2010 SHORT COURSE Coextrusion - TAPPITwin Screw. Viscous Encapsulation • Material of lowest...
Transcript of 2010 SHORT COURSE Coextrusion - TAPPITwin Screw. Viscous Encapsulation • Material of lowest...
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2010 SHORT COURSE2010 SHORT COURSE
CoextrusionCoextrusion
Gary OliverExtrusion Dies Industries, LLC
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Outline• Why Coextrude?
• Visco‐Elastic Concepts
– Laminar Flow
– Shear Viscosity, Elongational Viscosity
– Elastic Secondary Flows, Encapsulation
• Dynamic Coextrusion Problems
– Wave, Zig‐zag
• Approaches to Coextrusion
– Feedblock with Single Manifold Die
– Distribution Block with Multimanifold Die
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Why Coextrude?
• Utilize desirable properties of each polymer
• Avoid multi‐pass lamination or coating.
Properties (physical and esthetic)
Tota
l Cos
t: R
aw M
ater
ial +
Con
vers
ion
PE PS PVC PET ABS PVdC PC Nylon EVOH
Coextruded Structures
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Why Coextrude?
• Gas Barrier
• Moisture Barrier
• Heat Sealability
• Toughness
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Shear Viscosity
• Polymer molecules are large and entangled.• As the plastic is sheared, dis-entanglement and
alignment occurs, which reduces the resistance to flow.
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Shear Viscosity
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Elongational Viscosity
• Resistance to stretching. Melt strength is a related measure.
• Linear polymers (LLDPE, HDPE, PP) have poor melt strength.
• Branched polymers (LDPE) have high melt strength.
LDPE
LLDPE
HDPE
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Elastic Effects: Secondary Flows
• PS & LDPE are highly elastic (resist stretching) so the streamlines are rigid, which can result in vortex patterns.
• HDPE has low elasticity, so the flexible streamlines can fill square corners. Two layers of HDPE flowing together will remain less disturbed.
• Ref. TAPPI papers by Joe Dooley.
HDPE PS, LDPE
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Capillary Rheometer
Resin Blends20 Gram SampleNo Mixing
Dynisco LCR 7001
Single ResinsRapid SamplingMultiple Temperatures
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Rheological Analysis
Thermo Scientific HAAKE PolyLab System
Resin BlendsSingle ScrewTwin Screw
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Viscous Encapsulation
• Material of lowest viscosity seeks the area of the highest shear.
Flow Direction Viscosity A (Yellow) < Viscosity B (Blue)
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Shear Viscosity
• Shear Rate is highest at the wall• Lowest Viscosity material is preferred in outer layer• “Rule of Thumb” viscosity ratios of 3:1 maximum
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Wave Instability
•Appears most often in asymmetrical structures
• Most common with thin skin layers. Less likely to occur when the core layer is thick and has highest viscosity. The skin layers are in the area of highest shear.
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Zig-Zag Instability
• Can cause an intermixing of the interface boundary, which can reduce clarity.
• A lower shear-viscosity skin layer is more stable.
• High shear areas, like tight gaps can cause this instability.
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Feedblock with Single Manifold Die
Layers combine first, then spread through the die together.
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Feedblock with Single Manifold Die
Sequencing options: Spools or Plates
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Dow Style Modular Feedblock
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Dow Style Modular Feedblock
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Welex Feedblock
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Black Clawson Micro‐Layer Feedblock
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Cloeren Adjustable FeedblocksCloeren Adjustable Feedblocks
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Cloeren Fixed FeedblocksCloeren Fixed Feedblocks
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EDI Adjustable Feedblock & Single Manifold Die
Usually combine many layers in a sequential fashion. The combination geometry can be variable or fixed during production.
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EDI Fixed Geometry Coextrusion Feedblocks
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Cloeren InfiNanoCloeren InfiNano™™ FeedblockFeedblock
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Dow Multiplier TechnologyDow Multiplier Technology
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EDI Multiplier TechnologyEDI Multiplier Technology
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Multiplier TechnologyMultiplier Technology
First Multiplier
3
6
12
24
48
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Velocity Profile of StructureVelocity Profile of Structure
Highest Shear Area is at the wall
Polymer / Polymer InterfacesPolymer / Polymer Interfaces
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Multi‐Manifold Coextrusion
• Most accurate way to coextrude.
• Melt temperature differences can be accommodated.
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• Most precise way to create a naked edge or an encapsulated edge.
Multi‐Manifold Coextrusion
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Summary
Laminar flow makes coextrusion possible.
Shear viscosity and elongational viscosity can drive different types of interface deformations or coextrusion instabilities.
There are a number of tooling options for layer sequencing and control of layer width and uniformity.
Streamlined adjustable feedblocks provide versatility.
Multimanifold dies can accommodate large differences in melt temperature and viscosity.