Important Stages of the Production Process - Introduction to Polymeric Composites with Rice Hulls - page 93

types of screws are used for different grades of plastic resins (e.g., polyethylene (PE),
polypropylene and PVC) to obtain the maximum output and quality of product.
There are also universal screws that may be used for the processing of many plastic
resins without having to change from one polymer to another. For PCRH, specially
designed screws are required due to the generation of heat and abrasiveness because
of silica in the rice hulls and polymer mix. Strictly speaking, instead of an extruder,
the correct term should be an 'extrusion system' because an extruder cannot carry
out any processing by itself. An extrusion system will consist of several units, and
terms such as post-extrusion line or downstream equipment are associated with an
extrusion process. A typical extrusion line with automatic or semi-automatic operation
for processing polymeric composites with rice hulls would consist of the following
basic equipment:
• Material drying station.
• Particle size-reduction station.
• Mixing and compounding.
• Extruder, screw(s), feed hopper, zonal heaters, motor and drive, cooling fans,
control cabinet, extruder head, profiled die, extrudate sizing station, extrudate
cooling station, haul-off or take-up station and cutting station.
7.8 Extrusion Guidelines
Whichever type of extruder is used for polymer processing, there are certain basic
rules a producer should follow to maximise production and make quality products.
In the case of processing pure polymers, one may say it is straightforward, with the
material being introduced directly into the hopper, heated in the barrel and pushed
via a screw through a die to make a pre-determined profile. It is more complicated
in processing polymer composites and more so if other constituents such as wood
fibre or rice hulls are involved. Here, three additional conditions must be met before
extrusion of the mass:
• Adequate drying to remove moisture from the reinforcing constituents.
• Particle-size reduction of these constituents for easy mixing and compatibility.
• Specially designed screws are required to counter the abravesiness and extra heat
build-up.