Development of key technology of coaxial twin screw extruder

The torque transfer mode between screw and mandrel is the key to realize the deep screw groove. The torque transfer between the screw element and the mandrel was initially connected by flat keys, which not only limited the deepening of the screw groove and increased the free volume of the screw, but also the strength of the flat keys could not meet the requirements of some production operations.

1. Spiral groove depth

The depth of screw groove is an important parameter of the twin – screw extruder. Usually it is expressed indirectly by the ratio of screw diameter and screw root diameter. The value is an important parameter to indicate the technical level of the twin screw extruder. At present, the ratio of the external diameter of the screw to the root diameter of the screw in the latest generation (sixth generation) coaxial twin-screw extruder represented by WP of Germany is D/Di = 1.55.There are many factors limiting D/Di, in addition to the center distance of the gearbox output shaft of the driving screw, the screw itself also has a greater impact on D/Di: such as the shear resistance of the screw mandle-shaft, the torque transfer mode between the screw element and the mandle-shaft, the structure of the screw element itself, etc.

  1. The breakthrough in material selection, processing technology and heat treatment performance of mandrel makes the torque capacity of mandrel bearing greatly improved, and the length-diameter ratio of screw increases from the initial 36 to 50, or even higher.
  1. The technological breakthrough of the special gear drive system of the co-directional twin-screw extruder provides the power transmission guarantee for the torque input required by the deep groove screw under the condition of the constant center distance, making the screw D/Di goes to 1.55.
  1. The torque transfer mode between screw and mandrel is the key to realize the deep screw groove. The torque transfer between the screw element and the mandrel was initially connected by flat keys, which not only limited the deepening of the screw groove and increased the free volume of the screw, but also the strength of the flat keys could not meet the requirements of some production operations. Using multi – key torque transmission in the form of involute can greatly improve the depth of the screw groove under the same outer diameter of the screw. In recent years, while learning from foreign advanced technologies, China has made great breakthroughs in processing and manufacturing. At present, it has been able to manufacture involute spline structure with the same connection form with foreign screw, thus making great breakthroughs in the depth of screw groove.
  2. The breakthrough of element structure. The structure of the kneading block solves the key technology of manufacturing, its structure changes from the original single piece type, the assembling type to the present integral type, thus guarantees the deepening of the screw groove from the strength of the element, also makes the adjustment of the screw combination and the assembly of the screw more reliable and convenient.

To sum up, breakthroughs in the aspects of gear transmission system, shear strength of mandrel, torque transmission form of screw element and mandrel, and design and manufacture of screw element ensure the successful development of a new generation of high-speed and deep screw slot extruder.

  1. New screw element

Synthetic twin screw extruder is the core part of the screw, after the advent of synthetic double screw extruder, and development of screw and screw element has always been a developer research and the main direction, screw structure from the integral development early to “building block”, three thread from single head to head, later when the double structure, type of screw elements, structure and function differences. Develop self-cleaning type mixer type screw element, edge type mixer type screw element, toothed screw element, pressure regulating screw element, etc., of the twin screw extruder screw element made greater innovation, supplement and perfect, strengthen and improve the direction of the twin screw extruder mixing and plasticizing effect, dispersion, shear, pressure, and adjustable.

  1. Gear box torque rating parameters

The torque grade parameter M/A3 of the gear box is an important performance indicator parameter of the coaxial twin-screw extruder. Its value represents the technical level of the coaxial twin-screw extruder and represents the torque output capacity of the gear box under a certain screw center distance. Due to the particularity of the transmission form of the coaxial twin-screw extruder, the torque of the gear is input into the gear box by a high-speed shaft. Through deceleration and torque distribution, the torque is transmitted to the twin-screw by two output shafts with limited center distance under the condition of great axial force. Therefore, the ratio parameter M/A3 between the center distance of the output shaft and the output torque has become an important symbol to measure its technical level. So the design of gear box of twin screw extruder is very difficult compared with the design and development of general gear transmission system. At present, China has made great breakthroughs in gear transmission mode, torque uniform distribution and axial force solution. The torque grade parameters of the experimental extruder have been changed from 4.Seven goes up to eight.8. It lays a good foundation for the development of high speed and high torque gear box of production extruder.

In terms of the service life of the gear box, by improving and driven by new type of multi-channel way, not only greatly improve the transmission capacity, and through the reasonable arrangement of the axis, solved the gear and thrust bearing load distribution, greatly reduced the gear drive and work process of the additional force, making long-term restricting the service life of the gear box of the anti-fatigue performance of the gear, thrust bearing system performance are greatly improved, after adopts multiplex drive, output the stress of the bearing diameter of axle direction decrease greatly, therefore, in many ways to improve the gear, the thrust bearing system, the torque distribution part of the bearing service life, Designed life up to 10 a.

  1. Screw speed

Screw speed is another important parameter to measure the performance grade of the extruder with the same direction and it is an important way to improve the performance index of the extruder. It directly affects the torque grade parameters and production capacity of the equipment. The screw rotation speed increase mainly depends on the innovation and development of gear transmission system drive, the ability of the screw torque under and anti-fatigue performance, by developing a new type transmission way and optimize the innovation design, selection of key parts imported bearing, means, the strict screening of screw spindle materials and the improvement of heat treatment process, and the screw element and spindle, involute spline breakthrough made in the form of uniform stress distribution, making it possible to screw rotation speed increase, has been in a small high-speed experimental synthetic twin-screw extruder machine through industrial test.

  1. Production capacity

The technical level and performance of the equipment are aimed at improving production capacity, reducing energy consumption and improving reliability. Production capacity is the main factor to evaluate the performance of a unit: (1) improve the screw speed, the same type of production can be doubled; (2) the use of deep screw groove screw, increase the screw free volume, improve the unit speed and unit time transport capacity; (3) the use of twin screw and gear pump combined technical route, improve the reliability of the transmission box, reduce the axial force borne by the gear, eliminate the limit of the axial force to reduce the output of the factor, improve the output of the extruder.

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