May 02, 2019
Traceless Bending Technology of Sheet Metal
Sheet metal processing technology continues to improve, especially in some applications such as precision stainless steel bending, stainless steel decorative parts bending, aluminum alloy bending, aircraft parts bending, copper plate bending and so on, further put forward higher requirements for the surface quality of the formed workpiece. The traditional bending process is easy to cause damage to the surface of the workpiece, and the surface contacted with the die will form an obvious indentation or scratch, which will affect the aesthetics of the final product and reduce the value judgement of the product by users.
Reasons for Bending Indentation:
This article takes the example of bending a V-shaped part as an example. Sheet metal bending is a process in which a metal sheet is first elastically deformed under the pressure of a punch or a die of a bending machine, and then enters a plastic deformation process. At the beginning of the plastic bending, the sheet is free to bend. As the punch or the die presses the sheet, the inner surface of the sheet and the V-groove of the die gradually abuts, and the radius of curvature and the bending arm are gradually reduced, and the pressing is continued until the stroke ends, so that the die is made. Close contact with the plate at three points, at this point a V-bend is completed. When bending, the metal sheet is elastically deformed by being pressed by the bending die, and the contact point between the sheet and the die is slipped as the bending process proceeds. During the bending process, the sheet material undergoes two distinct stages of elastic deformation and plastic deformation. In the bending process, there is a pressure maintaining process (three points of contact between the mold and the sheet), so after the bending process is completed, Three indentation lines are formed. These indentation lines are generally produced by the friction between the sheet and the shoulder of the die V groove, so it is called the shoulder indentation.
Since the former said that the generation of shoulder indentation is related to the contact between the sheet metal and the v-groove shoulder of the die, in the bending process, the gap between the punch and the die affects the compressive stress on the plate, and the probability and degree of indentation will be different, as shown in figure 3. Under the condition of the same V groove, the greater the bending Angle of the bending workpiece is, the greater the stretching shape variable of the sheet metal will be, and the longer the friction distance of the sheet metal at the shoulder of the v-groove will be. In addition, the larger the bending Angle, the punch on the plate pressure will remain longer, the combination of the two factors caused by the indentation will be more obvious.
V groove structure of die
When bending the sheet metal with different thickness, the V groove width selected is also different. With the same punch, the larger the size of the V groove of the die, the larger the size of the indentation width. Correspondingly, the friction between the metal plate and the V groove shoulder of the die is smaller, and the indentation depth is naturally reduced. On the contrary, the thinner the plate is, the narrower the V groove is, and the more obvious the indentation is.
Now, when we talk about friction, the other thing that we think about in terms of friction is the coefficient of friction. The R Angle of V groove shoulder of die is different, and the friction is different during bending forming. On the other hand, it is the same from the perspective of the pressure exerted on the plate by V groove of the die. The larger R Angle of V groove of the die is, the smaller the pressure on the shoulder of V groove of the plate and the die will be, and vice versa.
Degree of lubrication in V groove of die
As mentioned above, the surface of the V groove of the die will come into contact with the sheet metal and cause friction. When the mold appears wear, the V groove and the contact part of the plate will become more and more rough, and the friction coefficient will become larger and larger. When the plate is sliding on the surface of the V groove, the contact between the V groove and the plate is actually countless rough convex points contacting the surface, so the pressure acting on the surface of the plate will increase correspondingly, and the indentation will be more obvious. On the other hand, the workpiece bending did not wipe the V groove of the die clean, often because of the residual debris on the V groove on the plate extrusion and produce obvious indentation, this situation is usually in the equipment bending galvanized plate, carbon steel plate and other workpieces.
Application of traceless bending technology
Since it is known that the main cause of flexure indentation is the friction between the plate and v-groove shoulder of the die, it is possible to reduce the friction between the plate and v-groove shoulder of the die through technology based on the cause-oriented thinking. According to friction formula f= ·N, the factors affecting friction include friction coefficient ·N and pressure N, which are proportional to friction. Correspondingly, the following process plans can be developed.
The v-groove shoulder of the die is made of non-metallic materials
Simply by increasing the R Angle of the v-groove shoulder of the mold, the traditional approach to improve the bending indentation effect is not large. From the perspective of reducing the pressure in the friction pair, the v-groove shoulder can be changed into a softer non-metallic material, such as nylon and PU elastomer, on the premise of ensuring the original extrusion effect.
Die V groove shoulder ball, roller structure
Also based on the principle of reducing the friction coefficient of the friction pair between the plate and the v-groove of the die, the sliding friction pair on the shoulder of the plate and the v-groove of the die can be transformed into a rolling friction pair, thus greatly reducing the friction force on the plate and effectively avoiding the occurrence of bending indentation. At present, this kind of technology has been widely used in the mold industry.
At present, there is a mold in the industry is the use of pivot rotation principle through the concave die shoulder to achieve bending parts. In this mold, the traditional V groove structure of the stereotypes die is changed, and the inclined planes on both sides of the V groove are set into a reversible mechanism. In the process of pressing the sheet metal under the punch, with the help of the pressure of the punch, the reversible mechanism on both sides of the punch is turned inward from the top of the punch, so that the sheet metal is bent into shape
The V groove of the die is isolated from the plate
The above mentioned is through the transformation of bending mold way to achieve traceless bending, for enterprise managers, in order to achieve the traceless bending of individual parts and the development and purchase of a new set of mold practice is not desirable. From the point of view of friction contact, as long as the mold and plate separated, friction will not exist. Therefore, on the premise of not changing the bending die, the indentation bending can be achieved by using soft film to make the v-groove of the die and the plate do not come into contact with each other. This kind of soft film is also called flexure indentation film, material is generally rubber, PVC (polyvinyl chloride), PE (polyethylene), PU (polyurethane) and so on. The advantage of rubber and PVC is low cost of raw materials, disadvantages are not pressure, poor protection performance, short life; PE and PU, as engineering materials with excellent performance, are used as the base material to produce traceless bending film with good tear resistance performance, so it has a higher life and better protection.
Bending protective film is mainly in the workpiece and die shoulder between the buffer, to offset the pressure between the mold and the plate, so as to prevent the workpiece in bending indentation, as long as the bending film on the die can be used, with low cost, convenient to use the advantages. At present, the general thickness of the bending indentation pressure film on the market is 0.5mm, the size can be customized according to the needs. In general, the non-indent bending film can reach the service life of about 200 bending times 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 solvent.