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Cold upsetting forming technology of fasteners

What is cold forging?

Cold forging, also known as cold volume forming, is a manufacturing process and a processing method. As the stamping process is basically the same, cold forging is also made up of materials, dies and equipment. Only the material in stamping is mainly plate, while the material in cold forging is mainly disc wire. Japan (JIS) called cold forging (cold forging for short), China (GB) called cold heading, outside screw factory like to call the head.

Cold forging refers to the volume forming of metals below the recrystallization temperature. According to the theory of metallurgy, the recrystallization temperature of various metal materials is different. The lowest recrystallization temperature of ferrous and nonferrous metals. Even at room temperature or room temperature, the forming process of lead and tin can not be called cold forging, but hot forging. But iron, copper, aluminum at room temperature forming processing can be called cold forging.
In metallurgy, the forging of materials heated to recrystallization temperature (steel is about 700 ℃) is called hot forging.
For steel forgings, forging below recrystallization temperature and higher than normal temperature is called warm forging.

Advantages of cold heading (extrusion)

Cold upsetting (extrusion) technology is a main processing technology in the forming process of fasteners. Cold upsetting (extrusion) belongs to the category of metal pressure processing. In production, under normal temperature, the metal is applied with external force to form the metal in the predetermined die. This method is usually called cold upsetting.
The forming of any fastener is not only a deformation method of cold upsetting, but also a variety of deformation modes, such as forward and backward extrusion, compound extrusion, punching and rolling, etc., can be realized in the process of cold upsetting. Therefore, the term “cold heading” in production is just a habitual term, rather it should be called cold heading (extrusion).
Cold upsetting (extrusion) has many advantages, it is suitable for the mass production of fasteners. Its main advantages are as follows:

  • High utilization rate of steel: cold upsetting (extrusion) is a kind of less cutting processing method, such as machining hexagon head bolt and cylindrical head hexagon screw of rod type. Using cutting method, the utilization rate of steel is only 25% – 35%, while the utilization rate of cold heading (extrusion) method can be as high as 85% – 95%, which is only the process consumption of head, tail and cutting hexagon head edge.
  • High productivity: compared with general cutting, the efficiency of cold upsetting (extrusion) is more than ten times higher.
  • Good mechanical properties: cold upsetting (extrusion) process parts, because the metal fiber is not cut off, so the strength is much better than cutting processing.
  • Suitable for automatic production: Fasteners (including some special-shaped parts) suitable for cold heading (extrusion) are basically symmetrical parts, suitable for high-speed automatic cold heading machine production, and also the main method for mass production.

In a word, cold upsetting (extrusion) is a processing method with high comprehensive economic benefits, which is widely used in the fastener industry and an advanced processing method widely used and developed at home and abroad.

Process of cold heading

Generally speaking, cold forging is to obtain the final shape of the part through the combination of various processes. Figure 2 is an example of cold forging. After the blank is cut off, the shaft is extruded forward, the cup tube is backward extruded, the cup tube is extruded forward, upsetting, punching and tube forward extrusion are formed.
It is divided into multiple processes to avoid excessive pressure during one-time forming. Because the less the process, the lower the cost. Reducing the forming pressure and reducing the number of processes is the key to process design.

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Fig. 1 example of cold forging process

Summary of main processing methods of cold forging


Fig. 2A shows the free upsetting of the outer surface without die constraint. The processing pressure increases with friction constraint. When the billet height h is greater than the diameter d0 (H / d0 > 1.0), C = 1.2, but when the billet becomes thinner, C will rise to about 2.5.
When the reduction rate increases, as shown in Fig. 3, cracks will appear in the oblique direction and longitudinal direction of the peripheral surface. The occurrence of crack depends on the ductility of the material, so it is necessary to use the material specially made for cold forging. When the reduction rate increases, the size of constraint coefficient for various upsetting is given in Fig. 2.

