DE102024108396A1 - Process for producing an air-laid fibrous web by means of a forming fabric designed as a perforated film covering - Google Patents

Abstract

The invention relates to a method for producing a fibrous web, in particular a paper, cardboard or tissue web, from a fibrous starting material, comprising the following steps:
a) low-water processing of the fibrous starting material in an air stream to produce individual fibres and/or fibre bundles;
b) forming the individual fibres and/or fibre bundles in the air stream to form a flat fibrous web by a dry forming process in which the individual fibres and/or fibre bundles are deposited on a forming fabric;
c) applying a maximum of 30% by weight, preferably a maximum of 20% by weight, more preferably a maximum of 10% by weight, of water and/or binders to the fibers of the formed fibrous web;
d) consolidating the formed fibrous web; wherein the forming fabric comprises a perforated film covering (54), preferably being formed substantially from a perforated film covering (54), wherein electrically conductive means (66) are arranged in and/or on the film covering (54). Furthermore, the invention relates to a forming fabric and a machine with such a forming fabric for carrying out the method.

Description

1. a) wasserarme Aufbereitung der faserstoffhaltigen Ausgangsware in einem Luftstrom zu Einzelfasern und/oder Faserbündel;
2. b) Formieren der Einzelfasern und/oder Faserbündel im Luftstrom zu einer flächigen Faserstoffbahn durch ein Trockenformierungsverfahren, bei welchem die Einzelfasern und/oder Faserbündel auf einem Formiersieb abgelegt werden;
3. c) Aufbringen von höchstens 30 Gew.%, bevorzugt höchsten 20 Gew.%, weiter bevorzugst höchstens 10 Gew.%, an Wasser und/oder Bindemitteln auf die Fasern der formierten Faserstoffbahn;
4. d) Verfestigen der formierten Faserstoffbahn.

The present invention relates to a process for producing a fibrous web, in particular a paper, cardboard or tissue web, from a fibrous starting material, comprising the following steps:

1. a) low-water processing of the fibrous starting material in an air stream to produce individual fibres and/or fibre bundles;
2. b) forming the individual fibres and/or fibre bundles in the air stream to form a flat fibrous web by a dry forming process in which the individual fibres and/or fibre bundles are deposited on a forming fabric;
3. c) applying a maximum of 30% by weight, preferably a maximum of 20% by weight, more preferably a maximum of 10% by weight, of water and/or binders to the fibers of the formed fibrous web;
4. d) Consolidation of the formed fibrous web.

Many fibrous webs, and in particular paper, board, and tissue webs, were and still are produced almost exclusively on an industrial scale using the wet process. Unless waste paper is used, baled pulp is usually dissolved in large quantities of water in a vat to produce a fiber suspension consisting of approximately 99% water by weight and only approximately 1% fiber by weight. The fiber suspension is then applied to a forming fabric via a headbox and pre-dewatered to form a sheet. The fibrous web is then dewatered or dried using pressure and heat until it can finally be wound up and/or processed in another way. The wet process has the advantage that hydrogen bonds form between the individual fibers during dewatering or drying, giving the fibrous web the necessary strength. A disadvantage of this process, however, is that large amounts of energy are required to dry the fibrous web. Especially in light of today's climate change, alternatives to this classic wet process are being intensively sought.

As an alternative to the wet process, for example, the publication US$4,167,378 A dry air-laying process is known in which fibers are laid down in a largely dry state to form a fibrous web. To impart the necessary strength to the fibrous web, only relatively small amounts of water (to form hydrogen bonds) and/or other binding agents are added. This results in significantly less energy being required for drying. The fibers are usually laid down on a woven forming fabric, which is permeable to air to draw in the fibers from below the forming fabric. Alternatively, air-permeable spiral fabrics can also be used as forming fabrics.

One of the challenges of this process is to reliably transport the fibrous web deposited on the forming fabric and then transfer it to a further treatment unit, in particular a bonding unit, without the fibers becoming entangled, accumulating locally, or becoming permanently stuck somewhere. As long as no water and/or binder has been applied to the fibers and the fibrous web has been bonded, there is virtually no cohesion between the fibers of the fibrous web. Currently, this is largely unproblematic, as the dry air-laying process is primarily used today to manufacture medical products such as diapers, and not to produce paper, board, or tissue webs, at least not on an industrial scale. However, with increasing fiber quantities and/or production speeds, problems increasingly arise, particularly with regard to fiber entanglement and/or fibers becoming stuck at the end of the forming fabric where the fibrous web is supposed to leave the forming fabric.

