1. Understanding Extrusion Chapter 5
Professor Joe Greene
CSU, CHICO
MFGT 144

2. 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

3. Solids Conveying
Gravity Induced Conveying: Material flows down the feed hopper into the feed throat and from there into the screw channel
Important Bulk properties
Bulk density- density of material including air voids
Compressibility- change in bulk density when pressure is applied
Internal coefficient of friction- between the plastic particles
External coefficient of friction- between plastic and hopper
Particle size and distribution- PCR material are difficult to handle due to large particle size distribution
Design of feed hopper effects flow in hopper to prevent stagnation and bridging
Circular hopper is better than square hopper (Fig 5.1)
Crammer feeder can be used for PCR materials
Diamondback feed hopper has a circular cross section to a oval cross section to a circular cross section. (Fig 5.4)

4. Solids Conveying Drag Induced Conveying Fig 5.5
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.

5. 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

6. How an Extruder Works
Conventional extruders without grooved barrel sections can be modified to include grooves
Modify the feed throat with a grooved liner (fig 5.6)
Most extruder stability problems occur in solids conveying section
Grooved feed throat can improve extruder performance
Improve barrel friction or reduce screw friction
Affected by screw design, screw temp, and screw material
Screw design features that reduce screw friction (Fig. 5.8)
Single flighted geometry (avoid multiple flights)
Large flight flank radius
Large Helix angle

7. 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 drool

8. 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

9. CSM Melting CSM Theory developed by Tadmor
Determine how plastic properties, processing conditions and screw geometry affect melting
Two sources of heat
Barrel heat conducts from heaters through barrel and to melt
Viscous heating caused by viscous movement of melt
Drag induced melt removal
Melted material is dragged away by rotation of screw
Thin melt film is essential to proper melting
Single screw extruders melt more efficiently than ram extruders
Similar to a stick of butter melting in frying pan; best if moved around
Thin melt film is essential parameter to high melting efficiency
Melt thickness determined from flight clearance. Larger flight clearance results in thicker melt film.
Important to keep flight clearance small
Increase in barrel temperature causes increased heating of plastic
causing viscosity to drop causing viscous dissipation to drop and less efficient melting. Thus, increase in barrel temp can reduce melting efficiency

10. CSM Melting Helix angle can have considerable effect on melting
Helix angle increases, melting efficiency increases Fig 5.11
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

11. 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.

12. Barrier Screw Advantages Disadvantages
Achieves more stable extrusion than simple conveying
No chance of unmelted material being beyond barrier section
Certain amount of dispersive mixing occurs as plastic flows over barrier into melt channel.
Disadvantages
No better performers than screws with mixing sections
More expensive than non-barrier screws
More susceptible to plugging due to solid material restricted to the solids channel.
Melting can not keep up with reduction in the size of the channel in compression section of the screw, resulting in the solid material getting stuck in the screw channel.
Creates a momentary obstruction to flow and leads to surging or variation in extruder output

13. 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

14. 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.

15. Distributed Mixing Takes place in melting and melt conveying zones
Due to plug flow behavior, little mixing occurs in solids conveying. Waits until all plastic is melted
Distributed Mixing
Extent determined from total shear deformation of plastic melt
Total shear deformation = shear rate · length of time exposed to shear rate . Where, shear rate is found from velocity/distance and length of time is Volume/flow rate
Example, plastic melt is exposed to shear rate of 100 sec–1 for 15 seconds the resulting strain is 1,500. (Dimensionless)
Mixing is determined by velocity in 2 directions Fig 5.18
Direction of channel (z-direction velocity is vz)
Direction across channel (x-direction velocity is vx)
The third direction, parallel to flight flank is usually small, vy.

16. Distributed Mixing
Down channel velocities (vz) depend upon Pressure gradient
Positive: pressure is increasing along melt conveying zone
Negative: pressure is decreasing along melt conveying zone
Fig 5.19
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 5.20
Shear rate
Determined from the slope of the velocity profile (velocity versus position)
Slope of velocity profile is also called velocity gradient
Fig 5.21
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

17. 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 5.24
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)

18. Cross Channel Shear Strain
Total cross channel shear strain can be determined
adding the shear strain of the upper portion to that of the lower portion of the channel
Fig 5.26:
Total shear strain vs. normal distance (thickness) at several throttle ratios (pressure flow to drag flow: rd=0 pressure flow or rd = then drag flow is 3 times pressure_ common)
Elements close to barrel wall experience high strain and mixing
Mixing in center increases with increasing rd by increasing resistance at end to flow by adding screen packs

19. 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.

