Patent Publication Number: US-11655591-B2

Title: Paper machine clothing and method of producing the paper machine clothing

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority, under 35 U.S.C. § 119, of European patent application EP 19 217 789, filed Dec. 19, 2019; the prior application is herewith incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention concerns a paper machine clothing comprising a substrate with an upper side, a lower side, two lateral edges and an usable region between the two lateral edges, wherein the usable region comprises a plurality of through-channels extending through the substrate and connecting the upper side with the lower side, wherein the through-channels are non-cylindrical with a cross-sectional area becoming smaller when going in a thickness direction of the substrate from the upper side to a middle region of the substrate between the upper side and the lower side and wherein an upper rim of at least one of the plurality of through-channels directly contacts an upper rim of at least one other neighboring through-channel of the plurality of through-channels. Another aspect of the present invention concerns a method of producing such a paper machine clothing. 
     In the sense of the present invention the term “paper machine clothing,” abbreviated “PMC”, refers to any kind of a rotating clothing used to transport a nascent or already formed fiber web in a machine that is designed to continuously produce and/or finish a fiber web, such as paper, tissue or board material. For historical reasons, PMC is sometimes also called wire, felt or fabric. In particular, PMC can be a forming wire or a dryer fabric or a press felt, depending upon its intended use in the corresponding machine. Furthermore, in the sense of the present invention the term PMC may also refer to any kind of clothing used in wet and/or dry production of fibrous nonwovens. 
     The term “substrate” in the sense of the present invention refers to some kind of foil material made of plastic. The substrate itself is usually impermeable to water, so that through-channels are needed to obtain a desired permeability, e.g. for dewatering the nascent fiber web or further drying the already formed fiber web. The substrate can be formed in a monolithic way or comprise several layers that might be co-extruded or produced separately and laminated together afterwards. After joining the longitudinal ends of the substrate to each other, e.g. by laser welding, to obtain an endless belt, the perforated substrate may already represent the final product, for example a forming wire. For other applications, further steps might be necessary to produce the final PMC, such as permanently attaching fibers thereto to form a press felt. Furthermore, the substrate may comprise a reinforcing structure, such as yarns, that may be imbedded therein. After joining the longitudinal ends of the substrate to each other, the “upper side” of the substrate shall be the radially outer side, sometimes also referred to as “paper side,” whereas the “lower side” of the substrate shall be the radially inner side, sometimes also referred to as “machine side”. 
     The idea of producing a PMC from a substrate that is perforated, especially by using a laser, has already been known for quite some time in the prior art and was described, by way of example, in the 1980s and 1990s in the documents U.S. Pat. Nos. 4,541,895 and 5,837,102, respectively. The content of these published patents is herewith incorporated by reference.  FIG.  1    illustrates the processes of perforating a substrate via laser drilling according to the U.S. Pat. No. 5,837,102 reference.  FIG.  1    only shows a portion of a substrate  20 ′ used to produce a PMC forming fabric. The substrate  20 ′ has a first surface  22 ′ and an opposite second surface that is not shown in the figure. Even though the first surface  22 ′ may be embossed it can be considered as being substantially plane and parallel to the second surface. The substrate  20 ′ is perforated using a laser beam LB from a laser that is connected to a controller so as to drill a plurality of discrete through-channels  30 ′ into the substrate  20 ′. The through-channels  30 ′ connect the side of the first surface  22 ′ with the side of the opposite second surface of the substrate  20 ′. The through-channels  30 ′ extend in the thickness direction TD of the substrate  20 ′, i.e. perpendicular to the first surface  22 ′ and the second surface. 
     In the sense of the present invention the term “usable region” refers to a region of the PMC that is actually used for the production and/or finishing of the fiber web. The usable region may span the complete width of the PMC, i.e. may reach from one lateral edge to the other lateral edge thereof. Alternatively, the usable region may refer only to a region that is located between the two lateral edges and is spaced apart from the two lateral edges. In the latter case, the PMC may have another configuration, such as permeability and thickness, outside the usable region compared to the usable region. 
     The term “cross-sectional area” of a through-channel in the sense of the present invention refers to an area of the through-channel that is obtained by cutting, or cross-sectioning, the through-channel with a plane that is perpendicular to the thickness direction of the substrate. 
     The term “non-cylindrical” in the sense of the present invention means that there are at least two different cross-sectional areas of a through-channel. For example, in the case of a non-cylindrical through channel that is substantially conical, a cross-sectional area taken at a first plane perpendicular to the thickness direction of the substrate may be substantially circular having a first radius, whereas another cross-sectional area taken at a second plane perpendicular to the thickness direction of the substrate may be also substantially circular but having a second radius that differs from the first radius. 
     Another paper machine clothing is known for example from the disclosure of U.S. Pat. No. 4,446,187 and German published patent application DE 10 2010 040 089 A1, the content of which is hereby incorporated by reference.  FIGS.  2 ,  3 A,  3 B and  3 C  are based on the disclosure of U.S. Pat. No. 4,446,187. 
