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MFGT 142Extrusion Manufacturing

Professor Joe Greene

CSU, CHICO

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Chapter 11: Extrusion Manufacturing

• Overview– 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.

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Extruder Capacity Example• Capacity- sized by the diameter of the screw

– Total flow rate of an extruder isTotal 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

cossin)21(_ 22 NHDflowDrag

Speed of the screw

Flight depthin metering section

Pitch angleScrew Diameter

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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

L

PDHflowP

12

sin_

23

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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

• 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

cossin)21(_ 22 NHDflowTotal

L

PDH

12

sin 23

P

NflowTotal _ cossin)21( 22 HD

L

DH

12

sin23

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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

P

NflowTotal _ cossin)21( 22 HD

L

DH

12

sin23

2)(_ DconstflowTotal

NtConsflowTotal )tan(_

Screw DiameterScrew Speed

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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

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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

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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

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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

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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

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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

• 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• 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

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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...

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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 SPECIFICATIONSPart Name Sheet

Width 10 cmMaximum Wall Thickness 37 mmAverage Wall Thickness 35 mm

External Surface Area 40 sq cmProjected Area 40 sq cm

Number of Cavities 1Number of Actions in Tool 0

Surface Finish [3=best] 1 [1,2 or 3]

Annual Production Volume 5000 (000/yr)Length of Production Run 4 yrs

MATERIAL SPECIFICATIONSMaterial Type HDPEMaterial Price $1.00 $/kg

Scrap Credit Value $0.00 $/kgDensity 0.94 g/cm^3

Thermal Conductivity 0.24 W/mKHeat Capacity 1675 J/kgK

Melt Temp 220 CTool Temp 45 C

Eject Temp 80 C

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Extrusion Costs• Spreadsheet

PROCESS RELATED FACTORSDedicated Investment 0 [1=Y 0=N]

Operation Rejection Rate 3.0%Material Scrap Rate 1.0%

Average Equipment Downtime 5.0%Direct Laborers Per Station 0.5

OPTIONAL INPUTSProduction Rate 9144 cm/minute

Equipment Cost per Station $330 (000)Tool Cost per Set $30 (000)

EXOGENOUS COST FACTORSDirect Wages 25 /hrIndirect Salary $50,000 /yr

Indirect:Direct Labor Ratio 0.4Benefits on Wage and Salary 30.0%

Working Days per Year 260Working Hours per Day 24Capital Recovery Rate 15.0%

Equipment Recovery Life 8 yrsBuilding Recovery Life 20 yrs

Working Capital Period 3 monthsPrice of Electricity $0.051 /kWh

Price of Natural Gas $6.50 /MBTUPrice of Building Space $600 /sq m

Auxiliary Equipment Cost 20.0%Equipment Installation Cost 50.0%

Investment Maintenance Cost 5.0%

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Extrusion Costs• Spreadsheet

per 100 m per year percent investmentVARIABLE 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% $66,270,000Building 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