Forming Roll , Suction Couch Roll

6.2.2.1 Forming Roll
The forming roll (Fig. 6.3) is a suction roll positioned at the beginning of the wire section e. g. in high speed twin wire formers. Here a high amount of white water has to be stored and low vacuum is applied (up to about 0.15 bar). The forming roll has a two-layered shell with an outer ring of high void volume (about 85 %) and a perforated inner ring. A sealed vacuum chamber inside the shell defines the suc-tion angle where the vacuum is applied. This is used for web formation control as regards for instance web symmetry in the z-direction.
6.2.2.2 Suction Couch Roll
This roll (Fig. 6.3) is placed at the end of the wire section where the web is already formed. It further increases the density of the web and increases its dry content. A small amount of white water is removed and the operating vacuum is about 0.3 to
0.7 bar. The vacuum chamber is divided into two zones with stepwise increase in vacuum. Shadow marking may occur on sensitive papers with too high vacuum application or inadequate drilling pattern. The open area is about 60 %.