Web Handling & Converting Blog

Calender Applications

Few pieces of web equipment, perhaps only printers, slitter (knives) and winders, have wider application than the calender. The calender is ubiquitous in metals but is called a rolling mill. Here the idea is to squash the hot strip into a thinner but longer web. Since the web can not be taken down to final thickness (either strip or foil) in one step, it is done by consecutive tandem calenders. Calendering is also ubiquitous in rubber but here the product is taken to final thickness in a single step. Calendering is also common in paper. At the high end of pressures of say one ton per inch of width of a hot nip, a fuzzy paper is ironed smooth so that a higher quality image can be placed on it during printing, such for glossy magazine photos. At the low end of pressures a kiss nip will ‘combine’ two plies of tissue together. Calenders are also used to laminate two plies together in converting. Here the process names the machine; laminator, even though it is a machine identical to that used in other applications. Calenders are also used for food products as varied as potato chips to cookie dough.

Despite this wide range of applications, the machine itself and the issues are remarkably similar. Thus people who think outside of the box will think outside of their own industry. I was once called into a food giant company who new product/process required a calender that needed nip loads far higher and yet maintain tolerances far more precise than anything they had ever seen in their own industry. In fact, higher than seen in most industries. They called in experts from the rubber and paper industries (yours truly). We taught them what we knew. They then took their own expertise from the stone age to the space age in one step by borrowing most of what they needed rather than taking the decades it would have required to ‘reinvent the wheel.’

Numerical Nightmare

I had it again last night. The most common version was the fear that someone found an error in my thesis and they took my doctorate back. Last night it was the final exam for Calculus III and I wasn’t ready. There is little cause for this because many have combed through my thesis and only found one tiny subscript error affecting just one page in the appendix. Still, the math was pretty stiff in grad school, especially for someone who had been out in the workforce for a bit before returning to compete with foreign students, especially for someone who was also working three quarters time (teaching undergraduate engineering), especially for someone who also had a wife and small child to care for.

I recall trying to pick what I thought would be the easiest advanced course required to satisfy the MS degree in Engineering Mechanics: elasticity. I audited it once, and then took it a second time and dropped it at the nine-week point. However, I had to bite the bullet and stick it out the third time else I would not get my degree for another six months, if ever. The version of elasticity taught at the UW Madison used tensor calculus instead of regular calculus. There was no textbook. The ancient prof filled the blackboards with equations several feet long. I didn’t follow a thing from day one. There was no homework. There was no test, except the final exam. I finished early. Not because I knew the answers, but rather because I didn’t even understand the questions. Somehow I passed.

I swore I would never do mechanics again, especially elasticity. Wrong. Within months of graduating, as a Manager of Winding Research at Beloit Corporation, I began to review the past and present winding models and try some of my own. Elasticity based mechanics. Later my doctorate coursework and thesis focused on elasticity based mechanics. Still, I have nightmares.

So, I will spare you the math nightmares. Enjoy the gold standard of web handling instruction WITHOUT the equations. (However, for those of you so inclined to the maths, I will tell you where to find them.) Join me in the last AIMCAL Converting School classes of the year on Web Handling and Winding on October 13 – 14 in Dallas or November 17 -18 in Brussels.

Know The Rules

The first time I was called into the metals industry was for a web break problem was well over a decade ago. This was the first experience I had with webs with over 100 PLI strength being broken. (Since then I have had numerous calls for web breaks and damage on some very stout webs.) My host and I were both hesitant bringing me into an industry where I had not trafficked before. However, we both had web-handling backgrounds and knew in our mind, body and soul that the laws of physics for breaking of tissue paper and aluminum can stock must be the same. I was considered a minor expert in paper breaks having worked on, taught about and published articles on the subject. So, we both decided to give it a chance.

Guess what? Nearly everything I learned about troubleshooting paper web breaks applied directly to troubleshooting web breaks on an aluminum line. I was called back into that metal plant regularly thereafter.

