p ti troubleshooting guidelines

World Class Extrusion Systems

SHEET EXTRUSIONProcess Troubleshooting Guidelines

Sheet Extrusion Process Troubleshooting Guidelines �

PTi’s Sheet Extrusion Process Trouble-shooting Guidelines offers solutions to many of the problems that sheet produc-ers face in their day-to-day operations. We trust this guide will assist you in creating quality products for today’s chal-lenging sheet extrusion applications.

Table of ContentsSHEET EXTRUSIONProcess Troubleshooting Guidelines Problem Type Page

Brittleness in Sheet ............................................................................................................................2Contamination On Sheet .....................................................................................................................2Curled Sheet in the Transverse Direction ..............................................................................................3Curled Sheet in the Machine Direction .................................................................................................3Degradation On One Layer ..................................................................................................................3Dimples on the Sheet Surface .............................................................................................................4Discolored Sheet ................................................................................................................................6Herringbone Patterns .........................................................................................................................6High or Inconsistent Sheet Orientation .................................................................................................7Inconsistent or Patchy Gloss ...............................................................................................................7Insufficient Sheet Toughness ...............................................................................................................8Layers Do Not Adhere.........................................................................................................................9Layer Too Thick ..................................................................................................................................9Layer Too Thin ...................................................................................................................................9Lines in the Machine Direction of the Sheet .........................................................................................9Lines in the Transverse Direction of the Sheet ......................................................................................10Low Gloss on Sheet ...........................................................................................................................11Non-Uniform Sheet Color ....................................................................................................................11Parabolic Lines in the Machine Direction .............................................................................................12Poor Transparency .............................................................................................................................12Rough Surface Texture .......................................................................................................................13Surging .............................................................................................................................................14Thickness Variation in the Machine Direction .......................................................................................15Thickness Variation in the Transverse Direction ....................................................................................16Uneven Layer Thickness in the Transverse Direction .............................................................................16Uneven Layer Thickness in the Machine Direction ................................................................................17Vent Flow ..........................................................................................................................................18Wave Instability ..................................................................................................................................18Zigzag Instability ................................................................................................................................18

PROCESSING TECHNOLOGIES, LLC

2655 White Oak CircleAurora, IL 60502-9674ph 630.585.5800fx 630.585.5855www.ptiextruders.com

© 2007 Processing Technologies, LLC

PTi reserves the right to make changes to this document without prior notice.

Note: This document is not intended as a substitute for professional judgement and operational expertise of the user. No war-ranty is implied. Actual results may vary.

Sheet Extrusion Process Troubleshooting Guidelines 2

Problem Cause SolutionBrittleness in Sheet Too Much Orientation Adjust roll gap to proper setting. ( Typically the first gap is

aproximately 0.001” to 0.002” UNDER the desired sheet thickness and the second gap should be the same or larger than the desired sheet thickness)See solutions to Orientation.

Cold Rolls Increase roll temperatures.

Poor Melt Consistency Use static mixer in the transfer line.

Reduce regrind levels.

Increase head pressure to improve mixing.

Reduce additives, i.e. color concentrate.

Contamination On Sheet Equipment Problem Check for ruptured screens.

Check for loss of plating, which can result in black specks.

See solutions to Discolored Sheet.

Material Problem Check regrind stream for contamination.

Check airway system and hopper loading procedures for contamination.

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

Remove excess fines from resin.

Static Electricity Thoroughly clean all the areas around the line.

Dissipate static charge on sheet.


Equipment Contamination Clean conveyor idler rolls.

Clean surface of chill rolls.

Degraded Material Falling On Sheet

Clean buildup from surface around die.

Clean die lips.

Curled Sheet in the Transverse Direction

Improper Roll Temperature Raise temperature of the concave sheet surface.(If sheet edges curl down and curling occurs “down stack,” raise middle roll temperature. If sheet edges curl up, lower middle roll temperature. Reverse recommendations when curl-ing occurs “up stack.” In both cases change third roll tempera-ture opposite to middle roll temperature changes.)

