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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. Another aspect of the present invention concerns a method of producing such a paper machine clothing.
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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.
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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 monolithic 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 some kind of 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.
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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".
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The idea of producing a PMC from a substrate that is perforated, especially by using a laser, is already known for quite a long time in the prior art and described e.g. in the 1980's and 1990's in the documents
US4541895A and
US 5,837,102 , respectively, the content of which is hereby incorporated by reference.
Figure 1 illustrates the processes of perforating a substrate via laser drilling according to the
US 5,837,102 reference.
Figure 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.
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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.
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A paper machine clothing according to the preamble part of claim 1 is known for example from the disclosure of documents
US 4,446,187 A and
DE 10 2010 040 089 A1 , the content of which is hereby incorporated by reference.
Figures 2, 3a, 3b and 3c are based on the disclosure of the
US 4,446,187 A reference.
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Figure 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 figures 3a, 3b and 3c. 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.
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Figures 3a, 3b and 3c show different possible configurations of the through-channels 30. In figure 3a the through-channel is cylindrical having the same cross sectional area at any location along the thickness direction TD of the substrate 20. In figure 3b 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 figure 3c 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.
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Furthermore, prior art document
EP3561176B1 , the content of which is hereby incorporated by reference, also already discloses a paper machine clothing according to the preamble part of claim 1. The following
figures 4 to 11 are based on the disclosure of this prior art document.
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Figure 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 figure 6), wherein the first surface 22 and the second surface 24 are substantially planar and parallel to each other.
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A single through-channel 30 is provided in the center of the section of the substrate 20. Figure 7 shows a cross sectional view which is taken through the through-channel 30 along line A-A or line B-B of figure 4. As can be seen from figures 4 and 7, the through-channel 30 extends through the substrate 20 in its thickness direction TD along a central axis CA of the through-channel 30, the central axis CA being indicated by a dashed line in figure 6. Thus, the through-channel 30 connects the first surface 22 with the second surface 24 of the substrate 20. The through-channel 30 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-channel 30 is obtained by cutting the through-channel 30 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-channel 30 is always circular, no matter at which height level of the substrate the cross sectional area is taken.
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The through-channel 30 has a circular upper rim 34 where a side wall of the through-channel 30 ends and the flat first surface 22 begins. The circular upper rim 34 has a diameter A, as shown in figure 5. Furthermore, the through-channel 30 has a circular lower rim 36 where the side wall of the through-channel 30 ends and the flat second surface 24 begins. The circular lower rim 36 has a diameter a, as also shown in figure 5. Diameter A of the upper rim is larger than diameter a of the lower rim.
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According to the teaching of
EP3561176B1 , to improve fiber retention, permeability and the degree of marking compared to previously known paper machine clothings, several of such non-cylindrical through-channels are arranged in such a close relationship that they partially overlap each other in the substrate.
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An example of such an arrangement for the through-channels is shown in figure 7. To be more precise, nine corresponding through-channels 30 arranged in a checkered pattern are shown in this figure. The term "checkered pattern" means that all through-channels have the same distance to all their neighboring through-channels and all through-channels are arranged in rows that are oriented perpendicular to each other. The through-channels 30 each have a respective lower rim 36. Furthermore, for the sake of clarity, also the corresponding upper rims 34 of the through-channels 30 are shown, even though these upper rims 34 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 34 that is at least partially delimited by the upper rim 34 of a neighboring through-channel 30. As shown in figure 7, the originally existing flat or planar first surface 22 of the substrate 20 has almost completely disappeared after the perforation of the substrate 20 in the usable region UR thereof. In alternative embodiments it may have completely disappeared. One reason for the complete disappearance of the originally flat first surface 22 of the substrate 20 could be that the distance between the through-channels 30 is chosen even smaller than shown in figure 7. An additional or alternative reason for the complete disappearance of the originally flat first surface 22 of the substrate 20 could be that the through-channels 30 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 condensates again on the first surface 22, thus forming some kind of hill or ridge thereon. Therefore, the upper rim 38 of a corresponding through-channel 30 does not necessarily 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.