       FIG.  2    shows a substrate  20 ′ that is placed under tension between two rollers R. The substrate  20 ′ has a radially outer, first surface  22 ′ and an opposite, radially inner, second surface  24 ′, as can be seen in  FIGS.  3 A,  3 B and  3 C . The first surface  22 ′ and the second surface  24 ′ are planar and parallel to each other. The thickness direction TD is oriented perpendicular to the first surface  22 ′ and the second surface  24 ′. The substrate  20 ′ further comprises a first lateral edge  26 ′ and a second lateral edge  28 ′. In this example, the usable region of the substrate  20 ′ extends in width direction WD of the substrate  20 ′ the full way from the first lateral edge  26 ′ to the second lateral edge  28 ′. In the usable region the substrate  20 ′ is perforated by a laser that is drilling a plurality of discrete through-channels  30 ′ into the substrate  20 ′. As indicated in  FIG.  2    the laser first makes the through-channels  30 ′ close to the first lateral edge  26 ′ in a first row and continues moving across the substrate  20 ′ to the through-channel  30 ′ close to the second lateral edge  28 ′ at the end of the same row. Thereafter, the laser is displaced by one row to make another through-channel  30 ′ close to the first lateral edge  26 ′ in a next row. 
       FIGS.  3 A,  3 B and  3 C  show different possible configurations of the through-channels  30 ′. In  FIG.  3 A  the through-channel is cylindrical having the same cross-sectional area at any location along the thickness direction TD of the substrate  20 ′. In  FIG.  3 B  the through-channel  30 ′ is conical wherein the cross-sectional area of the through-channel  30 ′ close to the first surface  22 ′ is larger than the cross-sectional area of the through-channel  30 ′ close to the second surface  24 ′. In  FIG.  3 C  the through-channel  30 ′ is neither cylindrical nor conical. Instead it resembles a hyperboloid having a cross-sectional area that is also always circular, like in the previous two examples, but the radius of this circle is first decreasing when going in thickness direction TD from the first surface  22 ′ to a middle region MR of the substrate  20 ′ situated in the thickness direction TD between the first surface  22 ′ and the second surface  24 ′, and is then increasing again when further going from the middle region MR of the substrate  20 ′ to the second surface  24 ′. 
     Fiber retention, permeability and the degree of marking are characteristic parameters of a PMC that are important in view of the quality of the fiber web that is to be produced and/or finished on the PMC. 
     A paper machine clothing according to the preamble part of claim  1  is already known from the disclosure of commonly assigned, prior-filed European published patent applications EP3348708 A1 and EP3561176 A1. In these documents it is proposed to place neighboring through-channels so close to each other that their upper rims directly contact each other. The through-holes preferably “intersect” or “overlap” each other and, thus, make the topography of the upper surface of the substrate resemble the topography of an “egg crate.” With such a PMC a good permeability can be achieved with a high open area ratio on the paper side. This is especially important for good quality results of a nascent paper web when the PMC is used as a forming fabric. 
     However, the nascent paper web formed on a forming fabric is generally very prone to markings. Markings occur when the nascent paper web is not equally well dewatered over its complete surface. Especially in view of these markings, it turned out that the paper machine clothing disclosed in the European patent applications EP3348708 A1 and EP3561176 A1 might be even further improved. 
     BRIEF SUMMARY OF THE INVENTION 
     Thus, it is an object of the present invention to provide a paper machine clothing with improved characteristics compared to the known paper machine clothing, thereby allowing to produce a fiber web of very high quality. 
     With the above and other objects in view there is provided, in accordance with the invention, a paper machine clothing, comprising: 
     a substrate having an upper side, a lower side, two lateral edges, and a usable region between said two lateral edges, the usable region having formed therein a plurality of through-channels each extending along a central axis through said substrate and connecting said upper side with said lower side; 
     said through-channels being non-cylindrical with a cross-sectional area becoming smaller along a thickness direction of said substrate from said upper side to a middle region of said substrate between said upper side and said lower side; 
     an upper rim of at least one of said through-channels directly contacting an upper rim of at least one neighboring through-channel of said plurality of through-channels; 
     said upper rims of both said neighboring through-channels having at least one common local maximum; 
     wherein a sectional plane parallel to the thickness direction of said substrate, including said at least one common local maximum and including or intersecting the central axis of at least one of said neighboring through-channels defines an intersecting line with a sidewall of said at least one of said neighboring through-channels; and 
     wherein said intersecting line includes a convexly shaped first portion, a concavely shaped second portion, and a convexly shaped third portion in the thickness direction of the substrate from the at least one common local maximum toward the middle region of said substrate. 
     In other words, according to the invention, a paper machine clothing is provided wherein the upper rims of both neighboring through-channels have at least one common local maximum, wherein a sectional plane being parallel to the thickness direction of the substrate, comprising the at least one common local maximum and comprising or intersecting the central axis of at least one of the two neighboring through-channels defines an intersecting line with a sidewall of the at least one of the two neighboring through-channels, and wherein the intersecting line comprises a first portion that is convexly shaped, a second portion that is concavely shaped and a third portion that is again convexly shaped when going in the thickness direction of the substrate from the at least one common local maximum toward the middle region of the substrate. 