It is therefore an object of the present invention to solve or at least mitigate the aforementioned problems of the prior art. In particular, a method and apparatus are to be provided with which a high-quality fibrous web, in particular a paper, board, or tissue web, can be reliably produced using the dry air-laid process. This should be possible on an industrial scale, i.e., with production volumes of several tons per day, preferably at least one ton per hour, with the produced fibrous web preferably being wound up as a roll at the end of the production machine.

This object is achieved by the features of the independent claims. The dependent claims describe advantageous developments of the present invention. In particular, according to a first aspect of the present invention, the object is achieved by the generic manufacturing method described above, which is particularly characterized in that the forming fabric comprises a perforated film covering, preferably being formed essentially from a perforated film covering, with electrically conductive means arranged in and/or on the film covering.

The inventors have recognized that perforated film coverings can lead to better release behavior at the end of the transport path, where the fiber web is transferred to the next treatment unit, compared to conventional woven fabrics. This is presumably due to the fact that the fibers are more likely to get caught at the intersection points of the threads of the woven fabrics. However, this advantage of perforated film coverings only becomes fully apparent if specific means are provided in or on the fabric to counteract the buildup of electrostatic forces or to dissipate electrostatic forces already present from upstream process steps. The causes and generators of electrostatic charging of the fibers can be diverse. Since the perforated film coverings have a larger contact surface with the rollers that guide them in a circular motion and possibly with other static components, such as the vacuum boxes, they experience increased friction compared to woven fabrics. This friction can – unlike the wet-laying process – quickly lead to electrostatic charges, which can have an undesirable impact on the formation and adhesion of the fibers. Alternatively or additionally, however, it can also happen that the fibers enter the forming heads already charged from the pipes. Furthermore, in dry climates, they can constantly change their charge through contact with the pipes, other fibers, etc.

The film covering can essentially consist of a monolithic film or be a film laminate. The term "film" refers to a flat structure, usually made of plastic, which is inherently waterproof. The film can be produced by casting, extrusion, or another process. The term "film laminate," on the other hand, refers to a structure consisting of multiple layers, which are themselves films and are bonded together over a large area, for example by gluing or welding, particularly without creating any cavities. The film layers can be made of the same or different materials. Furthermore, they can all have the same or different thicknesses. The total thickness of the film laminate should be between 0.5 mm and 1.5 mm. A film laminate is generally preferred because it has the advantage that the individual layers can be more easily stretched bi- or monoaxially prior to lamination than a monolithic film of the same thickness as the film laminate, so that a film laminate can exhibit higher tensile strengths than a monolithic film of the same thickness. Alternatively, however, the forming fabric can also consist, for example, of a combination of such a perforated film laminate and a conventional fabric, in which case the film laminate is preferably arranged on the side of the forming fabric facing the fibrous web during intended use.

For the purposes of the present invention, a paper web is understood to be a web made of “conventional” paper, in particular of so-called graphic paper. A paper web differs from a board or tissue web primarily in its specific basis weight (measured in g/m ² ), whereby the specific basis weight of a board web is higher and the specific basis weight of a tissue web is lower than that of a paper web. In particular, the fibrous web can be a tissue web with a specific basis weight of 28 g/m ² to 42 g/m ^2. With such lightweight fibrous webs, the problem of clean separation from the forming fabric is particularly pronounced and the advantages of the present invention therefore become particularly apparent. In addition to the different basis weights, paper, board and tissue also generally differ in their intended uses, in particular writing, protecting and absorbing.

For the purposes of the present invention, "low-water processing" means that essentially no water is used to separate the fibers from the starting material, which is a significant difference from the wet-laying process. The starting material can be, for example, pulp in the form of bales, also called "bale pulp," or in the form of "fluff pulp." Alternatively, it is also conceivable to use waste paper, for example. Even if the fibers are preferably only dissolved in an air stream during "low-water processing," this should not preclude the air in the air stream from being conditioned, in particular by adjusting its moisture content.

