FORMING SCREEN WITH UNPERFORATED LONGITUDINAL STRIPS, A MACHINE HAVING THE FORMING SCREEN AND METHOD OF PRODUCING A FIBROUS WEB

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation, under 35 U.S.C. § 120, of copending International Patent Application PCT/EP2024/068427, filed Jul. 1, 2024, which designated the United States; this application also claims the priority, under 35 U.S.C. § 119, of German Patent Application DE 10 2023 121 031.9, filed Aug. 8, 2023; the prior applications are herewith incorporated by reference in their entirety.

FIELD AND BACKGROUND OF THE INVENTION

The invention relates to a forming screen for a machine for producing a fibrous web, in particular a tissue web. The forming screen is substantially formed from a perforated, preferably laser-perforated, substrate and has the form of a continuous web extending in a longitudinal direction, and further has a first lateral edge and a second lateral edge opposite the first lateral edge. The forming screen contains a perforated useful region arranged between the first lateral edge and the second lateral edge, on which the fibrous web to be produced is formed when the forming screen is used as intended and which is spaced apart from the first lateral edge and the second lateral edge. The forming screen further contains a first edge strip arranged between the first lateral edge and the useful region and a second edge strip arranged between the second lateral edge and the useful region, wherein the first edge strip and the second edge strip are also perforated.

Machines for producing a fibrous web, such as a paper, cardboard or tissue web, usually comprise a forming section in which a fibrous suspension mainly consisting of water is applied, by means of a so-called headbox, onto a forming screen through which a considerable portion of the water is discharged, while the fibers remain on the screen where the so-called sheet formation takes place. The remaining machine then substantially only serves to dry the fibrous web. For this purpose, it usually comprises a press section in which the drying takes place by means of mechanical pressure, and a drying section in which the drying takes place through heat. Tissue webs are particularly light-weight fibrous webs, the surface density of which is not more than 25 g/m². For example, typical surface densities of tissue webs are around 15 g/m². Here, the mechanical pressing often occurs against a heated Yankee cylinder.

In the past and even today, most forming screens have been or are typically woven. However, since the upper side of the forming screen facing the fibrous web should be structured in a very fine manner to obtain a correspondingly fine formation, the production process for such a woven fabric is very time-consuming. Therefore, it has already been proposed in the past to use perforated substrates instead of a woven fabric. As such, the term “substrate” should generally be understood to mean a film material made of plastic. For the production of forming screens, it is expedient to use a laser for perforating as it can generate very fine perforations.

This idea has already been described in U.S. Pat. No. 5,837,102, the contents thereof being incorporated by reference herein. FIG. 1 illustrates the perforation method by means of laser drilling according to U.S. Pat. No. 5,837,102. As such, FIG. 1 only shows a portion of a substrate 20′ from which the forming screen is manufactured. The substrate comprises a first surface 22′ and an opposite second surface 24′ which cannot be seen in FIG. 1. Substrate 20′ is perforated by means of a laser beam LB from a laser connected to a controller to drill a plurality of through-channels 30′ into substrate 20′. Through-channels 30′ extend in thickness direction TD of substrate 20′, i.e., orthogonally to first surface 22′ and to opposite second surface, thus connection first surface 22′ to the opposite second surface.

During laser perforation, substrate 20′ may be clamped between two rollers R, as illustrated in FIG. 2. In the process, a substantially uniform arrangement of through-channels 30′ is introduced into the substrate across the entire extension in width direction WD of substrate 20′ between a first lateral edge 26′ and a second lateral edge 28′ opposite first lateral edge 26′. As shown in FIG. 3, the through-channels may have a cross-sectional form changing in thickness direction TD of substrate 20′, in particular a cross-sectional form tapered from first surface 22′, where the laser beam enters and which faces the fibrous web when the forming screen is used as intended, to second surface 24′, where the laser beam exits and which faces away from the fibrous web when the forming screen is used as intended.