20. 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

21. Cavity Mixers Cavity transfer mixer (CTM) Advantages Disadvantages
Consists of screw section and barrel section. Both containing hemi-spherical cavities (Fig 5.27)
Advantages
Good mixing capability
Disadvantages
No forward pumping capability and is pressure consuming
Reduces extruder output and increases temperature buildup
Streamlining is not very good, high cost, high installation $$
Barrel not completely wiped during processing
Twente Mixing Ring is easier to install, clean, and operate. Barrel is wiped

22. Pin Mixers Pin mixers are common and come in many sizes and shapes
Circular, square, rectangular, diamond-shaped. Fig 5.29
Pin barrel extruder common in rubber extrusion
Advantages
Good mixing capability
Disadvantages
Pins cause restriction and reduce extruder output
Pins create regions of stagnation at the corner of pin and root of screw

23. Slotted Flight Mixers Common ones are Axon, Dulmage and Saxton
Fig 5.31, 5.32, and 5.33
Advantages
Good mixing capability and high output
Disadvantages
Barrel not completely wiped

24. Variable Depth Mixers
Channel depth varies periodically in each channel
One channel decreases in depth, the other increases
Fig 5.34
Advantages
Improved mixing
Disadvantages
No strong mechanism for flow splitting and reorientation
Mixing capability is moderate

25. Summary of Distributive Mixers
Table 5.1: Ranking of various Mixers
Most important characteristic is Splitting and and reorientation
For distributive mixing the following are desirable
Mixing section should have high stress region, preferable elongational stresses
High stress region designed for short times
All fluid elements should pass through high stress many times
All fluid elements should pass through high stress the same number of times

26. Blister Ring
Circumferential shoulder on the screw with a small clearance between ring and the barrel (Fig 5.36)
All material must flow through a small clearance between ring and barrel where it is exposed to high stress
High pressure drop occur across the blister ring.
Stresses and mixing are not uniform

27. Fluted Mixing Section
Mixers have inlet and outlet flutes separated by barrier flights
Material passes through a narrow gap of barrier flights where mixing takes place.
Egan mixing section: flutes have helical orientation Fig 5.37
Poor Helix angle design of 30º. (Optimum is 50º) Fig 5.39
Leroy Union Carbide: has straight flights Fig 5.38
No forward pumping capability and thus high pressure drop
Inefficient streamlining at entry and exits
Most common for single screw extruders
Poor Helix angle design of 90º

28. Planetary Gear Mixers
Have six or more planetary screws that revolve around circumference of main screw. (Common in Europe)
Barrel section must have helical grooves corresponding to the helical flights of planetary screws. (Fig 5.41)
Benefits (Good for PVC, ABS, PU, acrylic, PE)
Good homogeneity of the melt at low temperature
Uniform shear exposure
High output per screw revolution
Low production cost per unit throughput
Self-cleaning action for easy material change
Good dispersive and distributive mixing of various additives

29. CRD Mixer
Uses slanted pushing flight flank to create wedge shaped lobal region (Fig 5.42)
Developed by Chris Rauwendaal to reduce problems
Relying too mush on shear stresses to disperse materials rather than elongational stresses
Material passes over high stress region only once.

30. Summary of Dispersive Mixers
Comparison of Dispersive Mixers
Table 5.2

31. Degassing Degassing is done on a vented extruder
Extruder with vent port in barrel (Fig 5.46)
Special design to insure there is zero pressure region under vent
Vent is need to rid the extruder of volatiles (Table 5.3)
Most common volatile is water. Plastics can tolerate about 0.1% moisture
Some hygroscopic (Water seeking) plastics degrade when exposed to heat and moisture
Polyester, Polycarbonate, nylon and polyurethane

32. Die Forming Shaping of molten plastic occurs in extrusion die
As plastic flows in die it takes the shape of the die. Fig 5.48
Exit region of the die flow channel is called the “land area”
Land region for pipe or tubing has an annular shape
Land region for sheet has a shape of a slit
Die Changes

33. Die Forming Shape of the die is changed due to
Drawdown: thinning of part caused by pulling extrudate
Extrudate swell: expansion of the dimensions of the plastic caused by normal stresses and viscoelastic nature.
Cooling: causing shrinkage in plastic.
Semi-crystalline materials shrink more than amorphous because of the higher density in semi-crystalline materials. Fig 5.51 and 5.52
Relaxation: gradual reduction of internal stresses causing the plastic to sag.
Warping: Caused by non-uniform stresses during flow. Fig 5.53

34. Tubing and Pipe Dies
Extrusion dies are categorized based upon the shape of the product produced
Annular dies
Tubing (< 1” OD), pipe (>1” OD), blown film (D can be > 30ft with thickness from 0.002” to 0.010”), wire coating
Inline Fig 5.55
Melt enters from the left and flows around a torpedo
Uniform stresses in extruded product

35. Tubing and Pipe Dies
Extrusion dies are categorized based upon the shape of the product produced
Annular dies:
Crosshead: Fig wire coating
Melt is split around the flow splitter, flows over a shoulder to tip and die
Allows for axial adjustment allows for concentricity of extruded tube
Spiral Mandrel die: Fig blown film
Plastic flows through central channel to a series of helical channels
Weld lines are eliminated, good flow distribution, hoop strength

36. Flat film and Sheet Dies
Same general shape for flat film and sheet
Sheet dies Fig 5.58
inlet channel
manifold
pre-land section
relaxation section
land section
Manifold designs
T-die:
No well-streamlined flow
Good for low viscosity plastics
No clam shelling (Fig 5.60)
coat hanger die: Fig 5.61
Complicated, but has streamlined flow
horse shoe die: Fig 5.62

37. Profile dies Many different shapes and sizes
Shapes other than rectangular, annular, or circular
Plate die: Fig 5.63
uses a plate with a cavity shaped to produce the extruded product
Easy to make, but is not streamlined and likely to have dead spots
Fully streamlined die: Fig 5.64
Has a flow channel that gradually changes to the exit geometry
Velocities slowly increase causing streamlined flow and no dead spots
Appropriate for long runs and plastics with limited thermal stability
More expensive to manufacture