     In the sense of the present invention the term “neighboring” could be replaced by the term “adjacent”, meaning that there is no other through-channel placed between two neighboring or adjacent through-channels. Furthermore, in the sense of the present invention the term “upper rim” of a through-channel refers to the rim of the through-channel on the upper side of the substrate. The rim itself may be defined as a closed line where the sidewall of the through-channel ends. In view of the previously described examples of the prior art shown in  FIGS.  1  to  3 C , the upper rim can be easily identified, always being completely surrounded by the first surface  22 ′. To be more specific, in these examples, the upper rim is always a circular line lying within the plane of the first surface  22 ′ of the substrate  20 ′. In contrast, according to embodiments disclosed in EP3348708 A1 and EP3561176 A1, the upper rim of a through-channel does not lie within a plane. In some of these embodiments, the upper rim is partially be surrounded or defined by portions of the still existing first surface of the substrate and partially by the sidewall of at least one neighboring through-channel. Especially the portions of the still existing first surface of the substrate can contribute to markings of the nascent paper web formed on the PMC when the PMC is used as forming fabric. The nascent paper web lies flat on these portions and dewatering there is consequently more difficult compared to other portions where the nascent paper web is “hanging” over the openings of the through-channels. 
     It is the merit of the inventors to have found out that this problem can be solved by providing some kind of “pin-like-structure” forming a common local maximum of the rims of neighboring through-channels and functioning as some kind of fiber-support-points for the nascent paper web. With this “pin-like-structure” there is only a very small contact area between the nascent paper web and the PMC allowing the nascent paper web to be substantially equally dewatered over its complete surface. Thus, markings can be avoided. 
     The “pin-like-structure” can be described by its geometrical properties as claimed. The “common local maximum” preferably represents a point of the topography of the upper side of the substrate that is like an apex or a mount peak and from which the surface of the upper side declines in all directions. Furthermore, the three portions of the intersecting line, namely the first portion that is convexly shaped, the second portion that is concavely shaped and the third portion that is again convexly shaped when going in the thickness direction of the substrate from the at least one common local maximum toward the middle region of the substrate are preferably directly connected to each other. In other words, the first portion is preferably directly connected to the second portion at a first inflection point and the second portion is directly connected to the third portion at a second inflection point. 
     Preferably, the above description of the three portions of the intersecting line does not only apply to the intersection line that is defined by a sectional plane that comprises or intersects the central axis of at least one of the two neighboring through-channels, but applies to all intersection lines that are defined by any sectional plane that is parallel to the thickness direction of the substrate and that comprises the at least one common local maximum, no matter if this sectional plane also comprises or intersects the central axis of at least one of the two neighboring through-channels. In other words, it is proposed that the intersecting line that is defined by intersecting the substrate with a sectional plane being parallel to the thickness direction of the substrate and comprising the at least one common local maximum comprises a first portion that is convexly shaped, a second portion that is concavely shaped and a third portion that is again convexly shaped when going in the thickness direction of the substrate from the at least one common local maximum toward the middle region of the substrate. 
     Furthermore, it is proposed that a first inflection point that is located between the first portion and the second portion of the intersecting line, and preferably also a second inflection point that is located between the second portion and the third portion of the intersecting line, is/are located close to the at least one local maximum, i.e. in the upper fourth, preferably in the upper fifth, more preferably in the upper sixth, of the substrate. In other words, the dimension or height of the “pin-like-structure” is preferably rather small compared to the overall dimension or height of the substrate in its thickness direction. It is not the aim of the “pin-like-structure” e.g. to contribute to the tensile strength of the substrate, but to provide some kind of fiber support point for the nascent fiber web, so as to allow for a good dewatering of the PMC substantially over the complete surface of the nascent fiber web. Consequently, the “pin-like-structure” does not need to be or even should not have a large dimension or height. 
     In a preferred embodiment of the present invention at least 90%, preferably all, of the through-channels in the usable region of the substrate have an upper rim that directly contacts an upper rim of at least one other neighboring through-channel, preferably of all other neighboring through-channels, of the plurality of through-channels in the usable region of the substrate, wherein the upper rims of the majority, preferably of all, of these directly neighboring through channels have at least one common local maximum, wherein a sectional plane being parallel to the thickness direction of the substrate, comprising the at least one common local maximum and comprising or intersecting the central axis of at least one of the corresponding neighboring through-channels defines an intersecting line with a sidewall of the one of the corresponding neighboring through-channels, and wherein the intersecting line comprises a first portion that is convexly shaped, a second portion that is concavely shaped and a third portion that is again convexly shaped when going in the thickness direction of the substrate from the at least one common local maximum toward the middle region of the substrate. In other words, it is preferred that almost all or all local maxima that are defined by corresponding neighboring through-channels in the usable region exhibit a “pin-like-structure” as described above. 
     Furthermore, it is advantageous if less than 5%, preferably 0%, of a surface on the upper side of the substrate in its usable region is flat and substantially orthogonal to the thickness direction of the substrate. In other words, it is preferred if hardly any portion of the original first surface of the substrate, that was existing before the perforation process, is left after the perforation process. 
     In contrast to the first surface, with respect to the second surface of the substrate, it is advantageous, if between 70% and 90%, preferably between 75% and 85%, and more preferably about 80%, of a surface on the lower side of the substrate is flat and substantially orthogonal to the thickness direction of the substrate. Such a result can be achieved if the cross-sectional area of the through-channels is smaller on the lower side of the substrate compared to the upper side of the substrate. For example, the through-channels may be substantially funnel-shaped tapering to the lower side of the substrate. 
