Springback is when the shape of the deformed body is partially restored after the load is removed, and the shape and size of the part do not match the shape and size of the working surface of the stamping die, causing the size of the part to be out of tolerance, affecting the assembly accuracy of the product. Engineering urgently requires solving the product error caused by springback.
Plastic deformation occurs in stamping, and elastic deformation also occurs. After the forming load is unloaded, the part will have a certain amount of springback. Springback is the deformation that will inevitably occur after the sheet is formed and the formed part is taken out of the die, which will affect the final shape of the part. The amount of springback directly affects the geometric accuracy of the workpiece, and it is also a forming defect that is difficult to overcome in the process.
Stress change curve after load unloading
Factors affecting the springback of stamping parts
1) Material properties Stamping parts of different strengths, from ordinary plates to high-strength plates, have different yield strengths. The higher the yield strength of the plate, the more likely it is to have springback. 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 sheets, hot-rolled thick sheets have poor surface quality, large thickness tolerance, unstable mechanical properties, and low elongation.
Stress changes before and after springback
2) Material thickness During the forming process, the thickness of the sheet has a great influence on the bending performance. As the thickness of the sheet increases, the springback phenomenon will gradually decrease. This is because as the thickness of the sheet increases, the material involved in plastic deformation increases, and the elastic recovery deformation also increases, so the springback becomes smaller.
Tangential stress at the sheet interface
With the continuous improvement of the material strength level of thick sheet parts, the problem of part dimensional accuracy caused by springback is becoming more and more serious. Both 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 plans.
For thick sheet parts, the ratio of the bending radius to the sheet thickness is generally very small, and the stress in the sheet thickness direction and its stress change cannot be ignored.
3) Part shape: The springback of parts of different shapes varies greatly. Parts with complex shapes generally have an additional order of shaping to prevent springback due to incomplete forming. Some parts with special shapes are more prone to springback, such as U-shaped parts. In the analysis of the forming process, springback compensation must be considered.
4) Bending center angle: The larger the value of the bending center angle, the greater the cumulative springback value, which will cause serious springback. The length of the stamping part deformation increases with the increase of the bending center angle.
5) Die clearance matching: When designing the die, a gap of one times the material thickness should be left in the relative working part to accommodate the product in the gap. In order to achieve better material flow, the local mold should be matched after the mold processing is completed. Especially for bending molds, the larger the gap in the working part, the greater the springback. If the error allowable range of the sheet thickness is larger, the springback will also be larger, and the gap of the mold cannot be well determined.
6) Relative bending radius: The value of the relative bending radius is proportional to the springback value, so the larger the curvature of the stamping part, the more difficult it is to bend and form.
7) Forming process The forming process is an important aspect that restricts its rebound value. Generally speaking, the rebound effect of corrected bending is better than that of free bending. If the same processing effect is to be achieved in the production of the same batch of stamping parts, the bending force required for corrected bending is much greater than that for free bending. Therefore, if the same bending force is used in the two methods, the final effect will be different. The greater the correction force required for corrected bending, the smaller the rebound of the stamping part. The corrective bending force will stretch the fibers inside and outside the deformation zone to achieve the forming effect. After the bending force is unloaded, the fibers inside and outside will be shortened, but the rebound directions of the inside and outside are opposite, so that the outward rebound of the stamping part can be alleviated to a certain extent.
Solutions to stamping rebound
1) Product design First of all, in terms of materials, on the premise of meeting the requirements of the product, it is necessary to choose materials with a small yield or appropriately increase the thickness of the material. Secondly, the design of the stamping part shape and the springback of the stamping part shape also have a very important influence. For complex stamping parts with curved shapes, it is very difficult to eliminate the springback in one bending due to the complex stress conditions in all directions and other factors such as friction. Therefore, when designing the shape of the product, for complex stamping parts, several components can be combined to solve the springback problem.
Anti-springback ribs can also be set to effectively solve the springback defect. On the premise of meeting the requirements of the stamping parts, anti-springback ribs can be added according to the product requirements and the springback requirements to change the product shape. Finally, the springback defect can also be solved by reducing the value of the R angle of the bending part.
2) Process design First, the mold design pre-forming process. Adding the pre-forming process can make the stamping parts formed in one time distributed in different processes, which can eliminate the internal stress in the forming process to a certain extent, thereby solving the springback defect. Secondly, reduce the gap between the concave and convex molds, which can be adjusted to about one times the material thickness to maximize the fit between the material and the mold. At the same time, hardening the mold can also effectively reduce the burring phenomenon of the stamping parts and reduce the wear on the mold.
There is also shaping of the product. If the design of the product cannot be changed at will, the stamping parts can be shaped at the end, which is also the last resort. Finally, there are some other methods to solve the springback defect, such as using hydraulic stamping equipment, setting footers on the punch, etc., which can solve the springback defect to a certain extent.
3) Part clamping force The clamping force stamping forming process is an important process measure. By continuously optimizing the clamping force, the material flow direction can be adjusted and the internal stress distribution of the material can be improved. Increasing the clamping force can make the part more fully drawn, especially the side wall and R angle position of the part. If the forming is sufficient, the internal and external stress difference will be reduced, thereby reducing the springback.
4) Draw ribs Draw ribs are widely used in today’s technology. Reasonable setting of the drawing position can effectively change the material flow direction and effectively distribute the feed resistance on the clamping surface, thereby improving the material formability. Setting draw ribs on parts prone to springback will make the parts more fully formed and the stress distribution more uniform, thereby reducing the springback.
Factors affecting springback simulation
Solutions to springback in bending
1) Correction bending
Correction bending force will concentrate the punching force in the bending deformation zone, forcing the inner metal to be squeezed. After correction, both the inner and outer layers are stretched. After unloading, the rebound trends of the two extrusion zones offset each other, which can reduce the rebound.
2) Heat treatment
Annealing before bending to reduce its hardness and yield stress can reduce the rebound, and also reduce the bending force. Harden again after bending.
3) Overbending
During bending production, due to elastic recovery, the deformation angle and radius of the sheet metal will increase. The sheet metal deformation degree can be used to reduce the rebound by exceeding the theoretical deformation degree.
4) Hot bending
By heating bending, selecting the appropriate temperature, the material has enough time to soften, which can reduce the amount of springback.
5) Stretch bending
This method is to apply tangential tension while bending the sheet metal, change the stress state and distribution inside the sheet metal, and make the entire section within the range of plastic tensile deformation. After unloading, the rebound trends of the inner and outer layers offset each other, reducing the rebound.
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 tendency of the inner and outer layers offset each other.
7) Multiple bending The bending forming is divided into multiple times to eliminate springback.
8) Inner corner blunting Compress from the inside of the bending part to eliminate springback. When the plate is U-shaped, this method is more effective because the two sides are symmetrically bent.
9) Change the overall drawing to partial bending forming The part is bent and then drawn to reduce springback. This method is effective for products with simple two-dimensional shapes.
10) Control residual stress When drawing, add a local convex shape to the surface of the tool, and then eliminate the added shape in the subsequent process, so that the residual stress balance in the material changes to eliminate springback.
11) Negative springback When processing the tool surface, try to make the sheet metal have negative springback. After the upper die returns, the workpiece reaches the required shape through springback.
12) Electromagnetic method: By using electromagnetic pulses to impact the surface of the material, shape and size errors caused by rebound can be corrected.
Post time: Sep-04-2024