Cold forming process introduction of fasteners, see a variety of cold forging methods

01 Advantages of cold heading

High utilization rate of steel: cold heading (extrusion) is a kind of less, no cutting processing method, such as processing rods hex head bolts, cylindrical head hex screws, using the cutting method, steel utilization rate is only 25% ~ 35%, and with cold heading (extrusion) method, its utilization rate can be as high as 85% ~ 95%, only some of the process consumption of the head, the tail and the cutting edge of the hex head.

High productivity: compared with the general cutting, cold heading (extrusion) forming efficiency is dozens of times higher than.

Good mechanical properties: Parts processed by cold heading (extrusion) are much stronger than those processed by cutting because the metal fibers are not cut.

Suitable for automatic production: Fasteners suitable for cold heading (extrusion) production (including some special-shaped parts), basically belong to symmetrical parts, suitable for high-speed automatic cold heading machine production, is also the main method of mass production.

In short, cold heading (extrusion) processing of fasteners and special-shaped parts is a kind of processing method with high comprehensive economic benefits, which is widely used in the fastener industry, and is also an advanced processing method widely used at home and abroad.

02 Cold forging order

Generally speaking, cold forging is a combination of various processes to obtain the final shape of the part. Figure 2 is an example of cold forging. After the billet is cut off, the shaft rod is extruded, the cup cylinder is extruded, the cup cylinder is extruded, the upsetting, the punching, the pipe is extruded and so on.

It is divided into multiple processes to avoid excessive pressure when forming at one time. The less the process, the lower the cost, reducing the forming pressure to reduce the number of processes is the key to process design.

03 Summary of the main processing methods of cold forging

FIG. 3a shows the free upsetting of the outer side without die constraints. The processing pressure increases with the friction constraint. When the height H of the billet is greater than the diameter D0 (H/D0 > 1.0), C= about 1.2, but when the billet becomes thinner, C will rise to about 2.5.

When the reduction rate increases, as shown in FIG. 4, cracks will occur in the oblique direction and longitudinal direction of the outer surface. The occurrence of cracks depends on the ductility of the material, which requires the use of materials specifically made for cold forging. FIG. 3 shows the size of the constraint coefficients for various upsetting forms as the reduction rate increases.

In free upsetting, when the initial height of the billet is more than 2 times the diameter, as shown in FIG. 5, material instability leads to bending of the billet, thus forming folding defects. In order to prevent material instability, preforming is usually done using a mold shaped like Figure 6.

Semi-closed forging

As shown in Figure 7, semi-closed forging is one way to increase the pressure in the cavity by creating a flying edge and promoting material filling. Technology gurus are known in Chinese screw net screws. When the flying edge part is compressed, the constraint coefficient C will increase to 6.0 ~ 9.0, and the thickness of the flying edge should be controlled above the necessary thickness as far as possible. Figure 8 shows an example of cold forging using a semi-closed forging.

Extrusion of shaft rod

Shaft extrusion is a machining method to reduce the diameter of a material, usually called positive extrusion. Extrusion of shaft rod can be divided into in-die constrained extrusion, in which billet is placed into the mold for extrusion as shown in Figure 9, and free extrusion, as shown in Figure 13b. Free extrusion is used in small machining degree of forming.

As shown in Figure 10, it is easy to produce internal cracking. In the last stage of shaft extrusion, the material flow is in an unsteady state, as shown in Figure 11, which is easy to produce central cavity or crack.

Cylinder extrusion

The extrusion of the cup barrel is to press the punch into the material to form a cylinder part with bottom while the outside diameter of the blank is constrained by the die, which is the most common method in cold forging.

Usually extrusion punch into the material, the direction of flow of the material and the direction of movement of the punch is opposite, so it is called reverse extrusion, but there are also punch does not move by extrusion material forming cylinder parts of the positive extrusion method. As is shown in Figure 12,

Block forging

Figure 15 shows the principle of block forging and bevel gear forging. The billet is put into the cavity formed by the upper and lower die, and the material is compressed and deformed by the upper and lower punch.

The contact area between the material and the punch remains almost constant. The material is squeezed to the radius direction. Compared with the compression flying edge in semi-closed forging, the forming force can be greatly increased.

The use of this method requires not only the movement and die force of the upper and lower punch, but also a specially designed die holder device. By closed forging method, such as bevel gear, constant speed universal joint has been successfully produced.

Shunt forging

The principle of shunt forging is to design a 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. 16a, an extrusion outlet is designed in the front, which is called abandon axis method. In FIG. 16b, a cavity is designed in the inside of the material to make the material flow inward at the same time, which is called hole setting method.