Troubleshooting WebTroubleshooting Web--handling & handling & Coating Problems in Flotation DryersCoating Problems in Flotation Dryers

By: Dan Bemi and Steve Zagar

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Unique Challenges

l Flotation dryers combine the heat transfer and web support/transport functions into the nozzle design.

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What is “good web handling”?

l Good web handling in a flotation dryer is understood to mean a stable, non-fluttering, non-wrinkling, uniform nozzle to web clearance

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Tension/Velocity Relationship

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Web Handling Problems

l Troughs and Wrinkling

l Web Flutter

l Web Shift

l Web Weave

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Web Troughs and Wrinkling

Problems

Machine direction waves or undulations

Troughs are precursors to “wrinkles”

Wrinkle usually forms when a trough passes over a roller or through a nip

Causes

Line tension too high

Non-uniform substrate thickness

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Troubleshooting Tools (Troughs/Wrinkling)

l Slightly reduce dryer zone tension and increase supply air nozzle velocity

l Damper positioning, or fan speed adjustments

l Check web gauge uniformity using calipers

l Spreader or bowed rolls may be only solution

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Web Flutter

Problems

Web flutter can be defined as “out-of-plane” motion, typically at the edges of the web, but sometimes, over the entire web width

Causes

Tension/velocity is unbalanced

Nozzle gap width is non-uniform

Nozzle-to-web distance is too great

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Troubleshooting Tools (Flutter)

l Adjust tension and nozzle velocity

l Adjust nozzle-to-web distance

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Troubleshooting Tools (Flutter)

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Troubleshooting Tools (Flutter)

l Check slot nozzle uniformity using a taper gauge

l Check for cross-machine nozzle velocity uniformity, using a manometer and a pitot tube fastened to an extension rod.

l Check nozzle/header alignment, by pulling a taut wire through the dryer on both the operator and gear side at the web height. Then measure the distance between the wire and nozzle with a taper gauge.

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Web Shift

Problems

Web shift is the movement of the web to a relatively steady position away from its centerline, either to the operator or gear-side of the machine

Causes

Substrate camber

Poor nozzle alignment

Non-uniform supply-air velocity

Poor cross-machine handling of spent air

Misaligned rollers

Non-uniform nip pressure

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Troubleshooting Tools (Web Shift)

l Eliminate non-dryer related causes by checking web for camber

l Roll out sample on floor (20X web width) and measure and record the camber using a taut nylon fishing line and tape measure

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Troubleshooting Tools (Web Shift)

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Troubleshooting Tools (Web Shift)

l Run your cambered roll noting the direction and amplitude of shift. Then flip the roll and run it again. If it shifts to the opposite side of the machine, camber is very likely the main cause

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Troubleshooting Tools (Web Shift)

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Troubleshooting Tools (Web Shift)

l If camber is a problem, increase line tension slightly

l Steering may be your only solution.

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Troubleshooting Tools (Web Shift)

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Troubleshooting Tools (Web Shift)

l If you suspect that the dryer is causing the shift then…

l Check the nozzles for cross-machine airflow uniformity, nozzle gap width and nozzle alignment using the same methods mentioned in the section on web flutter

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Web Weave

Problems

Web-weave is a cyclic, back and forth movement of the web inside the dryer

Causes

Under constant airflow conditions, a flotation dryer will not cause web weave. Rather, the dryer simply reacts to the varying cross-machine tension.

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Troubleshooting Tools (Web Weave)

l Check for poorly set up and/or improperly tuned web guides

l Check for indications of loss of roll traction (especially at the steering roll)

l Cross-machine tension variation can also be caused by unbalanced air pressure at nip points, in vacuum rolls or other gripper type rolls or belts.

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Coating Defect Problems

l Blistering

l Orange peel

l Mottle

l Dryer Bands

l Pinholes

l Cratering

l Curling/Cracking

l Blushing

l Scratching/Scuffing

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Blistering

Problems

“Boiling” under the surface of the coating layer that results in bubbles that may or may not burst at the surface

Causes

Evaporation rate is too high

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Troubleshooting Tools (Blistering)

l Slow evaporation and minimize skinning

l Reduce air temperature

l Reduce impingement air velocity

l If possible, add heat from the bottom-side only

l If these adjustments do not eliminate the defect, there may be no alternative but to slow down production or reformulate the coating.

