Patent Application
20220034036
The invention is based on a press cover, in particular for a pressing device for treating a fibrous web, for example to smooth or dewater the latter, in detail according to the independent claims. The invention also relates to a press roll, a shoe press and the use of a press cover in such, in detail according to the subordinate claims.
Pressing devices such as shoe presses have for a long time been a constituent part of modern paper machines. They substantially comprise a shoe that is arranged to be stationary (also called a press shoe), which extends in a cross-machine direction, and a press cover circulating around the stationary shoe. Said press cover is deformable and substantially assumes a tubular shape in operation. The shoe is shaped such that it forms a press nip with an opposing roll. The press nip is defined by the contact surface of the opposing roll in the shoe. The shoe is movable and can be moved onto the opposing roll.
Enormous requirements in relation to its stability are placed on the press cover, specifically with regard to surface hardness, resistance to pressure, temperature and hydrolysis. In addition, during operation the press cover is subjected to high alternating flexural loadings. As it runs in at the shoe edge—upstream of the press nip as viewed in the direction of rotation of the press cover—firstly flexure under a comparatively small radius takes place. This changes immediately into an opposite flexure as it passes through the press nip. As it runs out at the other shoe edge, that is to say—downstream of the press nip as viewed in the direction of rotation of the press cover—an opposite flexure takes place again. This deformation of the press cover as it runs in and out is also designated as an alternating nip. It can easily be seen that the tendency of the press cover to fracture, in particular at this point, as a result of the high mechanical stress is very high. Accordingly, the prior art discloses many measures which are intended to increase the stability of the press cover.
The press cover must thus be sufficiently flexible that it can be led around the shoe, it must be sufficiently rigid in order that it is not too highly deformed or compressed under the press load in the nip, and it must be sufficiently wear-resistant. Therefore, press covers consist of a single-layer or multi-layer polymer layer, preferably made of polyurethane, into which reinforcing threads in the form of laid or woven fabrics can be embedded.
The present invention relates to such generic objects mentioned at the beginning.
Press covers known from the prior art tend to premature failure during normal operation as a result of—often only local—overloading in the nip. Such overloading arises when, during what is known as a lump passage, a foreign body goes through the nip. Such overloading often leads to the reinforcing threads or the polymer layer in which they are embedded tearing. A press cover which, for example, is oil-lubricated from the inside, can become leaky, so that the oil comes into contact with the fibrous web to be produced. As a result of this, the press cover must be changed. In practice, this leads to unplanned stoppages of the pressing device and therefore to increased, costly downtimes.
It is accordingly an object of the invention to specify a press cover which avoids the disadvantages of the prior art. In particular, the total failure of the press cover as a result of even only local damage to the same because of overloading during normal operation is to be prevented or its consequences detected in good time. In particular, the stoppage times of a pressing device equipped with such a press cover are to be reduced.
The object is achieved by the features of the independent claims. Particularly preferred and advantageous embodiments of the invention are reproduced in the sub claims.
The inventors have recognized that local overloading that has already taken place can be detected better on a press cover if there is a sufficiently high optical contrast between the reinforcing structure and the at least one polymer layer in which the latter is embedded. This is because if the overload is sufficiently high, this normally leads to local damage to the reinforcing structure, for example in the form of tears in the threads. Torn threads can then be detected better by the human eye, so that a planned change of the press cover in good time can be carried out.
To this end, according to the invention, the materials of the reinforcing structure and of the at least one polymer layer are chosen such that the visible light transmittances thereof differ from each other. If the transmittance of the material of the at least one polymer layer is chosen to be higher than that of the reinforcing structure, then the reinforcing structure can shine through the polymer layer under appropriate lighting conditions. It would also be possible to say that the material of the at least one polymer layer is more transparent or more see-through (that is to say less opaque) than that of the reinforcing structure. Alternatively, the press cover can be illuminated or shone through with light that is visible (to the human eye), for example from an LED lamp. Local overloading can then be detected optically, for example with the machine at creep speed, at which the press cover rotates more slowly than in normal operation, in which the machine produces the fibrous web. Then, an impending loss of oil from the press cover can be avoided in good time by means of an appropriate replacement of the press cover.
