Page 92 - Engineering Plastics Handbook

66 Introduction

To calculate the compression ratio [3], use

CR = V feed zone ≅ H feed zone
V H
metering zone metering zone
where CR = compression ratio
3
V feed zone = feed zone channel volume, mm
V metering zone = metering zone channel volume, mm 3
H feed zone = feed zone channel depth, mm
H metering zone = metering zone channel depth, mm

The power to turn an extruder screw is determined in part by the helix
design. To calculate the power, first calculate the value of E by the following
formula, assuming an isothermal newtonian melt [2].
π 3 D sin θ
3
E = b b (1 + 3 sin 2 θ )
b
H
where E = extruder factor, mm 2
D = inside diameter of barrel, mm
b
θ = helix angle at barrel surface, rad
b
H = channel depth mm
2
P = EµN Z + AN∆P
where P = power required to turn screw
µ= melt viscosity, P
N = screw speed, rpm
Z = helical length, mm
AN = drag flow, N/h
∆P = pressure rise, MPa

The extruder thrust bearing is a critical component that contributes to
product quality and extrusion efficiency. It couples the gearbox to the
screw shank and gives support to the screw. It would be expected to last
the service life of the extruder, assumed to be 10 years or more. Thrust
bearing life expectancy is a function of head pressure and screw speed. Life
expectancy B-10 is the number of hours when 10 percent of the bearings
fail when tested at a standard pressure and screw speed. When the oper-
ating pressure and/or screw speed is greater than the B-10 standard,
thrust bearing life is reduced, and an estimated new life expectancy is
calculated by [3]

B-10 = B-10 standard  N standard  P standard    K



P
N