WO2025201841A1 - Method 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 material web, in particular a paper, cardboard or tissue web, from a fibrous-material-containing starting material, comprising the following steps: a) preparing, in a low-water manner, the fibrous-material-containing starting material in an air stream to form individual fibres and/or fibre bundles; b) forming the individual fibres and/or fibre bundles in the air flow into a flat fibrous web by means of a dry forming method in which the individual fibres and/or fibre bundles are deposited on a forming fabric; c) applying at most 30 wt.%, preferably at most 20 wt.%, more preferably at most 10 wt.%, water and/or binders onto the fibres of the formed fibrous web; d) solidifying the formed fibrous web; wherein the forming fabric 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). The invention also relates to a forming fabric and to a machine having such a forming fabric for carrying out the method.

Description

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

The present invention relates to a method for producing a fibrous web, in particular a paper, board 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 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 by a dry forming method in which the individual fibers and/or fiber bundles are laid down on a forming fabric; 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; d) consolidating 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 discloses a dry air-laying process in which fibers are laid 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 binders are added. This results in significantly less energy being required for drying. The fibers are usually laid onto 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 the object of the present invention to solve or at least reduce the aforementioned problems of the prior art. In particular, a method and a device are to be provided with which a high-quality fibrous web, in particular paper, cardboard or tissue web, can be can be reliably produced using a dry air-laying process. This should be possible on an industrial scale, i.e., with production quantities of several tonnes 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 effect on the formation and adhesion of the fibers. Alternatively or additionally, it can also happen that the fibers are already They enter the forming heads charged from the pipelines. In dry climates, they can also constantly change their charge through contact with the pipelines, 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 several layers, which are themselves films and which are bonded to one another over the entire surface, in particular without creating any cavities, for example by gluing or welding. The film layers can be made of the same material or different materials. Furthermore, they can all have the same thickness 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 a so-called graphic paper. A paper web differs from a cardboard or tissue web primarily in its specific basis weight (measured in g/m ² ), whereby the specific basis weight of a cardboard 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 28g/ ^m² to 42g/ ^m² . With such lightweight fibrous webs, the problem of cleanly separating 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, cardboard, and tissue also generally differ in their intended uses, particularly 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.

Preferably, no water and/or binder is applied to the forming fabric and/or the fibrous web while it is on the forming fabric. By providing the electrically conductive means in and/or on the film covering serving as the forming fabric, the application of water and/or binder at this point is not necessary to dissipate electrical charge or prevent the buildup of electrostatic charge. The less water and/or binder used, the less energy is required to dry the fibrous web. It is further proposed that the forming fabric be so flat on the side facing the fibrous web that the finished fibrous web does not have any texturing originating from the forming fabric that is visible to the naked eye. This distinguishes the forming fabric according to the present invention from so-called "texturing belts" or the like, which are specifically designed to exert a structuring effect on the fibrous web, e.g. to make its feel and/or appearance more appealing. However, the inventors have recognized that the use of such structuring clothing is disadvantageous as a forming fabric because they make it more difficult to detach the fibrous web from the forming fabric, e.g. during transfer to a next clothing. It is therefore advantageous if the forming fabric is as smooth or flat as possible on its upper side facing the fibrous web.

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 significantly 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, and no electric field builds up that could negatively influence 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 transported 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.

Alternatively to the coating, the electrically conductive means can also be incorporated into 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 them. 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 deliberately brought about 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, particularly 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 surface area in the film covering. This can lead to very even fiber absorption across the surface. With hot-needling, the film is pierced with a hot needle. Unlike punching, the film material is only displaced and not severed. This results in less or no waste during hot-needling, which is a benefit. Unlike punching, drilling, especially laser drilling, and hot-needling can result in perforation walls that are at an angle other than 90° to the fibrous web side and/or the 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 in such a way as to form a plurality of electrically conductive strips 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 arranged 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. electrostatically discharged. This increases the adhesion of the fibers or fibrous web to the forming fabric in this area.

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

Figures 1 + 2: the method according to the invention for producing an air-laid fibrous web and a machine for carrying out the method;

Figures 3+4: the production of a film covering perforated by means of a laser;

Figure 5: a section of a perforated foil covering in cross section, without electrically conductive means;

Figure 6: the foil covering as in Figure 5, but with a first

Formation of electrically conductive agents;

Figure 7: the foil covering as in Figure 5, but with a second

Formation of electrically conductive agents; and

Figure 8: the foil covering as in Figure 5, but with a third

Formation of electrically conductive agents.

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

Figure 1 shows a front and middle part of the machine 10, in which the following process steps are carried out: 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 process in which the individual fibers and/or fiber bundles are deposited on a forming fabric 16. Figure 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 fibrous 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 a maximum of 30 wt.%, preferably a maximum of 20 wt.%, more preferably a maximum of 10 wt.%, of water and/or binders to the fibers of the formed fibrous web 14; d) Consolidating the formed fibrous web 14.

For the sake of better clarity, a forming unit in which method step b) is carried out is shown in both Figure 1 and Figure 2, even if in the present embodiment it 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 large number 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), the bales in this embodiment can first be placed in a shaft 18, beneath which a conveyor belt 20 runs in a circle. The shaft 18 can be designed such that only 1 to 50, preferably 2 to 25, more preferably 3 to 10, of the cellulose boards are fed simultaneously in a substantially horizontal direction to a shredder 22. 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 be, for example, a so-called air sifter. In this, the chips 26 are fed into a riser pipe through which air is blown from below by means of a blower 30. The chips 26 are washed around and swirled around by the air flow, whereby impurities 32, such as sand or the like, are released. These contaminants 32 fall due to gravity against the air flow in the riser pipe, where they can exit 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, be returned to the chips 26 upstream of the cleaning device 28, as indicated by an arrow in Figure 1. The cleaned chips 26 fall down from the cyclone air separator device 34 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 a maximum of one quarter of the density of the pulp upstream of 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 can then be fed, preferably also 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 pass into an air-laying unit 42 in order 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 Fibre web 14 is simultaneously transported away.

