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 These hammers were superseded in the course of the 17th century by roll presses like those that had been used since the late Middle Ages for copper rotogravure print¬ing.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