Extrusion-The Center of all polymer processing
ExtrusionThe basic and most common component of all polymer processing equipmentInjection moldingBlown filmProfile extrusion
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: