Page 278 - Plastics Engineering

Processing of Plastics 26 1

barrel of the extruder in this case), the principal stresses which exist as a result
of an internal pressure are

PmmD
hoop stress, 01 =
2h
~
PmaxD
axial stress, a2 = -
4h
where h = wall thickness of the barrel.
The von Mises criterion simply states that yielding (failure) will occur if

where ay = tensile yield stress of material
FS = factor of safety.

In this case, therefore

8h2
h = 9.8 mm
Hence a barrel wall thickness of 10 mm would be appropriate.
Example 4.2 A single screw extruder is to be used to manufacture a nylon
rod 5 mm in diameter at a production rate of 1.5 ndmin. Using the following
information, calculate the required screw speed.
Nylon Extruder Die
Viscosity = 42.0 Ns/m2 Diameter = 30 mm Length = 4 mm
Density (solid) = 1140 kg/m3 Length = 750 mm Diameter = 5 mm
Density (melt) = 790 kg/m3 Screw flight angle = 17.7"
Metering channel
depth = 2.5 mm

Die swelling effects may be ignored and the melt viscosity can be assumed to
be constant.
Solution The output rate of solid rod = speed x cross-sectional area
= 1.5 x ~(2.5 1OW3)*/60
x
= 49.1 x m3/s
As the solid material is more dense than the melt, the melt flow rate must be
greater in the ratio of the solid/melt densities. Therefore

Melt flow rate through die = 49.1 x lop6 (g) = 70.8 x m3/s