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Figure 8 represents a view similar to the one shown in figure 6 but now with several neighboring through-channels 30 formed in the substrate 20 of the final product. In figure 8 a location (see reference sign 38) of the upper rim 38 of the through-channel 30 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 figure 8. The surface of the substrate 20 has a saddle point at this location of the upper rim 38.
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Figure 9 shows a section of a substrate 20 similar to the one shown in
figure 7 above, with the difference that the through-channels 30 are arranged in a non-checkered pattern. While in
figure 7 each through-channel 30 has eight neighboring other through-channels 30 wherein the distance to four of these eight neighboring through-channels 30 is larger than the distance to the remaining four neighboring through-channels 30, in
figure 9, each through-channel 30 has six neighboring other through-channels wherein the distance to all these neighboring through-channels 30 is substantially the same. These six neighboring through-channels 30 are arranged in a honeycomb pattern around a corresponding through-channel 30 in the middle thereof. In other words,
EP3561176B1 discloses the arrangement of through-channels 30 in the shape of regular hexagons, as shown in
figure 11, wherein each regular hexagon has one through-channel 30 in its geometrical center, as shown in
figure 10. Each through-channel 30 has the same distance to all other directly neighboring through-channels 30. The distance of two neighboring through-channels 30 shall be understood as the distance of their respective central axes CA. Connecting the axes CA of three directly neighboring through-channels 30 gives an equilateral triangle. With the honeycomb arrangement of
figure 9, the density of through-channels 30 in the final substrate 20 can be increased, as well as the open area on the upper side of the substrate 20, compared to the checkered pattern arrangement of
figure 7.
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Even though, the arrangements of the through-channels 30 shown in figures 7 and 9 are very well in terms of uniformity of dewatering, there is still room for improvements in terms of strength of the paper machine clothing. In practice, the paper machine clothing often must stand high tensile forces at elevated temperatures. This is particularly true for the machine direction of the paper machine clothing.
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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.
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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 while at the same time providing a relatively high tensile strength.
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This object is achieved by a paper machine clothing according to the features of independent claim 1, as well as by a method of producing the same according to the features of independent claim 12. Advantageous embodiments are the subject-matter of the dependent claims.
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Thus, according to the invention, a paper machine clothing as initially described and as recited in the preamble part of claim 1 is provided wherein at least one of the plurality of through-channels has exactly six directly neighboring through-channels of the plurality of through-channels surrounding it as a central through-channel in the shape of an irregular hexagon.
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By arranging the six directly neighboring through-channels in the shape of an irregular hexagon around the central through-channel it is possible to still have a relatively large open area of the paper machine clothing, e.g. an open area that is substantially the same as in the example shown in figure 7 with the checkered pattern arrangement, while at the same time providing the paper machine clothing with an improved tensile strength, especially in its machine direction, compared to the example of figure 7 and also to the example with the honeycomb arrangement of figure 9. The shape of an irregular hexagon is not an intuitive choice for a person skilled in the art because it automatically leads to an irregularity of dewatering. However, the inventors surprisingly found out that - at least in certain ranges - such irregularity is unproblematic not leading to visible markings or the like of the fiber web that is transported on the paper machine clothing according to the present invention. The advantages of gaining more tensile strength make it worth the switch from a traditional checkered pattern (like a chess board) or from a honeycomb pattern, i.e. a pattern of regular hexagons, to a pattern of irregular hexagons. It is the merit of the inventors to have found out that with the present invention it is possible to impart anisotropic properties to the substrate in a beneficial way.
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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.
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In a preferred embodiment of the present invention, two of the six surrounding through-channels that are located on opposite sides of the central through-channel each have a first distance from the central through-channel, whereas the remaining four through-channels of the six surrounding through-channels each have a second distance from the central through-channel, wherein the second distance differs from the first distance. In particular, the second distance can be larger than the first distance. To be more specific, the second distance can be more than 1,05 times, preferably more than 1,10 times, the first distance and/or the second distance can be less than 1,25 times, preferably less than 1,13 times, the first distance. Thus, the second distance can be for example between 1,10 and 1,13 times the first distance. The first distance and the second distance should be understood each as a substantially constant values, meaning that the corresponding values have only a small tolerance of less than 5%, preferably by less than 3%, even more preferably by less than 1%.