The forming fabric has a fiber web side facing the fiber web and a machine side facing away from the fiber web. The perforations extend from the fiber web side to the machine side, or vice versa, allowing the fibers to be sucked in. The perforations should be sized to prevent fibers from being sucked through them.

In an advantageous embodiment of the present invention, the electrically conductive means can be applied to the film covering in the form of a coating. The coating can be electrically conductive or have a have noticeably lower electrical resistance than the rest of the film covering. If the coating is also electrically conductively connected to the machine bed, electrical charge can be effectively dissipated, preventing the buildup of an electric field that could negatively affect the fibers' alignment and/or adhesion.

In particular, the coating can be applied to the film covering on the side of the film covering facing the fibrous web during normal operation. Alternatively or additionally, the coating can also be applied to the side of the film covering facing away from the fibrous web during normal operation. If a corresponding coating is provided on both sides of the film covering, these are preferably electrically conductively connected to one another, for example via the edges of the film covering. In this way, charge can be conducted from the fibrous web side of the covering to the machine side, from where it can then be very easily discharged into the machine bed, for example via electrically conductive deflection rollers.

A particularly preferred embodiment provides that the coating extends at least partially into the perforations of the film covering, preferably creating an electrically conductive connection between the side of the film covering facing the fibrous web during normal operation and the side of the film covering facing away from the fibrous web during normal operation.

Especially when the perforations are created using a laser in the film covering, it is advantageous to apply the coating after the perforation, as the coating can otherwise hinder the perforation process due to a different laser absorption rate. Furthermore, applying the coating after the perforation allows the coating to extend into the perforations and even cover the perforation walls.

The coating is preferably applied by spraying, similar to painting, but other processes such as electroplating and/or sputtering are also possible.

The coating may comprise or be formed from a metallic component. Alternatively, the coating may also comprise or be formed from carbon, preferably in the form of graphite or carbon black. Carbon is a very good electrical conductor. It can also be present, for example, in the form of so-called "carbon nanotubes." However, the concentration of such "carbon nanotubes" must be sufficiently high to achieve the desired conductivity.

As an alternative to the coating, the electrically conductive means can also be provided in a plastic material of at least one layer of the film covering. For example, soot particles or carbon nanotubes can be present in this at least one layer to increase electrical conductivity. Alternatively or additionally, in a film laminate, the electrically conductive means can also be provided in at least one adhesive layer between two adjacent layers of the film laminate.

Another alternative to the coating provides that the electrically conductive means can be in the form of electrically conductive threads arranged on at least one side of the film covering. They are preferably arranged on the fibrous web side of the film covering. For example, the threads can be very thin copper wires that are attached, for example, glued, to the fibrous web side of the film covering.

Regardless of the exact design of the electrically conductive means, it is advantageous if the electrically conductive means are arranged in and/or on the film covering such that a first side edge of the forming fabric is electrically conductively connected to a second side edge of the forming fabric opposite the first side edge. For example, the charge can be discharged into the machine bed at both side edges of the film covering, for example by a carbon fiber brush applied there. It is not absolutely necessary for the entire surface of the fibrous web to be electrically conductive. Instead, it is sufficient if electrically conductive threads or coating strips are present only at specific intervals and extend in the cross-machine direction of the forming fabric. In this case, the side edges of the forming fabric are preferably electrically conductive throughout.

A particularly advantageous development of the present invention provides that the electrically conductive means are arranged in and/or on the film covering in such a way that a plurality of electrically conductive strips are produced which extend between the two side edges of the forming fabric, preferably in the width direction of the film covering, wherein the individual strips do not have an electrically conductive connection between have. In particular, in this case the side edges of the forming fabric should not be electrically conductive throughout. The strips can be provided, for example, by electrically conductive threads or by an electrically conductive coating. The advantage of this embodiment is that the rotating forming fabric can be electrostatically charged or discharged to different degrees in different sections. In particular, it is possible to allow an electrostatic charge in a front section of the rotating forming fabric, where the fibers are laid down on the forming fabric, in order to deliberately hold the fibers on the forming fabric, whereas in a rear section of the forming fabric, where the fibrous web is to be transferred from the forming fabric to a further treatment unit, for example to a bonding fabric, an electrostatic discharge can be brought about in a targeted manner in order to avoid or at least reduce an attraction between the forming fabric and the fibrous web due to electrostatic forces at this point. The strips should have a width that is preferably no greater than the length of a transfer zone in which the fibrous web is transferred from the forming fabric to a subsequent treatment unit. A non-electrically conductive insulator strip can be provided between each two adjacent electrically conductive strips. The width of the insulator strip can be significantly smaller than the width of the electrically conductive strips. In the transfer zone, for example, electrically conductive brushes can touch the side edges of the forming fabric to specifically dissipate the electrostatic charge in this zone from the forming fabric, for example, into the machine bed.

