502867790-57919596666666666666677777.ppt

Polymer

1. Twin screw extruder

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

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
0 200 400 600 80010001200
0
20
40
60
80
100
120
Motor
power
Cooling
barrel
Cooling
screw

9. Relation output - screw diameter
0
200
400
600
800
1000
1200
1400
60 70 80 90 100 110 120 130
Screw diameter (mm)
O
u
t
p
u
t

(
k
g
/
h
)
30 D
25 D
22 D
8.1
~DQ

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).
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)
S
c
r
e
w

s
p
e
e
d

(
r
p
m
)

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.
N4
P0.85
= M
s
motor
s


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

16. Screw geometry
powder entrance
zone
first compression zone
first pump zone
powder lock
degassing zone
second compression
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
side gap
calandar gap

20. Gaps in the extruder screws
flight gap
side gap
calandar 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. 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. The powder lock
slots for recrushed PVC

26. 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 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

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

38. Screw cooling with oil
Heat is extracted from the melt in the second pump zone.
hot oil out
cold oil in

39. 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

40. 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
PVC grain (0.1 mm)primary particle (1 µ)
crystalline region (0.001 µ)

43. 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. 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. 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.
gelation level gelation level
impact pressure resistance
75 %

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

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

57. Creation of waviness
partially filled
completely
filled
completely
filled
partially filled

58. Creation of waviness
completely
filled
partially filled
forward speed of melt
backflow of melt Q
back
melt pressure
forward speed of screw flight
nett
back
Q
Q
W~
nettchannelback QQQ 
nett output Q
nett
transport capacity
channel Q
channel

59. Creation of waviness
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.
p
r e
s
s
u
r e
500 kg/h
500 kg/h
500 kg/h nett 500 kg/h
transport cap. 800 kg/h
pump zone
back flow 300 kg/h
screw speed 40 rpm

60. Creation of waviness
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
p re ssu re
500 kg/h
500 kg/h
500 kg/h nett 500 kg/h
transport cap. 600 kg/h
pump zone
screw speed 30 rpm
back flow 100 kg/h

61. 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. 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. 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. 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. 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.
0

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. 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. 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. Mixing by shear
speed profile of the melt
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.

73. Mixing by shear
deformed by shearing
of the melt
speed profile of the melt
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.

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.
hot oil out
cold oil in

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.
side gap
calandar 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
adapter
die
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
screw (front)
PVC hot at surface
PVC cold in centre
hot cold

86. 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.

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. 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.

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

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
p
r
e
s
s
u
r
e
intake of
powder
vacuum sealing
waviness
production
waviness
reduction
dirt
production
pressure creation for die
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
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.

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. Extruder design
screw torque
core diameter
shaft to shaft distance

98. 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.

99. •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.

100. •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.

101. 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.

102. 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

103. 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

104. 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

105. 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

106. Screw geometry
Spreadsheet screwdesign