Paper Calendering: Calender Types

The Different Calender Types 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). 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 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 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
 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 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.

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). 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. 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.