Web bagginess is a most common and troubling defect. Not surprisingly, there are many ideas about what may cause or cure baggy webs. Some of these ideas center on a common converting element, the spreader. My experience and understanding is that spreaders neither cause nor cure bagginess. Certainly there are exceptions to every rule, including this one. However, they are so rare that believing there might be a connection between spreading and bagginess is much more likely to be distracting than useful.

Though unlikely, the possibility exists that a bowed element could cause bagginess. Thus, we must use definitive tests to determine whether this possibility deserves further consideration in a specific situation. These tests are:

1. The bowed element must be so overbowed as to cause permanent differential yielding of the material. An appropriate bow for bowed rollers depends primarily on the extensibility (modulus) of the material and may be as little as 0.1% of width for stiff materials such as flat paper and high modulus films to more than 1% of width (e.g., 0.5" of bow for a 50" wide web) for very flexible webs such as creped tissue paper, low modulus nonwovens and rubbery compounds. The appropriate bow also depends on available MD troughing, traction and many other details. Appropriate bows are somewhat larger for bowed pipes, dual bow spreaders, and spreaders used for separating multiple slit lanes. By this numerical test, unfortunately, most spreaders are overbowed. However, this does not mean the material is yielding. We must look further.
2. The material must enter the bowed roller(s) without a gentle center bag. How can you tell? Simple—you must intercept the web before it hits the bowed roller. While this might be disruptive, it is vital troubleshooting technique. Wait until the end of a run so that this disruption is minimal and bust the web if you need to.
3. The web must leave the bowed rollers(s) with a gentle center bag. By this I mean that the looseness increases gradually from both ends of the web and is maximum at the center. Baggy lanes and baggy edges can not be created by a bowed roller because they do not have a smile/frown "shape." The shape tool, a most flexible and powerful problem solving technique, is described in more detail in my book on industrial problem solving, Critical Thinking in Converting.

I have worked on many cases of bagginess and have seen more spreaders than I can count. How many spreaders have I seen which caused bagginess? Only a few cases that I can recall. One was an overbowed pipe on a heavy unbonded fiberglass matte, and a couple more involving extremely delicate films of rare chemistry. Other cases were edge-pull spreaders on films and nonwovens that are extremely aggressive and easy to misadjust. The most common, however, is tenter frames that always distort material at the pins. I have yet to notice any of the more temperate spreaders such as compliant cover, concave roller or bowed roll misbehave to create bagginess on even the most delicate materials. The much more common spreader misbehavior is to cause wrinkling due to misapplication.

What then causes bagginess? It is a variation of manufacturing across the width or brutish handling afterward. The most common profile variations are basis weight, caliper, nip, moisture or temperature. To troubleshoot this problem we commonly use two techniques. First, we screen elements out that do not have sufficient pressure, tension or temperature such that the material could be permanently yielded. Second, we screen out elements that do not have the right "shape." Then we must use creative testing to determine which of the few remaining suspects need to be arrested. This subject is treated in far more detail in my article, "Baggy Webs: Making, Measurement and Mitigation Thereof," which is available on my website.

David Roisum, Ph.D.

Consulting Technical Editor

920/725-7671

DRroisum@aol.com

www.roisum.com