Usually, the fibrous suspension is applied onto the forming screen from the headbox across the entire width, even though only the fibrous web in a smaller useful region situated between the lateral edges is ultimately used. The formation at the edges of the forming screen is often of less good quality, so that these edges are removed. For this purpose, it is known to employ edge jet nozzles in the forming section, by means of which the edges are cut off by a water jet and passed on to a pulper. The remaining web in the central useful region then substantially corresponds to the width of the finished fibrous web at the end of the machine, when it is wound up there, wherein the shrinkage of the web caused by the drying should be disregarded here. The edge jet nozzles are high-maintenance as fibers can easily adhere thereto which, when falling off the fibrous web in lumps, may result in a flawed formation or even to the web breaking in the machine.

Further, international patent disclosure WO 2012/095251 A1, corresponding to U.S. Pat. No. 8,784,615, also discloses a perforated, in particular laser-perforated, film clothing which is usable as a forming screen for a machine for producing a fibrous web. Between the perforated useful region of this film clothing and the left and right lateral edges, respectively, an edge region extends, having a lower perforation density than the useful region of the film clothing. Therein, the respective edge region may not be perforated at all. The intent and purpose of this edge strip is to provide high safety against tearing at the clothing edge while the clothing is used as intended.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a forming screen and a method of producing a fibrous web which solve the problem described above. In this respect, the forming screen should preferably be producible as simply as possible, and the maintenance required during production of the fibrous web should be kept as low as possible.

This object is achieved by the features of the independent claims. Advantageous developments of the invention are the subject-matter of the dependent claims.

With the foregoing and other objects in view there is provided, in accordance with the invention, a forming screen for a machine for producing a fibrous web. The forming screen contains a perforated substrate having a form of a continuous web extending in a longitudinal direction, a first lateral edge and a second lateral edge opposite the first lateral edge. A perforated useful region is disposed between the first lateral edge and the second lateral edge, and on the perforated useful region the fibrous web to be produced is formed when the forming screen is used and the perforated useful region is spaced from the first lateral edge and the second lateral edge. A first edge strip is disposed between the first lateral edge and the perforated useful region and a second edge strip is disposed between the second lateral edge and the perforated useful region. The first edge strip and the second edge strip are also perforated. An unperforated first separating strip extending in the longitudinal direction is disposed between the first edge strip and the perforated useful region and/or an unperforated second separating strip extending in the longitudinal direction is disposed between the second edge strip and the perforated useful region. The unperforated first separating strip and/or the unperforated second separating strip has or have a width of between 1 mm and 8 mm. The forming screen further has an upper side facing the fibrous web and a lower side facing away from the fibrous web. The perforated useful region has a plurality of through-channels that extend through the perforated substrate and connect the upper side to the lower side. The through-channels are non-cylindrical and have a cross-sectional area decreasing along a thickness direction of the perforated substrate from the upper side to a central region of the perforated substrate disposed between the upper side and the lower side. 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.

Specifically, the object is achieved by the generic forming screen described in the beginning, which is particularly characterized in that an unperforated first separating strip extending in the longitudinal direction is arranged between the first edge strip and the useful region and/or an unperforated second separating strip extending in the longitudinal direction is arranged between the second edge strip and the useful region. The unperforated first separating strip and/or the unperforated second separating strip has or have a width of between 1 mm and 8 mm, preferably of between 2 mm and 6 mm, more preferably of between 3 mm and 5 mm.

The at least one unperforated separating strip achieves no drainage to occur in the area of this separating strip, so that there is no solidification or sheet formation of the fibrous web therein. Thus, this automatically results in a containment of the fibrous web in the useful region of the forming screen. As a result, the use of edge jet nozzles may be omitted completely, also eliminating the maintenance and cleaning work related to such edge jet nozzles.

In addition, the solution according to the invention also has the advantage of the forming screen being producible in a particularly quick and easy manner. That is, the at least one separating strip remaining unperforated saves the energy and time to introduce corresponding perforations. Alternatively, it may also be contemplated to subsequently close the through-channels 30′ in substrate 20′, described above with regard to FIGS. 1 to 3 relating to the prior art, in particular with a coating; however, this solution is more laborious and there not preferable. Moreover, coatings may be subject to wear due to abrasion.

Experiments have shown that it is important for the at least one unperforated separating strip to have a certain width, i.e., a certain extension in the width direction of the forming screen. This is the only way to ensure that there is no solidification or sheet formation in the at least one unperforated separating strip, however, without too much suspension not becoming drained. The latter may cause problems if the suspension flows into the useful region.

