Page 90 - Engineering Plastics Handbook

64 Introduction

used in vented barrel extruders. The five zones are the feed, transition,
first metering, vent, and second metering. Pressure is reduced in the vent
zone and increased in the second metering zone. Pump ratio ϑ indi-
cates the capability of the second metering zone Π to pump more resin
2
(compound or composite) than the first metering zone Π delivers [3].
1
∏
ϑ= 2
∏
1
(The two-stage screw is described in “Plasticating Components Technology,”
Spirex Corporation, Youngstown, Ohio, 1992.)
Melt flow near the screw is forced back into the cylinder. Extruder
output can be estimated by subtracting melt pressure flow from drag flow
Q which conveys melt along the cylinder wall to the die. Melt pressure
D
is highest in the metering zone immediately prior to entering the die,
where flow splits into two components: the pressure flow Q and a leak-
P
age flow Q . An order-of-magnitude net output Q of a single-screw extruder
L
is based on the following assumptions [3]:
Melt flow is newtonian.
Extruder is at a steady state.
Extruder is at a constant temperature.
Output is due to only the metering zone.

π 2 Dh sin cos φ
φ
2
Q = AN = N
D
2
π 2 Dh sinφ cos φ
2
A =
2
∆
3
Q = BP = π Dh sin 2 φ ∆ P
µ ( 12 L ) µ
P
m
π Dh sin 2 φ
3
B =
12 L
m
∆
φ
23
Q = CP = π D δ tan ∆ P
L µ µ
eL )
(
12
m
π D δ tan φ
23
C =
12 eL
m
∆ P
Q =
Kµ