Forming screen with unperforated longitudinal strips
Patent Information
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- VOITH PATENT GMBH
- Filing Date
- 2024-07-01
- Publication Date
- 2026-06-17
Smart Images
Figure EP2024068427_13022025_PF_FP_ABST
Abstract
Description
[0001] Forming screen with
[0002] The invention relates to a forming fabric for a machine for producing a fibrous web, in particular a tissue web, wherein the forming fabric is essentially formed from a perforated, preferably laser-perforated, substrate and has the shape of an endless belt extending in a longitudinal direction, with a first side edge and a second side edge opposite the first side edge, wherein the forming fabric comprises a perforated useful area arranged between the first side edge and the second side edge, on which, during the intended use of the forming fabric, the fibrous web to be produced is formed and which is spaced from the first side edge and the second side edge, and wherein the forming fabric further comprises a first edge strip arranged between the first side edge and the useful area, and a second edge strip,which is arranged between the second side edge and the useful area, wherein the first edge strip and the second edge strip are also perforated.,
[0003] Machines for producing fibrous webs, such as paper, board, or tissue webs, typically comprise a forming section in which a fiber suspension consisting predominantly of water is applied to a forming fabric by means of a so-called headbox, through which a significant portion of the water is drained away, while the fibers remain on the fabric, where the so-called sheet formation takes place. The rest of the machine then essentially serves only to dry the fibrous web. For this purpose, it typically comprises a press section, where drying takes place using mechanical pressure, and a drying section, where drying takes place using heat. Tissue webs are particularly lightweight fibrous webs with a basis weight of no more than 25 g / m².2 Typical basis weights of tissue webs, for example, are around 15 g / m 2 Here, mechanical pressing often takes place against a heated Yankee cylinder.
[0004] Traditionally, most forming fabrics were, and still are, woven. However, since the upper side of the forming fabric facing the fibrous web must be very fine to achieve a correspondingly fine formation, the manufacturing process for such a fabric is very time-consuming. Therefore, it has been suggested in the past to use perforated substrates instead of fabrics. The term "substrate" generally refers to a plastic film material. For the production of forming fabrics, it is advisable to use a laser for perforation, as this can create very fine perforations.
[0005] This idea has already been described in the document US5837102, to the content of which reference is made here. Figure 1 illustrates the perforation method by means of laser drilling according to US5837102. In this case, Figure 1 shows only a section of a substrate 20' from which the forming fabric is manufactured. The substrate comprises a first surface 22' and an opposite second surface 24', which is not visible in Figure 1. The substrate 20' is perforated by means of a laser beam LB from a laser that is connected to a control unit in order to drill a plurality of through-channels 30' into the substrate 20'. The through-channels 30' extend in the thickness direction TD of the substrate 20', ie orthogonal to the first surface 22' and the opposite second surface, and thus connect the first surface 22' to the opposite second surface.
[0006] During laser perforation, the substrate 20' can be tensioned between two rollers R, as illustrated in Figure 2. In this case, a substantially uniform arrangement of through-channels 30' is introduced into the substrate over the entire extent in the width direction WD of the substrate 20' between a first side edge 26' and a second side edge 28' opposite the first side edge 26'. As shown in Figure 3, the through-channels can have a cross-sectional shape that changes in the thickness direction TD of the substrate 20', in particular a cross-sectional shape that tapers from the first surface 22', where the laser beam enters and which faces the fibrous web during normal use of the forming fabric, to the second surface 24', where the laser beam exits and which faces away from the fibrous web during normal operation of the forming fabric.As a rule, the fiber suspension is applied from the headbox across the entire width of the forming fabric, although in the end only the fiber web in a smaller usable area located between the side edges is used. The formation at the edges of the forming fabric is often of lower quality, so these edges are removed. For this purpose, it is known to use edge jet nozzles in the forming section, with the help of which the edges are cut off using a water jet and fed into a pulper. The remaining web in the central usable area then essentially corresponds to the width of the finished fiber web at the end of the machine when it is wound up there, whereby the shrinkage of the web caused by drying should be disregarded here.The edge jet nozzles are maintenance-intensive because fibers can easily accumulate on them, which, if they fall onto the fiber web as clumps, can lead to poor formation or even a web break in the machine.
