At the end of this module, you will be able to:
- Identify the purpose of a roll press on a paper or board machine
- Describe the pressing phases in a roll press nip
- Identify and describe roll cover materials used in roll presses
- Describe a typical roll press arrangement
- Describe the purpose of a roll crown
- Describe swimming-type and shoe-type deflection- compensated rolls
The following key questions are answered in this module:
Why is the sheet frequently separated from one or both felts soon after a press nip?
To minimize "rewetting," wherein the sheet reabsorbs some water from the felt or felts
Are press rolls usually solid rolls?
In a conventional roll press, the roll surfaces can be solid, blind-drilled, grooved, or blind-drilled and grooved.
What are the limitations of fixed crown rolls?
A "fixed crown" roll has a crown ground into the roll face, and it is only appropriate for a single nip loading. On machines where nip loadings vary because different grades are made, a variable crown roll may be a better choice.
Are controlled-crown rolls used in top or bottom roll positions?
Controlled-crown rolls can be used in both top and bottom roll press positions. In bottom positions, they typically have softer covers that are grooved or blind-drilled to enhance water removal.
How do controlled-crown rolls create the crown?
CC rolls use hydraulic pressure in a sealed chamber or applied to cross-machine cylinders to create the crown.
Below is a transcript of the video sample provided for this module:
The amount of water removed in a press nip depends on two main factors. The nip pressure and dwell time, or the time the sheet is exposed to the nip pressure. Water removal occurs in two phases. During the compression phase, pressure on the sheet and felts increases. Air is forced out of the sheet and felts and water is pressed out of the sheet and absorbed by the felts. During the expansion phase, pressure decreases. The felts continue to absorb water from the sheet. However, the sheet reabsorbs some water or rewets after the nip. To minimize rewetting, the sheet and felts should be separated as soon as possible after the nip.
Use the additional resources and links below to learn more about this topic: