1. Twin screw extruder

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

3. Twin screw extruder
screws

4. 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

5. 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.

6. 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

7. 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

8. Motor power, barrel cooling and screw cooling

9. Relation output - screw diameter

10. 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

11. 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

12. 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).

13. 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.

14. 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

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

16. 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

17. Screw geometry

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

19. Gaps in the extruder screws
flight gap
calandar gap
side gap

20. Gaps in the extruder screws
flight gap
calandar gap
side gap

21. 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.

22. 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.

23. 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.

24. 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.

25. slots for recrushed PVC
The powder lock
slots for recrushed PVC

26. 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.

27. The degassing zone

28. The degassing zone

29. The degassing zone
air grooves

30. 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

31. SECOND COMPRESSION ZONE
Screw geometry
SECOND COMPRESSION ZONE
For efficient heat input the volume of the screw channels must be decreased again.

32. Screw geometry
SECOND PUMP ZONE
In this zone pressure is created to press the melt through the die. Mixing elements may be present.

33. Screw geometry
MIXING ELEMENT
The mixing element redistributes the melt over the screw channels. It reduces the pressure building capacity of the screws.

34. Screw geometry
MIXING ELEMENT
The mixing element redistributes the melt over the screw channels. It reduces the pressure building capacity of the screws.

35. Screw geometry

36. Screw pressure build-up
small gaps: high pressure building capacity
large gaps: low pressure building capacity

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

38. 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

39. 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

40. 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.

41. 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

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

43. 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”.

44. 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

45. Fusion of PVC grains
fusion 0 %
fusion 50 %
fusion 75 %
fusion 100 %

46. Fusion 0 %

47. Fusion 50 %

48. 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

49. 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

50. 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

51. 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

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

53. 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

54. Waviness in the pipe

55. 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)

56. 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

57. Creation of waviness completely filled partially filled

58. 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

59. 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 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.

60. 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 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

61. 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

62. 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

63. 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

64. 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

65. 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.

66. 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

67. 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

68. Screw without mixer

69. Screw with pinmixer

70. Mixing processes in a twin screw extruder
Mixing by shear
Mixing by screw cooling
Mixing by geometry changes
Mixing in the screw gaps

71. 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

72. 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

73. 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

74. 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

75. 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

76. 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

77. Examples of mixing sections

78. Examples of mixing sections
Slots: distributive mixing
Gaps: dispersive mixing

79. 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.

80. 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.

81. 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

82. From screw core to pipe
barrel
die
adapter
pipe

83. 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.

84. 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

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

86. 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.

87. 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

88. 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.

89. Wear rate screws 0.2 - 0.6 mm/year.
Screw wear
Wear rate screws mm/year.
Wear rate barrel 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.

90. 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

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

92. Possible causes for black spots
Wear of screws and barrel.
Too high barrel temperatures (> 200 °C).
Horizontal surfaces in the venting port.

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

94. 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

95. 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.

96. 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

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

98. 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°.

99. 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.

100. 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.

101. Powderlock. The pitch is very small (compression ratio 4.0 - 4.5).
Screw geometry
Powderlock.
The pitch is very small (compression ratio ).
The channels are always flooded with melt.
Usually single flighted.
Length minimum 2 pitches.

102. Degassing Large volume (compression ratio 0.5 - 0.8).
Screw geometry
Degassing
Large volume (compression ratio ).
Air can escape.
Vent opening will not be blocked with melt.
Flight angle 20°.
Friction losses at barrel are reduced.

103. 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.

104. 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.

105. 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.

106. Spreadsheet screwdesign
Screw geometry
Spreadsheet screwdesign