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Fig. 2 constraint coefficient during upsetting

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Fig. 3 upsetting crack
In free upsetting, when the initial height of the blank is more than 2 times of the diameter, as shown in Fig. 4, the bending of the blank is caused by the instability of the material, thus forming a folding defect. In order to prevent the instability of the material, the die with the shape of Fig. 5 is usually used for the preparatory forming.

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Figure 4 instability

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Fig. 5 Preparation upsetting to prevent instability

Semi closed forging

As shown in Fig. 6, semi closed forging is a method to increase the pressure in the die cavity and promote the material filling by producing flash. When the flash is compressed, the constraint coefficient C will increase to 6.0 ~ 9.0, and the thickness of the flash should be controlled above the necessary thickness. Figure 7 is an example of cold forging using semi closed forging.

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Figure 6 final stage of flash forging

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Figure 7 semi closed forging products

Extrusion of shaft rod

Shaft bar extrusion is a processing method to reduce the diameter of material, which is usually called forward extrusion. The extrusion of the shaft and rod can be divided into the in die constrained extrusion in which the blank is put into the die as shown in Fig. 8, and the free extrusion as shown in Fig. 12b. Free extrusion is applied to the forming with small machining degree.

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(a) In die extrusion                           (b) free extrusion
Figure 8 shaft bar forward extrusion
As shown in Figure 9, internal cracking is easy to occur. The material flow in the last stage of shaft rod extrusion is unsteady, as shown in FIG. 10, which is easy to produce central cavity or crack.

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Fig. 9 core extrusion cracking

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Fig. 10 unsteady extrusion defect

Cup barrel extrusion

Cup barrel extrusion is the most commonly used method in cold forging, when the outer diameter of the blank is constrained by the die, and the punch is squeezed into the material to form a cylinder part with bottom.

Generally, the material flow direction is opposite to the movement direction of the punch, so it is called back extrusion. However, there are forward extrusion methods in which the punch does not move to form cylinder parts by extruding the material. As shown in Figure 11:

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Figure 11 back extrusion of cup shell

Cold forging method of compound drive

Generally speaking, cold forging requires multiple processes, which is mainly due to the excessive pressure of the die when one process is used. High die surface pressure will not only cause the strength damage of the die, but also cause the elastic deformation of the die, resulting in the reduction of the precision of the forging products.
Recently developed gear cold forging process, if using the usual forging method, requires a high forming pressure to complete the filling of the tooth profile. In order to forge gears with as few working procedures as possible, it is necessary to use the closed forging or split forging method with compound motion function.

Block forging

Figure 12 shows the principle of closed forging and bevel gear forging. The blank is put into the cavity formed by the upper and lower dies, and the material is compressed by the upper and lower punches.
The contact area between the material and the punch is almost unchanged, and the material is extruded to the radius direction. Compared with the compression flash in semi closed forging, the forming force is large.
In addition to the motion and clamping force of the upper and lower punches, a specially designed die base device is needed to use this method. Through the closed forging method, the bevel gear and constant velocity universal joint are successfully produced.

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Fig. 12 principle of closed forging and forging of bevel gear

Split forging

The principle of split flow forging is to design space for material flow in both the main direction and the opposite direction of material flow, so as to reduce the forging pressure.
In the case of reverse extrusion as shown in Fig. 13a, the method of designing an extrusion outlet in the front is called the method of discarding the shaft. Figure 163 is to flow the material to the outside of the tooth shaped part, and at the same time, a cavity is designed inside the material to make the material flow inward at the same time, which is the so-called hole setting method.

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(a) Shaft method        (b) hole setting method

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    (a) Convex shaft forming   (b) split flow forging
Fig. 13 helical gear formed by split forging
This method is applied to gear forging by using compound action die. According to the characteristics of material flow, this method is called split flow forging method. In recent years, the precision forging of helical gear has been successfully developed by using this method and the compound motion of the die.

Source: China Fasteners Supplier:

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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