Presentation on theme: "1 MFGT 142 Extrusion Manufacturing Professor Joe Greene CSU, CHICO."— Presentation transcript:
1 MFGT 142 Extrusion Manufacturing Professor Joe Greene CSU, CHICO
2 Chapter 11: Extrusion Manufacturing Overview –Purpose, advantages, disadvantages, and cost elements –Extrusion problems and trouble shooting –Material and product considerations –Post-extrusion forming –Coextrusion
3 Plant concepts (Layout and Controllers) Layout –Extrusion lines are long, typically 45 feet. –Line should be straight to minimize stresses as material flow around a curve –Material fed from vacuum fed gaylords or from mezzanine above –Controllers –Feedback or automatic controllers and monitors are used extensively for monitoring portion of line including heat zones –Thermocouples 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 screw –Total flow rate of an extruder is Total flow = drag flow - pressure flow - leakage flow Drag 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 as Speed of the screw Flight depth in metering section Pitch angle Screw Diameter
5 Extruder Capacity Example You can increase output (capacity) of the extruder by –Increasing diameter of screw –Increasing screw speed –Increasing the flight depth –Optimum 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 flow where 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 flow In practice, the screw dimensional parameters (D,H, , L) are combined into two constants and which simplifies equation to Thus, output is increased by –increasing 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 diameters Output for Twin Screws Screw Diameter Screw Speed
8 Normal Operation and Control of Process Extrusion is a continuous, stable, steady-state process Achieving stable, steady-state operation requires considerable effort Disruptions to be avoided –shutdown and start-up –resin changes –die changes –screen pack changes
9 Start-up Start-up procedure is the bringing up the production line from a static condition Start-up steps –preheat extruder including the screw and the die –open end of extruder and remove screen pack for cleaning –load hopper with material (pre-dried if required) –rotate screw slowly at first –fill screw with desired material and flush out previous purge resin. –rotate screw at desired setting and bring extrusion to steady state –string up the extrudate by pulling extrudate into the puller –push extrudate into cooling bath and then into puller and take-up reel where start-up material is trimmed –puller speed is matched to extrudate speed at extruder exit –steady-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 shape Other factors that influence dimensional control –die swell: ratio of the diameter of the extrudate to the diameter of the die orifice after exiting the die (D x /D d ) –gap distance: distance between die face and water tank –drawdown ratio: ratio of the maximum diameter of the swell to the final part diameter (D x /D f ). High drawdown ratio = faster speeds –puller speed and the extruder speed faster puller speed thins down sheet and orients polymer as it cools oriented 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 orientation –temperature- lower temperature increases molecular orientation –material properties, e.g., Molecular weight and hydrogen bonding
11 Critical Operational Parameters Key operational parameters –screw diameter for each resin to optimize melting characteristics –polyethylene type screw: short feed section, long compression zone –general purpose screw: mid size feed and compression sections –nylon type screw: long feed section and short compression section –heating zone temperatures dependent on resin material and screw –short feed section yields less shear heating thus need to compensate –short 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 centipoise –Polymers have viscosities greater than 100,000 centipoise –For polymers viscosity is a function of shear rate and temperature Shear rate- is a measure of the shear imparted on a fluid = (Velocity)/Distance). Higher shear rate = lower viscosity = easy to flow Temperature- is a mesure of the thermal energy imparted on a material. Higher temperature = lower viscosity = easy to flow Mixing of materials is strongly dependent upon similar viscosities
13 Maintenance for Extrusion Maintenance –Base: extruder should be securely bolted to base –Drive: Fan is turning in proper direction. Clean inside periodically –Thrust bearing: Look for excessive wear or damage inside inspection space –Screw: Remove occasionally and inspected for wear on flights –Barrel: Inspect for excessive wear or contaminants –Heating or cooling system: Inspect contact surfaces of barrel heaters and temperature range of all heating units –Head and die: Inspect for leakages at joints and clean off carbonized material off breaker plate and insure it is flat. Calibrate pressure sensors and thermocouples Safety –Heated surfaces, hot material, take-up reel, safety guards
14 Extrusion Problems and Troubleshooting Melt fracture Sharkskin or alligator hide Uneven flow and surging Degradation Poor mixing Contamination Bubbles 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 extrudate Occurs because tensile forces exceed critical shear stress of material Caused by turbulent flow due to die not properly streamlines Reduced by streamlining dies, raising melt temp, lower Mw of resin, increase land (more laminar flow
16 Extrusion Problems and Troubleshooting Sharkskin or Alligator Hide –Extrudate 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 stresses Orange 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 extruder Broad molecular weight distribution
17 Extrusion Problems and Troubleshooting Uneven Flow and Surging –Cyclical variation in the extrudate thickness with cycle time between surges from 30 sec to 3 minutes. Ammeter records surges –Causes are Inadequate 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 density –Screw 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 puller Extruder speed too fast.
18 Extrusion Problems and Troubleshooting Degradation –Breakage of the molecular chains of the polymer Detected by discoloration or lower physical and mechanical properties Dark streaks or specs in extrudate. Caused by : Too much heat from heater or screw speed Solutions are: –reduce heat or reduce screw speed. –Reduce residence time. Poor Mixing –Streaks of particles in the extrudate –Caused by: Running the extruder too fast than it can mix the materials Too short a L/D ratio –Solved by: Slowing extruder Add mixing devices
19 Extrusion Problems and Troubleshooting Contamination –Has streaks or spots (dimples or fisheyes in the extrudate). –Found by using a microscope or examination of screen pack –Caused by: Resin quality or previous resin not purged Object dropping in extruder, including dust, other resins –Solved by: Keeping hopper covered or filters in conveying system Decrease opening size of screens Bubbles in Extrudate –Caused by Excessive moisture absorbed by resin, especially, PET, PA, PC –Solved by Dry resin in dryer to less than 0.1% for above resins Add only resin as needed in extruder by not having hopper full Slow extruder speed
20 Plant concepts (Layout and Controllers) Layout –Extrusion lines are long, typically 45 feet. –Line should be straight –Material fed from vacuum fed gaylords or from mezzanine above Controllers –Feedback or automatic controllers and monitors are used extensively for monitoring portion of line including heat zones
21 Extrusion Costs Material Cost –Lbs used per hour times $/lb cost –Determined from Design of sheet which requires volume of material per hour of sheet line Scrap rate which is excess material that is discarded Process Related Factors Machine cost: dedicated or non-dedicated Labor rate and number of operators per machine Tool and Die costs Cycle time or run rate (lbs per hour) Other Factors Plant overhead for building, rent, utilities, maintenance Management overhead for supervision, administrative, marketing, R&D Profit, shipping, packaging, secondary, painting, etc...
22 Extrusion Costs Process specific analysis –Blown Film Line –Pipe and Tube Line –Sheet Line: Use in Report available in the MFGT 142 Folder Excel Spreadsheet Analysis –Available from IBIS Associates. Input data per the specific job Total Operations Cost –Provides variable cost elements Material cost Direct labor cost Utility Cost –Provides Fixed cost elements Equipment, Tooling, Building, Maintenance, Overhead, Capial