Extrusion-The Center of all polymer processing

ExtrusionThe basic and most common component of all polymer processing equipmentInjection moldingBlown filmProfile extrusion

ExtrusionProfile shapes e.g. rods, fibers, tubes, etc.Films and sheets.Covering on wire and cables.Profile Extrusion Note: Die Swell, Orifice design and Post formingPipe ExtrusionSheet Extrusion Film Extrusion> 0.25mm( 0.01 in) sheet

ExtrusionHas many functionsMeltMixCompoundPressurize

ExtrusionOperation Principle: five stepsThe extruder: plasticated & forced out through the die The die: The hot molten of soft plastics takes shape.Forming: The hot material is further shaped.Post-forming: The material is cut or further shaped.Secondary processing

ExtrusionIs basically a feed screw with heated a heated barrel

ExtrusionWhyPlug heating is not effective because the thermal conductivity of most plastics is too low-proportional to electrical conductivityHeating from the outside-in is not efficient and results in high thermal gradients

ExtrusionThermal conductivityThe amount of heat conducted through a sample:Fourier Law of Conduction:

Heat flowHeat flux (qx) through a wall

.qx12dx1> 2

Heat flowHeat flux (qx)

.qxqx+dxdydx

Heat flowHeat flux (qx)

.qxqx+dxdydx

Solution to heat flowThin filmsLong slabsParticlesMelt removalPressure-induced melt removalDRAG-INDUCED MELT REMOVAL

Melt removalPressure induced removalHeated toolF

Melt removalDrag induced melt removalHeated toolV

ExtruderExtruderHopper Screw: The heart of the extruderThree-zone screw is the most used type (1) Feed zone greatest channel depth(2) Compression zone (transition zone)decreasing channel depth(3) Metering zoneAssures proper delivery amount

Extrusion zone

The Screw is a melt drag design

Screw flowNeed drag at barrel

Melt profileClassical melt cross section

Functions of a screwConveyMixPlasticating (melting)MeteringVenting

Transition (Compression) zonePromote both the compression and heating of the plastic granules. Uniformly tapered, Increasing root diameterReduces the available volume between flightsCompressing the granules.Air is purged back through the hopper. Heating, partly by conduction (15%)mainly by friction from rotary shear (85%)Mixed into a homogenous melt. one-fourth to one-third the entire screw length

Metering sectionAccurately controls amount of meltAssures smooth melt flow

Check valve

Prevent back flow during injectionBall check valveRing check valveLocated at tip of screwScreen pack maybe at final section to trap contaminants

Two stageRelease of entrapped volatiles & moistureBetter meteringBetter appearance, uniformity and properties

Twin ScrewsMore is better (but at a cost)

Twin screwsFrom SPE

Twin ScrewUse with reactive extrusionOften a modular designRelatively expensiveMore difficult to operateGood meltingGood ventingCan overload motor

Twin screwsModular design:

Twin screwsBetter conveying charteristics

Twin screwsFully-intermeshing-co-rotating

Twin screwsFrom SPENo wipingNo wipingWiping

Screw output:Flow (Assuming no back flow!)Vbz=Melt velocity in Z-DirectionFd and Fp=Shape factorW=Channel widthD=Screw diameterN=Screw speed (Hz)h=Channel depthL=Screw lengthP=Pressure=Pitch=Clearancefl=leakage0

Screw sizes15 in dia.!

Die swellExit flow is larger than die opening =D/DoTypically 1.12DDo

Die SwellDie orifice Extruded profile

Die swellAs a function of viscosity:D/DoFaster flow, more molecular alignment in flow channel.

Die swellAs a function of viscosity:D/Do.MW increaseLonger chains fold back on each other once outside die

Die swellAs a function of die temperature:D/Do.Decreasing temperature TLower temperature reduce folding possibility after existing

Die swellAs a function of die design (length/diameter)D/DoL/DChains recover random orientation with flow

Die swellDie designMore die swellLess die swell

Melt fractureShear stress at 105 N/m2Irregular flowLimit flowLimit productionLimit profits

Melt fractureTwo driving forcesSlip and stick-Melt sticks to wall then breaks free causing pulsation in pressureSkin rupture-Die swell causes pressure build up in melt at exist, then with sudden cooling the surface breaks

Melt fractureSlip and stickSkin rupture

Melt fracture-reductionsDecrease entrance angleIncrease temperatureReduce viscosityReduce shear stressIncrease die diameter (reduce stress)Reduce molecular weight

Die DesignThree major partsInlet channelManifoldLand

Die Design

Die Flow (Newtonian flow)For a circular die:

For a rectangular die:

R=Pipe radius=ViscosityL=Pipe lengthP=Pressure drop

W=Pipe widthH=Height of opening=ViscosityL=Pipe lengthP=Pressure drop

Cross over of screw and dieSmall die openingLarge die openingPQFlow of typical screw

Blown film

Calendering95% of sheet &film products are PVC.A series of heated, revolving rollers progressively squeezed thermoplastics stock to the desired thickness in the forms of sheet or film.Products: handbags, shoes, and luggage.Advantages: minimum of cleaningDisadvantages: expensive process

TroubleshootingMelt FractureStreamlining the flow channelReduce shear stressIncrease die temperatureOpening die at land regionReduce extrusion rateChange die wall material (Ceramic insert)Change material (add processing agents)

Troubleshooting VoidsVolatilesDegradation Not enough ventingCooling too fast

Trouble shootingVent flow-Material coming out of ventRoot cause is imbalance between stagesStarve feedingReduce screw speedCool first stageIncrease temperature in second stageOpen die gapCheck screen pack or use low mesh

TroubleshootingAir entrapmentRoot cause-air does not get out of screw in timeUse large sized pelletsUse high compression screwShorten feedUse vented extruderVacuum on hopper

TroubleshootingGels-cross linked particles-two sourcesP-Gels; from polymerization-call supplierE-Gels; from extrusionParticles sticking to screwLook for dead spotClean screw and look for scratches

TroubleshootingPoor mixingAdd mixing section to screw: