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Stamping spring back is that when the load is removed, the shape of the deformed body is partially restored, and the shape and size of the part do not match the shape and size of the working surface of the stamping die, resulting in the size of the part, not within the tolerance range, affecting the assembly accuracy of the product. Engineering There is an urgent need to solve the product error caused by the punching spring back phenomenon.
Plastic deformation occurs in stamping forming, and elastic deformation also occurs. After the forming load is unloaded, the part will spring back to some extent. Stamping spring back is the deformation that will inevitably occur after the formed part is taken out of the mold after the sheet is formed, which will affect the final shape of the part. The amount of punching spring-back directly affects the geometric accuracy of the workpiece, and it is also a forming defect that is difficult to overcome in the process.
Influence Conditions of Springback of Stamping Parts
- Material Properties
Stamping parts with different strengths, from ordinary sheets to high-strength sheets, have different yield strengths. The higher the yield strength of the sheet, the easier it is to spring back. The material of thick plate parts is generally a hot-rolled carbon steel plate or a hot-rolled low-alloy high-strength steel plate. Compared with a cold-rolled sheet, the hot-rolled thick sheet has poor surface quality, large thickness tolerance, unstable material mechanical properties, and lower material elongation.
- Material Thickness
In the forming process, the thickness of the sheet has a great influence on the bending performance, and as the thickness of the sheet increases, the spring-back phenomenon will gradually decrease. This is because as the thickness of the sheet increases, the material participating in the plastic deformation increases, and the elastic recovery deformation also increases, so the stamping spring-back becomes smaller.
With the continuous improvement of the material strength level of thick sheet parts, the problem of the dimensional accuracy of the parts caused by the spring-back is becoming more and more serious. The mold design and the later process debugging require an understanding of the nature and size of the spring-back of the parts in order to take corresponding countermeasures and remedial plans.
For thick plate parts, the ratio of the bending radius to the plate thickness is generally very small, and the stress in the plate thickness direction and its stress change cannot be ignored.
- Part Shape
The stamping spring-back of parts with different shapes is very different. The parts with complex shapes generally add a sequence shaping to prevent the spring-back phenomenon from being formed in place, and some special shape parts are more prone to spring-back phenomenon, such as U-shaped parts. When analyzing the forming process, spring-back compensation must be considered.
- Bending Center Angle
The larger the value of the bending center angle is, the greater the accumulated spring-back value will be, which will cause a serious spring-back phenomenon, and the deformation length of the stamping part increases with the increase of the bending center angle.
- Mold Clearance Fit
When the mold is designed, a gap of twice the thickness of the material should be left in the opposite working part to accommodate the product in the gap. In order to achieve a better flow of the material, after the mold is processed, the part of the mold should be ground. Especially for bending molds, the larger the gap of the working part, the greater the spring-back. If the tolerance of the thickness of the sheet is larger, the spring-back will be larger, and the gap of the mold will not be very good.
- Relative Bending Radius
The value of the relative bending radius is proportional to the spring-back value, so the greater the curvature of the stamping part, the less easy it is to bend.
- Forming Process
The forming process is an important aspect that restricts its spring-back value. In general, the punching spring-back 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 correction bending is much larger than that for free bending, so if the same bending force is used in the two methods, the final effect will be different. The greater the correction force required to correct the bending, the smaller the spring-back of the stamping part, and the correction of the bending force will elongate the fibers inside and outside the deformation zone to achieve the forming effect. After the bending force is unloaded, the inner and outer fibers will shorten, but the rebound direction of the inner and outer sides is opposite so that the outward rebound of the stamping part can be alleviated to a certain extent.
Solving Factors of Stamping Springback
- Product Design
First of all, in terms of materials, on the premise of meeting the requirements of the product, it is necessary to choose a material with a small yield or increase the thickness of the material appropriately; secondly, the design of the shape of the stamping part, the shape of the stamping part and its rebound are also very important. Influence, the complex stamping parts with curved shape are very difficult to eliminate the spring-back due to the complex stress in all directions and other factors such as friction in one bending, so when designing the shape of the product, for complex stamping The parts can take the form of a combination of several parts to solve the stamping spring-back problem.