Curled Sheet in the Ma-chine Direction

Excessive Pull-Roll Tension Reduce pull roll speed.

Reduce pull roll tension.

Degradation On One Layer Thermal Decomposition Reduce head pressure to make screw more efficient by clean-ing the screens, using a coarser mesh opening the die lip, and/or shortening the transfer lines.

Reduce temperature at the end of extruder barrel and down-stream equipment.

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

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

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

Problem Cause SolutionContamination On Sheet, Cont’d.

Chemical Reaction Use additives to stabilize the composition.


Problem Cause Solution

Dimples on the Sheet Surface (Also called pits, depressions, pock marks, buck shot and craters.)

Air Entrapment(Air collected in pockets causes voids in the hot sheet which contract upon sheet cooling leaving se-vere depressions. Can be more severe on vacuum forming because depres-sions become drawn out.)

Decrease screw speed.

Reduce temperatures in the first and second barrel zones.

Unplug the vent if using two-stage screw. Remove fines from regrind.

Check vacuum system for proper operation.

Reduce hopper drying temperature.

Maintain proper hopper level.

Raise back pressure if using two-stage screw.

Regrind feed stock Change regrind particle size.

Reduce regrind level to control bulk density factor.

Moisture Increase drying time.

Check dryer operation for proper dew point and desiccant regeneration.

Increase drying temperature.


Use nitrogen blanket on the feed hopper.

Use a drying hopper.

Unplug the vent if using two-stage screw.

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

Check for material compatibility in the coex structure.

Degradation On One Layer, Cont’d.

Chemical Reaction


Problem Cause SolutionVolatiles Unplug the vent if using two-stage screw.


Cool the melt to minimize volatilization due to decomposition.

Volatiles Entrapped Between the Sheet and the Roll

Adjust die-roll height to minimize the draping of the sheet on the roll.

Check for hot spots on the die.

Increase or decrease roll temperature.

Increase ventilation to prevent entrapment.

Reduce melt temperature.

Unplug the vent if using two-stage screw.


Adjust die lips in region corresponding to the entrapment.

Insufficient Melt Bank at the Roll Tip

Dimples on the Sheet Surface, Cont’d.

Increase extrusion rate to fill the bank.

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

Decrease roll speed to build bank at the roll nip.

Optimize die temperatures to improve die flow.

Check for extruder surging.

Move die closer to roll nip.

Increase extrusion rate to fill the bank.


Insufficiently Clean Equipment Dissipate static electricity to minimize dust pickup.

Clean chill-roll and polishing roll surface.

Clean idler rolls in conveyor.

Cooling the Sheet too Quickly Raise temperature of the roll in contact with the problem surface.

Problem Cause SolutionDimples on the Sheet Surface, Cont’d.

Discolored Sheet Excessive Temperature or Residence Times

Reduce last one or two barrel zone temperatures and/or reduce the die temperature.

Check for proper operation of the heaters and thermocouples.

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

Dirty Scrap or Regrind Re-extrusion Frequency

Streamline flow path by using smooth transitions and/or el-bows with longer bend ratio.

Clean behind deckles on regular basis. If that fails then remove deckles.

Check for loss of plating or other corrosion.

Insufficient Color Concen-trate or Segregation of the Color Concentrate in the Feed Hopper

Add color concentrate at feed throat.

Increase color concentrate to specified level.

Re-design hopper to get mass flow rather than funnel flow.

Herringbone Patterns (or “Chicken Tracks”), Discontinuous Lines

Inadequate Drying Increase hopper drying time and/or hopper temperature to improve drying of pellets


Problem Cause SolutionHigh or Inconsistent Sheet Orientation (Can reduce physi-cal properties of the sheet, resulting in localized stress in the thermoformed part. Also, sheet may pull out of clamps during thermoforming as the sheet shrinks.)

Roll Speed Too High Relative to Die Throughput Rate

Increase extrusion rate.

Reduce chill roll speed.

Decrease die gap.

Excessive Bank Size Decrease extrusion rate.

Increase chill roll speed.

