MFGT 142
Extrusion Manufacturing
Professor Joe Greene
CSU, CHICO
1
Chapter 11: Extrusion Manufacturing
• Overview
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Purpose, advantages, disadvantages, and cost elements
Extrusion problems and trouble shooting
Material and product considerations
Post-extrusion forming
Coextrusion
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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.
3
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
Drag _ flow  (1 2)  D NH sin  cos
2
2
Screw Diameter
Speed of the screw
Flight depth
in metering
section
Pitch angle
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Extruder Capacity Example
• You can increase output (capacity) of the extruder by
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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
DH P sin 
P _ flow 
12L
3
2
• 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
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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
Total _ flow  (1 2)  2 D 2 NH sin  cos
 DH 3 P sin 2  

 
12L


• In practice, the screw dimensional parameters (D,H, , L) are
combined into two constants  and  which simplifies equation to
 P 

Total _ flow  N  
  
  (1 2)  2 D 2 H sin  cos
 DH 3 sin 2  

  
12 L


• 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
6
Extruder Capacity Example
 P 

Total _ flow  N  
  
  (1 2)  2 D 2 H sin  cos
 DH 3 sin 2  

  
12 L


• 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 lowviscosity materials and will retard their advance.
• To compare the output of screws at various diameters
Total _ flow  (const ) D 2
Screw Speed
Screw Diameter
• Output for Twin Screws
Total _ flow  (Cons tan t ) N
7
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
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shutdown and start-up
resin changes
die changes
screen pack changes
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Start-up
• Start-up procedure is the bringing up the production line
from a static condition
• Start-up steps
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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
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– steady-state is obtained. Monitor temperature and thickness
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 10
Critical Operational Parameters
• Key operational parameters
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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
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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
12
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
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Extrusion Problems and Troubleshooting
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Melt fracture
Sharkskin or alligator hide
Uneven flow and surging
Degradation
Poor mixing
Contamination
Bubbles in extrudate
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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
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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
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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.
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Extrusion Problems and Troubleshooting
• Degradation
– Breakage of the molecular chains of the polymer
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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
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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
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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
20
• Material Cost
Extrusion Costs
– 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
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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...
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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
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Extrusion Costs
• Spreadsheet: sheet demo.xls in MFGT 142 folder
PRODUCT SPECIFICATIONS
Part Name
Width
Maximum Wall Thickness
Average Wall Thickness
External Surface Area
Projected Area
Number of Cavities
Number of Actions in Tool
Surface Finish [3=best]
Annual Production Volume
Length of Production Run
MATERIAL SPECIFICATIONS
Material Type
Material Price
Scrap Credit Value
Density
Thermal Conductivity
Heat Capacity
Melt Temp
Tool Temp
Eject Temp
Sheet
10
37
35
40
40
cm
mm
mm
sq cm
sq cm
1
0
1 [1,2 or 3]
5000 (000/yr)
4 yrs
HDPE
$1.00
$0.00
0.94
0.24
1675
220
45
80
$/kg
$/kg
g/cm^3
W/mK
J/kgK
C
C
C
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• Spreadsheet
PROCESS RELATED FACTORS
Dedicated Investment
Operation Rejection Rate
Material Scrap Rate
Average Equipment Downtime
Direct Laborers Per Station
Extrusion Costs
0 [1=Y 0=N]
3.0%
1.0%
5.0%
0.5
OPTIONAL INPUTS
Production Rate
Equipment Cost per Station
Tool Cost per Set
9144 cm/minute
$330 (000) EXOGENOUS COST FACTORS
Direct Wages
$30 (000)
Indirect Salary
Indirect:Direct Labor Ratio
Benefits on Wage and Salary
Working Days per Year
Working Hours per Day
Capital Recovery Rate
Equipment Recovery Life
Building Recovery Life
Working Capital Period
Price of Electricity
Price of Natural Gas
Price of Building Space
25
$50,000
0.4
30.0%
260
24
15.0%
8
20
3
$0.051
$6.50
$600
Auxiliary Equipment Cost
Equipment Installation Cost
Investment Maintenance Cost
20.0%
50.0%
5.0%
/hr
/yr
yrs
yrs
months
/kWh
/MBTU
/sq m
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• Spreadsheet
Extrusion Costs
per 100 m
per year
percent
investment
VARIABLE COST ELEMENTS
---------------------- ---------------------- ---------------------- ---------------------Material Cost
$52.21 $261,062,168
79.6%
Direct Labor Cost
$0.75
$3,732,592
1.1%
Utility Cost
$0.00
$5,178
0.0%
FIXED COST ELEMENTS
---------------------- ---------------------- ---------------------- ---------------------Equipment Cost
$0.01
$74,238
0.0%
$594,000
Tooling Cost
$3.31
$16,567,500
5.1%
$66,270,000
Building Cost
$0.07
$340,170
0.1%
$6,804,520
Maintenance Cost
$0.74
$3,683,365
1.1%
Overhead Labor Cost
$5.74
$28,712,245
8.8%
Cost of Capital
$2.76
$13,813,801
4.2%
============ ============ ============ ============
TOTAL OPERATION COST
$65.60 $327,991,255
100.0%
$73,668,520
25