The Main Calendering Methods for Various Paper and Board Grades
188.8.131.52.1 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.