CA2224298C - Method and apparatus including an extended nip for calendering a web - Google Patents

Abstract

A calender apparatus for treating a traveling web having opposite first and second surfaces. The apparatus includes a roll which is supported for rotation about an axis and which provides a generally cylindrical outer surface. The apparatus also includes a shoe including a smooth calendering surface located in opposed facing relation to the outer surface of the roll. The web is directed between the outer surface and the shoe. The apparatus also includes a mechanism for urging the shoe toward the roll such that the calendering surface directly contacts the second surface of the web and urges the first surface of the web toward the outer surface of the roll.

Description

METHOD AND APPARATUS INCLUDING AN EXTENDED NIP
FOR CALENDERING A WEB
BACKGROUND OF THE INVENTION
Technical Field The invention relates to methods and apparatus used for treating a traveling web. More particularly, the invention relates to methods and apparatus used tin the calender sections of a paper making machine.
Related Prior Art It is generally known to treat a traveling web, such as a web of paper or paper board by means of calendering, to provide desirable surface properties.
For example, the smoothness and degree of gloss provided by the web's surface can be varied by calendering.
Several methods of calendering a web are known, including super calendering and soft roll calendering.
In known soft roll calendering applications, the web is passed between a pair of rollers, one of which is relatively hard and one of which is relatively soft.
Among the critical factors that can be varied in soft roll calendering to effect the resultant surface characteristics are the nip pressure applied to the web as it passes between the rolls, the moisture content of the web being calendered, the type of fiber comprising the web, the amount of heat transferred to the web by the calendering assembly and the amount of time the web remains in contact with the nip between the pair of rollers, i.e, nip dwell time.
More particularly, with respect to the fiber content of the traveling web, paper is composed of various polymer structures, such as cellulose, hemicellulose, starches and lignin. In addition, paper webs may be coated and/or treated with Water in advance of calendering. Soft roll calendering is a desirable method for obtaining a high gloss and high smoothness surface for both coated papers webs and uncoated paper.
With respect to the factors of heat transfer and nip dwell time, it is also know to heat a component of the soft roll calendering assembly, such as the hard roll, so as to provide thermal energy to the traveling web. The application of heat to the web raises the paper surface temperature of the web such that the fibers comprising the web can be transformed from a relatively rigid material to a softer material more suitable for plastic deformation. When the fibers are heated to the threshold temperature for plastic molding, the paper fibers are more easily smoothed and tends to retain its smoothed configuration.
In the case of using super calenders to treat a paper web, relatively large shear forces are imposed on the surface of the traveling web by passing the web through multiple sets of hard and soft rolls. Large shear forces acting on the paper surface due to deformation of the relatively soft roll and slippage between the soft roll and hard roll produce high gloss and smoothness. The gloss and smoothness obtained from super calenders can be superior, but require multiple sets of rollers. With known single soft roll calendering apparatus, shear forces developed between the paper surface and the soft roll and heated metal roll are somewhat limited.
SUMMARY OF THE INVENTION
One of the problems associated with known soft roll calendering apparatus is that the degree of heat transfer experienced by a web passing through the nip of a soft roll calender assembly decreases with increased web speed. Because of the diminished nip dwell time, the amount of thermal energy transferred from the hot metal roll to the web is reduced.
Increasing metal roll surface temperatures to increase the transfer of thermal energy to the web is not a practical solution because of the inefficiency in transferring heat to the web and increases in operational cost, i.e., cost of energy to heat the hot >netal roll. In addition, the heating of a rotatable metal roll is relatively complex.
Another problem associated with known soft roll calendering apparatus is the reduction in web thickness or bulk due to relatively high nip pressures imposed on the web. Increasing nip pressure sometimes enhances heat transfer from the hot roller to the web. However, the increase in nip pressure can tend to thin the web, which can be undesirable in many applications, particularly applications using a paper board web.
The present invention provides a soft roll calendering apparatus including a stationary extended nip which directly contacts the traveling web and which, in combination with a soft calendering roll, can be used to calender a traveling web and produce paper having desirable surface characteristics. As described more particularly below, the present invention also provides a method for calendering a traveling web.
More particularly, the invention provides a calendering apparatus for treating a traveling web having opposite first and second surfaces. The apparatus includes a generally cylindrical roll supported for rotation about an axis and a nip shoe including a smooth calendering surface which is located in a opposed facing relation to the outer surface of the cylindrical roll. The web travels between the outer surface of the roll and the shoe. The apparatus also includes means for urging the shoe toward the roll such that the calendering surface directly contacts the web and urges the web toward the outer surface of the roll.
In one embodiment of the apparatus, the shoe includes a flexible member that provides the calendering surface. The flexible member is deflected against the web so as to apply pressure on the web and to force the web against the cylindrical roll. In another embodiment, the apparatus includes a felt web that travels in the direction of the web to uniformly distribute the pressure applied to the web by the roll and nip shoe. In still another embodiment, the nip shoe can be heated.
The invention also provides apparatus for treating a traveling web including a calendering roll providing a rotating surface, a nip shoe having a calendering surface extending along a portion of the outer surface of the calendering roll and a nip shoe support and loading assembly supporting the calendering surface in direct contact with the web at a predetermined pressure.
The invention also provides a method of treating a traveling web, the method comprising the steps of providing a rotating cylinder, providing a stationary and relatively smooth calendering surface, and the step of applying shear forces to the web by directing the web between the outer surface of the rotating cylinder and the calendering surface and by urging the calendering surface into direct contact with the web.
The invention provides a calendering apparatus which affords the elimination of expensive self-loading or controlled crown rollers by permitting calendering using relatively low nip pressures. Lower nip pressures can be used to achieve the results of conventional soft roll calendering processing because of the extended nip dwell times during which the effects of pressure and temperature act upon the web as it passes through the calender roll and nip shoe.
The invention also eliminates the need for expensive heating systems in the rotating nip calendering rollers.
Also, the apparatus and method according to the invention afford greater efficiency in heat transfer to the web and heating of the web to greater depths, also because of increased nip dwell times.

