Abstract: in the process of sheet metal bending, the traditional bending process is easy to damage the workpiece surface, and the surface in contact with the die will form obvious indentation or scratch, which will affect the beauty of the product. This paper will detail the causes of bending indentation and the application of traceless bending technology.
The sheet metal processing technology keeps improving, especially in some applications such as precision stainless steel bending, stainless steel trim bending, aluminum alloy bending, aircraft parts bending and copper plate bending, which further puts forward higher requirements for the surface quality of formed workpieces.
The traditional bending process is easy to damage the surface of the workpiece, and an obvious indentation or scratch will be formed on the surface in contact with the die, which will affect the beauty of the final product and reduce the user's value judgment of the product.
During bending, because the metal sheet will be extruded by the bending die and produce elastic deformation, the contact point between the sheet and the die will slip with the progress of the bending process. In the bending process, the sheet metal will experience two obvious stages of elastic deformation and plastic deformation. In the bending process, there will be a pressure maintaining process (three-point contact between the die and the sheet metal). Therefore, after the bending process is completed, three indentation lines will be formed.
These indentation lines are generally produced by the extrusion friction between the plate and the V-groove shoulder of the die, so they are called shoulder indentation. As shown in Figure 1 and Figure 2, the main reasons for the formation of shoulder indentation can be simply classified into the following categories.
1. Bending method
Since the generation of shoulder indentation is related to the contact between the sheet metal and the V-groove shoulder of the female die, in the bending process, the gap between the punch and the female die will affect the compressive stress of the sheet metal, and the probability and degree of indentation will be different, as shown in Figure 3.
Under the condition of the same V-groove, the larger the bending angle of the bending workpiece, the larger the shape variable 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 larger the bending angle is, the longer the holding time of the pressure exerted by the punch on the sheet will be, and the more obvious the indentation caused by the combination of these two factors.
2. Structure of V-groove of female die
When bending metal sheets with different thickness, the V-groove width is also different. Under the condition of the same punch, the larger the size of the V-groove of the die, the larger the size of the indentation width. Accordingly, the smaller the friction between the metal sheet and the shoulder of the V-groove of the die, and the indentation depth naturally decreases. On the contrary, the thinner the plate thickness, the narrower the V-groove, and the more obvious the indentation.
When it comes to friction, another factor related to friction that we consider is the friction coefficient. The R angle of the shoulder of the V-groove of the female die is different, and the friction caused to the sheet metal in the process of sheet metal bending is also different. On the other hand, from the perspective of the pressure exerted by the V-groove of the die on the sheet, the larger the R-angle of the V-groove of the die, the smaller the pressure between the sheet and the shoulder of the V-groove of the die, and the lighter the indentation, and vice versa.
3. Lubrication degree of V-groove of female die
As mentioned earlier, the surface of the V-groove of the die will contact with the sheet to produce friction. When the die is worn, the contact part between V-groove and sheet metal will become rougher and rougher, and the friction coefficient will become larger and larger. When the sheet metal slides on the surface of the V-groove, the contact between the V-groove and the sheet metal is actually the point contact between countless rough bumps and surfaces. In this way, the pressure acting on the surface of the sheet metal will increase accordingly, and the indentation will be more obvious.
On the other hand, the V-groove of the female die is not wiped and cleaned before the workpiece is bent, which often produces obvious indentation due to the extrusion of the plate by the residual debris on the V-groove. This situation usually occurs when the equipment bends the workpieces such as galvanized plate and carbon steel plate.
2、 Application of traceless bending technology
Since we know that the main cause of bending indentation is the friction between the sheet metal and the shoulder of the V-groove of the die, we can start from the reason oriented thinking and reduce the friction between the sheet metal and the shoulder of the V-groove of the die through process technology.
According to the friction formula F= μ· N it can be seen that the factor affecting the friction force is the friction coefficient μ And pressure n, and they are directly proportional to friction. Accordingly, the following process schemes can be formulated.
Figure 3 bending type
Only by increasing the R angle of the V-groove shoulder of the die, the traditional method to improve the bending indentation effect is not great. From the perspective of reducing the pressure in the friction pair, it can be considered to change the V-groove shoulder into a non-metallic material softer than the plate, such as nylon, Youli glue (PU elastomer) and other materials, on the premise of ensuring the original extrusion effect. Considering that these materials are easy to lose and need to be replaced regularly, there are several V-groove structures using these materials at present, as shown in Figure
2. The shoulder of V-groove of female die is changed into ball and roller structure
Similarly, based on the principle of reducing the friction coefficient between the sheet and the V-groove of the die, the sliding friction between the sheet and the shoulder of the V-groove of the die can be transformed into rolling friction, so as to greatly reduce the friction of the sheet and effectively avoid bending indentation. At present, this process has been widely used in the die industry, and the ball traceless bending die (Fig. 5) is a typical application example.
