Web Handling & Converting Blog

Humbled by Woodchucks and Web Handling

I have a half-century of experience with nature. I grew up on a farm, hunted/fished, have grown 1,000 different species of plants at my home, have given invited talks at local Master Gardener’s meetings and have had 500 visitors through my garden on a charity garden walk. I am also the current chair of the landscaping committee of our UU ‘church’ that has five acres of urban property to maintain, one acre of which that is wild.

When some of the members of our church were troubled by a woodchuck, I was asked to get rid of it. This is easy on a farm, throw a smoker down its hole and then shoot it when it comes out. In the city that is a very different matter, shooting is not permitted in the city and UU’s are noted for tolerance and peace. A live trap was suggested. I dismissed the idea under the assumption there is little motivation for an animal to climb into a live trap to get food when they already have an acre to feed on. One of the older ladies on our committee begged to differ; she had already caught a woodchuck on her own property in the summer using lettuce and carrots as bait. I never figured I knew everything and am constantly reminded/humbled by what people know that I don’t.

The same thing happens in web handling. Nearly every plant visit I learn something new from most of the team that I am working with. The most common learning is that my knowledge/experience is both incomplete and, in some cases, incorrect. Whether it is from operators or little old ladies, I am constantly humbled and, not on a few occasions, humiliated. As the 20th century Japanese Zen master Shunryu Suzuki put it, "in the beginner's mind there are many possibilities, but in the expert's mind there are few."

The Best SOP Ever

The cleverest SOP’s I’ve ever seen in my travels to something like 1,000 plants was in a printing department. It had the SOP’s with the signatures of the lead operators on it blown up into 25-foot tall banners that were hung on one wall. No excuses for not knowing and following the procedure when your signature is one foot tall and on the wall.

Steamed

Why is it best to cool a hot web before running it through a steam box?

Steam boxes are used for a variety of purposes such as controlling curl, adding moisture back in to replace what was lost in drying and so on. Steam boxes are fairly common in papermaking, paper converting and in other industries such as textiles.

The layman often draws the wrong conclusions when they think of a fancy steam iron where ‘steam’ aids getting the wrinkles out of clothes. However, when steam is desired it is usually added as room temperature water and then brought to high temperatures on the clothing by the hot iron. Laundrymen will often, quite effectively, sprinkle the cold clothes with cold water prior to ironing.

The chemist or physicist may also get the wrong impression because they know that the reaction rates of many processes speed up as temperature rises. In fact, some reactions will not proceed unless temperature is high enough (consider ignition as just one example).

However, these visions are not correct when it comes to steam boxes. Think about it like this. You want to get water into the web and are using steam instead of a liquid to do so. (Alternatives are to ‘coat’ liquid water onto the web with any number of coating methods such as a Dahlgren coater, treater with air knife, anilox rolls and so on.) Consider what happens if the web is warmed up, really warmed up, to say boiling. Then the steam cannot even get into the web because it is driven off by the boiling hot temperatures. The converse is what makes water get into the web, cool the web so that it more readily accepts steam. Contrary to common sense, but this is how the world works.

Name that Defect 03 - Core Burst

This defect mostly plagues newsprint and LWC rolls. It is extremely complicated, something like earthquakes, and as unpredictable. Thus we can only offer an ‘executive’ review here (perjorative intended as the devil is always in the details). Further details can be found in one of the few complete articles ever published on this subject, Nip Induced Defects of Wound Rolls. Unfortunately, this is one of the few white papers I’ve written that was not published by the TAPPI Journal so you will have to download this from my website.

