Sheet metal processing technology is constantly improving, especially in some precision stainless steel bending, stainless steel decorative parts bending, aluminum alloy bending, aircraft parts bending, copper plate bending and other applications. Further requirements are imposed on the surface quality of the formed workpiece.
The traditional bending process is more likely to cause damage to the surface of the workpiece, and a significant indentation or scratch will be formed on the surface in contact with the mold, which will affect the aesthetics of the final product and reduce the user’s value judgment on the product.
Reasons for bending marks
This article takes bending a V-shaped part as an example to discuss.
Sheet metal bending is the forming process of a metal sheet under the pressure of a punch or a matrix of press brake bending machine, first undergoing elastic deformation and then entering plastic deformation.
At the beginning of plastic bending, the sheet is free to bend. With the pressure from the punch or matrix on the sheet, the sheet and the inner surface of the V-groove of the matrix are gradually closer, meanwhile, the radius of curvature and the bending force arm are gradually reduced. Continue to pressurize until the end of the stroke, so that the matrix and the plate are in full contact at three points, and a V-shape bending is completed at this time.
During bending, since the metal sheet is elastically deformed by the pressing of the bending die, the contact point between the sheet and the matrix will slip as the bending process proceeds.
During the bending process, the sheet metal undergoes two distinct stages of elastic deformation and plastic deformation.
During the bending process, there will be a process of holding pressure (the mold and the sheet contact three points), so after the bending process is completed, three indentation lines will be formed.
These indentation lines are generally generated by the plate and shoulder of the V-groove of the matrix, so they are called shoulder marks, as shown in Figures 1 and 2.
The main reasons for the formation of shoulder marks can be simply classified into the following.
Figure 1 Schematic diagram of the bending process
Figure 2 Bending marks
Since it was said that the marks on the shoulder is related to the contact between the sheet and the V-groove shoulder of the matrix, then, during the bending process, the difference in the gap between the punch and the matrix will affect the compressive stress on the sheet, and the probability and degree of indentation will be different, as shown in Figure 3.
Under the same V-groove condition, the larger the bending angle of the bent workpiece, the larger the deformation amount of the metal sheet being stretched, and the longer the friction distance of the metal sheet at the V-groove shoulder.
Furthermore, the larger the bending angle, the longer the holding time of the punch for applying bending force on the sheet, and the more obvious the marks caused by the combination of these two factors.
The structure of the matrix V-groove
When bending metal sheets of different thicknesses, the V-groove widths chosen are also different.
Under the same conditions of the punch, the larger the size of the V-groove of the matrix, the larger the size of the indentation width. Correspondingly, the smaller the friction between the metal sheet and the shoulder of the V-groove of the matrix is, the more the indentation depth naturally decreases.
Conversely, the thinner the plate thickness, the narrower the V-groove, and the more obvious the marks.
When it comes to friction, another friction-related factor we consider is the coefficient of friction.
The radius of the V-groove shoulder of the matrix are different, and the friction caused to the board during the bending process of the board is also different.
On the other hand, from the perspective of the pressure exerted on the sheet by the V-groove of the matrix, the larger the radius of the matrix V-groove, the smaller the pressure between the sheet and the shoulder of the V groove of the die, and the slighter the bending marks.
Lubrication degree of the matrix V-groove
It was mentioned earlier that the surface of the V-groove of the matrix will contact the sheet material and cause friction.
When the mold is worn, the contact between the V groove and the sheet will become rougher and the friction coefficient will become larger and larger.
When the sheet slides on the surface of the V-groove, the contact between the V-groove and the sheet is actually the contact of countless rough bumps with the surface, so that the pressure on the surface of the sheet will increase accordingly, and the more obvious the indentation.
On the other hand, if the V-groove 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 pressing the sheet.
This situation usually occurs when the equipment bends workpieces such as galvanized sheet and carbon steel sheet.
Application of Mark Free Bending Technology
Now, it is known that the main reason for the occurrence of bending marks is the friction between the sheet and the V-groove shoulder of the matrix, then based on the cause-oriented thinking, we can use processing technology to reduce the friction between the sheet and the V-groove shoulder.
According to the formula of the friction force, F=μ×Fn, it can be known that the factors affect the friction force are the friction coefficient μ and the pressure Fn, and both are proportional to the friction force.
Correspondingly, the following 4 mark free bending solutions can be used.
1. The non-metallic material used on the shoulder of the matrix V groove
Figure 3 Bending type
The traditional method of merely increasing the radius of the V-groove shoulder is not effective in improving the bending indentation.
From the perspective of reducing the pressure in the friction pair, it can be considered to change the V-groove shoulder to a non-metal material that is softer than the sheet material, such as nylon, urethane (PU elastomer) and other materials, under the premise of ensuring the required extrusion effect.
Considering that these materials are easy to wear, they need to be replaced regularly.
At present, there are several types of V-groove structures that use these materials, as shown in Figure 4.
Figure 4 Schematic diagram of non-metallic V-groove structure
2. Change the matrix V-groove shoulder into ball and roller
Also based on the principle of reducing the friction coefficient of the friction pair between the sheet and the V groove, the sliding friction pair of the sheet and the shoulder of the V groove can be transformed into a rolling friction pair, thereby greatly reducing the friction force to which the sheet is subjected.