[0007] Furthermore, WO 2012 / 095251 A1 also discloses a perforated, in particular laser-perforated, film covering that can be used as a forming fabric for a machine for producing a fibrous web. Between the perforated usable area of this film covering and the left or right side edge, there is an edge area that has a lower perforation density than the usable area of the film covering. The respective edge area may also be unperforated. The purpose of these edge strips is to provide a high level of security against tearing of the edge of the covering during its intended use.
[0008] The object of the present invention is to provide a forming fabric and a method for producing a fibrous web that solves the problem described above. The forming fabric should preferably be as simple to manufacture as possible, and the maintenance effort during the production of the fibrous web should be kept as low as possible.
[0009] This object is achieved by the features of the independent claims. The dependent claims relate to advantageous developments of the invention. Specifically, the object is achieved by the generic forming fabric described at the outset, which is particularly characterized in that a longitudinally extending, imperforate first separating strip is arranged between the first edge strip and the useful area and / or a longitudinally extending, imperforate second separating strip is arranged between the second edge strip and the useful area, wherein the imperforate first separating strip and / or the imperforate second separating strip has or have a width between 1 mm and 8 mm, preferably between 2 mm and 6 mm, more preferably between 3 mm and 5 mm.
[0010] The at least one unperforated separating strip ensures that no dewatering occurs in the area of this separating strip, preventing consolidation or sheet formation of the fibrous web there. This automatically limits the fibrous web within the usable area of the forming fabric. Consequently, the use of edge jet nozzles can be completely eliminated, thus eliminating the maintenance and cleaning work associated with these edge jet nozzles.
[0011] In addition, the solution according to the invention also has the advantage that the forming fabric can be manufactured particularly quickly and easily. By leaving at least one separating strip unperforated, the energy and time required to create corresponding perforations are saved. Alternatively, it would also be conceivable to subsequently close the through-channels 30' in the substrate 20', which was previously described with reference to Figures 1 to 3 of the prior art, particularly by means of a coating, but this solution is more labor-intensive and therefore not preferred. Furthermore, coatings can be subject to wear due to abrasion.
[0012] Tests have shown that it is important that the at least one imperforate separating strip has a specific width, i.e., a specific extension in the width direction of the forming fabric. This is the only way to ensure that no solidification or sheet formation occurs in the at least one imperforate separating strip, while still ensuring that too much of the suspension is not dewatered.
[0013] The latter can cause problems if the suspension flows into the usable area.According to the invention, it is further provided that the forming fabric has an upper side facing the fibrous web in its intended use and an underside facing away from the fibrous web, wherein the useful region comprises a plurality of through-channels which extend through the substrate and connect the upper side to the underside, wherein the through-channels are non-cylindrical and have a cross-sectional area which decreases along a thickness direction of the substrate from the upper side to a central region of the substrate located between the upper side and the underside, wherein an upper edge of at least one of the plurality of through-channels is in direct contact with an upper edge of at least one other adjacent through-channel of the plurality of through-channels in such a way that the upper edge of the at least one of the plurality of through-channels does not lie in one plane.By allowing the upper edges of adjacent through-channels to virtually touch or intersect, a very close arrangement of through-channels can be achieved, which leads to a very fine formation of the fibrous web.
[0014] Preferably, the first imperforate separating strip is spaced from the first side edge by between 20 mm and 160 mm, preferably between 35 mm and 115 mm, and / or the second imperforate separating strip is spaced from the second side edge by between 20 mm and 160 mm, preferably between 35 mm and 115 mm. This allows good formation results to be achieved without removing too much fiber suspension from the first edge strip and / or second edge strip.