In optics, the transmittance describes the proportion of the incident radiation flux or light current which passes completely through a transparent component. The incident radiation flux which strikes the component can be transmitted by the latter, reflected and absorbed. Accordingly, the result is the following power balance of the radiation flux striking the component in a transmittance (T), a reflectance (R) and an absorptance (A):
R+A+T=1,
which are divided among one another in accordance with the material properties of the component. The transmittance therefore corresponds to that part of the incident radiation flux which is reduced by the reflectance (R) and the absorptance (A). Another designation for the transmittance is the term total transmission Tt. The last-named or transmittance corresponds the ratio of the reciprocal of the incident radiation flux T1 to the radiation flux T2 let through by the transparent component, that is to say T2/T1. Therefore, if it is mentioned that, for example, the material of the at least one polymer layer has a transmittance of 50%, then said layer lets through only half of the incident radiant flux. The transmittance or the total transmission are defined and can be measured in accordance with ASTM D 1003-00. The measurement can be carried out at any desired point on the press cover, for example even at its axial edges, thus, for example, in the region of the lugs by means of which such a press cover is held on the two lateral tensioning disks. The statement of the transmittance can relate to a factory-fresh, that is to say finished, press cover.
If it is mentioned that a polymer layer or the press cover is uncolored, then this means that it is free of colorants. Colorants can be dyes—which dissolve in the polymer layer at the molecular level or are adsorbed on the surface of the latter—or pigments, that is to say particles that are insoluble in the material of the polymer layer.
In the sense of the invention, a press cover is to be understood as a belt, hose or a cover which is closed, endless about its longitudinal axis in the circumferential direction, which, as illustrated, is guided through the nip (press nip) of a shoe press together with a fibrous web. To dewater the fibrous web, in normal operation the radially outermost surface (polymer layer) of the press cover can come into contact with a press felt, by which the fibrous web to be dewatered is carried directly. Depending on the embodiment of the pressing device, for example to smooth the latter, the press cover can also come directly into contact with the fibrous web in normal operation. At its axial ends—as viewed in the width direction (along the longitudinal axis)—it is open. Therefore, the press cover can be held at these axial ends by two lateral tensioning disks in order to form the shoe press roll. Instead of the guidance by the two lateral tensioning disks, the press cover can be guided over the press shoe and multiple guide rolls, as is the case in open shoe presses. Irrespective of whether the press cover is guided by the tensioning disks or the guide rolls, the press shoe (and the guide rolls) come (temporarily) into contact with part of the radially innermost surface of the press cover. The radially outermost surface of such a press cover, that is to say, for example, the radially outermost polymer layer of the same, can be provided with grooves and/or blind drilled holes.
Longitudinal direction means that direction which extends parallel to the longitudinal axis of the press cover. The longitudinal axis simultaneously corresponds to the axis of symmetry or rotation of the finished press cover and the press roll. The circumferential direction of the press cover, viewed around its radial limit, extends around the longitudinal axis. The term parallel also includes those angular deviations of two reinforcing threads lying in different planes of +/−5° relative to each other.
The press cover or the at least one polymer layer can be produced partly or completely from a polymer. Here, a castable, curable, preferably elastomeric polymer such as polyurethane can be used as the polymer. Consequently, the polymer can be set as a cast elastomer.
Polymer layer means a layer which comprises such a castable, curable, preferably elastomeric polymer or is produced entirely therefrom. Preferably, the polymer layer can be a cured layer produced in one piece by primary molding. In other words, this is molded monolithically, that is to say produced by casting, for example. The term in one piece also includes cases in which one layer has in turn been produced from multiple layers of the same material during the casting of the polymer. However, this only to the extent that these layers are substantially no longer visible following the curing, instead a single, preferably uniform layer results. The same is correspondingly true of the finished press cover.
If a plurality of polymer layers is provided, these can be arranged one above another, at least in some sections, over the width of the press cover—as viewed in the radial direction. At least some sections over the width of the press cover means that the press cover is, for example, only single-layered at its axial ends along the longitudinal axis of the press cover, whereas it is two-layered or multi-layered between the axial ends. However, the polymer layers can also extend over the entire width of the press cover. In addition, the thickness of the press cover—and thus the thickness of the individual polymer layers—can vary in some sections along the longitudinal axis in a section through the latter. Thus, for example, the radially outermost polymer layer in the region of the width edges of the press cover can be smaller than in the center of the press cover. In other words, in the region of the width edges, the radially outermost polymer layer can be less thick than a radially inner or radially innermost polymer layer. Preferably, exactly one, two or three polymer layers is/are provided. These can be designed identically with regard to their polymer or vary with regard to the hardness or stoichiometry of the pre-polymer.
A total thickness of the finished press cover in a section through the longitudinal axis of the same measured in the radial direction can be 5 to 10 mm, preferably 5 to 7, particularly preferably 5 to 6 mm. According to the invention, when a single layer is provided, the press cover can be produced from only one casting, i.e. monolithically, so that the single layer has the thickness just mentioned.
