The screw of the extruder

The screw is the heart of the extruder, is the key component of the extruder, the performance of the screw is good or bad, determines the productivity of an extruder, the plasticizing quality, the dispersion of the additive, melt temperature, power consumption, etc. Extruder is the most important component, it can directly affect the application range and production efficiency of extruder. Through the rotation of the screw to produce extreme pressure on the plastic, the plastic can move, pressurize and obtain part of the heat from the friction in the barrel. The plastic can be mixed and plasticized in the process of moving in the barrel. When the viscous flow melt is extruded and flows through the die, the required shape is obtained and formed. Like the barrel, the screw is made of a high strength, heat and corrosion resistant alloy.
Because there are many types of plastics, their properties vary. Therefore, in actual operation, in order to adapt to different plastic processing needs, the types of screw required are different, and the structure is also different. In order to maximize the efficiency of plastic transport, extrusion, mixing and plasticizing.
The basic parameters indicating the characteristics of the screw include the following points: diameter, length-diameter ratio, compression ratio, pitch, groove depth, screw Angle, clearance between the screw and the barrel, etc.
The most common screw diameter D is about 45~150 mm. With the increase of screw diameter, the processing capacity of extruder also increases correspondingly, and the productivity of extruder is proportional to the square of screw diameter D. The ratio of effective length to diameter of the working part of the screw (L/D for short) is usually 18~25. Large L/D, can improve material temperature distribution, is conducive to plastic mixing and plasticizing, and can reduce leakage and countercurrent. Improve the production capacity of extruder, L/D large screw adaptability is strong, can be used for a variety of plastics extrusion; But L/D is too large, will make the plastic department heating time growth and degradation, at the same time because of the screw weight increase, free end flexion sag, easy to cause between the material and the screw abrasion, and make manufacturing and processing difficult; The power consumption of extruder is increased. Too short screw, easy to cause bad plasticization mixing.
Half of the difference between the barrel diameter and the screw diameter is called gap δ, which can affect the extruder's production capacity. With the increase of δ, the productivity decreases. It is usually advisable to control δ around 0.1 to 0.6 mm. If δ is too small, the material is subjected to large shear action, which is conducive to plasticization. However, if δ is too small, the strong shear action is easy to cause thermal mechanical degradation of the material, and the screw is easy to be held or friction with the cylinder wall. Moreover, if δ is too small, the material leakage and countercurrent are almost no, which affects the mixing of melt to a certain extent.
Helix Angle Φ is the Angle of thread and screw cross section, with the increase of Φ, extruder production capacity, but the shear action and extrusion pressure of plastic, usually helix Angle between 10° to 30°, along the direction of screw length and change, often using equidistance screw, take pitch is equal to the diameter, the value of Φ is about 17°41 '
The larger the compression ratio, the greater the compression ratio the plastic receives. When the spiral groove is shallow, it can produce a higher shear rate for plastics, which is conducive to the heat transfer between the cylinder wall and the material. The higher the material mixing and plasticizing efficiency, the lower the productivity. Conversely, the spiral groove is deep. The opposite is true. Therefore, heat sensitive materials (such as polyvinyl chloride) should use deep groove screw; And for plastics with low melt viscosity and high thermal stability (such as polyamide), shallow groove screw should be used.