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To improve the tensile strength of the paper machine clothing especially in its machine direction it is advantageous if the two opposite through-channels each having the first (smaller) distance from the central through-channel are substantially aligned parallel to the two lateral edges of the paper machine clothing.
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To improve the tensile strength of the paper machine clothing efficiently, it is advantageous if at least one, preferably more than one, more preferably all, of the six surrounding through-channels is or are itself or themselves surrounded by exactly six directly neighboring through-channels of the plurality of through-channels. In other words, the complete usable region of the paper machine clothing, maybe apart from the lateral sides of the usable region, can have the same pattern of irregular hexagons.
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Expressed differently, at least half, preferably at least 90%, more preferably substantially all, of the plurality of through-channels in the usable region can have exactly six directly neighboring through-channels of the plurality of through-channels surrounding it as a central through-channel in the shape of an irregular hexagon.
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For the reasons described in detail in above-referenced document
EP3561176B1 , it is very beneficial if 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 if an upper rim of the central through-channel directly contacts an upper rim of at least two, preferably to all six, of the six surrounding through-channels.
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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 the through-channel with a plane that is perpendicular to the thickness direction of the substrate.
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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.
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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 with through-channels that are sufficiently spaced apart from each other to not overlap each other, the upper rim can be easily identified, always being completely surrounded by the first surface 22 (see figures 1 to 3c). 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 this preferred embodiment of present invention, the upper rim of a through-channel may not lie within a plane. This is particularly true when two neighboring through-channels partially "intersect" or "overlap" each other on the upper side of the substrate. The upper rim may then 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. As an alternative, the upper rim of a through-channel may be even completely surrounded or defined by the respective upper rims of the neighboring trough-channels. In the latter case, the original first surface of the substrate, i.e. the surface that was substantially plane and parallel to the second surface of the substrate before the perforation of the substrate, may have been completely lost in the usable region of the substrate. The topography of the substrate after the perforation process may somehow resemble the topography of an egg box.
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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 decrease when going in the thickness direction of the substrate from the upper side to the lower side of the substrate.
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Especially when the paper machine clothing is used as forming fabric a good dewatering capability is key for obtaining a fiber web of high quality and with a good formation. Therefore, it is proposed that less than 20%, preferably less than 10%, and more preferably less than 5%, of a surface on the upper side of the substrate 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.
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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.
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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 through-channels into a usable region of the substrate, wherein at least one of the plurality of through-channels has exactly six directly neighboring through-channels of the plurality of through-channels surrounding it as a central through-channel in the shape of an irregular hexagon.
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According to one embodiment of the present invention it is proposed that at least some, preferably all, of the plurality of through-channels that are neighboring each other are formed at such a close distance that they partially overlap each other.
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Furthermore, it is proposed that, when all the through-channels 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 the substrate, before it is perforated, has a caliper in its usable region between 0,5mm and 1,5mm and even more preferable between 0,8mm and 1,2mm. After perforating the substrate in its usable region, the caliper thereof may be different. In some embodiments the caliper of the perforated substrate may be smaller compared to the substrate before perforation. This may be particularly true when at least one of the first surface and the second surface in the usable region has completely disappeared. However, in other embodiments, the caliper of the perforated substrate may be even greater compared to the substrate before perforation. This can happen if part of the material that is evaporated e.g. by means of a laser condensates again, thereby forming some kind of hills or ridges. Anyway, as previously mentioned, the topography of the substrate after the perforation process may somehow resemble the topography of an egg box.
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Preferably the plurality of through-channels is formed into the substrate by using a laser, wherein preferably cold air is blown onto the substrate during the step of forming the through-channels 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-channels when the laser is advancing form the first of the two through-channels to the second one.
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In the following, the invention will be explained with respect to some schematic drawings that are not true to scale, wherein:
- Figure 12a
- shows a reference example, that does not form part of the present invention, of a plurality of through-channels arranged in a checkered pattern;
- Figure 12b
- shows an enlarged view of four through-channels from figure 12a;
- Figure 13a
- shows an example of a plurality of through-channels arranged according to the present invention;
- Figure 13b
- shows an enlarged view of four through-channels from figure 13a.