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Orange Peel

Problems

Puckering of the coating surface

Causes

Density or temperature gradients across the web or within the coating

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Troubleshooting Tools (Orange Peel)

l If caused by the dryer

l Reduce air temperature

l Reduce impingement air velocity

l If these adjustments do not eliminate the defect…

l Check mixer to insure good distribution of solids content

l Reformulate the coating

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Mottle

Problems

Non-uniform surface disturbance or pattern (usually only observable under magnification)

Causes

Non-uniform air distribution to the web

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Troubleshooting Tools (Mottle)

l Verify the dryer cross-machine airflow, velocity and temperature uniformity using the techniques previously described

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Dryer Bands

Problems

Continuous, machine direction disturbances in the coating visible as parallel lines across the web

Causes

Airflow emanating from rows of holes or circular nozzles above the coating.

Sometimes a non-uniform slot nozzle will create isolated dryer bands that correspond to variations in slot width across the web.

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Troubleshooting Tools (Dryer Bands)

l Reduce both line tension and impingement velocity

l Back the nozzles away from the web in addition to reducing the impingement velocity

l Check slot nozzles for uniform gap

l Replace old hole bars or circular jet style nozzles with Coanda style airbars

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Pinholes

Problems

Tiny voids in the coating where the substrate is often visible

Causes

Generally caused by contamination (air bubbles or particulate in the coating itself or oil, grease or other contamination on the substrate)

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Troubleshooting Tools (Pinholes)

l Check filters on the coating mixers and pumps

l Use additives to reduce foaming

l Fluid mixers and pumping systems may need cleaning or repairs

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Cratering

Problems

Similar to pinholes but voids are larger and easily visible

Causes

Caused by uneven wetting of the coating due to variability in surface tension of the substrate due to substrate contamination from oils or silicones

Often the result of non-uniform deposition of release coatings.

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Troubleshooting Tools (Cratering)

l If contamination is suspected

l Check filters on the coating mixers and pumps

l Fluid mixers and pumping systems may need cleaning or repairs

l If release coating is suspected

l Check coating operation for uniform deposition

l Increase solids content of release coating

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Curling and Cracking

Problems

Web curls toward the coated side or forms cracks in the coating itself

Causes

Caused by the build up of unequal stresses in the coating and substrate as they move through the dryer. As the coating dries, its volume is reduced and the resulting shrinkage either curls the web or cracks appear in the coating

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Troubleshooting Tools (Curling/Cracking)

l Edge curl is actually minimized in flotation style dryers due to sine wave strengthening the substrate section

l Increase supply air velocity and the flotation clearance

l Reduce drying temperatures to the greatest extent possible while meeting requirements for solvent removal

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Blushing

Problems

Blushing is a milky, opalescence of the coating

Causes

Caused by the surface temperature of the coating falling below the dew point of the air

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Troubleshooting Tools (Blushing)

l Simply increasing the air temperature will normally prevent this defect.

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Scratching and Scuffing

Problems

Scratching appears as machine direction grooves or striations in the coating that appear in the same position across the web

Causes

Physical contact with the nozzles or web slots is typically responsible for this defect

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Troubleshooting Tools (Scratches/Scuffs)

l Check line tension set points and supply air velocity set points to insure proper flotation clearance between the nozzles and web

l Check nozzle alignment

l Check entrance and exit roll alignment

l Adjust the impingement angle of the first nozzles inside the dryer at either end of the device

l If lead in and exit nozzle are dampered, adjust impingement velocity

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Summary

l Develop a good understanding of air flotation theory, air bar design, and the tension/velocity relationship

l Most common web handling problems (flutter, troughs and wrinkling) and common coating defects (curling, cracking, blushing and scratching) can be resolved with minor adjustments in tension, supply air velocity and air temperature

l Tougher problems may require dryer reconfiguration or physical modifications to nozzles or air-handling equipment or processing changes

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