Continuous Improvement of Sheet Metal Processing Techniques
Sheet metal processing techniques have been continuously refined, especially in applications such as precision stainless steel bending, stainless steel decorative component bending, aluminum alloy bending, aircraft component bending, and copper plate bending. These applications have raised higher demands for the surface quality of formed workpieces. Traditional bending processes often cause damage to the surface of the workpiece. Contact with the mold leaves noticeable pressure marks or scratches, which affect the aesthetics and diminish the perceived value of the final product.
Causes of Bending Pressure Marks
In this article, we will discuss the example of bending a V-shaped part. Sheet metal bending is a forming process where the metal sheet undergoes elastic deformation followed by plastic deformation under the pressure of a punch or die in a bending machine. In the initial stage of plastic bending, the sheet metal undergoes free bending. As the punch or die applies pressure to the sheet metal, it gradually comes into contact with the V-shaped groove of the die, reducing the curvature radius and the bending force arm. The pressure continues until the stroke ends, achieving a V-shaped bend with full contact between the die and the sheet metal at three points. During bending, the metal sheet undergoes elastic deformation due to the compression from the bending die, causing the contact point between the sheet and the die to shift during the bending process. The bending process involves both elastic and plastic deformation stages, as well as a holding process (full contact between the die and the sheet metal). Therefore, after the bending process is completed, three pressure lines are formed. These pressure lines are generally caused by the friction and compression between the sheet metal and the shoulder of the V-groove in the die, known as shoulder pressure marks. Refer to Figure 1 and Figure 2 for illustrations. The main reasons for the formation of shoulder pressure marks can be categorized as follows:
Bending Method
Since the formation of shoulder pressure marks is related to the contact between the sheet metal and the shoulder of the V-groove in the die, the gap between the punch and the die in the bending process affects the compressive stress on the sheet metal, leading to varying probabilities and degrees of pressure mark formation, as shown in Figure 3. Under the same V-groove conditions, as the bending angle of the workpiece increases, the amount of metal sheet stretching deformation and the friction distance on the shoulder of the V-groove also increase. Furthermore, with a larger bending angle, the pressure applied by the punch on the sheet metal lasts longer. The combination of these two factors results in more pronounced pressure marks.
Structure of the V-groove in the Die
When bending metal sheets of different thicknesses, the width of the V-groove in the die varies. Under the same punch conditions, a larger V-groove size in the die corresponds to a wider pressure mark width. Consequently, the friction force between the metal sheet and the shoulder of the V-groove in the die decreases, leading to a shallower pressure mark. Conversely, thinner sheets result in narrower V-grooves and more pronounced pressure marks.
Considering friction, another factor to consider is the coefficient of friction. The size of the R corner of the V-groove in the die affects the friction experienced by the sheet metal during the bending process. Similarly, the angle at which the V-groove in the die applies pressure to the sheet metal also plays a role. A larger R corner in the V-groove of the die results in less pressure on the sheet metal at the shoulder of the V-groove, leading to milder pressure marks, and vice versa.
Lubrication Level of the Die V-Groove
As mentioned earlier, the surface of the die V-groove comes into contact with the sheet metal, resulting in friction. When the die becomes worn, the contact area between the V-groove and the sheet metal becomes rougher, increasing the friction coefficient. When the sheet metal slides on the V-groove surface, the contact between the V-groove and the sheet metal actually occurs between numerous rough protrusions and the surface of the sheet metal. This increases the pressure exerted on the sheet metal surface, leading to more pronounced pressure marks. On the other hand, if the V-groove in the die is not wiped clean before bending the workpiece, noticeable pressure marks can occur due to the debris remaining on the V-groove, especially when bending galvanized sheets, carbon steel sheets, and similar workpieces.
Application of Markless Bending Technology
Since we know that the main cause of bending pressure marks is the friction between the sheet metal and the shoulder of the V-groove in the die, we can reduce the friction force between the sheet metal and the shoulder of the V-groove through process techniques. According to the friction force formula f = μ · N, it is evident that the friction force is influenced by the friction coefficient μ and the normal force N, both of which are directly proportional to the friction force. Based on this, the following process solutions can be formulated:
Use non-metallic materials for the shoulder of the die V-groove.