This can effectively avoid bending indentation.
At present, this process has been widely used in the mold industry, and the ball mark free bending mold (Figure 5) is a typical application example.
Figure 5 Ball mark free bending mold
In order to avoid rigid friction between the roller of the ball mark-free bending mold and the V groove, and at the same time, to make the roller easier to rotate and lubricate, balls are added to achieve the effect of reducing pressure and friction coefficient.
Therefore, the parts processed by the ball mark-free bending mold can basically achieve no visible marks, but the mark free bending effect of soft plates such as aluminum and copper is not good.
From an economic point of view, because the structure of the ball mark-free bending mold is more complex than the previous mold structures, the processing cost is high, and the maintenance is difficult, which is also a factor that needs to be considered by the enterprise manager when selecting the toolings.
3. Change the matrix V-groove shoulder into flip structure
Figure 6 Schematic diagram of the inverted V-groove structure
At present, there is another type of mold in the industry that uses the principle of fulcrum rotation to achieve part bending by turning the shoulder of the matrix.
This type of mold changes the traditional shaped V-groove structure, turning the inclined surfaces on both sides of the V-groove into reversible structure.
During the pressing of the sheet by the punch, the turning mechanism on both sides of the punch is turned inward from the top of the punch by the pressure of the punch, so that the sheet is bent and formed, as shown in Figure 6.
Under this working condition, there’s no obvious local sliding friction between the sheet and the matrix, but close to the turning plane and move closer to the vertex of the punch to avoid marks of parts.
The structure of this kind of mold is more complicated than the previous ones, with tension spring and flip plate structure, the maintenance cost and processing cost are higher.
4. Isolation between matrix V groove and sheet
The aforementioned methods are all used to achieve the mark free bending by changing the bending molds.
For enterprise managers, it is not advisable to develop and purchase a new set of molds to achieve the mark free bending of individual parts.
From the point of view of frictional contact, as long as the mold is separated from the sheet, friction does not exist.
Therefore, under the premise of not changing the bending die, a mark free bending can be achieved by using a soft film to prevent contact between the V-groove and the sheet.
This soft film is also called mark free bending film, and the materials are generally rubber, PVC (polyvinyl chloride), PE (polyethylene), and PU (polyurethane).
The advantages of rubber and PVC are lower raw material costs, but the disadvantages are non-pressure resistance, poor protection performance, and short service life.
PE and PU are excellent engineering materials, and the non-marking bending and pressing film produced by using them as the substrate has good tear resistance, so it has a long service life and good protection.
The bending protection film mainly acts as a buffer between the workpiece and the shoulder of the die, offsetting the pressure between the mold and the sheet, thereby preventing the workpiece from being scratched during bending.
When in use, the bending film can be placed on the matrix, which has the advantages of low cost and convenient use.
At present, the thickness of the traceless bending film on the market is generally 0.5mm, and the size can be customized according to needs.
The mark free bending film can generally reach the service life of about 200 bending times under the condition of 2t pressure, and has strong abrasion resistance, strong tear resistance, excellent bending performance, high tensile strength and elongation at break, resistance to lubricants and aliphatic hydrocarbon solvents.
The foregoing introduces several process methods for achieving mark free bending, and the following comparison of these process methods is shown in Table 1.
Table 1 Comparison of mark free bending process
Table 1 Comparison of mark free bending process
|Contrast item||Nylon type V groove||Urethane V-groove||Roller V groove||Flip type groove||Mark free bending foil|
|Bending angle||Various angles||Arc||Various angles||Commonly used at right angles||Various angles|
|Applicable sheet||Various plates||Various plates||Non-aluminum copper and other soft plates||Various plates||Various plates|
|Service life||15 ~ 200,000 times||10 ~ 150,000 times||/||/||200 times|
|Replacement maintenance||Replace nylon||Replacement of PU block||Replace the roller||Integral replacement or replace accessories such as extension springs||Integral replacement|
|Advantage||Low cost, suitable for non-marking bending of various plates, using the same method as the lower die of standard bending machine||Low cost, suitable for mark free bending of various plates||Long life||Suitable for mark free bending of various plates||Low cost, using the same method as standard die|
|Limitation||Shorter life than standard molds, segment size is limited to above 50mm||At present, it is only applicable to the mark free bending of arc products.||The cost is high, and the effect of mark free bending on soft plates such as aluminum and copper is not good. Because the friction and deformation of the ball are difficult to control, marks may also be produced on other rigid plates, and the length and slot are more restrictive.||Expensive, small scope, limited length and V opening||Shorter life than other solutions, frequent replacement affects production efficiency, and the cost increases significantly when used in large quantities|
The market competition for sheet metal processing industry is very fierce, and if companies want to occupy a place in the market, they need to continuously improve their processing technology. Not only to achieve the functionality of the product, consider the processability and aesthetics of the product, but also to consider the processing economy, by applying more efficient and economical process methods to make the product easier to process, more economical and more beautiful.