Method of working the surface of a machine element of a fiber web machine and a machine element of a fiber web machine

[001] The invention is related to working of the surface of a machine element of a fiber web machine according to the preamble of patent claim 1 and to a machine element of a fiber web machine according to the preamble of patent claim 12.

[002] Various coating materials are generally used in fiber web machines for example to create a certain desired surface property. This applies in particular to the surfaces of machine elements which are in contact with fiber web, in other words, mostly the surfaces of rolls, cylinders, felts, belts and wires. In the following the prior art is discussed mainly in the context of paper machine rolls although the invention is not limited to modifying the surface properties of rolls, only.

[003] Publication FI-A-950849 discloses adjusting the surface properties of surfaces of a paper machine by treating said surfaces with a liquid surface treatment agent to obtain the desired surface properties. The effect of the surface treatment agent is limited and the treatment must be repeated at certain intervals.

[004] Today, ceramically coated center rolls are commonly used in paper machine presses. When using ceramic center rolls, the adhering/detaching properties of the paper web are difficult to control. Publication US 5,569,142 discloses partial coating of a fiber web machine press section center roll for example with Teflon in order to influence the detaching properties of the web from the roll.

[005] Publication FI-A-955040 discloses a method of manufacturing a roll for use in paper production, according to which method the roll is coated by

thermal spraying. According to the method, sealing material solution is applied onto the roll coating manufactured by thermal spraying in order to seal the pores of the coating, the sealing solution is permitted to be absorbed to the pores of the coating and the roll is heated to remove water from the sealing material solution. While the water exits the sealing material solution crystallizes in the pores of the coating after which the roll is allowed to cool down.

[006] FI-B-116389 discusses a method of adjusting the properties of the surface of a paper machine roll, according to which a photocatalytic surface or a membrane is provided on the surface of the roll which under the influence of light increases the hydrophilic property of the roll surface which facilitates the detaching of the web from the roll surface.

[007] The above discussion discloses that the coatings often aim at influencing the behavior between the fiber web and the roll in certain situations. Currently the surface materials of a center roll are typically thermally sprayed ceramic coatings, polymer-based coatings or rubber coatings although also stone rolls are used. One of the factors influencing most the runnability and the increase of speed of the fiber web machine is the open feeding of the web from the center roll of the press section to the drying section or the subsequent separate press. Detaching the wet web from the roll requires subjecting the web to the tension created by traction difference. The conventional way of trying to influence the adhesion and the detaching of the web has been by way of the material of the center roll and the quality of the finishing grinding. Leaving the surface coarser creates as such a thicker water film on the center roll but it brings the drawbacks of the coarser surface such as changing surface properties due to wear of the roll surface during operation and also accelerated wearing of doctors.

[008] Prior art knows also grooves machined in the surfaces of fiber web machine rolls which are discussed in the two following publications.

[009] First, US-A-5,351 ,399 discusses producing grooves by laser in the roll surfaces of press rolls used for dewatering in a paper machine. In the solution described in the publication, grooves with a depth of at least 3 mm and a width of over 0.3 millimeters are melted in the surface of the roll with a carbon dioxide laser of at least one kilowatt. The purpose of the grooves is to carry the water separated from the web at the press by pressing away from the immediate vicinity of the web and thus to reduce the heating required at the drying section of the paper machine.

[0010] Also DE 2150 837 discusses a grooved roll. More precisely expressed the publication describes a pair of rolls provided in connection with a dewatering press, at least one of the rolls being provided with two grooves running spirally crosswise around the roll. The purpose of the grooves is to attach the web material to the surface of the roll and to prevent it from being torn.

[0011] Today it has become apparent that the surface properties of the machine elements of fiber web machines, such as apparatus in direct contact, or via a wire/felt in indirect contact, with the web, in particular rotating apparatus such as rolls and/or metal conveyor belts, must be improved. Thus the object of this invention is to raise the state of the art of the field by providing a method of working the surface of a machine element of a fiber web machine so that improved operation of a coated machine element of a fiber web machine and more versatile running possibilities of a fiber web machine are obtained.

