Solution to Springback in Stamping Dies

For various types of Stamping parts, deep drawing dies are the most difficult to handle because the material will flow. Other types will handle some of them, but for Stamping parts with high size requirements, the problem of springback can sometimes be very troublesome. Currently, the tribe has not seen any prepared formula for springback calculation. Generally, people rely on experience to compensate for different materials and R angles. Of course, We are quite clear about the factors that cause rebound. If we are repairing molds and targeting certain conditions, we can still find effective methods to control rebound

Rebound is a difficult problem to solve for automotive stamping parts. At present, only software analysis is used to analyze the theoretical rebound compensation amount, and reinforcement ribs are added to control rebound in the product. However, this cannot fully control rebound, and it is necessary to make up for the insufficient analysis of rebound compensation amount in the mold debugging stage and increase the shaping process.

The influencing factors of springback in stamped parts

1. Material properties

There are stamping parts with different strengths on the car body, ranging from ordinary sheet metal to high-strength sheet metal. Different sheets have different yield strengths, and the higher the yield strength of the sheet metal, the more likely it is to experience rebound.

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

2. Material thickness

During the forming process, the thickness of the sheet metal has a significant impact on the Bending performance. As the thickness of the sheet metal increases, the rebound phenomenon gradually decreases. This is because as the thickness of the sheet metal increases, the materials involved in plastic deformation increase, and thus the elastic recovery deformation also increases. Therefore, the rebound decreases.

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

For thick sheet metal parts, the ratio of Bending radius to plate thickness is generally very small, and the stress and its variation in the direction of plate thickness cannot be ignored

3. Part shape

There are significant differences in the springback of parts with different shapes. For complex shaped parts, a sequence of shaping is usually added to prevent springback from occurring due to inadequate forming. However, for some special shaped parts, such as U-shaped parts, springback compensation must be considered when analyzing the forming process.

4. Part edge pressing force

The stamping process with edge holding force is an important technological measure. By continuously optimizing the edge holding force, the direction of material flow can be adjusted and the internal stress distribution of the material can be improved. An increase in edge holding force can make the drawing of parts more complete, especially at the side walls and R-angle positions. If the forming is sufficient, it will reduce the difference in internal and external stresses, thereby reducing rebound.

5. Stretch reinforcement

Stretching ribs are widely used in today's technology. Reasonably setting the position of stretching can effectively change the direction of material flow and distribute the feed resistance on the pressing surface, thereby improving material formability. Setting stretching ribs on parts that are prone to springback can make the forming of parts more complete, stress distribution more uniform, and thus reduce springback.

Control method for springback of stamping parts

The best time to reduce or eliminate rebound is during the product design and mold development stages. Through analysis, accurately predict the amount of rebound, optimize product design and process, and reduce rebound by utilizing product shape, process, and compensation. During the mold debugging stage, it is necessary to strictly follow the guidance of process analysis to test the mold. Compared with ordinary SE analysis, the workload of rebound analysis and correction has increased by 30% to 50%, but it can greatly shorten the mold debugging cycle.

Rebound is closely related to the drawing process. Under different drawing conditions (tonnage, stroke, and feed rate, etc.), although there are no forming problems with stamped parts, the rebound after trimming will be more obvious. The rebound analysis and drawing forming analysis use the same software, but the key is how to set the analysis parameters and effectively evaluate the rebound results.

Spring back control of irregular parts

During the development process of the front floor left and right door sill components, there was a phenomenon of 4 ° rebound (see Figure 6), and Figure 6 marked the rebound position and degree of rebound of the components. Based on the rebound position and rebound degree of the workpiece, formulate the countermeasures as shown in Figure 7. On the process route, add 4 ° shaping and a third order shaping sequence. At the same time, the material of the mold shaping insert should be Cr12MoV, and the hardness should reach HRC58-62.

L-shaped part rebound control

The L-shaped parts of a certain car model's swing arm reinforcement plate are generally developed with the same mold for left and right countermeasures. In order to prevent lateral forces that may cause the formed parts to deviate, the left-right symmetrical development of L-shaped parts for rebound correction is basically the same as that of U-shaped parts.

U-shaped part rebound control

Generally, U-shaped parts are prone to rebound. Figure 1 shows the schematic diagram of the front body of the left/right front longitudinal beam inner plate of a certain vehicle model and its overlapping relationship on the entire vehicle. From Figure 1, it can be seen that this part encountered rebound problems during the development process. Figure 2 indicates the rebound location and specific rebound amount of the part. After repeated analysis and communication with the design personnel based on the overlap relationship, changes were made to the parts by increasing the length of the reinforcing ribs and adding a shaping sequence to the mold itself, with a predetermined shaping sequence of 1 to 3.5 mm

The process sorting increases the shaping sequence, and the entire side wall of the workpiece is shaped to ensure that there is no rebound phenomenon in the workpiece. As shown in Figure 5, the side punching sequence after assembly adds shaping inserts, and all mold inserts are made of Cr12MoV material, ensuring that the quenching hardness reaches HRC58-62. The final plan was determined, and the mold was modified according to this plan. On site verification showed that there was no rebound phenomenon in the formed parts.

Based on past experience in developing vehicle models, it is possible to determine the details of easily rebound parts and the development process for the application of such parts.

In addition, the current common process measures for solving sheet metal stamping rebound are as follows:

1. Correct bending

Correcting the bending force will concentrate the punching force in the bending deformation zone, forcing the inner layer of metal to be compressed. After correction, both the inner and outer layers will be elongated. After unloading, the rebound trend of the compressed two zones can be reduced by offsetting each other.

2. Heat treatment

Annealing before bending can reduce its hardness and yield stress, which can reduce rebound and also reduce bending force. After bending, it can be quenched again.

3. Excessive bending

In bending production, due to elastic recovery, the deformation angle and radius of the sheet metal will increase. It is possible to reduce rebound by using a method where the deformation degree of the sheet metal exceeds the theoretical deformation degree.

4. Hot bending

By using heating bending and selecting the appropriate temperature, the material has enough time to soften, which can reduce the amount of rebound.

5. Pulling and bending

This method applies tangential tension while bending the sheet metal, changing the stress state and distribution inside the sheet metal, so that the entire section is within the range of plastic tensile deformation. After unloading, the rebound trends of the inner and outer layers cancel out each other, reducing rebound.

6. Local compression

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

7. Multiple bends

Divide the bending process into multiple steps to eliminate springback.

8. Passivation of inner rounded corners

Compress from the inside of the curved area to eliminate rebound. When the U-shaped plate is bent, this method is more effective due to symmetrical bending on both sides.

9. Transforming integral drawing into partial bending forming

Part of the parts are bent and then stretched to reduce springback. This method is effective for products with simple two-dimensional shapes.

10. Control residual stress

During drawing, local convex hull shapes are added to the surface of the tool, and then eliminated in the subsequent process to change the residual stress balance in the material and eliminate rebound.

11. Negative rebound

When machining the surface of the tool, try to create negative rebound of the sheet metal. After the upper mold returns, the workpiece reaches the required shape through rebound.

12. Electromagnetic method

By using electromagnetic pulses to impact the surface of materials, shape and size errors caused by rebound can be corrected.