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6.9 Calendering 6.9.1
Objective and General Description of the Calendering Process
The objective of calendering is to modify the surface characteristics of paper with regard to its further use, e. g. printing. Depending on the individual grades, the focus is put on different technological properties. These are mainly
. • gloss
. • smoothness/roughness
. • density
. • blackening
. • brightness • opacity. Printed gloss and printing smoothness are the major prerequisites for a good print quality. Both are generally dependent on the gloss of the paper and its smooth-ness/roughness as well as its levelness and compressibility. High printed gloss gives the printed product the desired shiny appearance, while high (printed) smoothness is decisive for the evenness of print and print density, e. g. reduced number of missing dots.
6.9 Calendering
As to the theoretical basis of calendering, a series of explanations exist. Some scientists hold the view that smoothness and gloss result from slipping of the paper in the nips. Others maintain that calendering is a flattening process where the smooth surface of the hard rolls is replicated on the side of the web that contacts the hard roll. Still others argue that it is the shearing action in the nip which causes gloss and smoothness by “aligning” the surface particles of the web. Not in dispute is the influence of heat: thermal energy transferred to the web softens the cellulose fibers (glass transition point) and thus enhances the develop¬ment of gloss and smoothness.
Smoothing the surface and increasing gloss are accompanied by reduction in caliper, strength properties, brightness and opacity to a certain degree.
The strength properties of the paper are peremptory for the runnability of the web in the printing machine. Brightness and opacity have a distinct impact on the print quality. Blackening is found when parts of fibers have collapsed under pres¬sure. Under transmitted light the respective areas appear glassy, whereas under incident light they appear as darkened areas. This is still intensified by the printing process, i. e. the light full tone areas turn murky gray.
Calendering is done by pressing the paper web in one or more “rolling” nips formed by rolls with special properties.
The main factors in calendering – apart from furnish and paper properties such as moisture, temperature and coating – that influence the above-mentioned tech¬nological result are:
. • nip pressure/load
. • nip dwell time
. • roll elasticity
. • roll surface temperature and smoothness. Details concerning the rolls and roll covers, the roll configuration and other im¬portant components of the calenders will be treated below.
History of Calendering
From the very beginning there was a desire to glaze the rough surface of paper. The procedure then was to lay each hand-made sheet on a smooth surface and treat it with an agate or pumice stone. Nothing changed in this tedious procedure for centuries. Later, water-driven hammers came into use for smoothing. These hammers were superseded in the course of the 17th century by roll presses like those that had been used since the late Middle Ages for copper rotogravure print¬ing. This was the first step toward smoothing in the “rolling press nip”, i. e. cal¬endering. Today, this method still determines the surface treatment of paper.
In 1798, Nicolas Louis Robert invented the paper machine. It took more than fifty years, however, until calenders were installed in the paper machine. These calenders – also called machine calenders – consisted of at least two hard rolls. A decade later the supercalender appeared with a large number of alternating hard and resilient rolls. The resilient rolls were often termed filled or paper rolls since rounds made of fibrous material (cotton, wool) were pushed onto the roll shafts where they were pressed together under high pressure and secured with closure elements. Paper rolls are highly prone to marking. They must therefore be re¬placed at regular intervals and finish-turned. This is why these supercalenders could only be operated off-line.
Conventional rolls deflect under the influence of the load and their dead weight, which would result in a nonuniform distribution of linear load in the press nips. To avoid this, the rolls had to be crowned, i. e. ground with a camber. The selected crown does, of course, only apply to a certain load. If it were desired to alter the load, the rolls had to be re-crowned.
Hence, the introduction of the “Swimming Roll” in the 1950s by Küsters was of decisive importance for the further development of both the machine calender and the supercalender. This roll consists of a fixed shaft with a shell rotating around it. Between the shaft and the shell is an oil-filled chamber. By adjusting the oil pres¬sure in this chamber the shape of the roll shell can be changed (Fig. 6.66). Thus the “operating window” of calenders was suddenly expanded.
