Causes and solutions of springback in stamping forming

Springback is that when the load is removed, the shape of the deformed body is partially restored, and the shape and size of the parts do not match the shape and size of the working surface of the stamping die, resulting in the size of the parts not being within the tolerance range, which affects the assembly accuracy of the products. It is urgent to solve the product error caused by springback in engineering.

Plastic deformation occurs in stamping, and elastic deformation also occurs. After the forming load is unloaded, the parts will rebound to some extent. Springback is the deformation that must occur after the sheet metal is formed and the formed part is taken out of the die, which will affect the final shape of the part. The springback directly affects the geometric accuracy of the workpiece, and it is also a forming defect that is difficult to overcome in the process.

Influencing factors of springback of stamping parts

1) Stamping parts with different strength of material properties

From ordinary plates to Gao Qiangban, 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 stress change can not be ignored.

3) Part shape
The springback of parts with different shapes varies greatly. Parts with complex shapes will generally be added with 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) Bending center angle
The larger the bending center angle is, the greater the accumulated springback value is, which will cause serious springback phenomenon, and the deformation length of stamping parts will increase with the increase of bending center angle.

5) Mold clearance fit
When the mold is designed, it is necessary to leave a gap twice as thick as the material in the relative working part to accommodate the product in the gap. In order to realize the better flow of materials, the local parts of the mold should be researched and matched after the mold is processed. Especially for bending dies, the larger the gap at the working part, the greater the springback. If the allowable range of sheet thickness is larger, the greater the springback will be, and the gap of the die will not be well determined.

6) Relative bending radius
The value of relative bending radius is directly proportional to the springback value, so the greater the curvature of stamping parts, the less easy it is to bend and form.

7) Forming process

Forming process is an important aspect that restricts its springback value. Generally speaking, the springback effect of corrected bending is better than that of free bending. If you want to achieve the same processing effect in the production of the same batch of stamping parts, the bending force required for correcting bending is much larger than that for free bending, so if the same bending force is used in the two ways, the final effect will be different. The larger the correction force required for correcting bending, the smaller the springback of stamping parts, and the bending force will lengthen the fibers inside and outside the deformation zone to achieve the forming effect. After the bending force is unloaded, the fibers on the inner and outer sides will be shortened, but the rebound direction of the inner and outer sides is opposite, so that the outward rebound of stamping parts can be alleviated to some extent.

 

Solutions to Stamping Rebound

1) Product design

First of all, in terms of materials, under the premise of meeting the requirements of products, we should choose materials with small yield or increase the thickness of materials appropriately. Secondly, the shape design of stamping parts, the shape of stamping parts and its springback also have a very important influence. In a bending process, it is very difficult to eliminate the springback of complex stamping parts with curved shapes because of the complex stress situation in all directions and other factors such as friction. Therefore, in the shape design of products, the combination of several parts can be used to solve the springback problem.

In addition, anti-rebound ribs can be set, which can also effectively solve the rebound defects. On the premise of meeting the requirements of stamping parts, anti-rebound ribs can be added according to the requirements of products and resilience, which can change the shape of products. Finally, the springback defect can also be solved by reducing the value of the R angle of the bending part.

2) Process design

Firstly, the pre-forming process is designed for the die. Adding the pre-forming process can make the once-formed stamping parts distributed in different processes, which can eliminate the internal stress in the forming process to a certain extent, thus solving the springback defect. Secondly, reducing the gap between the concave and convex die can be adjusted to about double the material thickness to maximize the fit between the material and the die. At the same time, hardening the die can also effectively reduce the galling phenomenon of stamping parts and reduce the wear on the die.

There is also shaping the product. If the design of the product cannot be changed at will, the stamping parts can be shaped at last, which is also the last way. Finally, there are some other ways to solve the springback defects, such as using hydraulic stamping equipment and setting a footer on the punch, which can solve the springback defects to some extent.

3) 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.

4) 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.

 

Solutions to Bending Rebound

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) Multiple bending
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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