Dewatering element, and production method and use of such a dewatering element

Abstract

The invention relates to a method for producing a dewatering element (10) in the form of a roll cover or roll shell for dewatering a fibrous web, in particular a paper, cardboard, or tissue web, wherein the dewatering element (10) has an outer surface (22) which faces the fibrous web during normal operation, wherein the method comprises the step of introducing at least one dewatering groove (14) into the outer surface (22) of the dewatering element (10), wherein the at least one dewatering groove (14) comprises a groove base and two side walls extending from the groove base to the outer surface, wherein the average roughness value Ra of the two side walls (16) is less than 2.0 µm, preferably less than 1.0 µm, and wherein the average roughness value Ra of the groove base (18) is greater than 2.0 µm, preferably greater than 3.0 µm, more preferably greater than 3.5 µm. The invention also relates to a corresponding dewatering element and to the use of such a dewatering element.

Description

[0001] DRAINAGE ELEMENT AS WELL AS MANUFACTURING METHOD AND USE OF SUCH A DRAINAGE ELEMENT

[0002] The present invention relates to a method for producing a dewatering element in the form of a roll cover or roll shell for dewatering a fibrous web, in particular a paper, cardboard or tissue web, wherein the dewatering element has an outer surface facing the fibrous web during normal operation, wherein the method comprises the step of introducing at least one dewatering groove into the outer surface of the dewatering element, wherein the at least one dewatering groove comprises a groove base and two side walls extending from the groove base to the outer surface, and wherein the mean roughness value R aof the two side walls is less than 2.0 pm, preferably less than 1.0 pm. Furthermore, the invention relates to a corresponding drainage element and the use of such a drainage element.

[0003] During the production of a fibrous web, such as a paper, board or tissue web, a suspension consisting predominantly of water and a few fibers is usually applied via a headbox to a screen, through which a large portion of the water is already drained, so that the actual fibrous web is formed. The fibrous web must then be further dewatered until it has a dry content high enough to allow the fibrous web to be wound up or processed in some other way. Dewatering is usually carried out mechanically, in particular by pressure, and thermally, in particular by appropriately high temperatures. It is always important to design the production process as efficiently as possible, particularly with regard to the energy required to achieve the desired dry content.

[0004] One key factor for efficient drainage is the design of drainage grooves in drainage elements, such as shoe press sleeves. These typically consist essentially of a polyurethane material, into which the drainage grooves are created using a rotating cutting disc. The cutting disc has a specific diameter and a specific number of cutting teeth. Furthermore, it is operated at a specified speed, direction of rotation, and feed rate relative to the press element into which the drainage grooves are to be created. All of these parameters affect the surface properties of the groove base and the side walls of the drainage grooves.Because the polymer material is relatively soft, the material repeatedly yields slightly between the passage of two consecutive cutting teeth during the creation of the grooves, resulting in characteristic cutting marks on the side walls and in the groove base of the drainage grooves. Even though these marks may vary in severity, they are always detectable. Figure 1 shows an example of corresponding cutting marks in a side wall 16 of a correspondingly manufactured drainage groove. Similar information can also be seen in the illustrations of US Pat. No. 7,494,571 B2, which explains the usual manufacturing process for drainage grooves and whose content is hereby explicitly incorporated by reference.

[0005] In addition, it is difficult to achieve the smoothest possible side walls for the drainage grooves at the microscopic level using this manufacturing process. This is primarily due to the fact that a significant amount of frictional heat is generated during the cutting process, which causes the material into which the drainage groove is introduced to melt, at least superficially, and then solidify again. EP 1 818 446 B1 describes this phenomenon in more detail and explains that the resulting surface roughness is detrimental to good drainage results. The content of this document is also explicitly incorporated herein by reference. As a solution to this problem, EP 1 818 446 B1 proposes actively cooling the cutting disk during the cutting process. This is shown schematically in Figures 2 and 3.A drainage groove 14 is created in the outer surface 22 of a drainage element 10, in the form of a roller shell, using a rotating cutting disk 12. A coolant is sprayed onto the cutting point through a coolant tube 20. This reduces the resulting frictional heat and prevents the material from melting. This ensures that the mean roughness R is maintained. a , which is defined, for example, in the Japanese industrial standard JIS-B0601, is not more than 2.0pm on both the side walls 16 and the groove bottom 18 of the drainage groove 14.

[0006] Even if the smoother surfaces of the side walls 16 lead to better drainage behavior of the drainage groove 14 at the microscopic level, cooling does not solve the problem of preventing cutting marks in the side walls 16 at the macroscopic level. These also have a detrimental effect on the drainage behavior of the drainage groove 14. In particular, contaminants that are washed out of the fibrous web with the water can easily accumulate on the cutting marks, which then leads to an increase in drainage resistance or a reduction in the flow velocity in the drainage groove. In addition, providing the coolant represents an additional expense in the manufacturing process.