Fig. 5 ball traceless bending die
In order to avoid rigid friction between the roller of the ball traceless bending die and the V-groove, and also to make the roller easier to rotate and lubricate, the ball is added, so as to reduce the pressure and reduce the friction coefficient at the same time. Therefore, the parts processed by the ball traceless bending die can basically achieve no visible indentation, but the traceless bending effect of soft plates such as aluminum and copper is not good.
From the perspective of economy, because the structure of the ball traceless bending die is more complex than the above-mentioned die structures, the processing cost is high and the maintenance is difficult, which is also a factor to be considered by enterprise managers when selecting.
6 structural diagram of inverted V-groove
At present, there is another kind of mold in the industry, which uses the fulcrum rotation principle to realize the bending of parts by turning the shoulder of the female mold. This kind of die changes the traditional V-groove structure of the setting die, and sets the inclined planes on both sides of the V-groove as a turnover mechanism. In the process of pressing the material under the punch, the turnover mechanism on both sides of the punch is turned inward from the top of the punch with the help of the pressure of the punch, so as to bend the plate, as shown in Fig. 6.
Under this working condition, there is no obvious local sliding friction between the sheet metal and the die, but close to the turning plane and close to the vertex of the punch to avoid indentation of the parts. The structure of this die is more complex than the previous structures, with tension spring and turnover plate structure, and the maintenance cost and processing cost are greater.
Several process methods for realizing traceless bending have been introduced earlier. The following is a comparison of these process methods, as shown in Table 1.
|Comparison item||Nylon V-groove||Youli rubber V-groove||Ball type V-groove||Inverted V-groove||Traceless Pressure film|
|Bending angle||Various angles||arc||Various angles||Often used at right angles||Various angles|
|Applicable plate||Various plates||Various plates||Various plates||Various plates|
|service life||15-20 Ten thousand times||15-21 Ten thousand times||/||/||200 times|
|Replacement maintenance||Replace nylon core||Replace the Youli rubber core||Replace the ball||Replace as a whole or replace the tension spring and other accessories||Replace as a whole|
|advantage||Low cost and is suitable for traceless bending of various plates. The use method is equal to the lower die of standard bending machine.||Low cost and is suitable for traceless bending of various plates.||Longer service life||It is applicable to a variety of plates with good effect.||Low cost and is suitable for traceless bending of various plates. The use method is equal to the lower die of standard bending machine.|
|limitations||service life is shorter than standard die, and the segment size is limited to more than 50mm.||At present, it is only applicable to the traceless bending of circular arc products.||The cost is expensive and the effect on soft materials such as aluminum and copper is not good. Because the ball friction and deformation are difficult to control, traces may also be produced on other hard plates. There are many restrictions on length and notch.||The cost is expensive, the scope of application is small, and the length and notch are restrictive||The service life is shorter than other schemes, the frequent replacement affects the production efficiency, and the cost increases significantly when used in large quantities.|
Table 1 Comparison of traceless bending processes
4. The V-groove of the die is isolated from the sheet metal (this method is recommended)
The above mentioned methods are to realize traceless bending by changing the bending die. For enterprise managers, it is not advisable to develop and purchase a set of new dies to realize traceless bending of individual parts. From the point of view of friction contact, friction does not exist as long as the die and sheet are separated.
Therefore, on the premise of not changing the bending die, traceless bending can be realized by using a soft film so that there is no contact between the V-groove of the die and the sheet metal. This kind of soft film is also called bending indentation free film. The materials are generally rubber, PVC (polyvinyl chloride), PE (polyethylene), PU (polyurethane), etc.
The advantages of rubber and PVC are low cost of raw materials, while the disadvantages are no pressure resistance, poor protection performance and short service life; PE and Pu are engineering materials with excellent performance. The traceless bending and pressing film produced with them as the base material has good tear resistance, so it has high service life and good protection.
The bending protective film mainly plays a buffer role between the workpiece and the shoulder of the die to offset the pressure between the die and the sheet metal, so as to prevent the indentation of the workpiece during bending. When in use, just put the bending film on the die, which has the advantages of low cost and convenient use.
At present, the thickness of bending non marking indentation film on the market is generally 0.5mm, and the size can be customized according to needs. Generally, the bending traceless indentation film can reach the service life of about 200 bends under the working condition of 2T pressure, and has the characteristics of strong wear resistance, strong tear resistance, excellent bending performance, high tensile strength and elongation at break, resistance to lubricating oil and aliphatic hydrocarbon solvents.
The market competition of sheet metal processing industry is very fierce. If enterprises want to occupy a place in the market, they need to constantly improve the processing technology. We should not only realize the functionality of the product, but also consider the manufacturability and aesthetics of the product, but also consider the processing economy. Through the application of more efficient and economical technology, the product is easier to process, more economical and more beautiful. (selected from sheet metal and manufacturing, issue 7, 2018, by Chen Chongnan)
Post time: Feb-26-2022