The perforations can be introduced into the film covering by drilling, in particular laser drilling, and/or by punching and/or hot needling. Laser drilling in particular makes it possible to create very fine perforations with a high number of perforations per area in the film covering. This can lead to very even absorption of the fibers across the surface. Hot needling involves piercing the film with a hot needle, whereby, unlike punching, the film material is only displaced and not separated. This results in less or no waste, which is an advantage. Unlike punching, when drilling, in particular laser drilling, and hot needling, the walls of the perforations can have an angle other than 90° to the fiber web side and/or machine side of the forming fabric. In particular, the perforations can be tapered.

A further aspect of the present invention relates to a forming fabric for producing a fibrous web from a fibrous starting material according to the method according to the invention described above, wherein the forming fabric comprises a perforated film covering, preferably being formed substantially from a perforated film covering, and wherein electrically conductive means are arranged in and/or on the film covering.

A still further aspect of the present invention relates to a machine for producing a fibrous web from a fibrous starting material according to the above-described method according to the invention, comprising a above-described forming fabric according to the invention.

An advantageous development of the present invention provides that the electrically conductive means are arranged in and/or on the film covering such that a plurality of electrically conductive strips are produced which extend between both side edges of the forming fabric, preferably in the width direction of the film covering, wherein the individual strips have no electrically conductive connection between them, and that the machine comprises means, in particular electrically conductive brushes, in a transfer area in which the fibrous web is transferred from the forming fabric to a further treatment unit, in particular to a consolidation fabric, which are designed to specifically electrostatically discharge the forming fabric in the transfer area. In contrast, the forming fabric is preferably not electrostatically discharged in an area upstream of the transfer area in which the fibers are deposited on the forming fabric and the resulting fibrous web is transported on the forming fabric. This makes it possible to increase the adhesion of the fibers or the fibrous web to the forming fabric in this area.

• 1+2: das erfindungsgemäße Verfahren zur Herstellung einer luftgelegten Faserstoffbahn bzw. eine Maschine zur Durchführung des Verfahrens;
• 3+4: die Herstellung einer mittels eines Lasers perforierten Folienbespannung;
• 5: einen Abschnitt einer perforierten Folienbespannung im Querschnitt, ohne elektrisch leitfähige Mittel;
• 6: die Folienbespannung wie in 5, jedoch mit einer ersten Ausprägung elektrisch leitfähiger Mittel;
• 7: die Folienbespannung wie in 5, jedoch mit einer zweiten Ausprägung elektrisch leitfähiger Mittel; und
• 8: die Folienbespannung wie in 5, jedoch mit einer dritten Ausprägung elektrisch leitfähiger Mittel.

The invention will be explained in more detail below using an exemplary embodiment described with the aid of schematic figures. In the following:

• 1 + 2 : the method according to the invention for producing an air-laid fibrous web and a machine for carrying out the method;
• 3 + 4 : the production of a film covering perforated by means of a laser;
• 5 : a section of a perforated film covering in cross-section, without electrically conductive means;
• 6 : the foil covering as in 5 , but with a first expression of electrically conductive means;
• 7 : the foil covering as in 5 , but with a second form of electrically conductive means; and
• 8 : the foil covering as in 5 , but with a third form of electrically conductive means.

The 1 and 2 show in a schematic representation a machine 10 for producing a dry-laid fibrous web 14, in particular paper, board or tissue web, according to the method of the present invention.

