fundamental concepts of extrusion design - · pdf filefundamental concepts of extrusion design...
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Fundamental Concepts ofExtrusion Design
Presented By:Lou Piffer
Davis-Standard Corp.
Tappi PLACE Extrusion Coating and Blown/Cast Film Short Course
August 24th-26th, 2010Charleston, SC
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Extruder Components
• extruder screw• extruder barrel• feed throat• hopper
• extruder drive• reducer• barrel heating and cooling
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AC / DC Drives & Motors
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Drive Motor
• 6” - 400/500 HP• 4 1/2” - 250 HP• 3 1/2” - 150 HP• 2 1/2” - 75 HP
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GearBox
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Gearbox• Converts high speed output shaft from drive
(1750 RPM) to screw speed
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Gearbox• Closed loop oil
circulation system insures gears do not see excessive heat
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Thrust bearing
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B-10 Life
Expected Life of 9 out of ten bearings at a given
condition
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Predictive Maintenance
• Bronze in oil - out of round thrust bearing• Steel in oil - bad bearing or gear tooth• Cast iron powder in oil - moving part
against gear box casting
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Feedsection
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Feed Section
• Cast iron jacketed• Initial point where
resin is introduced• Space between gear
box and resin feeding point
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Resin Hopper
• Hopper often cooled to prevent bridging
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hopper
drive
belt drive
screw coolinggearbox
cylinder(barrel)
heaters-coolers feed section
cooling
screw
screenchanger
structure
PARTS OF THE EXTRUDER
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Extruder Barrel
• 1/16” Bimetalic Alloy• Bolted to feedsection• The barrel is harder
than the screw.• Sliding barrel support
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Barrel Types
• Non abrasive environments - Chromium-modified boron-iron alloy.
• Abrasive environments - Tungsten carbide in a matrix of nickel alloy.
• Abrasive style barrel may cost 25% more but last 3X longer!
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Barrel Wear
• 1/16” (.0625”) thick when new• Should look at replacing barrel when there
has be .030” of wear.
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Screw Wear
• Initial clearance of 1 mil/extruder diameter/side
• Example . 4.5”extruder will have initial clearance of approximately .0045/side
• Look to replacing screw when clearance doubles to triples. (In above example when clearance is .009 - .0135”)
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Barrel Heater Types
• Ceramic Band (heat only)• Electric Heat - Air Cooled• Electric Heat - Water Cooled
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Water cooled Heaters
• Jacketed Heater• Solenoid valves• Inlet/outlet
manifolds.• Water reservoir
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Water cooled Heaters
• Jacketed Heater• Solenoid valves• Inlet/outlet manifolds.• Water reservoir
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Air cooled Heaters
• Finned Heater• Air Blowers
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Air cooled Heaters
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Extruder Adapter
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Extruder Adapter
• Change resin flow • Change pressure• House screen pack
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screw
movable valve
melt channel
pressure sensor
screen pack
Valved Adapter
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hopper
drive
belt drive
screw coolinggearbox
cylinder(barrel)
heaters-coolers feed section
cooling
screw
screenchanger
structure
PARTS OF THE EXTRUDER
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Extruder Adapter
• Change resin flow • Change pressure• House screen pack
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INLETHeater Bands 180° helix elements
induce flow division
DispersionLug
Motionless mixers improves both melttemperature uniformity and color dispersion
(4 elements or more)
OUTLETStatic Mixer Principle
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Static Mixer Principle
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Fundamental Concepts ofScrew Design
Presented By:Lou Piffer
Davis-Standard Corp.
Tappi PLACE Extrusion Coating and Blown/Cast Film Short Course
August 24th-26th, 2010Charleston, SC
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What will I learn?
• Why can’t my screw run all resins?• Why are barrel profiles unique to a screw
design?• Why can increased barrel temperature
sometimes result in less output?• What happens when my screw wears out?
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Viscous Heat Generation
Heat = Viscosity x Shear Rate²
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Screw Terminology
Feed ZoneFeed Zone
Barrier (or Melting) ZoneBarrier (or Melting) Zone
Metering ZoneMetering Zone
Mixing ZoneMixing Zone
Flight LengthFlight Length
Overall LengthOverall Length
ShankShank
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Screw Terminology
LeadLead
FlightFlightWidthWidth
ChannelChannelDepthDepth
HelixHelixAngleAngleScrewScrew
O.D.O.D.
