Important Stages of the Production Process - Introduction to Polymeric Composites with Rice Hulls - page 94

7.8.1 Extrusion Screws
In general processing of plastics, different screws are used for different polymers or a
universal screw to achieve maximum homogenous mixing and good flow properties.
A screw comprises three main areas: feed zone, compression zone and metering zone.
All three areas combine to force a smooth, well-mixed hot melt through a profiled die.
The heat in each zone must be rigidly controlled with set temperatures to ensure smooth
mixing and flow as well as to avoid degradation due to excessive heat or excessive
dwell time in the extruder barrel. Most commercial plastics are poor conductors of
heat, have poor thermal stability but have high specific heat. This means that materials
degrade if overheated or if are not conveyed at the right speed along the barrel of the
extruder, and so the melt temperature of a particular polymer becomes important.
7.8.2 Melt Temperature
The melt temperature of a material or a mixture is, in general, measured in the die or
extruded into the air, in which case great care should be taken to prevent accidents
from the very hot melt. A hot melt is sufficiently hot to cause severe burns and also
to adhere on skin. Protective wear is needed, especially if starting an extruder or
purging at the end of a run.
7.8.3 Temperature Settings
The melt temperature is important in extrusion. A supplier of plastic resins will
give guide temperatures. In general, the first heat zone (zone 1) is set at the lowest
temperature and increased gradually along the barrel up to the die, where the
temperature may be set lower than the last zone. Temperature affects the final finish
and the output, so constant control of the temperature gradient should be monitored
and maintained throughout extrusion. Technological advances make this possible
without problems. The temperature settings for polymeric composite mixtures with
rice hulls differ from standard polymer extrusions, and a producer of composites
will have a choice between trial-and-error or a laboratory test, which will stop a lot
of material being wasted.
7.8.4 Extruder Warm-up
The heat supply to an empty extruder should be done with care so that the heating
is done within a reasonable time and does not overshoot. Before the material is
introduced into an extruder barrel, it should be allowed to equibrate and settle