What is the screw L/D ratio? How to choose the L/D ratio?

1. What is the screw L/D ratio?

The length of the working part of the screw refers to the length of the threaded part, and also refers to the length from the centerline of the feeding port to the end of the screw thread. The ratio of screw length to screw diameter is called the L/D ratio. Under certain other conditions (such as screw diameter), increasing the L/D ratio means increasing the length of the screw. A large L/D ratio and a reasonable temperature distribution are conducive to the mixing and plasticization of plastics. At this time, the plastics are heated in the barrel for a longer time, and the plasticization of the plastics will be more sufficient and uniform, thereby improving the plasticization quality.

2. The advantage of increasing the L/D ratio

(1) The screw is fully pressurized, which can improve the physical and mechanical properties of the product.

(2) The material is plasticized well, and the appearance quality of the product is good.

(3) The extrusion is stable, and the extrusion volume is increased by 20%-40%.

(4) It is beneficial to adjust the temperature gradient along the axial direction of the barrel to meet the needs of special polymer processing.

(5) Conducive to powder molding.

3. How to choose the L/D ratio?

If the plasticizing quality requirements remain unchanged, after the L/D ratio is increased, the rotational speed of the screw can be increased, thereby increasing the amount of plastic extrusion. Under the condition of a certain screw speed, the L/D 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 backflow and leakage of the melt and improve the production capacity.

However, if the L/D ratio is too large, the power consumed by the screw will increase accordingly, and the processing and assembly difficulty of the screw and barrel will increase, and the possibility of screw bending will also increase, which will cause scraping of the screw and the inner wall of the barrel, reducing the service life.

Therefore, the L/D ratio cannot be blindly increased. The selection of screw L/D 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 advisable to use a smaller screw L/D ratio, because an excessively large screw L/D ratio will easily cause the residence time to be too long and cause decomposition; For materials requiring higher temperature and pressure, a larger L/D ratio should be used; If the product quality requirements are not too high (such as recycling and granulation of waste materials), a smaller screw L/D ratio can be used; For materials with different geometric shapes, the requirements for the screw L/D ratio are also different. For granular materials, due to plasticization and granulation, the screw L/D ratio can be selected to be smaller. For powder materials that are not plasticized and granulated, the screw L/D ratio is required to be larger. Generally, the L/D ratio of the screw is 20-30.

In addition, if a screw with a larger L/D ratio is used, if the screw speed needs to be increased, the torque will be increased. For the small diameter screw, it is necessary to check its strength to check whether its strength meets the requirements.

Nanjing Haisi Extrusion is a professional manufacturer of twin screw extruder, we have our own fine processing fatory, which promise the screw elements are produced standardly and high quality.