1. a) wasserarme Aufbereitung der faserstoffhaltigen Ausgangsware 12 in einem Luftstrom zu Einzelfasern und/oder Faserbündel;
2. b) Formieren der Einzelfasern und/oder Faserbündel im Luftstrom zu einer flächigen Faserstoffbahn 14 durch ein Trockenformierungsverfahren, bei welchem die Einzelfasern und/oder Faserbündel auf einem Formiersieb 16 abgelegt werden.

1 shows a front and middle part of the machine 10 in which the following process steps are carried out:

1. a) low-water processing of the fibrous starting material 12 in an air stream to form individual fibres and/or fibre bundles;
2. b) Forming the individual fibers and/or fiber bundles in the air stream to form a flat fiber web 14 by a dry forming process in which the individual fibers and/or fiber bundles are deposited on a forming fabric 16.

• b) Formieren der Einzelfasern und/oder Faserbündel im Luftstrom zu einer flächigen Faserstoffbahn 14 durch ein Trockenformierungsverfahren, bei welchem die Einzelfasern und/oder Faserbündel auf einem Formiersieb 16 abgelegt werden;
• c) Aufbringen von höchstens 30 Gew.%, bevorzugt höchsten 20 Gew.%, weiter bevorzugst höchstens 10 Gew.%, an Wasser und/oder Bindemitteln auf die Fasern der formierten Faserstoffbahn 14;
• d) Verfestigen der formierten Faserstoffbahn 14.

2 shows the middle and a rear part of the machine 10, in which the following process steps are carried out:

• b) forming the individual fibers and/or fiber bundles in the air stream to form a flat fiber web 14 by a dry forming process in which the individual fibers and/or fiber bundles are deposited on a forming fabric 16;
• c) applying at most 30 wt.%, preferably at most 20 wt.%, more preferably at most 10 wt.%, of water and/or binders to the fibers of the formed fibrous web 14;
• d) Consolidation of the formed fibrous web 14.

For the sake of clarity, a forming unit in which process step b) is carried out is shown in both 1 as well as in 2 shown, even if in the present embodiment this is actually only included once in the machine 10.

In the present exemplary embodiment, the fibrous starting material 12 is preferably in the form of bales consisting of a plurality of stacked cellulose sheets, for example from the manufacturer "Mercer Stendal GmbH." This pulp can be NBSK pulp, as is typically used to produce a paper, board, or tissue web using the wet process. The bale pulp can have a density between 600 kg/m ³ and 1,000 kg/m ³ , in particular of around 920 kg/m ³ , wherein the individual cellulose sheets can have a thickness between 1.0 mm and 2.0 mm, in particular of approximately 1.5 mm.

To carry out process step a), in this exemplary embodiment, the bales can first be placed in a chute 18, beneath which a conveyor belt 20 runs in a circle. The chute 18 can be designed such that only 1 to 50, preferably 2 to 25, more preferably 3 to 10, of the pulp boards are fed to a shredder 22 at a time in a substantially horizontal direction. However, this is not mandatory. Entire bales could also be fed to the shredder 22 at once and/or the feeding can take place without the conveyor belt 20. The shredder 22 comprises at least one rotor 24 provided with projections, upstream of which are two driven feed rollers. The shredder 22 produces a plurality of chips 26 from the bale pulp fed to it. The chips 26 produced in the shredder 22 are next fed, preferably by air flow, to a cleaning device 28. This cleaning device 28 can, for example, be a so-called air separator. In this, the chips 26 are fed into a riser pipe through which air is blown from below by a fan 30. The chips 26 are washed around and swirled by the air stream, dislodging impurities 32, such as sand or the like. These impurities 32 fall downwards in the riser pipe due to gravity against the air flow, where they can leave the riser pipe. The chips 26, on the other hand, are carried upwards by the air flow and next reach a cyclone air separator device 34, where the air is separated from the chips. The separated air can, for example, as in 1 Indicated by an arrow, the chips 26 are returned to the cleaning device 28. The cleaned chips 26 fall from the cyclone air separator device 34 downward into a collection container 36. The density of the pulp in the collection container 36, where the pulp is present in chips 26, is preferably at most one-quarter of the density of the pulp before the shredder 22, where the pulp is in the form of baled pulp. In other words, the pulp in the form of chips 26 is preferably relatively loose in the collection container 36.