Moral: learning the rules that apply to all may be more useful than knowing the rules that are merely ‘known’ in your own plant, materials or industry.

You have one more chance this year to learn these rules in a public class. Mark your calendar for October 13-16 in Dallas for AIMCAL’s Converting School where I teach Web Handling and Winding.

How Many Knobs (do you want to give the night shift)?

I was asked to find out why a startup on what should have been a very mature winding process went so badly. One reason was controls. I counted no less than 50 knobs and dials on the machine. If one included keypads and program parameters accessible by the operators, the count would be on the order of 100. If you included control parameters buried in the several custom programmed black boxes the count would be closer to 200, almost none of which was calibrated or documented in any way. It turns out that several of the most prominent knobs actually did nothing at all (to the winding process).

I was asked to help several packaging film rewinding facilities to help determine what is the best settings for their winders. These German made winders had no less than 14 operator accessible parameters that affected winding tightness. In one of my client’s plants, they were struggling with a half dozen defects on the 300 grades of materials they processed. Do you know how many combinations of 14 knob settings you could have for 6 defects on 300 different grades?

Do the plant’s process engineers know what these knobs do? Do even the design engineers know what you should do with all of these knobs for any particular defect?

How many knobs do you want to give the night shift?

What Answer Do You Want?

I was once called into a nonwoven plant to troubleshoot roll width growth during winding. After careful study and measurement I concluded that this was a result of well-established laws of winding physics (interlayer pressure makes the roll wider due to Poisson expansion). I wrote a report referencing the literature and told them to wind as loose as possible and even then rolls would be wider at the bottom. A year later I got a call from the same plant (different people) complaining of roll width growth. On the phone I explained that I had already worked on that and had wrote a report and offered to send it to them. That was not good enough; the problem was quite serious and I had to come out and help. After careful study and measurement I concluded that this was a result of the laws of winding physics (interlayer pressure makes the roll wider due to Poisson expansion). I wrote a report referencing the literature and told them to wind as loose as possible and even then rolls would be wider at the bottom. Shortly after I heard through the grapevine that the same plant called in another consultant to work on roll width growth.

The Operator's Craft

No one likes the idea of being replaced by a machine.  This includes operators whose craft made it possible to run manual finicky machines.  Upgrading without considering the human element can backfire.

A story was told to me by a brake supplier who upgraded a manual regulated brake on an unwind to modern load cell tension control.  An old operator distrusted this new equipment that rendered his tension control craft obsolete.  Instead of reading tension by tapping on the web and controlling it by adjusting unwind brake pressure, the load cell computer controller did almost everything except select the tension.  He claimed that the new computer wasn’t as good as he was.

The supplier recognizing what the real issue was and addressed the operator’s concern.  He told the operator that the main tension setting knob was good enough for most operators.  However, for some who required even more control he would provide a vernier knob to fine tune the coarse main setting.  As you might have anticipated, the vernier knob was not connected to anything.

Stop Whining

There is a big difference between whining and complaining.  Whining is just futile noise.  Complaining is much different.  Complaining is rifle point focused.  Complaining is knowing what is wrong, who is responsible, how they are going to fix it and when they are going to fix it.

For example, almost any discussion of baggy webs would be whining.  Rarely does anyone know how to troubleshoot it much less fix it.  Baggy webs caused by stretching over gage bands may be entirely different.  If you are fortunate enough that roll hardness measurements are a useful predictor of that defect, such as is the case in at least half of all film grades, then you can complain.  First, find out the roll hardness difference across a roll which causes unacceptable bagginess.  This is your threshold of pain.  On the first occurrence document the offending roll and return it for credit.  On the second occurrence document the offending roll and quantify the maximum allowable roll hardness variation and request/require that the supplier check rolls before they are shipped.  On the third occurrence send back the entire truck and ask them to try again.  If they are not successful in screening defective gage varying material you will take your business to their competitor.