The swimming roll allows control of the linear load distribution across the width only in a given overall shape. This limitation was overcome with the next genera¬tion of nip control rolls introduced in 1974 by Escher Wyss, the Nipco roll (Fig. 6.67). Here the load distribution can be controlled locally zonewise. On this roll the rotating roll shell is carried by a large number of hydrostatic supporting elements, which in turn are supported on a fixed shaft. The hydraulic control unit combines several supporting elements into one zone. In all, there are six to eight effective hydraulic zones. As they can be controlled individually, the linear load can be specifically adjusted across the width of the calender. It is therefore possible not only to uniformly distribute the linear load across the roll width but also to in¬crease or reduce it locally. Zone-controlled deflection rolls are meanwhile available Fig. 6.68 Multizone roll, Nipcorrect roll .
in the marketplace under different names and designs by Küsters, by Metso and by Voith.
Since 1994 multi-zone control rolls have been in operation with up to sixty sup¬porting elements arranged horizontally close together. These can be individually controlled so that even more precise profile corrections can be made (Fig. 6.68).
Machine calenders and supercalenders with width about 5000 mm and above are today equipped with zone-controlled rolls as standard. Narrower machines still use the simpler overall control type rolls.
Supercalenders are classic off-line machines as they have downtimes of 25–30 % due to the filled roll change. To be able to keep pace with a high-speed paper machine at least two, sometimes even three, supercalenders were therefore re¬quired. This disadvantage of the supercalender led, at the beginning of 1980, to the development of the soft calender. The soft calender consists of at least one heated roll and one resilient roll covered with synthetic material. Because synthetic rolls are much more resistant to marking than the filled rolls of the conventional super¬calender, the soft calender was also able to be used on-line. In many cases, the on¬line soft calender was very successful. However, not all quality demands on the paper surface could be met with it. For demanding papers, the supercalender equipped with filled rolls and which could therefore only be operated off-line re¬mained the only alternative.
Things changed in the middle of the 1990s with the emergence of improved synthetic covers. In 1994 the first calender of the new type (Janus Concept calender of Voith Paper) was built with all resilient rolls covered with special synthetic materials. At a first glance, this new calender looks like a conventional superca¬lender. In reality, it differs in many respects, e. g. by the reduced number of nips, less energy input and – most noteworthy – suitability for on-line operation. Thanks to a suitable combination of pressure, roll surface temperature, roll sur¬face quality and number of nips, it was now possible to calender even highly demanding paper qualities on-line. Other machine builders followed (Küsters and Metso). The latest development is a calender (Voith’s Janus MK 2), on which the roll stack is no longer arranged vertically but at an angle of 45°. The modern on-line-capable multi-nip calenders have extensively ousted the classic supercalend¬ers. The few cases in which it is still used for technological reasons are treated in Section 6.9.4
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6.9.3
The Different Calender Types
6.9.3.1 Machine Calenders
Machine calenders consist of two or more hard rolls and are practically always installed on-line. They are primarily used for paper that only requires moderate finishing or for pre-calendering grades that need further finishing treatment in order to obtain higher gloss and/or smoothness. Machine calenders are equipped with overall deflection or zone-controlled deflection rolls (Fig. 6.69).
6.9 Calendering
6.9.3.2 Supercalenders
Supercalenders are off-machine multiroll-calenders consisting of an unwind stand, a roller stack and a rewind stand. The usual number of rolls is 9–12. For specialty papers – such as silicon based papers for instance – the number can reach 18. The rolls are hard and elastic in turn. When the number of rolls is even, there will be a so-called “reverse” nip having two adjacent elastic rolls. The elastic rolls are filled rolls. The filling consists of a multitude of specialty paper sheets slid onto a steel shaft, compressed to the required hardness and then locked by nuts. The hard rolls are steel or cast chilled iron rolls and are often heated. The top and bottom roll are either overall deflection or zone-controlled deflection rolls. To pre¬vent the filled rolls from getting marked by the web following a web break, the stack is equipped with a device for quickly opening the nips. Further important features are the spindle system, the overhanging load compensation system, doc¬tors, web cutting and oscillating devices, flying splice devices, inner and outer lift platforms etc. The maximum working speed of supercalenders is approximately 800 m min–1 and the maximum line load approximately 450 N mm–1. However, maximum speed, line load and maximum surface temperature cannot be applied at the same time because of the delicate nature of the filled rolls (Fig. 6.70).
