3. Problem Cause Solution
Brittleness 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.
Sheet Extrusion Process Troubleshooting Guidelines 2
4. Problem Cause Solution
Contamination Equipment Contamination Clean conveyor idler rolls.
On Sheet, Cont’d.
Clean surface of chill rolls.
Degraded Material Clean buildup from surface around die.
Falling On Sheet
Clean die lips.
Curled Sheet in the Improper Roll Temperature Raise temperature of the concave sheet surface.
Transverse Direction (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- Excessive Pull-Roll Tension Reduce pull roll speed.
chine Direction
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.
Chemical Reaction Use additives to stabilize the composition.
Sheet Extrusion Process Troubleshooting Guidelines 3
5. Problem Cause Solution
Degradation On One Layer, Chemical Reaction Remove troublesome additive such as color or regrind from the
Cont’d. extruder feed.
Check for material compatibility in the coex structure.
Dimples on the Sheet Surface Air Entrapment Decrease screw speed.
(Also called pits, depressions, (Air collected in pockets
pock marks, buck shot and causes voids in the hot Reduce temperatures in the first and second barrel zones.
craters.) sheet which contract upon
sheet cooling leaving se- Unplug the vent if using two-stage screw.
vere depressions. Can be
more severe on vacuum Remove fines from regrind.
forming because depres-
sions become drawn out.) 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.
Check vacuum system for proper operation.
Use nitrogen blanket on the feed hopper.
Use a drying hopper.
Unplug the vent if using two-stage screw.
Sheet Extrusion Process Troubleshooting Guidelines 4
6. Problem Cause Solution
Dimples on the Sheet Surface, Volatiles Unplug the vent if using two-stage screw.
Cont’d.
Check vacuum system for proper operation.
Cool the melt to minimize volatilization due to decomposition.
Volatiles Entrapped Adjust die-roll height to minimize the draping of the sheet
Between the Sheet and on the roll.
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.
Check vacuum system for proper operation.
Adjust die lips in region corresponding to the entrapment.
Insufficient Melt Increase extrusion rate to fill the bank.
Bank at the Roll Tip
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.
Sheet Extrusion Process Troubleshooting Guidelines 5
7. Problem Cause Solution
Dimples on the Sheet Surface, Insufficiently Clean Equipment Dissipate static electricity to minimize dust pickup.
Cont’d.
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.
Discolored Sheet Excessive Temperature or Reduce last one or two barrel zone temperatures and/or
Residence Times 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 Streamline flow path by using smooth transitions and/or el-
Re-extrusion Frequency 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- Add color concentrate at feed throat.
trate or Segregation of the
Color Concentrate in the Increase color concentrate to specified level.
Feed Hopper
Re-design hopper to get mass flow rather than funnel flow.
Herringbone Patterns Inadequate Drying Increase hopper drying time and/or hopper temperature to
(or “Chicken Tracks”), improve drying of pellets
Discontinuous Lines
Sheet Extrusion Process Troubleshooting Guidelines 6
8. Problem Cause Solution
High or Inconsistent Sheet Roll Speed Too Increase extrusion rate.
Orientation (Can reduce physi- High Relative to Die
cal properties of the sheet, Throughput Rate Reduce chill roll speed.
resulting in localized stress
in the thermoformed part. Also, Decrease die gap.
sheet may pull out of clamps
during thermoforming as the Excessive Bank Size Decrease extrusion rate.
sheet shrinks.)
Increase chill roll speed.
Incorrect Die/Nip Align die face with roll center line.
Distance
Reduce die-nip distance to minimum settings.
Pull Roll Tension Increase or decrease pull roll speed.
Incorrect Polymer If the differential between the melt temperature is too wide,
Cooling Rate 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 or Inconsistent Adjust Restrictor Bar.
Patchy Gloss Gauge
Check for extruder surging.
Adjust die lip.
Excessive Melt Change screen-changer screens.
Temperature
Reduce temperatures in the last one or two barrel zones.
Reduce die temperatures.
Sheet Extrusion Process Troubleshooting Guidelines 7
9. Problem Cause Solution
Inconsistent or Patchy Gloss, Non-uniform Roll Check for hot spots on roll circumference and clean flow.
Cont’d. Temperatures
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 Adjust die lips in region corresponding to the entrapment.
