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During the early stage of development of the EPZ of the co-rotating twin-screw extruder, an

untested hypothesis was that a uniform gap should be found between the intermeshing

elements as shown in Figure 1. This leads to a low shear rate in the regular screw channels

and a very high shear rate in the intermeshing zone of the screws. The magnitude of these

two shear rates can be expressed as follows.

Mean shear rate = 2

p

a N / 60 h

It may be noted that a/h is a constant for a particular Do/Di ratio. The mean shear rate is

diameter independent. Generally, this value lies between 200 to 500 1/s for screw speeds of

300 to 1200 RPM.

When the flank A crosses behind flank B, the two surfaces move in opposite directions and in

this space, material is trapped between the two surfaces and experiences shear. The

intensity of this shear is provided below

Peak Shear Rate = 2

p

a N / 60 c

Where a is the Center Distance in mm

N is the screw RPM

and c is the distance of separation of the two flank surfaces

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THE EXTRUDER TIMES / ISSUE 10 / AUGUST 2010

Game Changing

Technological Innovations in

MELT COMPOUNDING

Figure 1

Uniform Gap in Axial and Radial Directions

leads to Peak shear

Figure 2

Peak Shear in the intermeshing zone for

conveying screw and kneading blocks