Web Handling & Converting Blog

Dual Unwind Paths

Dual unwind paths should be standard. By this we mean that you can either unwind from the top of the roll or, by flipping the roll around, from the bottom. Even if you never think that you will never need to invert the web you should be able to do so any way. It gives you the flexibility to do future products not yet anticipated. It gives you the ability to do some troubleshooting. Finally, it is trivial to do in almost all cases if you think ahead, so why not?

Wound Roll Diameter Measurements

How can we measure wound rolls on the fly?

Continuous roll diameter measurements during winding and unwinding are essential for a variety of functions such as stop-to-diameter (before the unwinding roll runs off the core or the roll builds into the frame), roll structuring and drive inertia comp. No self-respecting winder or unwind lacks this essential sensor.

Roll diameter can be measured on the fly most crudely with a pivoting arm riding on the winding or unwind roll (accuracy worse than 0.1”, very maintenance intensive and in the way of the operator). Winders with layon rollers can infer diameter by layon-roller position as measured by linear pots (crude) or Temposonic probes (accurate and bullet-proof but very expensive.) You can also measure diameter ultrasonically (accuracy around 0.010 if set up well, however may not fit on some machines or sense some fluffy materials). However, the gold standard is the dual tach method. Here you can use sensors that already exist on most winders and unwinds to compute diameter very quickly and very accurately (something like 0.001”).

Roll Length Calculator

Can’t I calculate (predict) roll length using the formula L=Pi(Do^2-Di^2)/(4t) where the D’s are diameters of the roll and core outside and t is the thickness of the web measured in the test lab?

Yes you can, but …. The length that you measure will be under tension (footage counter) while the customer will usually define length as the number of units times the length of a unit (such as sheeted paper) under no tension. The tension inside the roll is yet different, also changing the definition of length of the web. Worse yet is that caliper or gage is measured under pressures quite different than the stresses inside a wound roll. For worse yet is that the web doesn’t stack the same in a wound roll as a single ply. (Smooth materials have air between the layers, fuzzy materials nest). For these and many other reasons equations like the above will only get you to a bit better than 1% accuracy for stiff materials (paper, foil) but could be more than 10% in error (discrepancy) for elastic products such as nonwovens, rubber and tissue.

A reader’s digest version of this discussion is found in my Converting Magazine Web Works article of Jan 2009.

A much more complete discussion is found as a chapter in my book Winding Machines, Mechanics and Measurements published by DEStech and TAPPI PRESS and available from AIMCAL's Converting Portal.

Telescoping During Curing

We have had a recent rash of telescoping in our hot curing room that began in November. The structure is a 3 ply poly foil laminate. Could the cause be related to a decrease in humidity?

I would be prepared to believe that temperature and/or humidity can affect the propensity to defects and other troubles, they are the strongest forces in web handling besides the nip.

Yet, I know of no connection between humidity and hygrophylic materials other than tangential concerns like sweating of cooling rolls in the summer and reduced static electricity (a extremely weak force) in the winter.

A more promising approach may be to look at known causes of telescoping in storage. It is very likely that the movement is driven by winding energies with a gage variation imbalance, but ENABLED by the lubrication of the yet-cured adhesive. The tiniest change in adhesive could make humungous changes in results.

Noisy Rolls

My printing press customer is asking why the rolls make noise when they are unwound. Do you know?

A very few grades of paper, LWC, news and so on, can make a muffled popping sounds. They can do this during core supported winding (reel spool) or unwinding (reel spool or printing press unwind). It is easiest to hear at thread speed when other machine noises are comparatively lower. This noise is caused because the ‘nip’ above the core causes a shear stress. By ‘nip’ I mean the pressure between the core and the inside of the roll caused by simple roll weight (as opposed to the more commonly thought of external nip.) This shear stress is reversing, one direction on one side of the nip and the opposite on the other side. What happens is that the paper layers just above the core do a micro slip back and forth as it is rotated. You will note that the noise decreases as the roll is unwound and will eventually disappear. This is because the cause, the nip caused by roll weight, gets below a threshold and movement stops.

On some grades, the paper ratchets more in one direction than the other and does not completely return. A J-line is one way to confirm this irreversible slippage. This is the cause of crepe wrinkles and core bursts. In most cases the noise is innocuous because the layers reverse completely. However, in the case of crepe wrinkles you are actually hearing the defect form.

There are equipment causes of noise as well. The most common is the lugs of expandable core shafts which work in and out a bit under the same kinds of loads, the internal nip of roll weight on shaft. Other causes are the clutch and other elements in the drive system.

Now the more important question. Why is your printing customer asking about noise? It is not a hearing protection consideration. I also seriously doubt that this is pure intellectual curiosity. They probably have issues that have not been clearly articulated (could be just about anything at this point) and they are wondering whether the noise is connected. Better do some digging.

Poor Winder Starts IV

As opposed to continuous winding, most offline winders will accelerate from the start at the core just when there are all kinds of other things going on. How do you separate a drive/tension problem from all of the rest? Very simple: accelerate/decelerate somewhere else in the roll. Find a mid-diameter and crank the speed pot. If you see nothing on the side, no offsets or rings there, then the drive is in perfect condition. If not, there may be room for improvement.

Poor Winder Starts III

Make sure that the winder nips uniformly across the width of the core. On two-drum winders, the cause will be often be core diameter/straightness variations or misalignment, particularly of the core chuck within the pocket. With duplex winders the problem could be misalignment or cores again, but now also look at core deflection. Nip uniformity can be checked by pulling on strips of paper that are nipped and judging force or by using a low-pressure nip impression paper.

Poor Winder Starts II

Make sure that the winder has zero side-play on the drums, rider roll and most particularly the chuck/shaft. Typically looseness here will show up as a somewhat quick move to one side though it may be more random and cause roll edge roughness throughout the winding cycle.

Poor Winder Starts I

Poor starts are an outcome, not a cause. Think of them as like a tummy ache; there could be 100 causes anywhere from eating too much to having a tape worm. You as a winder doctor must correctly diagnose the specific cause/causes of poor starts otherwise you waste time and cause frustration.

Make sure the operator threads the tail taut, square, dead flat on the core and in the equilibrium position else the web will wander back and forth a bit until it finally finds home. You can tell that this is the situation because there will be a statistical difference between extremes of operators and especially between the first set off of a parent roll or with rethreads and subsequent sets.

Continuous Defect Program

Many managers subscribe to continuous improvement programs of one sort or another. I subscribe to continuous defects. Let me explain. If you have a defect such as the crepe wrinkles above, air buckles on film, starring, etc, etc, I want to see them uniformly distributed and continuous across the width. If you have continuous defects there is a good chance that you only have one problem. If, however, the defects are not uniform across the width you almost certainly have two or more problems. The first is the crepe wrinkles, air buckles etc etc. The second is no less problematic, a non-uniformity of machine or material, most probably gage. In fact, some problems such as corrugations cannot exist with uniform profile and are solely the result of variation across the width. If you have problems that come and go with time, you not only have the problem itself (whatever that might be), but also MD variation in machine or material. Let’s hear it for continuous defects !