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6.2 Rolls in Paper and Board Machines
6.2.1
General Aspects
Different types of rolls are used throughout the paper or board machine as well as in off-line machines and fulfill a variety of functions:
. • Guide rolls give a stable run either to the paper web or to wires, felts or belts. They are about 400 to 1000 mm in diameter and are covered with coatings (Sec¬tion 6.2.4). They are also used in tension control devices to ensure that the fabrics operate at adequate tension in the machine direction and in wire and felt guide systems for continuous control of the fabric position in the cross machine direction.
. • Suction rolls apply controlled vacuum at a certain angle to the roll circumference to dewater the web or for pick up and transfer (Section 6.2.2).
. • Press rolls form a loaded press nip for mechanical dewatering of the paper web.
. • Calender rolls equipped with special covers and coatings generate web smooth¬ness and gloss (Sections 6.2.4 and 6.9).
. • Spreader rolls spread the paper web or fabrics in the width direction in order to avoid wrinkling or flutes. They consist of several short cylindrical roll sections which are covered by one common flexible hose. The overall axis can be bent for spreading.
. • Deflection control rolls (Section 6.2.3) overcome the disadvantages of deflection of the conventional rolls resulting from internal and external forces. Depending on their application these rolls are made from steel, cast iron, bronze or fiber reinforced plastics. Each roll may be driven by its individual drive, and thus may drive in turn the paper web, the wire, felt or belt. Alternatively, the rolls are driven through friction by a wire, felt or belt. Depending on their application the rolls are equipped with varying covers and coatings (Section 6.2.4). Increased ma¬chine speeds make good roll balancing ever more important in order to avoid machine vibrations with their negative effect on paper quality and machinery.
224
6.2.2
Perforated Rolls
Perforated rolls are used in the wire section as well as in the press and dryer sections. Their open area (2 to 85 %, total area of the openings/holes related to the overall roll surface) and their design vary considerably. Their tasks are
. • to dewater the paper web
. • to store the water and to release it in a controlled way
. • to remove the air between web and felt ahead of the press nip
• to hold the web to the roll surface for a certain circumference.
So perforated rolls are used under various operating conditions:
. • Without vacuum application, just storing water when it is pressed into the voids of the roll and releasing it afterwards. These rolls are used at lower machine speeds. A special application of such a roll type is the Dandy roll for formation improvement.
. • With vacuum application in the wire section and dryer sections.
. • With vacuum application and line load as suction press rolls in press nips. Most rolls with vacuum application emit a loud noise due to the siren effect when the holes under vacuum are suddenly refilled with ambient air. The noise level may be so high as to make ear protection measures necessary. The actual noise level depends on the roll drilling pattern, the vacuum level, machine speed and the design of the sealing of the vacuum chamber inside the roll. Some typical perfo¬rated rolls are described in more detail below.
6.2.2.1 Forming Roll
The forming roll (Fig. 6.3) is a suction roll positioned at the beginning of the wire section e. g. in high speed twin wire formers. Here a high amount of white water has to be stored and low vacuum is applied (up to about 0.15 bar). The forming roll has a two-layered shell with an outer ring of high void volume (about 85 %) and a perforated inner ring. A sealed vacuum chamber inside the shell defines the suc-
6.2 Rolls in Paper and Board Machines
tion angle where the vacuum is applied. This is used for web formation control as regards for instance web symmetry in the z-direction.
6.2.2.2 Suction Couch Roll
This roll (Fig. 6.3) is placed at the end of the wire section where the web is already formed. It further increases the density of the web and increases its dry content. A small amount of white water is removed and the operating vacuum is about 0.3 to
0.7 bar. The vacuum chamber is divided into two zones with stepwise increase in vacuum. Shadow marking may occur on sensitive papers with too high vacuum application or inadequate drilling pattern. The open area is about 60 %.
6.2.2.3 Suction Press Roll
This type of roll is an open press roll with vacuum application at part of the circumference. The vacuum removes the air between the web and the felt ahead of the press nip and holds the web on the felt. It enables the water squeezed out in the press nip to flow from the web and felt to the roll void volume where it is stored and released after leaving the vacuum zone.
