Bending dies are the main 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 companies increasingly demand greater precision and complexity in the shape of sheet metal products, as well as higher surface quality.
In particular, the bending process of stainless steel and aluminum plates requires scratch-free surfaces.
To avoid bend 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 press brake accuracy. headquarters.
1. Traditional Flexion Defects
Figure 1
Figure 1 represents the lower die structure in the traditional bending method, which typically has a V-shaped groove of varying sizes for bending plates of different thicknesses. During the bending process, the plate moves along the edge of the V-groove in the lower die, causing compression and friction generated by the bending force, resulting in distinct bend 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 appearance and quality of the product.
To solve 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.
2. Mark-free bending technology and die structure
If a sheet metal product without scratches or marks is required, the press brake operator must choose suitable solutions that prevent friction between the sheet metal and tools.
The most common problem is how to hide the two lines that may appear in the contact area between the die and the metal sheet. There are several ways to avoid them:
(1) Clean and lubricate the dies:
The friction of metals and the resulting scratches and marks can be reduced by removing debris such as scale or zinc from the dies and applying a lubricant to reduce friction.
(2) Matrices with larger radius:
Some dies have a larger radius of curvature between the top surface and the edge of the V. In practice, for a given V-die opening, the contact area between the die and sheet metal is rounded to decrease friction between the metals. .
This is a very effective solution, with no additional costs other than the cost of dies that have the same useful life as standard dies. The only disadvantage of rounding the support area is the need to slightly increase the minimum internal edge to prevent the sheet from falling.
in the V of the die
(3) Roll type unmarked die
During the bending process, the workpiece comes into contact with the carbide mandrel in the lower die and the mandrel rotates in the direction of the moving plate.
This transforms traditional sliding friction into rolling friction, reducing flex marks and extrusion-related abrasion on the part surface and improving its appearance and precision.
However, compression pressure still exists between the bottom roller and the plate during the bending process, so bend marks cannot be completely avoided, especially when bending soft metal plates.
Roller dies significantly reduce friction and prevent surface scratches on the sheet metal, but they are also quite expensive and require frequent cleaning to ensure that debris does not prevent the roller from rotating in its seat.
Roller dies are very useful for bending thick sheets, as they
reduce the required force.
(4) Die without turning mark and plate rotation
Figure 3
Figure 3 illustrates the die structure without rotation and plate rotation marks.
During the pressing process, the upper punch pushes the part into the lower die, and as the part comes into contact with the rotating surface, the rotating plate in the lower die rotates.
This eliminates relative slippage between the workpiece and the lower die, resulting in surface-to-surface contact between the workpiece and the rotating plate.
This eliminates bending marks and extrusion-related abrasion on the surface of the workpiece, effectively improving its appearance and accuracy.
(5) Dies with nylon inserts
Some dies made from steel have a seat for nylon inserts that are usually secured with grub screws.
These inserts can be V-shaped in the shape required for bending or rectangular in shape to press the sheet against the radius tool.
These tools are very effective in eliminating stains, but require replacement of the insert when it becomes worn through the sheet metal.
These dies can only be used with thin sheets because using an excessive load will soon permanently deform the V.
(6) Anti-scratch film
An anti-scratch film placed between the metal sheet and the matrix protects the surface against scratches and marks. Its useful life depends on the tools used, the angle and mainly the radius of the punch tip.
It must be made of a non-elastic material so that it does not deform under pressure.
When used, the operator must decrease the Y stroke of the ram by the same amount as the film thickness, which remains constant during bending.
Anti-scratch films can be supplied with tensioners for correct positioning of the film and easy replacement when it starts to crack due to wear.
(7) Proper sheet metal support
The metal sheet must be properly supported during bending and during the return of the upper beam. This is a key factor for large sheet metal products, the weight of which can cause bending deformation in the area near the die if the sheet is not supported properly.
(8) Punch tip radius
Although it is quite rare, sometimes the inside of a profile may need to be protected from scratches. In these cases, it is important to choose a punch with a tip radius very close to the required internal radius.
This is crucial if high force is required, as a punch with a very small nose radius will tend to penetrate the material leaving a groove in the sheet metal.
(9) Another non-marking bending technology
In addition to the above two non-marking mold structures, anti-marking mats, single V-shaped concave die pressure-proof rubber sleeves, polyurethane block liners + AT pad and hard rubber concave dies can also be used to avoid bend marks on the surface of metal plates during the bending process. These materials help meet product quality requirements.
However, rubber pads, 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 part. They are only suitable for parts that do not require high dimensional accuracy.
3. Brand-free magnetic bending technology
Benefits:
- Low cost: Users can make the tool (magnetic joint) themselves.
- Long service life: Longer replacement cycles.
- Effective prevention of indentation and curling.
