Can a spreader treat bagginess?

Baggy webs are trouble. First, they may refuse to go through a nip such as at a calender, coater, laminator, printer or winder. Because the baggy portion is longer, it may get "behind" the rest of the web as a bubble behind the nip. If it gets big enough or bad enough, the bubble will burp through the nip as a wrinkle with a diagonal or chevron orientation. The wrinkle at an angle at the baggy location is a nearly determining symptom of cause.

Second, baggy webs look ugly because they refuse to lay flat as a final product, something like a dent in the fender of a car. While the fender may still be practically functional, it is not perceptionally acceptable and any insurance company should agree to its repair. Thus, even if you get bagginess through your machine and your customer's machine, you could still have trouble. Perception is reality and if you can see it, you leave yourself open to both legitimate and illegitimate customer complaints.

So what can you do? Since we commonly put spreaders in front of nips to flatten the web, perhaps the spreader might help with bagginess. To understand how it might, we must distinguish between real bagginess and apparent bagginess. Real bagginess is when the web refuses to lie flat on an inspection table. Just because it does not lie flat in a machine does not mean it is baggy. Misaligned rollers, roller diametral variation across the width, and nip-pressure variation in that area can cause apparent bagginess. Just because it lies flat in a machine does not mean that the web is not baggy. Sometimes you can pull hard enough to make the web taut across its entire width.

The best we can do with the spreader is to make the web taut in the MD and CD such that the web enters the nip flat. It should be quite easy to pull in the CD to remove an MD-oriented trough. Taking up bagginess, though, is a bit harder. On a bowed roller, the nominal bow orientation is midway between the incoming and outgoing web directions. If we have a baggy center, we orient the bow slightly into the web from this nominal orientation. If we have baggy edges, we orient the bow slightly out of the web from the nominal. If we have baggy lanes we can do nothing.

What bow orientation does is to change the relative length of the paths of the web at the edges versus center, which can take up the slightly different internal "lengths" the baggy web has at the edges versus center. This effect is extremely short-lived. It does not extend beyond the next roller after the spreader because the relative span lengths are equal there. Thus, there is NO permanent fix of bagginess. In fact it is so ephemeral that it often doesn't even make it to the next roller.

Also, a bowed roller cannot spread baggy edges because of the following argument: Baggy edges mean no tension at the edges. No tension means no traction there. No traction means no normal entry that is the principle by which this spreader operates. A simpler argument goes like this: how can you grab hold of the loose, ruffly edges to pull on them? These arguments really need more explanation than I can give here. However, I can summarize thus. Work with the spreader, but do not be the least bit surprised if it does not help at all.

What can you do then to treat bagginess? If I were a manufacturer of webs, I would work on leveling my manufacturing profile. If I were a customer of webs, I would first complain about, and then send back, any baggy material that gave my process fits. If I were a machine operator caught in the middle, I would first try reducing tension until the web droops. Then I would try pulling like hell. Finally, I would pray for a new lot or grade, or the end of my shift...

David Roisum, Ph.D. Consulting Technical Editor 920/725-7671 DRroisum@aol.com www.roisum.com