Twin screw extruder

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

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 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

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

Relation output - screw diameter

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

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).

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.

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

The screw length varies from 22 to 30 D. Longer screws give a better melthomogeneity. Longer screws require a higher lubricated compound.

first compression zone Screw geometry powder entrance zone first compression zone powder lock first pump zone second compression zone degassing zone pump zone mixing elements

Screw geometry

Conical and parallel screw geometry powder entrance zone first pump zone degassing zone pump zone first compression zone powder lock second compression zone

Gaps in the extruder screws flight gap calandar gap side gap

Gaps in the extruder screws flight gap calandar gap side 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.

FIRST COMPRESSION 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.

slots for recrushed PVC The powder lock slots for recrushed PVC

In this zone air and volatiles are extracted from the melt. 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 removed by vacuum in vent zone pressure in polymer pressure in polymer air pressed away by compression of powder air pressed away by compression of powder

SECOND COMPRESSION ZONE 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

Cooling with heatpipes. No cooling. Screw cooling Cooling with oil. Cooling with heatpipes. No cooling.

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

Cooling with oil gives better control on the process. Screw cooling Cooling with oil gives better control on the process. Cooling with heatpipes reduces energy losses. Higher output capacity possible.

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

The PVC grain primary particle (1 µ) PVC grain (0.1 mm) crystalline region (0.001 µ)

PVC is processed at temperatures between 190 and 210 C. 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 of PVC grains fusion 0 % fusion 50 % fusion 75 % fusion 100 %

Fusion 0 %

Fusion 50 %

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

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. impact pressure resistance 75 % gelation level gelation level

Impact level versus fusion of PVC falling weight outside 100 % fusion crack inside falling weight outside 75 % fusion inside crack

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

Waviness in the pipe

Waviness in the pipe 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). length of waves (about equal to wallthickness)

Waviness increases approximately with the output squared. Waviness and output Waviness increases approximately with the output squared. maximum tolerable level waviness maximum output limited by waviness output

Creation of waviness completely filled partially filled

Creation of waviness melt pressure forward speed of screw flight backflow of melt Qback nett output Qnett transport capacity channel Qchannel completely filled partially filled forward speed of melt

Waviness is proportional to back flow / output. Creation of waviness pressure 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.

pressure 500 kg/h 500 kg/h 500 kg/h nett 500 kg/h back flow 100 kg/h Creation of waviness pressure 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 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

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. 3

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. 2

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). 1

Reduce the melt elasticity of the folds. 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 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

distributive dispersive 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

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. second fluid speed profile of the melt

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 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. 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. 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. calandar gap side 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 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

From screw core to pipe barrel die adapter 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 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

Screw marks PVC hot at surface hot cold screw (front) PVC cold in centre

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 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

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.

Wear rate screws 0.2 - 0.6 mm/year. 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 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

Production of dirt Burned PVC can accumulate at horizontal surfaces in the venting port. Burned PVC can accumulate in worn places of the barrel.

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 pressure creation for die vacuum sealing pressure dirt production waviness production intake of powder waviness reduction fusion of PVC grains

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

Conical versus parallel extruders The larger volume in the pumpzone gives more mixing and a more homogeneous melt. The construction of the screws and barrel is relatively cheap. 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 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

shaft to shaft distance Extruder design screw torque core diameter shaft to shaft distance

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. 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.

First pump zone. The section can be two, three or four-flighted. 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.

Powderlock. The pitch is very small (compression ratio 4.0 - 4.5). 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). Screw geometry 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.

Second pump zone Usually two to four-flighted. 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.

Second pump zone Usually two to four-flighted. 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.

Friction slots / Mixing elements 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 Spreadsheet screwdesign