Using Hardness Variation (CV%) to Identify Defective Rolls

“Papermaking would be easy, if only there were no edges!”


Problem: Converter rejects were traced back to certain rolls, but the mill had no reliable way to identify them before shipment.


Task: Find out a method to determine rolls that do not meet converter specs before roll is delivered to the customer.


Analysis: A study was conducted at the rewinder by running Tapio RQP Live  high resolution hardness profiles of each customer roll. Hardness profiles and profile key values clearly indicate that edge rolls had higher hardness variation compared to the rest of the rolls on each set of rolls. The result matched with the issues at the customer site. High CV% (Coefficient of Variation) values gave the best match to rejected rolls whereas hardness mean values were identical for all rolls and therefore showed weak correlation. 

Key finding: Average hardness did not differentiate good and bad rolls. However, the coefficient of variation (CV%) clearly separated problematic edge rolls from acceptable ones. Based on previous experience with rejected rolls, the alert limit for the Coefficient of Variation (CV %) was set to 5 %.


Investigation of a set of reels produced by a winder-cutter

A set of reels was measured with the Tapio RQP Live hardness tester and the continuous mode in Tapio RollView was used to investigate the entire set from a winder.   The results demonstrate how the edge rolls in have higher hardness variations.


Hardness profile across multiple rolls on a winder, with higher variation (CV) highlighted at the edges of specific rolls.
The figure shows the continuous hardness profile combined from a set of 8 rolls in a winder. The individual roll profiles are shown in and it can be clearly seen how the edge rolls have higher hardness variation.

What is the Coefficient of Variation (CV %)?

CV% describes how much the hardness varies within a roll. It is calculated as standard deviation divided by mean hardness. A higher CV% means more variation and typically poorer roll quality.


The RQP Live and Tapio RollView both produce multiple statistics of the roll hardness profile. All statistics are calculated based on all points in the profile measured with millimeter sample spacing. (More information: Roll Hardness Profiles – What Can the Statistics Tell Us?)




Bar chart of CV percentage by roll number alongside a table of standard deviation, mean, and CV values.
Statistics of the measured rolls. It can be observed that the edge rolls (Roll1 and Roll 8) have more hardness variation which is clearly visible in the statistics (CV %) of the hardness profile. All rolls have a very similar nominal mean hardness.

Conclusion

High resolution roll hardness profile key values can be used to separate good rolls from bad ones. In this study, the CV(%) value was a good indicator of the quality of a roll for the mill in question. In the above described case it was shown that rolls with CV(%) values below 5 are good to go whereas values above 5 are likely to cause issues in further processing.


Benefits of Tapio RQP Live

By using continuous hardness profiling measurements, customers gain significant understanding of the inner roll structure which single-point or quantitative hardness measuremens cannot reveal.  Benefits of using an RQP Live include:

• Reduced customer complaints

• Fewer rejected rolls

• Objective quality criteria at the rewinder and at any other stage in production




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