Transcript Der Doppeltschnecken Extruder
Twin screw extruder
die head barrel degassing hopper
Twin screw extruder
gearbox and thrustbearing box auxiliary equipment
screws
Twin screw extruder
Twin screw extruder
Motor power / Energy balance / Output
Why a twin screw extruder?
• • • •
Forced feeding of the powder.
High output at low screw speeds.
High pressure building capacity of the screws.
Low shearrates in the melt.
Twin screw extruder
Motor power / Energy balance / Output
ENERGY IN
Main motor Heating (barrel and dies)
Total in ENERGY OUT
Heating PVC Screw cooling Barrel cooling Gearbox and thrustbearingbox Pulley Convection
Total out Energy balance PVC extrusion
110 Wh/kg 40 Wh/kg
150 Wh/kg
80 Wh/kg 20 Wh/kg 25 Wh/kg 12 Wh/kg 4 Wh/kg 9 Wh/kg
150 Wh/kg
Motor power Cooling barrel Cooling screw
Motor power, barrel cooling and screw cooling
120 100 80 60 40 20 0 0 200 400 600 800 1000 1200
Relation output - screw diameter
1400 1200 1000 800 600 400 200 0 60 70
30 D 25 D 22 D
80 90 100
Screw diameter (mm)
Q
~
D
1 .
8 110 120 130
Twin screw extruder
Motor power / Energy balance / Output
Screw speed
The maximum circumferential velocity at the barrel is 0.2 m/s. This results in lower screw speeds for larger diameter screws (speed ~ 1/D).
0.2 m/s 0.2 m/s
Screw speed
The maximum circumferential velocity at the barrel is 0.2 m/s. This results in lower screw speeds for larger diameter screws (speed ~ 1/D).
100 90 80 70 60 50 40 30 20 10 0 40 50 60 70 80 90 100 Screw diameter (mm) 110 120 130 140
Screw torque
• •
The power of the main motor is transferred to the melt by screw speed and screw torque.
Larger extruders require much more torque on the screws due to the reduced screw speed.
M s = 0.85
4
P motor N s
Twin screw extruder
Motor power / Energy balance / Output
Screw length
• • •
The screw length varies from 22 to 30 D.
Longer screws give a better melthomogeneity.
Longer screws require a higher lubricated compound.
Screw geometry
powder entrance zone first pump zone degassing zone first compression zone powder lock second compression zone pump zone mixing elements
Screw geometry
Conical and parallel screw geometry
first pump zone first compression zone powder entrance zone powder lock second compression zone degassing zone pump zone
Gaps in the extruder screws
flight gap side gap calandar gap
Gaps in the extruder screws
flight gap side gap calandar gap
Screw geometry
INTAKE ZONE The PVC powder enters the extruder in the intake zone. The intake capacity is the same as the extruder output. It is determined by the screw speed and the volume of the screw channels in the intake zone.
Screw geometry
FIRST COMPRESSION ZONE The density of the PVC increases while being processed. For efficient heat input the volume of the screw channels must be decreased.
Screw geometry
FIRST PUMP ZONE The first pump zone presses the melt through the powderlock. All channels are filled in this section which prevents air to pass.
Screw geometry
POWDERLOCK The powderlock is a kind of a barrier for the passing melt. Pressure created by the first pump zone is required to move the melt forward.
The powder lock
slots for recrushed PVC
Screw geometry
DEGASSING ZONE In this zone air and volatiles are extracted from the melt.
The degassing zone
The degassing zone
air grooves
The degassing zone
PVC powder + air air pressed away by compression of powder air removed by vacuum in vent zone pressure in polymer
The degassing zone
pressure in polymer air pressed away by compression of powder
Screw geometry
SECOND COMPRESSION ZONE For efficient heat input the volume of the screw channels must be decreased again.
Screw geometry
SECOND PUMP ZONE In this zone pressure is created to press the melt through the die. Mixing elements may be present.
MIXING ELEMENT The mixing element redistributes the melt over the screw channels. It reduces the pressure building capacity of the screws.
Screw geometry
MIXING ELEMENT The mixing element redistributes the melt over the screw channels. It reduces the pressure building capacity of the screws.
Screw geometry
Screw geometry
Screw pressure build-up
small gaps: high pressure building capacity large gaps: low pressure building capacity
• • •
Cooling with oil.