In addition, the burnishing action between the web surface and the stationary calendering surface, at moderate and low nip pressures, results in superior surface properties comparable to those achieved through super calendering.
Thus, the invention affords improved smoothness and gloss of the paper product with retention of bulk and strength. The apparatus and practice of the invention thus will likely result in savings in manufacturing and capital costs, savings in operational costs and provide a simpler machine design.
Other features and advantageous of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims and drawings.
DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic side elevational view of a calender apparatus embodying the invention.
Fig. 2 is a view similar to Fig. 1 illustrating a calender apparatus which is a first alternative embodiment of the invention.
Fig. 3 is a view similar to Fig. 1 illustrating a calender apparatus that is a second alternative embodiment of the invention.
Fig. 4 is a view similar to Fig. 1 illustrating a calender apparatus that is a third alternative embodiment of the invention.
Before one embodiment of the invention is explained in detail, it is to be understood that the invention is not limited in its application to the details of the construction and the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or being carried out in various ways. Also, it is understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting.
DESCRIPTION OF THE PREFERRED EMBODIMENT
A calendering roll assembly 10 embodying the invention is illustrated in Fig.l. The calendering apparatus 10 is part of a paper making machine (not otherwise shown) having a machine direction (right to left in Fig. l) and a cross-machine direction (into and out of the plane of the page).
The calendering apparatus 10 is supported by a frame 14 supported by the paper making machine and is usable to treat a traveling web 18. The web 18 extends in the cross-machine direction and has a first or upper surface 22 and an opposite facing lower or second surface 26. The web 18 travels through the calender apparatus l0 in the direction indicated and follows a path defined, in part, by rollers 30 and 32.
In order to provide moisture or other pretreatment of the web, the paper making machine includes a preconditioning shower 34 located adjacent the web 18 and upstream of the calendering apparatus 10.
The calender apparatus 10 also includes a generally cylindrical calender roll 38 which is supported by the frame 14 for rotation about axis 42. The calender roll provides an outer surface 46.
In the embodiment of the calendering apparatus shown in Fig. l, the calendering apparatus 10 also includes a second traveling web 50 which is preferably made of felt and which also travels through the calender apparatus 10. The path of the felt web 50 lies between the upper surface 22 of the paper web 18 and the outer surface 46 of the calender roll 38.
The calender apparatus 10 also includes a nip shoe 58 supported by the frame 14. The nip shoe 58 provides a generally concave calendering surface 62 which complements the convex outer surface 46 of the calender roll 38. Importantly, the calendering surface 62 -7_ extends along an arcuate portion of the outer surface 46 generally in the machine direction, and directly contacts the lower surface 26 of the web 18 in a manner discussed below.
The calendering apparatus 10 also includes a nip shoe support and loading assembly for supporting the nip shoe 58 in position, to engage the web 18 and to apply a predetermined pressure on web 18 as it passes between the calender roll 38 and calendering surface 62. The nip shoe support and loading assembly comprises, firstly, a nip shoe support 70 which supports the nip shoe 58 and offords movement of the nip shoe 58 relative to the outer surface 46 of the calender roll 54 in a direction generally perpendicular to the axis of rotation 42. Otherwise, the nip shoe support 70 maintains the position of the nip shoe 58 in a fixed position relative to the rotating outer surface 46 of the calender roll 38 .
Secondly, the nip shoe support and loading assembly comprises biasing means 74 for urging the nip shoe toward the calender roll 38 such that the calendering surface 62 directly contacts the second surface 26 of the web 18 and urges the first surface 22 of the web 18 toward the outer surface 46 of the roll 54. The nip shoe support and loading assembly thus exerts a predetermined pressure on the surface 26 of the web 18 by pressing it against the felt web 50 and outer surface 46. The felt 50 acts to uniformly distribute the pressure applied to the web 18 across the nip width..
