Page 279 - Plastics Engineering

262 Processing of Plastics

The pressure necessary to achieve this flow rate through the die is obtained from
XPP
Q=-
8qLd

8 x 420 x 4 x x 70.8 x
P= = 7.8 MNIm’
n(2.5 10-3)4
At the operating point, the die output and the extruder output will be the same.
Hence

Q = 70.8 x = in2(30 x 10-3)2N(2.5 x sin 17.7 cos 17.7

n(30 x 1OP3)(2.6 x 10-3)3 sin 17 7.8 x lo6
- ( 0.75 )
12 x 420
N = 22 revlmin

4.2.6 General Features of Twin Screw Extruders
In recent years there has been a steady increase in the use of extruders which
have two screws rotating in a heated barrel. These machines permit a wider
range of possibilities in terms of output rates, mixing efficiency, heat genera-
tion, etc compared with a single screw extruder. The output of a twin screw
extruder can be typically three times that of a single screw extruder of the same
diameter and speed. Although the term ‘twin-screw’ is used almost universally
for extruders having two screws, the screws need not be identical. There are in
fact a large variety of machine types. Fig. 4.14 illustrates some of the possibil-
ities with counter-rotating and co-rotating screws. In addition the screws may
be conjugated or non-conjugated. A non-conjugated screw configuration is one
in which the screw flights are a loose fit into one another so that there is ample
space for material between the screw flights (see Fig. 4.15).

(a) Counter- rotating (b) Co-rotating (c) Counter-rotating (d) Co- rotating
(intermeshing) (intermeshing) (non-intemshlng) (non-intemhing)
Fig. 4.14 Different types of twin screw extruder