There are two ways to define bagginess. Both involve profile variation and both are equivalent. The word "profile" is shorthand for "variation of x with respect to CD position." One definition is a tension-profile variation across the width. The other definition is a variation of length across the width. The baggy lane is longer than its neighboring tight lane. The extra length in that lane can get behind as a bubble in front of a roller, especially a nipped roller. If the bubble accumulates the extra length because it has no place to go, it may burp through the nip as a wrinkle. Bagginess also steers the web and causes path control issues.

Some machines are more intolerant than others. The more rollers in the machine, the more trouble. Nipped rollers have almost no tolerance to slackness, whether caused by baggy webs or roller geometry error. The tighter the required path control, such as with multi-station printing registration, the more trouble. Finally, thin and high-modulus webs are more trouble.

Do spreaders help much to nurse a baggy web through? The experience of countless operators, using countless spreading devices on countless webs, suggests not. Even if spreaders do help, it is so localized as to provide no overall benefit. So what if you can nurse the web through one difficult span? What about the seven spans that precede the spreader and the five that follow it? Are you going to put a spreader in every span? While it may be only some roller positions that turn bagginess into real trouble, intolerant positions are likely to include all nips and most highly wrapped or grippy rollers.

Then there is the problem of how much spreading is needed and how to adjust for changing conditions. Severe bagginess may require more spreading than on a flatter web. Also, a low-modulus web can tolerate more spreading without misbehaving than a high modulus web. Is your spreader adjustable? Most are not, especially if you consider that the re-aiming bow roller orientation does not adjust spreading power for the traction mode.

So why doesn't spreading work very well, if at all, for bagginess? There are many reasons. First, the nature of bagginess is extra length in the MD. On the other hand, spreading creates extra width in the CD. It is hard to see how these different directions couple together.

Second, the defect is a profile problem—yet most spreaders are not profileable. In other words, each CD position is treated the same as its neighbor. Thus, you might have too much spreading in the non-baggy lane and too little in the baggy lane. Lastly, baggy edges are one of the more common profiles. Yet the two strongest spreaders, the bowed roller and the bent pipe, cannot even touch the edges if they are baggy.

So what should you do if you have to run baggy webs? The first thing to try is to adjust tensions. Next, try spreaders of various styles. However, if tension and spreading don't do it—and it probably won't—you are pretty much stuck. This is an unpleasant answer because people want some way to salvage less-than-perfect webs. However, we should spend less effort trying to save troubled webs and more effort in not making or buying them, in the first place.

920/725-7671, drroisum@aol.com, www.roisum.com