What are the basic parameters for the screw of a single screw extruder？

The screw diameter is used to indicate the size of the extruder, which in a certain sense indicates the output of an extruder. The screw diameter can generally be reasonably selected according to the sectional area of the product, the processing performance of the material and the required productivity and processing capacity, that is, the selection of the extruder model.

The diameter of the extruder screw has been serialized like 30mm, 45mm, 65mm, 90mm, 120mm, 150mm, 200mm and other specifications. The most widely used screw diameter is 65-150mm. Normally a large-diameter screw should be used for products with a large cross-sectional area and a small-diameter screw should be used for products with a small cross-sectional area.

The L/D ratio of an extruder refers to the ratio of the effective length of the screw to the diameter of the screw. This ratio is another important parameter of an single screw extruder. If the length-diameter ratio of the screw and the screw speed are considered together, it indicates the plasticizing ability and plasticizing quality of the screw in a certain sense. The national standard series stipulates that L/D ratio is of 15, 20, 25, 30, etc. and the maximum can reach 43. An extruder with a moderate L/D ratio should be selected according to raw material properties and production efficiency. If the ratio is increased, due to the increase of the length of the screw, the residence time of the plastic in the barrel is prolonged and the material is plasticized more fully and uniformly so that both the output and quality can be improved. However, this makes the processing and assembly of the screw and the barrel more difficult. The cost also increases accordingly and the screw is likely to get deformed. The clearance between the screw and the barrel tends to be uneven. Therefore, high quality and high yield should be obtained with an L/D ratio as small as possible. For example, when processing polyolefin and polyvinyl chloride materials, the length-diameter ratio of the screw is selected to be 20 and 25.

The ratio of the volume of the first groove in feeding section to the volume of the last groove in the homogenization section is called the compression ratio. The size of the compression ratio has a great influence on the compactness of the product and the ability to remove the air contained in the material. At the same time, it also affects the output of the extruder and the mechanical strength of the screw. The relationship between different materials and compression ratio is shown in the table.

The depth of the screw groove is related to the thermal stability of the material to be processed. The depth of the screw groove in the compression section of the conventional screw is most important and is a variable, which is represented by h2. The groove depth of the feeding section is represented by h1, which is generally a fixed value. The groove depth of the homogenization section is represented by h3, which is generally a fixed value. The depth of the screw groove directly affects the plasticizing efficiency of the screw and the maximum pressure in production. For example, the shallow screw groove is beneficial to plasticization, but the production efficiency is low; the extrusion volume of the deep screw groove is large, but the extrusion is not stable and the plasticization quality is poor.

The distance between two adjacent threads, usually S=D.

The angle between the helix and the vertical line of screw center is generally taken as Q=17°42′.

Usually the number of threads is 1.

Refers to the width of the thread top along the axial direction, in mm.

It refers to the gap between the outer diameter of the screw and the inner wall of the barrel, in mm.