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Question
#168
Subject: What is the cause of MD bands that are appearing on coated 48 gauge PET?
1st let me say we have run this product before several times without this problem.
Running 48 gauge PET through our coater the film is developing MD bands that appear to be heat related. The film only sees 105F for a brief period.
Typically we get 3 lanes, 3-6" wide with diagonal ripples.
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#169
Author: Phil Faria, Lamart Corp
Email: paf@lamartcorp.com
Subject: It might be misalignment of the rolls.
Diagonal ripples in polyester at that low a temperature at a regular
interval is a good indication of roll misalignment somewhere on the coater.
I would check that out especially if the PET your using is the same grade
and from the same source.
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#170
Author: Timothy J. Walker, TJWALKER+ASSOCIATES INC
Email: TJWALKER@TJWA.COM
Subject: Bagginess could be the problem.
The MD bands (w/ diagonal ripples) that you describe are likely associated
with a length dimensional variations (aka bagginess). Bagginess is defined
as a variation in product length across the web width. Some products have
baggy edges (like moisture dimensional changes in paper). Bagginess is seen
in the lanes where the product is dimensionally longer than the other lanes.
This may not be a large difference, but larger than the strain created by
web tension. (Bagginess is always present to some degree, but when it is
lower than the strain from tension it is hidden.)
The top source of bagginess is long-term crossweb caliper variations and
winding / wound roll life. Thick lanes build to larger than average
diameters, and may over time create permanent deformation in the lanes of
extra-stretched film. Upon unwinding, baggy lanes appear in line with
caliper (coating or substrate) variations. It seems you are describing a
bagginess not associated with wound rolls.
Your product must be going through a dimensional change, either differential
length increase or decrease vs. width. PET films are commonly biaxially
oriented to stretch the polymer chains creating strength, then heat quenched
to lock in the structure with a small percentage of polymer crystallization.
The heat quenching conditions may leave a small amount of residual stresses
in the film that may be relieved with later heat treatment. One of the
desirable properties of PET film is the good thermal stability (vs. PE or PP
films). PET can also be processes with large residual stresses
intentionally ready for shrinkage with reheating (e.g. window shrink film,
"shrinky-dink" films). PET can also be post-processed, in sheet or roll
form, to have even better thermal dimensional stability.
Coating can play a roll. Uniform coating does not usually play a role.
However, pattern coating, like stripes, can create a differing thermal
absorption or evaporative cooling, and be reflected in the web length.
Equipment may play a role. A drying nozzle or header with non-uniform flow
or temperature can combine with the film's thermal instability to create
crossweb variations.
I would agree that it is strange for a PET to have thermal dimensional
changes with a brief exposure to 105F. I would normally expect PET to be
quite stable up to 175F and above.
To fully understand why your film with previously no problems is now
demonstrating such I have a few questions. The problem source could be from
film changes, coating changes, equipment/process changes, or some
combination.
1. Do you have any film from previous shipments that didn't display this
defect? Can it still be run without showing the bagginess?
2. This is a coater. Does the bagginess develop with and without coating
(but with the dryer on)? More or less?
3. If yes, is the defect dependent on hot processing (i.e. no defect with no
coat, no heat web handling)?
4. Has the coater / dryer had any modifications between no problem and this
new defect? (Anything, even if it doesn't logically seem like a cause.)
5. Are the 3 lanes consistent or wandering?
6. What theories have been discussed among your engineers, operators, and
maintenance folks? (OK, what do the managers think, also?)
7. I assume the bagginess is not present at unwinding or post-coating /
pre-drying and seen post-drying / pre-winding. Correct?
As you can see, this is not a simple "turn this knob" or "send the product
back" answer.
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#171
Author: Dr. David Roisum , Finishing Technologies, Inc.
Email: DRoisum@aol.com
Subject: It could be misalignment of rollers
I have written a paper published by TAPPI that diagnoses the common causes of wrinkles. This paper is also a chapter in my book on the Mechanics of Web Handling, also published by TAPPI PRESS. Finally, a draft of the wrinkling paper is a downloadable on my website, roisum.com. I urge you to read one of these to flesh out the following brief comments.
Evenly spaced MD wrinkles can be caused by heat if the thermal expansion of width exceeds a few tenths of a percent on a thin webs. However, I doubt 105F is hot enough to accomplish that. Similar patterns can also be caused by a tension reduction in that zone or excessively high tension. Occasionally deflecting rollers or misbehaving spreaders can also make the same pattern. All of these subcases are ways that the web width is not as wide as it should be to keep it flat. The most common treatment (aside from changing the root cause) is effective spreading (also a subject that I've written upon in detail.
If the troughs/wrinkles are oriented at an angle with respect to the MD, it is because something is crooked. The most common subcase is misaligned rollers. Rollers may need to be aligned to a few hairsbreadths to avoid this. Other common cases of 'crookedness' are rollers which vary in diameter across the width (bulges or valleys) or baggy webs.
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#745
Author: Harold Seelig, P.E.,
Email: yasui@ix.netcom.com
Subject: True MD Bands develop over time starting with the outside of the wound roll
True MD Bands develop over time starting with the outside of the wound roll
"True" MD bands form as a result of hygroscopic expansion of particularly thin PET film. These bands are seen to form over a period of from a few hours to a few days after the film is wound.
Distinctive of True MD Bands is the path of the bands, always spiral, NEVER forming a ring (the beginning or "head" of an MD band will never connect back up to it's own tail). MD Bands can extend for less than a full turn, or can be more than a full turn around the wound roll.
An MD band is the excess width gained by very slight expansion of PET by water vapor pick-up.
Typically, PET film makers suffer MD bands in newly made films, up to about 1.5 mil (37 micron) thickness. Thicker PET films are stiff enough to resist MD bands.
Solutions to cure MD Bands consist of:
1) either a thick layer of PET, ie: 75 or 100 micron thickness wound onto the outside of a roll immediately at the end of the slitting cycle, or
2) a special tension/packing roll winding curve that minimizes residual stress in the outer layers of film.
The writer's question seems to be related to profile thick lanes, either from streaky coating, or "gauge bands" in the original feedstock (which would be seen on the un-winder).
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