# Research Progress on the Manufacturing of Screw-Shaped Parts in Screw Compressors
## 1 Introduction  
In 2013, a strategic policy of \"Industry 4.0\" was published in Germany to achieve the new industrial revolution [1; 2; 3]. \"A National Strategic Plan for Advanced Manufacturing\" was proposed in the US, targeted towards enhancing the merits of its modern manufacturing industry with intensified technology [4; 5]. The \"New Robot Strategy\" was published in Japan to enhance the international competitiveness of the manufacturing industry [2; 6]. \"Made in China 2025\" was published in China to implement the strategy of manufacturing power. Its fundamental objective is to change the situation of \"large but not strong\" in the manufacturing industry in China [7; 8]. Therefore, the manufacturing industry has attracted more and more attention in many countries.  
Mechanical basic parts are indispensable in the manufacturing industry for various pieces of equipment. The key characteristics of the equipment and related products, such as the machine performance, durability and reliability, are determined by the quality of the basic parts [9]. With the development of modern industrial technology, screw types of machinery, such as screw pumps, screw vacuum pumps, screw extruders and especially screw compressors, have become the key foundation and core parts in many high-end equipment manufacturing industries, playing a crucial role in the safe and reliable operation of high-end equipment.  
As a positive displacement compressor, the screw compressors shown in Figure 1a,b have mainly been applied as air compressors or freezing air conditioning compressors [10, 11, 12]. They are also widely used in aerospace, rail transit, ocean engineering, the petrochemical industry, new energy automobiles and other fields due to their advantages, including a long service life, high transmission efficiency, small floor area and low vibration [13, 14]. The screw compressor mainly includes female and male screw rotors and the compressor housing. The screw rotors rotate, meshing with each other. The compressor housing accommodates the screw rotors. An example of a screw rotor for the screw compressor with dimensions can be seen in Figure 1c. Therefore, the screw rotor, which belongs to typical long-shaft complex profile parts, is the key core component of the screw compressor. The processing method, manufacturing accuracy and quality have an extremely important impact on the performance of the whole screw compressor.  
The manufacture of screw rotors is often achieved by milling, roughing and subsequent machine grinding processes [15]. However, the shape of the screw rotors is complex, resulting in high difficulty in the manufacturing process. Consequently, it is difficult to guarantee machining accuracy, which is an extremely important factor to reduce the service life of screw machinery. Therefore, the manufacture of screw rotors is a challenging task because it has a high requirement for particular machines or instruments. It is essential to develop novel manufacturing technology for screw-shaped parts.  
Figure 1: Typical structure of screw compressor: (**a**) single-screw compressor; (**b**) twin-screw compressor; (**c**) specific dimensional and shape requirements of screw rotor for the screw compressor.  
This paper summarises the research progress on the manufacturing of screw-shaped parts from the aspects of the cutting process, solid plastic forming, casting and additive manufacturing. The pros and cons of different manufacturing processes are provided and discussed, which can contribute to the development of the high-efficiency, precise and high-performance forming process of complex screw-shaped parts. Moreover, a new process is proposed in this review to solve the problems of serious material waste and low production efficiency in forming complex screw-shaped parts.