QA routinely might do a dozen tests in a web plant. One large contract testing lab in Quebec has 2,000 tests in its arsenal! Some tests are quite popular. An example is MD strip tensile strength, though buoyed in part by misconceptions that strength implies runnability (when, in fact, Total Energy Absorption [TEA] is much more predictive, but only marginally so). Basis weight also is popular for much better reasons: Poor weight (or thickness) control is responsible for nearly all bagginess and the myriad resulting problems, as well as a host of winding defects. Basis weight thus is quite predictive.

However, many tests are not popular. An example is the Coefficient of Friction, or COF for short. It – as well as Poisson Ratio, which will be covered in my next column – is one of the most undertested and underappreciated of tests. Only a small fraction of web plants routinely test for it. Yet, it has predictive powers. Web-to-roller COF can predict when traction is lost. Web-to-web COF can predict many winding defects.

Traction is required to turn idler rollers, to control tension, for most spreaders and for most guides. Though the equation is a mere one liner, covering it in detail requires an entire 70-page chapter [1].

T₂/T₁ < e ^(mq)  

Sufficient for the moment is that the µ symbol in the equation, the Greek letter mu, pronounced like ‘mew’ in English, is the COF between web and roller. One simply can’t do design work without knowing its value.

A more complicated application is web-to-web COF, which is predictive for many winding defects. For example, if the COF is low, most of the types of telescopes (there are about a half dozen) will increase. Also found are some less common but no less debilitating defects, such as crepe wrinkles, which are the bane of newsprint. A mere pound per ton of friction modifiers can tame this tiger. If, on the other hand, COF is high, then blocking, slip dimples and winder vibration likely will increase. Again, a mere pound per ton of friction modifiers might tame these tigers. DFM, or better living through chemistry, sometimes is the strongest and sometimes is the only way to fix these problems.

So, how do you learn to love mu? First, go to the literature and note how many articles there are on the topic [2]. There are nearly 100, though one is a tribute just to this test and property [3]. Finally, review the test methods and select a couple for your plant from Appendix T, page 663 [1]. 

David R. Roisum, Ph.D.
920-312-8466; drroisum@aol.com
ARC Member, ARC TV Presenter,
R2R Presenter, Converting School Educator

References:

1. The Web Handling Handbook. Roisum, Walker and Jones, Destech 2024.
2. The Roisum Library literature database. https://www.stevenabbott.co.uk/abbottapps/RL/index.html
3. Understanding Friction. J. David McDonald. 5th Int’l Conf. on Web Handling Proc., Web Handling Research Center, Oklahoma State Univ, June 6-9, 1999.