CALENDER PROVIDED WITH AN EXTENDED NIP

Abstract: The invention concerns a calender provided with an extended nip for calendering of a paper or board web. The calender comprises at least one calendering nip (N), which is formed between a heatable hard roll (1) and a calendering belt (10) which has been formed as an endless loop and which is loaded against the heatable hard roll (1) by means of a backup roll (3) or a press shoe (24). The paper or board web (W) to be calendered is arranged to run through said calendering nip (N). According to the invention, in respect of at least a part of its thickness, the calendering belt (10) is made of a material of compressible volume. According to a second embodiment of the invention, the necessary resilience in the nip (N) is provided by means of a separate support belt, which runs underneath the calendering belt in relation to the web (W).

Description: Calender provided with an extended nip The invention concerns a calender provided with an extended nip for calendering of a paper or board web, which calender comprises at least one calendering nip, which is formed between a heatable hard roll, on one hand, and a calendering belt which has been formed as an endless loop and which is loaded against the heatable hard roll by means of a backup roll or a press shoe, on the other hand, the paper or board web to be calendered being arranged to run through said calendering nip. When it has been desirable to raise the level of calendering, with the present-day solutions, practically the only possibility has been to increase the number of calen- dering nips. This results in a more complex construction of the calender and in more difficult control and threading of the paper web. In particular in the case of on-line machines, it must be possible to solve the contradictions arising from high running speed and full-speed web threading. Attempts have been made to solve these problems by means of various belt and shoe calenders, by whose means the calen- dering nip is extended and, thereby, the operation of the nip is enhanced. In shoe calenders, as a rule, the technique very well known from extended-nip presses is employed, according to which the extended nip is produced so that the paper web is pressed against the backup roll by means of a shaped glide shoe and an endless belt running over the shoe. In a belt calender, by means of the belt, attempts are made to provide the necessary resilience in the calendering nip when the paper web is pressed between the rolls that form the calendering nip. Belt calenders in themselves are relatively new constructions, and they can also be formed so that the paper to be calendered is passed, by means of the endless belt, into a preliminary contact with a hot calender roll, in which case it is possible to provide a steep temperature gradient, which is favourable from the point of view of calendering. In such a case, by means of the belt, the effective length of the nip is increased because of the preliminary contact and because, as the belt material, it is possible to employ considerably softer polymers than in roll coatings without problems arising from deformations related to heat. With a nip more extended than in a supercalender or soft calender, the press impulse applied to the paper can be increased so that the pressure peak does not become excessively high and that the bulk is not reduced. With respect to the prior art related to belt calenders, reference is made, for example, to the Finnish Patent No. 95,061 and to the equivalent US Patent No. 5,483,873. The belt and shoe calenders that have been used so far have involved quite a signi- ficant problem, which has arisen from the structure of the belt that has been used. The belts that have been used have been almost completely non-compressible, and, as a rule, their structure has been such that there is a support fabric in the middle of the belts, and a polyurethane layer at both sides of the support fabric. Even though the belt in itself may have been elastic, its volume has not been compressible. Such a belt of substantially non-compressible volume behaves in a calendering nip, in which it is pressed with a high pressure to make it thinner, so that the material of the belt has to flow parallel to the face of the belt away from the nip. This has, among other things, the effect that a wave is formed ahead of the nip out of the belt material. Owing to the non-compressibility, in the belt material, forces parallel to the belt face are formed, which promote a collapse of the fibrous structure in the paper, i.e. a reduction in the thickness of the paper. In a nip, reversible and non-reversible compression of paper always take place. Shear forces in the nip increase the amount of non-reversible compression, which has a detrimental effect on the bulk of the paper. Further, a shear force that changes its direction abruptly in the nip tends to vibrate the paper, which mechanism can have quite a considerable effect on the calendered density of the paper. Thus, employment of a non-compressible belt makes the fibrous structure of the paper collapse and increases the strains applied to the belt face, the wear and the fatigue. The object of the present invention is to provide an essential improvement over the existing calenders provided with extended nips. In view of achieving this objective, the invention is mainly characterized in that, through the nip, at the side of the backup roll or of the press shoe, respectively, in relation to the web, a belt has been passed which is, in respect of at least a part of its thickness, made of a material of compressible volume. By means of the invention, a significant advantage is obtained over the prior art, and of the advantages obtainable by means of the invention, in this connection, among other things, the following can be stated. Thus, it is an essential feature of a first embodiment of the invention that the belt that is used in the calender is made of a material of substantially compressible volume. Similarly, it is an essential feature of a second embodiment of the invention that, in belt calendering, through the calendering nip, a calendering belt, favourably a conventional polyurethane belt, is passed, and additionally a separate, substantially fully compressible support belt is passed, which performs the major part of the compression work in the nip. In studies that have been carried out and in test runs, it has been noticed that, by means