Ningbo Fangli Technology Co., Ltd. is a mechanical equipment manufacturer with over 30 years of experience in plastic pipe extrusion equipment, as well as new environmental protection and new materials equipment. Since its establishment, Fangli has developed based on user demands. Through continuous improvement, independent R&D of core technologies, and the digestion and absorption of advanced technologies, we have developed PVC pipe extrusion lines, PP-R pipe extrusion lines, and PE water supply/gas pipe extrusion lines, which have been recommended by the Chinese Ministry of Construction to replace imported products. We have been awarded the title of "First-class Brand in Zhejiang Province."
What is the Screw Length-to-Diameter Ratio?
The ratio of the length of the working part of the screw (the threaded section, measured from the centerline of the feed port to the end of the screw flight) to the screw diameter is called the length-to-diameter (L/D) ratio. When other conditions, such as screw diameter, are fixed, increasing the L/D ratio means increasing the screw length. A higher L/D ratio, combined with a reasonable temperature distribution, facilitates better mixing and plasticization of plastics. The longer residence time in the barrel allows for more thorough and uniform plasticization, thereby improving plasticizing quality.
Advantages of Increasing the Screw L/D Ratio
(1) Enhanced pressurization by the screw improves the physical and mechanical properties of the product.
(2) Better material plasticization results in superior product surface quality.
(3) Stable extrusion with a 20%–40% increase in output.
(4) Facilitates adjustment of the axial temperature gradient along the barrel to meet specific polymer processing needs.
(5) Beneficial for powder forming.
How to Choose the Screw L/D Ratio?
If plasticizing quality requirements remain unchanged, increasing the L/D ratio allows for higher screw speeds, thereby increasing plastic extrusion output. At a given screw speed, a higher L/D ratio extends the material's residence time in the screw, promoting better plasticization and mixing, reducing melt backflow and leakage, and enhancing production capacity. However, an excessively high L/D ratio leads to increased power consumption, greater difficulty in screw and barrel machining and assembly, a higher risk of screw bending, and potential scraping between the screw and barrel inner wall, reducing service life. Therefore, the L/D ratio should not be increased indiscriminately. Selection should be based on the material properties and product quality requirements.
For processing heat-sensitive materials like PVC, a smaller screw L/D ratio is recommended, as an excessively high ratio can lead to prolonged residence time and material degradation. For materials requiring higher temperatures and pressures, a larger L/D ratio is preferred. If product quality requirements are not stringent (e.g., recycling and granulation of waste materials), a smaller L/D ratio may be chosen; otherwise, a larger ratio is advisable. Materials of different geometries also require different L/D ratios: for pelletized materials (pre-plasticized and granulated), a smaller L/D ratio can be used, while unprocessed powder materials typically require a larger ratio. The general screw L/D ratio ranges from 20 to 30.
Additionally, when using a screw with a larger L/D ratio, increasing the screw speed will raise torque. For small-diameter screws, strength verification is necessary to ensure it meets requirements.
If you need further information, Ningbo Fangli Technology Co., Ltd. welcomes your inquiry. We will provide professional technical guidance or equipment procurement suggestions.