Web Handling & Converting Blog

Name That Defect 11 - Crepe Wrinkle

This begins the first in a series of user submitted defect descriptions. Send me your pic and problem description and I will do my best to figure it out or tell you where to go (for more help). I promise to sanitize the description so thoroughly to protect the guilty.

We have many crepe wrinkles as well as defects such as shown in the picture above. The material is slippery uncreped tissue. We are winding this on an old two-drum winder that had been used for linerboard that has a rider roller with an 8 PLI minimum load. Could these defects be associated with baggy webs/lanes? - Anonymous

Possibly, bagginess already causes enough troubles on just about any material, especially when wound with a nip. However, lets start with what we can know for sure and then proceed to best guesses. This defect is certainly a web quality defect in that the locations of the defects are places where the web is different than its neighbors or the mean/average. In other words, the web rather than the winder mostly owns the locations of the defect. That is not to say the winder does not also have a role to play. It is most certainly also a winding defect because these defects show up after winding.

You say you also have many crepe wrinkles. I am also going to guess that the pictured defects are in fact just another way of a crepe-like behavior to develop, call it an incipient or baby crepe if you want. Certainly the picture resembles a partial core burst such as seen just above the unwinds on printing presses supporting large roll weights (if you cut the defect down its centerline to make an 'edge' much like a core has an 'edge’). The picture also resembles even more closely a 'caliper shear burst', a rare and debated separately listed defect in Roll and Web Defect Terminology. What connects all of these supposedly different defects together is mechanism, interlayer slippage under a heavy rotating nip.

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?

Cantankerous Coater-Laminator

Let’s do some problem solving. Let us say you have two different substrates that you bond together to make a structure. The problem is bagginess on the front side first seen after unwinding the now laminated wound roll. The problem is perennial. This means that while it may vary in magnitude, sometimes even disappearing from complaint, it stubbornly likes to misbehave just inside the front edge of the composite web. On close inspection you note that the diameter of the roll is ever so slightly larger there. The easiest way to see this is with a straight bar, it rocks over the tiny bump. You may even notice a relatively higher roll hardness reading via Rhometer, Schmidt Hammer or ParoTester. So whose fault is it?

Does Anybody Know if Skew ...?

Does anyone know if a skewed but otherwise uniform material will track straight through a machine?  This is especially important for the humungous paper printing industry.  Here, we know that paper on the edges of the paper machine will be skewed outward by as much as 20 degrees.  Paper people call skew fiber angle while mechanics people call this general form of anisotropy by the name orthotropy where the principle axis of the material (fiber angle) does not necessarily align with the principle axis of the stresses (MD).

 

We assume or grant that the rollers are aligned and that the normal entry law applies.  However, this does not necessarily mean the web will track straight as we know from ample theory and experience with baggy/cambered webs.  So, does skewed materials track straight through a straight machine?

Skrewed

The edges of the manufacturing line tends to be skewed, baggy/tight, as well as have the highest gage variation.  Those in the know won’t buy end rolls, at least not without demanding and receiving a discount – because end rolls are not as good.  The test lab and QA may bless these end rolls as conforming to spec, but that does not mean they are any good.  Just ask any operator about end rolls.  They know they don’t run as well.  It is QA that doesn’t know any better because their instruments do not pick up skew or bagginess at all nor even gage variation at a level of sensitivity that would detect problems their customers will see.  Wrinkling, path control and related troubles are the result of materials that have profile problems that fly under the radar of the test lab.  All of these tend to be worse at the edges of a machine.

Skewed

The principle axis of anisotropic materials is an ‘invisible grain’.  Just as you can judge the quality of poor lumber by grain that does not align and thus crosses the cut, so can you too with web materials; provided that you can detect the grain.  In paper this is called fiber angle and can be measured in a number of ways from off-axis tensile tests to ultrasonics.  The principle axis is the direction which has the highest tensile stress or greatest speed-of-sound in the material.  In paper, the fiber angle tends to have a fan shaped profile due to machine/process imperfections centering around the headbox but also has other contributors both upstream and downstream of this place where liquid first meets air.

 

 In some film manufacturing, such as linear casting dies, we also have a fan but here it is pointing inward.  Skew here is again caused by the edge effects where liquid first meets air.  The melt necks inward due to surface tension.  This not only tends to increase the relative gage (thickness) there, it also freezes the polymer chains at an inward angle.  Tentering or biaxial orientation also has similar edge effects.