Web Handling & Converting Blog

If I had a Hammer

New beginnings. What would I do ‘if I had a hammer?’ I would hammer down every gage band on every paper, film, foil, rubber, nonwoven and textile manufacturing line. While I was at it, I would hammer down the coating streaks, thick spots in extrusion and even printing ink. I would make webs everywhere as level as needed to satisfy the fussiest customer of all time, the wound roll.

Why? Well, I know that one of the top three problems facing (most of us) in the web industry is wrinkles and related defects. I know that bagginess is a major cause of wrinkling. I also know that gage variation is almost always the cause of bagginess on film and foil and is a major contributor elsewhere. I also know that gage variation causes many other quality issues and converting problems. It may be the largest root cause mechanics of troubles in our industries.

I would NOT start out my level pogrom with fine profile measurements and automatic gage controls. I would start so level on every component that these ‘patches’ to the root problem would not be needed. Indeed, some profile controls are in fact destabilizing and make the problem worse than it would otherwise have been. The reason people don’t realize this is because they paid big bucks for these systems figuring they would be helpful and never turn them off to find out for sure. The paper people who have the most expertise and experience have long known that even in the best of circumstances the operator usually has to take manual control over certain actuators (usually the ends of the machine) because she can do a better job than the computer.

THE Big Problem With Automated Data Acquisition

THE big problem with automated data acquisition is that it is almost always time-based sampling. However, our internal rejection (usually based on top of roll samples) and customer returns (roll # such and such) are lot based. Until data acquisition is constructed from the ground up to be lot based binning of time based data, our analysis will be slow, clumsy and crippled. (For the database aficionados such as myself this means a relational database instead of a ‘flat filer.’) If you don’t think this is the case, merely answer the question “Are products that are on the low end of acceptable basis weight made last month more likely to cause customer complaints?” and you will see just what a mess the typical systems are. Days of cut and paste before you can answer the simplest question.

Segmented Tension Rolls and Baggy Webs

Baggy and cambered webs still give us as much trouble as ever. The world is still looking for an accurate and affordable tension profile measurement that could be installed on web manufacturing and converting machines. While there are a few rare devices that are trustworthy, they tend to be either researchy (i.e., not mill duty) or expensive (as expensive as a house) so that only paper or steel people could afford them.

In the mean time, we can do what the big paper winders have been doing for some time. That is to measure loads at individual load cells on a segmented roller assembly. This gives you a crude and chucky picture of how variable the tension profile is for the web that you just made or just bought. A converting version of this device is described in the Converting Magazine. I have not personally tested this particular device. Even so, I can tell you that the end segments are enormously complicated by the web being narrower than the roller. The algorithm to do this web width correction correctly is far more difficult than you might think. I hope they got it right and, as we always expect from machine builders, that they have given us extremely detailed calibration procedures to make the readouts trustworthy and repeatable.

Baggy Web Rising Star

I get ecstatic on the all-to rare occasion when academia chooses their projects wisely because they: have widespread importance to the industry and where significant applicable/practical results can be obtained in a reasonable time frame, such as a year or so. Cecilia Land and her professors have certainly done this with their Modeling of bagginess due to storage of paper reels with ridges published in this month’s TAPPI Journal. Here they model and measure the amount of residual strain (bagginess) resulting from winding of gage varying material, in this case linerboard paper. (My apologies to the authors for oversimplifying the description of the work but, hey, I grew up on a farm where we didn’t have time for unnecessary complexity.) The bagginess problem is certainly important. It is right up there with manufacturing defects, wrinkling and winding as a top concern for the web industries. Moreover, winding of gage varying material is the overwhelmingly large root cause for bagginess in film and foil and is a major factor in most other materials as well. We know this to be the case even if these gage variations ‘fly under the radar ‘of conventional lab testing methods and online gauging/scanning instruments. (One wonders why we put so much effort into measuring systems that don’t connect to our major concerns.) We know that the gage-variation-to-bagginess connection is valid because we can sometimes measure these gage band causes directly and note the corresponding baggy lane of trouble, we can measure/infer them from other measurements most notably wound roll hardness or wound roll diameter variations, and we have modeled gage variations and the resulting stresses using 3D winding models. Again, congrats for a nice contribution to a longstanding problem.

