Understanding Extrusion - Polymer Processing - Lecture Slides, Slides of Software Engineering

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Understanding Extrusion

Chapter 5

MFGT 144 Docsity.com

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Chap 5: How an Extruder Works

• Solids Conveying

• Gravity Induced Conveying
• Drag Induced Conveying
• Starve Feeding
• Grooved Feed Extruders

• Melting

• Contiguous Solids Melting
• Dispersed Solids Melting

• Melt Conveying

• Melt Temperature

• Mixing

• Degassing

• Die Forming Docsity.com

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Solids Conveying

• Drag Induced Conveying Fig 5.
  • Plastic moves forward from rotation of screw due to friction with the barrel wall and not the friction with the screw.
  • Analogy is a nut on a screw. If the nut is free to rotate it will not move up the screw. If the nut is held the nut moves forward.
• Starve Feeding (Fig 6.5)
  • Method of feeding the extruder where the plastic is metered into the extruder at a rate below the flood feed rate.
  • The screw channel is partially empty in the first few diameters of the extruder.
  • Results in very little pressure buildup in the plastic and as a result very little frictional heating and mixing.
  • Effectively reduces the length of the extruder, e.g. a 25:1 L/D extruder may have an effective length of 21 L/D with the first 4 diameters partial
  • Used on high speed twin screw extruders.
  • Reduces motor load, melt temperature, and useful when adding several ingredients simultaneously through one feed port from several feeders. Docsity.com

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Solids Conveying

• Grooved Feed Extruders (Fig 5.6)
  • Driving force for the conveying process is the frictional force at the barrel surface.
  • Grooves effectively increase the barrel temperature.
  • Grooves typically run in the axial direction with the length of several screw diameters.
  • Advantages of grooved feed extruders (Fig 5.7) » Output is less dependent on pressure resulting in increased stability » Output tends to be higher than that of smooth bore extruders » Allows extrusion of very high molecular weight plastics, HMWPE
  • Disadvantages » Grooved barrel section has to be cooled well enough to avoid premature melting of plastic in the grooves reducing energy efficiency and adds to complexity of extruder » Stresses can be high in groove causing them to wear » Pressure required can be high, thus need strong barrel Docsity.com

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How an Extruder Works

– Reduce screw friction

• Internal screw heating
  • Coring the screw and circulating heat transfer fluid
  • Cartridge heater inside the screw
  • Apply a coating to the screw or a surface treatment. » PTFE impregnated nickel plating » PTFE/chrome plating » Titanium-nitride » Boron-nitride » Tungsten-disulfide (WS2) » Catalytic surface conversion (J-Tex)
  • Advantage of a low friction coating » Improves conveying along screw » Reduces tendency of plastic to build up on screw surface, is easier to clean » Coatings can be used for extrusion dies which reduce pressure drop » Reduces tendency for material to build up at exit or dye droolDocsity.com

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Melting

• Contiguous Solids Melting (Fig 5.9) (CSM)
  • Solid particles are compacted and form a solid plug that spirals along the length of the screw channel.
  • Thin film of plastic is located between solid bed and barrel
  • Most of the melting occurs at the interface between two
  • Newly melted material collects in the melt film then is dragged away
  • Most often observed in single screw extruders
• Dispersed Solids Melting (Fig 5.10) (DSM)
  • Solid particles are dispersed in a melt matrix, decrease in size till melted
  • Observed in high-speed twin extruders and reciprocating single screw compounding extruders.
  • Melting is more efficient than Contiguous solids melting (CSM)
  • Length to achieve melting in the axial direction is 1 to 2 screw Diameter
  • Versus single screw extruders length is 10 to 15 diameters
  • Versus twin screw extruders length is 5 to 6 diameters
  • Important when have a 25L/D extruder that won’t have length for melt conveying, mixing or degassing.
• Twin screw is more versatile than single screw Docsity.com

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CSM Melting

• Helix angle can have considerable effect on melting
  • Helix angle increases, melting efficiency increases Fig 5.
    • Highest melting efficiency (shorter length) is with 90º
      • Such angle not good for conveying since 90º means that screw flight is parallel to the axis of screw and conveying capability is zero.
      • Good range for helix angle is between 20º to 30º
  • Multiple flights can also improve melting (Fig 5.12)
    • Melt film is thinner than in a single flighted screw (Fig 5.13)
    • Drawback is that it reduces solids conveying and melt

conveying.

