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)
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
Screw Diameter
Flight depth
in metering
section
Pitch angle
Speed of the screw

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 Speed
Screw Diameter

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 (Dx/Dd)
gap distance: distance between die face and water tank
drawdown ratio: ratio of the maximum diameter of the swell to the final part diameter (Dx/Df). 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
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)
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 Process Related Factors Other Factors
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 Excel Spreadsheet 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

23. Extrusion Costs
Spreadsheet: sheet demo.xls in MFGT 142 folder

24. Extrusion Costs
Spreadsheet

25. Extrusion Costs
Spreadsheet