From the collecting container 36, a continuous flow of chips 26, preferably The fibers 26 are then fed, for example, by air flow, for which a further blower 38 can be used, to a defibration device 40, where the chips 26 are defibrated into individual fibers in a dry process. The individual fibers then enter an air-laying unit 42 to form the dry-laid fibrous web 14 from the fibers. The fibrous web 14 is formed on a rotating, air-permeable forming fabric 16, over which the fibrous web 14 is simultaneously transported away.

As in 2 As shown, the fibrous web 14 is next transferred from the forming unit to a consolidation unit, in which process steps c) and d) are carried out. For this purpose, the fibrous web 14 is transferred from the forming fabric 16 to a consolidation fabric 44. The consolidation fabric 44 transports the fibrous web 14 through a press nip 46 formed by two rollers. Beforehand, the fibrous web 14 is moistened by means of jet nozzles 48, wherein the amount of water and/or binder applied is relatively small, in particular less than 15% by weight, preferably less than 10% by weight, even more preferably less than 8% by weight, based on the weight of the fibrous web 14 before moistening. The fibrous web 14 is then passed through a dryer 50 before being wound up on a winder 52. The fibrous web 14 is preferably transported overhead by the consolidation fabric 44, for example, by means of negative pressure. Furthermore, the moistening of the fibrous web 14 by means of the jet nozzles 48 preferably takes place from below. In this way, neither the forming fabric 16 nor the consolidation fabric 44 is moistened, which significantly reduces the cleaning effort and preferably eliminates the need for cleaning these fabrics 16, 44 at all. The consolidation fabric 16 and/or at least one of the two rollers forming the press nip 46 can have a structure for deliberately forming local high-pressure zones in the fibrous web 14.

By applying water and/or a binder by means of the jet nozzles 48, in combination with the application of pressure in the press nip 46, wherein the fibrous web 14 is preferably also heated in the press nip 46, it is possible to impart sufficient strength to the fibrous web 14, even without large quantities of water and/or without large quantities of other strength agents, such as melt fibers or starch, and preferably without any other strength agents at all, in order to be able to wind up the fibrous web 14 at the end without any problems and for further processing. Preferably, just as much water is applied as is necessary to achieve a sufficient amount of hydrogen bonds for this purpose. Because only small amounts of water need to be removed from the fibrous web 14, the manufacturing process is particularly gentle on energy and water resources. In addition, the addition of chemical additives, such as melt fibers, can be dispensed with, so that the finished fiber web 14 can also be easily biodegraded or composted.

However, in order for the entire manufacturing process to function reliably at high process speeds, in particular at process speeds of more than 150 m/min, preferably more than 250 m/min, more preferably more than 400 m/min, it is necessary to transport the fibrous web 14 safely and without disruptions from the air-laying unit 42 to the bonding unit, where it essentially first acquires its strength. In this case, the fibrous web 14 can be imagined as a thin layer of dust until it is bonded. The dust layer is held on the forming fabric 16 by negative pressure. For this purpose, in the loop of the forming fabric 16 - in the 1 and 2 not shown - vacuum boxes are arranged and the forming screen 16 is designed to be permeable.

According to the invention, a film covering 54 is used as the forming fabric 16, which is perforated to make it permeable. 3 and 4 An example is shown of how such a film covering 54 can be perforated using a laser. For this purpose, a laser beam LB emitted by the laser is directed onto the film substrate. The laser is controlled by a controller. The laser drills a plurality of discrete through-holes or perforations 56 from a surface 58 facing the laser in the thickness direction TD of the film covering 54 to the opposite surface facing away from the laser.

During the perforating process, the film covering 54, which is preferably designed as an endless loop, can be stretched between two rollers R, as shown in 4 shown. The perforations 56 can, for example, be laser-drilled into the film covering 54 in rows, starting from a first side edge 60 of the film covering 54 to a second side edge 62 of the film covering 54 opposite the first side edge 60.

As in 5 As shown, the film covering 54 is preferably formed essentially from a film laminate comprising several layers 64. The layers 64 can consist of monolithic films which are connected to one another in a planar manner, preferably free of cavities between them, for example by means of thin, Adhesive layers. In the present embodiment, the film laminate comprises four layers 64, all of which are essentially the same thickness. For the sake of clarity, only two of the illustrated perforations 56 are provided with reference numerals.

