Basic principle of screw combination
For a twin-screw extruder, the screw is mainly divided into feeding section, melting section, mixing section, exhaust section and homogenization section. Screw elements mainly include conveying, melting, shearing, material mixing, residence time control and other functions. The screw elements of 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 twin-screw extrusion process.
The intermeshing co rotating twin screw extruder is mainly used for mixing. The screw combination should consider the performance and shape of main and auxiliary materials, feeding sequence and position, exhaust port position, barrel temperature setting, etc. At the same time, the mixing objects are very complex, and a reasonable screw combination is required for each specific mixing process. However, the screw combination of 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 blanking.
2. In the melting section, small lead screw threads shall be used to establish pressure, so as to compress and melt the materials. Kneading blocks with staggered angle of 90 ° can be set to balance the pressure, and kneading blocks with staggered angle of 30 ° can also be used to preliminarily distribute and mix the materials. Kneading blocks shall be set from the middle of the melting section. Note that kneading blocks shall be arranged at intervals.
3. In the mixing section, the main purpose is to shear, refine and disperse material particles. The setting of screw elements in this section is very complex, and designers need to have rich practical experience. In this section, kneading blocks with Staggered Angles of 45 ° and 60 ° are mainly used to strengthen the shear, and special elements such as tooth shaped elements or “s” shaped elements are 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 the materials in this section, screw conveying elements shall also be configured at intervals, that is, kneading blocks and screw conveying elements shall be staggered from each other.
4. Reverse thread elements or reverse kneading blocks shall be set before the exhaust port or vacuum port, large lead thread elements shall be set at the exhaust port or vacuum port, and small lead thread elements shall be set after the exhaust port or vacuum port.
5. In the homogenization section, the thread lead shall be gradually reduced to achieve pressurization and reduce the length of the back pressure section. At the same time, attention shall be paid to the use of single head thread and wide screw edge thread to improve the discharge capacity and avoid material overflow.