While various suitable arrangements for the biasing means 74 can be successfully used, in the embodiment of the biasing means 74. shown in Fig.l, the biasing means 74 includes a spring 78 which biases the nip shoe .58 toward the calender roll 38. Preferably, the biasing means 74 includes an adjustment mechanism to vary the force exerted by the biasing means 74 to _g_ urge the calendering surface 62 into contact with the web 18.
The calendering surface 62 may be provided by any highly polished surface, but should also be highly resistant to wear because of the direct engagement of the web 18 with the calendering surface 62. Thus, while a polished stainless steel surface can be provided by the nip shoe 58 as a calendering surface, the preferred calendering surface 62 is provided by a coating of ceramic material, such as a coating of flame sprayed aluminum oxide, chromium oxide or other similar ceramic materials or mixtures of such materials applied to the nip shoe 58. The ceramic coating provides a smooth, abrasion resistant surface, such that the frictional forces between either the outer surface 46 of roll 38 or felt web 50 and the upper surface 22 of the web 18 are greater that the frictional forces between the lower surface 26 of the web 18 and the calendering surface 62. Nevertheless, the direct contact of the web 18 and the shoe 58 provides enough friction between the web 18 and the calendering surface 62 such that the surface 26 of the web 18 is treated as it passes through the apparatus 10.
More particularly in this regard, the apparatus 10 applies shear forces to the lower surface 26 of the web 18 by providing direct contact between the calendering surface 62 and the lower surface 26 of the web 18 and by exerting a predetermined pressure on the web 18 through cooperation of the biasing means 74, nip shoe 58 and calender roll 38. These shear forces act on the ffibers of the web 18 to deform the fibers and provide the web 18 with a smooth surface finish and/or gloss.
Also, the heat of friction created by the passage of the web 18 directly on the calendering surface 62 heats the nip shoe 58, thereby creating a temperature differential between the calendering surface 62 and the web 18. The calendering apparatus 10 thus also generates heat which is transferred to the web 18 as it passes through the nip created by the shoe 58 and roll 38, which increases the effectiveness of the calendering process carried out by the apparatus 10.
If necessary, the additional heat transfer to the web 18 can be accomplished by heating the nip shoe 58 by means of an external heat source. In this regard, the nip shoe 58 is adapted to be operably connected to a heat source 82 (shown schematically in Fig. 1) such that the calendering surface 62 of the nip shoe 58 can be heated and thereby transfer thermal energy to the web 18 as it passes through the nip formed by the calender roll 38 and nip shoe 58. This arrangement provides a stationary component for transferring heat to the web 18.
In addition, the apparatus 10 provides an extended nip or zone in which the web 18 is engaged with the calender roll 38 and in contact with the calendering surface 62. The provision of an extended nip and, consequently, nip dwell time over enhances the mechanism by which heat can be transferred through direct contact between the calendering surface 62 and the web 18. In particular, the area of the web 18 engaged by the shoe 58 and the roll 38 is longer than that achieved in conventional soft roll calendering, and can be as long as three (3") inches.
The provision of an extended nip results in several advantages. One advantage is that the pressures applied to the web 18 by the shoe 58 and roll 38 can be greatly reduced compared to conventional soft roll calendering. In particular, pressures on the order of 16 pounds per square inch (psi) applied by the apparatus 10 on the web 18, which are equivalent to approximately 250 pounds per lineal inch (pli), can be used with a nip width of 3 inches to achieve results comparable to conventional soft roll calendering operating to apply pressures of 2000 pli. Because lower nip pressures can be used, there is no need for the apparatus 10 to include self-loading rolls or crowned rolls for the roll 38.
