rotating roller (1) and a second counter-rotating roller (2), in particular for influencing a particle mist (4) of a coating material or a printing ink in an inking unit of a printing press or in a roller coating machine, the device having the following:

- a first electrode with a multiplicity of electrode tips (10a) directed towards the first roller ( 1 ) ;

- a second electrode with a multiplicity of electrode tips (10b) directed towards the second roller ( 2 ) ; and

- a DC voltage source (14) with at least one high- voltage output;

each electrode tip (10a, 10b) of the multiplicity of electrode tips of the first and second electrodes being respectively connected to the at least one high-voltage output of the DC voltage source (14), and the first roller (1) being formed as a counter-electrode and interacting electrically with the multiplicity of electrode tips (10a) of the first electrode, and the second roller (2) being formed as a counter-electrode and interacting electrically with the multiplicity of electrode tips (10b) of the second electrode.

The device as claimed in claim 1, the multiplicity of electrode tips (10a) of the first electrode and the multiplicity of electrode tips (10b) of the second electrode being arranged at a distance from the respectively electrically interacting roller (1, 2) in such a way that, during the operation of the device, a first gas discharge (11a) is obtained between the multiplicity of electrode tips (10a) of the first electrode and the electrically interacting first roller (1), and in that, during the operation of the device, a second gas discharge (lib) is obtained between the multiplicity of electrode tips (10b) of the second electrode and the electrically interacting second roller (2) .

The device as claimed in claim 1 or 2, each electrode tip (10a) of the multiplicity of electrode tips of the first electrode being respectively connected by way of a current-limiting element (12a) to the at least one high-voltage output of the DC voltage source (14), and each electrode tip (10b) of the multiplicity of electrode tips in the second electrode being respectively connected by way of a dedicated current-limiting element (12b) to the at least one high-voltage output of the DC voltage source (14), the current-limiting elements (12a, 12b) being designed to limit the obtained current of the respective gas discharge (11a, lib), so that, during the operation of the device, a stable low- energy plasma discharge is in each case obtained.

The device as claimed in one of the preceding claims, the multiplicity of electrode tips (10a) of the first electrode being arranged in a first row (20a), which runs largely parallel to the axial direction of the first roller (1), so that the electrode tips (10a) are arranged substantially at a first distance (x) from the surface (16a) of the first roller, and the multiplicity of electrode tips (10b) of the second electrode being arranged in a second row (20b), which runs largely parallel to the axial direction of the second roller (2), so that the electrode tips (10b) are arranged substantially at a second distance (y) from the surface (16b) of the second roller (2) . The device as claimed in one of the preceding claims, the high voltage delivered to the respective high-voltage output of the DC voltage source (14) being at least 1.2 times, preferably at least 1.5 times, the dielectric voltage.

The device as claimed in one of the preceding claims, the first roller (1) and/or the second roller (2) being connected to earth potential.

The device as claimed in one of the preceding claims, the at least one high-voltage output of the DC voltage source (14) delivering a negative high voltage to the electrode tips (10a, 10b), so that, during the operation of the device, a stable, largely homogeneous low-energy plasma is in each case obtained, emanating from the respective electrode tips.

The device as claimed in one of the preceding claims, the distance (x, y) of the multiplicity of electrode tips (10a, 10b) of the first electrode and/or the second electrode from the respective surface (16a, 16b) of the respective electrically interacting roller (1, 2) lying in the range between 5 and 15 mm, preferably between 8 and 12 mm.

The device as claimed in one of claims 4 to 8, the distance of the electrode tips (10a, 10b) within a row (20a, 20b) lying in the range between 4 and 8 mm and preferably being approximately 5 mm.

The device as claimed in one of the preceding claims, the influencing taking the form of charging and changing the direction of the particles (4) of the particle mist, and the first gas discharge (11a) particularly causing a precipitation of particles (4) on the first roller (1), and the second gas discharge (lib) particularly causing a precipitation of particles (4) on the second roller (2) .

11. The device as claimed in one of the preceding claims, the first roller (1) and/or the second roller (2) having a surface (16a, 16b) which has a restrictedly conductive coating, preferably of rubber, NBR or Rilsan, with a resistivity of between 10⁶ and 10⁷ Ω x m.

12. The device as claimed in one of the preceding claims, the first electrode and the second electrode being arranged together in a housing (9) in such a way that the first row (20a) and the second row (20b) run at a distance from one another, the electrode tips (10a) of the first row (20a) preferably being arranged with respect to the electrode tips (10b) of the second row (20b) laterally offset from one another, particularly preferably laterally offset approximately 2.5 mm from one another.

The device as claimed in claim 12, the housing (9) being arranged in such a way that the electrode tips of the first row (20a) and of the second row (20b) are arranged parallel to the roller outlet (18) at a distance of approximately 0.4 to 0.6 times, preferably at a distance of approximately 0.5 times the roller radius (R) of the first and/or second roller (1, 2) with respect to a gauge axis (17) running perpendicular to the roller axes and parallel to the transporting direction in the tangent line in the roller outlet (18) .

14. The device as claimed in claim 12 or 13, the housing (9) additionally having at least one air duct (8a), at least one outlet opening (8b) and preferably a compressed-air connection (13), the at least one outlet opening (8b) being arranged between the rows (20a, 20b) of electrode tips (10a, 10b), for feeding air in centrally.

The device as claimed in claim 14, the housing (9) having a multiplicity of outlet openings (8b) with a diameter of in each case approximately 1 mm and a distance from one another in the range between approximately 5 and 15 mm, preferably approximately 10 mm.

A method for influencing liquid droplets or particles (4) at the roller outlet (18) of a pair of rollers (1, 2) comprising a first rotating roller (1) and a second counter-rotating roller (2), in particular for influencing a particle mist (4) of a coating material or a printing ink in an inking unit of a printing press or in a roller coating machine, a first electrode with a multiplicity of electrode tips (10a) directed towards the first roller (1) and a second electrode with a multiplicity of electrode tips (10b) directed towards the second roller (2) being provided, electrode tips (10a, 10b) of each electrode (1, 2) being respectively arranged in a row (20a, 20b) and having in each case a dedicated current-limiting element (12a, 12b), in particular in each case a resistor, the method comprising the following method steps:

- applying a high DC voltage to the multiplicity of electrode tips (10a) of the first electrode, so that a first gas discharge (11a) is obtained, preferably by choosing a high DC voltage of at least 1.2 times the dielectric voltage of a stable low-energy plasma discharge, between the electrode tips (10a) of the first electrode and the first roller (1);

applying a high DC voltage to the multiplicity of electrode tips (10b) of the second electrode, so that a second gas discharge (lib) is obtained, preferably by choosing a high DC voltage of at least 1.2 times the dielectric voltage of a stable low-energy plasma discharge, between the electrode tips (10b) of the second electrode and the second roller (2) .