[0007] In the two publications DE 102018 120 558 A1 and DE 102018 120 559 A1, it is proposed to introduce grooves into the surface of a roll shell or a roll cover by means of a laser. Furthermore, these publications disclose that, in particular when the inner surfaces of the grooves have an average roughness depth R z less than 50pm, this has a positive effect on the drainage performance.

[0008] Despite this improvement, there is still a need to further increase the drainage performance of such a drainage element.

[0009] The object of the present invention is to solve or at least mitigate the problems described above. In particular, it is intended to make it possible to provide a drainage element with drainage grooves that exhibit very good drainage capacity. This object is achieved according to the invention by the features of the independent claims. The dependent claims relate to advantageous developments of the present invention.

[0010] According to a first aspect of the present invention, the object is achieved by a generic manufacturing method as described above, which is particularly characterized in that the mean roughness value R a of the groove base is above 2.0 pm, preferably above 3.0 pm, more preferably above 3.5 pm. Preferably, the mean roughness R a However, the groove bottom should not exceed 10.0 pm, more preferably not exceed 5.0 pm.

[0011] Contrary to the teaching of the documents mentioned at the beginning, in particular the document EP 1 818 446 B1, which describes the groove base being as smooth as possible, namely with a mean roughness value R a of no more than 2 pm, the inventors surprisingly discovered that such a smooth groove base actually impairs the drainage capacity of the drainage groove. They explain this by saying that a rougher groove base is more likely to help expel the water from the drainage groove after it leaves the press nip.

[0012] For example, it is easily possible to use a laser to design the roughness of the groove base so that the mean roughness R a of the groove base is above 2.0 pm, preferably above 3.0 pm, more preferably above 3.5 pm. In particular, a mean roughness value R a The groove base between 3.5pm and 4.0pm has shown in tests that the drainage groove has a very good drainage performance.

[0013] Therefore, in an advantageous development of the present invention, it is proposed that the at least one drainage groove be introduced into the outer surface of the drainage element by means of a laser. The laser beam emitted by the laser allows the material into which the drainage groove is introduced to be relatively easily vaporized and then vacuumed away. Since there is no engagement of a mechanical cutting tool, in particular a rotating cutting disk, with the material, any cutting marks are eliminated. Furthermore, the inventors have recognized that by vaporizing the material using the laser, very smooth side walls of the drainage groove can be created, even at the microscopic level. Nevertheless, it is possible to make the groove base noticeably rougher than the side walls with the laser. The drainage grooves exhibit particularly good drainage performance.In particular, dirt particles cannot accumulate on the side walls, or only with noticeably greater difficulty than with conventional drainage grooves.

[0014] It should be noted at this point that the at least one drainage groove preferably has the previously described mean roughness value R a the side walls and the mean roughness R a of the groove base. However, this is not mandatory.

[0015] Some terms will be explained in more detail below: A “roll cover” is firmly connected to a rotatable roll core, whereas a “roll shell” is mounted rotatably on the inside relative to a fixed support element, as is the case, for example, with the shoe press shell of a typical shoe press.

[0016] In the context of the present invention, the term "paper web" refers to a web of "ordinary" paper, in particular so-called graphic paper. This differs from cardboard in particular by its lower basis weight and from tissue paper in its higher basis weight.

[0017] In the context of the present invention, the term "at least one drainage groove" means that the drainage element can also have more than one drainage groove. For example, there can be several drainage grooves running parallel to one another. Preferably, the at least one drainage groove extends continuously in a helical shape across the outer surface of the drainage element. However, this is not mandatory. For example, such an imaginary drainage groove can also be interrupted, resulting in several discrete drainage grooves.

[0018] The cross-sectional shape of the drainage groove, i.e., its shape orthogonal to its longitudinal direction, can be rectangular, as shown in Figure 3. However, this is not mandatory. For example, the two side walls can have an angle other than 90° to the outer surface, and / or the groove base can be rounded.

[0019] Tests have shown that the absorption rate of the laser in the material into which the drainage groove is to be introduced, particularly in a polyurethane, can be stabilized at a value between 90% and 98% if a laser with a wavelength of more than 1900 nm is used to introduce the at least one drainage groove. For example, a laser with a wavelength between 10.0 pm and 11.0 pm can be used. Particularly good results have been achieved with a CO2 laser, which is also readily available commercially. The energy introduced locally into the material leads to a large amount of heat being generated locally in a very short time, which in turn causes the material to evaporate immediately. The vapor can then be extracted, and the material contained in the vapor can be collected using filters.Furthermore, it has proven advantageous if the laser has an output power between 1 kW and 10 kW, preferably a power between 2 kW and 7 kW, more preferably between 3 kW and 5 kW.

