Plastic extruders have many varieties of plastics for extrusion molding, and it is impossible to form all plastics with one screw. The screw should be designed according to the characteristics of the raw materials and considering the commonality of various raw materials as much as possible, so that one screw can simultaneously extrude several kinds of plastics, which is economically significant in industrial production. The reverse thread on the rear end of the screw acts to prevent leakage.
The screw length to diameter ratio L/D, the screw diameter D refers to the outer diameter of the screw thread. The effective length L of the screw refers to the length of the working part of the screw, as shown in Figure 3-14. The effective length is different from the total length of the screw. The aspect ratio is the ratio of the effective length of the screw to the diameter. The length of the screw of the early extruder was relatively small, only 12-16. With the development of the plastic molding processing industry, the length-to-diameter ratio of the extruder screw is gradually increasing. Currently, the commonly used ones are 15, 20, and 25, and the maximum is 43.
Increasing the aspect ratio has the following advantages: 1. The screw is fully pressurized, and the physical and mechanical properties of the product can be improved. 2. The material is plasticized and the appearance quality of the product is good. 3. The extrusion amount is increased by 20-40%. At the same time, the slope of the screw characteristic curve with large aspect ratio is small, relatively flat, and the extrusion amount is stable. 4. Conducive to the formation of powder materials, such as PVC powder extruded tube. However, increasing the aspect ratio makes the manufacture of the screw and the assembly of the screw and the barrel difficult. Therefore, the aspect ratio cannot be increased without limit.