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Troubleshooting tips to cure your sheet extrusion headaches.

Sheet problems got you down? Here are some remedies from the extrusion experts at Dow Plastics Technical Service & Development that will help get you up and running again - whether you extrude monolayer or coex products.

Troubleshooting information for general extrusion problems and for some specific applications such as film has been widely available, but the same is unfortunately not true for sheet extrusion. Here at Dow Plastic's technical service department, we recently set out to fill this gap by compiling some of the knowledge we've gained from our experience with sheet lines. The resulting troubleshooting guide gives solutions to the commonplace problems affecting both monolayer and coextruded sheet manufacturing.

Any guide of this sort has to generalize some degree in order to accommodate the wide range of extrusion equipment available for sheet. A roll stack, for instance, may have three, four, or five rolls, and dies can be of the single-manifold or multimanifold type. Some of our recommended solutions may be more applicable to certain equipment configurations than to others, but the general principles behind each recommendation should hold true regardless of the equipment type.

The generalized nature of the suggested solutions also reflects the complexity of many extrusion problems - i.e., the solution to a given problem often requires more than one corrective action. With that in mind, the information in this guide has been presented with the most common causes and solutions first, but be aware that some combination of actions might be necessary to rid your line of certain ills.

If all else fails, you may have to consider changing to an improved equipment design. Our experience shows that design of screw, die, and roll stack can have a major influence on most of the problems listed in this guide. But before you think of replacing any equipment, the following guide shows there are plenty of possible remedies to try first.

Monolayer & Coextruded Sheet

Problem

Pock Marks, Pits, or Dimples

May be caused by air entrapment:

1. Reduce temperatures in first and second barrel zones. 2. Decrease screw speed. 3. Remove fines from regrind. 4. Unplug the vent if using a two-stage screw. 5. Raise backpressure if using a single-stage screw. 6. Check vacuum system for proper operation. 7. Maintain a constant hopper level.

May be caused by moisture:

1. Increase drying temperature. 2. Increase drying time. 3. Check dryer operation for proper dewpoint and desiccant regeneration. 4. Unplug the vent if using a two-stage screw. 5. Check vacuum system for proper operation. 6. Use a drying hopper. 7. Use a nitrogen blanket on the feed hopper.

May be caused by volatiles:

1. Cool the melt to minimize degradation. 2. Unplug the vent if using a two-stage screw. 3. Check vacuum system for proper operation.

May be caused by volatiles entrapment between the sheet and roll:

1. Increase the ventilation. 2. Adjust die lips in region corresponding to the entrapment. 3. Use an air jet to blow volatiles from nip area. 4. Adjust die-roll height to minimize draping of sheet on roll. 5. Reduce melt temperature. 6. Unplug the vent if using a two-stage screw. 7. Check vacuum system for proper operation. 8. Check for hot spots on the die.

May be caused by insufficient melt bank at roll nip:

1. Decrease the roll speed to build bank.

2. Decrease roll-nip gap to build bank.

3. Adjust die restrictor bar and/or die lips to optimize flow uniformity from the die.

4. Optimize die temperatures to improve die flow.

5. Increase extrusion rate to fill the bank.

6. Move die closer to roll nip.

7. Check for extruder surging.

May be caused by insufficiently clean equipment:

1. Clean chill-roll and polishing-roll surfaces.

2. Clean idler rolls in conveyor.

3. Dissipate static electricity to minimize dust pick-up.

May be caused by cooling the sheet too fast:

1. Raise the temperature of the roll in contact with the problem surface.

Lines in Machine Direction

May be caused by die imperfections:

1. Clean die-lip area with a soft tool - preferably wood or copper.

2. Disassemble and clean the die.

3. Polish or re-plate die surfaces.

4. Polish damaged die-lip face.

5. Refinish die lip face to 90 [degrees] angle.

May be caused by moisture:

1. See solutions to moisture problems in section above on Pock Marks, Pits, or Dimples.

May be caused by equipment damage:

