What is the length to diameter ratio of a screw? How to choose the screw L/D ratio?

Ningbo Fangli Technology Co., Ltd. is a mechanical equipment manufacturer with nearly 30 years’ experiences of plastic pipe extrusion equipment, new environmental protection and new materials equipment. Since its establishment Fangli has been developed based on user’s demands. Through continuous improvement, independent R&D on the core technology and digestion & absorption of advanced technology and other means, we have developed PVC pipe extrusion line, PP-R pipe extrusion line, PE water supply / gas pipe extrusion line, which was recommended by the Chinese Ministry of Construction to replace imported products. We have gained the title of “First-class Brand in Zhejiang Province”.

What is the length to diameter ratio of a screw?

The ratio of the length of the working part of the screw (including the length of the threaded part, also referring to the length from the centerline of the feeding port to the end of the screw thread) to the diameter of the screw, is called the aspect ratio. When other conditions are fixed, such as the screw diameter, increasing the aspect ratio means increasing the length of the screw. A large aspect ratio and a 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 the plastic will be more thorough and uniform, thereby improving the quality of plasticization.

Advantages of increasing the screw L/D ratio

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

（2）The material has good plasticization and the appearance quality of the product is good.

（3）Stable extrusion, with an increase of 20% -40% in extrusion volume.

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

（5）Beneficial for powder forming.

How to choose the screw L/D ratio?

If the L/D ratio increases while the plasticizing quality requirements remain unchanged, the 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 reflux and leakage of the molten material, and improve the production capacity. However, if the L/D ratio is too large, the power consumed by the screw will increase correspondingly, and the difficulty of processing and assembling of the screw and 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 its service life. Therefore, the L/D ratio should not be increased blindly. The selection of screw L/D ratio should be considered according to the performance of the material and the requirements for product quality.

For the processing of heat sensitive materials, such as PVC, it is advisable to choose a smaller screw L/D ratio, as excessive screw L/D ratio can easily cause excessive residence time and decomposition. For materials that require higher temperatures and pressures, a larger L/D ratio should be selected. If the product quality requirements are not too high (such as recycling and granulation of waste materials), a smaller screw aspect ratio can be selected, otherwise a larger screw L/D ratio should be selected. 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, while for powder materials without plasticization and granulation, the screw L/D ratio is required to be larger. The general screw L/D ratio is 20-30.

In addition, when using screws with a larger aspect ratio, if the screw speed needs to be increased, the torque will be increased. For small diameter screws, strength verification is required to examine whether their strength meets the requirements.

If you need more information, Ningbo Fangli Technology Co., Ltd. welcomes you to contact for a detailed inquiry, we will provide you with professional technical guidance or equipment procurement suggestions.