6.9.3.3 Softcalenders
The basic version of the softcalender is the two-roll softcalender. Its main compo¬nents are the soft covered deflection control roll and the heating roll. The linear pressure of a softcalender ranges from approximately 10–350 N mm–1 and the surface temperature of the heated roll can be up to 230 °C. For two-sided calender¬ing, two stacks with an inverted roll configuration are combined. In cases where one hot calendering nip per web side is not sufficient to obtain the desired finish¬ing result, more calendering capacity is achieved by adding further soft nips. In contrast to supercalenders, softcalenders can also be installed on line because the soft covers can withstand line loads, load cycles and temperatures that are much higher than witnessed with conventional filled rolls (Fig. 6.71).
6.9.3.4 Modern Multinip-calenders
Modern multinip-calenders are similar in function to supercalenders. The main difference is that the filled rolls are replaced by polymer covered rolls. As a result, modern multinip-calenders can be installed on-line and can be run more than
6.9 Calendering
twice as fast as supercalenders and with much higher surface temperatures and line loads.
Today, there are three calender designs which make use of the new technology (Janus MK 2 calender of Voith, ProSoft calender of Küsters, and the OptiLoad calender of Metso).
Voith’s Janus Concept calender was the first multinip-calender that could be integrated into a fast running paper machine. Various roll configurations were possible, i. e. 6–10 rolls in one vertical stack, 2 V 5-rolls in two vertical stacks etc. As the polymer cover of the elastic rolls is less thick than the filling of the conven¬tional paper rolls, no slideways and spindles are necessary. The intermediate rolls are supported by loading arms that incorporate the overhanging load compensa¬tion function. Depending on their design, the heated rolls can produce surface temperatures of up to 170 °C. Line loads in the range 250–500 N mm–1 are possi¬ble. When installed in-line with a paper machine, the Janus calender is featured with a special tail threading device. Similar parameters are found in the OptiLoad calender of Metso and Küsters’s ProSoft calender.
Based on the Janus Concept calender, Voith developed the Janus MK 2. This latest multinip-calender version is mainly characterized by the stack being no longer arranged vertically but inclined at a 45° angle offering operational and tech¬nological advantages (Fig. 6.72).
6.9.3.5 Extended Nip Calenders
Certain board grades have traditionally received their final surface properties by means of a Yankee cylinder, i. e. a heated cylinder having a large diameter and a highly polished surface, or by soft calenders. This technology is about to yield to extended nip calendering which provides two advantages, namely a speed that is much higher than that possible with the Yankee cylinder and a better relationship of smoothness versus bulk than can be obtained on a soft calender.
6.9 Calendering
The extended nip calender is based upon the well-established shoe-press tech¬nology. The machine consists of a heated metallic roll acting against a soft sleeve rotating around a shoe roll and a moistening device directed against the side of the web to be surface-treated. When passing the elongated nip, the web is calendered on the side contacting the heated metallic roll. Typically, the heated roll is operated at surface temperatures far above 200 °C. The nip length is determined by the length of the concave shoe. For board, the length varies between 130 and 250 mm. Technologically, extended nip calendering can be described as moisture and tem¬perature gradient contour calendering on the basis of reduced line loads and leads to improved micro-roughness (PPS) and high bulk preservation (Fig. 6.73).
6.9.3.6 Embossing Calenders
The objective of embossing is to give the paper a three-dimensional pattern. This is achieved by means of a single nip calender.
There are three different embossing methods, namely “matrix” embossing, “flat¬back” embossing and “union” embossing.
Machines for “matrix” embossing consist of an engraved ridged heated and sometimes chromium plated top roll and a soft covered bottom roll whose diame¬ter is exactly double that of the top roll. By pressing both rolls together and running them at low speed, the pattern of the top roll is imprinted on the bottom roll. As a result, a paper web passed through the nip will have an embossed laid pattern on both sides. Matrix embossing is applied for graphic papers, wallpapers etc.
Flat-back embossers are similar to the a. m. calenders for geared embossing except that the diameter relation of the top and bottom roll is bigger or smaller than 1 : 2. With this type of calender, only the top side of the web receives a pattern. If both sides require a three-dimensional structure, the web has to be passed through the nip again with the former bottom side now turned against the top roll and with a reduced nip pressure. Flat-back embossing is applied for writing and printing papers, photographic papers, car body boards etc.
