Research & Innovation
Troubleshooting Baggy Webs
Plenty of motivation
October 1, 2006 By Pulp & Paper Canada
Plenty of motivation
Baggy webs are big trouble. Perhaps the worst of these troubles is that they may refuse to go through nips without wrinkling. The reason is straightforward. The baggy lane is a bit longer than its neighbouring lanes. Thus, when the nip tends to meter the web through at an equal rate, the baggy or longer portion gets behind as a bubble. If modest, this bubble will remain stable and not cause a defect. However, if the bagginess is too much, the bubble will grow, burp through the nip as a wrinkle, grow, burp, grow and so on as a repeating defect. It does not matter whether the machine is a calender, coater, printer or winder, the nip will complain in any case if the bagginess is greater than incoming web tension can pull out.
Wrinkles are not your only concern, however. Even the slightest bagginess greatly increases the risk of web breaks. Also, the web will not track straight and may cause edge position troubles. While a customer might be able to compensate by setup, changing bagginess from one roll to the next is more than a nuisance. Bagginess is a major cause of printing registration errors as a result of this changing web path. Even if you do get the baggy web through the machine, however, you are still not home free. The end product will not lay flat. Flatness is important in the feeding of sheets through machines and is important for those products where appearance is important.
I have had endless questions from students in my web handling classes about how to get a baggy web through a machine. I say try changing the tension (usually up). They say they tried this and it does not eliminate the problem. Then they ask whether spreading or special rollers might help. I say “probably not, but go ahead and try anyway so you leave no stone unturned.” Only after you play all of these games will you be ready to listen to my advice. The best strategy for dealing with baggy webs is to return them to your customer. Don’t let their problems become yours.
Though this advice is well intended and pragmatic, it does little good for the papermakers who make the problem in the first place. They cannot pass the buck. They must either solve the problem or pay the claims. Before we pay any claims, however, it is vital to know for certain that the web is too baggy to run. Merely looking baggy in a machine is suggestive but not conclusive. A roller which is misaligned or has a diametral variation can make the web appear baggy when it is in fact a crooked machine that is the cause. A good operator can probably tell the difference between a crooked web and a crooked machine, but this is a bit presumptuous to make claims on. Also, just because it does not look baggy does not mean that it is not. If the customer can pull enough tension they may temporarily flatten the web. Bagginess will return just as soon as the tension is relaxed.
One way to separate the customer’s machine from your web is to flip the unwinding roll. This is best done right in the middle of a run. Do not be concerned about production or waste. Do not be concerned even if saleable product cannot be made that way. What we are doing here is problem solving, not production. If the problem, be it bagginess or anything else, flips to the other side, you are in trouble. If, on the other hand, flipping the unwind leaves the defect in the same place, you both are in trouble. This technique of inverting the web is so conclusive that it is hard for anyone to argue with the results. However, it is limited to asymmetrical problems. Thus, a baggy edge would respond to this test whereas baggy edges would be inconclusive. This test can work with symmetrical problems if you can move the web sideways far enough. This is a set-up challenge for sure, but most machines are capable of enough sideways movement to see whether the problem shifted a bit with the move. Again, we are concerned solely with problem solving rather than production. It is important to know without a doubt which building you need to work in next.
Baggy web measurement
Our eyes are the best baggy web detector. However, in situ reads can be risky. The mechanicals (crooked rollers) and controls (magnitude of web tension) will make the problem appear worse or better than it really is. The safest way to inspect a baggy web is to remove the influence of the machine. One excellent way is to use a light table such as shown in Figure 1. Alternatively, you might just lay a long section on a very flat floor. The web should lay flat and the (cut) edges should be straight. Ruffles on the edges mean baggy edges, and a puckered lane similarly means an area of extra long material. However, in many cases the baggy lane is not the defect. Look again at the figure. Note that it is the few tight lanes that cinch broad otherwise good web into puckers.
The ability to invert your view is an important skill in baggy web troubleshooting. Thus, a ‘smile’ shaped length profile is the same as a ‘frown’ shaped tension profile. The baggy lane is both looser and longer. Unfortunately, while tension profile is the nature of the bagginess, it is extremely difficult to measure. More than a decade of development has made a couple of trustworthy and mill-duty instruments. Unfortunately, the trustworthy instruments are too costly for most paper mills and virtually all converting plants.
The flip side of stress (tension) is strain (length). We can measure the variation of length across a web with the strip test. Here, we cut the webs into as many narrow strips as we need or have patience for. When freed from their neighbours, the individual strips will seek their own length. Not only is this affordable, all you need is a string, scissors and precision square; it is quantifiable. Baggy webs complaints on paper arise when the length differences are between 1 and 10 parts per 10,000. Thus, at the end of 50 feet of strip, the lengths may differ by a fraction of an inch. Unfortunately, while this test does work for most materials, it is extremely tedious and ill-suited for routine quality control. Imagine getting on your hands and knees cutting out hundreds of feet of strip just for one test?
What about scanner or test lab instruments? Well, we’ve had them for decades and they have not gotten us where we need to be. It is true that tiny changes in basis weight or moisture are often associated with bagginess, but they are not always. Besides, the amount of profile variation required to form or stretch a web into bagginess is well below the threshold of both instrument and sampling in almost all cases. Thus, they seldom do us much good for baggy web troubles that are subtle.