[0020] Similar to what was already stated in the aforementioned document EP 1 818 446 B1, the inventors have recognized that smoother side walls lead to improved drainage performance of the drainage grooves. Using the method according to the invention, it is easily possible to produce side walls with a mean roughness R a of less than 2.0 pm. In fact, the process even produces side walls with a mean roughness R a less than 1 .0pm, which is even better.

[0021] The mean roughness value R aindicates the mean distance of a measuring point - on the surface - from the center line. The center line intersects the actual profile within the reference section in such a way that the sum of the profile deviations in a plane parallel to the center line is distributed over the length of the measuring section. The mean roughness therefore corresponds to the arithmetic mean of the absolute deviation from the center line. The mean roughness (in one dimension) is somewhat easier to imagine as the height of the rectangle that has the same length as the section to be examined and the same area as the area between the reference height and the profile. As mentioned in the document EP 1 818 446 B1, for the definition and determination of the mean roughness R a for example, the Japanese industrial standard JIS-B0601 can be used.

[0022] As is often the case with dewatering elements in the form of roller shells or roller covers, the outer surface of the dewatering element can be provided by a layer of polyurethane. In other words, the at least one dewatering groove is introduced into the polyurethane layer. Not only is polyurethane easy to process, especially to cast, it can also be easily vaporized using a laser. Polyurethane is generally naturally transparent. Depending on the wavelength of the laser used, it can be advantageous to color the polyurethane, in particular, color it red, blue, or black, in order to achieve the highest possible absorption rate of the laser beam.

[0023] In one embodiment of the present invention, the dewatering element is a shoe press cover. Such a cover is guided in a circle around a stationary shoe having a substantially concave surface. The shoe presses the shoe press cover against a press element, such as a press roll or a Yankee cylinder, with a substantially cylindrical surface to form an extended press nip through which the fibrous web to be dewatered is guided, often together with at least one press felt.

[0024] For producing the drainage groove using a laser, it has proven particularly advantageous if, during the step of introducing the at least one drainage groove into the outer surface of the drainage element, the laser is essentially fixed in place, and the drainage element is moved relative to the laser, in particular rotated about a central axis of the drainage element and simultaneously translated in the direction of the central axis. In this way, the laser—once aligned—can remain firmly in place and no longer needs to be moved.

[0025] A further aspect of the present invention relates to a dewatering element in the form of a roll cover or roll shell for dewatering a fibrous web, in particular a paper, cardboard or tissue web, preferably produced according to the method according to the invention described above, wherein the dewatering element has an outer surface facing the fibrous web during normal operation, with at least one dewatering groove provided therein, and wherein the at least one dewatering groove comprises a groove base and two side walls extending from the groove base to the outer surface, wherein the mean roughness value R a of the two side walls is less than 2.0pm, preferably less than 1.0pm, and wherein the drainage element is particularly characterized in that the mean roughness value R a of the groove base is above 2.0pm, preferably above 3.0pm, more preferably above 3.5pm.

[0026] Preferably, the two side walls are free of any traces of mechanical cutting. In other words, the side walls of the drainage grooves preferably do not exhibit typical cutting marks, such as those that inevitably occur when the drainage groove is introduced into the drainage element by mechanical cutting, in particular by means of a rotating cutting disk, as shown schematically in Figure 2.

[0027] It should be noted that the characteristics and advantages stated for the manufacturing process according to the invention also apply mutatis mutandis to the drainage element according to the invention, and vice versa, unless the context indicates otherwise. To avoid repetition, reference is made to the above statements.

[0028] Instead of the mean roughness value R a can also be applied to the average roughness R zof the two side walls. This can be less than 20.0 pm, preferably less than 15.0 pm, more preferably less than 12.0 pm. The average roughness R z is defined according to DIN EN ISO 4287 as the sum of the height of the largest profile peak and the depth of the largest profile valley within a single measuring section.