of such a belt of compressible volume, and, similarly, by means of a solution in which, in addition to a polyurethane belt, a support belt of compressible volume is used, the forces parallel to the face of the paper and/or belt can be eliminated from the nip almost completely. Owing to the compressibility of the calendering belt or of the support belt placed underneath the calendering belt, the extent of non-reversible compression of the paper in the nip is reduced essentially, in which case the thickness and the bulk of the paper can be maintained as well as possible. Further, it is to be considered a significant additional advantage that the so-called "toleration of fibre string" is substantially better than in earlier solutions, because, owing to the compressibility of the calendering belt or of the support belt placed underneath the calendering belt, respectively, fibre strings can pass through the nip readily without causing a shear load. From the point of view of manufacturing technology, a multilayer belt of the novel type is considerably easier to manufacture than the earlier belt provided with a support fabric, and, for example, the belt can be provided with uniform thickness more easily than in the prior art. Similarly, a solution with two belts can be accomplished very easily, because, in the manufacture, it is possible to employ conventional prior-art techniques, in particular in the case that a polymer belt is used as the calendering belt and the support belt is made of a felt material. As the support belt, it is also possible to use other compressible materials, for example, cellular rubbers, cellular plastics, and equivalent. Uniform thickness can be achieved easily in a unit composed of a calendering belt and of a support belt in particular made of a felt material. On the other hand, it can also be stated as an advantage that, owing to the compressibility of the belt, the calendering process is not so sensitive to little variations in thickness of the belt as it was earlier. The further advantages and characteristic features of the invention will come out from the following detailed description of the invention. In the following, the invention will be described by way of example with reference to the figures in the accompanying drawing. Figure 1 is a fully schematic side view of a single-nip calender in which a calender- ing belt is employed. Figure 2 shows an enlarged detail from the area of the calendering nip in a calender as shown in Fig. 1. Figure 3 is an illustration corresponding to Fig. 1 of a shoe calender. Figure 4 is an enlarged detail from the nip area in a calender as shown in Fig. 3. Figure 5 is a schematic side view of a belt calender in which, in accordance with the second embodiment of the invention, a combination consisting of a calendering belt and a support belt is employed. Figure 6 shows an enlarged detail from the area of the calendering nip in a calender as shown in Fig. 5. Thus, Figs. 1 and 2 illustrate a belt calender, in which the calendering nip N is formed between a hot calender roll 1 and a backup roll 3. Further, through the nip N, a calendering belt 10 has been passed to provide the nip N with the necessary elasticity so that the paper web W runs through the nip N between the calendering belt 10 and the face of the hot roll 1. Fig. 2 is a more detailed illustration of the conduct of the calendering belt 10 in the nip, whose length is denoted with the reference a. Thus, in the case of a belt calender, the calendering belt 10 consists of a support layer 12 of compressible volume and of a tight polymer layer 11 fitted on one face of said support layer 12. As is shown in Fig. 2, the polymer layer 11 is placed on the face of the support layer 12 placed facing the paper web W. The support layer 12 itself is placed against the backup roll 3. Thus, it is an essential feature of the belt 10 that the structure of the support layer 12 is such that it can be compressed in the nip N, i.e. its volume is reduced, in which case no wave of the sort formed in prior- art solutions can be formed ahead of the nip. The support layer 12 of compressible volume is favourably a felt, even though other materials can also be employed in the support layer 12. Of these other materials can be mentioned, for example, cellular rubbers, cellular plastics, and equivalent which are compressible. The material of the tight polymer layer to be placed against the paper is, for example, thermosetting or thermoplastic resin, favourably polyurethane. Figs. 3 and 4 illustrate a shoe calender, in which the extended calendering nip N is formed between a hot hard roll 1 and a shoe roll 2. The shoe roll 2 again comprises a press shoe 24 supported by a stationary beam 25 as well as a calendering belt 20, which has been passed around the press shoe 24 and the beam 25 and which has been formed as an endless loop. By means of the press shoe 24, the necessary load is produced in the nip N. As is illustrated in Fig. 4, in this embodiment the calen- dering belt 20 consists of three layers fitted one above the other, i.e. of a support layer 22 of compressible volume placed in the middle and of tight polymer layers 21, 23 placed at both sides of the support layer. The materials of the support layer 22 and of the polymer layers 21, 23 can be the same as in the embodiment shown in Figs. 1 and 2, so that the polymer layers are preferably of polyurethane, and felt, cellular rubber, cellular plastic, or equivalent is employed as the support layer 22 of compressible volume. In the embodiment of Figs. 3 and 4, a tight polymer layer 23 is also required at the side of the press shoe 24, because it must be possible to make the friction between the press shoe 24 and the calendering belt 20 sufficiently low. The mode of reduction of friction that is known best and that is employed most commonly is producing an oil film between the press shoe and the calendering belt 20. Thus, the face of the calendering belt 20 that is placed against the press shoe 24 must be tight, in which case a polymer layer 23 is necessary. A similar belt can, of course, also be employed in connection with the embodiment shown in Figs. 1 and 2. As has been illustrated by means of Figs. 2 and 4, the support layer 12; 22 can be compressed over the length a of the nip N without causing any wave ahead of the nip. Thus, Figs. 5 and 6 illustrate a belt calender, in which the calendering nip N is formed between a hot calender roll la and a backup