How to choose the correct length to diameter ratio of the blow molding machine screw

1. Screw diameter Ds: The screw diameter is the outer diameter of the thread. The production capacity (extruding amount) of the extruder is approximately proportional to the square of the screw diameter. Under other conditions, a slight increase in the screw diameter will cause extrusion The significant increase in the amount of the screw has a greater impact than the increase in the number of screw revolutions on the extrusion rate. Therefore, the screw diameter is often used to characterize the technical parameters of the extruder specifications.

2. Screw length-to-diameter ratio L/Ds: The ratio of the length L of the working part of the screw to the screw diameter Ds is called the length-to-diameter ratio. Under other conditions (such as screw diameter), increasing the length-to-diameter ratio means increasing the length of the screw. . The large L/Ds value and reasonable temperature distribution are conducive to the mixing and plasticization of plastics. At this time, the plastics will be heated in extruder screw barrel the barrel for a longer time, and the plastics will be fully and more uniformly plasticized. Thereby improving the quality of machine plastics. If the plasticization quality requirements remain unchanged, after the length-to-diameter ratio is increased, the speed of the screw can be increased, thereby increasing the amount of plastic extrusion. However, if an excessively large aspect ratio is selected, the power consumed by the screw will increase correspondingly, and the processing and assembly of the screw and barrel will be more difficult; the possibility of screw bending will also increase, which will cause the screw and barrel inner wall Scrape, reduce service life. In addition, for heat-sensitive plastics, an excessively large aspect ratio is thermally decomposed due to a long residence time, which affects the plasticization and extrusion quality of the plastic. Therefore, in making full use of the advantages of the increased aspect ratio, the selection should be based on the physical properties of the processed plastic and the extrusion quality requirements of the product.

3. Compression ratio ε: Also known as the geometric compression ratio of the screw, it is the ratio of the volume of the first groove of the screw feeding section to the volume of the last groove of the homogenizing section. It is determined by the physical compression ratio of the plastic-the ratio of the density of the product to the apparent density of the feed material. The purpose of making the extruder compress larger is to make the granular plastic fully plasticized and compacted. When the types of processed plastics are different, the choice of compression ratio should also be different. According to the compression ratio, the screw types can be divided into three types: equidistant unequal depth, equal depth unequal distance, and unequal depth unequal distance. Among them, isometric unequal depth is the most commonly used one. This kind of screw is easy to process, the contact area between the plastic and the barrel is large, and the heat transfer effect is good.