Knowledge of the plastic extruder heating cooling system
There has two heat sources that the plastic extrusion process have gained. One is supplied by thecylinder¡¯s external heater, the other is the shear heat generated by friction from the plastic and the inner wall of the cylinder, the plastic and the relative motion between the screw and plastic The former part of the heat is from the heater power, the latter part of the heat is from the transformation of the electric motors into the mechanical energy of the screw. The proportion of these two parts heat and is related to the structure of screw rod, cylinder, process conditions, material properties and so on, also related to the extrusion process of stage (such as start stage, the stable operation stage).
In addition, the proportion of these two parts heat is also different in different stages of the extrusion process: In the period of feeding, due to the deeper spiral groove , the material has not yet been compacted, the friction heat is very little, more calories are from heater;While in the homogenization section, material has been melt with higher temperature, shallow screw groove. The heat generated by the friction shear has been increasing. Sometimes we not only don't need a heater to supply heat, but still need the cooler for cooling. In the compression stage, the material heated is the transition state above two cases, which produces more friction shear heat than the feeding section, and less than the homogenization section. Therefore thecooling system of the extruder is a segmented setting.
From the point of view of energy analysis, there is a heat balance problem in the extrusion process. Heater supply of heat and part of the friction heat generated by the friction shear is used to make plastic produces the change of state of matter, the other part is lost. Although there are many influencial factors in the balance, under the condition of stable extrusion, the balance should always be maintained.
Heating method of the extruder
There are three kinds of extruder heating method: liquid heating, steam heating and electric heating. Electric heating is mostly used among them while the steam heating has been rarely used (hot feeding rubber extruder steam heating). The principle is to heat liquid first (water, oil, PCB, etc.), then heat the cylinder. Now there are more applications of the electric heating,which is divided into the resistance heating and the induction heating.
The cooling of the extruder
Extruder cooling system (with heating system) is used to ensure that the plastic completes its extrusion molding processon under the technological requirements of temperature. During the extrusion process ,it will often produces more friction shear heat generated by the screw rotary than the material needed heat phenomenon, which results in a overhigh material temperature in the cylinder temperature. If not exhaust the exceeding heat in time, it will cause material decomposing(especially for the heat-sensitive plastic) , sometimes it makes the molding process difficult to continue. Therefore, the material barrel must be cooled and screwed.
The cooling of the material barrel
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