film coextrusion troubleshooting 7832
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FILM COEXTRUSIONA Troubleshooting Guide
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FILM COEXTRUSION:
A TROUBLESHOOTING GUIDE
Paul H. Jackson, Equistar Chemicals, LP
(This article originally appeared inConverting Magazine, November 1994.)
Successful production of coextruded products depends on several key factors,including polymer selections, hardware design (screw, feedblock/die, handling
equipment, layer construction and optimal processing conditions). Proper selectionand adjustment of each factor will minimize difficulties and ensure high quality
coextrusion results.Troubleshooting methods for coextrusion become increasingly complex as the
number of layers in the structure increases, as the asymmetry of multi-layer con-struction grows, or as processing and rheological characteristics of coextrudedmaterials differ greatly from one another. Understanding the problems associated
with nonuniform layer distribution and interfacial instability between layers or onfilm surfaces is very important when troubleshooting the coextrusion process.
THE UNIFORMITY PROBLEM
Nonuniform layer distribution is one of the more common problems encoun-
tered in film coextrusion. This nonuniformity may appear in either the direction ofextrusion or tangential to the direction of film production.
Layer uniformity in the machine direction can be influenced by die imperfec-
tions, poor die design or adjustment, excessive extruder pressure variation, variablefilm tension, or film bubble or web instability.
Layer uniformity tangential to machine direction can be influenced by poormelt temperature uniformity, viscosity mismatch between layers, poor hardware
design, or viscoelastic f low characteristics induced by excessive shear stress.Poor layer uniformity tangential to machine direction is caused by nonuniform
melt temperature across a melt pipe, feedblock and/or die, as well as poor melt-ing in an extruder. Melt temperature variance alters viscosity uniformity, whichexhibits a change in flow characteristics and layer distribution. Melt temperature
of a single polymer stream can of ten vary by as much as 30F. A general rule ofthumb is to achieve 2F or less variation in melt temperature for each extruder.
Homogenous melt temperatures can be achieved through installation of a staticmixer in the melt pipe, a dynamic mixer on the extrusion screw or a more efficient
screw design, or through adjustment of pipe, feedblock and/or die temperatures.Variation in the thickness of a film, which eventually reaches a steady-state
condition of nonuniformity (assuming homogenous melt temperature conditions
for each polymer), can be caused by a viscosity mismatch between layers. In acoextrusion system, lower-viscosity polymers migrate to the region of highest
shear stress (nearest the die wall) and tend to encapsulate higher-viscosity poly-mers. The amount of migration is dependent on the degree of viscosity mismatch,
the length of the flow path, and the shear stress in the system.
IMPROVING LAYER VARIATION
Improvements in layer variations that are caused by viscosity-induced flow
behavior can be achieved through adjustment of melt temperature, modification ofdistribution channels in the feedblock or die, or selection of a polymer with different
viscosity characteristics which most commonly are measured by melt index. Also,annular dies typically are more tolerant of viscosity mismatch than flat-die systems.
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Nonuniform layer distribution in the direction tangential to extrusion can also
be caused by poor hardware design. Improperly designed flow channels of thefeedblock or die can cause poor steady-state layer distribution of materials, evenwith the most closely matched viscosities.
Nonuniform distribution of layers, in the form of parabolic flow lines, inter-mixing of layers, roughness between polymer boundaries, melt fracture, or
uncharacteristically high haze, can be caused by interfacial instability between
layers or on film surfaces. The instabilities are believed to be a result of theviscoelastic behavior of polymers at the die land or region of highest shear stress.Improvements in layer instabilities can be achieved by reducing the shear
stress between coextrusion layers and/or the die-land surfaces. Shear stress can be
reduced by decreasing total output rate, increasing skin-layer melt temperature(decrease in viscosity), increasing the die gap, adding a process lubricant to the
skin material, or selecting a lower viscosity material.An increase in the thickness of the skin layer can also reduce instability
between polymer layers by moving the interface further from the high-shear-stressdie wall. This is especially significant for asymmetric coextrusion constructions.
Finally, if coextrusion layers exhibit dramatic differences in melt elasticity, then
choose materials that match more closely in extrudate elasticity as measured byextrudate swell.
