Presentation on theme: "MFGT 142 Extrusion Manufacturing"— Presentation transcript:
1 MFGT 142 Extrusion Manufacturing Professor Joe GreeneCSU, CHICO
2 Chapter 11: Extrusion Manufacturing OverviewPurpose, advantages, disadvantages, and cost elementsExtrusion problems and trouble shootingMaterial and product considerationsPost-extrusion formingCoextrusion
3 Plant concepts (Layout and Controllers) Extrusion lines are long, typically 45 feet.Line should be straight to minimize stresses as material flow around a curveMaterial fed from vacuum fed gaylords or from mezzanine aboveControllersFeedback or automatic controllers and monitors are used extensively for monitoring portion of line including heat zonesThermocouples are placed along outside of barrel to sense temperature and send signal to controller.Pressure in the melt is measured by a thin metal disk set flush with the wall at the tip of the screw inside the barrel, where it measures P at screen pack, thrush bearing load, and mixing conditions in the final turn of screw.
4 Extruder Capacity Example Capacity- sized by the diameter of the screwTotal flow rate of an extruder isTotal flow = drag flow - pressure flow - leakage flowDrag flow is a measure of the amount of material that is dragged through the extruder by the friction action of the barrel and the screw.Pressure flow is the flow that is caused by the back pressure inside the extruder. Pressure flow is counter to drag flow and thus is negative.Leakage flow is amount of material that leaks past the screw in the small space between the screw and the barrel. Leakage flow is counter to drag flow and thus is negative.Drag flow is calculated by classical fluid mechanics asScrew DiameterFlight depthin meteringsectionPitch angleSpeed of the screw
5 Extruder Capacity Example You can increase output (capacity) of the extruder byIncreasing diameter of screwIncreasing screw speedIncreasing the flight depthOptimum pitch angle depends strongly on the number of flights, flight width, and the screw diameter. Pitch angle is usually constant at 17.5°Pressure flow component is found from classical pipe flowwhere D is the diameter of the screw, H is the flight depth, P is the back pressure, is the pitch angle, is the viscosity, and L is the length between flights
6 Extruder Capacity Example Leak flow is small compared to drag and pressure flow and may usually be neglected in finding total flow. Then,Total flow = Drag flow - Pressure flowIn practice, the screw dimensional parameters (D,H, , L) are combined into two constants and which simplifies equation toThus, output is increased byincreasing extruder speed, N,decreasing back pressure, P, by keeping screen packs unclogged.Important to monitor pressure drop around screen pack
7 Extruder Capacity Example The output can be increased by increasing the viscosity.Thus, increasing the viscosity by raising the temp will decrease the output because the pressure flow is increased.Remember, back pressure has a greater influence on low-viscosity materials and will retard their advance.To compare the output of screws at various diametersOutput for Twin ScrewsScrew SpeedScrew Diameter
8 Normal Operation and Control of Process Extrusion is a continuous, stable, steady-state processAchieving stable, steady-state operation requires considerable effortDisruptions to be avoidedshutdown and start-upresin changesdie changesscreen pack changes
9 Start-upStart-up procedure is the bringing up the production line from a static conditionStart-up stepspreheat extruder including the screw and the dieopen end of extruder and remove screen pack for cleaningload hopper with material (pre-dried if required)rotate screw slowly at firstfill screw with desired material and flush out previous purge resin.rotate screw at desired setting and bring extrusion to steady statestring up the extrudate by pulling extrudate into the pullerpush extrudate into cooling bath and then into puller and take-up reel where start-up material is trimmedpuller speed is matched to extrudate speed at extruder exitsteady-state is obtained. Monitor temperature and thickness
10 Part Dimensional Control Geometry of the die, die orifice, is the major influence on setting the part size and shapeOther factors that influence dimensional controldie swell: ratio of the diameter of the extrudate to the diameter of the die orifice after exiting the die (Dx/Dd)gap distance: distance between die face and water tankdrawdown ratio: ratio of the maximum diameter of the swell to the final part diameter (Dx/Df). High drawdown ratio = faster speedspuller speed and the extruder speedfaster puller speed thins down sheet and orients polymer as it coolsoriented polymer has increased strength in machine direction and less in transverse direction (radial direction). In pipes is reduced burst strength.die land- longer land length increases molecular orientationtemperature- lower temperature increases molecular orientationmaterial properties, e.g., Molecular weight and hydrogen bonding
11 Critical Operational Parameters Key operational parametersscrew diameter for each resin to optimize melting characteristicspolyethylene type screw: short feed section, long compression zonegeneral purpose screw: mid size feed and compression sectionsnylon type screw: long feed section and short compression sectionheating zone temperatures dependent on resin material and screwshort feed section yields less shear heating thus need to compensateshort compression zone yields less shear heating need to compensate
