Important Stages of the Production Process - Introduction to Polymeric Composites with Rice Hulls - page 96

(which maintains a constant cross-section to the flow channel before reaching the die
exit lips). If designing dies, some of the important areas to consider are:
• Extrudate shape
• Degradation of melt
• Die design
• Required surface inish
• Blocking due to contamination
• Back pressure from die resistance
• Die maintenance
PCRH are extruded in various profiles such as solid sections, hollow sections and
various other profiles. Thus, the design and size of dies becomes important.
• Extrusion Speeds
Some of the heat needed to plasticise the melt is provided by rotation of the screw.
The faster the speed of the screw, the higher is the temperature. This phenomenon
is due to the increase in the speed of the screw surface, resulting in an increasing
amount of shear. To counter localised overheating in the barrel, as a general rule,
larger machines are worked at lower speeds, whereas smaller machines should
work at faster speeds, which will keep down the shear rate.
• Zonal Heating
An extruder barrel, in general, consists of three regions or zones: feed, compression
and metering. Each of these zones has its own heating and cooling systems. In
general, small machines will have three zones, whereas the large machines can
have up to 12 zones. Each zone can be controlled and monitored independently
using microprocessors and other equipment. The required parameters can be set
to adjust automatically but these are the barrel temperatures. The more important
parameter is the melt temperature of the plastic composite mix, which is also
dependent on the rotational speed of the screw and the back pressure.
• Extruder Venting
Some materials develop gases if heated, and an extruder barrel must be provided
with approriate venting to allow them to escape. In most cases, it could be air