Union embossing calenders differ from the a. m. machines in so far as they consist of two rigid rolls of the same diameter. The rolls mesh. The distance be¬tween the two rolls is always adjusted in such a way that it is identical with the thickness of the web to be embossed. The result of union embossing is a web having a corrugated shape.
6.9.3.7 Friction Calenders
The purpose of friction calendering is to impart glazing to the paper. Friction calenders are single or double nip machines in which all rolls are driven separately at speeds that differ by 10–30 %. They are mainly used for glazing playing cards.
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6.9.4
The Main Calendering Methods for Various Paper and Board Grades
6.9.4.1 Wood-containing Paper Grades
6.9.4.1.1 Newsprint
Newsprint belongs to the group of uncoated, woodcontaining printing papers. Nowadays it consists of up to 100 % DIP (deinked pulp), basis weight around 45gm–2. These papers are produced on machines at speeds of up to about 1900 m min–1 and are equipped with machine calenders or on-line soft calenders. Modern paper machines equipped with just two double-felted press nips produce a base paper whose top side is slightly rougher than the bottom side. If a soft calen¬der is used, its heated roll is always installed in the top position, since the contact of this roll with the rougher top side reduces the two-sidedness. The shrinkage-preventing, single-tier dryer section of a modern newsprint machine causes a more or less pronounced upward curl. To eliminate this curling, a steam moistur¬izer is installed in front of the first calender nip. Since optimal CD caliper profiles are a basic prerequisite for a smooth-running printing process, particularly with lightweight newsprint, deflection control rolls are used in calenders that allow zone-controlled, short-wave caliper profiling and insure 2-sigma values of below
0.5 m in the caliper. As far as the printing process is concerned, offset printing is becoming more and more popular. A differentiation is made between cold offset and heat offset. Each method places a different demand on the paper.
For cold offset printing, Bendtsen roughnesses of the order of 150–200 ml min–1 are required. This roughness level can easily be achieved on a two-roll on¬line soft calender. The linear loads here are between 40 and 100 N mm–1. The calendering temperature corresponds approximately to the web temperature.
For heat offset printing, lower roughness is required, i. e. Bendtsen values of approximately 100 ml min–1. To meet this requirement, on line soft calenders with a total of two nips are used, so that each side of the web contacts a heated roll once. With a suitable layout – linear load, roll surface temperature – even rotogravure-capable SC-B papers (see below) can be produced.
6.9.4.1.2 SC-B/Offset and Rotogravure
SC papers can be subdivided into two main grades, SC-B with a high percentage of recovered paper (up to 100 %) and SC-A with a high percentage of woodpulp (TMP or groundwood; up to 80 %).
In contrast to SC-A top grades (see below), SC-B papers with a partially 100 % DIP portion are already being calendered on-line today on 6, 8 or 10-roll calenders. The large percentage of DIP means that the papers are very quickly prone to increased black calendering. This trend continues with increasing web moisture. For this reason, cooling rolls are indispensable here. These reduce the moisture losses and therefore allow running into the calender at approximately 9 % mois¬ture. On line calendering of course places special demands on the availability of
6.9 Calendering
the machines, especially on the service life of the resilient rolls and thermorolls and presupposes a properly functioning tail threading system.
6.9.4.1.3 SC-A/Offset and Rotogravure
SC-A top grades are calendered on 10–12-roll multi-nip calenders in the off-line process, allowing speeds of up to 1500 m min–1. Due to the much higher calender¬ing temperatures compared to the supercalendering process, the web running into the calender must be significantly moister. To achieve approximately 5 % end mois¬ture after the calender, ingoing moistures of 8–10 % are necessary. Still higher web moistures do not produce any additional increase in the effect, on the contrary, they even worsen the optical properties. The moisture losses between the calender and the rewind should therefore be kept as low as possible. The use of cooling rolls has proven useful in these cases. These are installed shortly after the last nip. In this way, the web temperature is suddenly cooled down by approximately 20 °C and the moisture loss is reduced accordingly.
Due to the previously mentioned high calendering temperatures in conjunction with significantly higher steam rates – up to five steam moisturizers are used – offset papers can be produced that already come very close to the LWC grades and are characterized by high gloss, a very uniform surface – no more “print mottling”!
– and greatly reduced black calendering.
