Still, this does not get to two even more challenging issues that are almost never spoken of. The next one issample size. If you place any stock in statistics you will find that what you can ‘know’ about a lot or roll given a few lab specimens is surprisingly little (Frost, 1991). The final challenge is that surprisingly few customer complaints come from thickness being too big or too little or out of spec. Rather, customers complain that the wound rolls you supply look or perform poorly. While it can be unsafe to apply generalizations to a specific situation, I will opine that a good share of these are profile problems in disguise. By profile we mean the variation of thickness (or other property) across the width. The great majority of bagginess (and related problems) and many wound roll defects are caused directly by profile or made worse by profile variation. The problem is, however, we seldom have the sensors good enough to ‘satisfy’ the fussiest customer for level products, that is the winder. In round numbers, at 1% variation of thickness you have few problems and at 10% variation you have few customers. (Metals are tighter than that while blown film a bit looser because of 360 degree oscillation, but threshold of pain given here applies to many if not most webs). So what’s the problem? The problem is trying to find a sensor that can resolve thickness much better than 1% of your thin webs. So, if we can’t ‘see’ the problem with our sensors then we are really handicapped in manufacturing and QA. Fortunately, the wound roll tells you what you need to know in many cases about profile variation.
Come to my Web101 Web class for background and then get graduate training in Winding such as coming up inAugust 23-26 in Patterson NJ or as video-on-demand. Then there will be little mystery why your thickness measurements will not always protect your customer from thickness related issues.