The inventors have recognized that, with a perforated film covering 54 as the forming fabric 16, the fibrous web 14 can generally be transported reliably even at high speeds, with the fibrous web being easier to detach from the forming fabric 16 than with conventional, woven forming fabrics. However, it is important to correctly control the electrostatic forces, which build up much more strongly with perforated film coverings 54 due to friction than with conventional, woven fabrics. These forces can have a negative impact on the fibers deposited on the forming fabric. For example, the process for detaching the fibers from the forming fabric 16 can be negatively affected, and fibers can permanently accumulate as contamination in certain places. For this reason, according to the present invention, it is proposed to specifically arrange electrically conductive means 66 in and/or on the film covering in order to counteract the build-up of electrostatic forces between the forming fabric 16 and the individual fibers and/or fiber bundles or to dissolve already existing electrostatic forces.

In 6 A first embodiment of a film covering 54 according to the invention with electrically conductive means 66 is shown schematically. In this case, a coating 68 is applied to both sides of the film covering 54, which forms the electrically conductive means 66. The coating 68 is designed to be electrically conductive, for example by comprising carbon black or graphite. The coating 68 was only applied after the film covering 54 was perforated and therefore also extends into the perforations 56. Specifically, it covers the side walls of the perforations 56 such that an electrically conductive connection is created between the two sides of the film covering 54. The coating 68 can preferably be applied by spraying. However, other processes, such as in particular electroplating and sputtering, are also possible.

For the purposes of the present invention, "electrically conductive" means that it has an electrical resistance that is significantly lower than that of the film covering 54, or significantly lower than that of the layers 64 of the film laminate. Preferably, the specific electrical resistance of the electrically conductive means 66 is less than 250 ohms*cm. The specific electrical resistance (in ohms*cm) of the electrically conductive means 66 is understood to be the electrical resistance (in ohms) per length (in cm) and per reciprocal of the cross-sectional area of the electrically conductive means 66 (in 1/cm ² ).

In 7 A second embodiment of a foil covering 54 according to the invention with electrically conductive means 66 is shown schematically. In this embodiment, at least one layer 64, in this embodiment exactly one layer 64, namely the uppermost layer 64, is electrically conductive, for example by comprising carbon black or graphite. An additional process step, such as the coating of the foil covering 54 after perforation according to the embodiment of 6 is therefore not necessary.

In 8 A third embodiment of a foil covering 54 according to the invention with electrically conductive means 66 is shown schematically. Electrically conductive threads 70, in particular copper threads with a very small diameter, are applied, for example, glued, to one of the two surfaces of the foil covering 54. The threads 70 are preferably so thin and aligned that they do not obscure any perforations 56.

Regardless of the specific type of electrically conductive means 66 used, it is advantageous if they create a connection between the two side edges 60, 62 of the film covering 54. The side edges 60, 62 may also include electrically conductive means 66, so that electrical charge can be easily dissipated via the side edges 60, 62 into the bed of the machine 10.

The method according to the invention makes it possible to produce a dry-laid fibrous web 14 at a production speed of more than 400 m/min and/or a width of more than 200 cm, and/or at a quantity of more than 1 t/h. The fibrous web can, in particular, be a tissue web with a basis weight between 28 g/m ² and 42 g/m ² .

List of reference symbols:

10

machine

12

fiber-containing raw material

14

Fibrous web

16

Forming screen

18

shaft

20

conveyor belt

22

shredder

24

rotor

26

Schnitzel

28

Cleaning device

30

fan

32

Contamination

34

Cyclone air separator device

36

Collection container

38

additional fan

40

defibration device

42

Air-laying unit

44

Consolidation sieve

46

Press nip

48

Jet nozzles

50

dryer

52

Winder

54

Foil covering

56

perforation

58

surface facing the laser

60

first page margin

62

second page margin

64

layer

66

electrically conductive agents

68

Coating

70

electrically conductive threads

LB

laser beam

R

roller

TD

Thickness direction

WD

Latitude direction

QUOTES CONTAINED IN THE DESCRIPTION

This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US 4,167,378 [0003]

Claims (17)

