Forming process of fasteners

In the forming process of fasteners, cold heading (extrusion) technology is a main processing technology. Cold heading (extrusion) belongs to the category of metal pressure processing. In production, under normal temperature, external force is applied to the metal to form the metal in the predetermined die. This method is usually called cold heading. Today, let’s have a comprehensive understanding of the cold heading process of fasteners.

Definition of cold forging

Cold forging, also known as cold volume forming, is a manufacturing process and a processing method. Basically the same as the stamping process, the cold forging process is also composed of three elements: material, die and equipment. Only the material in stamping processing is mainly plate, while the material in cold forging processing is mainly disc wire. Japan (JIS) is called cold forging (cold forging for short), China (GB) is called cold heading, and screw factories outside like to call heading.

Basic concept of cold forging

Cold forging refers to various volume forming below the recrystallization temperature of metal. According to the theory of metalology, the recrystallization temperature of various metal materials is different; T re = (0.3 – 0.5) T melting. Minimum recrystallization temperature of ferrous and non-ferrous metals. Even under the condition of room temperature or normal temperature, the forming process of lead and tin can not be called cold forging, but hot forging. However, the forming of iron, copper and aluminum at room temperature can be called cold forging.
In metallurgy, the forging of materials heated above the recrystallization temperature (steel is about 700 ℃) is called hot forging.
For steel forgings, the forging below recrystallization temperature and higher than normal temperature is called warm forging.
The forming of any fastener can be realized not only by cold heading, but also by forward and backward extrusion, compound extrusion, punching, rolling and other deformation methods in addition to upsetting deformation.
Therefore, the name of cold heading in production is only a habitual name. More specifically, it should be called cold heading (extrusion).
Cold heading (extrusion) has many advantages. It is suitable for mass production of fasteners. Its main advantages are summarized as follows:

  • a. High utilization rate of steel. Cold heading (extrusion) is a kind of less and no cutting processing method, such as machining rod hexagon head bolts and cylindrical head hexagon head screws. With the cutting processing method, the steel utilization rate is only 25% ~ 35%, while with the cold heading (extrusion) method, its utilization rate can be as high as 85% ~ 95%, which is only some process consumption of material head, material tail and cutting hexagon head edge.
  • b. High productivity. Compared with general cutting, the efficiency of cold heading (extrusion) is more than dozens of times higher.
  • c. Good mechanical performance. The strength of the parts processed by cold heading (extrusion) is much better than that of cutting because the metal fiber is not cut off.
  • d. Suitable for automatic production. Fasteners suitable for cold heading (extrusion) production (including some special-shaped parts) basically belong to symmetrical parts. They are suitable for high-speed automatic cold heading machine production and are also the main method of mass production.

In a word, cold heading (extrusion) processing of fasteners and special-shaped parts is a processing method with high comprehensive economic benefits. It is not only a processing method widely used in the fastener industry, but also an advanced processing method widely used and developed at home and abroad.
Therefore, how to make full use of and improve the plasticity of metal, master the mechanism of metal plastic deformation, and develop a scientific and reasonable fastener cold heading (extrusion) processing technology is the purpose and purpose of the research.

Cold heading process

Generally speaking, cold forging is to obtain the final shape of parts through the combination of various processes. Figure 2 is an example of cold forging. After the blank is cut off, it is composed of shaft rod forward extrusion, cup barrel reverse extrusion, cup barrel forward extrusion, upsetting, punching, tube forward extrusion, etc.

It is divided into multiple processes to avoid excessive pressure during one-time forming. Because the fewer processes, the lower the cost. Reducing the forming pressure and reducing the number of processes is the key of process design.

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Figure 2 example of cold forging process

Overview of main processing methods of cold forging

Upsetting

Fig. 3a is a free upsetting in which the outer surface is not constrained by the die. The processing pressure increases with the friction constraint. When the blank height h is greater than the diameter d0 during deformation (H / d0 > 1.0), C = about 1.2, but when the blank becomes thinner, C will rise to about 2.5.
When the reduction rate increases, as shown in Figure 4, cracks will occur in the oblique and longitudinal directions of the peripheral surface. The occurrence of crack depends on the ductility of the material. Therefore, it is necessary to use the material specially made for cold forging. When the reduction rate increases, Fig. 3 shows the size of the constraint coefficient during various upsetting forming.