TROUBLESHOOTING AT A GLANCE
Problem: Lines in the film surfacePossible cause: Die imperfections
Solutions: Clean die buildup
Remove contaminants from polymer melt channel Repair die nicks and burrs
Problem: Gauge bands on film rollPossible cause: Poor die design
Solutions:
Install spiral-channel die design to eliminate weld lines Install rotating nip assembly in tower
Possible cause: Poor die adjustment
Solutions: Adjust concentricity of die gap Center air ring in relation to die gap
Problem: Repeating pattern of variation in thickness of layer(s)
Possible cause: Excessive extruder pressure variation (surging)Solutions:
Achieve 1 percent or less variation in total head pressure for each extruder Adjust extruder temperature profile (feed and transition zones) Increase back pressure with restrictor f low plug
Increase back pressure by installing f ine-mesh screen pack Change screw design of surging extruder(s)
Check for worn screw(s) and replace if needed Check extruder feedthroat(s) for bridging and correct if needed
Possible cause: Variable f ilm tensionSolution:
Eliminate variability in drive speed
Possible cause: Film bubble instability
Solutions: Protect bubble from atmospheric air turbulence Correct pressure instability of air ring and/or internal air flow
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Problem: Intermittent and somewhat random variation in thickness of layer(s)
Possible cause: Poor melt temperature uniformitySolutions:
Achieve 2F or less variation in melt temperature for each extruder
Adjust extruder temperature profile to ensure complete melting ofextrudate
Install new screw design with dynamic mixer for more eff icient melting
capacity Reduce screw speed for increased residence time to complete meltingof the polymer(s)
Adjust temperature of feed channels, die and/or feedblock
Problem: Variation in thickness of layer(s) that reaches steady-state distribution
Possible cause: Viscosity mismatch of polymer layersSolutions:
Select polymers with matching viscosities Adjust temperature of polymers to aid in matching viscosities
Possible cause: Poor hardware designSolution:
Change die and/or feedblock design
Problem: Uncharacteristically high film haze
Possible cause: Viscoelastic flow characteristics induced by excessive shear stressbetween layers and/or feedblock/die surfaces
Solutions: Select lower-viscosity skin layer(s) Increase melt temperature of skin-layer polymers
Increase die temperatures Reduce total extrusion output
Increase die-gap opening Add process lubricant to skin-layer polymer
Increase thickness of skin layers Select polymer(s) that exhibit similar melt elasticity behavior
(extrudate swell)
Problem: Parabolic-shaped flow lines in direction of extrusionPossible cause: Same as for uncharacteristically high, film-haze problemSolution:
Same as for uncharacteristically high, film-haze problem
Problem: Intermixing of polymer layersPossible cause: Same as for uncharacteristically high, film-haze problem
Solution: Same as for uncharacteristically high, film-haze problem
Problem: Roughness between polymer-layer boundariesPossible cause: Same as for uncharacteristically high, film-haze problem
Solution: Same as for uncharacteristically high, film-haze problem
Problem: Melt fracture of film surfacePossible cause: Same as for uncharacteristically high, film-haze problem
Solution: Same as for uncharacteristically high, film-haze problem
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FILM COEXTRUSIONA Troubleshooting Guide
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Coextrusion Processing Defined
The conversion of mult iple thermoplast ics,
f lowing through separate st reams, that are
combined into a common primary passage and
then shaped by a die. Multiple layers provide
propert ies that cannot be provided by a single
material for high barrier coextrusion processing.
The main classes are: f ilm; sheet ; tubing;
coat ing; and blowmolded shapes.
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Successful Product ion of Coext rudedProducts Depends on Three Key Factors
1.Polymer Selections
2.Design of Hardware
Screws, Feedblock/Die, Handling
3.Coextrusion Layer Const ruct ion
4.Opt imal Processing Condit ions
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Layer Uniformity is Inf luenced by:
Variations in ext rusion pressure
Nominal ext rusion melt t emperature
Viscosity-induced web f low
Bubble or melt instability
Variable f ilm tension
Poor die design or improper adjustment
Die imperfect ions or contaminants
Interfacial Flow Instabilityis Caused by:
Interfacial crit ical shear st ress
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Extrusion Pressure Variations
Variation in extrusion pressure, often referred
to as surging, is directly related to feeding
stability of an ext ruder.
Improving layer uniformity caused by pressurevariat ion can be achieved through:
Adjustment of back pressure
Screenback
Rest rictor f low plug
Ext rusion screw design of feed and t ransit ionsections
Adjustment of ext rusion screw-temperature
profile
Prevent polymer bridging in feedthroat
Replace/repair worn ext rusion screw
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Head Pressure Trace
0 15 30 45 60 75 90 105 120
TIME (seconds)
1850
1840
1830
1820
1810
1800
1790
1780
1770
1760
1750
HEAD
PRESSURE
1805
1795
1785
1775
1765
1755
1745
1734
1725
1715
1705
HEAD
PRESSURE
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Layer Thickness InstabilityCaused by Pressure Variation
FLOWDIRECTION
STABLE FLOW
Uniform thickness
ONSET OF INSTABILITY
Small gauge variat ion
SEVERE INSTABILITY
Large gauge variation
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Extrusion FlangeBack Pressure Adjustment
ADJUSTABLE VALVE
INSTRUM ENT PORT
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Typical Arrangement ofCoarse and Fine Screens Betw een
the Screw and Breaker Plate
BREAKERPLATE
COARSE SCREEN
FINE SCREENS
COARSE SCREEN
RESIN FLOW
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M elt Temperature Variations
Nonuniform melt temperature across a melt
pipe, as well as poor polymer melt ing in an
extruder, cause poor layer unif ormity.
