Talking about the seamless bending technology of sheet metal

The sheet metal processing technology is constantly improving, especially in some applications such as precision stainless steel bending, stainless steel decorative parts bending, aluminum alloy bending, aircraft parts bending, copper plate bending, etc., further improving the surface quality of formed workpieces. High demands.

The traditional bending process is easy to cause damage to the surface of the workpiece, and an obvious indentation or scratch is formed on the surface in contact with the mold, which affects the aesthetics of the final product and reduces the user's value judgment on the product.

Bending method

Since it is said that the generation of shoulder indentation is related to the contact between the sheet material and the V-groove shoulder of the die, then during the bending process, the difference in the gap between the punch and the die affects the compressive stress on the plate, resulting in indentation The probability and degree will also be different, as shown in Figure 3. Under the same V-groove condition, the greater the bending angle of the bent workpiece, the greater the amount of deformation of the metal sheet being stretched, and the longer the friction distance of the metal sheet at the shoulder of the V-groove; moreover, the bending angle The larger the punch, the longer the holding time of the pressure applied by the punch to the sheet, and the more obvious the indentation caused by the combination of these two factors.

The structure of the concave die V groove

When bending metal sheets of different thicknesses, the selected V groove width is different. Under the same condition of the punch, the larger the size of the die V groove, the larger the size of the indentation width. Correspondingly, the friction between the metal sheet and the shoulder of the die V groove is also smaller, and the indentation depth will naturally decrease. On the contrary, the thinner the plate thickness, the narrower the V groove, and the more obvious the indentation.

Lubrication degree of V groove

As mentioned earlier, the surface of the V-groove of the die and the sheet metal will contact each other and cause friction. When the mold wears out, the contact part between the V groove and the sheet material will become rougher and the friction coefficient will become larger and larger. When the sheet material slips on the surface of the V groove, the contact between the V groove and the sheet material is the point contact of countless rough bumps and the surface, so that the pressure on the surface of the sheet material will increase correspondingly, and the indentation will be greater. obvious. On the other hand, if the V groove of the die is not wiped and cleaned before the workpiece is bent, it will often cause obvious indentation due to the residual debris on the V groove squeezing the sheet. In this case, the equipment is usually bent and galvanized It appears when workpieces such as steel plates and carbon steel plates.

Application of non-mark bending technology

Now that it is known that the main reason for the bending indentation is the friction between the sheet and the V-groove shoulder of the die, we can start from reason-oriented thinking and reduce the friction between the sheet and the V-groove shoulder of the die through process technology force. According to the friction force formula f=μ·N, the factors that affect the friction force are the friction coefficient μ and the pressure N, and they are all proportional to the friction force. Accordingly, the following process plans can be formulated.

Non-metallic materials are used for the V-groove shoulder of the die

The traditional method of simply increasing the R angle of the V-groove shoulder of the mold is not effective in improving the bending impression. From the perspective of reducing the pressure in the friction pair, it can be considered to change the V-groove shoulder to a non-metallic material that is softer than the sheet material, such as nylon, unique, etc., while ensuring the originally required extrusion effect.  Considering that these materials are easy to wear and need to be replaced regularly, there are several types of V-groove structures currently using these materials, as shown in Figure 4.

The V-groove shoulder of the die is changed to the ball and roller structure

Also based on the principle of reducing the friction coefficient of the friction pair between the sheet and the die V-groove, the sliding friction pair between the sheet and the V-groove shoulder of the die can be converted into a rolling friction pair, thereby greatly reducing the friction force on the sheet. Effectively avoid bending and indentation. At present, this process has been widely used in the mold industry, and the ball non-mark bending mold is a typical application example.

To avoid rigid friction between the roller and the V groove, the ball is added to make the roller easier to rotate and lubricate, to reduce the pressure and reduce the coefficient of friction at the same time, so the ball is bent without marks The parts processed by the mold can achieve no visible indentation, but the non-marking bending effect of soft plates such as aluminum and copper is not good. From an economic point of view, because the ball non-marking bending mold structure is more complex than the aforementioned several mold structures, the processing cost is high, and the maintenance is difficult. This is also a factor that needs to be considered when selecting enterprise managers.

The V-groove shoulder of the die is changed to a flip structure

At present, there is another kind of mold in the industry that uses the principle of pivot rotation to realize the bending of parts by turning the shoulder of the die. This kind of mold changes the V-groove structure of the traditional shaping concave mold and sets the inclined surfaces on both sides of the V-groove is a reversible mechanism. During the process of pressing the sheet under the punch, the reversing mechanism on both sides of the concave mold is replaced The apex of the punch is turned inward so that the sheet is bent into shape, as shown in Figure 6. Under this working condition, the sheet material and the concave mold do not produce obvious local sliding friction, but close to the apex of the convex mold against the turning plane to avoid indentation of the parts. The structure of this mold is more complicated than the previous ones, with tension spring and a flip plate structure, and the maintenance cost and processing cost are greater.

The V groove of the die is isolated from the sheet material

From the perspective of frictional contact, as long as the mold and the sheet are separated, friction does not exist. Therefore, under the premise of not changing the bending mold, the non-marking bending can be achieved by using a soft film to prevent contact between the cavity V groove and the sheet material. This kind of soft film is also called bending non-marking film, and its materials are generally rubber, PVC (polyvinyl chloride), PE (polyethylene), PU (polyurethane), etc. The advantages of rubber and PVC are that the cost of raw materials is low, but the disadvantages are that they are not pressure resistant, poor protection performance, and short life; PE and PU are excellent engineering materials, and the non-marking bending film produced with them as the base material has good Tearing resistance, so it has a longer life and better protection.

The bending protective film mainly acts as a buffer between the workpiece and the shoulder of the die, offsetting the pressure between the die and the sheet, to prevent the workpiece from producing indentation during bending. When using, just put the bending film on It only needs to be on the concave mold, which has the advantages of low cost and convenient use.