The solution of rebound in stamping dies

2021/08/22


For all kinds of stamping parts, the deep-drawing die is the most difficult to deal with, because the material will produce flow, other types, will deal with some, but like the high size requirements of the stamping parts, the problem of rebound is sometimes very headache, at present the tribe has not seen where there is a prepared rebound formula, usually we are based on experience, for different materials, different R angle to compensate for processing. Of course, we know the factors that affect the rebound, and if we repair the die, we can still find effective ways to control the rebound for some conditions.

Rebound is a difficult problem to solve for automobile stamping parts. At this stage, we only use software to analyze the theoretical rebound compensation amount and add reinforcement bars on the product to control rebound, but this cannot completely control rebound, so we need to make up for the lack of analysis of rebound compensation amount and increase the shaping process at the stage of mold debugging.

Influencing factors of springback of stamping parts
1. Material performance
In the body of the car there are different strength of the stamping parts, from ordinary plate to high strength plate, different plates have different yield strength, the higher the yield strength of the plate, the more likely the phenomenon of rebound.
 The material of thick sheet material parts is usually hot rolled carbon steel plate or hot rolled low alloy high strength steel plate. Compared with cold-rolled thin plate material, hot-rolled thick plate material has poor surface quality, large thickness tolerance, unstable material mechanical properties, and low elongation of the material.

2. Material thickness
In the forming process, the thickness of the sheet has a great impact on the bending properties, with the increase in the thickness of the sheet, the rebound phenomenon will gradually reduce, because with the increase in the thickness of the sheet, the participation of plastic deformation material increases, and then the elastic recovery deformation also increases, therefore, the rebound becomes smaller. 
With the continuous improvement of the material strength level of thick sheet parts, the problem of dimensional accuracy of the parts caused by springback is becoming more and more serious, and the mold design and later process debugging require an understanding of the nature and size of the springback of the parts in order to take corresponding countermeasures and remedial solutions.
For thick plate material parts, the bending radius and the ratio of plate thickness are generally very small, the plate thickness direction of the stress and its stress changes should not be ignored.

3 parts shape
Different shapes of parts rebound differences, the shape of complex parts will generally increase a sequence of shaping to prevent the formation of the rebound phenomenon, and a part of the special shape parts are more likely to rebound phenomenon, such as U-shaped parts, in the analysis of the forming process, must consider rebound compensation matters.

4 parts crimping force
Crimping force stamping and forming process is an important process measures, through the continuous optimization of crimping force, can adjust the direction of material flow, improve the internal stress distribution of the material. An increase in crimping force can make the part more fully drawn, especially the part sidewall and R-angle position, if fully formed, will reduce the internal and external stress difference, thus reducing the rebound.

5. Ductile tendons
The reasonable setting of the position of the stretching bar can effectively change the direction of material flow and effectively distribute the feed resistance on the press surface, thus improving the material formability, and setting the stretching bar on the parts prone to rebound will make the parts more fully formed and the stress distribution more uniform, thus reducing rebound.

Stamped part rebound control methods
The best time to reduce or eliminate springback is during product design and tool development. With the help of analysis, the amount of springback can be accurately predicted, the product design and process can be optimized, and the product shape, process and compensation can be used to reduce springback. And during the mold commissioning stage, the mold must be tried strictly according to the guidance of the process analysis. Compared with ordinary SE analysis, the workload of analysis and correction of springback increases by 30% to 50%, but it can greatly reduce the mold commissioning cycle.
Rebound is closely related to the drawing and forming process. Under different drawing conditions (tonnage, stroke and feed volume, etc.), although the stamped parts have no forming problems, the rebound after cutting the edges will be more obvious. The same software is used for rebound analysis as for drawing and forming analysis, but the key is how to set the analysis parameters and to evaluate the rebound results effectively.

Rebound control for shaped parts
During the development of the front floor left and right door sill parts, the phenomenon of rebound 4° appeared (see Figure 6), and Figure 6 shows the part of the part with rebound and how many degrees of rebound. According to the rebound part and the rebound degree, the countermeasures shown in Fig. 7 were made. In the process line also increase the shaping 4 °, increase the third shaping sequence, while the mold shaping insert material application for Cr12MoV, hardness needs to reach HRC58 ~ 62.

L-shaped parts rebound control
A model of swing arm reinforcement plate L-shaped parts, general L-shaped parts are left and right countermeasures with the same mold development, in order to prevent the existence of lateral forces, resulting in shaped parts offset, left and right symmetrical development of L-shaped parts rebound rectification and U-shaped parts are basically the same.
Rebound control of U-shaped parts
Figure 1 is a schematic diagram of the front part of the left/right front longitudinal plate of a model and the lap relationship on the whole vehicle. After repeated analysis and communication with the designer according to the lap relationship, changes were made to the part by increasing the length of the reinforcement and adding a shaping sequence in the mold itself, with a scheduled shaping of 1 to 3.5 mm.
3.5 mm.
Process sorting to increase the shaping sequence, the entire side wall of the part all shaping, to ensure that the part without rebound phenomenon. As shown in Figure 5, the shaping block was added to the side punching sequence of the rear flap of the group, and all the mold blocks were made of Cr12MoV to ensure that the hardness of the treatment quenching reached HRC58-62. This plan was finally determined, and the mold was changed according to this plan, and it was verified on site that no rebound phenomenon occurred in the formed parts.
Based on the experience of the previous development models, we can determine the details of the parts that are prone to rebound and the development process for the application of such parts.

In addition, the following are some of the common measures to solve the rebound of sheet metal stamping:
1、Corrective bending
Corrective bending force will make the punching pressure concentrated in the bending deformation area, forcing the inner layer of metal to be extruded, after being corrected, the inner and outer layers are elongated, and the rebound tendency of the two areas of extrusion after unloading can be offset to reduce rebound.

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

3, overbending
Bending production, due to elastic recovery, the deformation angle and radius of the sheet will become larger, can be used to reduce the rebound of the degree of deformation of the sheet beyond the theoretical degree of deformation.

4、Heat bending
The use of heating bending, select the appropriate temperature, the material has enough time to soften, can reduce the amount of rebound.

5, pull bending
The method is to apply tangential tension while bending the plate, changing the internal stress state and distribution of the plate, so that the entire section is in the range of plastic tensile deformation, these unloading, the inner and outer layers of the rebound tendency to cancel each other out, reducing the rebound.

6、Partial compression
Local compression process is to increase the length of the outer sheet by thinning the thickness of the outer sheet, so that the inner and outer rebound tendencies cancel each other out.

7、Multiple bending
The bending is divided into several times to eliminate the springback.

8、Inner rounding
Compression from the inside of the bending part to eliminate rebound. When the plate U-shaped bending, due to symmetrical bending on both sides, using this method is more effective.

9、Change the whole pulling extension into part of the bending forming
Part of the part will be bent into shape and then through the stretch forming to reduce rebound. This method is effective for two-dimensional shape of simple products.

10、Control residual stress
When drawing, a local convex package shape is added to the surface of the tool, and then the added shape is eliminated in the post-process, so that the balance of residual stresses in the material is changed to eliminate rebound.

11、Negative springback
When processing the tool surface, try to make the plate material produce negative rebound. After the return of the upper mold, the part achieves the required shape by rebound.

12、Electromagnetic method
The use of electromagnetic pulse impact on the surface of the material can correct the shape and size errors caused by rebound.