Comparison of the advantages of parallel and conical twin screw extruders
Comparison of the advantages of parallel and conical twin screw extrudersHow to choose parallel twin-screw extruder and conical twin-screw extruder in production? It is a question that users often ask when purchasing twin-screw extruder.
Classification of twin screw extruder:
According to the rotation direction of the twin screw, the extruder can be divided into two kinds of extruders: co-directional and counter-directional. Co-rotating extruder means that two screws work in the same direction of rotation, and co-rotating extruder means that two screws work in opposite directions.
According to whether the axis line of the twin screw is parallel or not, it can be divided into two types of extruders in which the axis line is parallel and the axis line intersects. Parallel twin-screw extruders are parallel to the axis line, and conical twin-screw extruders are intersecting the axis lines.
Twin screw extruders are also divided into meshing and non-meshing.
The parallel and conical twin screw extruder are the same:
All have a conveying mechanism for plastics to force forward, good mixing plasticization ability and dehydration ability, and have basically the same adaptability to the molding process of materials and plastic products.
The difference between parallel and conical twin screw extruder:
1. Diameter: The diameter of the parallel twin screw is the same, the diameter of the small end of the conical twin screw is different from the diameter of the big end;
2. Concentric distance: the center distance of the flat twin screw is the same, the two axes of the conical twin screw are at an angle, and the size of the center distance changes along the axis;
3. L/D ratio: parallel double screw (L/D) refers to the ratio of the effective part length of the screw to the outer circle of the screw, and conical double screw (L/D) refers to the effective part length of the screw and the large end diameter and small The ratio of the average of the end diameters.
From the above points, we can clearly see that the most significant difference between parallel and conical twin screw extruders is the different geometry of the screw barrel, which leads to many differences in structure and performance. Although the two have different characteristics, But each has its own advantages.
Parallel twin screw extruders are limited by the small center distance between the two screws. In the transmission gear box, there is limited space for the radial bearings and thrust bearings supporting the two output shafts and the related transmission gears, although the designer has exhausted Brain, but can not solve the bearing capacity, gear module, small diameter, the diameter of the tail of the two screws is small, resulting in poor torque resistance. The output torque is small and the load resistance is poor, which is the most significant defect of the parallel twin screw extruder. However, the plasticity of the length-diameter ratio is the advantage of the parallel twin screw. It can increase and decrease the length-diameter ratio to meet the requirements of plastic processing technology according to the difference in molding conditions. Twin screw extruders are difficult to do.
The two conical screws of the conical twin screw extruder are arranged horizontally, and the two axes are inserted into the barrel at an angle. The center distance of the two axes gradually increases from the small end to the large end, so that the two output shafts of the transmission gear box have Larger center distance, the gears and gear shafts in these transmission systems, as well as the radial bearings and thrust bearings supporting these gear shafts, have a larger installation space, which can be equipped with larger specifications of radial bearings and thrust bearings, each The transmission shaft has a shaft diameter sufficient to transmit torque, so a large working torque and a large load-bearing capacity are a major feature of a conical twin-screw extruder. This parallel twin-screw extruder is unmatched. When the twin screw extruder is working, the melt will generate a very large pressure (head pressure) at the screw head. The pressure is usually around 14MPA, sometimes even as high as 30MPA. The pressure forms a strong axial thrust on the screw, and the thrust is the role of the anti-return bearing.
The parallel twin-screw extruder is restricted by the small center distance between the two screws. The bearing capacity of the anti-return bearing is related to its diameter. The large-diameter bearing capacity is large. Obviously, it is impossible to use a large-diameter non-return bearing. This contradiction is usually achieved by using several small-diameter anti-return bearings in series to work together to withstand a strong axial force. The core problem of using this method is that the load on each thrust bearing must be uniform and the same, otherwise, the large bearing Bearings are destroyed in advance due to overload, and the load they should bear is added to other bearings to make them overloaded. The consequences of this continuous destruction are very serious. It can be seen from this that the structure of the transmission system of the parallel twin-screw extruder is relatively complicated. Compared with the structure of the transmission system of the conical twin-screw extruder, the manufacturing cost of the gear box is high and the maintenance is complicated.
Conical twin-screw extruder because of the angle between the arrangement of the two screws, so the two output shafts of the transmission gear box have a larger center distance, and two large thrust spherical bearings staggered in the gear box are arranged in the gear box. It is enough to prevent the axial force formed by the pressure of the machine head. Its characteristic load carrying capacity is large, the manufacturing cost of the gear box is low, and the maintenance is more convenient.
For users, the purchase of twin-screw extruders is very important. Different types of twin-screw extruders have different performances and applications. Therefore, it is necessary to understand the performance and performance of various twin-screw extruders. Application occasions. For example, the intermeshing co-rotating twin-screw extruder is widely used in the modification of polymers that are not easily thermally decomposed ---- blending, filling, fiber due to its high speed and high shear rate. Reinforcement and reactive extrusion of materials. For example, the meshing type counter-rotating twin-screw extruder, because of its good mixing and plasticizing function, its biggest feature is the direct molding of PVC powder. If the geometric structure of the screw is changed, it can also be used for molding processing of other materials, but its strength is still PVC molding processing. According to the size of the plastic section, determine the amount of extrusion, and then select the specifications of the twin screw extruder from the amount of extrusion. Under the condition that the plastic processing and molding process conditions are basically the same, the conical twin-screw extruder can adapt to the larger head pressure, and the parallel twin-screw extruder can adapt to the smaller head pressure.
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