Any discussion of wrinkling to maintenance will fall on deaf ears as well.  They have no idea that crooked rollers can directly cause wrinkling.  However, if you can point to the specific roller that is crooked and which is kicking off wrinkles at an angle and show that it is out of parallel by pi tape or other measurement and that the misalignment value exceeds commonly published guidelines; then you can complain.

The paper industry has turned complaining into a high art form.  I recall the story of a coater upgrade/rebuild that failed to improve coating.  In fact, it made it worse.  After weeks of struggling with the new equipment the mill took it out and put the old coater back in.  They put the new coater on a flat bed semi-trailer truck.  It was so large and heavy it barely fit.  They took it back to the supplier’s town.  They went to the company president’s home and craned this huge piece of equipment onto his lawn; a one of a kind lawn ornament; and drove away.  Now that is complaining!

The Best of a Sorry Lot (of controls)

Customers can be so cruel.  When I worked at Beloit Corporation as a winder designer, there were three major machine suppliers to paper mills.   When the mills went shopping for new equipment, they would torture the machine builder’s salesmen to get the truth.  Every sin past and present would be intoned with an appropriate number of lashes meted out.  When the mills finally did place an order, they grumbled that they went with “the best of a sorry lot.”   I thought that was a bit overstated because this equipment was rock solid with a century of history behind it and it costed something like $5-15 million dollars and it was highly engineered.

Now I am sometimes on the other side of the fence when working with converters who are buying the small duplex rewinders that are perhaps the most common type in the world.  Now, however, I KNOW we are always stuck with the ‘best of a sorry lot.’  These controls are the worst piece of human engineering and always have been.  Tension as defined by differential shaft starting pressure and differential shaft ending pressure, and shaft overspeed ratio (a do nothing knob) instead of in proper units of PLI.  Nip pressure control nonexistent or at best in improper units of PSI of air instead of PLI of nip.  Taper instead of the four point method.  And the worst sins of all for controls: cryptic units with no conversion charts or calibration procedures to get in line with the rest of the world.

Human engineering?  Not any humans I know!  It looks like they took the I/O devices and put them in a bushel basked and tossed them up on the panel and where the stuck, there they were.  Computer screens made them even worse.  Pathetic operator’s instruction manuals that in many cases read like engineerese translated from Farsi into English using Babel Fish website.

And don’t get me started or I’ll tell you how I really feel …

Slack Web Silliness

Certain industries baffle me because they break all the rules of web handling by going slack web between sections.  Textiles do this when pulling out of a fan folded box or from a J-table.  Both textiles and the (paper) forms industry use slack webs between the unwind and the printer or other converting machine. 

Silly people.  Web tension in the unwinding roll is lost as it is unwound and then re-established on the next roller.  Position which was established on the unwinding roll is lost as it is unwound and re-established on the next roller.  Worse yet, the machine carries redundant parts because an extra motor and often extra guide are required which increases equipment costs.  Worse yet, the machines have poorer performance by re-establishing what was already there (tension and edge position).

Go figure.  Yet if it was good enough for grandpa, it was good enough for their new machines despite the fact that almost all other industries hold tension and position at the unwind and can do it under much more challenging situations such as on materials as weak as tissue, at widths from string-like to 40 feet wide and at speeds from crawl to exceeding 10,000 feet per minute.

Roll Hardness Safety Warning

Roll hardness as measured by an impact tester is one of the best tools we have for measuring web/roll quality.  Gage bands too small to pick up on the web, stack by the thousands in the roll to form hard/soft spots which can then generate defects.  Roll hardness has been found to statistically correlate to many defects and is so trustworthy that some plants reject rolls solely based on hardness variation.  Roll hardness works great on almost all paper except extremes of hardness such as tissue and coated food board and works on many grades of film and other materials.

However, many operators have been hurt by impact testers such as the Rhometer, Schmidt Hammer and perhaps the Parotester.  (See blog post of 6/27/06 and my new book Winding: Machines, Mechanics and Measurements for more info.)  I thought it was an urban legend until it was confirmed by students in many of my classes.  Operators got bored/creative and got into a contest to see who had the hardest head.  The first person to try won.