     According to one embodiment of the present invention, the cross-sectional area of at least one through-channel, preferably of all through-channels, of the plurality of through-channels in the usable region of the substrate may continuously decreases when going in the thickness direction of the substrate from the upper side to the lower side of the substrate. 
     According to an alternative embodiment of the present invention, the cross-sectional area of at least one through-channel, preferably of all through-channels, of the plurality of through-channels in the usable region of the substrate continuously increases again when going in the thickness direction of the substrate from the middle region of the substrate between the upper side and the lower side to the lower side of the substrate. With such a configuration, the respective through-channel resembles the through-channel shown in  FIG.  3 C  and the dewatering capability of the PMC may be enhanced by using the effect of a nozzle. 
     It is also possible to have in the same substrate a mixture of through-channels according to the two previously described embodiments. 
     Another advantageous feature of the present invention concerns the aspect that a shape of the cross-sectional area of at least one through-channel, preferably of all through-channels, of the plurality of through-channels can change when going in the thickness direction of the substrate from the upper side to the lower side. 
     Advantageously, the shape of the cross-sectional area is substantially more elliptical in an upper region of the through-channel than in a lower region of the through-channel. In mathematics, an ellipse is a curve in a plane surrounding two focal points such that the sum of the distances to the two focal points is constant for every point on the curve. As such, it is a generalization of a circle, which is a special type of an ellipse having both focal points at the same location. The shape of an ellipse (how “elongated” it is) is represented by its eccentricity, which for an ellipse can be any number from 0 (the limiting case of a circle) to arbitrarily close to but less than 1. Consequently, “the cross-sectional area being substantially more elliptical in an upper region of the through-channel than in a lower region of the through-channel” means that the shape of the cross-sectional area changes as the eccentricity of the substantially elliptically shaped cross-sectional area in the upper region of the through-channel is larger than the eccentricity of the substantially elliptically shaped cross-sectional area in the lower region of the through-channel, wherein the latter one might be even 0 (corresponding to a circle). Thereby, the value of the eccentricity may diminish continuously in thickness direction. 
     Of course, the terms “elliptical” and “circular” when used in view of the cross-sectional areas of the through-channels must not be understood in a strict mathematical way but some deviations, e.g. due to manufacturing tolerances, are allowed. Therefore, the term “elliptical” may be rather understood as “oval” as also described in prior art documents WO 91/02642 A1 and WO 2010/088283 A1. 
     In view of the through-channels  30 ′ described with respect to  FIGS.  3 A,  3 B and  3 C , the basic shape of the cross-sectional area of the through-channels  30 ′ is always the same, i.e. circular. However, it turned out to be advantageous—for reasons explained in more detail below—if the cross-sectional area of the through-channels  30 ′ changes along the thickness direction of the substrate, in particular if the cross-sectional area is more elliptical close to the upper side of the substrate than in the lower side of the substrate. If the through-channels are drilled by a laser, such a form of the through-channels can be achieved for example by not shutting off of the laser or by at least not shutting off completely the laser when advancing with the laser from one through-channel to the next neighboring through-channel in a row. Applying this method can result in that the upper rim of a through-channel is deeper below the original first surface of the substrate at a point between two neighboring through-channels in the direction of advancement of the laser compared to a point between two neighboring through-channels in a direction perpendicular thereto. 
     With the above described aspect of the present invention it is possible to impart anisotropic properties to the substrate in a beneficial way. For example, it is proposed that the shape of the cross-sectional area in the upper region of the through-channel has a first dimension extending in cross-machine direction and a second dimension extending in machine direction, wherein the first dimension is smaller than the second dimension. With such a configuration of the through-channels the substrate, and thus the final paper machine clothing, can stand higher stress in the machine direction compared to the cross machine direction, wherein stresses that act on the paper machine clothing are usually in fact much higher in the machine direction than in the cross machine direction. As it is clear to those skilled in the art, the term “machine direction” refers to the longitudinal direction of the PMC, i.e. the direction of transportation of the fiber web or the fibrous nonwoven when the PMC is installed in a corresponding machine, whereas the term “cross machine direction” refers to a direction within the plane of the PMC that is perpendicular to the machine direction. 
     In an alternative embodiment it is proposed that the shape of the cross-sectional area in the upper region of the through-channel has a first dimension extending in cross-machine direction and a second dimension extending in machine direction, wherein the first dimension is larger than the second dimension. Such a form of the through-channels is particularly beneficial if the fiber retention on the paper machine clothing, in particular a forming fabric, shall be enhanced. 
     The first dimension and the second dimension preferably differ from each other by at least 5%, more preferably by at least 10%, and even more preferably by at least 15%, of the respective smaller dimension. 
     Preferably, on the lower side of the substrate the shape of the cross-sectional area is substantially circular. 
     In order to increase the density of through-channels in the usable region of the substrate, and thus, to enhance the dewatering capability of the paper machine clothing, it is suggested that at least 90% of all through-channels in the usable region of the substrate are arranged in a non-checkered pattern. Arranging the through-channels in a checkered pattern would mean that the through-channels are evenly distributed in the usable region of the PMC like the fields of a classic chess-board. In contrast to this, arranging the through-channels in a non-checkered pattern means that the through-channels are distributed differently. 
