For all kinds of stamping parts, the drawing die is the most difficult to deal with, since the material will flow and other types can be dealt with, but like the stamping parts with high size requirements, spring back is sometimes a headache. At present, we have not seen any prepared solution for spring back.
Generally, everyone uses his experience to compensate for different materials and different R angles. We all analyze the factors that cause spring back. Clearly, if we repair the mold, we can still find an effective way to control the problem of spring back for some conditions.
Spring back is a difficult issue for automotive stamping parts. At this stage, only software is used to analyze the theoretical compensation amount for spring back, and ribs are added to the product to control it, but this cannot completely solve the issue.
In the debugging stage, we still need to make up for the lack of analysis of the amount of spring back compensation and increase the shaping process.
Influencing factors of spring back of stamping parts:
1, Material properties
There are stamping parts of different strength on the car body, including those for ordinary plates and high-strength plates. Different plates have different yield strengths, the higher the yield strength of the plates, the easier it is to rebound.
The material of thick sheet parts is generally hot-rolled carbon steel plate or hot-rolled low-alloy high-strength steel plate. Compared with cold-rolled thin sheet, hot-rolled thick sheet has poorer surface quality, large thickness tolerance, unstable material mechanical properties, and lower elongation of the material.
2, Thickness of the material
In the forming process, the thickness of the sheet has a significant impact on the bending performance. As the thickness of the sheet increases, there would be less spring back. This is because as the thickness of the sheet increases, the materials involved in plastic deformation also increase. Then, the deformation of elastic recovery also increases, decreasing spring back.
With the continuous improvement of the material strength of thick sheet parts, the problem of part dimensional accuracy caused by spring back is becoming more and more serious. Die design and later process debugging require an understanding of the nature and size of part spring back in order to take corresponding countermeasures and remedies.
For thick sheet metal parts, the ratio of the bending radius to the sheet thickness is generally very small, and the stress and stress variation in the sheet thickness direction cannot be ignored.
3, Shapes of the parts
The spring back of parts with different shapes is very different. Parts with complex shapes will generally be reshaped to prevent spring back if the forming is not in place, and some parts with special shapes are more prone to spring back, such as U-shaped parts. Spring back compensation must be considered when analyzing the forming process.
4, Blank-holding force of parts
The Blank-holding force of stamping forming process is an important technological measure. By continuously optimizing the blank-holding force, the flow direction of the material can be adjusted and the internal stress distribution of the material can be improved.
The increase of the blank holder force can make the part draw more fully, especially the side wall and the R angle position of the part. If the forming is sufficient, the internal and external stress difference will be reduced, thereby reducing the spring back.
Drawbeads are widely used in current technology. By reasonably setting the drawing position, one can effectively change the flow direction of the material and effectively distribute the feed resistance on the binder surface, thereby improving the formability of the material. Setting the drawbead on the part can make the part more fully formed, the stress distribution more uniform, and reducing spring back.
Control measures against spring back of stamping parts:
The best time to reduce or eliminate spring back is during the product design and tooling development stages. With the help of analysis, we can accurately predict the amount of spring back, optimize product design and process, and use product shape, process and compensation to reduce spring back.
In the mold debugging stage, the mold must be tested in strict accordance with the guidance of the process analysis. Compared with ordinary SE analysis, the spring back analysis and correction workload increases by 30% to 50%, but it can greatly shorten the mold debugging cycle.
Spring back is closely related to the drawing forming process. Under different drawing conditions (tonnage, stroke and feed rate, etc.), although there is no forming problem in the stamping parts, spring back after trimming would be more obvious.
The same software is used in spring back analysis and drawing forming analysis, but the key is how to set the analysis parameters to effectively evaluate the spring back results.
Spring back control of parts with special shape:
During the development process of the left and right threshold parts of the front floor, there was a spring back of 4° (see figure 6). It marked the rebound position and the degree of rebound of the part.
According to the spring back part and the degree of the part, we made countermeasures as shown in figure 7. In the process route, plastic 4° and the third sequence of plastic surgery are added. The material of the inserted mold plastic is Cr12MoV, and the hardness needs to reach HRC58-62.
Rebound control for L-shaped part:
The L-shaped part of the swing arm reinforcement plate of a certain car model, generally the L-shaped parts are developed with the same mold as the left and right countermeasures. In order to prevent the lateral force from causing the deviation of the formed part, the left and right symmetrical development of the L-shaped part is rectified. It is basically the same as the U-shaped part.
Spring back control of U-shaped parts
Generally, U-shaped parts are prone to spring back. Figure 1 is a schematic diagram of the front part of the inner panel of the left/right front side beam of a certain model and its overlapping relationship on the vehicle.
It can be seen from figure 1 that this part is in the development process. There is a spring back in the process. Figure 2 shows the rebound position of the part and the specific rebound amount.
After repeated analysis and communication with the designer according to the overlapping relationship, we made changes to the part, increased the length of the rib, added a shaping sequence to the mold itself, and planned to shape the shape by 1 to 3.5 mm.
The shaping sequence is added to the process sequence, and the entire side wall of the workpiece is completely shaped to ensure that there is no spring back of the workpiece.
As shown in figure 5, plastic inserts are added in the flanging and side punching sequence after assembly, and all mold inserts are made of Cr12MoV material to ensure that the quenching hardness reaches HRC58-62.
We finalized this plan, changed the mold according to this plan, and verified that there is no spring back of the formed part on site.
According to the experience of developing models in the past, the details of parts that are easy to spring back can be determined, and the development process for the application of such parts can be determined too.
In addition, the current common technological measures to solve the spring back of sheet metal stamping are as followed:
1, Correct bending
Correction of the bending force will concentrate the stamping force in the bending deformation area, forcing the inner layer metal to be squeezed. After being corrected, both the inner and outer layers would be elongated.
2, Heat treatment
Annealing before bending, reducing its hardness and yield stress can reduce spring back, and also reduce bending force, then harden after bending.
3, Excessive bending
In bending production, due to elastic recovery, the deformation angle and radius of the sheet metal will become larger, and spring back can be reduced by using a method in which the deformation degree of the sheet metal exceeds the theoretical deformation degree.
4, Heat bending
Using heating and bending, choosing the right temperature, the material has enough time to soften, which can reduce the amount of spring back.
The method is to apply tangential tension while bending the sheet, change the stress state and distribution inside the sheet, and make the entire section within the range of plastic tensile deformation. After unloading, the spring back of the inner and outer layers cancel each other out, reducing rebound.
6, Local compression
Local compression is to increase the length of the outer sheet by reducing the thickness of the outer sheet, so that the spring back tendency of the inner and outer layers cancels each other out.
7, Multiple bending
Bending forming is divided into several times to eliminate spring back.
8, Inner fillet passivation
Compress from the inside of the bend to eliminate spring back. When the plate is bent in a U shape, this method is more effective thanks to the symmetrical bending on both sides.
9, Change whole drawing into partial bending
A part of the part is bent and drawn to reduce spring back. This method is effective for products with simple two-dimensional shapes.
10. Control residual stress
When drawing, add a local convex shape on the surface of the tool, and then eliminate the added shape in the subsequent process, so that the residual stress balance in the material will change to eliminate spring back.
11, Negative rebound
When machining the surface of the tool, try to make the sheet negative spring back. After the upper mold returns, the workpiece reaches the required shape by spring back.
12, Electromagnetic method
Shape and dimensional errors due to spring back can be corrected by impacting the surface of the material with an electromagnetic pulse.