1. Polish or re-plate damaged chill rolls.

2. Check for scuffs. (See solutions to Scuffs in section below.)

May be caused by unstable bank size:

1. Adjust roll temperatures.

2. Adjust die-to-nip distance.

3. Check for extruder surging.

Lines in Transverse Direction

May be caused by sheet sticking to roll(s):

1. Reduce roll temperature.

2. Reduce melt temperature.

3. Move die closer to rolls to prevent sheet from sagging before the nip.

4. Clean the roll.

May be caused by a roll-drive problem:

1. Reduce pull-roll tension to avoid "pulling" sheet across chill rolls.

2. Raise tension to ensure that sheet does not drop off the bottom roll.

3. Adjust roll drive chain.

May be caused by winder overpowering the pull rolls:

1. Decrease winder tension. 2. Increase pull-roll-nip pressure.

May be caused by equipment variations:

1. Check for and eliminate electrical voltage variations throughout the line by isolating each drive's power supply.

2. Check for and dampen vibrations from all equipment. (First, isolate the roll-stack to see if it's at fault.)

May be caused by sheet cut-off:

1. Install a "hump" section in conveyor between pull rolls and shear.

May be caused by a rolling bank at nip roll:

1. Reduce size of bank by increasing polishing-roll speed, reducing extrusion rate, and/or opening roll-nip gap.

Thickness Variation in Machine Direction

May be caused by improper extruder operation:

1. Check for extruder surging.

May be caused by a rolling bank:

1. Reduce the size of bank by increasing polishing-roll speed, reducing extrusion rate, and/or opening roll-nip gap.

May be caused by a roll problem:

1. Clean the rolls.

2. Check roll bearings for wear.

3. Check rolls for concentricity.

4. Check roll-nip pressure ("pli") to ensure sufficient pressure to keep roll gap constant.

May be caused by surface-temperature variation across roll face:

(Tip: Cooling-fluid temperature should not change more than 2 [degrees] F from inlet to outlet.)

1. Increase roll coolant recirculation rate. 2. Decrease extruder output.

May be caused by winder overpowering pull rolls:

1. Adjust winder tension. 2. Increase nip pressure on the pull rolls.

May be caused by roll-drive variations:

1. Adjust roll drive.

Thickness Variation in Transverse Direction

May be caused by die May be caused by die adjustment:

1. Adjust die-lip gap.

(Tip: A typical starting gap is 110-120% of the desired sheet thickness.)

2. Adjust die restrictor bar, keeping all the bolts tight.

3. Cool die in thick areas; heat die in thin areas. (This action has only a limited effect.)

May be caused by roll adjustment:

1. Adjust roll gap to proper setting.

(Tip: A typical first gap is about 1-2% more than the desired sheet thickness. Typical second gap is the same as, or slightly less than, the desired sheet thickness.)

2. Balance and/or raise pressure to better distribute inconsistent die flow.

3. Reduce roll pressure to minimize roll bending.

4. Check roll bearings for wear.

May be caused by a non-uniform bank:

1. Adjust die restrictor bar and/or die lips to optimize flow uniformity from die.

2. Adjust die temperatures to optimize flow uniformity from die. (This action has only a limited effect.)

3. Check for extruder surging.

Curled Sheet in Transverse Direction

May be caused by improper roll temperature:

1. Raise temperature of the concave sheet surface.

(Tip: If sheet edges curl down and curling occurs "downstack," raise middle roll temperature. If sheet edges curl up, lower middle roll temperature. Reverse recommendations when curling occurs "upstack" In both cases, change third-roll temperature opposite to middle-roll temperature change. Crystallinity effects may alter these recommendations.)