[0015] In order to give the forming fabric greater stability, it is advantageous if an imperforate first edge protection strip extending in the longitudinal direction of the forming fabric is arranged between the first side edge and the perforated first edge strip, and an imperforate second edge protection strip extending in the longitudinal direction of the forming fabric is arranged between the second side edge and the perforated second edge strip, wherein the first edge protection strip and the second edge protection strip each have a width between 1 mm and 5 mm, preferably between 1 mm and 3 mm, and are directly adjacent to the first and second side edges, respectively.On the one hand, this increases the strength and dimensional stability of the forming fabric and, on the other hand, it can better protect the forming fabric against wear on the side edges, where the forming fabric frequently rubs against fixed components, such as a mechanical and / or pneumatic edge sensor, during normal use.
[0016] To further protect the side edges of the forming fabric against wear, it can further be provided that the first edge protection strip and the second edge protection strip are each coated with a protective layer. In this case, the protective layer can extend over a portion of the first surface of the substrate and / or a portion of the second surface of the substrate and / or over the edge or a side edge in the thickness direction of the substrate. Preferably, all of these cases apply.
[0017] It is particularly advantageous if the respective protective layer extends beyond the imperforate first edge protection strip or the imperforate second edge protection strip into the area of the perforated first edge strip or the perforated second edge strip. This allows the material of the protective layer to flow into the perforations or passageways of the substrate, where, upon solidification, it forms a positive connection with them. This ensures particularly good adhesion of the protective layer to the substrate.
[0018] The respective protective layer can comprise a polyurethane or, preferably, consist essentially of a polyurethane. Polyurethane has proven particularly suitable for this purpose. Furthermore, it is relatively inexpensive and easy to process.
[0019] An advantageous development of the present invention provides that the substrate is formed from a film or from a laminate of several films, wherein the one film or the several films are preferably stretched, wherein more preferably at least one stretching direction corresponds substantially to the longitudinal direction of the forming fabric. The stretching can take place uniaxially only in the longitudinal direction of the forming fabric, or biaxially in two directions, in which case one of the two stretching directions then preferably corresponds to the longitudinal direction of the forming fabric. Through stretching, the molecular chains from which the film is formed acquire a preferred orientation. If a tensile load later acts on the film in precisely this direction, it can no longer stretch as much as would otherwise be the case without prior stretching.
[0020] In order to achieve the most uniform formation of the fibrous web, it is proposed that the useful region have a substantially homogeneous permeability. The first edge strip and the second edge strip can also have a substantially homogeneous permeability, although this can differ from the permeability of the useful region. In particular, the clear width of the passage channels of the edge strips can be selected to be larger than that in the useful region, so that in the first and second edge strips of the forming fabric, not only the water is carried away through the passage channels, but also the fibers from the applied fibrous suspension. This means that no solid edge strips are formed on the fibrous web in the first place. This process can be promoted by suction elements that suck the water and the fibers through the passage channels.
[0021] Alternatively, however, it may also be advantageous if the first edge strip and the second edge strip have a substantially homogeneous permeability that substantially corresponds to the permeability of the useful area, and / or if the first edge strip and the second edge strip have a perforation density that substantially corresponds to the perforation density of the useful area. This also allows a paper web to form on the edge strips, which, in terms of its formation, substantially corresponds to the paper web in the useful area and which can be easily lifted off the forming fabric at the end of the forming section, for example, to be fed into a so-called pulper.In particular, the first and second edge strips, as well as the useful region, can comprise a plurality of through-channels that extend through the substrate and connect the top side to the bottom side, wherein the through-channels are non-cylindrical and have a cross-sectional area that decreases along a thickness direction of the substrate from the top side to a central region of the substrate located between the top side and the bottom side, wherein an upper edge of at least one of the plurality of through-channels is in direct contact with an upper edge of at least one other adjacent through-channel of the plurality of through-channels such that the upper edge of the at least one of the plurality of through-channels does not lie in a plane.
[0022] A further aspect of the present invention relates to a machine for producing a fibrous web, in particular a tissue web, comprising a previously described forming fabric according to the invention. Preferably, a forming section of the machine, more preferably the entire machine, is free of a device for edge trimming the fibrous web. Such a device can be dispensed with by using the forming fabric according to the invention, especially if it comprises both the first imperforate separating strip and the second imperforate separating strip.
