How do parallel twin screw extruder and conical twin screw extruder work?
According to whether the axis of the twin-screw is parallel or not, it can be divided into two types of extruders with parallel axis and intersecting axis. A parallel twin-screw extruder with parallel axis lines is a conical twin-screw extruder with its axis lines intersecting.
The parallel twin-screw extruder is limited by the small distance between the centers of the two screws. In the transmission gearbox, the space given to the radial and thrust bearings supporting the two output shafts and the related transmission gears is very limited.
Although the designers tried their best, they could not solve the reality of the bearing capacity of the bearing, the modulus of the gear, the small diameter, and the small diameter of the tail of the two screws, which led to the result of poor torsion resistance. Small output torque and poor load resistance are the most significant defects of parallel twin-screw extruders.
However, the plasticity of the aspect ratio is the advantage of the parallel twin-screw. It can increase and decrease the aspect ratio according to the difference of the molding conditions to meet the requirements of the plastic processing technology, and can expand the application scope of the parallel twin-screw. But conical twin-screw extruder is difficult to achieve this.
The two conical screws of the conical twin-screw extruder are arranged horizontally, and the two axes are installed into the barrel at an included angle, so that the two output shafts of the transmission gearbox have a larger center distance. The gears and gear shafts in these transmission systems and the radial and thrust bearings supporting these gear shafts leave a large installation space.
It can install radial bearings and thrust bearings of larger specifications, and each transmission shaft has a shaft diameter sufficient to meet the transmission torque. Therefore, large working torque and large load-bearing capacity are a major feature of conical twin-screw extruders.
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, and sometimes even as high as 30MPA. This pressure forms a strong axial thrust on the screw, and resisting the thrust is the function of the anti-return bearing.
Due to the limitation of the small center distance of the two screws, the bearing capacity of the anti-return bearing is related to the diameter of the parallel twin-screw extruder. The large diameter has a large bearing capacity. Obviously, it is impossible to use a large-diameter anti-return bearing. This contradiction is usually caused by the use of several small-diameter anti-return bearings in series to jointly bear 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 will be damaged in advance due to overloading, and the load it should bear will be added to other bearings to make it overloaded. The consequences of this continuous damage are very serious. It can be seen from this that the transmission system structure of the parallel twin-screw extruder is relatively complex. Compared with the transmission system structure of the conical twin-screw extruder, the manufacturing cost of the gearbox is high and the maintenance is more complicated.
The conical twin-screw extruder has an included angle due to the arrangement of the two screws, so the two output shafts of the transmission gearbox have a large center distance. Two large thrust spherical ball bearings arranged in a staggered front and rear are installed in the gear box, which is enough to prevent the axial force formed by the pressure of the machine head. It is characterized by large bearing capacity, low manufacturing cost of the gear box, and convenient maintenance.
When should a parallel/conical twin screw extruder be chosen?
For users, the selection of twin-screw extruders is very important. Different types of twin-screw extruders have different properties and applications. Therefore, it is necessary to clarify the performance and application of various twin-screw extruders.
For example, the co-rotating twin-screw extruder is widely used in the modification of non-thermally decomposable polymers due to its high speed, high shear rate and combined screw – blending, filling, fiber reinforcement and reactive extrusion of materials.
The counter-rotating twin-screw extruder has a good mixing and plasticizing function, and its biggest feature is the direct molding and processing of PVC powder.
If the geometry of the screw is changed, it can also be used for the molding of other materials, but its strength is still PVC molding. According to the size of the plastic section, the extrusion volume is determined, and then the specifications of the twin-screw extruder are selected by the extrusion volume.
Under the condition that the plastic processing and molding process conditions are basically the same, the conical twin-screw extruder can adapt to a larger head pressure, and the parallel twin-screw extruder can adapt to a smaller head pressure.
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