roll 3a. Through the nip N, a calendering belt 30 as well as a support belt 40 of a material of compressible volume have been passed so as to provide the nip N with the necessary resilience, so that the web W runs through the nip N between the calendering belt 30 and the face of the hot roll la. Both the calendering belt 30 and the support belt 40 have been formed as endless loops by means of alignment and tensioning rolls. The calendering belt 30 and the support belt 40 are passed into the nip N between the calendering rolls la and 3a by means of a first alignment roll 4, and similarly they are passed out of the nip N by means of a second alignment roll 5, said first and second alignment roll 4 and 5 being, thus, common of the calendering belt 30 and of the support belt 40. Thus, the calendering belt 30 and the support belt 40 run from the first alignment roll 4 through the nip N to the second alignment roll 5 as a joint run and while fitted one above the other. After the second alignment roll 5, the calendering belt 30 and the support belt 40 are, however, separated from one another so that they form separate loops of their own. From the second alignment roll 5, the support belt 40 is, thus, guided onto the alignment roll 6 of the support belt, from it onto the support belt tensioning roll 7, and from it further onto the first alignment roll 4, which is common of the support belt and the calendering belt, as was already described above. Similarly, the calendering belt 30 is guided from the second alignment roll 5 onto the belt align- ment roll 8, from it further onto the belt tensioning roll 9, from which the calendering belt 30 is passed, in the case shown in Fig. 5, onto the support belt 40 at the support belt tensioning roll 7. In this respect, the arrangement can. however, also be such that the calendering belt 30 is passed from the belt tensioning roll 9 directly onto the first alignment roll 4 common of the calendering belt 30 and of the support belt 40. For each loop, i.e. for the calendering belt loop 30 and for the support belt loop 40, alignment rolls 6,8 and tensioning rolls 7,9 of their own are needed in order that the tension of the calendering belt 30 and of the support belt 40 could be regulated appropriately. A solution with two belts is in itself known, for example, from presses, but there the paper web is always placed against a porous belt for the purpose of removal of water, and the smooth support belt is placed underneath the porous belt. Fig. 6 illustrates the conduct of the calendering belt 30 and of the support belt 40 in the nip N in more detail, the length of the nip being denoted with the reference a. Thus, through the nip N, the non-compressible calendering belt, favourably a polymer belt 30, and the support belt 40 of compressible volume, favourably a felt or a support belt of a similarly compressible material, are passed. In the way shown in Fig. 6, the polymer belt 30 has been arranged in the nip N at the side of the heatable calendering roll la, i.e. at the side of the web W to be calendered. The material of the polymer belt 30 is favourably polyurethane. The structure of the calendering belt 30 is such that it is provided with a thin fibre reinforcement and with a polyurethane layer at least at the side placed facing the heatable calendering roll la. The structure of the belt can also be such that the fibre reinforcement is placed, in relation to the thickness of the calendering belt 30, in the middle of the belt, and the smooth polyurethane layers are placed at each side of the calendering belt. Further, in the way shown in Fig. 6, the support belt that runs through the nip N, favourably a felt 40 or equivalent, is placed in the nip against the backup roll 3a. Thus, it is an essential feature of the invention that the belt 40 or equivalent can be compressed in the nip N, in other words, its volume is reduced, in which case a wave corresponding to that formed in the prior-art solutions cannot be formed ahead of the nip N. As was stated earlier, in stead of a felt material, it is also possible to use other materials, e.g. cellular rubbers, cellular plastics, and equivalent which are compressible. In the illustration in Figs. 5 and 6, the web W to be calendered is introduced into the nip N so that it does not enter into contact with the calendering belt 30 until in the nip N, and similarly the web W is taken out from the nip N so that it is separated from the calendering belt 30 directly. The web W can also be passed into the nip so that it is brought onto the calendering belt 30 before the nip N, and similarly, after the nip, the web is transferred on the calendering nip 30, from which it is taken to further processing. Thus, the running of the web W can be accomplished as a what is called closed draw. With a calendering belt of compressible volume in accordance with the invention, and similarly in an arrangement in which a tight calendering belt, in particular a polymer belt, and a support belt of compressible volume, such as a felt or equivalent, are employed, by means of a calender a thicker and stiffer paper or board can be calendered so that the gloss and smoothness of the calendered web are at an equally high level as with a non-compressible calendering belt alone. Also, it can be established that the runnability with a felt or equivalent of compressible volume is better, because, in particular with thin paper grades, the frequency of web breaks is substantially lower than earlier. In the case of a compressible multi-layer belt, the improved runnability was ascertained in test runs. It can be suggested as a reason for this that, when a non-compressible calendering belt is used alone, the flow of the belt material parallel to the belt face strains the paper to such an extent that the frequency of web breaks becomes higher. Above, the invention has been described by way of example with reference to the figures in the accompanying drawing. The invention is, however, not confined to the exemplifying embodiments illustrated in the figures alone, but a belt of compressible volume in accordance with the invention can also be used in other connections. As examples of such other applications can be mentioned various shoe presses, size presses that employ a belt, belt coating devices, belt winders, and equivalent. Also in the other respects, it must be stated that the invention can be varied within the scope of the inventive idea defined in the accompanying claims.