These rolls are very sensitive with regard to their stressing and material strength. Their main load is the linear force (up to about 120 N mm–1) in the press nip for dewatering. Additional loading occurs from the forces due to vacuum, felt tension and dead weight. These forces result in a dynamic stressing of the shell. Stress concentration due to the drilling pattern and open area as well as the shell thickness has an impact on the actual maximum stress which may be either more “beam bending” (rolls with small diameter, high wall thickness) or more “shell deflection” (rolls with large roll diameter, low wall thickness).
Suction press rolls run in a corrosive ambience, so corrosion fatigue strength is the important material property. Corrosion fatigue strength decreases with the number of cycles and time. As a rule of thumb it can be stated that 10 % more stress (or less corrosion fatigue strength) leads to a factor of about ten in lifetime reduction, which can be translated into a lifetime of either 10 years or 1 year. Suc¬tion press rolls are designed for a lifetime of at least 109 cycles. The material applied is bronze or special alloys exhibiting good corrosion fatigue character¬istics.
Shell thickness and open area define the amount of air to be removed constantly during vacuum build up which affects the amount of energy consumption. Hole diameters (about 4 mm, open area about 15 to 30 %) in the metal shell are larger than those in the cover (about 2.5 to 4 mm, open area about 10 to 20 %). The holes in the cover are drilled after the coating has been applied. The coating holes have to match the pattern in the shell which is easier with smaller hole diameters in the coating.
6.2.2.4 Dandy Roll
These rolls are driven and are used in fourdrinier wire sections, the aim being to improve formation and surface smoothness or to generate watermarks. Dandy rolls are wire covered rolls with a very open structure behind the wire, either built up of rings and bars or of a honeycomb design. They are placed between two flat suction boxes a short distance ahead of the “water line” (where there is no more free water on top of the web surface). Here the structure of the freshly formed web is still weak enough to be partly rearranged without being completely destroyed. The dandy roll dips into the wet web, dewaters it for a short moment during the contact and “rewets” the web on leaving the nip. At higher speeds more and more water is thrown out at the exit in the form of droplets. The application of these rolls is limited to machines speeds of below about 1000 m min–1 (see section 6.4.3).
6.2.3
Deflection Control Rolls
Rolls undergo a deflection under forces which may be due to roll dead weight, wire or felt tension, linear load in press or calender nips or low inside pressure at a certain circumference angle of the shell. However, most often a straight press nip or a uniform nip load in the cross machine direction is required with only small deviations allowed. One simple means of achieving this is to crown the roll. This is done by grinding a curve onto the originally cylindrical shell with larger diameter at the roll center than at the edges, thus compensating for the deflection and resulting in a uniform nip load across the width. One disadvantage of roll crown¬ing is that the local circumferential speed of the roll is different across the width due to the varying diameters. This may create problems in paper quality or in fabric operation.
The above nip conditions also have to be reached for different operating condi¬tions, such as varying linear loads in press nips. A given crown only really precisely fits for instance one line load. So changing operating conditions result in devia¬tions from a constant line load in the nip over the width or from a straight press nip. With increasing width of the paper machines the problem gets worse as de¬flection increases by the cube of the roll face.
With the introduction of deflection control rolls this general flaw can be over¬come. The principle of all such rolls is that the bending deflection is taken by an axial beam which supports the shell by means of a kind of hydraulic “cushion”, be it just one or several “cushions” across the width of the roll. This “cushion” presses the shell to the counteracting roll.
6.2.3.1 One-zone Rolls
6.2.3.1.1 Swimming Roll
The swimming roll (Fig. 6.4) was introduced into the paper industry in 1960 by Küsters (Germany). In principle the shell rotates around a fixed axial beam with
6.2 Rolls in Paper and Board Machines
bearings at each end. A pressurized oil chamber is placed between the shell and the beam which is sealed along the axis against the rest of the inside shell volume and against the roll ends. The oil pressure can deform the shell towards the coun¬teracting roll. The forces acting on the shell are carried by the axis supporting the oil chamber. High oil pressure tends to “blow up” the shell, resulting in more local load at the center of the press nip whereas reduced oil pressure will reduce the local load at the center compared to that at the edges. Loading of the roll to gen¬erate the nip pressure is done by external loading devices. A further example of this type of roll is the Voith Econip Roll.