[0012] A particular object of the invention is to provide a method of influencing the properties of the outer surface of a machine element, and a machine element to be installed in direct or indirect contact with a web, the application of which remarkably contributes to the control of the running of the web in a fiber web machine.

[0013] According to the method of the invention for working the surface of a machine element of a fiber web machine, the surface of the machine element is first formed smooth, the coarseness of the surface preferably being Ra < 10 μm, and subsequently the smooth surface is provided with a pattern comprising recesses having a depth of and a width of less than 100 μm. In this way the surface is very smooth and at the same time the water film being formed on the surface of the machine element can be influenced so that the fiber web or wire/felt containing water is more easily and more evenly detached from the surface.

[0014] The patterning is provided to cover less than 50 % but preferably more that 1 % of the surface of the machine element.

[0015] According to the invention the patterning is formed in the surface of the machine element by using laser micro machining with a high-frequency pulse laser.

[0016] A high-frequency pulse laser means a laser apparatus the laser pulse duration is approx. 10 μs (micro seconds) -10 fs (femto seconds) and the pulse frequency to be used is 100 Hz - 1000 MHz. The short pulse duration and the high repetition frequency enable the material to be vaporized without heat deformation zones whereby the quality of the surface is very good also after the treatment.

[0017] The patterning is preferably provided regular whereby the properties of the surface are uniform throughout the entire surface.

[0018] According to a preferred embodiment of the invention the patterning is provided as spotlike recesses. Then there are preferably 50000 - 2000000 recesses / m ² provided in the surface. When the number of recesses is relatively high and the size of individual recesses is relatively small the recesses may be provided also to some extent irregular in the surface.

[0019] According to another preferred embodiment of the invention the patterning is provided as intersecting grooves.

[0020] In a fiber web machine, the detaching of the web or other surface properties may be efficiently influenced by treating the surface, after the recesses have been formed, with a certain substance so that an amount of the substance remains at the bottom of the recesses.

[0021] The choice of the substance may be used to determine the influence created. When the surface is treated with a hydrophobic substance the detaching of the wet web is improved. When the surface is treated with a hydrophilic substance the detaching of the wet web/wire/felt is weakened. Depending on the final desired influence and the influence of the recesses of the surface, a substance may be selected according to the desired direction of the change.

[0022] The method is particularly favorable in the manufacture of a fiber web machine press section roll as the method allows providing a coating influencing the adhesion of the fiber web/wire/felt. A metal belt that can be used in a calender or a press may be mentioned as a particularly favorable application. Especially in calender applications it is advantageous to treat the surface of the metal belt according to the invention in order to obtain optimal web adhesion and detaching properties.

[0023] A machine element of a fiber web machine according to the invention comprises a substantially smooth operating surface, the surface having been provided with recesses of micrometric scale to crease patterning in the surface.

[0024] Although there is a clear visually detectable patterning which has a substantial influence for example on the detaching properties of the web, the patterning does not have a marking influence on the web.

[0025] The coarseness of the operating surface according to the invention is R _a < 10 μm; especially applied in the surface of a press section roll the coarseness is preferably < 5 μm.

[0026] The patterning of the machine element operating surface comprises for example individual cavities, or the patterning comprises intersecting grooves.

[0027] The diameter of the grooves and/or cavities of the patterning in the operating surface is preferably 1 - 50 μm and the depth 1 - 50 μm. Further, it is advantageous that the patterning of the operating surface covers less than 50 % of the total area of the operating surface, however, so that the patterning of the operating surface covers 1 - 50 % of the total area of the operating surface.

[0028] Other additional characteristic features of the invention will be disclosed in the appended patent claims.