Even though we are not able to predict them we can say a lot about the factors that are involved. The problem is seldom a ‘winding problem’ because it almost never has a winding solution. Even though in the comments below Catherine correctly told us to wind tight (including using over-dried cores which when taking moisture from the air to reach equilibrium will create much larger pressures that can be obtained by mere winding), this is not going to reliably get you out of deep do-do with your customers. This is only to convince the paper mill and their customers that they need to change their product. In the simplest terms possible, this defect is caused by carrying too much roll weight on too small of a core. (Other risks are also geometrical customer design choices, wide rolls. narrow core chucks and poor unwind design.) The best analogy is overloading tires, such as putting golf-cart tires on a pickup truck. If the tires don’t blow immediately, they will walk on the rims as the truck moves, shredding in the process. Most of us will have observed similar behavior on the tires on our bicycles if under-inflated; the tube walks, often tearing out the valve stem. Thus, this is not a torque or tension induced defect contrary to the comments below. You could cause the defect by merely rotating a core supported roll with the tail taped down. While the geometrical solutions are strongest, chemistry also should be considered. With newsprint, you might be able to turn this defect off like a light switch by backing off the calender or switching clays (natural to calcined) or dumping in a few pounds of chemicals in the wet end. (LWC and SC chemistry is not as well known). So here is my not-so-abbreviated offering.

Winder operators: wind tight at the start (using all of the TNT’s) on over-dried cores to convince everyone that product design solutions must be made, then ignore management as you have already done all that can be done. Winding machine builders: duplex winders have contributed to the problem because they can wind rolls so large they can not be unwound. Papermakers: consider calendering and chemistry. Printing customers: don’t ask for large diameter rolls on small cores and then blame the supplier for your poor choices. While you are at it, redesign your unwinds to load the belts from the bottom rather than the top. All: an honest dialogue between customer and supplier is the only way we are going to make progress on this terrible disease. Suppliers who are afraid of their customers and customers who don’t do their homework are barriers to progress.

Weight Induced Telescoping

We have telescoping on materials that have high friction. How can this happen given that the torque capacity of the layers should be more than enough?

The most common type of telescoping during center winding or center unwinding is caused by center torque exceeding the capacity of the layers to transmit the torque. This can be calculated. Obviously, winding/unwinding slippery materials that have large outside diameters and small cores would be most at risk.

However, your type of telescoping is almost identical in the list of remedies but is caused by a distinctly different type of physics. Instead of torque causing the layers to slip near the cores, it is weight. You can verify this by merely spinning a misbehaving roll without unwinding or winding. The solutions, however, are pretty much identical: change the material, reduce roll size, increase core diameter or use side guards.

Dishing in Newsprint Winder

We have dishing on our two drum newsprint winders. What can be the cause?

The cause is Poisson expansion of the roll width (due to pressures inside the winding roll) that well exceeds the gaps between the rolls caused by spreading. On newsprint, the increase in width is on the order of 1mm and gaps tend to be on the order of ½ mm. The name for this specific defect is progressive outward (symmetrical) dishing and is quite specific. This dishing defect common on common axis winders, such as two-drums. It is seen on grades as diverse as tissue, newsprint and LWC. The wider the machine, the greater the accumulation of width that drives the end rolls outward. The greater the number of rolls per set the greater the challenge as well for the obvious reason: more spreading is required.

Thus you have a limited set of choices that physics allows. To reduce the driving force: wind looser, and increase the spreading gaps (especially on the end rolls). To reduce the movement: make sure the end cores don’t dent on the chucks, make sure the hydraulics don’t give way and make sure the framework does not splay under axial loads that could approach 10,000#. Driving force can be measured with hydraulic pressure gages on some chucks. Motion can be measured with dial indicators and/or theodolites (alignment transits). Thus, the entire problem can be quantified so that you know how much each element is contributing so that a best cost-benefit remedy can be applied.

Troughs at the Exit of a Driven Roller: by Jerry Brown

 

It ‘s common knowledge that troughs can form at the exit of driven rollers. The web wants to expand laterally when moving from an area of higher to lower MD tension (reduction of Poisson contraction). While it’s on the entry side of a roller, the web is locked to it by friction. The MD stress doesn’t change and the lateral stress conditions remain the same as they were in the entering span (zero at an aligned roller with steady tension). But, when the web nears the exit of the roller, it comes under the influence of the reduced MD tension and lateral compressive stress develops. As the web enters the free span, it tries to expand but is still restrained at the roller and buckles. You can see it happening in this photo from my lab machine. A stretchy latex web was used so that the deformation would be more visible.