6.2.3.1.2 CC Roll
Introduced by Beloit in 1960, the shell of this crown control (CC) roll is supported by one shoe with hydrodynamic lubrication. The shoe can move in the nip direc¬tion in the fixed axial beam which takes the line load forces and deflects. Loading is external as with the swimming roll.
6.2.3.1.3 Profile Roll
Introduced by Voith in 1980, the shell support of this roll is similar to that of the CC roll. One main feature is that the distance between its end bearings equals the distance of the bearings of the counteracting roll, therefore it is also called the equidistance roll. Loading is external.
6.2.3.2 Multi-zone Rolls
6.2.3.2.1 Nipco Roll
The Nipco roll (Fig. 6.5) was introduced in 1971 by Escher Wyss. It not only com¬pensates for deflection but also allows one to control the line load in the press nip locally. Here the shell is supported by a number of hydrostatic pistons, which are grouped in at least six or eight zones with two counteracting zones at the edges. All zones are pressurized separately in order to “design” the CD line load curve. The pistons are supported by a fixed beam which takes the press force and deflects. One special feature of the Nipco roll is the self-loading characteristic of the F-type. Self-loading means that no external loading devices are required to load the press nip. The movement for loading or opening the nip is performed by the before mentioned hydrostatic pistons. This movement is made possible by special bear¬ings which allow the shell to move in the nip direction only.
6.2.3.2.2 Hydrein Roll
Introduced by Kleinewefers in 1979, the roll principle is similar to the Nipco Roll with the special feature of having “double pistons” in a circumferential direction.
6.2.3.2.3 Hydro Vario Roll
This roll was introduced by Küsters in 1980. It consists of a shell, a fixed axial beam and in between these a pressurized oil chamber. For local line load control hollow
6.2 Rolls in Paper and Board Machines
pistons placed inside the oil chamber are tightly pressed to the shell to eliminate the oil pressure and thus reduce the press force at that position. In addition these pistons can be applied with a higher oil pressure than in the pressure chamber itself which increases the line load locally.
6.2.3.2.4 Nipcorect Roll
Voith introduced the Nipcorect roll in 1994. These fine control rolls have been developed to control the line load profile in a much finer pattern, e. g. with a roll having more than 30 and up to 60 zones. In order to make this fine control effec¬tive the rigidity of the shell has to be reduced dramatically, for instance by using very thin metal or nonmetal shells. Further examples of this kind of roll are the multi-HV-roll of Küsters and the Sym CD roll of Metso.
6.2.3.2.5 Controls
Complicated calculations are needed to define the optimum oil pressure in the different zones in order to give the best approximation to the desired CD line load profiles. These have to take into account all elastic bending characteristics not only of the deflection control roll itself but also the whole roll system involved. A control system based on these models assists the operator or closes the control loop in automatic operation.
Roll Covers/Coatings
Norbert Gamsjäger
6.2.4.1 Objectives and Basic Design Criteria
Depending on their application, roll covers or coatings in the paper machine have to fulfill various functions. The main objective of roll coverings or coatings may be:
. • to protect the roll body against corrosion in a corrosive environment
• to protect the roll against wear from doctors or wires or felts
• to generate a soft – and thus wide – nip compared to a hard – and thus narrow – nip
. • to reduce the hydraulic pressure in a press nip
. • to ensure good release of the paper web
. • to transfer coating or sizing to the paper
. • to provide elastic support for the paper in calendering
. • to avoid the agglomeration of deposits
. • to support the paper in winding operations. The various applications in the paper industry require a considerable variety in the design and materials of roll covers and coatings. The materials used range from very hard metallic coatings to very soft elastomeric covers, the surface geometry from plain polished to profiled with a highly open surface. In the following the term cover is used for cover thicknesses of approximately 10 to 30 mm, the term coating is used for coating thicknesses of less than 1 mm. This is not a standard¬ized nomenclature, but is established in the industry.
