Important Stages of the Production Process - Introduction to Polymeric Composites with Rice Hulls - page 90

Another related trend in co-rotating intermeshing twin screws is their ability to run
at increasingly higher speeds. Some extruders can reach very high speeds, especially
on the larger ones. At the higher speed ranges, the limiting factors are process-related
(i.e., whether it is possible to maintain product quality at those high rates). Other
limiting factors may be wear-related, as with abrasive fillers. Composite processing
with rice hulls poses this problem because the mixture will contain silica from the rice
hull flour and will also generate extra heat. However, extruder manufacturers have
overcome this problem with specially designed screws. For non-abrasive mixtures,
higher speeds can be achieved easily. Shear stress can often be minimised by screw
design so that even some temperature-sensitive materials, such as polycarbonate (PC),
can benefit from higher outputs.
7.5.3 Counter-rotating Non-intermeshing Extruders
Suppliers of counter-rotating non-intermeshing twin screw machines bring several
advantages to feeding, mixing and venting, with some machines that can be
converted into all major operating modes. One advantage of this type of machine
is maximum free volume, which is the result of two related design factors inherent
in non-intermeshing screws: (i) greater centreline distance can be achieved between
screw shafts; and (ii) flight occlusion (in which the wiping function of screw flights
against the root of the opposite screw takes up some volume). A third point relates to
flexibility. Because the screws do not intermesh, it is possible to vary the root diameter
to provide much-needed shear energy for the application. Some process engineers
think that counter-rotating non-intermeshing machines may provide higher output
rates of ≤30% than co-rotating intermeshing machines of the same screw diameter.
The non-intermeshing screw configuration also has implications for power. This extra
available horsepower for the same screw diameter enables its machines to handle
greater speed, volume and length-to-diameter ratio (L/D).
Counter-rotating non-intermeshing machines are well suited to distributive mixing
and provide a high degree of flexibility and control of the shear energy imparted. One
advantage of the low-shear field in the apex is that it reportedly permits relatively
high screw speeds in compounding shear-sensitive materials. If more shear is needed,
compounder sections may be added. Staggered screw flights provide cross-channel
and radial back mixing. Interestingly, non-intermeshing extruders decouple mixing
efficiency from screw speed so that mixing remains relatively efficient at low speed.
These machines also have advantages in terms of feeding and venting. In feeding,
the downward motion of the counter-rotating screws is said to allow wider openings
across both screws and to promote better feed acceptance. The same principle applies
in venting, in which wider openings are provided directly above the apex.