Difference between cone twin and flat twin in twin screw extruder
Parallel twin-screw extruder or conical twin-screw extruder? It is a problem that users often ask when purchasing twin-screw extruder.
1、 Classification of twin screw extruders
According to the rotation direction of the twin-screw, the extruder can be divided into two types: the same direction extruder means that the rotation direction of the two screws is the same when working, and the opposite direction extruder means that the rotation direction of the two screws is opposite when working.
According to whether the axis line of twin-screw is parallel or not, it can be divided into two kinds of extruders with parallel axis line and intersecting axis line. The twin-screw extruder is parallel to the axis line, and the conical twin-screw extruder is intersected by the axis line.
Twin screw extruder can also be divided into meshing and non meshing.
The similarities between parallel and conical twin-screw extruder: it has the conveying mechanism of forced forward plastic, good mixing plasticization ability and dehydration ability, and basically the same adaptability to materials and plastic product forming process
2、 Differences between parallel and conical twin screw extruders
1. diameter: the diameter of parallel twin-screw is the same, and the diameter of small end of conical twin-screw is different from that of large end.
2. Center distance: the center distance of flat twin-screw is the same, the two axes of conical twin-screw are at an included angle, and the size of center distance changes along the axis.
3. Length diameter ratio: parallel twin screw (L / D) refers to the ratio of the effective part length of the screw to the outer circle of the screw. Conical twin screw (L / D) refers to the ratio of the effective part length of the screw to the small end diameter. Conical twin screw usually only indicates the diameter value (D / D) of the large end / small end of the screw.
From the above, we can clearly see that the most significant difference between parallel and conical twin-screw extruders is the different geometry of screw barrel, which leads to many differences in structure and performance. Although they have different characteristics, they have their own advantages.
3、 Parallel twin screw extruder
Due to the limitation of the small center distance between the two screws, in the transmission gearbox, the space for the radial bearing and thrust bearing supporting the two output shafts and the related transmission gears is very limited. Although the designer spends his brains, he can not solve the reality of the bearing capacity, the module and diameter of the gear and the tail diameter of the two screws, resulting in poor torque resistance. Small output torque and poor load resistance are the most significant defects of parallel twin-screw extruder. However, the plasticity of aspect ratio is the advantage of parallel twin-screw. It can increase and reduce the aspect ratio according to the difference of molding conditions to meet the requirements of plastic processing technology, and expand the application scope of parallel twin-screw, but it is difficult for conical twin-screw extruder to do this.
4、 Conical twin screw extruder
Two conical screws are arranged horizontally, and the two axes are installed into the barrel at an included angle. The center distance of the two axes gradually increases from the small end to the large end, so that there is a large center distance between the two output shafts of the transmission gearbox. There is a large installation space for the gears and gear shafts in these transmission systems and the radial bearings and thrust bearings supporting these gear shafts. It can be equipped with larger radial bearings and thrust bearings, 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 extruder. This parallel twin-screw extruder is incomparable.
5、 Non return bearing of twin screw extruder
When the twin-screw extruder works, the melt will produce a very large pressure (head pressure) at the head of the screw. The pressure is usually about 14MPa, sometimes even more than 30MPa. This pressure forms a strong axial thrust on the screw. The thrust is the function of the stop bearing.
1. Parallel twin-screw extruder is limited by the small center distance between the two screws. The bearing capacity of the stop bearing is related to its diameter. The diameter is large and the bearing capacity is large. Obviously, it is impossible to use the large-diameter stop bearing. In this contradictory situation, several small-diameter stop bearings are usually used in series to bear strong axial force. The core problem of using this method is that the load borne by each thrust bearing must be uniform and the same. Otherwise, the large bearing will be damaged in advance due to overload, and the load it should bear will be added to other bearings to overload it, The consequences of the destruction of this continuity are very serious.It can be seen that the transmission system structure of parallel twin-screw extruder is complex. Compared with the transmission system structure of conical twin-screw extruder, the manufacturing cost of gearbox is high and the maintenance is complex.
2. The conical twin-screw extruder has a large center distance between the two output shafts of the transmission gearbox due to the included angle between the two screws. Two large thrust self-aligning ball bearings staggered in front and back are installed in the gearbox, which is enough to prevent the axial force formed by the head pressure. It is characterized by large bearing capacity, low manufacturing cost of the gearbox and convenient maintenance.
6、 Selection of twin screw extruder
For users, the purchase of twin-screw extruder is very important. Different types of twin-screw extruders have different performance and application occasions. Therefore, it is necessary to understand the performance and application occasions of various twin-screw extruders. For example, the meshing co rotating twin-screw extruder is widely used for the modification of polymers that are not easy to thermal decomposition - blending, filling, fiber reinforcement and reactive extrusion of materials because of its high speed, high shear rate and combined screw. For example, the meshing counter rotating twin-screw extruder is characterized by direct molding of PVC powder because of its good mixing and plasticizing function. If the geometric structure of the screw is changed, it can also be used for the molding processing of other materials, but its strength is still PVC molding processing. According to the size of the plastic section, the extrusion amount is determined, and then the specification of the twin-screw extruder is selected by the extrusion amount. 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.