UNIVERSITY OF VICTORIA MECH 450D PULP AND PAPER TECHNOLOGY CALENDERING AND REELING 1 Introduction After the dryer, paper passes through a calender stack and then is wound into a reel of paper as shown in Figure 1. These two operations are linked and therefore will be discussed together in this lecture. Figure 1 Calendering, shown in Figure 2, is a unit operation which compresses the paper web between one or more rolling nips. When carried out on a paper machine, it is called machine calendering. Reeling is the subsequent operation of uniformly winding paper into a jumbo roll at the end of the paper machine. Calendering compresses paper to a uniform thickness and imparts smoothness to the paper surface. The thickness uniformity is necessary to build a uniform reel, which affects the runnability of paper in the press room. The smoothness affects paper printability, that is, how well print and images can be reproduced on the paper surface. Both runnability and printability are key quality factors for press rooms. Of these, runnability is the more important. If paper cannot get through presses without breaking, it is worthless to printers, particularly those running to tight deadlines such as newspapers.
Figure 2 Figure 3
2 Calendering 2.1 Thickness Reduction and Smoothness Typical machine calendering takes place in a vertical column of steel rolls resting on one another, supported by a king roll at the bottom. The roll next up is the queen roll, which is often the driven roll. The others run by friction from the driven roll. The paper web is threaded down the stack, and thereby sees a series of nips of increasing pressure. This compresses the web thickness as shown in Figure 3. The flat rolls surface impose smoothness on the paper. 2.2 Major Variables The major variables in calendering are moisture and temperature for paper, and the number of rolls, roll diameter, and roll temperature for the calender stack. The relative importance of these is shown in Figures 4 and 5. Figure 5 shows that while a target thickness reduction can be obtained in a number of ways, high temperature calendering gives slightly better smoothness, much better gloss, and a lower strength loss. 2.3 Nip Mechanics Figure 4 The degree of paper compression in a nip is governed by the paper compressive properties and the applied pressure and time. In a calender nip, the latter are not known. The independent machine variables are line loading and speed. These, together with paper deformation properties, determine nip pressure and dwell time, as shown in Figure 6.
Figure 5 Figure 6
Table I: The Calendering Equation = A+ µ B i where the permanent relative compression is defined as: and the nip intensity factor µ is defined as: ( ) = B B B i f i µ = a + a log L + a log S + a log R + a θ + a M o L s R θ M Limits: - The equation applies between the limits: - Outside these limits: i ( ) A µ B 1 A 2µ B = B for B < A µ f i i f 2 ( ) ( ) B = 1 A 4µ for B > 1 A 2µ Parameters Coefficients B i Initial bulk (cm 3 /g) B f Final bulk (cm 3 /g) L Nip load (kn/m) a L S Machine speed (m/min) a S R Equivalent roll radius (m) a R θ Average mid-nip web temperature ( C) a θ M Web moisture (%) a M Intercepts A, a o The coefficents must be determined experimentally. They are a function of the furnish properties. Also, a R can be approximated as i ( ) a = a + a 2 R L S Figure 7
2 δ ϕp ρcp δϕ = 2 δz kp δt (1) φ p = temperature of paper at Z t = time k p = thermal conductivity ρ = density c p = specific heat K = k p /ρc p thermal diffusivity Bi s = Biot Number = Ch/k p C = thickness of paper h = contact resistance of paper k p = as above Temperature, φ p T c ( ) ( s ( z )) φ 2e cos Bi 1 p = 1 φ 2 + Bi cos Bi R s s c (2) when T c > 0.05 and T c kpbiss = C WCV p S = contact time with roll V = speed W = basis weight Figure 8
