Troubleshooting Tip: Curl

Curl is an issue that can creep up on you from time to time.  There are three types of curl to be concerned with; machine direction (MD), cross direction (CD), and diagonal.

For MD and CD curl adjustments to the jet-to-wire ratio or rush/drag can be made to alleviate the issue.  Generally if you are having a MD or CD curl issue, moving the jet-to-wire ratio closer to one or moving the rush/drag closer to zero will help.  This adjusts the fiber orientation to alleviate the curl issue.  Reducing refining and reducing broke content will also help.

Other things to look at when dealing with MD and CD curl are the size press and coater.  Make sure that the sheet is picking up the same amount at the size press.  The same should be done for the coater.  Also check to make sure that the mixtures in the run tanks are consistent with each other.  Unless you are running out of one run tank, this can be a source of trouble.

Diagonal curl is a different animal altogether.  It can come seemingly out of nowhere.  The first thing to look at when this issue pops up is to see what has changed.  For example, was there an outage prior to the issue and if so what was changed out during the outage that could have caused the problem.  The lexan sheets in the headbox can be a source of trouble.  Make sure that the new sheets are the same as the old sheets and that the manufacturer hasn't changed anything in their production process.  Other causes could be uneven wear of metering blades or rods at the size press.  This will cause an uneven coating that could cause diagonal curl.  Uneven press loading may also cause diagonal curl.

Out of the three curl types, diagonal is the most difficult to troubleshoot.  Remember, curl like any other issue is caused by a change in the process.  Sometimes the change is something that can be controlled, and sometimes it is due to the raw materials you have to work with.  Either way make the necessary adjustment and make a note for future reference.