A method for producing a fibrous web (14), in particular a paper, cardboard or tissue web, from a fibrous starting material (12), comprising the following steps: a) low-water processing of the fibrous starting material (12) in an air stream to form individual fibers and/or fiber bundles; b) forming the individual fibers and/or fiber bundles in the air stream to form a flat fibrous web (14) by a dry forming method in which the individual fibers and/or fiber bundles are laid down on a forming fabric (16); c) applying at most 30% by weight, preferably at most 20% by weight, more preferably at most 10% by weight, of water and/or binders to the fibers of the formed fibrous web (14); d) consolidating the formed fibrous web (14); characterized in that the forming screen (16) comprises a perforated film covering (54), preferably is formed substantially from a perforated film covering (54), wherein electrically conductive means (66) are arranged in and/or on the film covering (54). Procedure according to Claim 1 , characterized in that the electrically conductive means (66) are applied in the form of a coating (68) on the film covering (54). Procedure according to Claim 2 , characterized in that the coating (68) is applied to the film covering (54) on the side of the film covering (54) facing the fibrous web (14) during normal operation. Procedure according to Claim 2 or 3 , characterized in that the coating (68) is applied to the film covering (54) on the side of the film covering (54) facing away from the fibrous web (14) during normal operation. Method according to one of the Claims 2 until 4 , characterized in that the coating (68) extends at least partially into the perforations (56) of the film covering (54), preferably creating an electrically conductive connection between the side of the film covering (54) facing the fibrous web (14) during normal operation and the side of the film covering (54) facing away from the fibrous web (14) during normal operation. Method according to one of the Claims 2 until 5 , characterized in that the coating (68) is applied to the film covering (54) only after perforation. Method according to one of the Claims 2 until 6 , characterized in that the coating (68) is applied by spraying and/or galvanizing and/or sputtering. Method according to one of the Claims 2 until 7 , characterized in that the coating (68) comprises or is formed from a metallic component. Method according to one of the Claims 2 until 7 , characterized in that the coating (68) comprises or is formed from carbon, preferably in the form of graphite and/or soot. Procedure according to Claim 1 , characterized in that the electrically conductive means (66) are provided in a plastic material of at least one layer (64) of the film covering (54). Procedure according to Claim 1 , characterized in that the electrically conductive means (66) are designed in the form of electrically conductive threads (70) which are arranged on at least one side of the film covering (54). Method according to one of the preceding claims, characterized in that the electrically conductive means (66) are arranged in and/or on the film covering (54) in such a way that a first side edge (60) of the forming fabric (16) is electrically conductively connected to a second side edge (62) of the forming fabric (16) opposite the first side edge (60). Method according to one of the preceding claims, characterized in that the electrically conductive means (66) are arranged in and/or on the film covering (54) in such a way that a plurality of electrically conductive strips are produced which extend between the two side edges (60, 62) of the forming fabric (16), preferably in the width direction (WD) of the film covering (54), the individual strips having no electrically conductive connection between them. Method according to one of the preceding claims, characterized in that the perforations (56) in the film covering (54) are introduced by drilling, in particular laser drilling, and/or by punching and/or by hot needling. Forming fabric (16) for producing a fibrous web (14) from a fibrous starting material (12) according to the method according to one of the preceding claims, characterized in that the forming fabric (16) has a perforated film covering (54), preferably formed substantially from a perforated film covering (54), wherein electrically conductive means (66) are arranged in and/or on the film covering (54). Machine (10) for producing a fibrous web (14) from a fibrous starting material (12) according to the method according to one of the Claims 1 until 14 , comprising a forming fabric (16) according to Claim 15 . Machine (10) after Claim 16 , characterized in that the electrically conductive means (66) are arranged in and/or on the film covering (54) in such a way as to result in a plurality of electrically conductive strips which extend between the two side edges (60, 62) of the forming fabric (16), preferably in the width direction (WD) of the film covering (54), the individual strips having no electrically conductive connection between them, and in that the machine, in a transfer area in which the fibrous web (14) is transferred from the forming fabric (16) to a further treatment unit, in particular to a consolidation fabric (44), comprises means, in particular electrically conductive brushes, which are designed to specifically electrostatically discharge the forming fabric (16) in the transfer area.