BarrelBarrel
Bimetallic LinerBimetallic Liner
FlightFlightClearanceClearance
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Typical Screw Classifications
• Metering Screw
• Mixing Screw
• Barrier Screw
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Important Ratio Terms
• L/D = Length/Diameter• CR = Compression Ratio
CR = Feed Channel DepthMeter Channel Depth
– Compression Ratio has no meaning for barrier type screw designs
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Primary Functions of a Single Screw Extruder
• Optimized screw design must accomplish– Solids Conveying– Melting– Pumping– Mixing
• Each component is critical to the extrusion process
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Primary Functions of a Single Screw Extruder
Feeding Melting Pumping / Mixing
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Feeding/Solids Conveying
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Solids Conveying• Formation of a compacted solid bed
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Some of the Most Important Bulk Properties:
• bulk density• compressibility• internal coefficient of friction• external coefficient of friction• particle size and particle distribution• particle shape and particle distribution
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Plastic Forms for Extrusion
Powdergranules
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Resin Hopper
• Hopper often cooled to prevent bridging
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Drag Induced Conveying
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The frictional force at the barrelis responsible
for moving the material forward
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For efficient forward conveying we wanthigh friction on the barrel and low friction
on the screw
If there is no barrel friction, then there can be no forward conveying
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Solids Conveying
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A wide distribution in particle sizemakes the material more difficult
to convey and can lead toseparation
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Melting
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Melting• Formation of a compacted solid bed• Development of a melt film
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Melt Film / Solid BedInterface
Melt Film / Barrel WallInterface
MechanicalDeformation
Conductive Meltingw/ Forced Melt Removal
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Melting• Formation of a compacted solid bed• Development of a melt film• Removal of melt into an accumulating
melt pool
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Contiguous Solids Melting
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Viscous Heat Generation
Heat = Viscosity x Shear Rate²
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Factors that effect the melt rate
• Melt Film Thickness
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Contiguous Solids Melting
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Factors that effect the melt rate
• Melt Film Thickness• Barrel Temperatures
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Contiguous Solids Melting
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Factors that effect the melt rate
• Melt Film Thickness• Barrel Temperatures• Screw Speed
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Contiguous Solids Melting
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Barrier vs. Single Screw Melting
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Single Stage Mixing Screw vs. Barrier Screw
Mixing and Barrier Screws
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B A R R E L
Melting Model – Single Flighted Screw
Material Flow
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BARRIER SCREW
CONVENTIONALSCREW
MELTING MECHANISM SEEN IN SCREW MELTING MECHANISM SEEN IN SCREW PUSHPUSH--OUTSOUTS
MELTPOOL
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Solid Bed Break-UpSolid Bed Break-Up
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1600
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0 5 10 15 20 25 30
Time (seconds)
Pres
sure
(PSI
)
Cyclic variation in pressure profile
Movement of solid bed
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Solid Bed Break-Up
Solid Bed Break-Up
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Time (seconds)
Pres
sure
(PSI
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Movement of Solid Bed
Solid bed in solids channel Full melt in solids channel
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MeltingBarrier Screw
• Barrier flight isolates solid bed from melt pool• Solids and melt channel depths vary independently
to accommodate accumulating melt pool and diminishing solid bed
• Critical design variables:– Solids channel compression– Barrier flight undercut
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Melting - Barrier Screw
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Solids Channel
Solids Channel
Barrier Flight
Melt Channel
Barrier Flight
Melt Channel
Mid Barrier Cross-Sectional Photos
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Melt TemperatureInfluenced primarily by two factors:
• Conduction= f (residence time, barrel set-points, limitations
of heating/cooling units...)• Viscous Energy Dissipation (VED)
= f (viscosity, shear rate...)
