A U.S. mill specializing in packaging could not achieve repeatable coat-weight correlation results between its lab and online sensors. While operators expected a deviation of up to 0.5 lbs/3,000 ft2, they in fact observed up to 10 times this level.
They initially believed that the problem derived from the online sensor, but further research established that the machine’s high process variability and inconsistent lab test results were the root causes.
Two online scanners (measuring basis weight before and after coating) were placed in “single-point mode,” following which, basis weight measurements were collected.
There was short-term basis weight variability of approximately 5 lbs/3,000 ft2 between the maximum and minimum values. However, in under a week, having resolved an upstream process issue, the basis weight variability was significantly reduced, down to 1.3 lbs/3,000 ft2.
This confirmed that the process variability was affecting the correlation results.
Operators analyzed lab variability by taking cross-direction and machine-direction samples measuring one by three inches and weighing about 13 grams from three locations on the reel: front, middle and back. The intention was to investigate how repeatable the lab results were from these adjacent paper strips. The lab tests showed variation of up to 2 lbs/3,000 ft2 of coat weight between the samples. To decrease variability, larger samples were recommended to reduce the effect of random lab and sampling errors and provide improved accuracy.
The paper manufacturer was surprised to find that responsibility for inconsistent correlation results was not due to its online sensor.
Helpfully, the project opened discussions on how to improve machine-direction control for coat weight, subsequently enabling savings in coating usage: if reducing coat weight by 0.5 lbs while maintaining high product quality is possible, the manufacturer could make 4.1 per cent coating material savings.
Some other reasons commonly found for poor correlation between lab and online measurements include not following TAPPI or PAPTAC measurement standards, not protecting paper samples immediately by sealing them in a plastic bag, since paper is hygroscopic, or not measuring the correct side of the paper for measurements that are affected by two-sidedness.