Characteristics of co-rotating twin-screw extruders and basic principles of screw combination

There are many types of twin-screw extruders, among which the intermeshing co-rotating twin-screw extruder is a production and processing equipment widely used in the plastics industry. This type of extruder is composed of two intermeshing "building block" screws, a barrel, a power unit, a temperature control device, etc. The body can have multiple feeding ports and vacuum/non-vacuum devolatile ports.

The meshing co-rotating twin-screw extruder mainly has the following characteristics.
(1) The two screws rotate in parallel and in the same direction, producing a uniform shearing effect between the contact parts and the barrel, and the strength of this shearing effect 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 capabilities, making it suitable for mixing operations. After the material enters the barrel and is softened, since the twin screws have opposite directions at the meshing point, one screw will pull the material into the meshing gap, and the other screw will push it out of the gap, so the material will be pushed out by one screw here. It is transferred to another screw and moves in an "∞" motion. This motion has a large relative speed at the meshing point, which is very conducive to the mixing and homogenization of the material. Moreover, the gap in the meshing area is very small, and the threads and grooves at the kneading point are On the contrary, the speed has a high shearing effect, thereby achieving uniform plasticization.

(3) The screw and barrel are both combined. There are many types of threaded elements, including conveying elements, kneading elements, shearing elements, reverse threaded elements and boosting threaded elements, etc., each of which plays a different role. According to the needs of material processing, various elements are combined by building blocks. Together, and through optimized design, it can be adapted to the processing of various process formula materials.

(4) The co-rotating twin-screw extruder has reaction capabilities and is a dynamic reactor. After the material is melted in the barrel, a series of chemical reactions can occur, such as polymerization, grafting, etc. Reactive extrusion processing is mainly used for: polymerization of monomers or oligomers (free radical polymerization, addition polymerization, condensation polymerization and copolymerization); controlled cross-linking and degradation of polyolefins; graft modification of polymers ( Functionalizing or polarizing polymers to achieve the purpose of 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 conveying, melting, shearing, material mixing, residence time control and other functions. The threaded elements of the twin-screw extruder are combined in a "building block" manner. In practice, they can be adjusted according to different production needs. Therefore, screw combination is the key to customizing the twin-screw extrusion process.

The meshing co-rotating twin-screw extruder is mainly used for mixing. The screw combination should take into account the performance and shape of the main and auxiliary materials, the order and position of the feeding, the position of the exhaust port, the 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. Despite this, the screw combination of meshing co-rotating twin-screw extruders still has its basic rules to follow.

The following are several basic principles of screw combination.

(1) A large lead thread should be used at the feeding port to ensure smooth discharging.

(2) Small-lead threads should be used in the melting section to build up pressure to compress and melt the material. Kneading blocks with a staggered angle of 90° can be set up to balance the pressure, or kneading blocks with a staggered angle of 30° can be used. The kneading block performs preliminary distribution and mixing of the materials. The kneading block should be installed from the middle of the melting section. Note that the kneading block should be arranged at intervals.

(3) The main purpose of the mixing section is to shear, refine and disperse 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 enhance shearing, supplemented by special elements such as toothed elements or "S"-shaped elements. However, be careful not to set too many kneading and shearing elements, nor to arrange them too closely to avoid excessive shearing. In addition, in order to enhance the material conveying capacity of this section, threaded conveying elements should be arranged at intervals, that is, the kneading block and the threaded conveying elements are staggered from each other.

(4) A reverse threaded component or a reverse kneading block should be installed in front of the exhaust port or vacuum port, a large-lead threaded component should be installed at the exhaust port or vacuum port, and a small-lead threaded component should be installed after the exhaust port or vacuum port. Process threaded components.

(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-start threads and wide-rib threads to improve the discharge capacity and avoid material leakage.