Another advantage achieved by the provision of an extended nip width is the above mentioned increased area of contact of the roll 36 and calendering surface 62 with the web 18. This increased area results in a longer nip dwell time as the web passes through the apparatus 10. Consequently, the web 18 can receive thermal energy from the shoe 58 over a longer duration, thereby reducing the need for high energy thermal sources and operational costs.
Another advantage achieved by the provision of an extended nip width is the retention of bulk or web thickness as the web 18 passes through the apparatus 10. Because lower pressures are exerted of the web, the web 18 is not as likely to be crushed or weakened by excessive thinning. In conventional soft roll calendering, pressures of 2000 pli and higher can result in such desirable losses in web bulk.
Accordingly, the processing of heavier grades of paper by the apparatus 10 will be particularly advantageous if bulk retention and relatively smooth surface finishes are required results. The apparatus 10 calenders such grades of paper to provide a product having sufficient bulk so as to retain strength and to provide a surface finish suitable for direct ink printing.
Figure 2 illustrates a calender apparatus 90 which is a first alternative embodiment of the invention.
The calender apparatus 90 is similar to the calender apparatus 10 and common reference numerals will be used to identify like features.
The calender apparatus 90 includes a calender roll 38 providing an outer surface 46 but does not include a felt web such as the felt web 50 shown in Fig. 1.
However, it will be readily understood that such a felt web can be successfully incorporated in the apparatus _11_ and in connection with the treatment of web 18 through operation of calender apparatus 90.
Calender apparatus 90 includes a nip shoe 94. The nip shoe 94 includes, in the embodiment showm in Rig.2, a flexible member or belt 98 having a first end 102, and a.second end 106 and intermediate portion 110 extending between the first and second ends 102, 106. The flexible belt 98 has a width extending in the machine direction and a length (not shown) extending in the. cross-machine direction. In the apparatus 14 90, the calendering surface 62 is provided by the.intermediate portion 110 of the flexible belt 98 which is preferably coated with a ceramic coating. Such a ceramic coating can be applied to a flexible belt 98 and serves to directly contact the surface 26~ of the web 18 in the manner describe above in connection with the calendering surface 62 and apparatus 10.
The calender assembly 90 also includes a nip shoe support and loading assembly 114 including a first shoe support 118 and a second shoe support 122. The first shoe support 118 is fixed to the first end 102 of the belt 98 and the second shoe support 122 includes a roller or other support surface engaged with the flexible belt 98. The first and second shoe supports 118, 122 support the belt 98 and extend toward the outer surface 46 of the calender roll 38 such that the distal ends of the supports 118, 122 are in a plane intersecting or lying nearly tangentially to the outer surface 46 of the roller 38 .
The nip shoe support and loading assembly 114 also includes biasing means 126 for urging the nip shoe 94 toward the calender roll 38. The biasing means 126 places the flexible belt 98 under tension so that the second end 122 of the belt is pulled away from the first. support 118 to which the first end 102 of the belt 98 is fixed and so that the intermediate portion of the belt 98 is pulled across the supports 118, 122 and directly contacts the lower surface 26 of the web 18. While various suitable constructions for the biasing means 126 can be used, in the embodiment of the biasing means 126 illustrated in Fig.2, the biasing means 126 includes a spring 134 which is operably connected to the second end 106 of the S belt 98 to pull the belt 98 taut. By placing the belt 98 under tension, the intermediate portion thereof is pulled tight between the first and second supports 118, 122 yet remains deflected by contact with the web 18 and engagement with the outer surface 46 of the roll 38. The belt 98 is thus in contact with the web 18 along an arcuate portion of the outer surface 46 of the roll 38 in the machine direction, thereby providing an extended nip.