[0023] Furthermore, the present invention relates to a method for producing a fibrous web using a previously described forming fabric according to the invention and preferably in a previously described machine according to the invention, wherein a fibrous suspension is applied to the forming fabric by means of a headbox substantially across the entire width of the forming fabric, and wherein the excess portion of the fibrous suspension applied to the first edge strip and the second edge strip is discharged into a pulper. The excess portion of the fibrous suspension can be discharged substantially through the perforations in the first edge strip and the second edge strip.
[0024] The present invention is illustrated below using schematic and not-to-scale figures. They show:
[0025] Figure 4: a section of a forming fabric according to the invention; and
[0026] Figure 5: a high magnification of the top of the forming fabric in Figure 4.
[0027] Figure 4 schematically shows a small section of a forming fabric 10 according to the invention with a first side edge 26 and a second side edge 28 opposite the first side edge 26 in the width direction WD of the forming fabric 10. Between the two side edges 26, 28 and at a distance from them is a perforated useful area 32. During normal use of the forming fabric 10, a fiber suspension is applied to an upper side of the forming fabric 10 across its entire width, however, only the fiber suspension applied in the useful area 32 is used for the fibrous web actually to be produced. Between the useful area 32 and the first side edge 26 there is a first edge strip 34 of the forming fabric 10, and between the useful area 32 and the second side edge 28 there is a second edge strip 36 of the forming fabric.The first edge strip 34 and the second edge strip 36 are also perforated. The portion of the fiber suspension applied to the first edge strip 34 and the second edge strip 36 does not contribute to the finished fiber web. Instead, this portion of fiber is returned to a pulper.
[0028] A first imperforate separating strip 40 is located between the usable area 32 and the first edge strip 34. A second imperforate separating strip 42 is also located between the usable area 32 and the second edge strip 36. Both separating strips 40, 42 have a width of at least 2 mm, but no more than 6 mm. Since the two separating strips 40, 42 are imperforate, no dewatering of the fiber suspension occurs on them, and thus no sheet formation occurs. This automatically limits the fiber web forming on the usable area 32 along the separating strips 40, 42, without the need for edge-cutting nozzles, as is required in the prior art.
[0029] In the useful area 32, the through-channels 30 preferably have a cross-sectional shape which changes, in particular tapers, in the thickness direction TD of a substrate 20, from which the forming fabric 10 is essentially formed, from a first surface 22, to which the fiber suspension is applied during intended use, to a second surface facing away from the first surface 22, similar to that shown in Figure 3. As shown in Figure 5, which is an enlargement of section V in Figure 4, the through-channels 30 can be located so close to one another that their upper edges 44, i.e. the edges on the first surface of the substrate 20, at least partially overlap one another, whereas their lower edges 46, i.e. the edges on the second surface of the substrate 20, are noticeably spaced from one another.This results in an egg-crate-like topography on the upper side of the forming fabric 10 facing the fibrous web in the usable area. Only small areas, if any, remain of the originally essentially flat first surface 22 of the substrate 20. In addition, the clear width of the through-channels 30 in the usable area 32 is very small, allowing for effective filtration of the fibrous suspension to be applied to the forming fabric 10. In other words, a significant portion of the water from the fibrous suspension passes through the through-channels 30, whereas the fibers on the upper side of the forming fabric 10 are retained in the usable area 32, where so-called sheet formation then occurs. The result is a very fine formation of the fibrous web.
[0030] Whether or not sheet formation occurs on the first edge strip 34 and the second edge strip 36, however, depends significantly on how the perforations or through-channels are designed there. In a preferred embodiment, the permeability of the first edge strip 34 and the second edge strip 36 differs from the permeability of the useful area 32 of the forming fabric 10. In particular, a preferred embodiment of the forming fabric 10 according to the invention provides that the clear width of the through-channels in the two edge strips 34, 36 is noticeably larger than the clear width of the through-channels 30 in the useful area, so that not only the water but also the fibers can pass through the through-channels in the two edge strips 34, 36. In this case, sheet formation does not even occur there.At the same time, the through-channels in the two edge strips 34, 36 can or should be spaced further apart from each other than the through-channels 30 in the useful area in order not to adversely affect the required structural strength of the forming fabric 10 there or, if necessary, even to increase it there compared to the useful area.