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Fig. 3 constraint coefficient during upsetting

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Fig. 4 upsetting crack
In free upsetting, when the initial height of the blank is more than twice the diameter, as shown in Fig. 5, the blank is bent due to material instability, resulting in folding defects. In order to prevent the instability of materials, the mold in the shape of Fig. 6 is usually used for preliminary forming.

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Figure 5 instability

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Fig. 6 preliminary upsetting to prevent instability

Semi closed forging

As shown in Figure 7, semi closed forging is a method to increase the pressure in the die cavity by generating flash and promote material filling. When the flash part 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 as much as possible. Fig. 8 is an example of cold forging using semi closed forging.

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Figure 7 final stage of flash die forging

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Fig. 8 semi closed forging products

Extrusion of shaft rod

Shaft and rod extrusion is a processing method to reduce the diameter of materials, which is usually called forward extrusion. The extrusion of shaft rod can be divided into in die constrained extrusion in which the blank is put into the die for extrusion as shown in Fig. 9 and free extrusion as shown in Fig. 13b. Free extrusion is used in forming with small machining degree.

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(a) In die extrusion

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(b) Free extrusion
Fig. 9 forward extrusion of shaft rod
As shown in Figure 10, internal cracking is easy to occur. In the final stage of shaft rod extrusion, the flow of material is in an unsteady state, as shown in Figure 11, which is easy to produce central cavity or crack.

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Figure 10 core extrusion cracking

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Figure 11 unsteady extrusion defects

Cup barrel extrusion

Cup barrel extrusion is to squeeze the punch into the material to form a cylindrical part with a bottom while the outer diameter of the blank is constrained by the die. It is the most commonly used method in cold forging.
Usually, the extrusion punch presses the material, and the material flow direction is opposite to the movement direction of the punch, so it is called back extrusion. However, there is also a forward extrusion method of forming cylindrical parts by extruding the material without moving the punch. As shown in Figure 12:

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Figure 12 cup shell back extrusion
High strength stainless steel parts (SUS630)

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Forming process

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Cold forging texture flow diagram

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Hardness test

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High voltage sensor port

Composite driven cold forging method

As shown in Figure 2, generally speaking, cold forging requires multiple processes, which is mainly due to the excessive pressure of the die when forming with one process. High die surface pressure will not only destroy the strength of the die, but also cause the elastic deformation of the die, resulting in the decline of the precision of the finished forging.
The recently developed gear cold forging process requires high forming pressure to fill the tooth profile if the usual forging method is used. In order to forge the gear with as few processes as possible, it is necessary to use the closed forging or split forging method with composite motion function.

Block forging

Figure 15 shows the principle of block forging and bevel gear forging. Put the blank into the die cavity formed by the upper and lower dies, and compress and deform the material through the upper and lower punches.
The contact area between the material and the punch remains almost unchanged, and the material is squeezed to the radius direction, which can greatly reduce the forming force compared with the compression flash in semi closed forging.
Using this method requires not only the movement of the upper and lower punches and the clamping force, but also a specially designed mold base device. The closed bevel gear is produced successfully through the forging method.

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FIG. 15 principle of block forging and forging of bevel gear

Split flow forging

The principle of split flow forging is to design a space for material flow in both the main and opposite directions of material flow, so as to reduce the forging pressure.
In the case of reverse extrusion as shown in Fig. 16a, the method of designing an extrusion outlet in the front is also called the shaft abandonment method. In Fig. 16b, the purpose is to flow the material to the tooth shaped part on the outside, and at the same time, a cavity is designed inside the material to make the material flow to the inside at the same time, which is the so-called hole setting method.

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(a) Axis method

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(b) Hole setting method

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(a) Convex shaft forming

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(b) Split forging
Figure 16 helical gear formed by split forging
This method is applied to the forging of gears by using compound action dies. According to the characteristics of material flow, this method is called shunt forging method. In recent years, the precision forging of helical gear has been successfully developed by using this method and the compound movement of die.

Processing calculation method of cold forged products

1. Single weight calculation.

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2. The calculation of model, material wire diameter and material length l shall be selected according to the product.

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3. Calculate the beam strength ratio according to the product and confirm whether the cold forging can be processed.

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4. Calculate the expansion ratio according to the product and confirm whether the head can be in place.

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5. Calculate the reduction of area according to the product and confirm whether the perforated parts can be processed.

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Source:  China Fasteners Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.com)

(Yaang Pipe Industry is a leading flange 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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