The Main Calendering Methods for Various Paper and Board Grades
220.127.116.11.4 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.
18.104.22.168.5 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.