Is parallel twin-screw extruder better or conical twin- […]
Is parallel twin-screw extruder better or conical twin-screw extruder better? It is a question that users often raise when they choose twin-screw extruder.
Classification of twin screw extruders
According to the rotation direction of twin-screw, the extruder can be divided into two kinds of extruders: the same direction and the opposite direction. The same direction extruder has the same rotation direction when two screws are working, and the opposite direction extruder has the opposite rotation direction when two screws are working.
According to whether the axis of twin screw is parallel or not, it can be divided into two kinds of extruders with parallel axis and intersecting axis. The twin-screw extruder with parallel axis and the conical twin-screw extruder with intersecting axis.
The twin-screw extruder can also be divided into meshing and non meshing.
Conical twin screw extruder
Two conical screws are arranged horizontally, and the two axes are installed in the barrel at an included angle. The center distance of the two axes gradually increases from the small end to the large end, which makes the two output shafts of the transmission gearbox have a larger center distance. The gears and gear shafts in these transmission systems, as well as the radial and thrust bearings supporting these gear shafts, have a larger installation space, which can be installed with a larger diameter To the bearing and thrust bearing, each transmission shaft has enough shaft diameter to transmit torque, so large working torque and large load-bearing capacity is a major feature of conical twin-screw extruder. This parallel twin-screw extruder is incomparable.
Stop bearing of twin screw extruder
When the twin-screw extruder is working, 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. To counter the thrust is the function of the stop bearing.
1. Due to the limitation of the small center distance between the two screws, the bearing capacity of the stop bearing is related to its diameter. The large diameter bearing capacity is large, so it is obviously impossible to use the large diameter stop bearing. In this contradictory situation, several small-diameter backstop bearings are usually used in series to bear strong axial force. The core problem of using this method is that each thrust bearing must bear the same load uniformly, otherwise, the bearing large bearing will be damaged in advance due to overload, and the load it should bear will be added to other bearings to overload them, which will destroy the continuity The consequences are very serious. It can be seen 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 complex.
2. Because of the angle between the two screws in the conical twin-screw extruder, there is a large center distance between the two output shafts of the transmission gearbox. Two large thrust aligning ball bearings arranged in staggered front and back in the gearbox are enough to prevent the axial force formed by the head pressure. Its special point bearing capacity is large, the gearbox manufacturing cost is low, and the maintenance is convenient.
For users, the selection of twin-screw extruders is very important. Different types of twin-screw extruders have different performance and application occasions. Therefore, it is necessary to make clear the performance and application occasions of various twin-screw extruders. For example, the intermeshing co rotating twin-screw extruder is widely used in the modification of polymers that are not easy to decompose, such as blending, filling, fiber reinforcement and reactive extrusion of materials due to its high rotation speed, high shear rate and combined screw. Because of its good mixing and plasticizing function, the biggest feature of the intermeshing counter rotating twin screw extruder is the direct molding of PVC powder. If the geometry of the screw is changed, it can also be used for forming other materials, but its strength is still PVC forming. According to the size of plastic section, determine the extrusion amount, and then select the specification of twin-screw extruder by the extrusion amount. Under the same processing conditions, the extrudability of conical twin-screw is suitable for larger head pressure, while that of parallel twin-screw is suitable for smaller head pressure.
The parallel twin-screw extruder and the conical twin-screw extruder have the same features: they have the conveying mechanism to push the plastic forward forcibly, good mixing plasticizing ability and dehydration ability, and basically have the same adaptability to the molding process of materials and plastic products.
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. Concentric distance: the center distance of the flat twin screw is the same, the two axes of the tapered twin screw show an included angle, and the size of the 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, while tapered twin screw (L / D) refers to the ratio of the effective part length of the screw to the uniform value of the large end diameter and the small end diameter.
From the above, we can clearly see that the most obvious difference between parallel twin-screw extruder and conical twin-screw extruder is the different geometric shape of the screw barrel, which leads to many differences in structure and performance. Although they have different characteristics, they have their own advantages.
Parallel twin screw extruder
Due to the small size of the center distance between the two screws, there is limited space for the radial bearing, thrust bearing and related transmission gear supporting the two output shafts in the transmission gearbox. Although the designer has spent his brains, he cannot solve the reality that the bearing capacity, the module and diameter of the gear are small, and the tail diameter of the two screws is small, resulting in the result of poor torsional resistance. The most obvious defects of parallel twin screw extruder are small torque and poor load resistance. But the plasticity of length diameter ratio is the upper hand of parallel twin screw. According to the difference of molding conditions, it can increase and reduce the length diameter ratio to meet the requirements of plastic processing technology, and expand the application scope of parallel twin screw, but this point is difficult for conical twin screw extruder to achieve.