A finished press cover in the sense of the invention is one of which the at least one polymer layer is cured and possibly finally machined, that is to say is ready to use for the purpose mentioned at the beginning, for example in a shoe press. In an analogous way, a finished polymer layer means a layer which is cured.
In the sense of the invention, a reinforcing thread is understood to be a flexible, textile linear structure which has a dominant extent and a uniformity in its in longitudinal direction. If fibers are mentioned, then this means a single, endless fiber in the manner of a monofilament. If, on the other hand, mention is made of a fiber bundle in the sense of the invention, this is not monofilaments but, for its part, an individual thread such as a twine or yarn, that is to say a bundle of endless fibers or monofilaments. The fiber bundles themselves can entirely possibly be produced from fibers twisted with one another.
The definition that at least the longitudinal threads are produced as reinforcing threads according to the invention means that only the longitudinal threads are designed in this way or, in addition, the longitudinal threads and at least one further circumferential thread are produced in this way. If, preferably, for example a laid fabric made of circumferential and longitudinal threads is present, then this means that at least the longitudinal threads are designed according to the invention.
In the sense of the invention, the term reinforcing structure means a reinforcement of the at least one layer containing the polymer or consisting of the latter—that is to say the polymer layer. Here, the reinforcing structure can be embedded completely in the polymer layer, so that the reinforcing structure does not go beyond the limit of the polymer layer. In other words, the polymer layer performs the role of a matrix, which surrounds the reinforcing structure and bonds it to the matrix as a result of adhesive or cohesive forces. Such a reinforcing structure can comprise textile linear structures—e.g. yarns or twines—and/or two-dimensional textile structures—such as, for example, woven fabrics, knitted fabrics, crocheted fabrics, braided fabrics or laid fabrics—and be producible from an appropriate starting material, for example by winding. In other words, a single reinforcing thread according to the invention, viewed on its own, can be a textile linear structure. A plurality of such reinforcing threads can be designed, for example, as longitudinal and/or circumferential threads such that they together form a textile two-dimensional structure. The at least one reinforcing thread which is embedded in the at least one polymer layer then represents the reinforcing structure of the press cover or the polymer layer of the latter.
Starting material is understood to be that material or semifinished product by means of which the reinforcing structure of the finished press cover according to the invention is produced, that is to say in the present case the at least one reinforcing thread.
The reinforcing thread or the reinforcing structure can be produced from a polymer or comprise one such. Suitable polymers are polyesters, polyethylene naphthalate or polyamides such as aramides. Therefore, the materials of the at least one polymer layer and that of the at least one reinforcing thread or the reinforcing structure embedded therein differ.
In the sense of the invention, a pressing device means, for example, a shoe press, for example for dewatering or treating, such as smoothing, a fibrous web. The shoe press comprises a shoe press roll and an opposing roll, which together form or delimit a press nip. The shoe press roll further comprises a circulating press cover and a stationary pressing element, the so-called press shoe. The latter is supported on a load-bearing, likewise stationary, yoke—for example via hydraulic pressing elements—and is pressed onto the circulating press cover. The press cover circulates relative to the stationary press shoe and yoke and, as a result, is pressed onto the opposing roll in the press nip. The press shoe and yoke are arranged radially within the press cover. The term stationary is understood to mean that the pressing element does not circulate relative to the shoe press roll or the opposing roll but can move translationally—toward the opposing roll and away from the latter, preferably in the radial direction thereof—and therefore relative to the opposing roll. In addition to the fibrous web and press cover, one or more press felts circulating endlessly in the circumferential direction and/or further endlessly circulating press belts can be guided through the press nip of the shoe press. Of course, such a shoe press can comprise more than one press nip.
In the sense of the invention, a fibrous web is a laid fabric or tangled fabric of fibers, such as wood fibers, plastic fibers, glass fibers, carbon fibers, additional materials, additives or the like. Thus, the fibrous web can be formed, for example, as a paper, board or tissue web. It can substantially comprise wood fibers, wherein small quantities of other fibers or else additional materials and additives can be present. This is left up to those skilled in the art, depending on use.
If, preferably, a plurality of reinforcing threads as longitudinal threads and at least one reinforcing thread as a circumferential thread, which surrounds the longitudinal threads in the circumferential direction, are embedded into the polymer layer as a laid fabric, then the advantages according to the invention are particularly well achieved. This is because a laid fabric is capable of absorbing local overloads particularly well.