[0029] A further aspect of the present invention relates to the use of a previously described dewatering element according to the invention and / or a dewatering element which was produced according to the previously described method according to the invention, in a dewatering device for dewatering a water-containing fibrous web, in particular a paper, cardboard or tissue web, wherein the dewatering device further comprises a pressing element, in particular a pressing roll or a Yankee cylinder, which pressing element together with the dewatering element forms a pressing nip, in particular an extended pressing nip,wherein the fibrous web is guided through the press nip, and a portion of the water contained in the fibrous web is pressed out of the fibrous web and removed from the fibrous web via the at least one dewatering groove in the outer surface of the press element. The dewatering element can, in particular, be the roll cover of a conventional press roll or the roll shell of a shoe press roll. An alternative aspect of the present invention relates to the use of a previously described dewatering element according to the invention and / or a dewatering element produced according to the previously described method according to the invention in a dewatering device for dewatering a water-containing fibrous web, in particular a paper, board, or tissue web, wherein the dewatering device comprises at least one curved dewatering zone.in which the dewatering element in the form of a roll shell is guided over a support element with a radius of curvature, wherein the fiber suspension to be dewatered is guided between two wires over the at least one curved dewatering zone, wherein the degree of curvature of the curve of the at least one curved dewatering zone increases in the running direction of the dewatering element such that an increasing dewatering pressure is applied to the fiber web running between the wires at the at least one dewatering zone, wherein the increasing dewatering pressure depends on a tension of the wires and the radius of curvature of the at least one curved dewatering zone. In this case, the dewatering element can be used in particular in the forming section of a machine for producing the fiber web, where the water content in the fiber web is still very high.

[0030] The invention will be explained in more detail below using an exemplary embodiment described with the aid of schematic figures. In the following:

[0031] Figure 4: the inventive manufacturing method of the pressing element with a

[0032] laser; and

[0033] Figure 5: the roughness of side walls of two dewatering grooves produced according to the present invention compared to the roughness of side walls of three conventionally produced dewatering grooves. Figure 4 shows a very schematic illustration of the method according to the invention for producing a dewatering element 10 in the form of a roll cover or roll shell for dewatering a fibrous web, in particular a paper, cardboard, or tissue web, wherein the dewatering element 10 has an outer surface 22 facing the fibrous web during normal operation, wherein the method comprises the step of introducing at least one dewatering groove 14 into the outer surface 22 of the dewatering element 10, wherein the at least one dewatering groove 14 comprises a groove base 18 and two side walls 16 extending from the groove base 18 to the outer surface 22 (cf. Figure 3).The at least one drainage groove 14 is introduced into the outer surface 22 of the drainage element 10 by means of a laser 24. In the present example, a CO2 laser 24 with a wavelength of 10.6 pm was used. During the introduction of the at least one drainage groove 14 into the outer surface 22 of the drainage element 10, the laser 24 is preferably substantially fixed in place, and the drainage element 10 is moved relative to the laser 24, in particular rotated about a central axis of the drainage element 10 and simultaneously translated in the direction of the central axis.

[0034] By introducing the at least one drainage groove 14 by means of a laser 24, it is possible to form the side walls 16 of the drainage groove 14 free of the typical cutting marks that arise during mechanical production of the drainage grooves 16, and at the same time to form them very smoothly, both of which benefit the drainage capacity of the drainage groove 14 of the drainage element 10.

[0035] Figure 5 shows a diagram in which the roughness of the side walls 16 of various samples is shown. The roughness was measured as the mean roughness value R a and once as averaged roughness depth R zmeasured. The first two pairs of measured values ​​(left in the diagram) belong to samples of drainage elements 10 according to the invention, which were manufactured according to the method of the present invention. The two samples were manufactured with the same output power of the same laser 24. In both cases, the mean roughness R a under

[0036] I ,0pm, to be more precise, between 0.80pm and 0.90pm. The average roughness R z is in both cases below 11,0pm, to be more precise, between 9,0pm and

[0037] II ,0pm.

[0038] The last three pairs of measured values ​​(on the right in the diagram) belong to samples from the applicant and from competitors, in which the drainage grooves 14 were conventionally produced using a mechanical cutting process. This can be recognized by the characteristic cutting marks on the walls 16. In all three cases, the measured mean roughness R aover 2.0pm, to be more precise, between 2.0pm and 3.5pm. The measured average roughness R z is in all three cases above 20.0pm, to be more precise, between 20.0pm and 31.0pm.

[0039] The measurement results show that the roughness of the side walls 16 of the drainage grooves 14 produced by the method of the present invention is significantly lower than the roughness of the side walls 16 of the drainage grooves 14 produced by a conventional mechanical cutting process.

[0040] However, the roughness of the groove bottom 18 of the drainage grooves 14 produced by the method of the present invention, at over 2.0 μm, is in a similar range to the roughness of the groove bottom 18 of the drainage grooves 14 produced by a conventional mechanical cutting process.

