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Filed under: Guiding, Spreading, Wrinkling --- Tim Walker @ 09:01 AM

What's the difference between registering a new patterned feature (print, die cut, laminate) to an upstream integrated pattern vs. registering to a pre-patterned web?

How about putting it another way? If you have a choice of developing a new Excel spreadsheet (or other computer program) or modifying a spreadsheet someone else has written, which is easier?

It is almost always harder to work on something that someone else has started. It was hundreds of years between the beginning of manufacturing processes to the creation of the assembly line. Having to modify someone else's work requires an initial step of figuring out where they left off. When you do a job yourself, you know what's been done (unless you put it on the shelf for too long, then it's like starting over).

Let's get back to registration.

Whenever possible, always integrate multiple printing, die cutting, and laminating steps in a single process. It may seem that creating a complex integrated process will lead to more waste (it often does), but in the case of registration, a great deal of startup and process variation waste is actually reduced by integration.

More...

Continue reading"Registration to Pre-Printed Webs"

Filed under: Guiding, Spreading, Wrinkling --- Tim Walker @ 11:58 AM

I got an email from a fellow web handler recently saying they were visiting a plant and had just spent the better part of an hour removing spiral-wound tape from many idler rollers on a thin film laminator that were used to prevent wrinkles.

This web handler knew the theory that spiral tape has no spreading effect, but wanted to know how to battle the strong local opinion that the tape spirals were required to prevent wrinkles.

My reply:

What can I say about helically wrapped tape on rollers? I’m not a fan and yes, Ron Swanson of 3M proved (and presented a paper at IWEB in 1997) that there is NO SPREADING from spiral tape.

So why is it popular? It there something more happening than the pleasing and mesmerizing optical illusion? I think ridged rollers, either ridges formed by the gap between spiral tape bands or the ridges of one tape banded stepped on top of another, can have a wrinkle disturbing or wrinkle dispersing benefit. If the unwrapped roller is prone to wrinkles from the combination of traction, misalignment, diameter variations, baggy web in long spans, or other wrinkle sources, then the spiral tape can help by turning a tendency to form one big creasing wrinkle into several smaller non-creasing wrinkles.

Do I like spiral tape or recommend it? No, but it may seem like the easiest way forward.

My preferred plan of attack is the same as the person who sent this question.
1. Remove the tape and see what's up.
2. Diagnose where the wrinkles are forming, starting with the first, most upstream wrinkle.
3. Look for the tale-tell signs of shear wrinkles caused by poor alignment (diagonal troughs and walking wrinkles).
4. Measure and maintain alignment.
5. If wrinkles persist in a few locations (rarely do all rollers have wrinkles), try using masking or similar tape to create a concave roller with a percent diameter variations appropriate to the web strain.
6. If concave rollers don't work, feel free to try your favorite spiral tape pattern.
7. If you want to eliminate all tape on rollers, which is a great idea for contamination-sensitive products and for an operator-independent process, there is more hard work ahead, but it can be done. This harder way forward includes work towards optimized traction (wrap, tension, and traction coefficient), optimized span lengths, reducing bagginess, and tactical use of concave rollers.

The best argument against tape is contamination. If tape is left on a roller long enough, eventually you see small pieces missing. Where did they go? Off to you customer and won’t they be happy!?

tjw

For an update on this story, check the comment below:

Filed under: Guiding, Spreading, Wrinkling --- Tim Walker @ 02:13 PM

Contributed by Tim Walker

Several specialty rollers have spiraling patterns on their surface. Why are spiral roller surfaces so common and what is their advantage?

Some examples of spiraling roller surfaces are:
1. Spiral tape applied to a roller.
(Spirals are almost always begin at the roller's center and helix outward in opposite directions to either end of the roller.)
2. Spiral ridges cut into a roller surface, either as square or triangle waves cut into a hard metal surface.
3. Spiral grooves at angles from almost parallel to the machine direction to 45 degrees or more off MD parallel.
4. Spiral grooves cut into a rubber covering at an biased angle off perpendicular to the axis of rotation.
5. Diamond groove pattern with two spirals running the full length in both directions.

Why so many spirals?
1. The mesmerize engineers and operators into think they provide spreading, much like the optical illusion of a spinning barber's pole.
2. Grooves and ridges an effectively channel an entrained air layer, much like grooves of a car tire help channel water when driving through a puddle. In this case, the spiral shape doesn't add anything to the traction benefits.
3. Ridges can effectively break up a large wrinkle into several non-damaging micro-wrinkles, but will not have any spreading effect.
4. A spiral roller that is slipping relative to the web, such as dragging a fabric over a spiral roller with the V-shape pointing upstream is a spreader. A rotating spiral that matches the web speed does not spread, like a snow plow doesn't move snow if the truck isn't moving.
5. The angled cut in the rubber roller spreaders (such as Finzer's FlexSpreader or American Roller's ArcoStretcher) don't need to be spiraling to spread the web. The mechanics of spreading is related to the lateral flexing of the flutes between the spirals.
6. Spiral ridges and grooves will run quieter than axial ridges and grooves that may create a beat or harmonic noise (and be surprisingly loud).
7. Spiral ridges and grooves will break up wrinkles better than MD ridges or grooves that may catch and hold wrinkles, especially with thinner webs.