The design of a cover or coating is either defined by the application or by the design needs of the cover material used. Covers and coatings for the paper in¬dustry normally have a multilayer design (see Fig. 6.6) where the outermost layer provides the function of the cover for the papermaking process and the innermost layer ensures the bonding to the metal roll body. The use of additional intermedi¬ate layers is often required in order to withstand the shear stresses within the cover in loaded positions. A gradual adjustment of material properties like hardness, Young’s modulus, thermal expansion etc. in the radial direction is also desirable in order to avoid residual stresses in the cover. These residual stresses are caused by the differences in material properties of the metallic roll shell and the cover/ coating.
Covers/coatings have to reach extremely high load cycles during their opera¬tional lifetime. In double nip installations, where the cover is loaded twice per revolution of the roll, the number of load cycles goes up to 109. This dynamic load, in combination with temperature and the, eventually wet, chemical environment, requires high fatigue resistance of the cover/coating. Covers are usually ground several times before recovering is required. Depending on the application, opera¬tion lives of a cover of between 18 months and several years are standard.
6.2.4.2 Application and Function
6.2.4.2.1 Corrosion and Wear Protection
Covers for paper, wire or felt guide rolls have to protect the metallic roll body against corrosion or wear. In general, these covers are plain and relatively hard materials are used. In drive roll applications the cover has to transfer the torque to
6.2 Rolls in Paper and Board Machines
the driven wire or felt, depending on machine design, softer elastomeric covers are also applied.
6.2.4.2.2 Nip Design in the Press Section
In the press section the individual rolls are hard or soft covered. The press nip (Fig. 6.7) is created either by two profiled rolls or by a combination of one profiled and one plain roll. Covers with high open surface area, similar to tyre technology, offer the necessary storage capacity for the water squeezed out in the press nip. The various surface designs include grooves, blind drilled holes or suction holes as well as combinations of these.
The press nip geometry is mainly defined by the diameters of the press rolls, the deformation characteristics of their covers and the compressibility of the felts. The deformation characteristics depend on the dynamic Young’s Modulus of the cover material, the thickness of the cover and its surface design. The press nip geometry defines the nip pressure profile, which in turn affects dewatering in the press nip and thus the final paper sheet properties. Optimization of press dewatering means optimum press nip geometry as well as optimum water storage capacity and in¬cludes the rolls with their covers and surface geometries as well as the felts.
6.2.4.2.3 Release Properties
In some press designs with hard plain press rolls, the paper web comes into direct contact with a cover or coating. It needs to be pulled off the roll surface uniformly with preferably low force as it exhibits low strength due to its high water content. Web elongation in the machine direction also needs to be reduced to a minimum. Therefore, easy release of the paper web from the cover or coating is required for good paper quality and high machine runnability. This can be achieved by a special cover or coating material design, which has been optimized with regard to hydro-philic/hydrophobic areas and defined surface porosity.
Coating of drying cylinders in some positions aims to reduce the deposition of stickies or color on the roll surface. The release characteristics of the coating allow easier removal of the deposits by doctoring.
6.2.4.2.4 Nip Design in Coating and Sizing
For coating, sizing or pigmenting in film transfer presses the primary purpose of the cover is both to provide a soft nip and to transfer the applied liquid film uniformly to the paper web. Abrasion resistance is important for keeping the nip geometry constant, as the film thickness of the applied coat/size is extremely thin and any nonconformity in the nip results in nonuniform film application. Good wettability and film splitting properties are also required.
Figure 6.8 shows a film press cover.
In conventional puddle size presses the hard and elastic covers must mainly ensure a uniform nip, in which the size is pressed into the paper web. Both tem¬perature and chemical resistance of the covers are essential.
Coater backing rolls give elastic support and ensure transport of the paper web without slipping of the web, which is coated on the opposite side. Elasticity and optimized deformation behavior are the crucial parameters for roll cover design.
6.2.4.2.5 Nip Design in Calenders
The functions of covers or coatings in soft nip calenders are twofold. The hard heated roll in the calender is either a chilled iron roll with a hard wear-resistant surface, or it is coated for even more improved wear resistance. The surface of the hard roll needs to be extremely smooth, as the roll surface is imprinted onto the paper surface. Since thermal energy must be transferred to the paper through the coating, its thermal conductivity and thickness are important.
