Calendering – Wet Stack
Calendering is a process used in the production of paper and board to smooth the surface and control the caliper or thickness of the sheet. Some sheet compaction or pressing also occurs. Wet stack calendering uses moisture to enhance calendering results. Wet calender stacks must be followed by dryers to remove the added moisture before they can be wound onto the reel.
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Learning Objectives
• Describe the purpose of wet stack calenders • Identify the safety hazards associated with calender stacks • Describe the wet stack calendering process • Identify and describe the main components of a wet calender stack • Describe the design and operation of deflection compensated rolls in wet stacks
Specs
| Course Level | Intermediate |
| Languages | English |
| Compatibility | Audio, Video |
| Based on: | Industry Standards and Best Practices |
Key Questions
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.
Sample Video Transcript
Dry calendaring or hard nip calendaring can be done with two or more rolls and one or more nips. This type of calendaring is detailed in a separate training module. Wet stack calendaring is typically done with three or more rolls and two or more nips. In a wet stack, water is added to the sheet surface one or more nips using water boxes. Uniform nip pressure profiles are critical to prevent water pockets from forming, which can cause sheet breaks. The added moisture and applied nips pressure develop good smoothness of the sheet. Dyes, lubricants, and binders like starch can be added to the water boxes if desired. After the wet stack, the sheet must be dried before it can be wound onto the reel.
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