How To Choose The Length To Diameter Ratio?

If the aspect ratio increases while maintaining the same plasticization quality requirements, the screw speed can be increased to increase the extrusion amount of plastic. Under the condition of a constant screw speed, an increase in the aspect ratio means that the material's movement time in the screw increases, which is beneficial for the plasticization and mixing of plastics, reducing the backflow and leakage of molten materials, and improving production capacity. However, choosing an excessively large aspect ratio will increase the power consumption of the screw, and the difficulty of machining and assembling the screw and barrel will also increase. The possibility of screw bending will also increase, which will cause scraping between the screw and the inner wall of the barrel, reducing its service life. Therefore, we cannot blindly increase the aspect ratio. The selection of screw length to diameter ratio should be based on the performance of the added material and the requirements for product quality.

For the processing of heat sensitive materials such as PVC, it is advisable to use a smaller screw aspect ratio, as an excessively large screw aspect ratio can cause prolonged residence time and decomposition; For materials that require higher temperatures and pressures, a larger aspect 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 used, otherwise a larger screw aspect ratio should be used; For materials with different geometric shapes, the requirements for the aspect ratio of the screw are also different. For granular materials, due to plasticization granulation, the aspect ratio of the screw can be selected to be smaller, while for powder materials that have not been plasticized granulation, a larger aspect ratio of the screw is required. Generally, the length to diameter ratio of the screw is 20-30.

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