Curled Sheet in Machine Direction

May be caused by too much pull-roll tension:

1. Reduce pull-roll tension. 2. Reduce pull-roll speed.

Low Gloss

May be caused by improper processing conditions or equipment:

1. Increase melt temperature by raising temperatures of the last one or two barrel zones.

2. Raise die temperatures to equal the melt temperature.

3. Increase roll temperatures.

4. Adjust die to obtain uniform flow.

5. Adjust roll gaps to maintain good contact with sheet.

6. Increase roll-nip pressure.

7. Add glazing unit between die and rolls.

May be caused by poor melt consistency:

1. Increase head pressure to improve mixing.

2. Use a static mixer in transfer line.

Inconsistent Or Patchy Gloss

May be caused by inconsistent gauge:

1. Adjust die lips.

2. Adjust restrictor bar.

3. Check for extruder surging.

May be caused by excessive melt temperature:

1. Reduce temperatures in the last one or two barrel zones. 2. Reduce die temperatures. 3. Change screen-changer screens.

May be caused by non-uniform roll temperatures:

1. Check [Delta]T across roll face.

(Tip: Cooling fluid should have a [Delta]T of 2 [degrees] F or less from roll inlet to outlet.)

2. Check for hot spots on roll circumference and clean flow.

May be caused by volatile entrapment between sheet and roll:

1. Increase ventilation. 2. Adjust die lips in region corresponding to the entrapment. 3. Use an air jet to blow volatiles from the roll-nip area. 4. Adjust die-roll height to minimize draping of sheet on roll. 5. Reduce melt temperature. 6. Unplug the vent if using a two-stage screw. 7. Check for proper operation of vacuum system. 8. Check for hot spots on die.

Rough Sheet Surface

May be caused by a rolling bank:

1. Reduce size of bank by increasing polishing-roll speeds, reducing the extrusion rate, and/or opening the roll-nip gap.

May be caused by a material problem:

1. Remove any incompatible regrind.

2. Increase mixing in extruder with higher head pressure from valving or finer screen-changer screens.

3. Use a static mixer.

4. Make sure previous materials have been thoroughly purged.

(Tip: Proper purging covers all melt channels - screw, barrel screen packs, transfer lines, and dies - as well as solids-handling equipment - pneumatic-conveying system, dryers, and grinders.)

5. Eliminate moisture. (See solutions to moisture problems in section above on Pock Marks, Pits, or Dimples.)

May be caused by roll set-up conditions:

1. Increase roll temperature to slow solidification.

2. Decrease roll gap to polish sheet.

3. Increase roll-nip pressure to "roll out" roughness.

4. Clean roll surfaces.

5. Re-surface damaged rolls.

High or Inconsistent Sheet Orientation

May be caused by too high a roll speed relative to die throughput rate:

1. Reduce chill-roll speed. 2. Increase extrusion rate. 3. Decrease die gap.

May be caused by excessive bank size:

1. Increase chill-roll speed. 2. Decrease extrusion rate.

May be caused by incorrect die/nip distance:

1. Reduce die-nip distance to minimum setting. 2. Align die face with the roll centerline.

May be caused by excessive pull-roll tension:

1. Decrease pull-roll speed. 2. Increase chill-roll speed.

May be caused by incorrect polymer cooling rate:

1. If melt temperature is too low, raise temperature on the last one or two barrel zones and/or raise die temperatures.

2. If the differential between the melt temperature and roll temperature is too wide, raise the roll temperature and/or reduce melt temperature.

Brittleness in Sheet

May be caused by too much orientation:

1. See solutions to Orientation problems above.

2. Adjust roll gap to proper setting.

(Tip: A typical first gap is about 1-2% over the desired sheet thickness. A typical second gap is the same as, or slightly less than, the desired sheet thickness.)

May be caused by cold rolls:

1. Increase roll temperatures.

May be caused by poor melt consistency:

1. Increase head pressure to foster better mixing. 2. Use static mixer in the transfer line. 3. Reduce regrind level. 4. Reduce additive levels - for example, color concentrate.

Discolored Sheet

May be caused by excessive temperatures or residence times:

1. Check for proper operation of heaters and thermocouples.

2. Reduce last one or two barrel-zone temperatures and/or reduce die temperatures.

3. Reduce head pressure by changing to clean or coarser screens, and/or by opening up available valving, the die restrictor bar, and/or die lips.