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Mixing in Single Screw Extrusion
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Mixing• DISTRIBUTIVE
– Increase of interfacial area between two miscible components– Splitting and recombination of flow streams
• DISPERSIVE– Shear induced particle size reduction of immiscible components
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Mixing and Quality
MicroMicro--graphs 100X 0.15 MI HDPE+5% CBMBgraphs 100X 0.15 MI HDPE+5% CBMB
• Poor mixing can effect:– Physical properties– Product Uniformity– Aesthetics
Excellent PoorDispersive
PoorDispersive &Distributive
PoorDistributive
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Mixing Sections
DISPERSIVE• Union Carbide Mixer• Helical UCC• Cavity Transfer Mixer
DISTRIBUTIVE• Pineapple Mixer• Saxton Mixer• Dulmage Mixer
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Pineapple Mixing Section
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Pineapple Mixing Section
Streamlines Showing Distributive Mixing
100X HDPE+5% CBMB100X HDPE+5% CBMB
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Helical Union Carbide Mixing Section (UCC-T)
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Helical Union Carbide MixingFlow Pattern
Channel Cross Section
100X HDPE+5%CBMB100X HDPE+5%CBMB
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EGAN Mixer
• Similar to UCC mixer• Higher pressure drop• Intensive shearing• Surface of barrel not wiped by flight
(completely under-cut)
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Screw Designs forExtrusion Coating Applications
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Conventional Mixing Screws• Workhorse design for processing extrusion
coating resins since the 1960’s• Typical Design
– 28:1 - 32:1 L/D– Shallow Metering Depth– 4:1 Compression Ratio (CR)– Single mixing section (Egan or UCC)
Feed Transition Meter Mix Meter
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Double Mixing Screw• A conventional screw with two barrier mixing
sections introduced in the 1980’s– Boost output rate (10-15%)– Improve reliability of adhesion at higher outputs
• Higher output applications necessitate the use of a static mixer to improve melt temperature homogeneity across the melt curtain
Feed Transition Meter Mix Meter Mix
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Barrier Screw Designs
• Recommended for LLDPE and engineering resins in extrusion coating applications
• Also beneficial for lower temperature / higher output polyolefin applications
• Offers superior melting rates and process stability
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Performance Comparison
8”, 30L/D ExtruderResin: LLDPE Dowlex 3310
Conventional vs. Barrier
8", 30L/D PerformanceLLDPE Dowlex 3310
285
290
295
300
305
310
315
0 10 20 30 40 50 60 70
Screw Speed (RPM)M
elt T
emp
(C)
Barrier ScrewConventional Screw
8", 30L/D PerformanceLLDPE Dowlex 3310
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0 10 20 30 40 50 60 70
Screw Speed (RPM)
Out
put R
ate
(KG
/HR
)
Barrier ScrewConventional Screw
Barrier screw providesBarrier screw provideshigher rate and lower higher rate and lower melt temperaturesmelt temperatures
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SCREW COMPARISON
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What will I learn?
• Why can’t my screw run all resins?• Why are barrel profiles unique to a screw
design?• Why can increased barrel temperature
sometimes result in less output?• What happens when my screw wears out?
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LDPE PPSolid Density (kg/m3) 920 907Melt Density (kg/m3) 750 735Thermal Conductivity, Solid(J/s.m.°C)
0.28 0.22
Thermal Conductivity, Melt(J/s.m.°C)
0.24 0.15
Heat Capacity, Solid (J/kg.°C) 2300 1700Heat Capacity, Melt (J/kg.°C) 2300 2100Melting or Softening Temp, °C 110 163Freezing or Hardening Temp, °C 90 138Frictional Coefficient, f 0.3 0.22Degree of Crystallinity Low HighMolecular Weight Distribution Narrow Broad
LDPE VS PP COMPARISON
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Solid Conveying - PP vs. LDPE
• PP more crystalline, pellets are harder and less elastic. Lower friction between PP and barrel.
• PP has higher melt temperature. Requires longer feed before compression starts.
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Melt Conveying - PP vs. LDPE
• Melting rate is limiting factor for PP.• Melt conveying rate is limiting factor in
LDPE
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General Screw Design - PP vs. LDPE• LDPE screw would have shorter feed and
faster transition.• PP screw would have a shallower metering
section to help in melting.