Sheet and Roll
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.
Insufficient Sheet Toughness 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.
Excessive Regrind Use additives to restore toughness.
Reduce regrind level.
Orientation See solutions to Orientation Problems.
Sheet Extrusion Process Troubleshooting Guidelines 8
10. Problem Cause Solution
Layers Do Not Adhere Adhesive 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.
Improper Equipment Operation Set chill roll speed to match polymer die-exit speed.
Layer Too Thick Over-Pumping Decrease output of the extruder feeding the layer.
the Feedblock
If output reduction in this extruder is not possible, speed up
the other extruders.
Layer Too Thin Under-Pumping Increase output of the extruder feeding the layer.
the Feedblock
If output increase in this extruder is not possible, slow down
the other extruders.
Lines in the Machine Direction Die Imperfections Disassemble and clean the die.
of the Sheet
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 9
11. Problem Cause Solution
Lines in the Machine Direction Moisture Increase hopper drying time and/or temperature.
of the Sheet, Cont’d.
Equipment Damage Check scuffs (see Scuffs section).
Polish or re-plate damaged chill rolls.
Unstable Bank Size Adjust die-to-nip distance.
Check for extruder surging.
Adjust roll temperatures.
Lines in the Transverse Sheet Sticking to Roll(s) Reduce melt temperature.
Direction of the Sheet
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 Increase pull roll nip pressure.
the Pull Rolls
Decrease winder tension.
Sheet Cut Off Install a “hump” section in conveyor between pull rolls and
shear.
Sheet Extrusion Process Troubleshooting Guidelines 0
12. Problem Cause Solution
Lines in the Transverse Equipment Variation Check and dampen variations from all equipment. (First isolate
Direction of the Sheet, the roll-stack to see if it is at fault.)
Cont’d.
Check for and eliminate electrical voltage variation throughout
the line, by isolating each drive’s power supply.
Rolling Bank at Reduce extrusion rate by reducing screw rpm.
the Nip Roll
Reduce size of bank by increasing polishing roll’s speed.
Open roll-nip gap.
Low Gloss on Sheet (Gloss Improper Process Conditions or Increase roll temperatures.
is partly a material property Equipment
and can be affected by the Increase roll nip pressure.
process temperatures.)
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 Increase head pressure to improve mixing.
Consistency
Use a static mixer before die.
Non-Uniform Sheet Variation in Eliminate thin spots in skin layer—see Uneven Skin Layer
Color the Skin Layer Thickness Problems.
Thickness
Increase skin layer thickness.
Sheet Extrusion Process Troubleshooting Guidelines
13. Problem Cause Solution
Non-Uniform Sheet Poor Regrind Mixing Increase head pressure.
Color, Cont’d.
Use static mixer.
Add compatibilizer to regrind—see Rough Sheet Surface and
Discolored Sheet.
Parabolic Lines in Roll Bank Too Large and If bank is not uniform, adjust die for better gauge.
the Machine Direc- Non-Uniform
tion 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 Redesign hopper to prevent stratification.
mixing regrind with virgin with
inadequate blending the extru- Eliminate stratification of materials by blending and/or segre-
sion rate may vary and pulse.) 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.
Sheet Extrusion Process Troubleshooting Guidelines 2
14. Problem Cause Solution
Poor Transparency, Poor Equipment Clean screw and downstream equipment.
Cont’d. Maintenance
Polish roll surfaces.
Improper Ensure complete melting of the resin by raising the
Temperatures 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 Rolling Bank Reduce the extrusion rate.
is not to be confused with
gloss or sheen. Texture can Reduce bank size by increasing polishing roll speeds.
be seen as microscopic
bumps on the sheet. Grain is Open the roll-nip gap.
a natural property, however
it can be emphasized by the Material Problem Increase mixing in extruder with higher head pressure
conditions of processing that from valving or finer screen mesh.
will affect the gloss.)
Remove any incompatible regrind.
Use a static mixer.
Sheet Extrusion Process Troubleshooting Guidelines 3
15. Problem Cause Solution
Rough Surface Texture, Material Problem Make sure previous materials have been thoroughly purged.
Cont’d. (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.)
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.