So spirals do have advantages, but don't let them mesmerize you into benefits that aren't there.

-tjw

Filed under: Guiding, Spreading, Wrinkling --- Tim Walker @ 08:00 AM

Contributed by Tim Walker:

Question:
When I have web thickness variations, I see meandering at my nipped drive roller. How do I prevent this?

Answer:
Two things with nipped roller I almost always recommend will reduce this problem. First, make sure the web wraps one of the two rollers at least 20-30 degrees before reaching the nip contact point. Second, keep the span length ahead of the nip short. The web wrap and traction it creates between the web and roller will help to isolate the upstream span from the nip force variations. The short span will be stiffer than a long span and have less meandering for the same stress variations.

Filed under: Guiding, Spreading, Wrinkling --- Tim Walker @ 08:00 AM

Contributed by Tim Walker -

Question:
When plotting a film's shear wrinkle sensitivity vs. tension or strain, thinner films wrinkle easier for the same tension, but thicker webs wrinkle easier for the same strain. Explain this.

Also, what is the effect of crossweb thickness variations on wrinkle sensitivity?

Thinner web will have fewer wrinkles if you compare two webs at the same tension (tension defined in force per width). For a given tension, a thinner web will have more tensile stress and more strain. This may add enough tension stiffening effect to make a thinner web stiffer.

I don't think that small web thickness variations will create more wrinkles. Thickness variations at winding will create wound roll stress variations that often lead to making baggy web and baggy web will have more wrinkles, so indirectly, thickness variations may promote more wrinkles.

Filed under: Guiding, Spreading, Wrinkling --- Tim Walker @ 06:30 PM

I was crunching some number this weekend on tension, misalignment, and shear wrinkles. (I know...exciting weekend...but it was raining.)

Did you know sometimes a thinner web is LESS sensitive to roller misalignment and shear wrinkles? It goes against intuition, but it is backed up by theory and experiments (most of these out of OK State Univ and their WHRC).

For example, what misalignment creates a wrinkle for 48" long by 24" wide span, polyester web with modulus of 640kpsi? Tension is 1 PLI and thickness is either 0.5 mil or 1.0 mil.

Surprisingly, the 1.0 mil wrinkles at a misalignment of 15 mils/ft and the 0.5 mil doesn't wrinkle until 18 mils/ft.

Why would the 0.5 be less wrinkle sensitive?

My lateste saying is "What is the secret to web handling?" The answer: strain.

That's why 0.5 mil at 1 PLI is less sensitive to misalignment than 1.0 mil. The 0.5 mil is at twice the strain at that tension. More strain means more stiffness and less percent stress variation from the same misalignment.

If you look at the 1.0 mil sensitivity at 2 PLI, at an equivalent strain to the 0.5 mil at 1 PLI, the 1.0 mil now wrinkles at 26 mils/ft. So once you compare the two thicknesses at equal strains, you see the benefit of thickness.

Just one of many tidbits will be giving out at the AWEB conference Wed. PM short course on wrinkling. Hope you can make it.

tjw

Filed under: Guiding, Spreading, Wrinkling --- Tim Walker @ 09:14 AM

Laying out the web path of a complex web process is a combination of engineering and art. (Where "art" is more complex engineering, difficult to put numbers to.)

One small tidbit I advise it how to make equipment less wrinkle sensitive:

When your web moves from station to station, say from an unwind stand to a pull roller station, when possible, make this transition in vertical span (or mostly vertical).

Why? Because it is much easy to ensure you have parallelism in the vertical direction. Just pull out your master level and measure it. Checking for parallelism in the horizontal direction is more difficult, requiring either a pi tape, tramming stick, optical transit, or a laser system. All these options are more time consuming and potentially less accurate than a level measurement.

So why make the station-to-station transitions vertically? Within a station, you probably have good allignment. Equipment builders will make some effort to machine sideframes as a set or other tricks so the alignment within a station is generally pretty good. The biggest alignment errors often happen between units or station. If this transition is vertical, alignment is easy. Measure with a leve. Shim to parallel. End of story.

Filed under: Guiding, Spreading, Wrinkling --- Tim Walker @ 08:56 AM

A post today asked:

Can you point out any major differences between stepper-controlled offset pivot guides and servo-controlled offset pivot guides?

There is a cost differential that is significant w/ large order. We are desiring to control the web to +/-.005".

First off, I'm not a EE, so I'd love to have a qualified answer to this question.

If a guide is used simply to correct a constant offset, then actuator performance is a big deal, but this is rarely the case. Webs move around and the guide needs to repond to this.

