The Main Calendering Methods for Various Paper and Board Grades
184.108.40.206.2 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 the machines, especially on the service life of the resilient rolls and thermorolls and presupposes a properly functioning tail threading system.
220.127.116.11.3 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.