What is springback?
Let’s see the definition of springback first:
“Springback is when the load is removed, the shape of the deformed parts gets restored partially. ”
The shape and size of the parts do not accord with the shape and size of the working surface of the stamping die, so the size of the parts is not within the tolerance range, which affects the assembly precision of the products.
In engineering, it is urgent to solve the product error caused by springback.
Stamping not only generates plastic deformation, but also elastic deformation.
When the forming load is unloaded, the parts will generate springback.
Springback is the deformation that will inevitably occur after the sheet metal is formed and the forming part is removed from the mold, which will affect the final shape of the part.
The springback value will directly affect the geometric precision of the workpiece, which is also a forming defect that is difficult to overcome in the technique.
Stress change curve after load unloading
Influencing factors of springback of stamping parts
From an ordinary plate to high strength plate, the different sheet has different yield strength.
The higher the yield strength of sheet metal, the more likely it is to rebound.
The materials of thick plate parts are usually hot-rolled carbon steel or hot-rolled low-alloy high-strength steel.
Compared with cold-rolled sheet materials, the surface quality of hot-rolled sheet materials is poor, the thickness tolerance is large, the mechanical properties of materials are unstable, and the elongation rate of materials is low.
Stress changes before and after springback
In the forming process, the sheet material thickness has a great influence on the bending performance. With the increase of plate thickness, the rebound phenomenon will decrease gradually.
This is because as the thickness of the sheet increases, the amount of plastic deformation material involved increases, and the elastic recovery deformation also increases. Therefore, the resilience decreases.
Tangential stress of sheet metal interface
With the continuous improvement of the material strength level of thick plate parts, the problem of dimension precision caused by rebound becomes more and more serious.
Both the mold design and the later technique commissioning require to know the nature and size of the part resilience, so as to take the corresponding countermeasures and remedial measures.
For thick plate parts, the ratio of bending radius to plate thickness is generally small. The stress and its change in the direction of plate thickness cannot be ignored.
The springback of parts with different shapes is very different. The manufacturing of the parts with complex shape will generally add a process (shaping), to prevent the springback caused by the incomplete forming process.
However, some special shaped parts are more likely to rebound, such as u-shaped parts. The springback compensation must be considered in the process of forming.
Bending Center Angle
The greater the value of the bending center angle, the greater the springback cumulative value. This can cause serious springback.
The deformation length of stamping parts increases with the increase of bending center angle.
Matched Condition of Mold Clearance
The die shall be designed with a gap of twice the thickness of the material in the relative working part, and the product shall be accommodated in the gap.
In order to achieve better material flow, the parts of the mold should be developed after the completion of mold processing. Especially the bending mold, the greater the clearance of the working part, the greater the springback.
If the error tolerance range of plate thickness is larger, the rebound will be larger, and the clearance of the mold will not be well determined.
Relative Bending Radius
The relative bending radius is proportional to the springback value. Therefore, the greater the curvature of stamping parts, the less likely it is to be bent.
The forming process is an important aspect that restricts its springback value.
Generally speaking, the springback effect of correcting bending is better than that of free bending.
If the same machining effect is to be achieved in the production of the same batch of stamping parts, the bending force required to correct the bending is much greater than that required for free bending.
Therefore, if the bending force is the same in both ways, the final effect will be different.
The greater the correction force required to correct bending, the smaller the springback of stamping parts. The correcting bending force can elongate the inner and outer fiber in the deformation zone and achieve the forming effect.
When the bending force is unloaded, both the inside and outside fibers shorten. However, the internal and external springback direction is opposite, so that the springback of stamping parts to the outside can be alleviated to a certain extent.
The solution of stamping springback
First, in the aspect of materials, on the premise of meeting the requirements of the product, materials with low yield should be selected or the thickness of materials can be appropriately increased.
Secondly, in the aspect of stamping parts design, the shape of stamping parts also has an important influence on its springback.
In the single bend of complex stamping parts with the curved shape, since the stress in all directions is complex and there are other factors such as friction, it is very difficult to eliminate the springback.
Therefore, in the shape design of products, several parts can be combined to solve the springback problem for complex stamping parts.
Also, you can set up anti-springback bead, which can also effectively solve springback defects.
On the premise of meeting the requirements of stamping parts, the product shape can be changed by increasing the setting of anti-springback beads according to the requirements of product and springback value.
Finally, the springback defect can be solved by reducing the value of R angle of the bending part.
First, add die design preforming process. By adding preforming process, the one-time forming stamping parts can be distributed in different processes, and the internal stress in the forming process can be eliminated to a certain extent, so as to solve the springback defect.
Secondly, the gap between the concave and convex molds can be reduced, which can be adjusted to about twice the thickness of the material, so as to maximize the bonding degree between the material and the mold.
At the same time, the hardening treatment of the die can also effectively reduce the drawing of stamping parts and reduce the wear of the die.
Besides, product shaping can also be adopted. If the design of the product cannot be changed at will, then the stamping parts can be reshaped at the end, which is also the final method.
Finally, there are some other methods to solve the springback defect, such as using hydraulic stamping equipment, which can solve the springback defect to some extent.
Blank Holder Force of Parts
The forming process of blank holder force is an important technological measure. The flow direction of materials can be adjusted and the internal stress distribution of materials can be improved by continuous optimization of the blank holder force.
The increase of blank holder force can make the parts more fully extended, especially the position of the side wall and R angle. If fully formed, the internal and external stress difference will be reduced, and thus the resilience will be reduced.
The draw bead is widely used in today’s technology. The reasonable setting of the bead drawing position can effectively change the material flow direction and effectively distribute the feed resistance on the pressing surface, so as to improve the material formability.
The setting of drawing beads on the parts prone to springback will make the parts more fully formed and the stress distribution more even, thus reducing the springback.
The solution of bending springback
Correcting bending forces will concentrate the impact pressure on the bending deformation zone, forcing the inner metal to be compressed. After adjustment, both inner and outer layers are elongated, and the springback trend of the two areas after unloading can be offset to reduce the springback.
Anneal before bending to reduce hardness and yield stress, which can not only reduce rebound, but also reduce bending force. After bending, do the hardening process.
In bending production, the deformation angle and radius of sheet metal will be increased due to elastic recovery. The springback can be reduced by the way that the deformation of plate exceeds the theoretical deformation degree.
Use heating bending, choose the right temperature to make the material has enough time to soften which will reduce the amount of resilience.
The method is to apply the tangential tension when the sheet metal is bent, change the stress state and distribution inside the sheet metal, and make the whole section within the scope of plastic tensile deformation. After these unloading, the internal and external springback offset each other which cause the springback reduced finally.
The local compression technique is to increase the length of the outer plate by reducing the thickness of the outer plate so that the springback of the inner and outer layers offset each other.
The bending is divided into many times to eliminate springback.
Passivation of Inner Corner
Compress from the inside of the bending part to eliminate springback. When the plate is u-shaped, the effect is better because both sides are symmetric.
Adopt Partially Bending in Stretching Forming
Partially of the part is formed by bending and then by stretching forming to reduce springback. This method is effective for products with simple two-dimensional shape.
Control Residual Stress
During stretching process, add the local convex hull shape to the tool surface first, and then eliminate the added shape in the later process to change the residual stress balance in the material, so as to eliminate the springback.
When machining the tool surface, try to make the sheet material produce negative springback. After the return of the upper die, the parts will achieve the required shape by springback.
The shape and size errors caused by springback can be corrected by using the electromagnetic pulse to impact the material surface.