Mark-Free Sheet Metal Bending: Tips and Solutions

Bending dies are the primary processing equipment used in sheet metal forming.

With the growth of industries such as engineering machinery, automobiles, ships, aerospace, electrical instrumentation, and building decoration, processing enterprises are increasingly demanding higher precision and complexity in the shape of sheet metal products, as well as higher surface quality.

In particular, the bending process for stainless steel and aluminum plates requires scratch-free surfaces.

To prevent bending marks on the surface of sheet metal parts during the press brake bending process, we will examine the causes of these marks from four perspectives: bending method, material hardness, concave die structure, press brake selection, and die precision.

Defects of traditional bending

Fig.1

Figure 1 depicts the lower die structure in the traditional bending method, which typically has a V-shaped groove of varying sizes to bend plates of different thicknesses. During the bending process, the plate moves along the edge of the V-groove in the lower die, leading to squeeze and friction generated by the bending force, resulting in distinct bending marks on the surface of the formed sheet metal. The width of these defects is approximately 0.414 times the width of the V-groove, seriously affecting the product’s appearance and quality.

To address the defects in traditional bending, it is necessary to overcome the plate movement, friction, and extrusion pressure generated by the V-groove in the lower die during the bending process.

Mark-free bending technology and die structure

Roller-type mark-free die

Fig.2

Figure 2 shows the structure of the roller-type mark-free die. During the bending process, the workpiece comes into contact with the cemented carbide mandrel on the lower die, and the mandrel rotates in the direction of the moving plate. This transforms the friction from traditional sliding friction to rolling friction, reducing bending marks and extrusion-related abrasion on the surface of the workpiece and improving its appearance and precision.

However, squeeze pressure still exists between the lower die roller and the plate during the bending process, so bending marks cannot be completely avoided, especially when bending soft metal plates.

Plate rotate-turning mark-free die

Fig.3

Figure 3 illustrates the structure of the Plate Rotate-Turning Mark-Free Die. During the pressing process, the top punch pushes the workpiece into the lower mold, and as the workpiece comes into contact with the rotating surface, the rotating-turning plate in the lower die rotates. This eliminates the relative sliding between the workpiece and the lower die, resulting in surface-to-surface contact between the workpiece and the rotating-turning plate. This eliminates bending marks and extrusion-related abrasion on the surface of the workpiece, thus effectively improving its appearance and precision.

Other mark-free bending technology

Fig.4

In addition to the above two mark-free mold structures, anti-mark mats, single V-shaped concave die pressure-proof rubber sleeves, block polyurethane+AT pad linings, and hard rubber concave dies can also be used to prevent bending marks on the surface of metal plates during the bending process, as shown in Figure 4. These materials help meet the product quality requirements.

However, rubber cushions, rubber sleeves, strong rubber blocks, and hard rubber are prone to deformation during the bending process, have a short service life, and can affect the dimensional accuracy of the bent workpiece. They are only suitable for workpieces that do not require high dimensional accuracy.

Magnetic markless bending technology

Advantages:

• Low cost: Users can make the tool (magnetic gasket) themselves.
• Long lifespan: Longer replacement cycles.
• Effective prevention of indentations and curling.