As shown in Figure 2, 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 to process it further. Preferably, just as much water is applied as is necessary to achieve a sufficient amount of hydrogen bonds for this purpose. By only small amounts of water being released from the fiber Since the fibers must be removed from the fabric web 14, the manufacturing process is particularly energy and water-efficient. Furthermore, the addition of chemical additives, such as melt fibers, is eliminated, so the finished fiber web 14 is easily biodegradable and compostable.

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 disruption from the air-laying unit 42 to the bonding unit, where it essentially only acquires its strength. Until it is bonded, the fibrous web 14 can be imagined as a thin layer of dust. The dust layer is held on the forming fabric 16 by vacuum. For this purpose, vacuum boxes (not shown in Figures 1 and 2) are arranged in the loop of the forming fabric 16, and the forming fabric 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. Figures 3 and 4 show, by way of example, how such a film covering 54 can be perforated using a laser. For this purpose, a laser beam LB emanating from 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 perforation process, the film covering 54, which is preferably designed as an endless loop, can be stretched between two rollers R, as shown in Figure 4. The perforations 56 can, for example, be drilled into the film covering 54 by the laser 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 shown in Figure 5, the film covering 54 is preferably formed essentially from a film laminate comprising several layers 64. The layers 64 can consist of monolithic films that are bonded to one another in a planar manner, preferably without any voids between them, for example, by means of thin adhesive layers provided between the films. In the present exemplary 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 perforations 56 shown 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.

Figure 6 schematically shows a first embodiment of a foil covering 54 according to the invention with electrically conductive means 66. A coating 68 is applied to both sides of the foil covering 54, forming the electrically conductive means 66. The coating 68 is electrically conductive, for example, by comprising carbon black or graphite. The coating 68 was applied only after the perforation of the foil covering 54 and extends therefore also into the perforations 56. Specifically, it covers the side walls of the perforations 56 in such a way that an electrically conductive connection is established between the two sides of the foil covering 54. The coating 68 can preferably be applied by spraying. However, other processes, such as 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 ).

Figure 7 schematically shows a second embodiment of a foil covering 54 according to the invention with electrically conductive means 66. In this embodiment, at least one layer 64—in this exemplary embodiment, precisely 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 coating the foil covering 54 after perforation according to the exemplary embodiment of Figure 6, is thus unnecessary.

Figure 8 schematically shows a third embodiment of a foil covering 54 according to the invention with electrically conductive means 66. 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 which type of electrically conductive means 66 is used for

are used, it is advantageous if they have a connection between the two Side edges 60, 62 of the film covering 54. The side edges 60, 62 can also include electrically conductive means 66, so that electrical charge can be easily discharged 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 with 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 machines

12 fiber-containing raw materials

14 Fibrous web

16 Forming screen

18 shaft

20 conveyor belt

22 shredders

24 rotors

26 schnitzels

28 Cleaning device

30 blowers

32 contaminants

34 Cyclone air separator device

36 collection containers

38 additional fans

40 defibration device

42 Air laying unit 44 Consolidation sieve

46 Press nip

48 jet nozzles

50 dryers

52 winders

54 foil covering

56 Perforation

58 surface facing the laser

60 first page margin

62 second page margin

64 shift

66 electrically conductive agents

68 Coating

70 electrically conductive threads

LB laser beam

R roller

TD thickness direction

WD Latitude Direction

Claims

Patent claims 1 . 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 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 (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). 2. Method according to claim 1, characterized in that no water and/or binding agent is applied to the forming fabric (16) and/or to the fibrous web (14) while the latter is on the forming fabric (16). 3. Method according to claim 1 or 2, characterized in that the forming fabric (16) is formed so flat on the side facing the fibrous web (14) that the finished fibrous web (14) has no texturing originating from the forming fabric (16) that is visible to the naked eye. 4. Method according to one of the preceding claims, characterized in that the electrically conductive means (66) are applied in the form of a coating (68) on the film covering (54). 5. Method according to claim 4, 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. 6. Method according to claim 4 or 5, 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. 7. Method according to one of claims 4 to 6, 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. 8. Method according to one of claims 4 to 7, characterized in that the coating (68) has been applied to the film covering (54) only after perforation, and/or that the coating (68) has been applied by spraying and/or galvanizing and/or sputtering. 9. Method according to one of claims 4 to 8, characterized in that the coating (68) comprises a metallic component or is formed therefrom, or that the coating (68) comprises carbon, preferably in the form of graphite and/or carbon black, or is formed from it. 10. Method according to claim 1 to 3, 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). 11. Method according to claim 1 to 3, 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). 12. 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) such 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). 13. 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), wherein the individual strips have no electrically conductive connection between them. 14. 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. 15. 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) 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). 16. Machine (10) for producing a fibrous web (14) from a fibrous starting material (12) according to the method according to one of claims 1 to 14, comprising a forming fabric (16) according to claim 15. 17. Machine (10) according to claim 16, 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 both side edges (60, 62) of the forming fabric (16), preferably in the width direction (WD) of the film covering (54), wherein the individual strips have 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.