Figure 3 illustrates a calender apparatus 150 which is a second alternative embodiment of the invention. The calender apparatus 150 is similar to the calender apparatus 10 and common reference numerals are used to identify similar elements.
The calender apparatus 150 includes a nip shoe 154 which directly engages the lower surface 26 of web 18 and exerts a predetermined pressure on the web 18 as the web 18 passes between the nip shoe 154 and the outer surface 46 of calender roll 38.
More particularly, the nip shoe 154 includes a flex member or belt 158 having first and second ends 162, 166. The belt 158 also includes an intermediate portion 170 extending between the first and second ends 162, 166 and engaging the lower surface 26 of web 18. The intermediate portion of the belt 158 provides a calendering surface 62 which directly contacts the web 18 in a manner discussed below and which is preferably coated with a ceramic coating.
The calender apparatus 150 also includes a nip shoe support and loading assembly 174 including a first support 178 and a second support 182. The first and second supports 178, 182 have distal ends which are respectively fixed to the first and second ends 162, 166 of the belt 158. The nip shoe support and loading assembly 174 also includes biasing means 186 associated with the first and second supports 178, 182 for urging the belt 158 toward the calender roll 38 so that the belt 158 and calendering surface 62 provided thereby directly contacts the web 18. While various suitable constructions for the biasing means 186 can be successfully used, in the illustrated embodiment of the biasing means 186 a pair of hydraulic springs 190 support the first and second supports 178, 182 for moving the supports toward the calender roll 38 so as to apply a predetermined pressure on the web 18.
Figure 4 illustrates a calendering apparatus 200 that is a third alternative embodiment of the invention. Calender apparatus 200 is similar to calender apparatus 10 and common reference numerals will be used to identify similar components. Calender apparatus 200 includes a nip shoe 202 that directly contacts the lower surface 26 of the web 18 and applies a predetermined pressure on the web 18 through cooperation with the calender roll 38.
More particularly, the nip shoe 202 includes an edge 206 and a blade 210 extending from the edge 206 and providing a calendering surface 62. The calendering surface 62 directly engages the lower surface 26 of the web 18 in a manner discussed below, and preferably includes a ceramic coating for contacting and treating the surface of the web 18.
The calender apparatus 200 also includes a nip shoe support and loading assembly 212 having a nip shoe support 214 which holds the edge 206 of the nip shoe 202. The nip shoe support and loading assembly also includes biasing means 218 for urging the blade 210 and calendering surface 62 into engagement with the lower surface 26 of the web and against the outer surface 46 of the calender roll 38. In the embodiment of the biasing means 218 shown in Fig. 4, the biasing means includes a pneumatic or hydraulic spring 222 which engages the blade and urges the blade into a deflected position against the web 18.
The surfaces of web 18 can therefore be treated according to the following method. The web 18 is trated by first providing a rotating cylinder having outer surface. The method also includes the step of providing a stationary and relatively smooth calendering surface. The method also includes the step ofapplying shear forces to the surface of the web to be treated by directing the web between the outer surface and the calendering surface and by urging the calendering surface into direct contact with the surface of the web at a predetermined pressure. The method can be succesfully used when the step of providing a calendering surface includes the step of providing a flexible member providing the calendering surface. In addition, the step of applying shear forces to the surface of the web to be treated can include the step of deflecting the flexible member into contact with the web.
The method of treating the web also includes, as part of the step of providing a calendering surface, the step of providing a shoe and the step of coating a portion of the shoe with a ceramic coating.
In addition, the step of providing a calendering surface includes the step of providing a shoe having a calendering surface extending along an arcuate portion of the outer surface to provide an extended nip.
Various features of the invention are set forth in the following claims.

