The screw is the heart of the plastic extrusion machinery and is a key component of the extruder. The performance of the screw determines the productivity, plasticizing quality, dispersion of the filler, melt temperature, power consumption, etc. of the extruder. Plastic extrusion line is the most important part of the extruder, which can directly affect the application range and production efficiency of the extruder. The rotation of the screw exerts an extreme pressure on the plastic. The plastic can move, pressurize and obtain some heat from the friction in the cylinder. The plastic is mixed and plasticized during the movement of the cylinder, and the viscous flow state The melt is shaped while being extruded to flow through the die to obtain the desired shape. Like the barrel, the screw is also made of a high strength, heat and corrosion resistant alloy.
Due to the variety of plastics, their properties are also different. Therefore, in actual operation, in order to adapt to different plastic processing needs, the types of screws required are different, and the structures are also different. In order to maximize the efficiency of the plastics to maximize transportation, extrusion, mixing and plasticization. The picture shows several more common screws.
The basic parameters indicating the characteristics of the screw include the following: diameter, aspect ratio, compression ratio, pitch, groove depth, helix angle, screw and barrel clearance, and the like.
The most common screw diameter D is approximately 45 to 150 mm. As the screw diameter increases, the processing capacity of the extruder increases accordingly, and the productivity of the extruder is proportional to the square of the screw diameter D. The ratio of the effective length to the diameter of the working part of the screw (referred to as the aspect ratio, expressed as L/D) is usually 18~25. Large L/D can improve the material temperature distribution, facilitate the mixing and plasticization of plastics, and reduce leakage and backflow. Improve the production capacity of the extruder, the L/D large screw has strong adaptability and can be used for extrusion of various plastics; however, when the L/D is too large, the plastics will be degraded by the heating time and the screw When the self-weight increases, the free end flexes and sag, which easily causes the material to be scratched between the screw and the screw, and makes the manufacturing process difficult; the power consumption of the extruder is increased. A screw that is too short may cause poor plasticization of the kneading.
The 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 productivity of the extruder. As the δ increases, the productivity decreases. Generally, the control δ is preferably about 0.1 to 0.6 mm. δ is small, the material is subjected to a large shearing effect, which is beneficial to plasticization, but δ is too small, strong shearing action easily causes thermal mechanical degradation of the material, and at the same time, the screw is easily hugged or rubbed against the wall of the cylinder, and When δ is too small, there is almost no leakage or backflow of the material, which affects the mixing of the melt to some extent.
The helix angle Φ is the angle between the thread and the cross section of the screw. As the Φ increases, the productivity of the extruder increases, but the shearing effect and the pressing force on the plastic are reduced. Usually, the helix angle is between 10° and 30 degrees. Between °, along the direction of the change of the length of the screw, often using an equidistant screw, taking the pitch equal to the diameter, the value of Φ is about 17 ° 41 '
The greater the compression ratio, the greater the compression ratio that the plastic receives. When the groove is shallow, plastic extrusion machinery can produce a high shear rate for the plastic, which is beneficial to the heat transfer between the wall of the barrel and the material. The higher the material mixing and plasticizing efficiency, the lower the productivity; on the contrary, the groove is deep. The situation is just the opposite. Therefore, heat-sensitive materials (such as polyvinyl chloride) should be used with deep screw screws; for plastics with low melt viscosity and high thermal stability (such as polyamide), shallow screw screws should be used.
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