6.2 Rolls in Paper and Board Machines
The soft elastic roll presses the paper web against the heated roll. This cover must exhibit a certain local deformation ability in order to at least partly follow the “microscale” topography of the paper web in the calender nip. The compression modulus of the cover is of similar magnitude to that of the paper. The local de¬formation of the cover ensures that the paper is not just calibrated in the nip, but that the thinner areas of the web are calendered as well. Thus soft nip calendering results in better printability compared to hard nips for most printing processes.
6.2.4.2.6 Other Applications
In winding, reeling and cutting operations the covers mainly have to support the paper without affecting the paper surface negatively. Soft, sometimes compressible covers are used, providing lower shear stresses in the nip compared to incompress¬ible elastomeric covers.
6.2.4.2.7 Application Overview
The functions of roll covers and coatings are various and significantly influence paper quality as well as the runnability of a paper machine. This explains the need for a variety of materials which enables one to have custom-designed covers and coatings for all applications. Each of the major materials has its unique advan¬tages, nevertheless an overlap of the different cover materials exists for many ap¬plications. Table 6.1 gives an overview of the materials presently used as roll covers and coatings, their main properties, as well as their major applications in paper machines. In the following sections these materials are described in more detail.
Future development of covers and coatings must take into account ever increas¬ing production speeds as well as the demand for improved runnability of paper machines. Here reliability, predictable operation (grinding) intervals, predictable lifetime and safety aspects in the case of damage are the main goals. The main issues in paper technology are innovations in coating and calendering to give im¬proved printability of the paper.
6.2.4.3 Materials
6.2.4.3.1 Rubber Roll Covers
Rubber roll covers are the covers with the longest tradition. The hardness ranges from very soft elastomeric covers to hard rubber. Both natural rubber and synthetic rubber polymers are used. Typical rubber formulations consist of approximately 8 to 15 ingredients, such as polymers, fillers, processing materials, tackifiers, anti¬degradant components, colorants, activators and vulcanizing agents. The possibil¬ity of compounding standard formulations as well as high performance formula¬tions makes rubber an excellent material for roll covers.
Hard rubber covers are used for guide rolls and hard press rolls. For guide rolls the compounds are cost optimized due to the large number of guide rolls in a
234
6 Paper and Board Manufacturing
Table 6.1 Roll cover applications. Wet section
Press section
Special press
Wire guide roll covers
(p)
Wire drive roll covers
(p)
Wire suction roll covers
(sd) Lumpbreaker roll covers
(p)
Paper guide roll covers
(p)
Wire and felt guide rolls
(p)
Hard press roll covers
(p)
Hard press roll covers
(g)
Soft press roll covers(g, bd, bdg)
Suction press roll covers(s, sg, sbd, sbdg)Smoothing press, soft covers
(p)
Shoe press counter roll covers(p, bd)
Long nip presses(Jumbo press) (bd)
Thermal coatings
(x)
(x)
(x)
(x)
x
x
(x)
x
x = standard p = plain bd = blind drilled
(x) = special applications = suction drilled g = grooved
paper machine and the relatively low mechanical demands. Therefore a large amount of conventional inexpensive fillers is used. Depending on the application, single layer designs are usually used. Bonding of the covers to the metal shell is a combination of shrink fit of the hard rubber and chemical bonding, resulting in good corrosion resistance, which is the main reason for applying the cover.
For hard press roll applications the demand on surface quality and abrasion resistance is higher, therefore more expensive fillers and combinations of fillers are used. Release properties of the cover can be adjusted by addition of polymer fillers like PTFE. Two layer designs are frequently used, a cost optimized hard rubber bonding layer and a performance optimized functional layer.
Soft press roll covers are formulated for good dynamic properties in combina¬tion with high wear resistance. Chemical resistance and swelling characteristics are adjusted by selection of the polymer. The higher loads of the nipped positions require two or three layer designs in order to withstand the shear forces in opera¬tion. Bonding is achieved by chemical bonding supported by the shrink fit when using a hard underlayer.
for example result in good chemical resistance. Properties like heat build up, dampening, wettability, roughness, and low compression set for marking resis¬tance must be considered for development of the compound in these applica¬tions.
