Key Questions
The following key questions are answered in this module:
Why is it called "wet stack" calendering?
In a wet stack, water is added to the sheet surface in one or more nips. This added moisture improves the transfer of the smooth calender roll surface to the surface of the sheet, so it enhances calendering results.
How is the water applied to the sheet on a wet calender stack?
"Water boxes" are positioned at one or more nips. They contain heated water which contacts the sheet briefly to moisture only the sheet surface, in order to concentrate the calendering action on the surface.
Can anything be added to the water that is used to moisten the sheet?
Dyes, lubricants, and binders like starch can be added to the water going to the water boxes, if desired.
How are wet calender stacks loaded?
Stack loading is typically provided by the weight of the rolls, which means that the nip loads increase from top to bottom. Jacking bolts or compensators may be provided on the ends of the upper rolls to adjust edge loads, if necessary.
Are variable crown rolls used in wet calender stacks?
To maintain uniform cross-machine nip pressure profiles in a wet stack, the king roll and queen roll (if there is one) are either fixed crown or variable crown rolls. Fixed crown rolls are designed for a single nip loading, while variable crown rolls can be adjusted for different nip loadings.