One important thing to note is that all components have bores or channels drilled inside them for cooling purposes. This indicates that the temperature rise inside the extruder is formidable and sufficient measures must be taken to prevent scorching.
Also excessive temperature makes the rubber to stick on the screw and make mixing inefficient. Too low a temperature gives uneven flow, bad surface, large expansion and large shrinkage on the length of the product.
Also the mixing chamber or the barrel is divided into a number of zones depending on whether it is cold (more zones) or preheated (fewer zones) rubber. Each zone has its own temperature control circuit. The zones are roughly 375 – 450 mm long i.e a 4.5 inch extruder(~112.5 m) will have 4 –6 barrel zones. The number of zones also depends on the L/D ratio, the number of feed points for additives, and the type of material being extruded.
Temperature control is complicated due to several factors. These include heating due to shear rate (It is worthy to note that 80 – 100% of the heat produced throughout the extruder can be generated by the screw shear alone) caused by screw speed, feed rate changes, resistance offered by the die and difference in process gains for heating and cooling. Say, the screw speed increases; the higher rate of shear in the screw channel increases mechanical work, raising the compound temperature. To aggravate the situation, cooling rate is reduced as screw speed and output rate increase. But suppose the pressure is low a sin the case of low resistance die size, the amount of frictional heat may not be sufficient to overcome the decrease in conducted heat, in this case an increase in screw speed results in consequent drop in temperature.
At higher pressures, the temperature may show an initial increase before dropping and at still higher pressures, the same screw and operating conditions can give an increased temperature with increasing screw speed.
