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Processing of Plastics 263

lo) Non-conjugated screws showing some I bl Conjugoted screws showing closed
passages around each screw passages around each screw
Fig. 4.15 Two types of twin screw extruder

In a counter-rotating twin screw extruder the material is sheared and pres-
surised in a mechanism similar to calendering (see Section 4.5), i.e. the material
is effectively squeezed between counter-rotating rolls. In a co-rotating system
the material is transferred from one screw to the other in a figure-of-eight
pattern as shown in Fig. 4.16. This type of arrangement is particularly suit-
able for heat sensitive materials because the material is conveyed through the
extruder quickly with little possibility of entrapment. The movement around
the screws is slower if the screws are conjugated but the propulsive action is
greater.

Table 4.1
Comparison of single-screw, co-rotating and counter-rotating twin-screw extruders
Co-rotating screw
type I
Single screw Low speed High speed Counter-rotating
twin screw
type
Friction between Mainly depend on the Forced mechanical
cylinder and frictional action as in the conveyance based
Principle materials and the case of single screw on gear pump
same between extruder principle
material and screw
Conveying
efficiency LOW Medium High
Mixing efficiency Low Mediudigh High
Shearing action High Medium High Low
Self-cleaning effect Slight Medium/High High Low
Energy efficiency LOW Medium/High High
Heat generation High Medium High LOW
Temp distribution Wide Medium Narrow Narrow
Max. revolving
speed (rpm) 100-300 25 - 35 250-300 35-45
Max. effective
length of
screw L/D 30-32 7- 18 30-40 10-21