Most roll cover manufacturers carry out their own rubber compounding because of the variety of the formulations, the demand for high quality and the small batch sizes. Mixing of the compounds is frequently done on open mills (Fig. 6.9) due to the small batch sizes, the high number of different compounds as well as hardness variations and the good quality of this compounding procedure. The use of inter¬nal mixers is not always justified due to high equipment costs.
After mixing the rubber compound, it is strained to remove impurities and converted into either a feeding strip for an extruder or a calendered sheet for direct application.
The surface of the roll is carefully prepared by cleaning and sandblasting before the application of chemical adhesives or rubber cements.
There are three basic methods for applying the rubber cover to the roll core:
• Extrusion: an extruded strip of rubber is spirally wound around the rotating roll body (Fig. 6.10)
. • Knott method: a narrow strip of calendered rubber is spirally wound around the rotating roll body
. • Hand build: large calendered sheets of rubber are manually applied on the roll.
With each of these build up methods, the desired layers of different materials are applied. The choice of application method depends mainly on the compound and the requirements for cover homogeneity. The hand build method, as an example, is the most sophisticated and most expensive build method. For these reasons it is only used for applications with extremely high demands on surface quality such as sizing or coating.
Vulcanisation of the covers is done in steam autoclaves, where the rubber poly¬mer is crosslinked to the elastomeric network. The cover is finished by mechanical tooling, drilling and grinding to the required geometrical dimensions.
6.2 Rolls in Paper and Board Machines
6.2.4.3.2 Polyurethane Roll Covers
Polyurethane roll covers were introduced to the paper industry at the end of the 1980s. The outstanding mechanical and dynamical properties make polyurethane the most suitable material for soft elastomeric covers in press positions. Depend¬ing on paper quality and press design, high open surfaces up to 45 % with grooves, blind drilled holes or suction holes, as well as combinations thereof are used. An example of surface design is shown in Fig. 6.11.
For these types of covers in modern machines only polyurethane elastomers are able to provide enough wear resistance to reach acceptable grinding intervals. High performance polyurethane formulations are used, giving the required me¬chanical strength and elasticity as well as outstanding hydrolytic stability.
Usually the cover has a multilayer design in order to ensure excellent bonding of the polyurethane functional layer to the core as well as safe running properties. State of the art are composite base-layers with endless fiber reinforcements, which allow a gradual adjustment of the mechanical and thermal properties of the cover in a radial direction. This results in a gradual adjustment of the shear forces cre¬ated by the nip load within the cover layers. Two layer designs with a harder polyu¬rethane underlayer are also used to provide bonding to the shell.
In fast running machines the dynamic heat build up in the cover limits the application of elastomeric materials. Polyurethane polymers, based on special for¬mulations with excellent dynamic properties, ensure good performance in these applications with minimal heat build up. An example of such an application is the substitution of grooved steel rolls by elastomeric covers in shoe presses.
The functional layer of polyurethane covers is manufactured by casting proc¬esses, thus ensuring very homogeneous material properties.
Roll preparation is similar to that for rubber, chemical adhesives are used to bond the base layer. The base layer can be a fiber reinforced composite, which is applied in a wet impregnation process of the reinforcement material. The glass fiber reinforcement (tapes or rovings) is impregnated in a resin bath and then spirally wound onto the roll shell. Curing of the base layer with infrared heating and/or ovens and tooling to a defined geometry are the next steps.
The polyurethane elastomer itself is a reaction polymer, created by the mixing of prepolymers and chain extenders or hardeners. This reactive mixture, which is created in the desired mixing ratio by special casting machines, is cast into a mold built around the roll shell with the base layer. The casting procedure can be carried out horizontally or vertically. Bonding to the base layer is done chemically with adhesives or reactive layers. The polyurethane material itself is not normally filled, thus providing outstanding elastomeric material properties.
Another method for manufacturing polyurethane covers is the moldless rota¬tional casting process (Fig. 6.12). Here the reacting polyurethane mixture is cast as a spiral on the rotating roll, the fast reaction of the polyurethane mixture prevents the material dripping off the roll. Layers of different hardness can be applied easily with this method.
Most polyurethane formulations need to be post-cured at elevated temperatures for optimum material properties. The final mechanical manufacturing steps are similar to those of rubber covers.
