# Research Progress on the Manufacturing of Screw-Shaped Parts in Screw Compressors
## 3 Summary of Common Screw Manufacturing Processes  
The manufacturing processes of screw-shaped parts mainly include cutting (milling and grinding), solid plastic forming, casting, additive manufacturing forming, etc. Additionally, these processes can also be used in the production of long-axis complex profile parts such as worm gear and steering gear components, high-pressure common rail pipes, crankshafts, gear racks and gear shafts [57; 58; 59], which are shown in Figure 14. Among the above processes, the main manufacturing method adopted by many enterprises is the cutting process, which has the advantages of overall manufacturing and a short process flow, but it has the following obvious shortcomings:  
Figure 12: Female and male screw rotors fabricated by the SLM process [55]. Figure 13: The fabricated process of CPAS [56]. Fused Deposition Modelling was the additive manufacturing (3D-printing) method used in this process. The material was polylactic acid (PLA). In the cutting process, the material is greatly wasted. Moreover, the metal fibre is cut off, resulting in a reduction in length; thus, the mechanical property of the part reduces. The bending deformation of the workpiece is risky to form. 2. The actual production process of screw rotors needs multiple pieces of equipment, such as lathes, milling machines and grinding machines. Each process requires the replacement of tools and fixtures. The processing process is cumbersome. 3. The manufacturing cost of screw-cutting tools is high. A group of tools shall be replaced for each type of screw, where the corresponding cutter edge grinding equipment shall be provided. Consequently, the equipment investment is high. The solid plastic forming process of screw-shaped parts mainly includes the WCR process, the MRRFF process, the FEF process, the RCEF process, the IDFF process, etc. The screw-shaped parts produced via the solid plastic forming process have high mechanical properties. However, it is challenging to ensure the accuracy of producing the screw profile in the forming process. There are numerous disadvantages, such as a large machining margin and high energy consumption. In addition, due to the large forming force and high requirements for equipment tonnage during forming, the service life of the mould is short. Casting has high production efficiency, through which the screw profile is easy to be formed. However, casting defects such as shrinkage, shrinkage cavities and segregation are inevitable in the forming process, resulting in poor mechanical properties of the formed products [60]. Meanwhile, the high temperature of the molten metal in the forming process will shorten the die life. Additionally, the additive manufacturing process can achieve the precise forming of various complex-shaped screws, but it is necessary to improve the production efficiency. Moreover, there are significant limitations on the types of forming materials. The merits and demerits of the common screw manufacturing processes from the above analyses are summarised in Table 1. To summarise, although cutting is the production mode adopted by most enterprises currently, more than 60% of the materials in the manufacturing process are removed by cutting, where the material utilization rate is low. Solid-state plastic forming, casting, additive manufacturing and other processes can effectively improve the material utilization rate. Nevertheless, their forming quality and size specifications are subject to certain restrictions. Therefore, it is necessary to research the new process of long-axis complex screw-shaped parts, such as female and male screw rotors. Only by carrying out in-depth research on the new process, structure and performance control technology of long-axis complex parts can the processing and manufacturing problems of the screw-shaped parts be effectively solved. Figure 14: The long-axis complex profile parts: (**a**) screws; (**b**) worm gear and steering gear components; (**c**) camshaft; (**d**) cranshaft; (**e**) gear rack; (**f**) gear shaft [57; 58; 59].