Calender barring

1. One cause of barring is that the calender rolls are corrugated when removed from the roll grinder.
— unfortunately, the corrugations frequently do not show as the roll is removed.
2. The irregularities left after grinding can grow if conditions on the machine stack are right.
— scrupulous attention to roll grinding is essential.
— avoid grinding down below previously stress-hardened corrugations.
3. A calender stack with a sheet running through it forms a system where vibration proceeds from the top downward, resulting in visible bars across the sheet. Elimination of the causes of calender vibrations is the principal method of correcting a barring condition. This can be done by:
(a) ensuring that the stack is in excellent mechanical condition with no loose bearings in the ways.
(b) staggering the calender rolls (offsetting).
(c) ensuring that the pressure is even across the stack.
(d) using a breaker stack.
(e) improve the cross-machine moisture uniformity.
(f) check stack condition with vibration analyzer.
4. Barring may come from the wet end of the machine, i.e., from pulsations through the headbox.
— check for any movement of the dry line.
— check for whip or vibration in any rolls on the forming table.
See also Barring.