The advantages according to the invention are achieved particularly well if the transmittance of the material of the at least one polymer layer is between 50% and 90%—even up to 100%—and preferably between 50% and 75% and, preferably, the transmittance of the material of the at least one polymer layer is higher than the transmittance of the material of the reinforcing structure, which is preferably more than 1.1 to 1.5 times the last-named transmittance.
The advantages according to the invention are achieved particularly well if the press cover is built up from preferably a plurality of polymer layers arranged one above another in the radial direction. If two polymer layers are provided, then the radially inner one is that having the reinforcing structure according to the invention. This means that the reinforcing structure is arranged only in the radially innermost polymer layer. If three or more polymer layers are provided, then the reinforcing structure is preferably arranged in the second lowest polymer layer, that is to say in that which lies radially above the radially innermost polymer layer.
The invention also relates to a press roll, such as a shoe press roll, for a shoe press for dewatering a fibrous web, wherein the press roll has at least one press cover according to the invention.
The invention also relates to a shoe press for dewatering a fibrous web, preferably a paper, board, tissue or pulp web, comprising a press roll and an opposing roll, which together form or delimit a nip, wherein the press roll comprises a circulating press cover, wherein the press cover is formed according to the invention.
Finally, the invention relates to the use of a press cover according to the invention for a press, such as a shoe press for dewatering a fibrous web, preferably a paper, board, tissue or pulp web.
The invention will be explained in more detail below with reference to the drawings without restricting the generality. In the drawings:
While the opposing roll 14 here comprises a cylindrically configured roll rotating about its longitudinal axis, the shoe press roll 12 is assembled from a shoe 16, a stationary yoke 18 carrying the latter and a press cover 20. The shoe 16 and yoke 18 are arranged to be stationary in relation to the opposing roll 14 and the press cover 20. This means that they do not rotate. The shoe 16 is supported by the yoke 18 and, via hydraulic press elements not illustrated, is pressed against the radially innermost surface of the press cover 20 circulating relative thereto. The press cover 20, surrounds the shoe 16 and the yoke 18 in the circumferential direction and in the process rotates about its longitudinal axis in the opposite direction of rotation to the opposing roll 14. Because of the concave configuration of the shoe 16 on its side facing the opposing roll 14, the result is a comparatively long nip 22.
The shoe press 10 is suitable in particular for dewatering fibrous webs 24. During the operation of the shoe press, a fibrous web 24 is guided through the press nip 22 with one or two press felts 26, 26′. In the present case, there are exactly two press felts 26, 26′, which accommodate the fibrous web 24 between them in the manner of a sandwich. During the passage through the nip 22, a pressure is exerted indirectly on the fibrous web 24 by the press felts 26, 26′ in the nip 22. This takes place as a result of the fact that the radially outermost surface of the opposing roll 14, on the one hand, and the radially outermost surface of the press cover 20 come directly into contact with the corresponding press felts 26, 26′. The liquid escaping from the fibrous web 24 is picked up temporarily by the press felt or press felts 26, 26′ and any depressions (not illustrated) provided in the press cover surface. After leaving the nip 22, the liquid picked up by the depressions of the press cover 20 is thrown off before the press cover 20 enters the press nip 22 again. In addition, the water picked up by the press felt 26, 26′ can be removed by suction elements after leaving the press nip 22.
In a further embodiment of the invention, not illustrated in the figures, the press felts 26, 26′ can be omitted. In such a case, the fibrous web 24 is in direct contact on one side with the press cover 20 and on the other side with the opposing roll 14, which together form a press nip. Said opposing roll 14 can be designed as a heated drying cylinder.
As illustrated in the following figures, the press cover illustrated in
In
According to
As illustrated, a reinforcing structure 20″ can be provided in the second polymer layer 20.2. In the present case, this is embedded completely in the polymer layer 20.2. This is indicated by the hatched circles, which can be textile two-dimensional or linear structures such as fibers. This means that the reinforcing structure 20″ does not extend beyond the limits of the polymer layer 20.2.
Here, the reinforcing structure 20″ comprises a plurality of reinforcing threads 21 serving as longitudinal threads 21.1. These are arranged in the longitudinal direction of the press cover 20, extending parallel to one another at a distance over its circumference. In addition, here at least one further reinforcing thread 21 is provided as a circumferential thread 21.2, which preferably extends in the form of a helix in the circumferential direction of the press cover within the same polymer layer 20.1, 20.2, 20.3 in which the longitudinal threads 21.1 are also arranged. The longitudinal threads 21.1 and the circumferential thread 21.2 form a laid fabric with one another, specifically in such a way that the longitudinal threads 21.1 are arranged radially within the at least one circumferential thread 21.2—viewed in relation to the longitudinal axis 20′ of the press cover 20.