The rheology of paper in compression is time-dependent and non-linear with both pressure and time. Thus, prediction of paper compression in rolling nips is not simple. To accomplish this, empirical expressions have been developed from platten press data for pressure and time, and these have been converted to speed and loading in the calendering equation developed by Kerekes and Crotogino and shown in Figure 7. Thickness reduction in a stack is calculated by applying this equation from nip to nip, i.e. the thickness emerging from one nip becomes the entry thickness at the next nip. 2.4 Heat Transfer Heating paper makes it more pliable, and therefore easier to calender. Accordingly, one or more hot rolls are often included in calender stacks to transfer heat to paper. Given the high speeds of modern paper machines, this heat transfer is often incomplete in raising the temperature through the thickness of paper to the roll temperature. Instead, the paper surface in contact with the roll surface is at high temperature while the outer thickness remains at a lower temperature. These temperature gradients can be estimated from transient-state heat conduction, suitably modified for paper, as shown in Figure 8. Temperature gradients have been exploited to achieve a desired an often-desired objective in calendering: high surface smoothness with minimal thickness reduction. The concept is called temperature gradient calendering and is illustrated in Figure 9. Figure 9
3 On-Line Soft-Nip Calendering A recent development in calendering has been the use of on-line soft-nip calendering shown in Figure 10. Here, paper is passed through two calender nips without roll wrap. One of the rolls is a soft polymeric material which deforms to give a wider nip (see later in supercalendering). The other roll is a high-temperature, heated roll. These calenders achieve a superior finish by a more pliable nip and by temperature gradient calendering. They are used on-line. Figure 10 4 Reeling In addition to imparting smoothness to paper, the calender stack serves an important role in producing a uniform reel. When paper of non -uniform thickness in the CD direction is wound into a reel, the local roll diameter of the paper roll becomes slightly larger. This requires that paper stretch more in this zone compared to adjacent zones. This is defined as a hard spot, as shown in Figure 11. This in turn induces more permanent tensile deformation in paper in this zone compared to adjacent zones. Consequently, when the paper is unwound, this zone will be under less tension than adjacent zones under a given tension. Indeed it may be sag, i.e. be baggy, relative to the adjacent taut zones. To overcome this, press
rooms must increase the average tension, which leads to stress gradients in the web, which greatly increase the probability of breaks. Figure 11 For these reasons, press rooms require that all delivered rolls meet a standard of uniformity, and monitor this by routinely measuring roll hardness profiles. CD non-uniformities may originate in the headbox, slice, or press, e.g. plugged fabric). Whatever the source, often the only place for immediate remedial action is at the calender stack. Evening out paper thickness is accomplished by local heating or cooling of calender rolls in specific CD zones. This causes a slight roll diameter expansion or contraction locally, which in turn causes the roll loading to shift to or away from this zone. This in turn changes the level of paper compression and thereby the level of local thickness reduction. For example, local cooling increases local web thickness as illustrated in exaggerated form in Figure 12. Figure 12 Local heat or cooling of calender rolls is attained by air showers or by local induction heating. These are illustrated in Figure 13.
Figure 13 Figure 14
5 Other Issues in Calendering 5.1 Blackening Over-calendering of paper leads to calender blackening. Here calendering is so severe that fibres fuse together, causing a loss of light scattering surface. Incident light therefore passes through the paper, giving a dark appearance to the observer. This is illustrated in Figure 14. This frequently becomes a problem if paper moisture content is too high, for example 10% or more. Blackening can be remedied by lowering moisture or reducing calender loading. 5.2 Calender Barring A calender stack is a spring-mass-dashpot system, with the paper as a spring/dashpot and the rolls as the mass. Consequently, the stack has natural vibration frequencies and may therefore be excited to vibrate. The source of excitation may be regular MD variations in paper or vibrations coming through the floor from other equipment. When caused to vibrate, the rolls over-compress paper on the downward portion of their cycle, in essence blackening the web in a line across the machine. This is visible to the eye and is called barring. Remedial action consists of reducing the number of rolls, precompressing the paper in a breaker stack, or offsetting rolls. All these are aimed at changing the frequency response of the stack, i.e. the spring constant, the number of harmonics, the excitation frequency.