4. Clean behind deckle on regular basis. If that fails, remove deckles.

5. Streamline flow path by using smooth transitions and/or elbows with longer bend radii.

6. Check for loss of plating or other corrosion.

May be caused by insufficient color concentrate or segregation of concentrate in feed hopper:

1. Increase color concentrate to specified level. 2. Add color concentrate at feed throat. 3. Redesign hopper to get mass flow rather than funnel flow.

Contamination In Sheet

May be caused by equipment problems:

1. See solutions to Discolored Sheet. 2. Check for ruptured screens. 3. Check for loss of plating, which can result in black specks.

May be caused by material problems:

1. Check incoming resin for dirt, packaging scraps, gels, etc.

2. Check regrind stream for contamination.

3. Check airvey system and hopper-loading procedures for possible sources of contamination.

4. Remove excess fines from resin.

Contamination On Sheet

May be caused by static electricity:

1. Dissipate static charge on sheet. 2. Improve cleanliness around line.

May be caused by equipment contamination:

1. Clean chill roll and polish roll surfaces. 2. Clean idler rolls of conveyor.

May be caused by degraded material falling onto sheet:

1. Clean die lips. 2. Clean build-up from surfaces around die.

Poor Transparency

May be caused by contamination:

1. Reduce regrind level. 2. See above sections on solutions to Contamination.

May be caused by poor equipment maintenance:

1. Polish roll surfaces. 2. Clean screw and downstream equipment. 3. See section below on solutions to Scuffs.

May be caused by improper temperatures:

1. Adjust roll temperatures to get proper cooling rate while maintaining appropriate level of crystallinity.

2. Ensure complete melting of resin by raising temperature of last one or two barrel zones and/or by raising die temperatures.

May be caused by poor mixing:

1. Increase head pressure. 2. Use a static mixer in the transfer line.

May be caused by a rolling bank:

1. Reduce bank size by increasing polishing-roll speeds, reducing the extrusion rate, or opening the roll-nip gap.

Scuffs

May be caused by damaged or dirty sheet-handling equipment:

1. Clean idler rolls.

2. Remove burrs or rough spots on idler rolls.

3. Coat rollers with non-stick surface.

4. Clean pull rolls.

5. Replace or repair damaged pull rolls.

6. Look for damage to shear in-feed and out-feed areas and make repairs as necessary.

7. Insert slip sheet at shear in-feed and discharge.

Coex Sheet Only

Problem

Zigzag Instability

May be caused by excessive interfacial shear stress in the die land:

1. Increase relative thickness of the skin layer.

2. Increase die-lip opening.

3. Reduce total throughput.

4. Decrease the viscosity of skin layer by raising its temperature or by using a resin with a higher MFR.

5. Raise die-lip temperatures.

Wave Instability

May be caused by a mismatch in the rheological properties of different layers at a given set of process conditions:

1. Make the structure more symmetrical.

2. Increase skin-layer viscosity by switching to a lower-MFR resin or by lowering the skin layer's melt temperature.

3. Decrease core-layer viscosity by changing to a higher-MFR resin or by raising the core layer's melt temperature.

4. Use a different die design. For example, a multimanifold die often helps.

5. Correct velocity mismatches in feed-block.

Uneven Layer Thickness in Transverse Direction

May be caused by non-uniform melt temperature:

1. Set die, feedblock, and transfer-line temperatures close to melt temperature.

2. Use a static mixer or add a mixing section to the screw.

3. Reduce screw rpm.

4. Increase head pressure to improve mixing.

May be caused by rheology/equipment interactions:

1. Optimize skin-layer rheology by changing melt temperature in the extruder, die, and feedblock.

2. Optimize core-layer rheology by changing melt temperature in extruder, die, and feedblock.

3. Modify feedblock's slot shape or flow configuration.

4. Modify die design. Multimanifold dies can help.

May be caused by improper equipment set-up or operation:

1. Check for proper operation of the temperature-control system

2. Retract the die's restrictor bar to minimize flow disruption.

3. Remove remaining flow restrictions - such as sharp corners, melt thermocouples.

4. Clean build-up from equipment.

Uneven Layer Thickness in Machine Direction

May be caused by improper extrusion operation:

1. Check for extruder surging.

2. Reduce size of rolling bank at the chill roll.

3. Check to see that roll stack's pull rolls are functioning properly. (See roll-drive problems in solutions to Lines in Transverse Direction above.)