Claims (17)

1. A calendar apparatus for treating a traveling web having opposite first and second surfaces, said apparatus comprising:
a roll supported for rotation about an axis and including a generally cylindrical outer surface;
a shoe including a smooth calendering surface located in opposed facing relation to said outer surface, the web being directed between said outer surface and the shoe: and biasing means for urging said shoe toward the roll such that the calendering surface directly contacts the second surface of the web and urges the first surface of the web toward the outer surface of the roll, said calendering surface and outer roll being moved into engagement with the web at a pressure of approximately 250 pounds per lineal inch.

2. An apparatus as set forth in claim 1, wherein said calendering surface is heated.

3. An apparatus as set forth in claim 1, and further including a traveling felt located between the first surface of the web and said outer surface of said roll.

4. An apparatus as set forth in claim 1, wherein said shoe includes a flexible member providing said calendering surface.

5. An apparatus as set forth in claim 4, wherein said shoe includes a first support and wherein said flexible member is a belt having a first end fixed to said first support, a second end, and an intermediate portion extending between said first and second ends, and wherein said urging means moves said intermediate portion into contact with said web.

6. An apparatus as set forth in claim 5, wherein said second end of the belt is fixed to said shoe and wherein said urging means is operable to place said belt under tension.

7. An apparatus as set forth in claim 4, wherein said flexible member is a blade having a first edge and a second edge, wherein said shoe includes a blade support fixed to said first edge, and wherein said biasing means urges said second edge toward said outer surface.

8. An apparatus as set forth in claim 1, wherein said shoe includes a ceramic coating on said calendering surface.

9. An apparatus as set forth in claim 8, wherein said ceramic coating is made of a material selected from the group consisting of aluminum oxide and chromium oxide and mixtures thereof.

10. An apparatus as set forth in claim 1, wherein said calendering surface extends along said outer surface and engages the web in the machine direction along a nip width of approximately 3 inches.

11. A calender apparatus for treating a traveling web, said apparatus comprising:
a calendering roll providing a rotating outer surface;
a nip shoe including an edge and a blade extending from said edge, said blade providing a calendering surface contacting a portion of said outer surface, the web being directed between said outer surface and said calendering surface; and a nip shoe support and loading assembly supporting said calendering surface in direct contact with the web at a predetermined pressure.

12. An apparatus as set forth in claim 11, wherein said nip shoe is flexible and wherein said nip support and loading assembly deflects said nip shoe.

13. An apparatus as set forth in claim 11, wherein said calendering surface is provided by a ceramic coating.

14. A calender apparatus for treating a traveling web, said apparatus comprising:
a calendering roll providing a rotating outer surface;
a flexible belt fixed to a first support and extending to a second support, said belt providing a fixed calendering surface extending along a portion of said outer surface, wherein the calendering surface is fixed with respect to said rotating outer surface, the web being directed between said rotating outer surface and said fixed calendering surface, the web in direct contact with said calendering surface; and a loading assembly supporting said calendering surface in direct contact with the web at a predetermined pressure.

15. An apparatus as set forth in claim 14, wherein said flexible belt includes a ceramic coating on said flexible belt calendering surface.

16. An apparatus as set forth in claim 15, wherein said ceramic coating is made of a material selected from the group consisting of aluminum oxide and chromium oxide and mixtures thereof.

17. An apparatus as set forth in claim 11, wherein said outer surface is convex and wherein said calendering surface is concave.