Twin screw extruder. hopper degassing barrel die head auxiliary equipment gearbox and thrustbearing box

Twin screw extruder

Twin screw extruder

hopper

degassing

barrel

die head

auxiliary equipment

gearbox and thrustbearing box

Twin screw extruder

screws

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

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


Energy balance PVC extrusion

ENERGY IN

Main motor 110 Wh/kg

Heating (barrel and dies) 40 Wh/kg

Total in 150 Wh/kg

ENERGY OUT

Heating PVC 80 Wh/kg

Screw cooling 20 Wh/kg

Barrel cooling 25 Wh/kg

Gearbox and thrustbearingbox 12 Wh/kg

Pulley 4 Wh/kg

Convection 9 Wh/kg

Total out 150 Wh/kg

Motor power, barrel cooling and screw cooling

0 200 400 600 800 1000 12000

20

40

60

80

100

120

Motor power

Cooling barrel

Cooling screw

Relation output - screw diameter

0

200

400

600

800

1000

1200

1400

60 70 80 90 100 110 120 130

Screw diameter (mm)

Ou

tpu

t (k

g/h

)

30 D

25 D

22 D

8.1~ DQ

Twin screw extruder


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).

0

10

20

30

40

50

60

70

80

90

100

40 50 60 70 80 90 100 110 120 130 140

Screw diameter (mm)

Scr

ew s

peed

(rp

m)

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.

N4P0.85

= Ms

motors

Twin screw extruder


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 compression zone

first pump zone

powder lock

degassing zone

second compression zone

pump zone

mixing elements

Screw geometry

Conical and parallel screw geometry

powder entrance zone

first pump zone

degassing zone pump zonefirst compression zone

powder lock

second compression 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

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

air pressed away by

compression of powder

pressure in polymer

pressure in polymer

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.

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

MIXING ELEMENT

The mixing element redistributes the melt over the screw channels. It reduces the pressure building capacity of the screws.

Screw geometry

Screw pressure build-up

small gaps: high pressure building capacity

large gaps: low pressure building capacity

Screw cooling

• Cooling with oil.

• Cooling with heatpipes.

• No cooling.

Screw cooling with oil

Heat is extracted from the melt in the second pump zone.

hot oil out

cold oil in

Screw cooling with heatpipes

Heat is transferred from the melt in the second pump zone to the powder in the entrance zone.

thermal isolationcopper netting

evaporating watercondensing water vapour

copper netting

Screw cooling

• Cooling with oil gives better control on the process.

• Cooling with heatpipes reduces energy losses.– Higher output capacity possible.

Twin screw extruder


The PVC grain

PVC grain (0.1 mm)primary particle (1 µ)

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”.


• 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 50 %

fusion 75 % 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

• The fusion level decreases due to friction at low melt temperature.– Low barrel temperatures.– Very low die temperatures (surface effect).

higher melt temperature

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 melt + 2 to 4 °C).– The speed of the screws (+ 10 % 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 melt ± 1 °C– Last barrelzone ± 10 °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.

gelation level gelation level

impact pressure resistance

75 %

Impact level versus fusion of PVC

falling weight

falling weight

outside

outside

inside

inside

100 % fusion

75 % fusion

crack

crack

Twin screw extruder


Waviness in the pipe

Waviness in the pipe

length of waves (about equal to wallthickness)

height of waves

light

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).

Waviness and output

Waviness increases approximately with the output squared.

maximum tolerable level

waviness

output

maximum output limited by waviness

Creation of waviness

partially filled completely filled

completely filled

partially filled


completely filled

partially filled

forward speed of melt

backflow of melt Qback

melt pressure

forward speed of screw flight

nett

back

Q

QW ~

nettchannelback QQQ

nett output Qnett

transport capacity channel Qchannel


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.

pres

sure

500 kg/h

500 kg/h

500 kg/h nett 500 kg/h

transport cap. 800 kg/hpump zone

back flow 300 kg/h

screw speed 40 rpm


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

pressure

500 kg/h

500 kg/h

500 kg/h nett 500 kg/h

transport cap. 600 kg/hpump zone

screw speed 30 rpm

back flow 100 kg/h

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.

4





Reduction of screw speed at the same output reduces waviness.

The screw torques will increase.

3





Requires new screw geometry.

2





May impact on the final quality of the pipe (MC attack).

1





Increasing chalk from 2 to 10 % reduces waviness two times.

Not applicable for pressure pipes.

0

Twin screw extruder


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

speed profile of the melt


Mixing by shear

deformed by shearing of the melt speed profile 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.

hot oil out

cold oil in

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

Examples of mixing sections

Slots: distributive mixing

Gaps: dispersive mixing

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


From screw core to pipe

barrel

adapter

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


Screw marks

screw (front)

PVC hot at surface

PVC cold in centre

hot cold

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


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


Production of dirt

Burned PVC can accumulate in worn places of the barrel.

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

pre

ssur

e

intake of powder

vacuum sealing

waviness production

waviness reduction

dirt production

pressure creation for die

fusion of PVC grains

Twin screw extruder


Conical versus parallel extruders

Parallel 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.

The larger volume in the pumpzone gives more mixing and a more homogeneous melt.

The construction of the screws and barrel is relatively cheap.

Twin screw extruder


Extruder design

screw torque

core diameter

shaft to shaft distance

Screw geometry

• 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°.

Intake 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

First pump zone.

Screw geometry

• 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.

Powderlock.

Screw geometry

• 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.

Degassing

Screw geometry

• Usually two to four-flighted.– The number of flights determine the friction per unit screw length.

• Compression ratio 1.5 to 1.8.

Second pump zone

Screw geometry

• Usually two to four-flighted.– The number of flights determine the friction per unit screw length.

• Compression ratio 1.5 to 1.8.

Second pump zone

Screw geometry

• 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.

Friction slots / Mixing elements

Screw geometry

Spreadsheet screwdesign

Documents

Twin screw extruder. hopper degassing barrel die head auxiliary equipment gearbox and thrustbearing box