Incorrect Die/Nip Distance

Align die face with roll center line.

Reduce die-nip distance to minimum settings.

Pull Roll Tension Increase or decrease pull roll speed.

Incorrect Polymer Cooling Rate

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

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

Inconsistent Gauge

Adjust Restrictor Bar.


Adjust die lip.

Inconsistent or Patchy Gloss

Excessive Melt Temperature

Change screen-changer screens.

Reduce temperatures in the last one or two barrel zones.

Reduce die temperatures.


Inconsistent or Patchy Gloss, Cont’d.

Problem Cause SolutionNon-uniform Roll Temperatures

Check for hot spots on roll circumference and clean flow.

Check the delta temperature across the roll’s face. (Typically, the cooling fluid should vary 2°F or less from roll inlet to roll outlet.)

Volatile Entrapment Between Sheet and Roll

Adjust die lips in region corresponding to the entrapment.

Increase ventilation.

Adjust roll height to minimize draping of sheet on roll.

Reduce melt temperature.

Check for hot spots on die.

Unplug vent if using a two stage screw.

Check for proper operation of vacuum system.

Brittle Layers Too Thick Improve transverse sheet-thickness uniformity, especially at edges of the sheet. (See solutions to Uneven Layer Thickness in Transverse Direction.)

Switch to tougher polymers.

Reduce thickness of brittle layers.

Insufficient Sheet Toughness

Excessive Regrind Use additives to restore toughness.

Reduce regrind level.

Orientation See solutions to Orientation Problems.


Problem Cause SolutionAdhesive Problems Use an adhesive with higher tensile strength.

Increase adhesive layer thickness.

Use a separate resin layer for an adhesive or another type of adhesive (a lower viscosity adhesive, for example, will give better wetting).

Increase the adhesive melt temperature.

Increase the melt contact time.

Layers Do Not Adhere

Improper Equipment Operation Set chill roll speed to match polymer die-exit speed.

Over-Pumping the Feedblock

Decrease output of the extruder feeding the layer.

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

Layer Too Thick

Layer Too Thin Under-Pumping the Feedblock

Increase output of the extruder feeding the layer.

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

Lines in the Machine Direction of the Sheet

Die Imperfections Disassemble and clean the die.

Clean die lip area with a soft tool (preferably wood or copper).

Polish damaged die-lip face.

Refinish die lip face to 90 degree angle.

Polish or replate die surface.

Sheet Extrusion Process Troubleshooting Guidelines �0

Moisture Increase hopper drying time and/or temperature.

Equipment Damage Check scuffs (see Scuffs section).

Polish or re-plate damaged chill rolls.

Unstable Bank Size Adjust die-to-nip distance.


Adjust roll temperatures.

Problem Cause SolutionLines in the Machine Direction of the Sheet, Cont’d.

Lines in the Transverse Direction of the Sheet

Sheet Sticking to Roll(s) Reduce melt temperature.

Reduce roll temperature.

Clean the rolls.

Move die closer to the rolls to prevent sheet from sagging.

Roll Drive Problem Raise tension to ensure that the sheet does not drop off the bottom roll.

Reduce pull-roll tension to avoid “”pulling”” the sheet across the sheet rolls.

Adjust drive chain.

Winder Overpowering the Pull Rolls

Increase pull roll nip pressure.

Decrease winder tension.

Sheet Cut Off Install a “hump” section in conveyor between pull rolls and shear.

Sheet Extrusion Process Troubleshooting Guidelines ��

Problem Cause SolutionEquipment Variation Check and dampen variations from all equipment. (First isolate

the roll-stack to see if it is at fault.)

Check for and eliminate electrical voltage variation throughout the line, by isolating each drive’s power supply.

Rolling Bank at the Nip Roll

Reduce extrusion rate by reducing screw rpm.

Reduce size of bank by increasing polishing roll’s speed.

Open roll-nip gap.

Lines in the Transverse Direction of the Sheet, Cont’d.

Low Gloss on Sheet (Gloss is partly a material property and can be affected by the process temperatures.)

