Presses – Methods – With heating or cooling
Reexamination Certificate
1999-03-12
2001-09-18
Gerrity, Stephen F. (Department: 3721)
Presses
Methods
With heating or cooling
C100S153000, C100S16200R, C100S313000, C100S329000, C100S334000, C162S206000
Reexamination Certificate
active
06289797
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for calendering a paper web, wherein the paper web is conveyed through a calendering nip formed by two calender rolls. At least one of the calender rolls is a soft-faced roll including a coating of a polymeric or corresponding material or arranged so that a belt passes over the roll and runs through the nip. The profile of the calendering nip is controlled to compensate for defects in the web entering the calender. The present invention also relates to a calender comprising at least one calendering nip formed between two calender rolls, at least one of which is equipped with a resilient (soft) surface layer in the form of a coating or a belt brought over the roll and running through the nip. The calender is provided with at least one control device to control the profile of the calendering nip to compensate for defects in the web entering the calender.
BACKGROUND OF THE INVENTION
In the paper web entering the calender, there are variations due to functional inaccuracies in the wet end of a paper machine, i.e., the headbox, the forming section and the press section, and in the drying section. In addition to the primary function of the calender, i.e., the finishing of the surface properties of the web, the calender is also used to correct the above-mentioned inaccuracies in the quality of the web. Thus, the function of the calender must be adjustable, i.e., the profile of the calendering nip must be controllable to compensate for these defects.
Conventionally, the profile adjustment at the calender is conducted by changing the diameter of a metal-faced nipping roll or the nip force locally and/or by using so-called variable-crown rolls, whereby the nip force profile can be adjusted as desired. However, these rolls have a relatively large need for rotational power, due to sliding frictions inside the roll, and furthermore, the rolls are structurally very complicated and thereby also expensive to manufacture.
The change of the diameter is conventionally conducted by heating the mantle of the metal roll locally from the outside or cooling the mantle of the metal roll locally from the inside. Attempts in local heating of the roll mantle include, e.g., hot air blowing, infrared heating and various electric heating elements. Disadvantages of this local heating method include, however, the large size of the heating devices used in relation to the roll, and the need for separate supports and transfer devices. After a web break, cleaning up of the calender is rendered difficult by the small gap between the roll and the heat control device(s), web scraps being easily accumulated in the gap in connection with web breaks.
Another important drawback has been that a change in the diameter of the roll mantle has required relatively large temperature differences, wherein even large variations have been possible in the temperature of the nipping roll in the axial direction. This, in turn, has clearly affected the gloss of the paper, i.e., more glossy and less glossy stripes may have occurred in the paper web.
With respect to related prior art, reference is made to U.S. Pat. No. 4,658,716 which describes a calender roll equipped with several infrared heaters spaced from each other in the axial direction of the roll. The purpose of the infrared heaters is to adjust the diameter of the calender roll to compensate for cross-directional variations in the paper web entering the calender nip. In other words, the profile of the calender nip is adjusted by heating the metal roll from the outside. However, problems associated herewith are generated, as discussed above.
Furthermore, as an example of one method according to the prior art, reference is made to a method described in Finnish Patent Application No. 961816, (corresponding to International Publication No. 97/41298) wherein the local properties of the polymeric surface are influenced by heating and cooling the coating with an external temperature control device.
An example of internal heating of the roll mantle in zones is described in Finnish Patent Publication No. FI 70358, which corresponds to German Patent Application No. DE 3033482 and U.S. Pat. No. 4,425,589.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the present invention to provide a novel method in calendering of a fibrous web as well as a calender applying the method.
It is another object of the present invention to provide a novel method in calendering of a paper web and calender in which drawbacks related to the prior art, e.g., as discussed above, are avoided.
It is yet another object of the present invention to provide a novel method for calendering a paper web and calender which provides improved control of the profile of the calender nip.
In order to achieve these objects, and others, in a method in accordance with the invention, the nip force profile is controlled during operation of the machine by zonewise changing the temperature of the tube used as the body of the soft-faced roll in the cross machine direction in a controlled manner by means of external and internal temperature profiling devices of the metal-surface rolls.
A calender in accordance with the invention includes a profile control device arranged in connection with, and preferably inside of, a calender roll equipped with a soft surface layer and which profile control device is arranged to change the temperature of the tubular body of the roll in the cross machine direction and thereby the local diameter thereof.
By adjusting the temperature of the roll body, a small local difference in the temperature produces a significant profiling effect, as shown by the following calculation example. Assuming a polymer roll having a diameter of about 735 mm, metal body wall thickness of about 100 mm and polymer coating thickness of about 25 mm, if in such a roll there is a temperature difference of about 50° C. between two adjacent areas in the body and the coating, then the diameter of the polymer roll is approximately 65 &mgr;m greater in the warmer area. The coefficient of thermal expansion of the body is approximately 1.17 E-05 1/° C., and the coefficient of the coating, as is typical for polymer materials, is higher, about 5.0 E-05 1/° C. With a densely-zoned zonewise adjustable roll used today, it is possible to achieve an equivalent diameter change of only a few 10 &mgr;m.
In the following, an advantageous embodiment of the invention will be described in more detail. The local heating/cooling of the body of the polymer roll can be accomplished by a cool air blowing through a blow nozzle (e.g., filtered hall air which is air from the machine hall in which the calender is situated), in which every zone is provided with a respective controllable electric resistor. The minimum width of the zone is determined, in practice, by the width of the electric resistor. Thus, it is possible to adjust the blowing temperature in different zones (for example from about 30° C. to about 70° C.). When the air is blown at a rate of about 10 m/s to about 200 m/s, preferably from about 30 m/s to about 60 m/s, towards the inner body of the roll, the air provides good heat transfer properties (from about 200 W/m
2
° C. to about 350 W/m
2
° C.) to the body. The blowing of air is preferably implemented on a narrow sector in the direction of the periphery of the roll in order to avoid the need of excessive air quantities. During the blowing, the polymer roll rotates all the time. It is advantageous to bring the air to the roll through the first end to the inside of the polymer roll, and to remove it through the second end. In addition to the blow nozzle, each control zone is advantageously provided with a suction nozzle to remove the air used in the area of the zone and to prevent its entry to the area of other zones. The control of the electric resistors can be coupled to the gloss or thickness profile control. Furthermore, the heating can be implemented by heating the roll body inductively, in a zonewise controlled manner, or uniformly over its full length.
A heating de
Gerrity Stephen F.
Steinberg & Raskin, P.C.
Valmet Corporation
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