[0029] The invention and its operation will be described below with reference to the attached schematic drawing figures of which

Figure 1 illustrates the press section of a fiber web machine according to a preferred embodiment of the invention,

Figure 2 illustrates the surface of a machine element according to a preferred embodiment of the invention,

Figure 3 illustrates the surface of a machine element according to another preferred embodiment of the invention, and Figure 4 illustrates the influence of the invention with contact angle determination on a surface according to a preferred embodiment of the invention.

[0030] Figure 1 illustrates schematically a press section 10 of a fiber web machine. Web W being produced with the fiber web machine has been arranged to run through a press section. The web W arrives supported by a wire 12 to the first roll and is wound around it. The first roll 18 forms a nip N 1 with a center roll 22. The web W runs through the nip N1 , is detached from the wire 12 and subsequently follows the surface of the center roll 22 to a second nip N2 formed by the center roll 22 and second roll 20. There is another wire 14 arranged to run over the second roll and through the second nip N2 whereby the web runs in the second nip N2 between the center roll and the second wire 14. After the second nip N2 the web has been arranged to follow the surface of the center roll for a distance which depends for example on the machine geometry, the traction difference used and the surface properties of the center roll 22.

[0031] The method according to the invention provides a very advantageously operating center roll 22 of a press section of a fiber web machine by working the surface 24 of the center roll 22 of the fiber web machine press section. In this way the surface 24 of the center roll 22 is caused to function so that the adhesion of the web is sufficient in order to first cause the web to adhere to the surface of the roll 22 after the nip N1 which in this kind of a position of the fiber web machine is essential for the operation. On the other hand the surface 24 of the center roll 22 is caused to function also so that the adhesion of the web is sufficiently weak so that the web can be detached in a controlled manner from the surface of the roll after the second nip N2. Also in the application where the web is detached to the surface of a so-called transfer nip roll (not illustrated) the approach of the invention creates a situation where the adhesion at the center roll is so low that the web may be transferred by pressing to the surface of the transfer nip roll.

[0032] According to a preferred embodiment of the invention, in the detaching of the fiber web from the operating surface 24, where there is water

present, the water film between the fiber web and the surface is caused to break in a very controlled and regular manner, when the surface 24 of the center roll 22 is first provided smooth, the coarseness being R _a < 10 μm, and after this patterning is applied to the smooth surface, the pattern comprising recesses 30 having a depth and width of less than 100 μm. Then the recesses provided at the level of the generally smooth surface function as efficient water film cutting points. When the patterning extends essentially regular and uniform over the entire surface area in contact with the fiber web the fiber web is detached in a very controlled manner from the surface of the roll 22. Although according to the invention the surface properties of the machine element are influenced by the recesses in the surface, the dimensions of the recesses are for example in relation to the thickness of the fiber web being produced insignificantly small whereby the existence of the recesses does not affect the surface of the fiber web, in other words for example the patterning is not to the least extent transferred to the surface of the fiber web.

[0033] A surface produced in this way is in a fiber web machine advantageous also in view of maintenance. As the machine element surface produced according to the method of the invention is very smooth and the surface properties are influenced by recesses the supporting surface remains large and thus the surface properties of the surface change for example due to doctoring relatively little.

[0034] Figures 2 and 3 illustrate test objects the operating surface 24 of which has been produced according to the method of the invention. The surface has been provided with micro meter scale recesses 30 which form the patterning of the surface. The invention is preferably applied in a surface the basic material of which is hard and wear-resistant such as a ceramic or cermet material.

[0035] The patterning is preferably produced according to the invention by using high-frequency pulse laser instead of the conventional finishing grinding

or in addition to it. The laser pulse duration is preferably 10 μs - 10 fs and the pulse frequency used is 100 Hz - 1000 MHz. In this way the heat brought to the machine element during the manufacture is minimal and thus the quality of the surface does not essentially suffer from the treatment. A very short pulse duration and a high repetition frequency enable the material to be vaporized without heat-affected zones. In figure 2, the recesses 30 in the surface 24 are intersecting grooves 30'. The groove width in the exemplary surface 24 illustrated in figure 2 is approx. 20 μm but in practice the suitable groove width varies typically between approx 10 and 50 μm. The depth of the grooves is preferably 2 - 20 μm. During operation of the machine element the grooves are preferably at an angle α relative to the average detaching line 32 of the fiber web, whereby their influence on the detaching process is more even.

