METHOD IN THE CALENDERING OF A PAPER WEB AND A CALENDER THAT MAKES USE OF THE METHOD

Abstract: The invention relates to a method in the calendering of a paper web, wherein the paper web (W) is passed through a calendering nip (N) formed by two rolls (11, 12). Of the nip rolls at least one roll (12) has been made soft-faced by means of a coating (13) of a polymer or equivalent material or by means of a band passed over said roll and running through the nip (N). The profile of the calendering nip (N) is regulated in order to compensate for any flaws present in the web (W) entering the calender (10), and in the invention the regulation of the profile is carried out during the running of the machine by varying the properties of the coating (13) or, correspondingly, of the band on the soft-faced roll (12) locally in a direction transverse to the machine direction. Preferably, the properties of the coating (13) or the band are varied by means of heat. The invention also relates to a calender that makes use of the method.

Description: Method in the calendering of a paper web and a calender that makes use of the method The invention relates to a method in the calendering of a paper web, wherein the paper web (W) is passed through a calendering nip formed by two rolls, of which nip rolls at least one roll has been made soft-faced by means of a coating of a polymer or equivalent material or by means of a band passed over said roll and running through the nip, the profile of the calendering nip being regulated in order to compensate for any flaws present in the web that enters the calender. The invention also relates to a calender that comprises a calendering nip formed between two calender rolls, of which calender rolls forming the nip at least one roll is provided with a soft surface layer of a polymer or equivalent material in the form of a coating or a band passed over said roll and running through the nip, the calender being provided with regulation devices for regulating the profile of the calendering nip in order to compensate for any flaws present in the web that enters the calendering nip. In the paper web entering a calender there occur variations that originate in the wet end and the dryer section of a paper machine. It must be possible to control the operation of the calender, ie the profile of the calendering nip must be adjustable in order to compensate for these defects. Conventionally, the profile adjustment that takes place in the calender has been carried out by varying the diameter of a nip roll locally. The diameter has been conventionally varied, for instance, by locally heating the mantle of a metal roll or by cooling correspondingly. Among other things, hot- air blowing, infrared heating and different electric heating members have been tried in the local heating of the roll mantle. However, a drawback of the method is, among other things, its slowness especially when a heated section in the roll is cooled. A further drawback is that it has been possible for heat to be conducted in the mantle of a metal roll, whereupon the heating has not been so local as intended. A significant drawback has been additionally the fact that the varying of the diameter of the roll mantle has required relatively high temperature differences, whereupon the temperature of the nip roll in the axial direction may have varied even to a considerable extent. This in turn has had a clear effect on the gloss of paper, that is, the paper web may have had more glossy or less glossy streaks after calendering. A roll adjustable in zones represents a newer technology in the regulation of the calender profile, but said roll does not allow very narrow adjustment zones. With respect to prior art, reference is additionally made to US Patent No. 4,658, 716, which discloses a calendering roll provided with a plurality of infrared heaters arranged in the axial direction of the roll at a distance from each other. The purpose of said infrared heaters is specifically to adjust the diameter of the calender¬ ing roll in order to compensate for the variations that occur in the transverse direction of the paper web entering the calendering nip, ie said publication discloses specifically the adjustment of the profile of the calendering nip by heating a metal roll externally. The problems associated therewith were, however, already discussed above. An object of the present invention is to provide a novel method in the calendering of a paper web as well as a calender that makes use of the method, by which method and by which calender the disadvantages associated with prior art are avoided and which provide an improvement in the adjustment of the profile of a calendering nip. In order to attain the objects of the invention the method according to the invention is mainly characterized in that the profile regulation is carried out during the running of the machine by varying the properties of a coating or, correspondingly, of a band on a soft-faced roll locally in a direction transverse to the machine direction. The calender that makes use of the method according to the invention is in turn mainly characterized in that profile regulation devices are disposed in connection with a calender roll provided with a soft surface layer and they are arranged to vary the properties of the surface layer in a desired way in a direction transverse to the machine direction. The invention provides a considerable advantage over prior art and of the advantages attainable by the invention may be mentioned, for instance, that local profile regulation can be performed in a simpler fashion in the axial direction, among other things, because the temperature differences required in the axial direction are very small, substantially smaller than previously. The temperature differences may be even of the order of less than 1 °C. Since in the method according to the invention a metal roll or a metal mantle of a roll is not sought to be heated for the purpose of profile adjustment, it is possible to place heating devices closer to each other than previously, because in the arrangement according to the invention it is possible to heat a very narrow area of a roll coating in such a way that heat from this area does not spread elsewhere in the axial direction of the roll. This in turn is due to the fact that the roll coating functions as effective insulation. A significant advantage over prior arrangements is additionally that profile regulation can be carried out substan¬ tially more quickly in both directions when compared with previous heat profiling. The further advantages and characteristic features of the invention 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. Fig. 1 is a fully schematic illustration of a two- roll calender, in which one of the calender rolls is provided with a coating. Fig. 2 is an illustration corresponding to Fig. 1 of another embodiment of a calender in which a resilient band is arranged to run about one of the calender rolls. In Fig. 1, the calender is denoted generally with the reference numeral 10. The calender 10 is a two-roll calender, where calender rolls 11, 12 form a calendering nip N therebetween. A paper web W passes through the nip N in the manner shown in the figure. The first calender roll 11 is a hard-faced roll, or a metal roll, in the embodiment of Fig. 1, whereas the other roll 12 of the calender rolls is provided with a resilient, soft coating 13. The coating is, for instance, made of a polymer material or of an equivalent material suitable for the purpose. The reference numeral 14 in Fig. 1 denotes a heating device, and since Fig. 1 is a side view of the calender 10, only one heating device 14 is seen in said figure. However, it is to be under¬ stood that there are a plurality of such heating devices 14 in the axial direction of the roll 12 in such a way that the heating devices 14 are evenly spaced over the axial length of the roll 12. It is possible to dispose heating devices 14 even at 100 mm intervals or even at shorter intervals than that in the axial direction of the roll 12. The properties of the coating 13 are varied by means of the heating devices 14 such that the linear load distribution in the nip N can be made as desired. The coating 13 is softened locally by the heat produced by the heating devices 14 in accordance with the properties of the web W entering the nip N. The heat effect does not spread in the transverse direction of the web W, because the coating 13 insulates well against heat. Desired effects are achieved by lower heat amounts and temperatures than previously and the necessary local temperature differences are very small, even of the order of below 1 °C. Thus, the energy requirement of the heating devices 14 is very slight. Regulation can be made very rapidly in both directions, as compared, for instance, with a situation where the metal roll 11 would be heated and cooled in a corresponding way. In the arrangement shown in Fig. 2, the calender is denoted generally with the reference numeral 20. A calendering nip N is formed in the embodiment of Fig. 2 between rolls 21,22, of which the first roll 21 is a hard-faced roll. The run of the web W is corresponding to that described in connection with Fig. 1. The second calender roll 22 is also a hard-faced roll, but over said roll is passed an endless belt, a band or equivalent 23, which runs over the second calender roll 22 and a guide roll 25. The material of the band 23 is, for instance, a reinforced polymer band, in which the polymer material corresponds to that used as the coating 13 of the roll 12 shown in Fig. 1. Thus, the properties of said band 23 correspond to the properties of the coating 13. The calender 20 is provided further with heating devices 24, whose intended use and operation correspond to that described in connection with the heating devices 14 in Fig. 1. The profile adjustment in the above description was proposed to be performed by means of heat. However, it is possible that the coating 13 or the band 23 is, for instance, layered in such a way that it contains a layer that is sensitive to a profile regulation device. In this kind of case the regulation might be carried out, for instance, by means of microwaves, a magnetic field or the like. In the profile regulation carried out by means of heat, heating can be provided, for instance, by infrared heaters by means of heat blowing or in some other equivalent fashion. Among the currently conceivable heating means, infrared heating is probably the most usable. 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 shown in the figures alone, but different embodiments of the invention may vary within the scope of the inventive idea defined in the accom¬ panying claims.

