What is the screw L/D #pvc screw barrel #extruder screw

The ratio of the length of the working part of the screw (the length of the threaded part, also refers to the length from the center line of the feed port to the end of the screw thread) and the diameter of the screw is called the aspect ratio. When other conditions are constant (such as screw diameter), the ratio increases The aspect ratio means increasing the length of the screw.

The large aspect ratio and reasonable temperature distribution are conducive to the mixing and plasticizing of plastics.

At this time, the plastic is heated in the barrel for a longer time, and the plasticizing of the plastic will be more complete and more uniform, thus improving the plasticizing quality.

Advantages of increasing screw L/D

(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 helpful 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.

How to choose the screw L/D

If the plasticizing quality requirements remain unchanged, after the aspect ratio is increased, the screw speed can be increased, thereby increasing the plastic extrusion volume. Under the condition that the screw speed is constant, the aspect ratio increases, which means that the movement time of the material in the screw increases, which is beneficial to the plasticization and mixing of plastics, can reduce the backflow and leakage of melt, and improve production capacity. However, if you choose an aspect ratio that is too large, the power consumed by the screw will increase accordingly, and the processing and assembly of the screw and barrel will increase. The possibility of screw bending will also increase, which will cause scraping of the screw and barrel inner wall. Grinding reduces service life. Therefore, the aspect ratio cannot be increased blindly. The selection of screw length-to-diameter ratio should be considered based on the performance of the materials to be added and the requirements for product quality.

For the processing of heat-sensitive materials, such as PVC,  it is better to use a smaller screw length-to-diameter ratio, because an excessively large screw length-to-diameter ratio can easily cause the residence time to be too long and cause decomposition; for materials that require higher temperature and pressure, a smaller screw length-to-diameter ratio should be used. Larger aspect ratio; if the product quality requirements are not too high (such as waste material recycling and granulation), a smaller screw aspect ratio can be used, otherwise a larger screw aspect ratio should be selected; for different geometric shapes Materials and screw length-to-diameter ratio requirements are also different. For granular materials, since they have been plasticized and granulated, the screw length-to-diameter ratio can be selected to be smaller. For powdery materials that have not been plasticized and granulated, the screw length-to-diameter ratio is required. Bigger than. The general screw length-to-diameter ratio is 20-30.

In addition, when using a screw with a larger aspect ratio, if the screw speed needs to be increased, the torque will increase. For a small diameter screw, it is necessary to perform a strength check to see whether its strength meets the requirements.