Working principle of twin-screw extruder

1. Material conveying:
Meshing twin-screw extruder (counter-rotation): The two screws mesh with each other. At the meshing point, the thread of one screw is inserted into the screw groove of the other screw, so that the continuous screw groove is divided into isolated C-shaped chambers. When the screw rotates, the C-shaped chamber also moves along the axial direction as the meshing part moves axially. Since the two screws rotate in different directions, the material will be squeezed and sheared in the meshing area, so that the material is subjected to a strong thrust, thereby achieving positive conveying. This conveying method has a strong compulsion and can ensure that the material is steadily pushed forward, and it is not easy to produce backflow or stagnation.
Meshing twin-screw extruder (co-rotation): The two screws rotate in the same direction, and the material is conveyed forward in the gap between the two screws. In this case, the design of the screw usually has a special structure, such as a closely matched screw rib shape, which can make the material better sheared and mixed during the conveying process. The co-rotating twin-screw extruder has a strong mixing and plasticizing ability when running at high speed. It is often used to highly mix materials or perform some special processing operations.
Non-meshing twin-screw extruder: The center distance between the two screws is greater than the sum of the two screw radii. Its conveying mechanism is quite different from that of the meshing extruder and is closer to that of the single-screw extruder. Under the push of the screw, the material moves forward mainly by friction and its own gravity, but the efficiency and stability of this conveying method are relatively low.
2. Material plasticization:
When the material enters the barrel of the twin-screw extruder, it is gradually transformed from a solid state to a molten state by the rotation of the screw and the heating of the barrel. The rotation of the screw generates shear force and friction on the material, which weakens the force between the molecules of the material and gradually unfolds the molecular chain, thereby achieving plasticization.
The interaction between the two screws of the twin-screw extruder can make the material more evenly sheared and mixed during the plasticization process, improving the plasticization effect and quality. Compared with single-screw extruders, twin-screw extruders have shorter plasticizing time and better plasticizing effect, and can better adapt to various materials of different properties.
3. Material mixing:
During the rotation process, the two screws of the twin-screw extruder will continuously divide, stir and mix the materials. Driven by the screw, the material is transferred from the screw groove of one screw to the screw groove of another screw, or flows back and forth in the gap between the two screws, thereby achieving full mixing of the material.
The structure of the screw thread, screw ridge and other structures, as well as the design of the screw rotation speed, steering and other parameters can affect the mixing effect of the material. By reasonably selecting and adjusting these parameters, different degrees of mixing of materials can be achieved to meet the processing requirements of various products.
4. Extrusion molding:
After plasticization and mixing, the material is extruded through the die die under the drive of the screw to form a product of the required shape and size. The design and structure of the die die will be selected and adjusted according to different product requirements to ensure that the extruded products have good quality and precision.
During the extrusion process, the twin-screw extruder can maintain stable extrusion pressure and extrusion speed, making the size and shape of the product more uniform, while also reducing product defects and defective rates.