According to the invention, it is also provided for the forming screen to have, when used as intended, an upper side facing the fibrous web and a lower side facing away from the fibrous web. The useful region contains a plurality of through-channels extending through the substrate and connecting the upper side to the lower side. The through-channels are non-cylindrical and have a cross-sectional area decreasing 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 lower side. 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. By the upper edges of adjacent through-channels virtually contacting or intersecting one another, a very close arrangement of through-channels can be achieved which results in a very fine formation of the fibrous web.

Preferably, the first unperforated separating strip is spaced apart from the first lateral edge by between 20 mm and 160 mm, preferably by between 35 mm and 115 mm, and/or the second unperforated separating strip is spaced apart from the second lateral edge by between 20 mm and 160 mm, preferably by between 35 mm and 115 mm. This allows to achieve good formation results, without too much fibrous suspension being removed from the first edge strip and/or second edge strip.

To add stability to the forming screen, it is advantageous for an unperforated first edge protective strip extending in the longitudinal direction of the forming screen to be arranged between the first lateral edge and the perforated first edge strip and for an unperforated second edge protective strip extending in the longitudinal direction of the forming screen to be arranged between the second lateral edge and the perforated second edge strip. The first edge protective strip and the second edge protective strip each have a width of between 1 mm and 5 mm, preferably of between 1 mm and 3 mm, and directly adjoin the first and second lateral edges, respectively. On the one hand, this increases the strength and dimensional stability of the forming screen and, on the other hand, this can better protect the forming screen against wear at the lateral edges where, when used as intended, the forming screen often rubs against fixed components, such as a mechanical and/or pneumatic edge sensor.

To even better protect the lateral edges of the forming screen against wear, it may further be provided for the first edge protective strip and the second edge protective strip to each be coated with a protective layer. In this case, the protective layer may extend across a part of the first surface of the substrate and/or a part of the second surface of the substrate and/or across the periphery or a lateral edge in the thickness direction of the substrate. Preferably, all of these cases apply.

It is particularly advantageous for the respective protective layer to extend beyond the unperforated first edge protective strip and the unperforated second edge protective strip into the area of the perforated first edge strip and the perforated second edge strip, respectively. In this way, the material of the protective layer can flow into the perforations or through-channels of the substrate where, when solidifying, it establishes a form-fit connection therewith. This causes particularly good adhesion of the protective layer to the substrate.

The respective protective layer may comprise a polyurethane or preferably even substantially consist of a polyurethane. Polyurethane has proven to be particularly suitable for this purpose. In addition, it is relatively low-cost and easy to process.

An advantageous development of the present invention provides for the substrate to be formed from a film or from a laminate of multiple films, wherein the one film or the multiple films are preferably stretched, wherein more preferably at least one stretching direction substantially corresponds to the longitudinal direction of the forming screen. As such, the stretching may be uniaxial only in the longitudinal direction of the forming screen, or biaxial in two directions, wherein, in this case, one of the two stretching directions then preferably corresponds to the longitudinal direction of the forming screen. The stretching gives the molecular chains from which the film is formed a preferred orientation. If, at a later point in time, a tensile load acts on the film in precisely that direction, it can no longer expand to the same extent as would have been the case without prior stretching.

To achieve a formation of the fibrous web which is as uniform as possible, the useful region is proposed to have substantially homogeneous permeability. The first edge strip and the second edge strip may also have substantially homogeneous permeability which, however, may be different from the permeability of the useful region. In particular, the clear diameter of the through-channels of the edge strips may be chosen to be greater than that in the useful region, so that, with the first and second edge strips of the forming screen, not only is the water discharged through the through-channels but also the fibers from the applied fibrous suspension. Thus, edge strips are not created on the fibrous web from the start. This process may be aided by suction members drawing the water and the fibers through the through-channels.

