From the experience of developing and producing various metal stamping dies, it can be seen that the drawing die is difficult to operate, because the material will move, and the problem of stamping springback can sometimes cause headaches. There is no accurate and credible springback calculation formula yet. Generally, everyone compensates for different materials and different R angles based on experience. We are very clear about the influencing factors of springback. If we address some situations in mold repair, we can still find effective ways to control springback.
Influencing factors of springback of metal stamping parts
1. Mechanical properties of stamping materials
There are metal stampings of different strengths on the body. From ordinary sheets to high-strength sheets, different sheets have different yield strengths. The higher the yield strength of the sheet, the easier it is to spring back, especially the DP series of dual-phase high-strength steels.
2. Thickness of stamping material
During the forming process, the thickness of the sheet has a great influence on the bending properties. With the increase of the thickness of the sheet, the springback phenomenon will gradually decrease, because with the increase of the thickness of the sheet, the material involved in the plastic deformation is increased, and then the elastic recovery deformation is also increased, 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 is becoming more and more serious. Mold design and post-process debugging require understanding of the nature and size of part springback in order to adopt corresponding countermeasures and remedial plans.
3. Shape and disorder of stamping parts
The springback of parts with different shapes is quite different. When the parts with messy shapes are not formed in place, a series of shapings are generally added to avoid the springback phenomenon, while some parts with special shapes show the springback phenomenon more easily, such as U-shaped parts, Therefore, springback compensation needs to be considered in the analysis and forming process.
4. Holder force of blanking plate for die parts
The blank holder force stamping process is an important process method. After continuous optimization of the blank holder force, the material movement direction can be adjusted and the internal stress distribution of the material can be improved. Adding blank holder force can make the part draw more fully, especially the side wall and R angle of the part. If the forming is sufficient, the difference between the internal and external stress will be reduced, and the springback will be reduced.
5. Die drawbeads
Die drawbeads are widely used in today's technology. Reasonable setting of the stretching direction can effectively change the direction of material movement, effectively distribute the feed resistance on the pressing surface, and improve the formability of the material. Setting drawbeads on parts with simple springback will make the part more fully formed, the stress distribution will be more uniform, and then the springback will be reduced. Metal stamping parts springback control method
The best opportunities to reduce or eliminate punch springback are during product design and tool development. With early CAE springback analysis, accurately predict springback, optimize product design and process, and reduce springback using product shape, process and compensation. In the hardware mold debugging stage, the mold needs to be tested in strict accordance with the guidance of process analysis. Compared with general SE analysis, the workload of springback analysis and correction increases by 30% to 50%, but it can greatly shorten the mold debugging cycle.
Springback is closely related to the forming process of drawing under different drawing conditions (tonnage, stroke and feed speed, etc.). ), although the stamping part has no forming problem, the springback after trimming will be more obvious. Springback analysis and draw forming analysis use the same software, but the key is how to set the analysis parameters and effectively evaluate the springback results.
Rebound control of stamping shaped parts and L-shaped parts
The L-shaped parts of a certain vehicle's swing arm reinforcement plate are generally developed in the same way as the left and right countermeasures. In order to avoid the deflection of the forming part caused by the lateral force, the springback correction of the left-right symmetrically opened L-shaped part is basically the same as that of the U-shaped part.
Rebound control of U-shaped parts Generally speaking, U-shaped parts simply show rebound. For example, the parts at the front of the left/right front longitudinal beam inner panel of some models have a rebound problem during the development process. After repeated analysis and communication with the designer according to its stacking relationship, the parts are changed, the length of the reinforcing ribs is added, and the molding sequence is added to the mold itself, and the shape is set to 1~.
3.5 mm. The forming sequence is added to the stamping process sequence, and the entire side wall of the part is formed to ensure that the part does not spring back. As shown in Figure 5, forming inserts are added to the flanging and side punching sequence after installation, and all mold inserts are made of Cr12MoV material to ensure that the quenching hardness reaches hrc58 ~ 60. The plan was determined later, and the mold was replaced according to the plan, and it was verified on site that the formed parts had no springback phenomenon. According to the experience of developing car models in the past, the springback of some car molds can be judged.
In addition, the general process to solve the springback of sheet metal stamping is as follows:
1. Correct twists
Correcting the zigzag force concentrates the punching force in the zigzag deformation zone, forcing the inner metal to be squeezed. After correction, both the inner and outer layers are stretched. After unloading, springback can be reduced by counteracting the springback tendency of the two squeeze zones.
2. Excessive twists and turns
In tortuous production, due to elastic recovery, the deformation angle and radius of the sheet will become larger, and the springback can be reduced by the method that the deformation degree of the sheet exceeds the theoretical deformation degree.
3. Stretch and Twist
This method applies tangential tension when the sheet is bent, which changes the stress state and distribution inside the sheet, so that the entire section is within the range of plastic tensile deformation. After unloading, the springback trends of the inner and outer layers cancel each other out, reducing the springback.
4. Partial tightening The partial tightening technology is to increase the length of the outer plate by thinning the thickness of the outer plate, so that the rebound trends of the inner and outer layers can cancel each other out.
5. Multiple twists and turns
The twists are divided into several times to eliminate springback.
6. Inner fillet passivation
Compression is done from the inside of the zigzag section to eliminate punch springback. When the plate is U-shaped, this method works better because the two sides are symmetrically bent.
7. Turn the whole pull into a partial zigzag shape
A portion of the metal stamping is bent and then drawn to reduce springback. This approach is useful for products with simple two-dimensional shapes.
8. Control residual stress
When the metal mold is being pulled and delayed, a part of the convex hull shape is added to the surface of the tool, and the added shape is eliminated in the subsequent process, and then the residual stress balance in the material is changed and springback is eliminated.
9. Let the negative rebound
The stamped workpiece attempts to give the sheet a predetermined negative springback. After the upper die is returned, the workpiece is rebounded to the required shape.
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