Page 275 - Plastics Engineering

258 Processing of Plastics

0.96
0.94

0.82 E

0.8 Ld-LL s

0 0.05 0.1 0.15 0.2 0.25 0.3
Ratio (hiT)
Fig. 4.11 Flow correction factors as a function of screw geometry

are conflicting. The extruder has a high output if the pressure at its outlet is
low. However, the outlet from the extruder is the inlet to the die and the output
of the latter increases with inlet pressure. As will be seen later the output, Q,
of a Newtonian fluid from a die is given by a relation of the form

Q=KP (4.12)
nP
where K = -
for a capillary die of radius R and length Ld.
87&
Equation (4.12) enables the die characteristics to be plotted on Fig. 4.12
and the intersection of the two characteristics is the operating point of the
extruder. This plot is useful in that it shows the effect which changes in various
parameters will have on output. For example, increasing screw speed, N, will
move the extruder characteristic upward. Similarly an increase in the die radius,
R, would increase the slope of the die characteristic and in both cases the
extruder output would increase.
The operating point for an extrudeddie combination may also be determined
from equations (4.9) and (4.12) - ignoring leakage flow

So for a capillary die, the pressure at the operating point is given by
2nqD2NH sin 4 cos 4
POP = (4.13)
(R4/2Ld) + (OH3 sin2 4)/3~