     According to another aspect, the present invention also refers to a method of producing the paper machine clothing as previously described comprising the following steps: providing a substrate having a first surface and a second surface, wherein the first surface and the second surface are preferably planar and parallel to each other; and forming a plurality of non-cylindrical through holes into a usable region of the substrate by using a laser, wherein at least some, preferably all, of the plurality of through holes that are neighboring each other are formed at such a close distance that they partially overlap each other, wherein during the formation of the plurality of non-cylindrical through holes the laser is controlled in such a way that the upper rims of the overlapping through-holes have at least one common local maximum, wherein a sectional plane being parallel to the thickness direction of the substrate, comprising the at least one common local maximum and comprising or intersecting the central axis of at least one of the overlapping through-holes defines an intersecting line with a sidewall of the at least one of the overlapping through-holes, and wherein the intersecting line comprises a first portion that is convexly shaped, a second portion that is concavely shaped and a third portion that is again convexly shaped when going in the thickness direction of the substrate from the at least one common local maximum toward the middle region of the substrate. 
     The inventors have found that the “pin-like-structure” can be created relatively easily during the perforation of the substrate via a laser, by correspondingly adjusting the power of the laser, the pulse length and the location of the focus of the laser. Thus, it is possible to make part of the material that is evaporated by the laser to condense again, thereby forming “pin-like-structure.” 
     The term “through hole” in the sense of the present invention refers to the form of a hole that is formed in the substrate neglecting the neighboring through holes that may partially overlap. In contrast, the term “through-channel” refers to the geometric form of the channels in the finally drilled substrate. Due to the fact that neighboring through holes may overlap each other according to the present invention, its form, especially in view of its upper rim, can differ from the form of the through-channels. 
     According to one embodiment of the present invention it is proposed that, when all the through holes have been formed into the usable region of the substrate, at least one of the first surface and the second surface in the usable region has disappeared by at least 90%, preferably by 100%. As result the finally drilled substrate has none or hardly any opposite surface portions that are planar and parallel to each other. 
     Preferably cold air is blown onto the substrate during the step of forming the through holes into the substrate. The cold air inhibits overheating and damaging of the substrate material, which is particularly important for the material region between two neighboring through holes when the laser is advancing from the first of the two through holes to the second one. 
     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 paper machine clothing and a method of producing the same, 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. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG.  1    is an illustration of the processes of perforating a substrate via laser drilling according to U.S. Pat. No. 5,837,102; 
         FIG.  2    is a plan view of a substrate  20 ′ that is placed under tension between two rollers R; 
         FIGS.  3 A,  3 B,  3 C  are cross-sectional views illustrating a radially outer, first surface and an opposite, radially inner, second surface; 
         FIG.  4    shows a section of a substrate comprising a single through hole of a first type; 
         FIG.  4 A  shows an enlarged view of the through hole in  FIG.  4   ; 
         FIG.  5    shows a section of a substrate comprising a single through hole of a second type; 
         FIG.  5 A  shows an enlarged view of the through hole in  FIG.  5   ; 
         FIG.  6    shows a sectional view along the lines A-A and B-B in  FIG.  4    and along the line C-C in  FIG.  5   ; 
         FIG.  7    shows a sectional view along the line D-D in  FIG.  5   ; 
         FIG.  8    shows a section of a substrate comprising a plurality of through holes of the first type; 
         FIG.  9    shows a section of a substrate comprising a plurality through holes of the second type; 
         FIG.  10    shows a sectional view along the lines E-E and F-F in  FIG.  8    and along the line G-G in  FIG.  9   ; 
         FIG.  11    shows a sectional view along the line H-H in  FIG.  9   ; 
         FIG.  12    shows a sectional view similar to the sectional view of  FIG.  10   , but with a third type of through holes; 
         FIG.  13    shows a section of a substrate similar to the one shown in  FIG.  8   , but with the through holes arranged in a non-checkered pattern; 
         FIG.  14    shows a section of a substrate similar to the one shown in  FIG.  9   , but with the through holes arranged in a non-checkered pattern; 
         FIG.  15    shows a section of a substrate comprising a plurality of through holes of a fourth type; 
         FIG.  16    also shows a section of a substrate comprising a plurality of through holes of the fourth type; 
         FIG.  17    shows a sectional view along the lines J-J and K-K in  FIG.  15   ; 
         FIG.  17 A  shows an enlarged view of a “pin-like-structure” in  FIG.  17   ; 
         FIG.  18    shows a sectional view along the lines L-L and M-M in  FIG.  16   ; and 
         FIG.  18 A  shows an enlarged view of a “pin-like-structure” in  FIG.  18   . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG.  4    shows a section of a substrate  20  which section is indicated by a dashed square. The substrate  20  comprises a first surface  22  and an opposite second surface  24  (see  FIG.  6   ), wherein the first surface  22  and the second surface  24  are substantially planar and parallel to each other. 