The web is 26 mils thick and the angle of wrap is 155 degrees. The coefficient of friction for this combination of materials ranges from 0.75 to 1.0 depending on cleanliness (wiping the web and rollers with isopropyl alcohol will produce the higher value for a while. In this instance, it was probably close to the 0.75 value.

The web was being pulled at constant speed and the input tension was held constant as the roller drive torque was varied. At low values of torque (entry and exit tensions close to the same value) the troughs didn’t form. As the drive torque was increased, there would be a value where the troughs were deepest. Then, as the torque was increased further, the exit tension (and the normal force that creates friction) would become low enough that the web would do all it spreading while still on the roller surface and the troughs disappeared.

It seems unlikely that troughs like this could turn into fully formed wrinkles, because the material where they could develop is always being moved off the roller. However, wrinkles might develop on the next roller if the span in between were very short. 

Jerry Brown

Name that Defect 02- Crushed Core

The first thing you must realize is that core crush is not a single defect, but rather are three distinctly different defects that share the same name. A similar situation exists in biology where the common name ‘lily’ is used for hundreds of plants that do not belong to the genus lilium (flowering bulbs). Perhaps a better analogy would be a headache that, for example, could be caused by stress or a brain tumor. You better know which case you have.

True, all core crushes all have a similar outcome, the core gives way, and increasing core strength is a possible treatment option for all. However, we should first consider lower cost options, such as changing winding tightness by diagnosing which type of core crush we have. Here, strangely as it turns out, Type I core crush is a tight defect, Type III is a loose defect and Type II is not even a winding problem, it is a process/product design issue. So I offer my short answer.

First, reduce winding tightness (considering all of the TNT’s) for Type I and Type II core crush but increase winding tightness for Type III core crush. Next, else, increase core strength by using a stronger material, increasing wall thickness (usually quickest/cheapest), decreasing diameter or adding tight fitting very strong core plugs for cores shorter than about 10X the nominal diameter.

Further explanation will simply have to wait until my September 2009 Web Works column in the Converting Magazine. If you need quicker answers, the Web101CD also carries the explanation.

Contest particulars. Two winners will be announced in next week’s post where we give a new post for your consideration. At the end of the contest this fall the top scorer will receive a set of all of my books in current publication, a retail value of nearly $1,000, which will be provided by TAPPI PRESS. This will also include my Web101 CD that includes 1,000 slides of instruction in 50 different areas of web handling, winding, converting, critical thinking and related topics, essentially everything I teach. This will also include the one must-have reference in our industry; Web and Roll Defect Terminology book. Second and third place winners will also receive publications.

The first person to correctly identify the preferred name for this defect gets two points. Simply enter your guess in the comment box below. If you use an accepted but not preferred alias you only get one point. The Roll and Web Defect Terminology book will be the final judge on proper naming. A second and quicker route to points will be identifying treatment. Here the most concise phrase, a sentence or so at most, that describes the single strongest treatment (few holds barred) will get up to 5 points. I will be the final judge here and appeals can only be made by citing evidence or literature. This contest is open to all except those who are authors of web handling articles. Good luck!

Packing Peanuts and the Man on the Moon

Materials engineer Edgar Burchard working for BASF invented an alternative to the ubiquitous packing peanut. It was W shaped foam that was manufactured in die-cut sheets. It trounced other packaging materials in performance: transmitting up to 90 percent less shock and four times better cushioning than the peanut. It was also cheaper because it was compact to ship because the sheets have a high packing density. All the user has to do is a mild crumble of the sheets to release the individual ‘peanuts.’ Essentially, better faster and cheaper.

3M bought the design in 2004 and renamed the product the Packaging Noodle. I have used it and absolutely loved it. This summer, five years later, it was taken off the market. Why? Consumers (mostly white collar office people) could not figure out that the blocks broke apart into hundreds of noodles. Just how the hell did we put a man on the moon 40 years ago?

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.