Claims: Claims 1. A method in the calendering of a paper web, wherein the paper web (W) is passed through a calendering nip (N) formed by two rolls (11 , 12;21 ,22), of which nip rolls at least one roll (12;22) has been made soft-faced by means of a coating (13) of a polymer or equivalent material or by means of a band (23) passed over said roll (22) and running through the nip (N), the profile of the calendering nip (N) being regulated in order to compensate for any flaws present in the web (W) that enters the calender (10;20), characterized in that the regulation of the profile is carried out during the running of the machine by varying the properties of the coating (13) or, correspondingly, of the band (23) on the soft-faced roll (12;22) locally in a direction transverse to the machine direction. 2. A method as claimed in claim 1, characterized in that the properties of the coating (13) or the band (23) are varied by means of heat. 3. A method as claimed in claim 1 or 2, characterized in that the coating (13) or the band (23) is softened by locally heating it and hardened by locally cooling it. 4. A calender that makes use of the method as claimed in any of the preceding claims, comprising a calendering nip (N) formed between two calender rolls (11,12; 21,22), of which calender rolls forming the nip (N) at least one roll (12;22) is provided with a soft surface layer of a polymer or equivalent material in the form of a coating (13) or a band (23) passed over said roll (22) and running through the nip (N), the calender (10;20) being provided with regulation devices (14;24) for regulat¬ ing the profile of the calendering nip (N) in order to compensate for any flaws present in the web (W) that enters the calendering nip (N), characterized in that the profile regulation devices (14;24) are disposed in connection with the calender roll (12;22) provided with a soft surface layer (13;23) and they are arranged to vary the properties of the surface layer in a desired way in a direction transverse to the machine direction. 5. A calender as claimed in claim 4, characterized in that the profile regulation devices (14;24) are arranged to vary the temperature of the soft surface layer (13;23) locally in order to vary the hardness of said surface layer. 6. A calender as claimed in claim 4 or 5, characterized in that the profile regula¬ tion devices (14;24) comprise infrared heaters. 7. A calender as claimed in claim 4, characterized in that the soft surface layer (13;23) of the calender roll (12;22) is layered in such a way that it contains a layer sensitive to the profile regulation device (14;24).

Inventors: KERTTULA REIMA, KIVIOJA PEKKA, LINSURI ARI, NYBERG TIMO R, KERTTULA, REIMA, KIVIOJA, PEKKA, LINSURI, ARI, NYBERG, TIMO, R.
Applicants: VALMET CORP, VALMET CORPORATION
Application year: 1997
Main classification: D21G1
IPC codes: D21G1