[0031] Optionally, an imperforate first edge protection strip 48 extending in the longitudinal direction LD of the forming fabric 10 can be arranged between the first side edge 26 and the perforated first edge strip 34, and an imperforate second edge protection strip 28 extending in the longitudinal direction LD of the forming fabric 10 can be arranged between the second side edge 28 and the perforated second edge strip 36, wherein the first edge protection strip 26 and the second edge protection strip 28 each have a width between 1 mm and 5 mm, preferably between 1 mm and 3 mm, and directly adjoin the first and second side edges 26, 28, respectively. The edge protection strips 26, 28 primarily serve to protect the side edges 26, 28 of the forming fabric 10 against wear.
[0032] Additionally, to provide even better wear protection, the first edge protection strip 26 and the second edge protection strip 28 can each be coated with a protective layer, for example made of a polyurethane. The protective layer preferably extends both over a portion of the first surface 22 of the substrate 20 and over a portion of the second surface of the substrate 20, as well as over the edge or side edge 26, 28 in the thickness direction TD of the substrate 20. It is particularly advantageous if the respective protective layer extends beyond the imperforate first edge protection strip 48 or the imperforate second edge protection strip 50 into the region of the perforated first edge strip 34 or the perforated second edge strip 36, respectively. This allows the material of the protective layer to flow into the perforations or through-channels of the substrate, where, when it solidifies, it forms a positive connection with them.This ensures particularly good adhesion of the protective layer to the substrate. For example, the protective layer can be applied with a width of 5 mm to 10 mm, so that it is wider than the imperforate first edge protection strip 48 or the imperforate second edge protection strip 42.
[0033] It should also be noted that the total width of the forming fabric according to the invention is typically between 2.50 m and 3.00 m, or between 5.00 m and 6.00 m for double-width machines. The forming fabric according to the invention is preferably used in a machine for producing a tissue web. This machine, or at least its forming section, can or should be free of a device for edge trimming the fibrous web or tissue web.
[0034] 10 forming screen
[0035] 20, 20' substrate
[0036] 22, 22' first surface
[0037] 24' second surface
[0038] 26, 26' first page margin
[0039] 28, 28' second margin
[0040] 30, 30' through channel
[0041] 32 usable area
[0042] 36 first verge
[0043] 38 second verge
[0044] 40 unperforated first separator strips
[0045] 42 unperforated second separating strip
[0046] 44 upper edge
[0047] 46 lower edge
[0048] 48 unperforated first edge protection strip
[0049] 50 unperforated second edge protection strip
[0050] LB laser beam
[0051] LD longitudinal direction
[0052] R roller
[0053] TD thickness direction
[0054] WD latitude direction
Claims
1. A forming fabric (10) for a machine for producing a fibrous web, in particular a tissue web, wherein the forming fabric (10) is formed essentially from a perforated, preferably laser-perforated, substrate (20) and has the shape of an endless belt extending in a longitudinal direction (LD), having a first side edge (26) and a second side edge (28) opposite the first side edge (26), wherein the forming fabric (10) comprises a perforated useful area (32) arranged between the first side edge (26) and the second side edge (28), on which, during the intended use of the forming fabric (10), the fibrous web to be produced is formed and which is spaced from the first side edge (26) and the second side edge (28), wherein the forming fabric (10) further comprises a first edge strip (34) arranged between the first side edge (26) and the useful area (32),and a second edge strip (36) which is arranged between the second side edge (28) and the useful area (32), wherein the first edge strip (34) and the second edge strip (36) are also perforated, characterized in that an imperforate first separating strip (40) extending in the longitudinal direction (LD) is arranged between the first edge strip (34) and the useful area (32) and / or an imperforate second separating strip (42) extending in the longitudinal direction (LD) is arranged between the second edge strip (36) and the useful area (32), wherein the imperforate first separating strip (40) and / or the imperforate second separating strip (42) has or have a width between 1 mm and 8 mm, preferably between 2 mm and 6 mm, more preferably between 3 mm and 5 mm,wherein the forming fabric (10) has an upper side facing the fibrous web in the intended use and a lower side facing away from the fibrous web, wherein the useful area (32) comprises a plurality of through-channels (30) which extend through the substrate (10) and connect the upper side to the lower side, wherein the through-channels are non-cylindrical and have a cross-sectional area which is along a thickness, direction (TD) of the substrate (20) decreases from the top side to a central region of the substrate (20) located between the top side and the bottom side, wherein an upper edge (44) of at least one of the plurality of through-channels (30) is in direct contact with an upper edge (44) of at least one other adjacent through-channel (30) of the plurality of through-channels (30) such that the upper edge (30) of the at least one of the plurality of through-channels (30) does not lie in one plane.
