# Research Progress on the Manufacturing of Screw-Shaped Parts in Screw Compressors
## 2 Common Manufacturing Process for Screw-Shaped Parts
### Solid Plastic Forming Process  
Compared with the traditional cutting process, the solid plastic forming process has almost no material loss, and can also significantly improve production efficiency. Therefore, many scholars have conducted a lot of research on the solid plastic forming process for screw-shaped parts. In general, the screw-shaped part is first rough produced through the solid plastic forming process, and then finished through machining with the grinding and polishing process. The cross-wedge rolling (CWR) process is a new forging method for metal materials with a near-net-shape for the production of shaft parts, such as screw rotors, steeped axes and shafts [27; 28; 29; 30; 31]. Figure 4 shows in detail how the CWR process is achieved [32]. As shown in Figure 4, the upper and bottom dies move oppositely, driving the billet rotation, where the screw rotors can be formed under the hole of the dies during the CWR process. [PERSON] et al. [32] carried out a finite element analysis considering the plastic deformation phenomenon for the CWR process to produce the spiral shaft parts using the DEFORM-3D software with the version of 10.2. Simulation results indicated that the forming quality of spiral shaft parts was good and the CWR process of spiral shaft parts was feasible. [PERSON] et al. [26] proposed the fixed axis cross-rolling (FACR) process, as shown in Figure 5, to form a screw compressor rotor. The metal materials of the billet produced continuous plastic deformation under the pressure of rotating rolling tools arranged parallel to the axis to obtain the required section shape. The screw compressor rotor with asymmetric spiral teeth and steps can be directly formed by the FACR process, which obviously improve production efficiency and the material utilization rate. The multi-roller rotary feed forming (MRRFF) method [33; 34] is a modified method for forming screw-shaped parts, and its schematic diagram is shown in Figure 6. During the MRRFF procedure, rolling wheels are radially located at the side of the workpiece. The workpiece rotates at a constant speed around itself and simultaneously moves in the direction of its axis. [PERSON] et al. [34] examined the effect of process parameters on the product quality of the MRRFF process for uniform-wall-thickness spiral tubes. The experimental result revealed that a relatively high forming precision and relatively low damage value of 304 stainless was obtained under the rolling depth of 3.3 mm and formation speeds below 15 mm/s. Figure 4: Principle of the CWR of screw rotor [32]. [PERSON] et al. [35; 36] analysed the forward extrusion forming (FEF) process of twist drills, as shown in Figure 7. During the extrusion process, the starting materials were heated to a suitable temperature for hot forming. When the material reached the required temperature, it was pushed by the punch into the special hole formed via the fixed and movable dies. Then, the formation of the drill in the cross-section was achieved by twisting. Based on the results of the theoretical-experimental tests, they revealed that the production of twist drills is accomplished by this extrusion procedure. It was also found that a small extrusion force was required with a small helix angle. Figure 8 shows the working principle of the rotary cold extrusion forming (RCEF) procedure for processing screw rods. In the RCEF process, the push rod shifts in the direction of the rod axis, as represented by the green arrow, while the die circumferentially rotates at a specific speed, as indicated by the red arrow. [PERSON] et al. [37] investigated the RCEF process of screw rods by simulations via the Deform-3D software. The results demonstrated the feasibility of the production of either hollow or solid screw rods with a high uniformity in wall thickness by the RCFF process. They found that when the belt height where the die works was large, the required force and torque for the extrusion process were also high.