Basic Knowledge About Screw Assembly: Thread Elements

1. Conveying elements of extruder screw

The conveying element is threaded, and its function is to convey materials (including liquid materials). The shape of the screw groove can be rectangular or a special shape (meshing type) generated according to the principle of relative motion. The threaded elements are divided into two types: forward and reverse. It can also be divided into single-head, double-head, and three-head threaded components.

2. Single threaded element of extruder screw

It has high-efficiency solid conveying capacity, and is generally used in the feeding section to improve the extrusion volume that is limited by the feeding volume and to convey materials with poor fluidity, such as low-density materials. It is generally used to transport materials with a particle size similar to water in the reaction process, and can also be used in the discharge section. The output capacity of a single-threaded thread is greater than that of a multi-threaded thread, and the torque is also greater than that of a multi-threaded thread. Its mixing characteristics are more than multi-start threads.

3. Comparison of double-thread and three-thread screw of extruder screw

Under the same central moment, D/D. is larger and the groove is deeper. Therefore, under the same extruder screw speed, a lower shear rate can be provided. It is more suitable for processing powder materials, especially low bulk powder materials, glass fiber and other materials that are sensitive to shear.

Compared with three-threaded components, under the same shear stress and torque, two-threaded components can work at a higher speed and have higher productivity.

The three-threaded element can apply a higher average shear rate and shear force to the material under the same extruder screw speed. In addition, because the screw groove is shallow and the material layer becomes thinner, the heat transfer performance of the three heads is better than that of the two heads, which is conducive to the plasticization and melting of the materials. However, because of the strong shearing, it is generally not easy to process materials sensitive to shearing, such as glass fiber and PVC.

4. Lead change and characteristics of extruder screw

In the extruder screw combination, for the output-oriented occasions, choosing a larger lead thread is beneficial to increase the output. For the extrusion of heat-sensitive polymers, choosing a larger lead can shorten the residence time of the material And reduce the thermal degradation of materials.

Thread lead has a great influence on extrusion volume, mixing characteristics and torque. Generally speaking, with the increase of thread lead, the extrusion capacity of extruder screw increases. The residence time of the material is shortened, the mixing effect on the material is relatively reduced, and the torque is also smaller.

For the occasion dominated by mixing, the thread with medium lead shall be selected. Moreover, the lead of the threads in different working areas of the extruder screw is a combination of gradually decreasing, which is mainly used for the output and pressurization of solid materials, so as to improve the melting speed or mixing speed and extrusion stability.

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