Page 91 - Engineering Plastics Handbook

Processing 65

where Q , Q , Q = drag flow, pressure flow, leakage flow, N/h
L
P
D
A = screw geometry constant
N = screw speed, rpm
D = screw diameter, mm
h = channel depth in metering zone, mm
φ= helix angle of screw, rad
B = screw geometry constant
L = length of metering zone, mm
m
C = screw geometry constant
∆P = head pressure, MPa
−1
µ= apparent viscosity in metering zone, s
δ= flight clearance, mm
e = flight width, mm
K = die constant
Drag flow is primarily determined by helix angle, barrel, and chan-
nel design; pressure flow is determined by helical length, channel design,
pressure rise, and melt viscosity. Extruder output, therefore, is a func-
tion of all these factors. Assuming newtonian, isothermal melt flow [2],
Drag flow = AN

A = n (π DWH cos θ b )
b
2
where A = geometry constant
N = screw speed, rpm
n = number of parallel channels
D = inside diameter of barrel, mm
b
W = width of screw channel, mm
H = channel depth, mm
θ = helix angle at barrel surface, rad
b
Z = helical length, mm
B = geometry constant
∆P = pressure rise, MPa
µ= melt viscosity, s −1
Q = net output of extruder, N/h
The compression ratio enhances melt mixing and uniform melt deliv-
ery. The compression ratio ranges from about 1.5:1 to 4:1. It is gener-
ally the ratio of the volume of the channel of the screw at the hopper end
to the volume of the channel at the die end. It can be measured in other
ways. The feed (hopper) zone channel volume depth is greater than the
metering zone. The compression ratio for more viscous resins is lower
to avoid overheating. Higher compression ratios are used when large
amounts of low bulk density are in the feed.