Improper Process Conditions or Equipment

Increase roll temperatures.

Increase roll nip pressure.

Increase melt temperature by raising temperatures in last one or two barrel zones.

Raise temperatures to equal the melt temperature.

Adjust to obtain uniform flow.

Adjust roll gaps to maintain good contact with sheet.

Poor Melt Consistency


Use a static mixer before die.

Non-Uniform Sheet Color

Variation in the Skin Layer Thickness

Eliminate thin spots in skin layer—see Uneven Skin Layer Thickness Problems.

Increase skin layer thickness.



Non-Uniform Sheet Color, Cont’d.

Poor Regrind Mixing Increase head pressure.

Use static mixer.

Add compatibilizer to regrind—see Rough Sheet Surface and Discolored Sheet.

Parabolic Lines in the Machine Direc-tion

Roll Bank Too Large and Non-Uniform

If bank is not uniform, adjust die for better gauge.

Reduce pressure on roll where the bank is occurring.

Extruder Surging Check regrind if used. Variable bulk density causes non-uni-form feed.

Starved Feeding Reduce drying time and/or temperature.

Check hopper and feed section temperatures and clean out plug.

Poor Compound Mixing (If mixing regrind with virgin with inadequate blending the extru-sion rate may vary and pulse.)

Redesign hopper to prevent stratification.

Eliminate stratification of materials by blending and/or segre-gating. (An agitator helps.)

Poor Transparency Material Contamination Check regrind for contamination.

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

Remove excess of fines from resin.

Check airway system and hopper loading procedures for possible sources of contamination.

Check for loss of plating which can result in black specks.


Problem Cause SolutionPoor Transparency, Cont’d.

Poor Equipment Maintenance

Clean screw and downstream equipment.

Polish roll surfaces.

ImproperTemperatures

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

Adjust roll temperatures for proper cooling and maintaining the level of crystallinity.

Poor Mixing Use static mixer in transfer line.

Increase head pressure.

Rolling Bank Reduce the extrusion rate.

Reduce bank size by increasing polishing roll speeds.

Open the roll-nip gap.

Rough Surface Texture (This is not to be confused with gloss or sheen. Texture can be seen as microscopic bumps on the sheet. Grain is a natural property, however it can be emphasized by the conditions of processing that will affect the gloss.)




Material Problem Increase mixing in extruder with higher head pressure from valving or finer screen mesh.

Remove any incompatible regrind.

Use a static mixer.



Rough Surface Texture, Cont’d.

Roll Setup Condition Decrease roll gap to polish sheet.

Increase roll temperature to slow solidification.

Increase roll nip pressure to “roll out” roughness.

Clean roll surfaces.

Re-surface damaged rolls.

Make sure previous materials have been thoroughly purged. (Proper purging covers all melt channels–screw, barrel, screen packs, transfer lines, and die, as well as solids handling equipment - pneumatic conveying system, dryers and grinder.)

Eliminate moisture. (See Dimples, Pits, and Pock Marks.)

Material Problem

Surging(This is the most common problem in extrusion, especially when frequent material changes are made and/or only one “general purpose feed screw” is used for all materials. Surging is present when screw is not delivering a constant ma-terial flow and is noted by gross thickness and/or width changes in the product.)

Starving Screw(Condition is more prominent on multistage screws in which the forward screw flights are not full of melt.)

Check for material blockage. (Note flow of pellets at throat and flow at vent.)

Lower rear barrel zones. (This moves the melt forward and will increase material flow.)

Lower dryer temperature to reduce the feed temperature.

Insufficient Back Pressure Close die manifold and/or R-bar.

Close gate valve to increase back pressure.

Increase screen pack.

Lower heat in the gate and/or the screen changer.



Non-Uniform Feed Stock Temperatures

Check regrind level.

Check dryer.

High Stock Temperatures Lower the barrel temperature profile. (High stock temperatures may cause the material to be too soft.)

Poor Screw Design In some cases or for some materials, another screw or a recut of the existing one may be required.

Surging, Cont’d.

