Difference between conical twin screw and parallel twin screw extruder

Is a parallel twin-screw extruder better, or a conical twin-screw extruder? This is a question that users often ask when purchasing a twin-screw extruder.

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 and the opposite direction.

The co-rotating extruder means that the two screws rotate in the same direction when they work, and the counter-rotating extruder means that the two screws rotate in opposite directions when they 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. The twin-screw extruder with parallel axis lines is the conical twin-screw extruder with the axis lines intersecting.

Twin-screw extruders are also divided into meshing and non-meshing.

Parallel and conical twin-screw extruders in common:

It has a conveying mechanism that forces the plastic forward, good mixing and plasticizing ability and dehydration ability, and has basically the same adaptability to the molding process of materials and plastic products.

The difference between parallel and conical twin-screw extruders

1. Diameter: The diameter of the parallel twin screw is the same, and 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 varies along the axis.

3. Length-to-diameter 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. Conical twin screw (L/D) refers to the ratio of the effective part length of the screw to the average of the diameter of the large end and the diameter of the small end.

From the above, we can clearly see that the most significant difference between parallel and conical twin-screw extruders is the geometry of the screw barrel, which leads to many differences in structure and performance. Although the two have different characteristics, they have their own advantages.

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. The small modulus and diameter of the gear and the small diameter of the tail of the two screws lead to the result of poor torque 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. This is difficult to achieve with a conical twin-screw extruder.

The two conical screws of the conical twin-screw extruder are arranged horizontally, and the two axes are placed 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 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 drive shaft has a shaft diameter sufficient to meet the transmission torque, so large working torque and large load capacity are the major features of the conical twin-screw extruder.

Anti-return bearing for twin-screw extruder

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 or more. This pressure forms a strong axial thrust on the screw. , resisting the thrust is the function of the anti-return bearing.

1. Due to the limitation of the small center distance between the two screws of the parallel twin-screw extruder, the bearing capacity of the anti-return bearing is related to its diameter, and 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 structure of the transmission system of the parallel twin-screw extruder is relatively complex. Compared with the structure of the transmission system of the conical twin-screw extruder, the manufacturing cost of the gearbox is high and the maintenance is more complicated.

2. 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, and two large thrust self-aligning balls are installed in the gearbox. The bearing is enough to prevent the axial force formed by the pressure of the machine head. It has the characteristics of large bearing capacity, low manufacturing cost of the gear box and convenient maintenance. 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 performance of various twin-screw extruders. application. For example, the intermeshing co-rotating twin-screw extruder is widely used in the modification of polymers that are not easy to thermally decompose due to its high rotational speed, high shear rate, and combined screw - blending, filling, fiber Reinforcement and reactive extrusion of materials. For example, the intermeshing 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.