[0041] 10 Drainage element

[0042] 12 Cutting disc 14 Drainage groove

[0043] 16 Side wall

[0044] 18 groove base

[0045] 20 coolant pipe

[0046] 22 Outer surface 24 Laser

Claims

Patent claims 1. A method for producing a dewatering element (10) in the form of a roll cover or roll shell for dewatering a fibrous web, in particular a paper, cardboard or tissue web, wherein the dewatering element (10) has an outer surface (22) facing the fibrous web during normal operation, wherein the method comprises the step of introducing at least one dewatering groove (14) into the outer surface (22) of the dewatering element (10), wherein the at least one dewatering groove (14) comprises a groove base (18) and two side walls (16) extending from the groove base (18) to the outer surface (22), and wherein the mean roughness value R a of the two side walls (16) is less than 2.0pm, preferably less than 1.0pm, characterized in that the mean roughness value R a of the groove base (18) is above 2.0pm, preferably above 3.0pm, more preferably above 3.5pm.

2. Method according to claim 1, characterized in that the at least one drainage groove (14) is introduced into the outer surface (22) of the drainage element (10) by means of a laser (24).

3. Method according to claim 2, characterized in that a laser (24) with a wavelength of more than 1900 nm is used to introduce the at least one drainage groove (14).

4. Method according to claim 2 or 3, characterized in that a CO2 laser (24) is used to introduce the at least one drainage groove (14).

5. Method according to one of claims 2 to 4, characterized in that the laser (24) has an output power between 1 kW and 10 kW, preferably a power between 2 kW and 7 kW, more preferably between 3 kW and 5 kW.

6. Method according to one of the preceding claims, characterized in that the outer surface (22) of the drainage element (10) is provided by a layer of polyurethane, wherein the polyurethane is preferably colored.

7. Method according to one of the preceding claims, characterized in that the dewatering element (10) is a shoe press jacket.

8. Method according to one of the preceding claims, but at least according to claim 2, characterized in that during the step of introducing the at least one drainage groove (14) into the outer surface (22) of the drainage element (10), the laser (24) is substantially fixed in place and the drainage element (10) is moved relative to the laser, in particular rotated about a central axis of the drainage element (10) and simultaneously displaced translationally in the direction of the central axis.

9. Dewatering element (10) in the form of a roll cover or roll shell for dewatering a fibrous web, in particular a paper, cardboard or tissue web, preferably produced according to the method according to one of the preceding claims, wherein the dewatering element (10) has an outer surface (22) facing the fibrous web during normal operation, with at least one dewatering groove (14) provided therein, and wherein the at least one dewatering groove (14) has a groove base (14) and two side walls (16) extending from the groove base (14) to the outer surface (22), and wherein the mean roughness value R a of the two side walls (16) is less than 2, μm, preferably less than 1, 0pm, characterized in that the mean roughness value R a of the groove base (18) is above 2.0 μm, preferably above 3.0 μm, more preferably above 3.5 μm.

10. Drainage element (10) according to claim 9, characterized in that the two side walls (16) are free of traces of mechanical cutting.

11. Drainage element (10) according to one of claims 9 or 10, characterized in that the average roughness depth R z of the two side walls (16) is less than 20, Opm, preferably less than 15, Opm, more preferably less than 12, Opm.

12. Use of a dewatering element (10) according to one of claims 9 to 11 and / or produced by the method according to one of claims 1 to 8 in a dewatering device for dewatering a water-containing fibrous web, in particular a paper, cardboard or tissue web, wherein the dewatering device further comprises a pressing element, in particular a pressing roll or a Yankee cylinder, which pressing element together with the dewatering element (10) forms a pressing nip, in particular an extended pressing nip, wherein the fibrous web is guided through the pressing nip and a portion of the water contained in the fibrous web is pressed out of the fibrous web and is discharged from the fibrous web via the at least one dewatering groove (14) in the outer surface (22) of the pressing element (10).

13. Use of a drainage element (10) according to one of claims 9 to 11 and / or produced by the method according to one of claims 1 to 8 in a drainage device for draining a water containing fibrous web, in particular a paper, cardboard or tissue web, wherein the dewatering device comprises at least one curved dewatering zone in which the dewatering element (10) in the form of a roll shell is guided over a support element with a radius of curvature, wherein the fibrous suspension to be dewatered is guided between two wires over the at least one curved dewatering zone, wherein the degree of curvature of the curve of the at least one curved dewatering zone increases in the running direction of the dewatering element (10) such that an increasing dewatering pressure is applied to the fibrous web running between the wires at the at least one dewatering zone, wherein the increasing dewatering pressure is dependent on a tension of the wires and the radius of curvature of the at least one curved dewatering zone.

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