Claims: Claims 1. _A_calenderj)rovided with an extended nip for calendering of a paper or board web, which calender comprises at least one calendering nip (N), which is formed between a heatable hard roll (1,1a), on one hand, and a calendering belt (10; 20; 30) which has been formed as an endless loop and which is loaded against the heatable hard roll (l;la) by means of a backup roll (3; 3a)) or a press shoe (24), on the other hand, the paper or board web (W) to be calendered being arranged to run through said calendering nip (N), characterized in that, through the nip (N), at the side of the backup roll (3; 3 a) or of the press shoe (24), respectively, in relation to the web (W), a belt (10;20;40) has been passed which is, in respect of at least a part of its thickness, made of a material of compressible volume. 2. A calender as claimed in claim 1, characterized in that the belt of a material of compressible volume is a calendering belt (10;20) provided with a multi-layer structure, which comprises a support layer (12;22) of compresj>ibl^volume and, at least on its face to be placed against the material web (W) to be calendered, a tight polymer layer. 3. A calender as claimed in claim 1 or 2, characterized in that, in the calendering belt (20), in the direction of thickness, there is a tight polymer layer (21,23) at both sides of the support layer (22) . 4. A calender as claimed in any of the preceding claims, characterized in that the material of the support layer (12;22) of compressible volume in the calendering belt (10j20) is felt. 5. A calender as claimed in any of the claims 1 to 3, characterized in that the material of the support layer (12;22) of compressible volume in the calendering belt (10;20) is cellular rubber, cellular plastic, or equivalent. 6. A calender as claimed in any of the preceding claims, characterized in that the tight polymer layer (11) or tight polymer layers (21;23) in the calendering belt is/are made of thermosetting or thermoplastic resin. 7. A calender as claimed in any of the preceding claims, characterized in that the material of the tight polymer layer (11) or of the tight polymer layers (21; 23) in the calendering belt is polyurethane. 8. A calender as claimed in claim 1, characterized in that the belt of a material of compressible volume is a support belt (40) formed as an endless loop and running at the side facing the backup roll (3a) in relation to the calendering belt (30). 9. A calender as claimed in claim 8, characterized in that the calendering belt (30) and the support belt (40) of compressible volume have been formed as separate _ endk^j^psj y neans of alignment and tensioning rolls (4...9). 10. A calender as claimed in claim 8 or 9, characterized in that the calendering belt (30) and the support belt (40) have been passed through the nip (N) so that the calendering belt (30) and the support belt (40) run from the first alignment roll (4), preceding the nip (N), into the nip (N) and from the nip further onto the second jtlignment roll (5), following after the nip, as a joint run. 11. A calender as claimed in any of the claims 8 to 10, characterized in that the support belt (40) of compressible volume is made of a felt material. 12. A belt calender as claimed in any of the claims 8 to 10, characterized in that the material of the support belt (40) of compressible volume is cellular rubber, cellular plastic, or an equivalent material. 13. A calender as claimed in any of the preceding claims 8 to 12, characterized in that the material of the calendering belt (30) is polyurethane, and the belt is provided with a fibre reinforcement.

Inventors: HASANEN KARI, KOIVUKUNNAS PEKKA, TANI MIKKO, HASANEN, KARI, KOIVUKUNNAS, PEKKA, TANI, MIKKO
Applicants: HASANEN KARI, KOIVUKUNNAS PEKKA, TANI MIKKO, VALMET CORP, HASANEN, KARI, KOIVUKUNNAS, PEKKA, TANI, MIKKO, VALMET CORPORATION
Application year: 1998
Main classification: D21G1
IPC codes: D21G1