Alternatively, it may also be advantageous for the first edge strip and the second edge strip to have substantially homogeneous permeability, which substantially corresponds to the permeability of the useful region, and/or for the first edge strip and the second edge strip to have a perforation density which substantially corresponds to the perforation density of the useful region. This also causes a paper web to form on the edge strips, the formation of which substantially corresponds to the paper web in the useful region and which can be easily lifted from the forming screen at the end of the forming section, to be passed on to a so-called pulper, for example. In particular, just like the useful region, the first and second edge strips may comprise a plurality of through-channels extending through the substrate and connecting the upper side to the lower side. The through-channels are non-cylindrical and have a cross-sectional area decreasing 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 lower side. 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.

A further aspect of the present invention relates to a machine for producing a fibrous web, in particular a tissue web, containing a forming screen according to the invention as described above. Preferably, in this case, a forming section of the machine, more preferably the entire machine, is free from a device for edge trimming of the fibrous web. Such a device may be omitted by using the forming screen according to the invention, in particular if it comprises both the first unperforated separating strip and the second unperforated separating strip.

Further, the present invention relates to a method of producing a fibrous web using a forming screen according to the invention as described above and preferably a machine according to the invention as described above. By means of a headbox, a fibrous suspension is applied onto the forming screen substantially across the entire width of the forming screen, and the excess part of the fibrous suspension applied to the first edge strip and the second edge strip is discharged towards a pulper. As such, the discharge of the excess part of the fibrous suspension substantially occurs through the perforations in the first edge strip and in the second edge strip.

Other features which are considered as characteristic for the invention are set forth in the appended claims.

Although the invention is illustrated and described herein as embodied in a forming screen with unperforated longitudinal strips, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

In the following, the present invention will be illustrated with reference to schematic figures not drawn to scale.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is an illustration showing a portion of a substrate from which a forming screen is manufactured according to the prior art;

FIG. 2 is an illustration showing the substrate being clamped between two rollers according to the prior art;

FIG. 3 is an illustration of through-channels having a cross-sectional form changing in a thickness direction of the substrate according to the prior art;

FIG. 4 is an illustration showing a portion of the forming screen according to the invention; and

FIG. 5 is a detailed close-up of an upper side of the forming screen shown in FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the figures of the drawings in detail and first, particularly to FIG. 4 thereof, there is shown a small portion of a forming screen 10 according to the invention, having a first lateral edge 26 and a second lateral edge 28 opposite first lateral edge 26 in a width direction WD of the forming screen 10. Between the two lateral edges 26, 28 and spaced apart therefrom, there is a perforated useful region 32. When the forming screen 10 is used as intended, a fibrous suspension is applied onto an upper side of the forming screen 10 across its entire width, wherein, however, only the fibrous suspension applied in useful region 32 is utilized for the fibrous web to actually be manufactured. Between the useful region 32 and the first lateral edge 26, there is a first edge strip 34 of the forming screen 10, and between the useful region 32 and the second lateral edge 28, there is a second edge strip 36 of the forming screen. The first edge strip 34 and the second edge strip 36 are also perforated. The fraction of the fibrous suspension which is applied onto the first edge strip 34 and onto the second edge strip 36 does not contribute to the finished fibrous web. Instead, this fraction of fibers is returned to a pulper.

Between the useful region 32 and the first edge strip 34, there is a first unperforated separating strip 40. Similarly, between the useful region 32 and the second edge strip 36, there is a second unperforated separating strip 42. Both separating strips 40, 42 have a width of at least 2 mm, but not more than 6 mm. As both separating strips 40, 42 are unperforated, drainage of the fibrous suspension and thus sheet formation do not occur thereon. This automatically results in a containment of the fibrous web to form on the useful region 32 along the separating strips 40, 42, without requiring edge cutting nozzles - like in the prior art.