Cooling with heatpipes.
No cooling.
Screw cooling
Screw cooling with oil
Heat is extracted from the melt in the second pump zone.
cold oil in hot oil out
Screw cooling with heatpipes
Heat is transferred from the melt in the second pump zone to the powder in the entrance zone.
copper netting thermal isolation copper netting condensing water vapour evaporating water
Screw cooling
• •
Cooling with oil gives better control on the process.
Cooling with heatpipes reduces energy losses.
– Higher output capacity possible.
Twin screw extruder
Motor power / Energy balance / Output
primary particle (1 µ)
The PVC grain
PVC grain (0.1 mm) crystalline region (0.001 µ)
Fusion of PVC grains
• •
PVC is processed at temperatures between 190 and 210 C.
– – Glass transition point 82 C.
Crystalline melting point ~ 270 C.
Processing of PVC is done in the rubbery state!
– – – Strong elastic effects compared to other polymers.
No melt: PVC grains have to be fused together.
Fusion is often called “gelation”.
Fusion of PVC grains
•
The fusion of PVC is mainly done by friction induced by the rotating screws.
– Depending on the process the level of fusion can be lower or higher.
– – Most friction is generated in the pressurize regions of the extruder.
The level of friction will also influence the final melt temperature.
Regions of high friction level
fusion 0 % fusion 75 %
Fusion of PVC grains
fusion 50 % fusion 100 %
Fusion 0 %
Fusion 50 %
Fusion
• •
The fusion level of the melt equals the fraction of fused grains in the melt.
The fusion level increases due to friction at high melt temperature.
– Friction slots in screws.
– – – High barrel temperatures.
High screw speed Less lubricants
higher melt temperature
Fusion
• • •
The fusion level of the melt equals the fraction of fused grains in the melt.
The fusion level increases due to friction at high melt temperature.
– Friction slots in screws.
– – – High barrel temperatures.
High screw speed Less lubricants
higher melt temperature
The fusion level decreases due to friction at low melt temperature.
– – Low barrel temperatures.
Very low die temperatures (surface effect).
Fusion in the extruder
• • The fusion of the pipe is mainly determined by the amount of friction (= temperature) in the extruder.
– – – – – – The total surface of the screw (length, number of flights).
The length of friction slots (+ 1 D melt + 3 to 6 °C).
The amount of lubricants in the compound.
The pressure of the die (+ 100 bar The speed of the screws (+ 10 % melt + 2 to 4 °C).
melt + 2 to 4 °C).
The output of the extruder (+ 10 % melt + 2 to 4 °C).
The fusion of the pipe is partially determined by thermal conduction from the barrel.
– – Any barrelzone ± 20 °C Last barrelzone ± 10 °C melt ± 1 °C melt ± 1 °C
Quality of pipe versus fusion of PVC
The optimal fusion level is 70 to 75 %. It is reached at a melt temperature of about 190 ºC. This means no attack in methylene chloride of 10 ºC during half an hour.
impact pressure resistance 75 % gelation level gelation level
outside inside outside inside
Impact level versus fusion of PVC
falling weight 100 % fusion crack falling weight 75 % fusion crack
Twin screw extruder
Motor power / Energy balance / Output
Waviness in the pipe
Waviness in the pipe
height of waves The eye sees the light scattered by the waves. The scattering is proportional to the slope of the waves (height of waves / length of waves).
length of waves (about equal to wallthickness) light
Waviness and output
Waviness increases approximately with the output squared.
maximum tolerable level waviness maximum output limited by waviness output
Creation of waviness
partially filled completely filled partially filled completely filled
Creation of waviness
melt pressure forward speed of screw flight
Q back
Q channel
Q nett
backflow of melt Q back
W
~
Q back Q nett
nett output Q nett partially filled forward speed of melt completely filled transport capacity channel Q channel
Creation of waviness 500 kg/h 500 kg/h 500 kg/h nett 500 kg/h back flow 300 kg/h
screw speed 40 rpm transport cap. 800 kg/h pump zone
Waviness is created by the back flow of melt in the screw. Hot melt is folded into cold melt.
Waviness is proportional to back flow / output.
Waviness is strongly dependant on fusion level of folds.
