Important Stages of the Production Process - Introduction to Polymeric Composites with Rice Hulls - page 95

down by running a small batch of the material through the die and checking the melt
temperature with a melt probe before starting the production run.
In the event of a stoppage, for example, due to a power failure or a stoppage overnight
(which in the case of low-density polyethylene (LDPE) can be left in the barrel),
care should be taken to ensure that decomposition or degradation of the material
in the barrel does not occur. If this happens, pressurised gases will blow or shoot
out hot plastic melt through the die, which can cause serious accidents to operators.
The correct procedure is to heat up the barrel below the actual melt temperature of
the mixture and equilibrate and slowly increase the temperature of the die to
above the melt temperature. The temperatures of the front zone and back zone must
be increased to above the melt temperature and then one must work towards the
middle zone. Finally, the desired temperatures can be set for all and the extruder
allowed to equilibrate before extrusion starts. The time factor is important to avoid
material degradation in the barrel.
7.8.5 Heat Supply
The actual heat needed to plasticise the composite mixture comes from the barrel
heaters and the screw speed. Sometimes the heat generated by the rotating screw is
so great that the heat is excessive. In this case a balance must be maintained between
the heaters and cooling of the barrel to maintain the required temperatures.
7.8.6 Die Heaters
In polymeric composite extrusions, most dies could be profiles of various cross-
sections. If a die is divided into more than one zone, better temperature controls
can be achieved. In general, electrical heating elements are used with thermocouples
as sensors and electronic controllers. There are different types of heaters, such as
cartridge heaters, band heaters, and plate heaters. Selection and use of the correct
power rating for a heater is important to ensure that dies heat up to the desired
temperature in a reasonable time. The three main factors involved in the calculation
are: the weight of metal in each zone of the die; the target time desired to reach the
required temperature; and the specific heat of the metal used. To compensate for heat
losses from the surface of the die, the calculated power rating in kilowatts should be
increased by 30-40%.
Dies are made from metals and are mounted on extruder heads. A basic design
comprises three sections: the adaptor (which links with the extruder), the distributor
section (which speeds the hot melt into the correct shape or profile) and the land