Known from the extrusion process, temperature is one of the necessary conditions for the extrusion process, and the heating and cooling system of the extruder is set to ensure this necessary condition.
There are two sources of heat for plastics during extrusion; One is the heat supplied by the external heater of the barrel, and the other is the frictional shear heat generated by the relative movement between the plastic and the inner wall of the barrel, the plastic and the screw, and the plastic. The first part of the heat is converted by the electrical energy of the heater, and the latter part of the heat is converted by the mechanical energy transferred by the motor to the screw. The size of the heat ratio of these two parts is related to the screw, the structural form of the barrel, the process conditions, the nature of the material, etc., and also related to the stage of the extrusion process (such as the start-up stage, the stable operation stage).
In addition, the proportion of heat in these two parts is 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, most of the heat comes from the heater. In the homogenization section, the material has been melted, the temperature is higher, the screw groove is shallow, and the heat generated by friction shear is more, and sometimes not only does not need heater heating, but also needs cooler cooling. In the compression section, the heating of the material is a transitional state of the above two cases, that is, the heat generated by frictional shear is more than that of the feeding section, and less than 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 thermal equilibrium problem. Part of the heat supplied by the heater and the frictional heat generated by frictional shear is used to change the state of the plastic, and the other part is lost. Although there are many factors affecting this balance, it should always be maintained under conditions of stable extrusion.
Heating method of extruder
There are usually three heating methods for extruders: liquid heating, steam heating and electric heating. Among them, electric heating is used the most. Steam heating has rarely been used (hot feed rubber extruders steam heating is more).
The principle of liquid heating is to first heat the liquid (water, oil, biphenyl, etc.), and then they heat the barrel. At present, the most widely used extruder is electric heating, which is divided into resistance heating and inductive heating.
Cooling of the extruder
The extruder has a cooling system (together with the heating system) to ensure that the plastic is extruded at the temperature required by the process. In the extrusion process, the frictional shear heat generated by the screw rotation is often more than the heat required by the material, which will lead to the temperature of the material in the barrel being too high, if too much heat is not discharged in time, it will cause the material (especially heat-sensitive plastics) to decompose, and sometimes it will make molding difficult. For this purpose, the barrel and screw must be cooled.
1. Cooling of the barrel
The barrel of modern extruders is equipped with a cooling system. Barrel cooling methods include air cooling and water cooling.
Air cooling
Air cooling is relatively soft, uniform and clean, and is more used in extruders produced at home and abroad. However, the fan occupies a large space volume, and if the quality of the fan is not good, it is easy to make a sound. It is generally considered to be suitable for small and medium-sized extruders.
Water cooling
Compared with air cooling, water cooling has a fast cooling speed, small size and low cost, but it is easy to cause quenching, thereby disrupting the stable flow of plastics. If the seal is not good, there will be running, running, dripping, leakage. The cooling system wrapped around the barrel with water pipes is easy to generate scale surfaces to block the pipes and is also prone to corrosion. Therefore, the water used in the perfect water cooling system is not tap water, but chemically treated water. Softened water and deaerated treatment. It is generally believed that water cooling is better for large extruders.
2. Cooling of the screw
Cooling screw purpose
First, obtain maximum solids transfer rates
It is known from the solid conveying theory that the solid conveying rate is related to the difference between the friction coefficient of the material to the screw and the friction coefficient of the material to the barrel. That is, the greater the friction coefficient between the barrel and the material, the smaller the friction coefficient between the material and the screw, the more conducive to the transportation of solid materials. In addition to opening a longitudinal groove on the inner wall of the barrel feeding section to improve the surface finish of the screw, this can also be achieved by controlling the temperature of the barrel and screw.
This is because the frictional properties of solid plastics are greatly affected by temperature under different conditions. In some cases, for a certain plastic, its friction coefficient increases with the increase of temperature, and in other cases decreases, therefore, we can control the temperature of the barrel and screw in the solid conveying zone by controlling the temperature of the barrel and screw in the solid conveying zone, so that the difference between the friction coefficient of the barrel and the material and the friction coefficient of the screw and the material is the largest to obtain the maximum solid conveying rate.
Second, the screw is cooled to control the quality of the article
Experience has proved that if the cooling hole of the screw is driven to the homogenization section for cooling, the material plasticization is better, and the quality of the product can be improved. However, the amount of extrusion will be reduced, and the outlet temperature of the cooling water will be lower; The lower the extrusion volume. This is because the cooling homogenization section screw makes the material close to the screw surface become glued and difficult to flow. This is equivalent to reducing the depth of the screw groove in the homogenization section.
Materials with high viscosity should pay special attention to mastering that the temperature of the cooling water outlet should not be too low, otherwise there will be an accident of screw twisting. From the point of view of energy utilization, the cooling screw loses some heat.
Cooling medium
There are also those that use oil and air as cooling media. The advantages of oil and air are that it does not have a corrosive effect, the temperature control is more accurate, and it is not easy to block the pipeline. However, the large extruder has a better cooling effect with water.
3. Hopper seat cooling
The plastic temperature of the feeding section should not be too high, otherwise an “arch” will be formed at the feeding port, making the material difficult to add. For this, the dosing hopper seat must be cooled. In addition, the cooling hopper seat prevents heat from the extruded section from being transferred to the thrust bearing and gearbox, thus ensuring their normal working conditions. The cooling medium of the hopper seat is mostly water.