2. Forming screen (10) according to claim 1, characterized in that the imperforate first separating strip (40) is spaced from the first side edge (26) between 20 mm and 160 mm, preferably between 35 mm and 115 mm, and / or that the imperforate second separating strip (42) is spaced from the second side edge (28) between 20 mm and 160 mm, preferably between 35 mm and 115 mm.
3. Forming fabric (10) according to claim 1 or 2, characterized in that an imperforate first edge protection strip (26) extending in the longitudinal direction (LD) of the forming fabric (10) is arranged between the first side edge (26) and the perforated first edge strip (34), and an imperforate second edge protection strip (50) extending in the longitudinal direction (LD) of the forming fabric (10) is arranged between the second side edge (28) and the perforated second edge strip (36), wherein the first edge protection strip (48) and the second edge protection strip (50) each have a width between 1 mm and 5 mm, preferably between 1 mm and 3 mm, and directly adjoin the first and second side edges (26, 28), respectively.
4. Forming screen (10) according to claim 3, characterized in that the first edge protection strip (48) and the second edge protection strip (50) are each coated with a protective layer.
5. Forming fabric (10) according to claim 4, characterized in that the respective protective layer extends over the imperforate first edge protection strip (48) or the imperforate second Edge protection strip (50) extends into the area of the perforated first edge strip (34) or the perforated second edge strip (36).
6. Forming screen (10) according to claim 4 or 5, characterized in that the respective protective layer has a width between 4 mm and 12 mm, preferably between 5 mm and 10 mm.
7. Forming screen (10) according to one of claims 4 to 6, characterized in that the respective protective layer comprises polyurethane, preferably consists essentially of polyurethane.
8. Forming screen (10) according to one of the preceding claims, characterized in that the substrate (20) is formed from a film or from a laminate of several films, wherein the one film or the several films are preferably stretched, wherein more preferably at least one stretching direction corresponds substantially to the longitudinal direction (LD) of the forming screen (10).
9. Forming screen (10) according to one of the preceding claims, characterized in that the useful area (32) has a substantially homogeneous permeability.
10. Forming fabric (10) according to claim 9, characterized in that the first edge strip (34) and the second edge strip (36) have a substantially homogeneous permeability which differs from the permeability of the useful region (32).
11. Forming fabric (10) according to claim 9, characterized in that the first edge strip (34) and the second edge strip (36) have a substantially homogeneous permeability which substantially corresponds to the permeability of the useful area (32), and / or the first edge strip (34) and the second edge strip (36) have a perforation density which substantially corresponds to the perforation density of the useful area (32).
12. Machine for producing a fibrous web, in particular a tissue web, comprising a forming fabric (10) according to one of the preceding claims.
13. Machine according to claim 12, characterized in that one forming section (10) of the machine, preferably the entire machine, is free of a device for edge trimming the fibrous web.
14. A method for producing a fibrous web using a forming fabric (10) according to one of claims 1 to 11 and preferably in a machine according to claim 12 or 13, wherein a fibrous suspension is applied to the forming fabric (10) by means of a headbox substantially over the entire width of the forming fabric (10) and wherein the excess part of the fibrous suspension which is applied to the first edge strip (34) and to the second edge strip (36) is discharged into a pulper.
15. The method according to claim 14, characterized in that the removal of the excess part of the fiber suspension takes place essentially through the perforations in the first edge strip (34) and in the second edge strip (36).