Homogenous polymer melt temperatures
can be achieved through:
Stat ic and/or dynamic mixers
Adjustment of pipe and die temperatures
Adjustment of ext rusion screw temperature
profile
Replace/repair worn ext rusion screw
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414
410
406
402
398
F 394
390
386
382
378
374
0 1/8 1/4 3/8 1/2 5/8 3/4 7/8 0
BARRELWALL
BARRELCENTER LINE
BARRELWALL
Typical Temperature Profileof Polymer M elt Stream in Pipe
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Layer Thickness Instability Caused byNonuniform M elt Temperature
FLOWDIRECTION
STABLE FLOW
Uniform thickness
ONSET OF INSTABILITY
Small gauge variat ion
SEVERE INSTABILITY
Large gauge variation
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Adjustable Depth ProbeM elt Thermocouple
EXPOSEDTHERMOCOUPLE
JUNCTION
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Distributive Mixing Sections
Dulmadge mixing sect ion (Dow)
Saxton mixing sect ion (DuPont)
Pin mixing section (Barmag)
Pineapple mixing sect ion
Cavit y-t ransfer mixing sect ion
(Davis-Standard)
Slot ted-screw f light (Axon)
Two types of stat ic mixers(Kenics and Ross ISG)
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Viscosity Related Behavior
Melt index select ion and the related viscosit y-
induced f low behavior affect layer uniformity.
Improvement of layer variations causedby viscosity-induced flow behavior can be
achieved through:
Adjustment of melt t emperature
Modif icat ion of dist ribut ion channels
Select polymer of dif ferent viscosity or
viscoelast ic characterist ic
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Viscosity-Induced Flow Behavior
P1
P2
P1
P2
FLOW DIRECTIONTHROUGH A PIPE
Migrat ion of low er viscosit y polymer
to t he region of highest shear st ress.The effect is encapsulation of P2 by P1.
The amount of migration or encapsulat ionis dependent on the degree of viscositymismatch, the length of the f low path andthe cri t ical shear st ress for the given system.
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Viscosity vs. Shear Ratefor Various Polymers @ 230C
100000
10000
1000
100
10 100 1000 10000SHEAR RATE (1/sec)
VISCOSITY (poise)
EVOH
1.5 MI
EVOH5.5 MI
PPHIPS
LDPE
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Interfacial Flow Instability
Excessive shear stress between layers, feedblock
and/or die surfaces causes interfacial flow
instability. Drag f low and dif ferences in polymer
velocities of multiple layers create shear stresses.
Reduction of interfacial shear stress near a die
wall can be achieved through:
Decreasing skin-layer viscosit y or increasing
melt temperature
Increasing die temperature
Increasing skin-layer thickness
Reducing total ext rusion output
Increasing die-gap opening Adding process lubricant to skin layer
Select ing polymers that exhibit similar
melt elasticity
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Critical Shear Stress of PolymersViscosity vs. Shear Rate
Non-Newt onian Behaviorof Polymers
LOG
Viscosity(po
ise)
Newtonian Fluid Behavior
Smooth Extrudate
Irregular Distort ion Occursat Critical Shear Stress
(Melt Fracture)
LOG Shear Rate (1/sec)
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Interfacial Instabilit y in CoextrusionCaused by Critical Shear Stress
FLOWDIRECTION
STABLE FLOW
Smooth interface
ONSET OF INSTABILITY
Wavy interface develops
SEVERE INSTABILITY
Propagat ion of wavinessto surface
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Polymer Interfaces and Critical ShearStresses Encountered in Coextrusion
THEORETICALSHEAR STRESS
Highest
Intermediate
Zero
METAL
METAL
POLYMER 1
POLYMER 2
POLYMER 1
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Coextrusion Melt-Flow Model
Melt Velocit y
Shear Rate
POLYMER 1
POLYMER 2
POLYMER 3
A
B
A = Stable Behavior
B = Crit ical Behavior(Occurrence of Instability)
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An Important Note
Pressure variat ion, nonuniform melt
temperatures, viscosity mismatch and the
effects of critical shear rate instabilities seldom
occur independently of one another. They
most of ten occur simultaneously, wit h variance
in the degree of severit y.
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National and InternationalSales OfficeEquistar Chemicals, LP1221 McKinney Street, Suite 1600P.O. Box 2583
Houston, TX 77252-2583Sales(800) 615-8999Customer Service(888) 777-0232
Plexar is a registered trademark of Equistar Chemicals, LP.
The information in this document is, to our knowledge, trueand accurate. However, since the particular uses and theactual conditions of use of our products are beyond ourcontrol, establishing satisfactory performance of ourproducts for the intended application is the customerssole responsibility. All uses of Equistar products and anywritten or oral information, suggestions or technical advicefrom Equistar is without warranty, express or implied, andis not an inducement to use any process or product inconflict with any patent.
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