Anti-rebound ribs can also be set, which can effectively solve the rebound defect. On the premise of meeting the requirements of stamping parts, anti-rebound ribs can be added according to product requirements and rebound volume requirements, which can change the shape of the product. Finally, the spring-back defect can also be solved by reducing the value of the R angle of the curved part.
- Process Design
First of all, the mold design pre-forming process, adding the pre-forming process can make the one-time forming stamping parts distributed in different processes, can eliminate the internal stress in the forming process to a certain extent, so as to solve the spring-back defect. Secondly, to reduce the gap between the concave and convex molds, it can be adjusted to about double the thickness of the material to maximize the fit between the material and the mold. At the same time, hardening the mold can also effectively reduce the phenomenon of stamping parts.
There is also shaping of the product. If the design of the product cannot be changed at will, the stamping part can be shaped at the end. Finally, there are some other methods to solve the spring-back defect, such as using hydraulic punching equipment, punch setting footer, etc., which can solve the spring-back defect to a certain extent.
- Parts Blank Holder Force
The blank holder force stamping process is an important technological measure. By continuously optimizing the blank holder force, the flow direction of the material can be adjusted and the stress distribution inside the material can be improved. The increase of the blank holder force can make the part drawing more fully, especially the sidewall and the R angle position of the part. If the forming is sufficient, the difference between the internal and external stress will be reduced, so that the spring-back will be reduced.
- Draw Beads
Draw beads are widely used in today’s technology. The reasonable setting of the drawing position can effectively change the material flow direction and effectively distribute the feeding resistance on the pressing surface, thereby improving the material formability. Setting the draw bead on the part will make the part more fully formed, the stress distribution will be more uniform, and the spring-back will be reduced.
The Specific Process to Solve The Stamping Springback
- Bending Correction
Correcting the bending force will concentrate the punching force in the bending deformation area, forcing the inner layer of metal to be squeezed. After being corrected, the inner and outer layers are stretched. After unloading, the spring-back tendency of the two extrusion areas can be offset to reduce the spring-back.
- Heat Treatment
Annealing before bending reduces the hardness and yield stress to reduce spring-back and also reduces bending force and then hardens after bending.
- Excessive Bending
In bending production, due to elastic recovery, the deformation angle and radius of the sheet will become larger, and the spring-back can be reduced by the way that the deformation of the sheet exceeds the theoretical deformation degree.
- Hot Bending
By heating bending and selecting the appropriate temperature, the material has enough time to soften, which can reduce the amount of spring-back.
- Deep Drawing And Bending
When the sheet is bent, tangential tension is applied to change the stress state and distribution inside the sheet, so that the entire section is within the range of plastic tensile deformation. After these unloadings, the spring-back tendencies of the inner and outer layers cancel each other out, reducing the stamping spring-back.
- 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 spring-back trends of the inner and outer layers can cancel each other out.
- Multiple Bending
Divide the bending into multiple times to eliminate spring-back.
- Inside Rounded Corner Passivation
Compression from the inside of the bend to eliminate rebound. When the plate-shaped U-shape is bent, the effect of this method is better due to the symmetrical bending on both sides.
- Change The Overall Drawing Into Partial Bending Forming
Part of the part is formed by bending and then drawing to reduce spring-back. This method is effective for products with simple two-dimensional shapes.
- Controlling Residual Stress
During deep drawing, a local convex hull shape is added to the surface of the tool, and the added shape is eliminated in the subsequent process to change the residual stress balance in the material to eliminate punching spring-back.
- Negative Rebound
When machining the surface of the tool, try to make the sheet produce a negative punching spring-back. After the upper die is returned, the part can reach the required shape by spring-back.
- Electromagnetic Method
Shape and dimensional errors due to punching spring-back can be corrected by impacting the material surface with electromagnetic pulses.