(Note: Zig-Zag and Wave Instabilities will both give machine-direction thickness vacation.)

Insufficient Sheet Toughness

May be caused by brittle layers being too thick:

1. Reduce thickness of brittle layers.

2. Improve transverse sheet-thickness uniformity, especially at edges of sheet. (See solutions to Uneven Layer Thickness in Transverse Direction.)

3. Switch to tougher polymers.

May be caused by excessive regrind level:

1. Reduce regrind level. 2. Use additive to restore toughness.

May be caused by orientation:

See solutions to Orientation problems above.

Non-Uniform Sheet Color

May be caused by variation in skin-layer thickness:

1. Increase thickness of skin layer. (See solutions below to Too-Thin Layer problems.)

2. Eliminate thin spots in skin layer. (See solutions above to Uneven Skin Layer Thickness problems.)

May be caused by poor regrind mixing:

1. Use a static mixer.

2. Increase head pressure.

3. Add compatibilizer to regrind. (See also solutions above to Rough Sheet Surface and Discolored Sheet.)

Degradation In One Layer

May be caused by thermal decomposition:

1. Reduce temperatures at the end of the extruder barrel and of downstream equipment.

2. Reduce head pressure to make screw more efficient by cleaning screens, using a coarser mesh, opening the die lip, and/or shortening the transfer lines.

3. Check for dead spots and streamline flow passages by employing sweep elbows and/or removing melt-immersion thermocouples.

4. Reduce contact with hot equipment. For example, encapsulate heat-sensitive polymer with another polymer.

5. Arrange sheet so that heat-sensitive layer is cooled on the chill roll first

May be caused by a chemical reaction:

1. Remove troublesome additive - such as color or regrind - from the extruder feed.

2. Use additive to stabilize the composition.

3. Check for compatibility between all materials in the coex structure.

Layer Too Thin

May be caused by under-pumping the feedblock:

1. Increase output of the extruder feeding the layer.

2. If output increase in this extruder is not possible, slow the other extruders.

Layer Too Thick

May be caused by over-pumping the feedblock:

1. Decrease output of the extruder feeding the layer.

2. If output reduction in this extruder is not possible, speed up the other extruders.

Sheet Curling

May be caused by differential cooling of layers on one side of the sheet:

1. Set roll-stack temperatures to minimize curl.

2. Adjust melt temperature of individual layers.

3. Adjust layer-thickness uniformity. (See solutions above to Uneven Layer Thickness and Non-Uniform Sheet Color.)

Layers Do Not Adhere

May be caused by adhesive problems:

1. Use a separate resin layer for an adhesive or another type of adhesive. A lower-viscosity adhesive, for example, will foster better wetting.

2. Increase adhesive-layer thickness.

3. Use an adhesive with higher tensile strength.

4. Increase the melt contact time.

5. Increase the adhesive melt temperature.

May be caused by improper equipment operation:

1. Set chill-roll speed to match polymer die-exit speed.

Joseph Powers of Dow Plastics, Midland, Mich., is a technical-service engineer specializing in applied fabrication technology. Troubleshooting information for this article was also contributed by his fellow technical-service engineers, including John Hagen, Joe Dooley, Bill Kirksey, Todd Hogan, Phil Wagner, and Bob Wrisley.
COPYRIGHT 1996 Gardner Publications, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1996, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Author:Powers, Joseph
Publication:Plastics Technology
Date:Aug 1, 1996
Words:3294
Previous Article:Polyolefins get tough with metallocenes.
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