The Main Calendering Methods for Various Paper and Board Grades
22.214.171.124 Wood-containing Paper Grades
Newsprint belongs to the group of uncoated, woodcontaining printing papers. Nowadays it consists of up to 100 % DIP (deinked pulp), basis weight around 45gm–2. These papers are produced on machines at speeds of up to about 1900 m min–1 and are equipped with machine calenders or on-line soft calenders. Modern paper machines equipped with just two double-felted press nips produce a base paper whose top side is slightly rougher than the bottom side. If a soft calen¬der is used, its heated roll is always installed in the top position, since the contact of this roll with the rougher top side reduces the two-sidedness.
The shrinkage-preventing, single-tier dryer section of a modern newsprint machine causes a more or less pronounced upward curl. To eliminate this curling, a steam moistur¬izer is installed in front of the first calender nip. Since optimal CD caliper profiles are a basic prerequisite for a smooth-running printing process, particularly with lightweight newsprint, deflection control rolls are used in calenders that allow zone-controlled, short-wave caliper profiling and insure 2-sigma values of below 0.5 m in the caliper. As far as the printing process is concerned, offset printing is becoming more and more popular. A differentiation is made between cold offset and heat offset. Each method places a different demand on the paper.
For cold offset printing, Bendtsen roughnesses of the order of 150–200 ml min–1 are required. This roughness level can easily be achieved on a two-roll on¬line soft calender. The linear loads here are between 40 and 100 N mm–1. The calendering temperature corresponds approximately to the web temperature.
For heat offset printing, lower roughness is required, i. e. Bendtsen values of approximately 100 ml min–1. To meet this requirement, on line soft calenders with a total of two nips are used, so that each side of the web contacts a heated roll once. With a suitable layout – linear load, roll surface temperature – even rotogravure-capable SC-B papers (see below) can be produced.