Transcript Understanding Extrusion Chapter 5 Professor Joe Greene CSU, CHICO

Understanding Extrusion
Chapter 5
Professor Joe Greene
CSU, CHICO
1
MFGT 144
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
2
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)
3
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
4
ingredients simultaneously through one feed port from several feeders.
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
5
» Stresses can be high in groove causing them to wear
» Pressure required can be high, thus need strong barrel
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
6
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
7
» Reduces tendency for material to build up at exit or dye drool
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.
8
– Twin screw is more versatile than single screw
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
2
• 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
9
– causing viscosity to drop causing viscous dissipation to drop and less efficient
melting. Thus, increase in barrel temp can reduce melting efficiency
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
10
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.
11
Barrier Screw
• Advantages
– 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
12
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  13.5  16.5n
• 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
13
reduced to obtain good melt conveying
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.
14
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.
15
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
16
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)
17
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 = 0.333 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
18
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.
19
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
20
Cavity Mixers
• Cavity transfer mixer (CTM)
• 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
21
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
22
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
23
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
24
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
25
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
26
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
27
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
28
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.
29
Summary of Dispersive Mixers
• Comparison of Dispersive Mixers
– Table 5.2
30
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
31
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
32
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
33
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
34
Tubing and Pipe Dies
• Extrusion dies are categorized based upon the shape
of the product produced
– Annular dies:
• Crosshead: Fig 5.56 - 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 5.57 - blown film
– Plastic flows through central channel to a series of helical channels
» Weld lines are eliminated, good flow distribution, hoop strength
35
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
» Complicated, but has streamlined flow
36
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
37