As one of the important parts of the main extrusion system of plastic extruder, plastic extruder screw design is very exquisite: not only plays the role of conveying plastics but also plays an extremely important role in the extrusion, plasticization and molding of plastics. Therefore, the rational selection of screw structure and parameters is an important link to obtain the ideal quality and output of products. In order to meet the different needs of plastic processing, the plastic extruder screw design will be somehow different, so there are many extruder screw types. Some common ones are as following: gradual change ( Isometric deepening screw/Isobathic varying-pitch screw), mutation change, torpedo head and etc. The selection of screw type is mainly determined according to the physical nature and of plastics and the technical specifications production of extruder.
1. In small plastic extruder screw design, such as φ45 extruder screw is a full thread type with Isometric deepening screw: the length and the diameter of screw is relatively small; mainly used to extrude insulation layer and sheath layer with small section; the extrusion speed is faster.
2. The softening of amorphous polymers is done at a relatively wide temperature; the isometric deepening screw is generally used. The melting temperature range of crystalline polymer is narrow, and isometric mutation screw is generally used.
3. Diameter of large screw is generally more than 150 mm, such as extruders of φ150, φ200, φ250. The large screw adopts two types, one is isometric deepening screw, such as extruder of φ150, φ200; the other is the screw of isometric and isobathic, isometric deepening and deepening and varying-pitch which can be divided into three types, that is, such as φ250 extruder with the compression ratio between 2~3 and the ratio of length to diameter is about 15:1. It is mainly used to produce wire or cable insulation layer and sheath layer with large section.
4. Medium screw adopts full thread type with isometric but deepening gradient. Its LDR is larger than that of smaller screw, and the pitch of thread is the same with a varied depth from root to the up. The thread at the end of the screw is deeper and the thread at the root is shallow so that the plastic extrusion quantity is high but no influence to the screw strength and the extrusion speed is fast with great plastic effects. As far as the plastic extruder screw design, the medium screw is the ideal one for the medium and small extruder to produce insulating layer and protection layer.