The calendering of rotogravure-capable SC papers proves to be more difficult, because rotogravure calls for very smooth paper surfaces in order to minimize the number of missing dots. A certain compaction is necessary for this, which is produced by the number of nips, linear load, temperature and addition of steam. With increasing calendering speed it is of course more and more difficult to achieve the desired compaction. For high quality rotogravure papers the maximum possible calendering speed is therefore around 1100 to 1200 m min–1.
6.9.4.1.4 Blade-coated LWC/Offset and Rotogravure
LWC papers are either blade-coated or film-coated. In both cases it is recom¬mended to calender the base paper before coating. The objective of this pre-cal-endering is to optimize the CD caliper profile, to compact the sheet structure and to easily smooth the web surface. In this way, the end result of calendering can be improved in Gardner gloss by up to 5 percentage points and in PPS-10S roughness by up to 0.2 mm. The pre-calendering mostly takes place on a two-roll machine calender, i. e. in a hard nip. It is, however, also possible to use a two-roll soft calender. Both types of calender are equipped with a deflection control roll. The calendered LWC papers go either to offset printing or to rotogravure printing.
LWC papers with a typical basis weight of 50–70 g m–2, which are blade-coated on off-line coating machines, are still calendered off-line today. The extreme de¬mands made on the smoothness of these papers provided with coating of only 6–8gm–2 per side – the PPS-10S roughness values are around 0.7–0.8 mm – have until now prevented on line calendering at high speed. For the off-line calendering of these papers generally modern 10 to 12-roll multi-nip calenders are used. The calendering temperatures are up to 130 °C. The linear loads range between 300 and 400 N mm–1.
6.9.4.1.5 Film-coated LWC Offset
On LWC offset grades of typically 45–70 g m–2 basis weight, the on line-capable two-sided simultaneous film coating method is becoming more and more popular, even at the highest speeds. Due to film splitting, the surface of the paper is dis¬tinctly rougher than with the blade coating method and hence harder to smoothen. As offset papers do not require such high smoothing as rotogravure papers, they can today already be calendered on-line. Modern 6, 8 or 10-roll multi-nip calenders are used for this purpose. The calendering temperatures partially lie above 160 °C. The linear loads can reach 450 N mm–1. Gardner gloss values of 55 % are not infrequent. The PPS-10S roughnesses are between 1.4 and 2.0 mm. Today’s base papers for LWC offset may contain up to 90 % recovered paper. It should be noted that greater efforts are also being made to produce LWC rotogravure grades by the film-coating method. These film-coated LWC rotogravure papers can then be cal¬endered on line.
6.9.4.2 Woodfree Paper Grades
6.9.4.2.1 Woodfree Uncoated Papers
Woodfree uncoated papers include, above all, office papers, which are in turn subdivided into writing and printing papers as well as copy papers. In the past, these papers were not supercalendered. Simple machine calendering was con¬sidered satisfactory. However, the demands made on the surface quality of these papers have now increased considerably.
Besides the standard copy papers with a Bendtsen roughness of 220–280 ml min–1 there is also a smooth paper for color copies with a roughness of less than 80 ml min–1. For standard copy papers a low-loaded machine calender with two hard rolls continues to be sufficient. As these papers require only a low sheet compaction, calendering in the hard nip does not lead to detrimental mottling. In the case of copy papers with a Bendtsen roughness of below 150 ml min–1, sheet compaction by a two-roll machine calender is already so high that distinct mottling occurs here. To avoid this, a 2 V 2-roll on line soft calender is required. As, in this case, each side of the web contacts a hot nip once, the roughness values are low for the top and bottom sides. It has been found that linear loads of 150–250 N mm–1 and roll surface temperatures above 140 °C provide the optimal calendering condi¬tions. Copy papers are cut-size papers, therefore uniform calipers are of special significance. Thus, irrespective of whether a machine calender or a soft calender is used, it is recommended to equip the calender with a multi-zone deflection control roll to optimize the caliper profile.
6.9 Calendering
6.9.4.2.2 Woodfree Coated Paper Grades
With these papers a differentiation must be made between single-coated, double-coated and triple-coated papers. In addition, it is also of significance whether the papers are to be used in rolls or as cut-size sheets.