Now what? Back to where we started. Look again at Figure 1. It tells us everything we need to know to begin problem solving. It tells us the ‘shape’ of the problem. In other words, we can put our finger on the width and location of every single lane if we but take a close look. Width and position are, as it turns out, far more important than magnitude in real-world problem solving. We can also note the change in position from one set to the next. We will use the shape tool to screen components from consideration. However, we get ahead of ourselves.
The first thing we need to know is where we are going. The goal of good problem solving, whether it is bagginess or anything else, is extremely specific. You want to be able to touch the thing that made the problem. Well, at least point at it safely from a distance anyway. Knowing why it creates problems is a later step. Right now we just want to know what creates the problem. People love to make lists. But lists are just a beginning. In the end it is just one item on that list that makes that specific baggy lane. Multiple (component) causes for a single lane are a sign of problem solving laziness or exhaustion. As Martin Fischer puts it “A conclusion is where you got tired of thinking.” That is not to say that only one thing can make bagginess
, only that one thing made that particular lane of bagginess. Thus, it may be a basis weight streak today and a moisture streak tomorrow.
Step 1 – Initial Screening by Form and Deform
So we start with a list. The list initially includes everything that touches the rollers. Obviously this list is so long that failure is guaranteed, unless it is shortened. More than shortened, it must eventually be pared down to but one item. The first thing we do is to remove all elements that do not form or deform the web. Paper machines are a bit involved because nearly every roller passes this screen, so I will use a coater laminator to illustrate the principle. As seen in Figure 2, the shortened list would be the two unwinding rolls (perhaps already distorted), the adhesive coater (perhaps uneven water add-on), the laminator (temperature, pressure), dryer (uneven heat) and winder (though gage variation.) Though the list is still too long, we have taken it down from perhaps a hundred (mostly idler) rollers to a half dozen more likely candidates. You say “what about spreaders and pull rollers, could they not deform the web?” Perhaps, but this cut is not final. We can always change our mind if new information comes in. However, both web handling and experience indicate that these common elements almost never distort the web.
Step 2 – screening again by the shape tool
The next screen is the shape tool. By shape I mean profile of the bagginess across the width which we can adequately determine merely by using our eyes. So, for example, if you knew for a fact that the bagginess was smile-shaped, you could eliminate ideas such as misalignment (taper shaped) or drives (changes level but not shape). If the problem was come and go, calendar wear would not be a good candidate but perhaps changing calender temperature profile might be. While we have all heard that every idea is a good one in brainstorming, that is as far as it goes. The real world is much more brutal. The shape tool is the very best screen to keep those initially good ideas from becoming counterproductive as they leave the meeting room and pass into the plant.
Step 3 – Screening yet again by creativity, exaggeration and other hard things
Now, hopefully, our list is quite a bit shortened. It is doubtful at this time that there is but a single item remaining. Now, we must move something. This is the hardest step of all. It requires creativity to know which thing can be moved most easily. By move I mean bending the shape. An example of bending the shape easily would be to take the coating roll out of level (if coat weight responds to that move). Alternatively, we could make a temporary lane of higher basis weight easily enough. On the other hand, changing the shape of a dryer is difficult. The number one trait of a good problem-solver is laziness. They move the easy stuff rather than the likely stuff which presumes you already know more than you do about what is likely. Now that we have found what can be moved, we need only ask how far to move it. The answer is as far as you can without breaking the machine. Breaking the product is not only allowed, it is what we need to do. It is usually much easier to make a problem worse than to make it better, even though both would lead you to the same conclusion. We try moving something enough to clearly see a response without the discipline of long timid trials and statistics.
I work on hundreds of problems in a variety of industries and only put a handful of problems on my personal ‘run and hide’ list. “Baggy webs” is on that list. The reasons, as we have seen, are many. Lacking trustworthy measurements is one. Having faith in close observation is another, especially for those with a scientific bent. But the hardest is simply the creativity and bravery that is needed to identify the offending element. Nearly always we are going to break something, such as the web, in order to get the answers. Even then, the answer may be unpleasant. What if, for example, you found that baggy edges on the paper machine were caused by unconstrained drying? This would be in the nature of that process to make edge bagginess for which a counter measure may not be found. Back to bravery. Take a good swing at it and just do your best.
Roisum, David R. Baggy Webs: Making, Measurement and Mitigation thereof. 6th Int’l Conf on Web Handling, Oklahoma State Univ., June 10-13, 2001.
Roisum, David R. Why can’t I spread out a baggy web? Converting Magazine, Web Works Column, pp 78, May 2006.
Roisum, David R. Must I endure baggy webs? Converting Magazine, Web Works Column, pp 66, March 2006.
Linna, Hannu and Moilanen, P. and Koskimies, J. Web Tension Measurements in the Paper Mill. 1st Int’l Conf. on Web Handling, Oklahoma State Univ, pp 196-209, May19-22, 1991.
Roisum, Dave R. Critical Thinking in Converting. TAPPI PRESS, Atlanta, 2002.
David R. Roisum, PhD, is a well-known authority in the area of web handling and converting, having authored six books and co-authored many more. He is now a principal of Finishing Technologies Inc., providing consulting services to more than 300 clients who convert or manufacture: paper, film, foil, non-wovens and many other materials. Dr. Roisum can be reached by e-mail at: DRroisum@aol.com
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