Case Study: Surface Defect on Speciality Paper

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To eliminate an edge defect so that the mill would no longer have to remove the edges prior to sending reels to the customer.

The cause of the sudden onset of the defect, known as “cockling”, could not be found. The mill was required to trim 15% of the width off each edge and had been doing so for several months. There were periods when the defect would disappear but it would return some time later for no apparent reason.

There was no record of cockling severity, so an index of the defect had to be developed, and new process data had to be logged. The outcome was an assessment on a scale from 1 (worse) to 20 (best) to be made by the operator studying an end-of-reel sample. A process mapping exercise was then conducted identifying all the parameters that could possibly relate to the defect, and methods of logging them were determined if systems were not already in place. Data was recorded alongside the quality index for each reel of paper produced for a period of six weeks prior to the analysis.

The multi-variable analysis quickly revealed that there were four key variables showing two important attributes:

  • They are easily controlled by the paper machine operators
  • Allowing for very few exceptions, when certain upper and lower limits are respected, a quality index of 16 or greater is achieved

The mill applied these rules eradicating the defect and consequently the 30% broke. The exercise highlighted key process parameters that needed to be better controlled. The logging was continued and subsequent analysis enabled tuning of the ranges according to paper grade. 

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