extruder screw

The screw is the heart of the extruder and a key component of the extruder. The performance of the screw determines the productivity, plasticizing quality, dispersion of fillers, melt temperature, power consumption, etc. of an extruder. . It is the most important component of the extruder, which can directly affect the application range and production efficiency of the extruder. The rotation of the screw produces extreme pressure on the plastic, so that the plastic can move, pressurize, and obtain part of the heat from friction in the barrel. The plastic is mixed and plasticized during the movement of the barrel, and the melt is in a viscous flow state. When the body is extruded and flows through the die, it obtains the desired shape and is formed. Like the barrel, the screw is also made of high-strength, heat-resistant and corrosion-resistant alloys.
Since there are many types of plastics, their properties also vary. Therefore, in actual operation, in order to adapt to different plastic processing needs, different types of screws are required, and the structures are also different. In order to maximize the transportation, extrusion, mixing and plasticizing of plastics with maximum efficiency.
The basic parameters representing screw characteristics include the following points: diameter, aspect ratio, compression ratio, pitch, groove depth, helix angle, screw and barrel clearance, etc.
The most common screw diameter D is approximately 45 to 150 mm. As the screw diameter increases, the processing capacity of the extruder also increases accordingly. The productivity of the extruder is proportional to the square of the screw diameter D. The ratio of the effective length and diameter of the working part of the screw (referred to as the length-to-diameter ratio, expressed as L/D) is usually 18~25. Large L/D can improve the temperature distribution of materials, facilitate mixing and plasticizing of plastics, and reduce leakage and backflow. Improve the production capacity of the extruder. A screw with a 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, the plastic will be heated for a long time and degrade. At the same time, due to the screw The dead weight increases and the free end flexes and sag, which easily causes scratches between the material web and the screw and makes manufacturing and processing difficult; it also increases the power consumption of the extruder. A screw that is too short can easily cause poor plasticization during mixing.
Half of the difference between the inner diameter of the barrel and the diameter of the screw is called the gap δ, which can affect the production capacity of the extruder. As δ increases, the productivity decreases. It is usually appropriate to control δ between 0.1 and 0.6 mm. If δ is small, the material will be subject to greater shearing, which is conducive to plasticization. However, if δ is too small, strong shearing will easily cause thermomechanical degradation of the material. At the same time, it is easy for the screw to be caught or rub against the barrel wall, and , if δ is too small, there will be almost no material leakage and backflow, which will affect the mixing of the melt to a certain extent.
The helix angle Φ is the angle between the thread and the cross section of the screw. As Φ increases, the production capacity of the extruder increases, but the shearing effect and extrusion force on the plastic decreases. Usually the helix angle is between 10° and 30°. °, changes along the direction of change of screw length. Equidistant screws are often used, and the pitch is equal to the diameter. The value of Φ is about 17°41′
The greater the compression ratio, the greater the extrusion ratio the plastic receives. When the screw groove is shallow, it can produce a higher shear rate for the plastic, which is beneficial to the heat transfer between the barrel wall and the material. The higher the mixing and plasticizing efficiency of the material, the productivity will decrease; conversely, when the screw groove is deep. The opposite is true. Therefore, heat-sensitive materials (such as polyvinyl chloride) should use deep groove screws; while plastics with low melt viscosity and high thermal stability (such as polyamide) should use shallow groove screws.