Fife put out a nice article in Converting magazine recently on web guide accuracy. You may ask a supplier, can your guide control to +/-0.005"? (a fairly common spec). The guide manufacturer can't say 'yes.' The guide performance depends on the characteristics of the error it is correct. The range of error is easy to design for, but the rate of error is the tough one. How fast does your web shift off center? For the same position change of shifting laterally 1 inch over 100 inches, for a slow speed process (100 fpm or 20 in/sec), this is a shift of 1 inch over 5 seconds or 0.2 in/s, not too bad. But this same web shift at 1000 fpm is 2 in/s and starts to push the limit of any acuation plan.

I hope we can get some more comments from actuator and motor specialist.

tjw

Filed under: Guiding, Spreading, Wrinkling --- Tim Walker @ 11:03 AM

Want to know more about web guiding?

Two guiding experts from Fife, Ken Hopcus and John Plumb, have combined to publish a 4-part series in Converting Magazine to answer just about any web guiding question you can think of.
What determines web guide accuracy?
How are actuator sized?
How do you select the right web position sensor?
and the big one...
Which type of guide is right for my application?

Parts 1, 2, and 3 are out already. Part 4 will be out next month.

Full Disclosure: Of course you know that Maxcess Int'l that host this site is the parent company for Fife Corp. Though convertingblog.com isn't intended as an advertising outlet for Maxcess and Fife, I felt this series of articles that emphasize fundamentals and aims a education rather than product promotion.

Filed under: Guiding, Spreading, Wrinkling --- Tim Walker @ 09:35 AM

Please post your questions or comments on wrinkles here.

I tend to divide wrinkles into four types, each with a characteristic shape and mechanism (see extended comments)

Continue reading"Got Wrinkles?"

Filed under: Guiding, Spreading, Wrinkling --- Tim Walker @ 09:33 AM

Please post your questions on web guiding, whether displacement (a.k.a. offset pivot guides), steering, or sidelay style.

Also, feel free to post questions on automatic guiding control systems or 'passive' guiding (like crowned rollers).

Filed under: Guiding, Spreading, Wrinkling --- Tim Walker @ 12:41 PM

I've been teaching classes for over 15 years now and I was surprised to find I haven't included the bowed roller spreading equations in my class notes. This partially stems from mild distaste for bowed rollers from my many years in polyester film-making where they usually did more harm than good. But that shouldn't be an excuse for not having this formula in my class notes.

OK. I'll just dig it up from my web handling library and make a new PPT slide. This turned out to be the typical home project gone bad. It seems that instead of finding what I needed in my junk drawer, this project was going to take multiple trips to the hardware store before it was fixed (analogy man strikes again).
(continued... you won't believe this trip...)

Continue reading"Finding the Bowed Roller Equation"

Filed under: Guiding, Spreading, Wrinkling --- Tim Walker @ 01:10 PM

In response to a post on differential bars, another winding topic came in from a reply:

Coated aluminum is prone to form wrinkles at the beginning of the coil winding (first wraps around the core). Does differential winding give a smoother winding start than lock-bar winding which help to eliminate those first wraps wrinkles?

Put another way:
What causes first wrap wrinkles?
Does differential winding help?

Let's go over each: (continued)...

Continue reading"Wrinkles at Roll Starts"

Filed under: Guiding, Spreading, Wrinkling --- Tim Walker @ 11:44 AM

Mike Flanagan wrote in asking this great question.
The short answer is YES.

Here's Mike's question in whole "Are there not some dynamics involved in guiding that can create wrinkles downstream from guides that are improperly installed? Seems to me I have seen wrinkles show up on some steering guide applications that have to do with overly short entry spans. Is that the case?"

Many wrinkles are created by grossly uneven tensions across the web. Usually this is caused by misalignment, but any thing that overly bends or twists the web can do it. How can a guide do this?

1. A guide may be over steering. If a guide is poorly tuned or the geometry is off (like a edge sensor too far downstream) the lateral oscillation of the web at the guides output can be worse than the error it was meant to correct.

2. If spans are set short and corrections are large, the severe bending or twisting may be too much for the web. This can happen when two sequential guides fight each other or the web lines has a dominate pull to one side. If an unwind guide sensor is misplaced and guides the web to one side, the next correctly set guide will pull the web back, but if the correction is extreme, the result may be a wrinkle. A smart positioning of an upstream guide can reduce the work of a downstream guide and eliminate the problem. Usually guides don't talk to each other, but this is a case where teamwork among web guides can solve a problem.

3. Get the geometry right (cont'd)

Continue reading"Can a Web Guide Create Wrinkles?"

Filed under: Guiding, Spreading, Wrinkling --- Tim Walker @ 12:44 PM

Maybe this is obvious. Traditionally, I've grouped these three web handling sub-topics into 1) guiding and 2) wrinkling/anti-wrinkle. Guiding is about lateral registration of the web, whether with active automatic web guides or passively with crowned, tapered, and skewed rollers at one topic. Wrinkling is about keep the web from buckling, either laterally or in the machine direction, and the use of spreader or anti-wrinkle rollers (not the same thing) to prevent them.

Why group them together?

Continue reading"Why group guiding, spreading, and wrinkling together?"