In the useful region 32, through-channels 30 preferably have a cross-sectional form changing, in particular tapering, in a thickness direction TD of a substrate 20, from which the forming screen 10 is substantially formed, from a first surface 22 to which the fibrous suspension is applied when used as intended to a second surface facing away from first surface 22, similar to that shown in FIG. 3. As shown in FIG. 5 which shows a close-up of detail V from FIG. 4, through-channels 30 may then be so closely packed that their upper edges 44, i.e., the edges on the first surface of substrate 20, at least partially overlap one another, while their lower edges 46, i.e., the edges on the second surface of substrate 20, are clearly spaced apart from one another. This creates an egg-box-like topography on the upper side of forming screen 10 facing the fibrous web in the useful region. Of the originally substantially planar first surface 22 of substrate 20, only small spots remain, if at all. Moreover, the clear diameter of through-channels 30 in useful region 32 is very small, so that good filtration of the fibrous suspension to be applied onto forming screen 10 can take place. In other words, a significant fraction of the water from the fibrous suspension passes through through-channels 30, while the fibers are retained on the upper side of forming screen 10 in useful region 32, where the so-called sheet formation will then occur. The result is a very fine formation of the fibrous web.

However, whether sheet formation occurs on the first edge strip 34 and the second edge strip 36 or not mainly depends on how the perforations or through-channels are formed therein. In a preferred embodiment, the permeability of the first edge strip 34 and the second edge strip 36 is different from the permeability of the useful region 32 of the forming screen 10. In particular, a preferred embodiment of the forming screen 10 according to the invention provides for the clear diameter of the through-channels in both edge strips 34, 36 to be significantly greater than the clear diameter of the through-channels 30 in the useful region, so that not only the water but also the fibers can pass through the through-channels in both edge strips 34, 36. In this case, sheet formation does not occur from the start. At the same time, the through-channels in both edge strips 34, 36 can or should be spaced apart from one another farther than the through-channels 30 in the useful region to not impair the required structural strength of forming screen 10 or to even increase it compared to the useful region if needed.

Optionally, an unperforated first edge protective strip 48 extending in longitudinal direction LD of the forming screen 10 may be arranged between the first lateral edge 26 and the perforated first edge strip 34 and the unperforated second edge protective strip 50 extending in longitudinal direction LD of forming screen 10 may be arranged between second lateral edge 28 and perforated second edge strip 36, wherein first edge protective strip 48 and second edge protective strip 50 each have a width of between 1 mm and 5 mm, preferably of between 1 mm and 3 mm, and directly adjoin first and second lateral edges 26, 28, respectively. Edge protective strips 48, 50 primarily serve to protect lateral edges 26, 28 of forming screen 10 from wear.

Additionally, to provide even better wear protection, the first edge protective strip 48 and the second edge protective strip 50 may each be coated with a protective layer, for example of a polyurethane. The protective layer preferably extends across both a part of the first surface 22 of the substrate 20 and a part of the second surface of substrate 20 and across the periphery or lateral edge 26, 28 in the thickness direction TD of the substrate 20. It is particularly advantageous for the respective protective layer to extend beyond the unperforated first edge protective strip 48 or the unperforated second edge protective strip 50 into the area of perforated first edge strip 34 or the perforated second edge strip 36, respectively. In this way, the material of the protective layer can flow into the perforations or through-channels of the substrate where, when solidifying, it establishes a form-fit connection therewith. This causes particularly good adhesion of the protective layer to the substrate. For example, the protective layer may be applied in a width of 5 mm to 10 mm such that it is broader than the unperforated first edge protective strip 48 or the unperforated second edge protective strip 42.

It should be noted that the entire width of the forming screen according to the invention is either between 2.50 m and 3.00 m or, in double-width machines, between 5.00 m and 6.00 m. Preferably, the forming screen according to the invention is employed in a machine for producing a tissue web. This machine, or at least the forming section thereof, may or should be free from a device for edge cutting of the fibrous web or tissue web.

The following is a summary list of reference numerals and the corresponding structure used in the above description of the invention:

• • 10 Forming screen
  • 20, 20′ Substrate
  • 22, 22′ First surface
  • 24′ Second surface
  • 26, 26′ First lateral edge
  • 28, 28′ Second lateral edge
  • 30, 30′ Through-channel
  • 32 Useful region
  • 34 First edge strip
  • 36 Second edge strip
  • 40 Unperforated first separating strip
  • 42 Unperforated second separating strip
  • 44 Upper edge
  • 46 Lower edge
  • 48 Unperforated first edge protective strip
  • 50 Unperforated second edge protective strip
  • LB Laser beam
  • LD Longitudinal direction
  • R Roller
  • TD Thickness direction
  • WD Width direction