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Simplified Flow Theory for Screw Extruders The flow behavior of a viscous liquid in the channel of an extruder screw is shown to be similar to the flow behavior of viscous liquids between infinite parallel plates， one of which is station... Simplified Flow Theory for Screw Extruders
The flow behavior of a viscous liquid in the channel of an extruder screw is shown to
be similar to the flow behavior of viscous liquids between infinite parallel plates， one of
which is stationary and the other moving. Assuming Newtonian behavior of the
liquid， a differential equation was derived which relates the rate of extrusion and the
die pressure to the screw and die geometry and to the operating variables. Integrated
flow equations are given for the special case in which the viscosity of the liquid is constant
throughout the screw channel (isothermal extrusion). Equations are also given
for the case in which the dimensions of the screw channel are functions of their position
along the length of the screw.
IN THE preceding paper ( 1 )o f this symposium the literature pertaining to the problem of viscous flow in extruders was reviewed.
In this paper the development of simplified but more
useful flow equations is presented. The synibols and nomenclature
used in this paper are defined in the preceding paper (1).
The flow mechanism of the viscous liquid in the helical channel
of the screw can be better understood if one imagines that the
channel be unrolled and laid out on a flat surface. Figure 1
shows this concept of the screw channel. If the lower plate，
representing the screw surface， is held stationary and the upper
plate， representing the barrel surface， is moved in the direction of
the arrow， the relative motions will be the same as those existing
in an extruder where the barrel is stationary and the screw rotates.
Assuming that the liquid wets both surfaces， the motion of the
barrel drags the viscous liquid along with it， while the stationary
plate exerts an equal and opposite drag. The velocity of the
liquid， relative to the screw， is a maximum at the barrel surface
and zero at the screw surface. There is also a directional factor
involved， since the channel is inclined at angle p to the direction
of motion. Therefore， in computing the flow rate in the channel
we break up the velocity into two components: one of these acts
directly down the channel， and the other acts at right angles to it.
We call the component which acts down the channel drag velocity，
and the component which acts at right angles to this transverse
velocity. At the end of the channel there is generally a die
or some other restriction to flow. This sets up a pressure gradient
down the channel causing a flow in the reverse direction to the
drag flon. There is one other
flow that must be considered. Generally the screw does not fit
perfectly inside the barrel. In other words， there is a clearance
between the top of the screw threads and the barrel surface.