Thickness Variation in the Machine Direction

Improper Extruder Operation Check for extruder surging.




Roll Problem Check roll bearings for wear.

Clean rolls.

Check rolls for concentricity.

Check roll nip pressure “PLI” to ensure sufficient pressure to keep roll gap constant.

Surface Temperature Variation Across the Roll Face

Increase roll coolant circulation rate. (Tip: Cooling fluid temperature should not change more than 2°F from inlet to outlet.)

Decrease extruder output.


Winder Overpowering the Pull Rolls

Increase pressure on pull rolls.

Adjust winder tension.

Drive Variations Adjust roll drive.

Problem Cause SolutionThickness Variation in the Machine Direction, Cont’d.

Thickness Variation in the Transverse Direction

Die Adjustment Adjust die restrictor bar, keeping all the bolts tight.

Adjust die lip gap. (Typically the starting gap is between 110-120% of the desired sheet thickness.)

Cool die in thick areas; heat die in thin areas.

Roll Adjustment Balance and/or raise the roll nip pressure to better distribute inconsistent die flow.

Adjust roll gap to proper setting. (Typically the first gap is about 1-2% more than desired sheet thickness. Typical second gap is the same or slightly less than desired sheet thickness.)

Reduce roll pressure to minimize roll bending.

Check roll bearings for wear.

Non-Uniform Bank Check for extruder surging.

Adjust die temperatures to optimize flow uniformity from die.

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

Uneven Layer Thickness in Transverse Direction

Non-Uniform Melt Temperatures

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

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


Reduce screw RPM.


Problem Cause SolutionUneven Layer Thickness in Transverse Direction, Cont’d.

Non-Uniform Melt Temperatures

Rheology or Equipment Interactions

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

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

Modify feedblock’s slot shape or flow configuration.

Modify die design. Multi-manifold die can help.

Improper Equipment Setup or Op-eration

Retract the die restrictor bar to minimize flow disruption.

Check for proper operation of the temperature control system.

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

Clean buildup from equipment.

Uneven Layer Thick-ness in the Machine Direction (Zigzag and Wave Instability will both give machine direction thickness variation.)

Improper Extrusion Operation

Reduce rolling bank at the chill roll.


Check to see that roll stack’s pull rolls are functioning properly. (See roll drive problems in Lines in Transverse Direction.)


Vent Flow (Considered the opposite of surging. The screw is melting and pump-ing more material than can be handled further down-stream so the excess mate-rial comes out of the vent.)

Output of Screw (First Stage Higher than Second Stage Can Handle)

Reduce output of the first stage by raising temperature of the first and/or second extruder barrel zone.

Increase output of second stage by opening gate valve or raising the front barrel zone and die temperature or raise the restrictor bar.

Redesign screw.

Wave Instability (Zigzag and Wave Instability will both give machine direction thickness variation.)

Mismatch in the Rheological Properties of the Different Layers at a Given Set of Process Conditions

Increase skin layer viscosity by switching to a lower MFR resin or lowering the skin layer’s melt temp.

Decrease core layer viscosity by changing to a higher MFR resin or by raising the core layer’s melt temp.

Make the structure more symmetrical.

Use a different die design. (A multi-manifold die helps.)

Correct velocity mismatches in the feedblock.


Zigzag Instability (Zigzag and Wave Instability will both give machine direction thickness variation.)

Excessive Interfacial Shear Stress in Die Land

Increase die lip opening.

Increase relative thickness of skin layer.

Reduce throughput.

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

Raise die lip temperature.

Corporate Headquarters2655 White Oak CircleAurora, IL 60502T. 630-585-5800F. 630-585-5855

PTi EuropeFibichova 4, Jihlava586 01 Czech RepublicT. +420 567 214 464F. +420 567 214 474

www.ptiextruders.com

USA - CANADA - EUROPE - ASIA - LATIN AMERICA

p ti troubleshooting guidelines

Technology

sheet surface

discolored sheet

sheet toughness

produccurled sheet

inconsistent sheet orientation

machine direction

transverse direction

nonuniform sheet color