Surging Starving Screw Check for material blockage. (Note flow of pellets at throat
(This is the most common (Condition is more prominent and flow at vent.)
problem in extrusion, on multistage screws in which
especially when frequent the forward screw flights are Lower rear barrel zones. (This moves the melt forward and
material changes are made not full of melt.) will increase material flow.)
and/or only one “general
purpose feed screw” is used Lower dryer temperature to reduce the feed temperature.
for all materials. Surging is
present when screw is
not delivering a constant ma-
terial flow and is noted
by gross thickness and/or Insufficient Back Pressure Close die manifold and/or R-bar.
width changes in the product.)
Close gate valve to increase back pressure.
Increase screen pack.
Lower heat in the gate and/or the screen changer.
Sheet Extrusion Process Troubleshooting Guidelines 4
16. Problem Cause Solution
Surging, Cont’d. Non-Uniform Feed Stock Check regrind level.
Temperatures
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.
Thickness Variation in the Improper Extruder Operation Check for extruder surging.
Machine Direction
Rolling Bank Reduce the extrusion rate.
Reduce bank size by increasing polishing roll speeds.
Open the roll-nip gap.
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 Increase roll coolant circulation rate. (Tip: Cooling fluid
Across the Roll Face temperature should not change more than 2°F from
inlet to outlet.)
Decrease extruder output.
Sheet Extrusion Process Troubleshooting Guidelines 5
17. Problem Cause Solution
Thickness Variation in the Winder Overpowering Increase pressure on pull rolls.
Machine Direction, Cont’d. the Pull Rolls
Adjust winder tension.
Drive Variations Adjust roll drive.
Thickness Variation in the Die Adjustment Adjust die restrictor bar, keeping all the bolts tight.
Transverse Direction
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 Non-Uniform Melt Use a static mixer or add a mixing section to the screw.
Transverse Direction Temperatures
Set die, feedblock and transfer line temperatures close to
melt temperatures.
Sheet Extrusion Process Troubleshooting Guidelines 6
18. Problem Cause Solution
Uneven Layer Thickness in Non-Uniform Melt Reduce screw RPM.
Transverse Direction, Cont’d. Temperatures
Increase head pressure to improve mixing.
Rheology or Optimize core-layer rheology by changing the melt temperature
Equipment in extruder, die and feedblock.
Interactions
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 Retract the die restrictor bar to minimize flow disruption.
Equipment
Setup or Op- Check for proper operation of the temperature control system.
eration
Remove remaining flow restrictions such as sharp corners,
melt thermocouples.
Clean buildup from equipment.
Uneven Layer Thick- Improper Extrusion Reduce rolling bank at the chill roll.
ness in the Machine Operation
Direction (Zigzag and Check for extruder surging.
Wave Instability will
both give machine Check to see that roll stack’s pull rolls are functioning properly.
direction thickness (See roll drive problems in Lines in Transverse Direction.)
variation.)
Sheet Extrusion Process Troubleshooting Guidelines 7
19. Problem Cause Solution
Vent Flow (Considered the Output of Screw (First Stage Reduce output of the first stage by raising temperature of the
opposite of surging. The Higher than Second Stage first and/or second extruder barrel zone.
screw is melting and pump- Can Handle)
ing more material than can Increase output of second stage by opening gate valve or
be handled further down- raising the front barrel zone and die temperature or raise the
stream so the excess mate- restrictor bar.
rial comes out of the vent.)
Redesign screw.
Wave Instability (Zigzag and Mismatch in the Rheological Increase skin layer viscosity by switching to a lower MFR resin
Wave Instability will both give Properties of the Different or lowering the skin layer’s melt temp.
machine direction thickness Layers at a Given Set of
variation.) Process Conditions 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 Excessive Interfacial Shear Increase die lip opening.
Wave Instability will both give Stress in Die Land
machine direction thickness Increase relative thickness of skin layer.
variation.)
Reduce throughput.
Decrease viscosity of skin layer by raising its temperature or by
using a resin with a higher MFR.
Raise die lip temperature.
Sheet Extrusion Process Troubleshooting Guidelines 8
20. Corporate Headquarters
2655 White Oak Circle
Aurora, IL 60502
T. 630-585-5800
F. 630-585-5855
PTi Europe
Fibichova 4, Jihlava
586 01 Czech Republic
T. +420 567 214 464
F. +420 567 214 474
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