Solution of Springback in Stamping Die

Springback is a difficult problem for automobile stamping parts. At present, we only use software to analyze the theoretical springback compensation and add stiffeners to the products to control springback, but this can’t completely control springback. It is necessary to make up for the lack of analyzing springback compensation in the die debugging stage and increase the shaping process.

Influencing factors of springback of stamping parts

1. Material properties

There are stamping parts with different strengths on the automobile body. From ordinary plates to Gao Qiangban, different plates have different yield strengths. The higher the yield strength of plates, the more likely it is to rebound.

The material of thick plate parts generally adopts hot rolled carbon steel plate or hot rolled low alloy high strength steel plate. Compared with cold-rolled thin plate, hot-rolled thick plate has poor surface quality, large thickness tolerance, unstable mechanical properties and low elongation.

2. Material thickness

In the forming process, the thickness of sheet metal has a great influence on the bending performance. With the increase of sheet metal thickness, the springback phenomenon will gradually decrease, because with the increase of sheet metal thickness, the materials participating in plastic deformation increase, and then the elastic recovery deformation also increases, so the springback becomes smaller.

With the continuous improvement of the material strength level of thick plate parts, the problem of dimensional accuracy of parts caused by springback becomes more and more serious. Mold design and later process debugging require an understanding of the nature and size of springback of parts in order to take corresponding countermeasures and remedies.

For thick plate parts, the ratio of bending radius to plate thickness is generally small, and the stress in the direction of plate thickness and its change can not be ignored.

3. Part shape
The springback of parts with different shapes varies greatly. Parts with complex shapes generally add a sequence of shaping to prevent springback from occurring when the forming is not in place, while some parts with special shapes are more prone to springback, such as U-shaped parts. In the analysis and forming process, springback compensation must be considered.

4. BHF of parts
Blank holder force stamping process is an important technological measure. By continuously optimizing blank holder force, the material flow direction can be adjusted and the stress distribution inside the material can be improved. The increase of blank holder force can make the drawing of parts more complete, especially the position of side wall and R angle. If the forming is sufficient, the internal and external stress difference will be reduced, thus reducing the springback.

5. Drawbead
Drawbead is widely used in today’s technology. Reasonable setting of drawing position can effectively change the direction of material flow and effectively distribute the feeding resistance on the blank pressing surface, thus improving the formability of materials. Setting drawbead on parts prone to springback will make the parts more fully formed and the stress distribution more uniform, thus reducing the springback.

Springback control method of stamping parts

The best time to reduce or eliminate springback is in the stage of product design and mold development. With the help of analysis, the springback is accurately predicted, the product design and process are optimized, and the springback is reduced by product shape, process and compensation. In the mold debugging stage, the mold must be tested in strict accordance with the guidance of process analysis. Compared with ordinary SE analysis, the workload of springback analysis and correction increases by 30% ~ 50%, but it can greatly shorten the mold debugging period.

Springback is closely related to the drawing process. Under different drawing conditions (tonnage, stroke and feed rate, etc.), although the stamping parts have no forming problems, the springback after trimming will be more obvious. The springback analysis uses the same software as the drawing forming analysis, but the key is how to set the analysis parameters and evaluate the springback results effectively.

Springback control of special-shaped parts

During the development of the left and right doorsill parts of the front floor, there is a phenomenon of 4 rebound (see Figure 6), and Figure 6 indicates the rebound position and the rebound degree of the parts. According to the rebound position and degree of the product, the countermeasures as shown in Figure 7 are made. In the process route, the shaping is also increased by 4 degrees, and the third shaping sequence is added. At the same time, the material of the mold shaping insert is Cr12MoV, and the hardness needs to reach HRC 58-62.

Rebound control of L-shaped parts
L-shaped parts of a certain type of swing arm reinforcing plate are generally developed by the same die for left and right countermeasures. In order to prevent the lateral force from causing the deviation of the formed parts, the springback rectification of L-shaped parts developed symmetrically is basically the same as that of U-shaped parts.

Springback control of U-shaped parts
Generally, U-shaped parts are prone to rebound. Figure 1 is a schematic diagram of the front part of the inner panel of the left/right front longitudinal beam of a certain vehicle and its overlapping relationship on the whole vehicle. As can be seen from Figure 1, the rebound problem occurred in the development process of this product, and Figure 2 shows the rebound position and specific rebound amount of the product. After repeated analysis and communication with designers according to the overlapping relationship, the parts were changed, the length of the reinforcing ribs was increased, and the shaping sequence was added to the mold itself, and the shaping was scheduled to be 1 ~ 3.5 mm. 。
The shaping sequence is added to the process sequence, and the whole side wall of the part is shaped to ensure that the part does not rebound. As shown in fig. 5, shaping inserts are added in the flanging side punch sequence after assembly, and the die inserts are all made of Cr12MoV material, so as to ensure that the quenching hardness reaches HRC 58 ~ 62. Finally, this scheme is determined, and the die is changed according to this scheme, and it is verified on the spot that there is no rebound phenomenon in the formed parts.

According to the previous experience of developing vehicle models, the details of parts that are easy to rebound and the development process of such parts can be determined.

In addition, the current general technological measures to solve the springback of sheet metal stamping are as follows:

1. Correct bending
Correcting the bending force will concentrate the punching force on the bending deformation zone, forcing the inner metal to be extruded. After being corrected, both the inner and outer layers will be elongated, and the rebound tendency of the two extrusion zones will be offset after unloading, which can reduce the rebound.

2. Heat treatment
Annealing before bending, reducing its hardness and yield stress can reduce the rebound, but also reduce the bending force, and then hardening after bending.

3. Excessive bending
In bending production, due to elastic recovery, the deformation angle and radius of sheet metal will become larger, and the springback can be reduced by the way that the deformation degree of sheet metal exceeds the theoretical deformation degree.

4. Hot bending
By heating and bending, choosing the appropriate temperature, the material has enough time to soften, which can reduce the springback.

5. Stretch bending
In this method, tangential tension is applied while the sheet metal is bending, so as to change the stress state and distribution inside the sheet metal, so that the whole section is in the range of plastic tensile deformation. After unloading, the springback trends of the inner and outer layers cancel each other and reduce the springback.

6. Local compression
The local compression process is to increase the length of the outer sheet by thinning the thickness of the outer sheet, so that the rebound trends of the inner and outer layers cancel each other out.

7. Bending for many times
Bending forming is divided into several times to eliminate springback.

8. Passivation of inner fillet
Compression is carried out from the inside of the bending part to eliminate springback. When the plate is bent in a U-shape, this method is effective because both sides are symmetrically bent.

9. Changing the whole drawing into partial bending forming.
Part of the part is bent and then drawn to reduce springback. This method is effective for products with simple two-dimensional shapes.

10. Control the residual stress
The local convex hull shape is added to the surface of the tool during drawing delay, and then the added shape is eliminated in the next process, so that the residual stress balance in the material changes to eliminate springback.

11. Negative rebound
When machining the tool surface, try to make the sheet metal rebound negatively. After the upper die returns, the parts reach the required shape through springback.

12. Electromagnetic method
Using electromagnetic pulse to impact the material surface can correct the shape and size errors caused by springback.

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