12 Viscosity Viscosity is a measure of the material’s resistance to flow Water has a viscosity of 1 centipoisePolymers have viscosities greater than 100,000 centipoiseFor polymers viscosity is a function of shear rate and temperatureShear rate- is a measure of the shear imparted on a fluid = (Velocity)/Distance). Higher shear rate = lower viscosity = easy to flowTemperature- is a mesure of the thermal energy imparted on a material. Higher temperature = lower viscosity = easy to flowMixing of materials is strongly dependent upon similar viscosities
13 Maintenance for Extrusion Base: extruder should be securely bolted to baseDrive: Fan is turning in proper direction. Clean inside periodicallyThrust bearing: Look for excessive wear or damage inside inspection spaceScrew: Remove occasionally and inspected for wear on flightsBarrel: Inspect for excessive wear or contaminantsHeating or cooling system: Inspect contact surfaces of barrel heaters and temperature range of all heating unitsHead and die: Inspect for leakages at joints and clean off carbonized material off breaker plate and insure it is flat. Calibrate pressure sensors and thermocouplesSafetyHeated surfaces, hot material, take-up reel, safety guards
14 Extrusion Problems and Troubleshooting Melt fractureSharkskin or alligator hideUneven flow and surgingDegradationPoor mixingContaminationBubbles in extrudate
15 Extrusion Problems and Troubleshooting Melt fracture (Fig 10.9)Extrudate has a rough surface with short cracks or ridges that are oriented in the machine direction or helically around the extrudateOccurs because tensile forces exceed critical shear stress of materialCaused by turbulent flow due to die not properly streamlinesReduced by streamlining dies, raising melt temp, lower Mw of resin, increase land (more laminar flow
16 Extrusion Problems and Troubleshooting Sharkskin or Alligator HideExtrudate is rough with lines running perpendicular to the flow direction.Causes tearing of the surface of the surface of the melt.Occurs because tensile stresses in Laminar flow exceed the tensile stress of the material causing a crack.Flow profile with center of material flowing too fast compared to the edges where the walls hinder the flow.As material exits die, edge material has to speed up to the center velocity, causing fracture and sharkshin. Effect.Bambooing occurs when the outer edge material snaps back to relive stressesOrange peel (Small dimples) can occur when differences is small between applied stresses and tensile strength.Effects are relieved by:Heating the resin or heating the die,Reducing pressure or reducing speed of extruderBroad molecular weight distribution
17 Extrusion Problems and Troubleshooting Uneven Flow and SurgingCyclical variation in the extrudate thickness with cycle time between surges from 30 sec to 3 minutes. Ammeter records surgesCauses areInadequate screw speed control. Motor could be undersized (Get new one)Major contaminate form piece of metal. (Clean screw and purge)Mismatch between screw dimension (depth of flights) and resin bulk densityScrew design for fluffy pellets. (Change density of resin or new screw)Starve feeding leads to uneven flow.Partial bridging with the resin clinging to screw in feed zone.(lower heat in feed zone)Feed from hopper can be uneven due to clumps. (Use Auger to feed)Slippage of pullerExtruder speed too fast.
18 Extrusion Problems and Troubleshooting DegradationBreakage of the molecular chains of the polymerDetected by discoloration or lower physical and mechanical propertiesDark streaks or specs in extrudate.Caused by : Too much heat from heater or screw speedSolutions are:reduce heat or reduce screw speed.Reduce residence time.Poor MixingStreaks of particles in the extrudateCaused by:Running the extruder too fast than it can mix the materialsToo short a L/D ratioSolved by:Slowing extruderAdd mixing devices
19 Extrusion Problems and Troubleshooting ContaminationHas streaks or spots (dimples or fisheyes in the extrudate).Found by using a microscope or examination of screen packCaused by:Resin quality or previous resin not purgedObject dropping in extruder, including dust, other resinsSolved by:Keeping hopper covered or filters in conveying systemDecrease opening size of screensBubbles in ExtrudateCaused byExcessive moisture absorbed by resin, especially, PET, PA, PCSolved byDry resin in dryer to less than 0.1% for above resinsAdd only resin as needed in extruder by not having hopper fullSlow extruder speed
20 Plant concepts (Layout and Controllers) Extrusion lines are long, typically 45 feet.Line should be straightMaterial fed from vacuum fed gaylords or from mezzanine aboveControllersFeedback or automatic controllers and monitors are used extensively for monitoring portion of line including heat zones
21 Extrusion Costs Material Cost Process Related Factors Other Factors Lbs used per hour times $/lb costDetermined fromDesign of sheet which requires volume of material per hour of sheet lineScrap rate which is excess material that is discardedProcess Related FactorsMachine cost: dedicated or non-dedicatedLabor rate and number of operators per machineTool and Die costsCycle time or run rate (lbs per hour)Other FactorsPlant overhead for building, rent, utilities, maintenanceManagement overhead for supervision, administrative, marketing, R&DProfit, shipping, packaging, secondary, painting, etc...
22 Extrusion Costs Process specific analysis Excel Spreadsheet Analysis Blown Film LinePipe and Tube LineSheet Line: Use in Report available in the MFGT 142 FolderExcel Spreadsheet AnalysisAvailable from IBIS Associates.Input data per the specific jobTotal Operations CostProvides variable cost elementsMaterial costDirect labor costUtility CostProvides Fixed cost elementsEquipment, Tooling, Building, Maintenance, Overhead, Capial
23 Extrusion CostsSpreadsheet: sheet demo.xls in MFGT 142 folder