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EXTRUSION

TroubleshooterBy Paul Waller

Plastics Touchpoint Group, Inc.

How to Fix Ugly Rolls�Appearance counts. Ugly rolls offilm are often rejected, even when thegauge variation of the film itself is wellwithin specification. It is often impos-sible to measure the slight gauge vari-ation that can build up over thousandsof layers to produce a bump in the rolldiameter. So operators must under-stand how to diagnose the cause orcauses of deformed rolls and adjustwinding for changes in raw materials,processing conditions, and equipmentfunction.

All winders use some combinationof nip pressure, web tension, and drivetorque to control film winding. Nip-rollpressure controls roll hardness byremoving air entrapped between lay-ers as the film winds. Web tension con-trols roll hardness as well, but cancause other problems, especially whenwinding elastic materials like poly-ethylene. PE film relieves stress bychanging shape over time, so rolldeformity may not show until severalhours after the roll is completed. Drivetorque from the center is a better wayto control stress as film winds.

winder helps as well. The taper strat-egy depends on the modulus of elas-ticity of the web. For thin, stretchyfilms, apply a negative tension taper,starting high, dropping rapidly, andflattening as the roll gets full. Heavy,stiff films need a positive tension taper,starting high on the empty roll, main-taining the higher level, and droppingoff more quickly as the roll nears com-pletion. Final tension is typically 50%to 75% of starting tension, dependingon roll diameter.

No stars, spokes, bucklesThese defects are most visible at

the ends of the roll, though ridgesalong the roll axis also may be visibleat the outer surface. In “starring” or“spoking,” the outside film layers com-press inner layers as the roll builds.The roll deforms into a star or spokedpattern because the layers bucklewhen compression force is too high.Film shrinks as it cools, so this prob-lem may not appear until severalhours after the roll is completed.

The most common processing

Prevent tin canningRolls that exhibit a defect known

as “tin canning” are softer near themiddle of the roll than at the edges.The result is regular corrugationsevery 1 to 2 in. around the roll like ribson a tin can. This problem occurs whenextruders or dies are unable to homog-enize materials of very different vis-cosity or density. Using materials thatare more similar helps. Adjusting tem-perature profiles or backpressure toincrease mixing of the melt before itenters the die may also solve this prob-lem.

From a machinery perspective,the most common cause in blown filmextrusion is excessive drag resistancein the collapsing frame, which can per-manently stretch the film near themiddle of the roll. Increasing the open-ing at the bottom of the collapsingframe helps. Film can also be stretchedpast its yield point if the film is too hotor tension is too high. So adding cool-ing before pulling the film past idlerrollers will help.

Tapering tension inside the

Winding problems create roll defects like tin canning, starring, spokes, buckled cores, and tapered rolls.

FIG. 1—“Tin Canning” FIG. 2—Starred or Spoked Rolls

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EXTRUSION Troubleshooter

causes are winding film too warm orapplying too much web tension orlay-on pressure. This problem can alsooccur with coextruded film where thedensity in each layer can be very dif-ferent. The film tends to curl towardthe higher density layer. Selecting lay-ers with more similar densities orchanging layer ratios may reducethis problem.

Buckled rolls are a more severecase of starred or spoked rolls, wherecompression becomes so intense thatthe core itself is deformed, often mak-ing it impossible to remove the rollsfrom the winding shaft. Causes areidentical to those for starred or spokedrolls. A core can also collapse at oneend if the web overhangs the core. Justa little extra pressure at one end canbe sufficient to crush the core.

Stop tapered rollsTapered rolls are larger in diam-

eter at one end than the other. Blownfilm lines, even with rotating dies oroscillating nips, can still taper becauserotating or oscillating dies only ran-domize TD gauge variation up to thefrost line, while oscillating nips onlyrandomize TD gauge up to the col-lapsing frame. MD gauge variationisn’t randomized at all. The most com-mon cause of tapered rolls is unbal-anced cooling, which can be causedby air drafts; a misaligned die or airring; misaligned haul-off nips or idler

rolls; or uneven nip pressure.Telescoped rolls have a problem

of the outside layers sliding to oneend. Telescoping can be caused by toomuch slip additive, which blooms tothe surface and changes the coeffi-cient of friction between film layers.The COF changes most during thefirst hour after extrusion. If idler, nip,or lay-on rollers aren’t parallel to thewinding core or don’t apply pressureevenly, stresses will pull the web toone side.

Air trapped between layers isanother cause of telescoping, so it’simportant to maintain either nip pres-sure or web tension high enough tokeep air out. Entrapped air can also

cause “edge wander,” making roll endsragged instead of smooth.

The most common operator mis-take is to compensate for poor rollgeometry by increasing web tension.The best strategy is to wind with theleast tension possible because you arewinding a rubber band. Instead,increase nip pressure or drive torquefirst, then increase web tension if aproblem occurs. When winding in gapmode, increase drive torque to raiseweb tension. Tapering web tension asthe roll increases should also be usedif the winder has that capability.

Roll appearance becomes a biggerissue as converters buy larger andlarger rolls to minimize waste duringroll splicing. Larger rolls make anyvariation in roll geometry much moreapparent. A temporary fix may be towind small-diameter rolls, but ulti-mately the causes of non-uniform rollsneed to be identified and corrected.

—Edited by Jan H. Schut

FIG. 3—Buckled Rolls FIG. 4—Tapered Rolls

Paul Waller consults on film extrusionand flexible packaging. He is presidentof Plastics Touchpoint Group Inc. inToronto (www.plasticstouchpoint.com),which offers engineering, plant design,machine installation, and operatortraining. He welcomes comments by e-mail: paul@plasticstouchpoint.com.Previous articles in this series are foundat www.ptonline.com/extrusion.

The most commonoperator mistake isto compensate forpoor roll geometryby increasing webtension. It’s best toincrease nip pressureor drive torque first.