Creation of waviness 500 kg/h 500 kg/h 500 kg/h nett 500 kg/h back flow 100 kg/h
screw speed 30 rpm transport cap. 600 kg/h pump zone
When the screw speed is reduced then the transport capacity is reduced.
– The back flow becomes less and the waviness reduces.
– The pressure building capacity reduces
4 • • Reduce the backflow.
– – Low screw speeds.
Higher compression in screws.
Reduce the melt elasticity of the folds.
– – Reduce the fusion level.
Increase the filler level.
Reduction of waviness
3
Reduction of waviness
• • Reduce the backflow.
– – Low screw speeds.
Higher compression in screws.
Reduce the melt elasticity of the folds.
– – Reduce the fusion level.
Increase the filler level.
Reduction of screw speed at the same output reduces waviness.
The screw torques will increase.
2
Reduction of waviness
• • Reduce the backflow.
– – Low screw speeds.
Higher compression in screws.
Reduce the melt elasticity of the folds.
– – Reduce the fusion level.
Increase the filler level.
Requires new screw geometry.
1
Reduction of waviness
• • Reduce the backflow.
– – Low screw speeds.
Higher compression in screws.
Reduce the melt elasticity of the folds.
– – Reduce the fusion level.
Increase the filler level.
May impact on the final quality of the pipe (MC attack).
0
Reduction of waviness
• • Reduce the backflow.
– – Low screw speeds.
Higher compression in screws.
Reduce the melt elasticity of the folds.
– – Reduce the fusion level.
Increase the filler level.
Increasing chalk from 2 to 10 % reduces waviness two times.
Not applicable for pressure pipes.
Twin screw extruder
Motor power / Energy balance / Output
Mixing of melt
• •
Distributive = Mixing of fluids by exchange of layers.
– Temperature differences are reduced.
Dispersive = Mixing of a fluid with a solid filler.
– The particle size of the filler must be broken down. The created shear stress must exceed the yield stress of the filler. The filler must be evenly distributed throughout the melt.
distributive dispersive
Screw without mixer
Screw with pinmixer
Mixing processes in a twin screw extruder
• • • •
Mixing by shear Mixing by screw cooling Mixing by geometry changes Mixing in the screw gaps
Mixing by shear
The second fluid will be deformed by shearing of the melt. This way some sort of mixture is obtained. The striation thickness of the second fluid will diminish, the surface will enlarge.
second fluid speed profile of the melt
Mixing by shear
The second fluid will be deformed by shearing of the melt. This way some sort of mixture is obtained. The striation thickness of the second fluid will diminish, the surface will enlarge.
speed profile of the melt
Mixing by shear
The second fluid will be deformed by shearing of the melt. This way some sort of mixture is obtained. The striation thickness of the second fluid will diminish, the surface will enlarge.
speed profile of the melt deformed by shearing of the melt
Mixing by screw cooling
• •
The screw (and barrel) cooling will reduce the slip of the melt against the screw and barrel surfaces.
This effectively increases the shear from the rotating screws.
cold oil in hot oil out
Mixing by geometry changes
•
A change from a two-flighted to a three-flighted section will redistribute the melt.
two-flighted section three-flighted section
Mixing in the screw gaps
• •
Melt is dragged through the gaps of the screws.
– Especially in the pressurized part of the pump section.
The high shear forces in calandar and side gaps will redistribute and break down filler particles in the melt.
– Combination of distributive and dispersive mixing.
side gap calandar gap
Examples of mixing sections
Slots: distributive mixing Gaps: dispersive mixing
Examples of mixing sections
Rules for mixing elements
• • • • •
The pressure drop must be as low as possible.
The flow through the mixing section should be streamlined.
The mixing section should completely wipe the surface.
– – – Good heat transfer.
Reduction of temperature increase.
Prevention of degradation.
The mixing section should be easy to clean.
The mixing section should be easy to manufacture and not too expensive.
Efficient dispersive mixing
• • •
High shear stresses must be created in the melt. They must exceed the yield stress of the filler.
The shear stresses must be present for only a short time in order to reduce temperature increase.
Every part of the melt should receive the same shear stress to reduce temperature differences.
