As we know,temperature is one of the necessary conditions for the extrusion processing , and the heating and cooling system of the extruderis to ensure the temperature condition.
There are two ways to earn heat in the extrusion processing,one is the heat supplied by the external heater of the barrel, and the other is the frictional shear heat generated by the relative motion between the plastic and the inner wall of the barrel, the plastic and the screw, and the plastic. . The former part is converted from the electric energy of the heater, and the latter part is converted from the mechanical energy transmitted by the motor to the screw. The proportion of these two parts of heat is related to the structure of the screw and barrel, the process conditions, the properties of the material, etc., and is also related to the stage of the extrusion process,such as the start-up stage and the stable operation stage.
In addition, the proportion of these two heating ways is also different in different sections of the extrusion process: in the feeding section, due to the deep screw groove, the material has not been compacted, the frictional heat is very little, and most of the heat comes from the heater; In the homogenization section, the material has been melted, the temperature is high, the screw groove is shallow, and the heat generated by friction and shear is more. Sometimes not only the heater is not needed for heat supply, but the cooler is also needed for cooling. In the compression section, the heating of the material is a transitional state between the above two situations, that is, the heat generated by friction and shear is more than that in the feeding section, but less than that in the homogenization section. Therefore, the heating and cooling system of the extruder is mostly set in sections.
From an energy point of view, the extrusion process has a heat balance problem. Part of the heat supplied by the heater and the frictional heat generated by frictional shearing are used to change the physical state of the plastic, and the other part is lost. Although there are many factors affecting this balance, under the condition of stable extrusion, this balance should always be maintained.
Heating method of the extruder
There are usually three heating methods: liquid heating, steam heating and electric heating. Among them, electric heating is used the most. Steam heating has been rarely used (hot feed rubber extruder steam heating more).
The principle of liquid heating is to heat the liquid (water, oil, biphenyl, etc.) first, and then heat the barrel by them. At present, electric heating is the most widely used in extruders, which is divided into resistance heating and induction heating.
Cooling method of the extruder
The cooling system of the extruder is to ensure that the plastic completes the extrusion molding process in required temperature conditions. During the extrusion process, the friction and shear heat from the screw is often more than that required,then the temperature of the material in the barrel will be too high. If the temperature didnt cool in time, it will make the platic decompose and difficult to form.So the barrel and screw must be cooled.
Barrel cooling includes air cooling and water cooling.
Air cooling is relatively soft and clean,but the cooling fan is too large.If the quality of the cooing fan is not good,it can make noise.Usually it is more suitable for small and medium-sized extruders.
Compared with air cooling,water cooling is faster,small size and low cost.But rapid cooling often happens,which will influence the stable flow of the plastic.If the sealing of the water pipe is not good, there will be running, popping, dripping and leaking.The cooling system with a water pipe on the barrel would have lime scale and be easy to corrode.Therefore, the water used in water cooling system is not tap water, but chemically treated water. The water is softened and treated to remove oxygen.Water cooling is better for large extruders.