Name That Defect 10 - Gage Variations

Those of you in-the-know, know that the winder person can usually read gage better than even the people who made the material and those responsible for testing and qualifying the material. We can also read gage variation far faster, usually at a glance. We can also often read gage patterns cheaper than capitol or labor-intensive scanners and test labs. So, here you can get some practice.

This roll is full of gage related defects. Name the three narrow width defects and the gage pattern that causes each of these three. Also, name the one global, large scale or overall gage pattern that this roll must have and why you know this must be the case.

The three narrow width defects are: ridges (a.k.a. gage bands), valleys (a.k.a. low spots) and corrugations (a.k.a. ropes). Corrugations are a subset of valleys but that in addition to merely having a narrow band of relative thinness also has wrinkles at an angle.

The roll must have a global carrot or taper shape. The reasoning is thus. Corrugations are a narrow low spot with high-low-medium gage thickness pattern. The angle tells us that the two sides of the low spot must be different as opposed to a pure valley with identical height neighboring material. The consistent angle of the many corrugations in this photo show that the high-to-medium or medium-to-high trend is consistent across the width of the rolls meaning that the web is, overall and generally, ever thicker or ever thinner as one proceeds from left to right across the width of the roll. (We can tell the direction only by knowing the winding direction that is not shown in this figure because we can’t see the tail.)

Come to my web and winding schools so that you can learn to read gage at a glance.

Name That Defect 09 - Tin Canning

This defect, as our commenters below correctly state, is called ‘tin-canning.’ Unfortunately, many people also (incorrectly) use the word ‘tin-canning’ to refer to ‘corrugations’ while others call ‘tin-canning’ by the name ‘corrugations.’ Not to confuse the issue but the words ‘ridges’ are thrown about for these and other defects. This is why we have the Roll and Web Defect Terminology book, so that everyone knows what you are talking about. ‘Tin-canning’ is defined by a near-regular spacing of MD ridges, while ‘gage bands’ are irregularly spaced MD ridges, while a corrugation is an annular band in which the ridges inside are at a slight angle. There is nothing at all connecting these defects in terms of mechanics except that they are all made by or exacerbated by gage variations and that they are all tight defects. How they take their shape is an entirely different matter. Their remedies are also somewhat different beyond the obvious loosen the rolls by decreasing the TNT’s (decrease tension, nip and torque; increase speed).

This defect, as far as I know, is confined to wound rolls of thin film. (Though in principle it could happen to other materials because winding is determined directly by physical properties. The role of chemistry is quite minor here; chemistry merely influences physical properties.) Tin-canning is caused by constrained expansion. Specifically, web just under the surface wants to expand laterally due to its MD compression (caused by the hoop stress of winding physics). This web just under the surface can not expand laterally because it is held back by layers above. So instead of taking its extra width by going out, it does so by going up into repeated buckles. Those of you familiar with wrinkling can immediately recognize the near-uniform spacing of MD wrinkles as a fingerprint of expansion-induced buckling. So let’s get beyond the complex physics and dive into practical stuff that everyone should know and remember.

It is a tight defect as pointed out by the commenters, thus loosen using all of the TNT’s at your disposal. Katherine adds using a slip agent, which just might have a bit of truth to it. She also describes a similar appearance on the top of slender wound rolls that sag when supported by their ends. The buckling outcome is similar, but it got there not by winding tightness but rather by simple beam deflection. The difference is obvious; tin-canning goes all the way around the wound roll while the sag defect (this has no official name) shows up only on the top of the wound roll and then only in the middle.

What all of the commenters miss, as does most everyone else in the film and other industries is a more complete discussion of tightness. While winding (via the TNT’s of tightness + material properties) affects the average tightness, the distribution of tightness is determined solely by gage variations. Thus, merely leveling the gage will smooth out the tight and loose areas and thus reduce tin-canning in the high gage area without doing a thing with the winder. That is why you see the tin canning in this picture showing up at the 10%, 80% and 90% of the distance from the left edges. Those are the high gage areas. In fact, gage variation is usually much more responsible for tightness and looseness than is the winder itself. Thus leveling the gage will help this defect, as it also does for ridges and corrugations, but for entirely different reasons. Also, while oscillation would help enormously for ridges and corrugations, it will do next to nothing for tin-canning because the gage bands are too broad, much wider than ridge spacing. The exception is bubble oscillation where we are not limited in ‘stroke.’ The next defect will give you more practice in reading gage.