Single-coated, woodfree papers are generally calendered on-line. The machines used for this are mostly 2 V 2-roll soft calenders. At speeds up to 1300 m min–1, Gardner gloss values of 55–65 % can be achieved. To insure good flatness – this applies above all to cut-size sheets – the papers must be subjected to only moderate linear loads.
Up to the mid-1990s, double or triple blade-coated woodfree high-gloss papers with coating per side of 20–30 g m–2 were calendered on supercalenders. Even today, they are still calendered off-line. However, 8–10-roll multi-nip calenders, whose resilient rolls are covered with synthetic material, have ousted the classic supercalenders. In contrast to conventional supercalenders, these new types can be operated at higher temperatures, i. e. temperatures of 100–130 °C. Conse¬quently, the desired calendering parameters – PPS-10S roughness 0.7–0.9 mm and Gardner gloss 75–85 % – can now be achieved at much higher speeds, around 700–1000 m min–1. A further increase in speed would theoretically be possible, but presupposes an increase in the linear load. With rising linear load, however, the flatness, which is particularly important for cut size paper, would be negatively affected. Linear loads of 200–300 N mm–1 have proven to be optimal. Usually, these calenders are equipped with two cooling rolls.
It goes without saying that in this premium segment there is, in addition to high-gloss paper, always a certain portion of matte or satin papers. Two different calendering processes exist for these. Either an on-line soft calender with two synthetic-covered rolls is built into the coater – by this soft/soft calendering the matte character of the paper is hardly changed, but the abrasion resistance is significantly improved – or the a. m. multi-roll off-line calender is used by cal¬endering the web in its uppermost and lowermost nips while the intermediate nips are “run through” – in this case both the gloss and smoothness of the web are increased.
6.9.4.3 Specialty Papers
6.9.4.3.1 Silicone Base Paper
Silicone base papers are used as carrier papers for the later application of a one-sided or two-sided layer of silicone, onto which, in the following operations, fur¬ther layers, e. g. adhesive in combination with label paper, are applied. Silicone base papers presuppose special quality features. These include, above all, high compaction, uniform caliper and moisture, high transparency, strength, good flat¬ness and a low surface roughness which ensures that the later application rates of the cost-intensive silicone are minimized. The basis weight of the silicone base paper is between 60 and 160 g m–2. As in the past, these papers are calendered off¬line on special supercalenders with at least 15 rolls. The resilient rolls consist of a cotton/linen mixture. The surface temperatures are around a maximum of 150 °C and the linear loads around 450 N mm-1. The operating speed is 600 m min–1. It is only in instances where not such high demands are made that modern off-line multi-nip calenders with synthetic-coated resilient rolls are taken into considera¬tion. 6.9.4.3.2 Laminated Base Paper
Laminated base papers are highly absorbent papers for the production of layered materials saturated with synthetic resins. The basis weight of these papers with an extremely high percentage of fillers – up to 40 % – and correspondingly low strength, lies between 40 and 80 g m–2. Laminated base papers are very dry. Their moisture content is only 1–2 %. Characteristic of the production process of lami¬nated base papers is that the trim widths change frequently and to a great extent. The base papers must be opaque and require one-sided smoothing to Bekk smoothness values of 400–600 sec. For this purpose, 5-roll on-line multi-nip calen¬ders with a linear load of 300 N mm–1 are used. The roll surface temperatures of the heated rolls are around 150 °C. The resilient rolls are covered with synthetic materials. A steam blow box is also used. 6.9.4.4 Board
6.9.4.4.1 Uncoated Board
Uncoated boards include a large variety of sub-grades like test liner, kraft liner, white top liner, liquid container board etc. Some of these grades need no calender¬ing at all. Where calendering is required to achieve better surface properties, Yan¬kee cylinders and soft calenders have been used. As with coated grades, this tradi¬tional technology is about to be superseded by extended nip calendering – see Section 6.9.3.5.
6.9.4.4.2 Coated Board
Coated board grades range from one- to five-ply boards and can consist of virgin fibers and/or recycled fibers. The most important properties are high bulk, stiff¬ness and smoothness. The board is usually one-side coated. In some cases, how¬ever, it can also be two-sided.
Traditionally, calendering has been performed with Yankee cylinders or soft cal¬enders. In view of the bulk saving, this classical technology is increasingly being replaced by extended nip calendering – see Section 6.9.3.5.
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