In the present case, the first and a second polymer layer 20.1, 20.2 are produced from a polyurethane. This can be obtained, for example, from a prepolymer and a cross-linker. The respective prepolymer itself can be obtained by reacting an isocyanate with a polyol.
It has transpired that the advantages according to the invention can be implemented particularly well if the polyurethane of the at least one polymer layer 20.1, 20.2, 20.3 or all the polymer layers is chosen in such a way that it is more transparent than the material of the reinforcing structure 20″. Therefore, in other words, that the material of the reinforcing structure 20″ and that of the at least one polymer layer 20.1, 20.2, 20.3 are chosen in such a way that their respective transmittance for visible light differs from one another and, preferably, the transmittance of the material of the at least one polymer layer 20.1, 20.2 exceeds the transmittance of the material of the reinforcing structure 20″.
The illustration shows a starting stage of the production process. In the present case, for this purpose one end of the starting material 20″′ is fixed to a polymer, which is arranged on the outer circumference of the mandrel 4. Apart from the schematic illustration shown, one end of the starting material 20″′ could also be laid or applied directly to the mandrel 4 without a polymer initially being provided between the starting material 20″′ and the mandrel 4. The starting material 20″′ can be a textile two-dimensional structure or linear structure.
The mandrel 4 is mounted such that it can rotate about its longitudinal axis 20′, which corresponds to the longitudinal axis of the press cover to be produced. The longitudinal axis 20′ here extends perpendicularly into the drawing plane. Via a line 5, a casting material, such as castable, curable elastomeric polymer, for example polyurethane, is discharged through a casting nozzle 6 from above onto the radially outermost lateral surface of the mandrel 4 or onto the starting material 20″′. Such a casting material can be chosen, for example with regard to its pot time and viscosity, in such a way that it does not drip off the mandrel 4 during the casting. During this, the mandrel 4 is rotated about its longitudinal axis in the direction of the arrow. At the same time as this rotation, the casting nozzle 6 is guided along on the mandrel 4 along the longitudinal axis 20′ and parallel to the latter via a suitable guide, not further illustrated in
The casting material emerging from the casting nozzle 6 in the present case is a mixture of a prepolymer and a cross-linker. The first is provided from a prepolymer container, not shown, in which it is stored or stirred. The prepolymer is the reaction product of an isocyanate according to the invention and a polyol. In the prepolymer container, it can be present, for example, in the form of a prepolymer made of the materials just mentioned.
The cross-linker can be provided in a cross-linker container.
The prepolymer container and cross-linker container are assigned to the device for producing a press cover 20. They are connected in a flow-conducting manner to a mixing chamber (not illustrated) connected upstream of the casting nozzle 6 in the flow direction via lines, likewise not illustrated. The prepolymer-cross-linker mixture is therefore produced upstream and outside the casting nozzle 6, that is to say mixed in the mixing chamber. Irrespective of the production of the mixture, this is then applied to the surface of the mandrel 4 to form the at least one polymer layer of the press cover 20.
In principle, it would be conceivable that two or more casting nozzles 6 could be provided. These could be connected via appropriate lines to separate prepolymer and cross-linker containers, in order also to supply different polymers to the plurality of casting nozzles 6 independently of one another. The casting nozzles 6 could then be arranged along the longitudinal axis of the press cover 20 at a distance from one another in order to produce a plurality of polymer layers 20.1, 20.2, 20.3 simultaneously in one casting by the simultaneous discharge of the polymer out of the casting nozzle 6.
By means of such a continuous casting operation, which is also known as rotational casting, an endless, intrinsically closed cylindrical press cover 20, the inner circumference of which corresponds closely to the outer circumference of the mandrel 4, is therefore produced gradually over the width of the mandrel 4.
In principle, it would be conceivable to wind the starting material 20″′ onto more than the one mandrel 4 shown in
As illustrated in
Although this is not illustrated in the figures, the reinforcing structure 20″ of the at least one polymer layer 20.1, 20.2 could also be built up from a plurality of starting materials 20″′ laid over one another in the radial direction, each extending in the longitudinal axial direction and in the circumferential direction of the press cover 20.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1 02018 122 779.5 | Sep 2018 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2019/068484 | 7/10/2019 | WO | 00 |