6.2.4.3.3 Composite Roll Covers
Two basic composite designs for the functional layer of roll covers are used in the paper industry: Composite covers with a fiber reinforced functional layer and com¬posite covers with a cast functional layer (resin covers).
6.2.4.3.3.1 Fiber Reinforced Functional Layer
Composite covers with fiber reinforcement in the functional layers are mainly used in calender applications. The use of fibers significantly improves the mechan¬ical and thermal robustness of the cover. High performance materials like Ara¬mide fibers are used, where a high damage tolerance is required. Compared with
6.2 Rolls in Paper and Board Machines
structural composites, where the fiber characteristics dominate the mechanical properties, the fiber volume content of the functional layer of roll covers is rela¬tively low. The major physical properties like hardness, compressive modulus or strength of the covers are mainly dominated by the particle filled resin matrix.
The base-layers of the multilayer designs are mostly glass fiber reinforced, in these layers the fiber content is higher, and therefore the fiber properties dominate the properties of the composite.
For soft calendering different hardnesses – or more accurately different Young’s moduli – of the covers are required in order to ensure the required paper proper¬ties. This can be achieved primarily by varying the amount and combination of different fillers in the fiber reinforced composite.
The surface of the covers must be as smooth as possible, the wear resistance excellent. These properties can be influenced for instance by the modulus of the fibers, particle size distribution and hardness combinations of the fillers and, to a minor extent, by the properties of the resin. Temperature resistance and low heat build up of the cover under dynamic load is mainly a function of the resin matrix system. Operating conditions of calenders such as 25 Hz load frequency, 90 °C surface temperature and 50 MPa nip pressure as well as expected load cycles of >109 make the elastic calender covers the most demanding applications of roll covers. Figure 6.13 shows an elastic composite cover in a multi-nip calender.
For lower demand applications composite covers with conventional fiber rein¬forcements (glass or polyester) are used. These covers are for applications such as guide rolls or special press positions.
6.2.4.3.3.2 Cast Functional Layer (Resin Covers)
Composite covers with cast functional layers are applied in the press section or in calenders. Typically, mineral filled resin systems are applied onto a fiber reinforced base layer. The advantages of the cast resin systems are extreme homogeneity of the functional layer and good abrasion resistance. These properties are important for applications such as soft calender covers or center press roll positions. Special formulations of the resin/filler composite allow the adjustment of sheet release properties as well as wear resistance. Limitations for this type of cover are the sensitivity to mechanical damage and thermal shocks due to the relatively brittle functional layer.
For the manufacture of composite covers wet impregnation processes are pri¬marily used. The reinforcement fibers (tapes, rovings or nonwovens) are impreg¬nated in formulated resin mixtures and wound onto the roll body. Multilayering with different fiber reinforcements and different fiber orientations is applied. The winding angle of the reinforcement material mainly governs the reinforcement direction of the fibers, creating anisotropic (direction dependent) material proper¬ties. These special material properties are used for design purposes, e. g. to influ¬ence the strength, thermal expansion behavior or modulus of the composite structure.
Particle filled functional layers are applied by casting processes similar to polyu¬rethane covers.
The curing of the resin systems which are primarily epoxy-based, is done by infrared heating and/or by heated curing ovens. Mechanical processes are similar to those used for the other cover types. Final surface grinding requires special techniques to reach the desired smooth surface properties.
6.2.4.3.4 Thermal Coatings
Thermally sprayed coatings are gaining wider use in modern paper machines due to their outstanding wear resistance. Even thin coatings of 0.2–0.7 mm provide both excellent resistance and long lifetime. Different thermal spraying processes
(e. g. plasma, HVOF, flame/arc spraying) allow the use of a large variety of mate¬rials.
6.2.4.3.4.1 Ceramic Coatings
Oxide ceramic coatings based on Al, Ti, Cr Oxides or combinations, are used in hard press roll positions. The outstanding wear resistance of the ceramic surface as well as excellent sheet release make these coatings suitable for high speed paper machines. Due to the well defined porosity structure of the ceramic, a hydrophilic coating surface is created. This results in excellent sheet release and a low ten¬dency for deposition of hydrophobic stickies on the roll surface. The surface topog¬raphy of the coating, an important factor for sheet release, is kept constant over its lifetime, even under doctored conditions.