Merely increasing the R corner of the V-groove in the die to improve the effect of bending pressure marks is not very effective. From the perspective of reducing the pressure in the friction pair, it is possible to consider using non-metallic materials that are softer than the sheet metal for the shoulder of the V-groove, such as nylon, polyurethane elastomer (PU), and other materials. Considering that these materials are prone to wear and require regular replacement, there are currently several V-groove structures utilizing these materials, as shown in Figure 4.
Replace the shoulder of the die V-groove with a ball or roller structure.
Similarly, based on the principle of reducing the friction coefficient between the sheet metal and the V-groove in the die, the sliding friction pair between the sheet metal and the shoulder of the V-groove can be transformed into a rolling friction pair. This significantly reduces the friction force on the sheet metal, effectively preventing the occurrence of bending pressure marks. This technique is widely used in the die industry, and the ball markless bending die (Figure 5) is a typical application example.
Roller Markless Bending Die
In order to avoid rigid friction and facilitate the rotation and lubrication of the roller, ball bearings are added between the roller and the V-groove. This achieves the dual effect of reducing pressure and lowering the friction coefficient. Therefore, parts produced with roller markless bending dies can generally achieve invisible pressure marks. However, the markless bending effect is not ideal for soft materials such as aluminum and copper. From an economic perspective, the structure of roller markless bending dies is more complex compared to the previously mentioned die structures, resulting in higher manufacturing costs and maintenance difficulties. These factors need to be considered by company managers when selecting the appropriate die.
Shoulder Flip Structure for the V-Groove in the Die
Currently, there is another type of die that utilizes the principle of pivot rotation to achieve part bending by flipping the shoulder of the die. This type of die deviates from the traditional fixed V-groove structure and incorporates a flip mechanism on the inclined surfaces of the V-groove. During the process of the punch pressing the sheet metal, the flip mechanism on both sides of the die is flipped inward by the pressure from the punch, allowing the sheet metal to bend, as shown in Figure 6. In this scenario, there is no significant local sliding friction between the sheet metal and the die. Instead, the sheet metal closely adheres to the flip surface and approaches the apex of the punch, avoiding the occurrence of pressure marks. The structure of this type of die is more complex than the previous structures, involving tension springs and flip plates, resulting in higher maintenance and manufacturing costs.
Isolating the V-Groove in the Die from the Sheet Metal
The methods mentioned earlier involve changing the bending die to achieve markless bending. However, from the perspective of friction contact, as long as the die and the sheet metal are separated, friction can be eliminated. Therefore, without changing the bending die, markless bending can be achieved by using a soft thin film to prevent contact between the V-groove in the die and the sheet metal. This soft thin film, also known as markless bending protection film, is typically made of materials such as rubber, PVC (polyvinyl chloride), PE (polyethylene), and PU (polyurethane). Rubber and PVC have the advantage of low raw material costs but are not pressure-resistant, have poor protective performance, and have a short lifespan. PE and PU, as high-performance engineering materials, produce markless bending protection films with excellent tear resistance, resulting in a longer lifespan and better protection.
The primary function of the bending protection film is to provide cushioning between the workpiece and the shoulder of the die, counteracting the pressure between the die and the sheet metal, thereby preventing pressure marks during bending. The film is simply placed on the die, offering the advantages of low cost and easy use. Currently available markless bending protection films on the market are generally 0.5mm thick and can be customized according to specific requirements. These films can typically withstand approximately 200 bending cycles under a pressure of 2t. They are characterized by strong wear resistance, excellent tear resistance, superior bending performance, high tensile strength, high elongation at break, and resistance to lubricating oils and aliphatic hydrocarbon solvents.
The sheet metal processing industry is highly competitive, and companies aiming to establish a foothold in the market must continuously improve their processing techniques. It is crucial to not only achieve product functionality but also consider the craftsmanship and aesthetics of the product, as well as the cost-effectiveness of the manufacturing process. By applying more efficient and economical processing methods, products can be easier to manufacture, more cost-effective, and visually appealing.
