The ratio of the length to diameter of the screw and the diameter of the screw to the working length. In the process of extrusion tube production, if the screw speed is fixed, the length diameter ratio of the screw increases, which means that the moving time of the material increases in the screw, is beneficial to plastic plasticizing and mixing, and can reduce the reverse flow and leakage of the molten material, and improve the production capacity. The selection of screw length to diameter ratio should be considered according to the properties of the processed materials and the quality requirements of the products. The processing of heat sensitive materials, such as PVC, should be selected for smaller screw length and diameter ratio, because the oversize ratio of screw length to diameter is easy to cause decomposition; for materials requiring higher temperature and pressure, such as fluoroplastics, it is necessary to use a screw with a larger length and diameter ratio, which can be used when the quality of the product is not too high (such as recycling granulation for waste materials). The smaller screw length diameter ratio, otherwise the larger screw length diameter ratio should be selected; for the material with different geometric shapes, the length diameter ratio of screw is not the same. For granular material, the length diameter ratio of screw is smaller because of plasticizing granulation, and the length diameter ratio of screw is higher than that of the powder which has not been plasticized and granulation, and the length diameter ratio of screw is 20-30.