How does the screw work in the extruder?

- Apr 13, 2020-

The screw is the heart of the extruder and the key component of the extruder. The performance of the screw determines the productivity, plasticization quality, dispersibility of the additives, melt temperature, power consumption, etc. of an extruder. . It is the most important part of the extruder, it can directly affect the application range and production efficiency of the extruder. The rotation of the screw produces an extreme pressure on the plastic, and the plastic can move, pressurize, and obtain some heat from the friction in the barrel. The plastic gets mixed and plasticized during the movement of the barrel. When the melt is extruded and flows through the die, the desired shape is obtained and shaped. Like the barrel, the screw is made of high-strength, heat-resistant and corrosion-resistant alloy.

Because there are many types of plastics, their properties are also different. Therefore, in actual operation, in order to adapt to different plastic processing needs, the required screw types are different, and the structure is also different. In order to maximize transport, extrusion, mixing and plasticizing of plastics. The picture shows several common screws.

The basic parameters that characterize the screw include the following: diameter, length-to-diameter ratio, compression ratio, screw pitch, screw groove depth, helix angle, clearance between screw and barrel, etc.

The most common screw diameter D is about 45 to 150 mm. As the diameter of the screw increases, the processing capacity of the extruder also increases accordingly. The productivity of the extruder is proportional to the square of the diameter D of the screw. The ratio of the effective length of the working part of the screw to the diameter (abbreviated as the length-to-diameter ratio, expressed as L / D) is usually 18 ~ 25. Large L / D can improve the temperature distribution of the material, is conducive to the mixing and plasticization of plastics, and can reduce leakage and counterflow. Improve the production capacity of the extruder. The screw with large L / D has strong adaptability and can be used for the extrusion of a variety of plastics. However, when the L / D is too large, it will cause the plastics department to degrade due to the increase in heating time. Increased self-weight and deflection of the free end easily cause scratches between the material and the screw, and make manufacturing and processing difficult; increase the power consumption of the extruder. Screws that are too short can easily cause poor plasticization of the compound.