     A single through hole  31  of a first type is provided in the center of the section of the substrate  20 .  FIG.  6    shows a cross-sectional view which is taken through the through hole  31  along line A-A or line B-B of  FIG.  4   . As can be seen from  FIGS.  4  and  6   , the through hole  31  extends through the substrate  20  in its thickness direction TD along a central axis CA of the through hole  31 , the central axis CA being indicated by a dashed line in  FIG.  6   . Thus, the through hole  31  connects the first surface  22  with the second surface  24  of the substrate  20 . The through hole  31  is substantially funnel shaped with a cross-sectional area becoming continuously smaller when going in the thickness direction TD from the first surface  22  to the second surface  24 . The cross-sectional area of a through hole  31  is obtained by cutting the through hole  31  with a plane that is oriented perpendicular to the thickness direction TD of the substrate  20 . In this embodiment the shape of the cross-sectional area of the through hole  31  is always circular, no matter at which height level of the substrate the cross-sectional area is taken. 
     The through hole  31  has a circular upper rim  34  where a sidewall of the through hole  31  ends and the flat first surface  22  begins. The circular upper rim  34  has a diameter A, as shown in  FIG.  4 A . Furthermore, the through hole  31  has a circular lower rim  36  where the sidewall of the through hole  31  ends and the flat second surface  24  begins. The circular lower rim  36  has a diameter a, as also shown in  FIG.  4 A . Diameter A of the upper rim is larger than diameter a of the lower rim. 
       FIG.  5    shows another section of a substrate  20  which section is also indicated by a dashed square. The substrate  20  comprises a first surface  22  and a second surface  24  (see  FIG.  7   ), wherein the first surface  22  and the second surface  24  are substantially planar and parallel to each other. 
     A single through hole  32  of a second type is provided in the center of the section of the substrate  20 .  FIG.  6    shows a cross-sectional view which is taken through the through hole  32  along line C-C of  FIG.  5    and  FIG.  7    shows a cross-sectional view which is taken through the through hole  32  along line D-D of  FIG.  5   . As can be seen from  FIGS.  5 ,  6  and  7   , the through hole  32  extends through the substrate  20  in its thickness direction TD along a central axis CA of the through hole  32 , the central axis CA being indicated by a dashed line in  FIGS.  6  and  7   . Thus, the through hole  32  connects the first surface  22  with the second surface  24  of the substrate  20 . The through hole  32  is substantially funnel shaped with a cross-sectional area becoming continuously smaller when going in a thickness direction TD from the first surface  22  to the second surface  24 . The cross-sectional area of the through hole  32  is obtained by cutting the through hole  32  with a plane that is oriented perpendicular to the thickness direction TD of the substrate  20 . In this embodiment the shape of the cross-sectional area of the through hole  32  is not constant but changes when going along the thickness direction TD of the through hole  32 . In an upper region of the substrate  20 , i.e. in a region close to the first surface  22 , the through hole  32  is more oval or elliptical, whereas in a lower region of the substrate  20 , i.e. in a region close to the second surface  24 , the through hole  32  is more or completely circular. The shape of the cross-sectional area of the through hole  32  preferably changes continuously along the thickness direction TD of the substrate  20 . 
     Thus, the through hole  32  has an elliptical upper rim  35  where a sidewall of the through hole  32  ends and the flat first surface  22  begins. The elliptical upper rim  35  has a first diameter A and a second diameter B measured orthogonally thereto, as indicated in  FIG.  5 A . Furthermore, the through hole  32  has a circular lower rim  36  where the sidewall of the through hole  32  ends and the flat second surface  24  begins. The circular lower rim  36  has a diameter a, as also shown in  FIG.  5 A . The second diameter B of the upper rim  35  is larger than the first diameter A of the upper rim  35 . The first diameter A of the upper rim  35  is larger than the diameter a of the lower rim  36 . Preferably, the second diameter B of the upper rim  35  is at least 5%, more preferably at least 10%, even more preferably at least 15% larger than the first diameter A of the upper rim  35 . 
     Several of such non-cylindrical through holes are arranged in such a close relationship that they partially overlap each other in the substrate. Examples of such arrangements for the through holes  31  of the first type and the through holes  32  of the second type are shown in  FIGS.  8  and  9   , respectively. To be more precise, nine corresponding through holes  31 ,  32  arranged in a checkered pattern are shown in these figures. The through holes  31 ,  32  each have a respective lower rim  36 . Furthermore, for the sake of clarity, also the corresponding upper rims  34 ,  35  of the through holes  31 ,  32  are shown, even though these upper rims  34 ,  35  do not exist anymore as such in the final product. Instead, in the final product, i.e. in the finally perforated substrate  20 , through-channels  30  are formed having a respective upper rim  38  that is at least partially delimited by the upper rim  38  of a neighboring through-channel  30 . As shown in  FIGS.  8  and  9   , the originally existing flat or planar first surface  22  of the substrate  20  has completely disappeared after the perforation of the substrate  20  in the usable region UR thereof. The reason for the complete disappearance of the originally flat first surface  22  of the substrate  20  is that the through holes  31 ,  32  have been laser-drilled and that the material of the substrate  20  that has been evaporated by the energy of the laser at least partially condenses again on the first surface  22 , thus forming a “pin-like-structure”  40  that will be explained in more detail below. As a consequence, the upper rim  38  of a corresponding through-channel  30  does not extend within a plane but is rather a closed line that extends three-dimensionally. It should be noted that the upper rim  38  of the through-channel  30  may extend partially below the originally flat first surface  22  of the substrate  20  and/or extend partially above the originally flat first surface  22  of the substrate  20 . 
