What is the screw length to diameter ratio? How to choose the length to diameter ratio?

1

What is the screw length to diameter ratio?

The length of the working part of the screw (the length of the threaded part, also refers to the length of the center line of the feeding port to the end of the screw thread) and the ratio of the screw diameter is called the length-diameter ratio, when other conditions are certain (such as the diameter of the screw), increasing the length-diameter ratio means increasing the length of the screw. Large length-diameter ratio and reasonable temperature distribution are conducive to the mixing and plasticization of plastics, at this time, the plastic is heated in the barrel for a longer time, and the plasticization of plastics will be more adequate and more uniform, thus improving the quality of plasticization.

2

Increase the advantage of length-to-diameter ratio
(1) screw pressure is sufficient, can improve the physical and mechanical properties of the product.
(2) The material is plasticized well, and the appearance quality of the product is better.
(3) Extrusion stability, extrusion capacity increased by 20%-40%.
(4) It is beneficial to adjust the temperature gradient along the cylinder axis to meet the needs of special polymer processing.
(5) Conducive to powder molding.

3

How to choose the length to diameter ratio?

If the plasticizing quality requirements remain unchanged, the length-diameter ratio increases, the speed of the screw can be increased, thereby increasing the amount of plastic extrusion. Under the condition of a certain screw speed, the length-diameter ratio increases, which means that the movement time of the material in the screw increases, which is conducive to the plasticization and mixing of plastics, which can reduce the countercurrent and leakage of the molten material, and improve the production capacity. However, if the ratio of length to diameter is too large, the power consumed by the screw will increase correspondingly, and the processing and assembly difficulty of the screw and the barrel will increase, and the possibility of bending the screw will also increase, which will cause the grinding of the screw and the inner wall of the barrel and reduce the service life. Therefore, the length-diameter ratio should not be increased blindly. The selection of screw length-diameter ratio should be considered according to the performance of the material to be added and the requirements for product quality.

For the processing of heat-sensitive materials, such as PVC, it is appropriate to choose a smaller screw length-diameter ratio, because too large screw length-diameter ratio is easy to cause too long residence time and decomposition; For materials requiring higher temperature and pressure, a larger length-diameter ratio should be selected; If the product quality requirements are not too high (such as waste material recycling granulation), you can choose a smaller screw length-diameter ratio, otherwise you should choose a larger screw length-diameter ratio; For materials with different geometric shapes, the requirements of the screw length-diameter ratio are not the same, for granular materials, due to plasticizing granulation, the screw length-diameter ratio can be selected to be smaller, and for powdered materials without plasticizing granulation, the screw length-diameter ratio is required to be larger. General screw length to diameter ratio of 20-30.

In addition, the use of large length-diameter ratio of the screw, if the need to increase the speed of the screw, it will increase the torque, for small diameter screw, it is necessary to check its strength to see whether its strength meets the requirements.