In short, the ‘tin-canning’ defect is recognizable by even spacing of ridges and is caused by constrained expansion. Wind looser, level the gage.

A lesson on testing from musicians

Dave's last entry reminded me of a related experience.

As a physicist and a consultant, I've spent a lifetime involved with measurements of many kinds. One of the most unusual involved violin bows.

I was contacted by the owner of a very small company in Chicago who said he needed help with an ultrasonic measurement. He explained that their main product was violin bows and they needed a replacement for an ultrasonic test instrument that was no longer manufactured. It seems these bows were made of a rare, expensive wood (I've forgotten the variety). They received the wood as unfinished logs. Some logs made good bows; but most didn't. Apparently, this had been a big problem since the days of Stradivarius, until someone in the 20th century discovered, through years of trial and error, that measuring the speed of sound in the wood helped them choose the best logs.

Wrinkling Due To Anisotropy?

I have a new material that is more anisotropic than the previous material. It also wrinkles more. Any connection?

Many of our materials are anisotropic yet do not wrinkle. Perhaps the best-known exception is when the material is skewed. Skew means that the principle axis of the material is not precisely aligned with the MD. Think of the grain of poor quality wood that may be crooked with respect to the edge of a sawed board. Materials at the edges of cast film lines, all tentered films and all paper machines exhibit this property to one degree or another. This makes the edges more wrinkle prone.

However, I believe the reasons for your wrinkling may be more mundane and should be figured out. Rollers (alignment and diametral variation across the width) and web (bagginess, possibly at the local level) are the most likely reasons provided that there is absolutely no hygro-thermal expansion whatsoever.

A roller does not have to be kicking off a continuous wrinkle to be a problem. If slightly misaligned, a crooked roller may be ok on a perfect web. Conversely a slightly baggy web can run on a perfect machine. What might not work is a slightly baggy web on a slightly misaligned machine. In other words, insults add up.

After you have made every reasonable effort to identify and correct the root cause, then is the time to pursue effective spreading. Effective means the right type (not too much, not too little) and in the right location (where the wrinkle is being initiated; one roller upstream won't work, one roller downstream is too late)

Did Blown Blow It?

A starring pattern that differs on the two sides of the same wound roll is often due to a profile problem in manufacturing; i.e. gage variation. (Other causes are usually easy to distinguish such as an extremely crooked winding machine that would yield consistent results and dropping one side of a roll). So let us suppose you have two materials bonded together: film A and film B. A working theory might be that one of the two web materials or one adhesive layer is uneven, causing the roll to collapse (star) in a local area where the relative gage is ever-so-slightly smaller. One other observation is that the starring is not the classic pattern involving the bottom of the roll. Rather, it is vividly banded and these bands vary in width and position for every roll and are different on all sides of all rolls within a set. Further let us constrain the problem even more. Let us say that this is a medical grade so that substituting raw material supplier is not permitted. (Swapping materials is a strong problem solving technique because if the problem moves with the swap, it is that material). Who is at fault?

How Much Gauge Variation is Too Much?

Often, less than you can measure on the web. Gauge variation that can cause the web to be stretched into bagginess might range from less than 1% to occasionally more than 10%.

Alternatively, we may use the wound roll itself to measure the accumulated variation. The wound roll is often the most sensitive measure of gauge variations. More importantly, the wound roll is often the fussiest customer for gauge variation. More about this topic can be found in my 2001 paper ‘Secrets of a Level Process and Product’ that can be downloaded from my website, Roisum.com, as well as through the organizations that hosted the talk such as AIMCAL (members only) and TAPPI.

You may not, however, need to go measuring stuff. If the finished rolls are inconsistent across the width, then it is very likely that the web is as well. If the winder operator complains that the web runs like crap, it may be that the web is crap. If the end-use customer is complaining that the roll looks and runs like crap, it is possible that the root cause is in fact that the web is crap. More specifically, the web has a profile problem.