Jun 20, 2017
If you work with supercalenders, you've probably at one time or another experienced the lazy "W" roll profile at the windup of the calender. The ends and center of the jumbo roll are hard, and the quarter points are soft. If the hard to soft areas are different enough, corrugations, wrinkles, and baggy lanes can be created.
What actually creates this roll profile needs to be determined in order to correct it. On most supercalenders, the fly rolls used to transport the paper from nip to nip are normally segmented rollers. Their bearing supports are located around the quarter points to allow for adjusting the ends so the sheet doesn't wrinkle as it goes into the next nip.
These quarter point bearings tend to heat up during operation and will transfer heat to the sheet. The hot sheet then transfers its heat to the cotton or polymer calender rolls. As the calender rolls heat up at the quarter points, their relative diameters increase, which creates more nip pressure. Higher nip pressures reduce the caliper in those areas which results in a softer roll at the reel. Infrared thermography can track these temperature bands down the stack very easily.
There are a few things that mills have done to reduce this thermal banding. First, they have replaced the fly roll roller bearings with ball bearings. Ball bearings don't generate as much heat as roller bearings.
Second, they have replaced some of the segmented fly rolls with solid fly rolls with pillow block bearings on the ends, outside of the sheet path. These solid rolls are a little larger in diameter than the segmented rolls for stiffness purposes.
Finally, for polymer covered calender rolls, a mixture of 50% glycol can be injected into the roll body through the end heads, about 25% of the roll shell volume. This had been found to have the greatest impact on the lazy "W" roll profile. The glycol helps distribute the heat of the rolls from the inside.
There are also other mechanical reasons for this type of roll profile which are dependent on static roll deflections and overhung bearing weights. However, there is little you can do to change those variables outside of modifying all of the calender rolls with nip relieving cylinders. Even that will not eliminate the thermal banding at the quarter points though.
For assistance with supercalender issues, contact your Valmet representative.