film coextrusion troubleshooting 7832

8/3/2019 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


Problem: Melt fracture of film surfacePossible cause: Same as for uncharacteristically high, film-haze problem


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FILM COEXTRUSIONA Troubleshooting Guide


6/29FILM COEXTRUSION: A Troubleshooting Guide 02

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.



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



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



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



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



Layer Thickness InstabilityCaused by Pressure Variation

FLOWDIRECTION

STABLE FLOW

Uniform thickness

ONSET OF INSTABILITY

Small gauge variat ion

SEVERE INSTABILITY

Large gauge variation



Extrusion FlangeBack Pressure Adjustment

ADJUSTABLE VALVE

INSTRUM ENT PORT



Typical Arrangement ofCoarse and Fine Screens Betw een

the Screw and Breaker Plate

BREAKERPLATE

COARSE SCREEN

FINE SCREENS

COARSE SCREEN

RESIN FLOW



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



Layer Thickness Instability Caused byNonuniform M elt Temperature

FLOWDIRECTION

STABLE FLOW

Uniform thickness


Small gauge variat ion

SEVERE INSTABILITY

Large gauge variation



Adjustable Depth ProbeM elt Thermocouple

EXPOSEDTHERMOCOUPLE

JUNCTION



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)



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



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.



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



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



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)



Interfacial Instabilit y in CoextrusionCaused by Critical Shear Stress

FLOWDIRECTION

STABLE FLOW

Smooth interface


Wavy interface develops

SEVERE INSTABILITY

Propagat ion of wavinessto surface



Polymer Interfaces and Critical ShearStresses Encountered in Coextrusion

THEORETICALSHEAR STRESS

Highest

Intermediate

Zero

METAL

METAL

POLYMER 1

POLYMER 2

POLYMER 1



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)



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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Polyolefins Sales Offices

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.

Responsible C

film coextrusion troubleshooting 7832

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