[0036] The method of the invention may be applied also in existing already installed machine elements by treating with laser on site for example the roll or a metal belt in a fiber web machine. This is an advantageous process also in the respect that the surface need not be cleaned separately for the laser treatment before the treatment but the laser vaporizes also the adhered material from the surface to be treated.

[0037] In the embodiment illustrated in figure 3, individual spotlike round recesses 30" with a diameter of about 10 μm have been used instead of grooves. The recesses have here been arranged regular so that the recesses are in a row, however, so that the direction of the rows deviates from the direction of the average detaching line of the fiber web. The patterning is arranged to cover less than 50 % but, however, at least 1 % of the operating surface 24, whereby the detaching properties of the fiber web are particularly good. Ip this way individual water layer thicknesses of adequate size are created while the rest of the surface remains smooth. There are preferably about 50000 - 2000000 spotlike recesses per square meter in the surface. In some applications the spotlike recesses may be arranged also to deviate to some extent from the organized pattern form.

[0038] In the test objects illustrated in figures 2 and 3 the surface coarseness (Ra) in an untreated object is a little less than 1 μm and after the method has been applied in it the coarseness is about 1.7 μm. Irrespective of the patterning of the surface the method of the invention produces a very smooth surface which, however, gives very good detaching properties of the web. The advantages of the invention are to be seen already with surface coarsenesses of R _a < 10μm but for example in a press section center roll a surface of R _a < 5 μm is preferably used.

[0039] The efficiency of the invention can be demonstrated with the illustration in figure 4. The hydrophilic property of the surface is often evaluated by contact angle or moistening angle measurement. Then the spreading of a liquid drop, often water, brought to the surface is observed. If the moistening angle is wide a non-spreading drop is formed of the liquid onto the surface of the solid material. The smaller the moistening angle is the more the drop has spread on the surface. Figure 4 illustrates a relative moistening angle compared with a reference surface (100 %). Sample 1 (Sped) illustrates an operating surface of figure 3 provided with recesses with a coverage of 10% and sample 2 (Spec.2) with a coverage of 50 %. The reference surface is a corresponding surface without recesses. The figure indicates that a machine element manufactured with the method of the invention clearly produces a wider moistening angle which in practice means a correspondingly larger tear of the water film from the surface. As a consequence of this also the water- containing fiber web is detached from the surface of the machine element manufactured according to the invention more easily and from the area of the whole surface uniformly. In practice this allows for example using a smaller tensile difference between the press section and the subsequent section.

[0040] The recesses may be further treated with a suitable substance whereby the influence of the recesses may be further intensified or adjusted. For example the recesses may be treated with a chemical promoting the

detaching of the web/wire/felt, for example a hydrophobic material such as fluoropolymers.

[0041] It must be understood that only a few most preferred embodiments of the invention have been presented above. Thus it is evident that the invention is not limited to the embodiments described above but it may be applied in many ways within the scope of protection defined by the appended patent claims. For example the invention may be applied in addition to the press section center roll described above also in other types of web treatment apparatus, such as belt-type web presses or belt-type web surface treatment devices such as calenders and for example in other such fiber web machine positions where the presence of water influences the properties between the web/wire/felt and the operating surface such as the first drying cylinders where also a certain kind of web/wire/felt detaching property is required from the cylinder surface. Features described in connection with different embodiments may further be used according to the basic idea of the invention in connection with the other embodiments and/or various entities may be combined from the features presented, if that is desired and the technical possibilities for that exist.