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Figure 12a shows a comparative example, not forming part of the present invention, with a section of a substrate 20 having a plurality of through-channels 30 that are arranged in a checkered pattern. Thus, this comparative example substantially corresponds to the prior art example of figure 7. However, for the sake of simplicity, the through-channels 30 in figure 12a are shown as cylindrical through-channels instead of non-cylindrical, in particular funnel-shaped through-channels partially overlapping each other. Nevertheless, the through-channels 30 of figure 12a could also be funnel-shaped, particularly overlapping each other like in figure 7.
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Figure 12b shows an enlarged view of four through-channels from figure 12a that form a square. The upper left through-channel 30 has a directly neighboring through-channel 30 on its right side in figure 12b, which neighboring through-channel 30 is located at a first distance d1. Furthermore, the upper left through-channel 30 has a directly neighboring through-channel 30 on its lower side in figure 12b, which neighboring through-channel 30 is also located at the first distance d1.
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Figure 13a shows an example according to the present invention, with a section of a substrate 20 having a plurality of through-channels 30 that are arranged in the shape of hexagons. Thus, this inventive example is similar to the prior art example of figure 9. Again, for the sake of simplicity, the through-channels 30 in figure 13a are shown as cylindrical through-channels instead of non-cylindrical, in particular funnel-shaped through-channels partially overlapping each other. Nevertheless, the through-channels 30 of figure 13a could also be funnel-shaped, particularly overlapping each other like in figure 7.
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More importantly, the inventive example of figure 13a differs from the prior art example of figure 9 in that figure 13a shows a pattern of irregular hexagons instead of regular hexagons. This pattern could be simply created by taking the checkered pattern of the comparative example of figure 12a and by shifting every second column of through-channels 30 upwards or downwards by half of the first distance d1. The distance between the columns of through-channels 30 stays constant, i.e. the first distance d1. Doing so, the open area in the section of the substrate 20 shown in figures 12a and 13a is the same. However, the tensile strength of the paper machine clothing is raised in the inventive example of figure 13a compared to the comparative example of figure 12a. This applies for the direction to which every second column of through-channels 30 was shifted, i.e. for the up-down-direction in figure 13a. If this direction substantially corresponds to the machine direction of the paper machine clothing, the paper machine clothing can better stand the higher tensile forces that are typically applied to the paper machine clothing in that direction.
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Figure 13b shows an enlarged view of four through-channels 30 from figure 13a that form a parallelogram but not a square. The upper left through-channel 30 has a directly neighboring through-channel 30 on its upper right side in figure 13b, which neighboring through-channel 30 is located at a second distance d2. Furthermore, the upper left through-channel 30 has a directly neighboring through-channel 30 on its lower side in figure 13b, which neighboring through-channel 30 is located at the first distance d1 (like in the comparative example of figures 12a and 12b). The second distance d2 is larger than the first distance d1. To be more precise, in this exemplary embodiment the second distance d2 is about 1,12 times (square root of 1,25) the first distance d1. In other words, the second distance d2 is about 12% larger than the first distance d1. To remember: The distance between two through-channels 30 is defined as the distance between their respective central axis CA. Even though 12% does not seem to be a lot, there is significantly more material of the substrate 20 (e.g. more than twice) between two directly neighboring columns of through-channels 30 along the line of the second distance d2, compared to the comparative example according to figure 12a and 12b along the line of the first distance d1 there. This results in a much higher tensile strength of the paper machine clothing according to the present invention compared to the examples known from the prior art.
list of reference signs:
-
- 20
- substrate
- 22
- first surface
- 24
- second surface
- 26
- first lateral edge
- 28
- second lateral edge
- 30
- through-channel
- 34
- upper rim of a through-channel
- 36
- lower rim of a through-channel
- a
- diameter of lower rim
- A
- diameter of upper rim
- CA
- central axis
- d1
- first distance
- d2
- second distance
- LB
- laser beam
- MR
- middle region
- R
- roller
- TD
- thickness direction
- WD
- width direction