For a twin screw extruder, the screw is mainly divided into feeding section, melting section, mixing section, exhaust section and homogenizing section. Screw elements mainly include conveying, melting, shearing, material mixing, residence time control and other functions. The screw elements of the twin screw extruder are combined in the way of “building blocks”, which can be adjusted according to different production needs in practice. Therefore, screw combination is the key to the customization of the twin screw extrusion process.
The intermeshing co-rotating twin screw extruder is mainly based on mixing. The screw combination shall take into account the performance and shape of main and auxiliary materials, the feeding sequence and position, the position of exhaust port, the temperature setting of the barrel, etc. At the same time, the objects of mixing are very complex and a reasonable screw combination is required for each specific mixing process. Nevertheless, the screw combination of the intermeshing co-rotating twin screw extruder still has its basic rules to follow.
The following are the basic principles of screw combination.
1. Large lead thread shall be used at the feeding port to ensure smooth discharging.
2. The small lead screw shall be used in the melting section to establish the pressure, so as to compress and melt the material. The kneading block with a staggered angle of 90 °can be set to balance the pressure and the kneading block with a staggered angle of 30 °can also be used to initially distribute and mix the material. The kneading block shall be set from the middle of the melting section, and the kneading block shall be arranged at intervals.
3. In the mixing section, the main purpose is to shear, refine and disperse the material particles. The setting of threaded elements in this section is very complex and requires designers to have rich practical experience. In this section, kneading blocks with staggered angles of 45 °and 60 °are mainly used to strengthen shearing, and special elements such as toothed elements or “S” shaped elements are used as auxiliary.
However, it should be noted that the kneading and shearing elements should not be set too much or arranged too closely to avoid too strong shearing. In addition, in order to enhance the conveying capacity of this section of materials, screw conveying elements should also be configured at intervals, that is, kneading blocks and screw conveying elements should be staggered.
4. A reverse threaded element or reverse kneading block shall be set in front of the exhaust port or vacuum port, a large lead threaded element shall be set at the exhaust port or vacuum port, and a small lead threaded element shall be set after the exhaust port or vacuum port.
5. In the homogenization section, the thread lead should be gradually reduced to achieve pressurization and reduce the length of the back pressure section. At the same time, attention should be paid to the use of single head thread and wide screw thread to improve the discharge capacity and avoid material overflow.