The coating itself is usually a two or three layer design. The functional layer made of ceramics is, by its nature, porous. To ensure a corrosion resistant coating either chemical sealing of the pores is carried out or corrosion resistant nonporous underlayers are applied.
6.2 Rolls in Paper and Board Machines
The main applications are for hard press rolls/center press rolls in fast running paper machines.
6.2.4.3.4.2 Hard Metal Coatings
Hard metal coatings are carbides, nitrides and borides of transition metals. Car¬bides of the Cr-group are mainly used for thermal coatings. These wear resistant layers are used for grooved or drilled press rolls providing increased grinding intervals. Optimized formulations of these multiphase coatings are used to coat heated calender rolls. These coatings can be polished to extremely fine surface smoothness. The durability of these systems allows continuous doctoring of the coatings.
6.2.4.3.4.3 Metals and Alloys
Metals and/or alloys are sprayed mainly as base or underlayers in combination with oxide ceramics and hard metal coatings. The difference in thermal expansion of the oxide layer and the metallic roll is partly compensated by these layers. The major features of these materials are ductility and toughness, improving the over¬all coating performance.
Thermal spraying is the build up of a coating on a substrate from particles sprayed at defined temperature and kinetic energy onto the roll surface. The ther¬mal energy is required to melt the powder particles or the wire while the gas flow is necessary for the particle acceleration. Energy sources can be either electrical (arc, plasma) or chemical: H2, propane or kerosene (flame spraying). The melted and accelerated particles hit the surface, re-solidify and build up the coating layer by layer (approx. 10–20 mm/pass).
Arc and plasma spraying can be performed either in vacuum (VPS) or under atmospheric (APS) conditions, but only the latter is applicable for roll coatings. Differences in flame spraying mainly concern the particle velocity. High velocity spraying can be continuous (HVOF) or discontinuous (D-Gun). The plasma coat¬ing of a press roll is shown in Fig. 6.14.
6.2.4.3.5 Chromium Coatings
Galvanic chromium coatings are used in special applications for coating the hard calender rolls. The supreme surface finish of an extremely dense homogeneous coating stays is balanced by its sensitivity to damage and limited doctorability. This is why thermal coatings compete in these applications.
6.2.4.3.6 Thermoplastic Covers, Sleeves and Coatings
Pure thermoplastic materials are applied only in niche applications. Cast or extruded polyamide tubes are used as counter rolls in marking presses,
e. g. for cigarette papers, thin PTFE sleeves are applied as hoses on bow rolls. High performance thermoplastic covers are applied in niche calendering applications. Limitations of thermoplastic materials are their moderate wear resistance and the dimensional instability of thermoplastics under load.
Combinations of thermoplastic coatings with an extremely open layer of a hard metal coating combine the properties of both material classes. Release coatings of PTFE sintered on thermally sprayed hard metal coatings for good release proper¬ties are examples of these hybrid coatings. The hard metal underlayer ensures wear resistance, the thermoplastic PTFE layer fills the pores and creates the good release. This type of coating is applied e. g. in the dryer section for the first drying cylinders after the press or size press, as well as for critical guide roll applica¬tions.
6.2.4.3.7 Granite Rolls and Calender Paper Shafts
Granite rolls and paper or cotton filled paper shafts are not roll covers or coatings in the usual sense. Here the material which is providing the function is also part of the load bearing structure of the roll itself.
Certain properties of granite rolls or of cotton filled paper bowls were of great advantage to the paper maker and these were also targets for the steadily improved coatings and covers which in turn could provide additional advantages. Granite rolls, for instance, have become history, regardless of their excellent release charac¬teristics, due to the operational risks at high machine speeds or the technical and economic problems encountered when building this equipment for modern paper machines of 10 m width or more. The release properties in the meantime were matched by ceramic coatings or synthetic composite or rubber covers.
In existing supercalenders the competition from composite covers is increasing due to the limited marking resistance of paper bowls.
These roll types have been replaced in most cases by rolls with modern coatings and covers.
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