       FIGS.  10  and  11    represent views similar to the ones shown in  FIGS.  6  and  7   , respectively, but now with several neighboring through holes  31 ,  32  that form the through-channels  30  in the substrate  20  of the final product. In  FIG.  10    a location (see reference sign  38 ) of the upper rim  38  of the through-channel  30  of  FIG.  8    is shown that represents an absolute minimum of the upper rim  38 . In other words, the upper rim  38  has the largest distance to the originally flat first surface  22  of the substrate  20  which surface  22  is indicated by a dotted line in  FIG.  10   . The surface of the substrate  20  has a saddle point at this location of the upper rim  38 . 
     In  FIG.  11    a location (see reference sign  38 ) of the upper rim  38  of the through-channel  30  of  FIG.  9    is shown (according to the section along line H-H of  FIG.  9   ) that represents an absolute minimum of the upper rim  38  of this through-channel  30 . In other words, the upper rim  38  has the largest distance to the originally flat first surface  22  of the substrate  20  which surface  22  is also indicated by a dotted line in  FIG.  11   . The surface of the substrate  20  has a saddle point at this location of the upper rim  38 . A section along line G-G of  FIG.  9    is represented by the drawing of  FIG.  10   . At the location of the upper rim  38  shown in this figure, the upper rim only has a local minimum. Thus, the ridges that separate two neighboring through-channels  30  from each other are higher when following the line G-G compared to the ridges when following the line H-H of  FIG.  9   . Consequently, the substrate has anisotropic properties. 
     These anisotropic properties can be used in a beneficial way. For example, the substrate that is perforated in a way as shown in  FIGS.  9 ,  10  and  11    is more stress resistant in the direction parallel to line H-H compared to the direction parallel to line G-G. If line H-H substantially represents the machine direction of the final paper machine clothing the relatively high forces in the machine direction can be absorbed by the substrate  20  while at the same time the substrate  20  provides a relatively large open area on its upper side. Alternatively, if line H-H substantially represents the cross machine direction of the final paper machine clothing the nascent paper web in a forming section can adhere better to the substrate  20  since ridges formed in the substrate  20  between neighboring rows of through channels  30  that extend in cross machine direction are higher than those extending in the machine direction. Consequently, the properties of the substrate  20  can be adjusted to the intended use or the requirements of the paper machine clothing. 
       FIG.  12    shows a sectional view similar to the cross-sectional view of  FIG.  10   , but of a third type of through holes. This third type of through holes differs from the first and second type of through holes  31 ,  32  in that the cross-sectional area of the through hole of the third type and, thus, the cross-sectional area of the corresponding through-channel  30  that is created thereof, continuously increase again when going in the thickness direction TD of the substrate  20  from the middle region MR of the substrate  20  between the upper side and the lower side to the lower side of the substrate  20 . In an extreme case, neighboring through holes may not only partially overlap each other on the first side  22  of the substrate  20  but also on the second side  24  thereof. 
       FIGS.  13  and  14    show a section of a substrate  20  similar to the one shown in  FIGS.  8  and  9   , respectively, with the difference that the through holes  31 ,  32  are arranged in a non-checkered pattern. In  FIGS.  8  and  9    each through hole  31 ,  32  has eight neighboring other through holes  31 ,  32  wherein the distance to four of these eight neighboring through holes  31 ,  32  is larger than the distance to the remaining four neighboring through holes  31 ,  32 . 
     In contrast, in the examples shown in  FIGS.  13  and  14   , each through hole  31 ,  32  has six neighboring other through holes  31 ,  32  wherein the distance to all these neighboring through holes  31 ,  32  is substantially the same (for example corresponding to the smaller distance of the embodiments shown in  FIGS.  8  and  9   ). These six neighboring through holes  31 ,  32  are arranged in a honeycomb pattern around a corresponding through hole  31 ,  32  in the middle thereof. With such an arrangement, the density of through-channels  31  in the final substrate  20  can be increased, as well as the open area on the upper side of the substrate  20 . 
     Each of  FIGS.  15  and  16    shows a section of a substrate comprising a plurality of through holes of a fourth type.  FIGS.  15  and  16    are substantially identical to  FIG.  8    which shows a section of a substrate comprising a plurality of through holes of a first type. However, the holes of the fourth type are longer (or the substrate has a larger thickness) compared to the holes of the first type as can be seen by comparing  FIGS.  17  and  18    with  FIG.  10   . It should be noted that this difference is not decisive for the effect of the present invention, especially when taking into account that the figures only represent only schematic drawings anyway. Therefore, the following description of  FIGS.  16 - 18 A  may equally refer to the embodiment shown in  FIG.  8   .  FIG.  17    represents a sectional view along lines J-J and K-K of  FIG.  15    and  FIG.  18    represents a sectional view along lines L-L and M-M of  FIG.  16   . In  FIGS.  17  and  18    a detail referring to the “pin-like-structure”  40  is emphasized with a dashed circle and this detail is shown in enlarged views in  FIGS.  17 A and  18 A , respectively. 
