How can I wind material with bad gauge profile?
David Roisum, Ph.D. Consulting Technical Editor -- Converting Magazine, 12/1/2007 12:00:00 AM
You really only have a few options: Wind very short rolls (a few dozen wraps ought to be okay), send the ugly raw material back to the supplier, or wind soft and live with the remaining gauge-related defects.
Our greatest fear in winding is being asked to wind a material with a profile problem. The word “profile” means variation of ____ across the width, where ___ could be thickness (gauge), MD tension (bagginess) or any other property. Some people would say this is like trying to make a silk purse out of a sow's ear. However, that is not really a good analogy because in many cases the web itself is fine enough if the winding step could be avoided, such as by sheeting.
The fear is that a number of serious defects are directly caused by gauge variation. The most common is the baggy lane created by yielding (stretching) over the ever-so-slightly bigger diameter at a gauge band. However, any defect that is sensitive to winding tightness is also sensitive to gauge bands. Whether you have a tight or loose defect, all are made more frequent and severe in the presence of gauge variation. Consider blocking, a tight defect, which is the sticking together of layers in the wound roll. The defect will form first at the high-gauge area, it will form far earlier than it would have otherwise, and it will be more frequent than if the web were level. While a winder might be able to have a useful range of 2:1 on tightness, gauge variation could easily cause a pressure variation of 20:1 or even 200:1 across the width of the roll on stiff materials. Thus, many pressure-sensitive defects are more determined by (gauge) variation than the (winding) mean. The same principle applies to loose defects. If there's a bubble behind the nip during winding, it will be much worse with gauge variation because the air will seek the low spot in the roll. If the web were level, the air would distribute uniformly and seldom be a problem itself.
Not only do gauge variations limit the defect-free window, they limit the range over which the TNTs (tension, nip, torque, speed) of winding can be useful. If you had a dead-level product, you would have a wide useful range of nip-load adjustment. At the low end, you could apply low (surface wind) or literally zero (center wind) nip load without the nip misbehaving. At the high end with a dead-level product, most papers and films could withstand a nip of dozens of PLI without direct damage from the nip. Thus with a level product, the range is low to dozens of PLI. Now consider a mild gauge variation. You might not be able to apply even a couple of PLI without wrinkling near the gauge band. With a more vivid gauge variation, you may not even be able to touch the wound roll with the nip without it misbehaving. Thus, just as the material challenge rises, the useful range of the winder adjustment closes up.
The experience of operators and the science of the web handlers are quite clear and quite consistent on gauge variation. All you can do at the winder is to wind as loose as you can get away with without causing even worse problems. After that, the problem is no longer a winding problem because there is no winding solution. You then have to live with the problems or redesign the web or roll product. Specifically, the web might have to be made more level or the roll made much, much smaller.
920/312-8466drroisum@aol.com, www.roisum.com
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