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Monday, April 8, 2013

Roll Hardness vs. Reel Weight and Moisture (Part 1)

This was submitted by Jon Saatvedt, Manager-Technical Services.

Early in my career, while working with coated paper grades, I was part of a team that worked to reduce “mechanical condition” defects such as ridges and corrugations. We worked to identify as many production variables as possible that contributed to CD profile variations in weight, caliper and moisture since these characteristics impacted coat weight profiles and corresponding defects such as ridges and corrugations, also known as ropes. The tools from 25+ years ago were limited to mechanical concrete hardness testers and the trained ear with a wooden club.

About ten (10) years ago, I was called in to investigate roll quality issues on an off machine coater producing specialty coated papers. The operators on the coater were naturally concerned with perceptions that their coating basestock quality was a key piece of this problem. The coated grades had a saw-tooth pattern of density variation which was manifest as severe ridges in the finished, coated rolls of product. We could measure and observe roll variation in the specialty coated finished product. We marked the wrapper of the coated paper rolls and were able to measure roll hardness profiles at ~ ½” intervals.

The following day, we visited the paper mill manufacturing the coating base. Rolls coming off the winder appeared uniform and weight and caliper profiles from the machine looked to be satisfactory. However, a spray boom using water spray immediately after the size press to help deliver uniform reel moisture profiles was controlling in a distinct pattern characterized by one actuator being on and the next actuator being off. The root cause of the problem had been going on for months. Several months prior, a new slice had been installed on this paper machine and mapping of the slice and control actuators was off by just fractions of an inch. The online weight measurement profiles averaged across “zones” which masked or averaged out the underlying weight variation. However, control strategy on the machine looked closer at raw profiles and corrected for variation, which required rewetting from the spray boom in an attempt to deliver a uniform moisture, weight and caliper profile to the reel. The end result was a reel made with a high frequency weight variation (which was masked or averaged out in profiles) and a corresponding moisture variation. It was interesting that rolls initially coming off the machine winder still had uniform roll density and roll hardness profiles.

Read the rest of this story next week...

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