• Use multiple flights if extrusion is limited by melt capacity

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Barrier Screw

• Barrier screw has two flights: main and barrier

– Main flight: is to separate the solid material from melted

• Barrier flight (Fig 5.14) separates solid bed from the melt pool.
• Barrier screw melting capability is same as a single flighted

screw without a barrier flight.

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Melt Conveying

• Melt Conveying starts when melting is completed
  • Melt conveying zone is region where all plastic is melted
  • Mechanism for melt conveying is viscous drag
    • Viscous force at the barrel is responsible for conveying
    • Viscous force at the screw is responsible for retarding force
    • Melt conveying is improved by reducing barrel temperature and increasing screw temperature
  • Optimum screw geometry for conveying
    • Optimum helix angle is dependent on degree of non_Newtonian behavior of plastic melt, n (power-law index, which is slope of log viscosity-log shear rate)
    • Optimum helix angle for melt conveying decreases as power law index decreases; in other words, when the plastic is more shear thinning
    • Optimum depth depends upon:
      • Viscosity, pressure gradient, and power law index
      • When plastic becomes more shear thinning, the channel depth should be reduced to obtain good melt conveying

optimum _ helix _ angle = 13. 5 + 16. 5 n

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Melt Temperature

• Temperatures vary considerably in melt conveying

• Due to low thermal conductivity
• Local temperatures are difficult to measure due to screw
• Numerical techniques can predict temperatures with finite element

analysis with flow and pressures (Fig 5.15)

• Hottest near center of channel and coolest at screw

• Temperature distribution due to curling flow pattern

• (Fig 5.16)
  • Fluid close to barrel surface flows in direction of channel
  • Recircualting flow (Fig 5.17) causes inner layer is trapped
  • Outer layer insulates inner hot layer
  • Important to keep non-uniform heating layers away from end of screw with mixing sections in design of screw to achieve thermally homogeneous.

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Distributed Mixing

• Down channel velocities (v (^) z ) depend upon Pressure gradient
  • Positive: pressure is increasing along melt conveying zone
  • Negative: pressure is decreasing along melt conveying zone
  • Fig 5.
• Cross Channel flow
  • At top of channel, material flows to the left by drag flow
  • At bottom of channel, material flows to the right by pressure flow
  • Fig 5.
• Shear rate
  • Determined from the slope of the velocity profile (velocity versus position)
  • Slope of velocity profile is also called velocity gradient
  • Fig 5.
• Pressure affects on shear rate
  • Press gradient is positive then the shear rates increase toward the barrel surface
  • Press gradient is zero then the shear rates is constant
  • Press gradient is negative then the shear rates decrease toward the barrel surface
  • Cross channel shear rates can be determined in the same way.
  • Fig 5.22 Docsity.com

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Shear Rate

• Channel flow is helical (Fig 5.23)

• If unroll screw channel onto flat plane material follows helical path
  • At top, the fluid element travels in the direction of the barrel
  • At bottom, the fluid elements travel across the channel.
  • Resident time is the length of time the material spends in the channel
    • Dependent on velocities and geometry of channel (Volume of channel/ Volumetric flow rate)
    • Resident time as function of distance: Fig 5.
    • Fluid elements at center have shortest resident time
    • Residence time increases toward the screw and barrel surfaces
    • Residence time is very long at barrel and screw surfaces
    • Outer Region A and inner region B (Fig 5.25)

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Mixing

• Best way to improve mixing in single screw

extruders is to incorporate mixing sections

• Desirable characteristics for mixing section

– Minimum pressure drop with forward pumping capability

– Streamlined flow and no deadspots

– Barrel surface wiped completely with no circumferential

grooves.

– Operator friendly and easy to install, run, clean, etc.

– Easy to manufacture and reasonably priced.

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Distributive Mixing Sections

• Specific characteristics for distributive mixing

– Plastic melt subjected to significant shear strain

– Flow should be split frequently with reorientation of melt

– Types

• Cavity mixers
• Pin mixers
• Slotted flight mixers
• Variable channel depth mixers
• Variable channel width mixers

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