There are many types of twin-screw extruders, among which the meshing co rotating twin-screw extruder is a widely used production and processing equipment in the plastic industry. This type of extruder is composed of two interlocking "building block" screws, a barrel, a power device, a temperature control device, etc. It can have multiple feeding ports and vacuum/non vacuum volatilization ports on the body.
The main characteristics of a co rotating twin-screw extruder are as follows.
(1) Two screws rotate in parallel and in the same direction, producing uniform shear between the contact area and the barrel, and the strength of this shear can be adjusted through screw combination, spacing design, and other means.
(2) The geometric shape and co rotation of the screw block enable the screw to have good material distribution and mixing ability, making it suitable for mixing operations. After the material is softened in the material barrel, due to the opposite direction of the twin-screw at the meshing point, one screw needs to pull the material into the meshing gap, while the other screw pushes it out of the gap. Therefore, the material is transferred from one screw to the other screw here in an "∞" motion. This motion has a high relative velocity at the meshing point, which is very conducive to the mixing and homogenization of the material. The gap in the meshing area is small, and the speed of the threads and grooves at the kneading point is opposite, which has a high shear effect and achieves uniform plasticization.
(3) The screw and barrel are both combined. There are many types of threaded components, including conveying components, kneading components, shearing components, reverse threaded components, and pressure boosting threaded components, each playing a different role. According to the needs of material processing, various components are combined together by building blocks, and through optimized design, they can adapt to the processing of various process formula materials.
(4) The co rotating twin-screw extruder has reaction capability and is a dynamic reactor. After the material melts in the barrel, a series of chemical reactions such as polymerization and grafting can occur. Reactive extrusion processing is mainly used for polymerization reactions of monomers or oligomers (free radical polymerization, addition polymerization, condensation polymerization, and copolymerization reactions); Controllable crosslinking and degradation of polyolefins; Grafting modification of polymers (functionalization or polar functionalization of polymers to achieve material modification and preparation of compatibilizers); Forced blending modification of multiple materials. It also includes physical modification of materials, such as filling, mixing, toughening, and reinforcement.
Basic principles of screw combination
For a twin-screw extruder, the screw is mainly divided into a feeding section, a melting section, a mixing section, an exhaust section, and a homogenization section. Threaded components mainly include functions such as conveying, melting, shearing, material mixing, and residence time control. The threaded components of twin-screw extruders are assembled in a "building block" manner, and can be adjusted according to different production needs in practice. Therefore, screw combination is the key to customizing twin-screw extrusion processes.
The co rotating twin-screw extruder is mainly used for mixing, and the screw combination should consider the performance and shape of the main and auxiliary materials, the feeding sequence and position, the position of the exhaust port, the temperature setting of the barrel, and so on. At the same time, the objects of mixing are very diverse, and reasonable screw combinations are required for each specific mixing process. However, the screw combination of the co rotating twin-screw extruder still has its basic rules to follow.
The following are several basic principles of screw combination.
(1) Large lead threads should be used at the feeding port to ensure smooth feeding.
(2) Small lead threads should be used in the melting section to establish pressure, thereby compressing and melting the material. Kneading blocks with a misalignment angle of 90 ° can be set to balance the pressure, or kneading blocks with a misalignment angle of 30 ° can be used for preliminary distribution and mixing of the material. Kneading blocks should be set from the middle of the melting section, and attention should be paid to the spacing arrangement of kneading blocks.
(3) In the mixing section, the main purpose is to shear, refine, and disperse material particles. The setting of threaded components 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 enhance shear, supplemented by special elements such as tooth shaped elements or "S" - shaped elements. However, it should be noted that there should not be too many kneading and shearing elements, nor should they be arranged too tightly to avoid excessive shearing. In addition, to enhance the conveying capacity of this section of material, threaded conveying elements should be installed at intervals, that is, the kneading block and the threaded conveying elements should be staggered from each other.
(4) Reverse threaded components or reverse kneading blocks should be installed before the exhaust or vacuum port, large lead threaded components should be installed at the exhaust or vacuum port, and small lead threaded components should be installed after the exhaust 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 using single head threads and wide ribbed threads to improve the discharge capacity and avoid material spillage.