Twin screw extruder
Motor power / Energy balance / Output
barrel adapter
From screw core to pipe
die pipe
Influence of screw cooling on processing
High screw temperature: The PVC stays at the core of the screw. The transport of this layer of melt is slow. Local MC attack due to low temperature. Rough regions at left and right side of pipe. Degradation is possible due to long residence time.
Low screw temperature: The thickness of the cooled PVC layer grows and becomes larger than the calandar gap. This cooled layer of PVC cannot pass the calandar gap and is mixed into the melt.
Twin screw extruder
Motor power / Energy balance / Output
screw (front)
Screw marks
PVC hot at surface hot cold PVC cold in centre
Screw marks
• • •
Screw marks in the pipe are caused by temperature differences.
Screw marks are reduced by: – – Low barrel and screw temperature.
Mixing elements at the end of the screws.
Screw marks are not influenced by the die.
Twin screw extruder
Motor power / Energy balance / Output
Screw wear
Most screw wear is generally observed in the compression sections of the screw. This is caused by the calander force.
The wear in the first compression section is often higher because the PVC is relatively cold.
Screw wear
• • • •
Wear rate screws 0.2 - 0.6 mm/year.
Wear rate barrel 0.05 - 0.15 mm/year.
The gap between the barrel and the screws should be less than 1 mm.
Otherwise: – Black spots in the pipe from the barrel wall.
– – Increased melt inhomogeniety.
Increased melttemperature.
Twin screw extruder
Motor power / Energy balance / Output
Burned PVC can accumulate in worn places of the barrel.
Production of dirt
Burned PVC can accumulate at horizontal surfaces in the venting port.
Possible causes for black spots
• • •
Wear of screws and barrel.
Too high barrel temperatures (> 200 °C).
Horizontal surfaces in the venting port.
Overview PVC processing
vacuum sealing pressure creation for die intake of powder dirt production waviness production fusion of PVC grains waviness reduction
Twin screw extruder
• • • • • • • • • • • • • Why a twin screw extruder?
Motor power / Energy balance / Output Screw speed and torque Screw geometry Gelation of PVC Waviness Mixing “Ten to two” effects Screw marks Screw wear Dirt Conical / Parallel Extruder design
Parallel The larger volume in the pumpzone gives more mixing and a more homogeneous melt.
The construction of the screws and barrel is relatively cheap.
Conical versus parallel extruders
Conical The large shaft to shaft distance results in a relatively cheap gear system with a large torque available.
The large volume and surface at the intake zone gives a better thermal influence and a better intake capacity.
Twin screw extruder
Motor power / Energy balance / Output
screw torque core diameter shaft to shaft distance
Extruder design
Screw geometry
Intake zone.
• • • •
The intake capacity should be 115 % of the required output.
Usually a two-flighted section is used for parallel screws (single flighted for conical).
The section length is about 2 pitches (parallel) to 6 pitches (conical).
The flight angle is about 20 °.
First pump zone.
• • •
The section can be two, three or four-flighted.
The compression ratio is 1.3 to 1.7.
More flights give more shear per unit screw length.
Screw geometry
First pump zone.
• • •
The section can be two, three or four-flighted.
The compression ratio is 1.3 to 1.7.
More flights give more shear per unit screw length.
Screw geometry
Screw geometry
Powderlock.
• • •
The pitch is very small (compression ratio 4.0 - 4.5).
– The channels are always flooded with melt.
Usually single flighted.
Length minimum 2 pitches.
Degassing • •
Large volume (compression ratio 0.5 - 0.8).
– Air can escape.
– Vent opening will not be blocked with melt.
Flight angle 20 °.
– Friction losses at barrel are reduced.
Screw geometry
Screw geometry
Second pump zone • •
Usually two to four-flighted.
– The number of flights determine the friction per unit screw length.
Compression ratio 1.5 to 1.8.
Screw geometry
Second pump zone • •
Usually two to four-flighted.
– The number of flights determine the friction per unit screw length.
Compression ratio 1.5 to 1.8.
Screw geometry
Friction slots / Mixing elements • • • • •
The slots must be cut through the flights down to the core of the screw.
Width of slots = Channel depth.
Not all the slots should be placed behind eachother.
– This would lead to excessive wear.
A N-flighted screw requires N rows of slots.
Only one of every N flights (1 pitch) should be slotted.
Spreadsheet screwdesign
Screw geometry