     Lines J-J and K-K in  FIG.  15    each describes a sectional plane that is parallel to the thickness direction TD of the substrate  20 , that comprises the central axis CA of at least one of two neighboring through-channels  30  the upper rims of which have at least one local maximum  42  in common, and that comprises the at least one local maximum  42 . The local maximum  42  is illustrated in detail in  FIGS.  17 A and  18 A , and might be compared to an apex or a mount peak from which the surface of the upper side of the substrate  20  declines in all directions. 
     According to the present invention, the outline of the substrate  20  in the sectional view of  FIGS.  17  and  17 A  comprises a first portion  44  that is convexly shaped, a second portion  46  that is concavely shaped and a third portion  48  that is again convexly shaped when going in the thickness direction TD of the substrate  20  from the at least one common local maximum  42  toward the middle region MR of the substrate  20 . In this exemplary embodiment, the first portion  44  is directly connected to the second portion  46  at a first inflection point  50  and the second portion  46  is directly connected to the third portion  48  at a second inflection point  52 . In in the sectional view of  FIGS.  17  and  17 A  the “pin-like-structure”  40  has a substantially symmetrical outline. Therefore, the outline does not only comprise a first, second and third portion  44 ,  46 ,  48  as described above on the left hand side in  FIG.  17 A  but also on the right hand side in this figure. It should be noted, however, that the outline of the “pin-like-structure”  40  as shown in  FIGS.  17  and  17 A  does not have to be symmetrical. For example it is possible that the outline is somehow deformed to one side. 
     Lines L-L and M-M in  FIG.  16    each describes a sectional plane that is parallel to the thickness direction TD of the substrate  20 , and that comprises the at least one local maximum  42 . However, this sectional plane—in contrast to the one shown in  FIGS.  17  and  17 A — does neither comprise nor intersect the central axis CA of any of the shown through-channels  30 . As shown in  FIGS.  18  and  18 A  also the outline of the substrate  20  in this sectional plane comprises a first portion  44 * that is convexly shaped, a second portion  46 * that is concavely shaped and a third portion  48 * that is again convexly shaped when going in the thickness direction TD of the substrate  20  from the at least one common local maximum  42  toward the middle region MR of the substrate  20 . In this exemplary embodiment, the first portion  44 * is directly connected to the second portion  46 * at a first inflection point  50 * and the second portion  46 * is directly connected to the third portion  48 * at a second inflection point  52 *. In in the sectional view of  FIGS.  18  and  18 A  the “pin-like-structure”  40  has also a substantially symmetrical outline. Therefore, the outline does not only comprise a first, second and third portion  44 *,  46 *,  48 * as described above on the left hand side in  FIG.  18 A  but also on the right hand side in this figure. It should be noted, however, that the outline of the “pin-like-structure”  40  as shown in  FIGS.  18  and  18 A  does not have to be symmetrical. For example it is possible that the outline is somehow deformed to one side. Preferably, any outline of the substrate in the region of the “pin-like-structure”  40  comprises a first portion that is convexly shaped, a second portion that is concavely shaped and a third portion that is again convexly shaped when going in the thickness direction TD of the substrate  20  from the at least one common local maximum toward the middle region MR of the substrate  20 , no matter what sectional plane has been chosen to define the outline, as long as the sectional plane is parallel to the thickness direction TD of the substrate  20  and comprises the local maximum  42 . 
     In  FIGS.  17 ,  17 A,  18  and  18 A , a dotted line indicates the original first surface  22  of the substrate  20 . Preferably, the material that is located above this dotted line in the final product is material that has first been evaporated during the formation of the through-channels  30  by laser-drilling and has then been condensed again. The inventors have found out that by correspondingly adjusting parameters, such as the power of the laser, the pulse length and the location of the focus of the laser, it is possible to create the “pin-like-structure”  40  relatively easily during the perforation of the substrate. 
     In laser drilled substrates known from the prior art, there is either no material above the dotted line that represents the original first surface  22  of the substrate  20 , or there is material above this line, but only in the form of a smooth hill or ridge as indicated by a dashed line in  FIGS.  17 A and  18 A . However, the formation of the “pin-like-structure”  40  is not know from the prior art. 
     The “pin-like-structure”  40  is advantageous because—especially when the laser drilled substrate is used as a forming fabric—it supports the fiber web punctually, thus providing a very good and equal dewatering for the fiber web substantially over its complete surface, thus, avoiding markings. 
     The following is a summary list of reference numerals and the corresponding structure used in the above description of the invention:
       20 ′,  20  substrate     22 ′,  22  first surface     24 ′,  24  second surface     26 ′ first lateral edge     28 ′ second lateral edge     30 ′,  30  through-channel     31  through hole of first type     32  through hole of second type     34  circular upper rim of through hole     35  elliptical upper rim of through hole     36  circular lower rim of through hole     38  upper rim of through-channel     40  pin-like-structure     42  local maximum     44 ,  44 * first portion that is convexly shaped     46 ,  46 * second portion that is concavely shaped     48 ,  48 * third portion that is convexly shaped     50 ;  50 * first inflection point     52 ;  52 * second inflection point   a, b diameter of lower rim   A, B diameter of upper rim   CA central axis   LB laser beam   MR middle region   R roller   TD thickness direction   WD width direction