Since the twin screw extruder was developed in Italy in the late 1930s, it has been greatly improved after more than half a century of continuous improvement and improvement. In foreign countries, twin-screw extruders have been widely used in the field of polymer processing, accounting for 40% of the total number of extruders. Rigid PVC pellets, pipes, profiles, and plates are almost all processed using a twin-screw extruder. As a continuous mixer, twin-screw extruders have been widely used for polymer blending, filling, and reinforcement modification, as well as reactive extrusion. In the past 20 years, the development of polymer blending and reactive extrusion technology has further promoted the increase in the number and types of twin-screw extruders.
Structure and classification of twin screw extruder
The twin-screw extruder is composed of several parts, such as the transmission device, the feeding device, the barrel and the screw. The functions of each component are similar to the single-screw extruder. The difference from the single screw extruder is that in the twin screw extruder, two parallel screws are placed in the same barrel, as shown in Figure 6 continued on the next page).
There are many different forms of twin-screw extruders, and the main difference lies in the screw structure. The screw structure of the twin-screw extruder is much more complicated than that of the single-screw extruder, because the screw of the twin-screw extruder also has problems such as the direction of rotation, the degree of meshing and so on.
Twin-screw extruders commonly used for PVC profile extrusion are usually close-meshing and counter-rotating screws. A few also use co-rotating twin-screw extrusion, but generally can only be operated at low speeds, about 10 r / min. Within range. The high-speed meshing co-rotating twin-screw extruder is used for mixing, exhaust granulation, or as a continuous chemical reactor. This type of extruder has a large screw speed ranging from 300 to 600 r / min. The conveying mechanism of non-meshing extruder and intermeshing extruder is very different, which is closer to the conveying mechanism of single screw extruder. There is a fundamental difference between the two.
The twin-screw extruder process can be divided into three stages: feeding and solid transportation, melting and melt transportation. Regarding the working principle of the twin-screw extruder, it should be mentioned that the twin-screw extruder is transported in a forward direction, forcing the material to be pushed forward. On the other hand, the twin-screw extruder has a strong shearing effect on the material at the meshing point of the two screws, increasing the mixing and plasticizing effect of the material.
When the screw rotates in the same direction, the screw teeth of one screw extend into the screw of the other screw like a wedge. From the groove of one screw to the groove of another screw. In this way, the material is repeatedly forced to turn between the two screws, and is subject to good shear mixing. If the screw is rotating in the reverse direction, the material must pass through the nip, and the material seems to pass through the nip of the two rollers, so the shearing effect will be better.