Important Stages of the Production Process - Introduction to Polymeric Composites with Rice Hulls - page 89

mixing for a wide range of materials. Forces on bearings and components are radial
and not axial because in twin-screw extruders shearing takes place mainly between
the tip of the rotor to the barrel wall. Because there is little thrust, more energy can
be directed into mixing.
7.5.2 Co-rotating Intermeshing Extruders
The largest group of new suppliers of machines in recent years has been for co-rotating
intermeshing twin-screw extruders. One important overall trend in twin-screw
compounding is the ability to process increasingly greater volumes for a given size
machine. Several manufacturers of co-rotating intermeshing twin-screw machines
claim to offer machines that maximise free volume or the free space in the barrel
that determines volumetric processing capacity. Free volume may be defined by the
centreline distance between the screw shafts and the outer diameter (OD)/inner
diameter (ID) ratio of each screw.
Three distinct benefits if higher free-volume processing are:
• Fluidisation can be minimised when feeding powdery materials, which permits
higher throughput rates.
• Residence time can be maximised, allowing higher throughput rates on some
chemical reactions and formulations of highly divergent viscosities.
• Vacuum entrainment can be minimised, allowing higher volatile levels to be drawn
from the process. In addition, its clamshell barrel design can be an effective screw
design tool.
According to specifications, some screws have two channel depths at the same screw
diameter whereas in some, the screw flights are deeper and the root of the screw is
reduced. According to the manufacturers, the larger free volume, coupled with high-
torque capability, results in higher throughput.
Another important aspect of the free-volume issue is the ability to transmit
power through the shaft to the material. The relationship between free volumes and
power varies with different machines and suppliers. Technical reports claim that a
1.55 OD/ID ratio for the latest super compounders optimises the balance between
free volume and power for a wide range of materials, and also provide high torque,
which aids faster processing. An added benefit is that the power per unit of free
volume is constant, making scale-up easier because of consistent shear rate from one
size to the other, assuming the same degree of channel fill.