Springback is one of the most common forms of scrap in sheet metal processing, as well as one of the technical difficulties in bending processes.
At the same time, it is also one of the main defects in the sheet metal stamping process, seriously affecting the dimensional accuracy and appearance quality of parts. It is a difficult defect to control in practical production processes.
1. Sheet Metal Springback Phenomenon
Springback is a reverse elastic deformation that occurs during unloading and is a common phenomenon in the sheet metal stamping process.
Springback is particularly severe during the bending and drawing processes, which has a significant impact on the dimensional accuracy, production efficiency and economic benefits of parts.
2. Sheet Metal Springback Mechanism
When a metal sheet is subjected to an external bending moment, it first undergoes elastic flexural deformation.
In the elastic bending stage, the deformation of the sheet metal is minimal when the radius of curvature is large and the inner radius of curvature of the sheet metal does not coincide with the radius of the punch corner.
In the bending deformation zone, the material on the inner side of the bend (near the punch side) is compressed and shortened, and the stress state is uniaxial compression.
The outer side of the bend (near the die side) of the sheet metal is stretched and elongated, and the stress state is uniaxial stress.
From the inside to the outside of the folded surface, the degree of shortening and stretching gradually decreases, and there is a layer of fibers between the two deformation zones where the length remains constant and the deformation is zero, called the neutral layer.
Likewise, between the transition from tensile stress to compressive stress, there is a stress layer where the tangential stress is zero, called the neutral stress layer.
In general, these two neutral layers of different properties are considered to overlap into a single neutral layer.
As the bending moment increases, the bending deformation of the sheet metal increases and the metal on the inner and outer surfaces of the sheet reaches the yield strength first.
The metal sheet begins to transition from the elastic deformation stage to the elastic-plastic deformation stage, and the stress distribution changes with the increase in bending moment.
The plastic deformation zone expands from the surface inward, and the elastic deformation zone in the middle of the sheet gradually decreases, until the entire cross section enters the plastic state.
The second image in Figure 1 shows the stress change caused by the reverse bending moment. The third image shows the residual stress that can cause springback. The main reason for the elastic return to bending is due to the elastic deformation of the material.
When sheet metal is bent, the inner layer is subjected to compressive stresses and the outer layer is subjected to tensile stresses.
Although these two stresses exceed the yield stress during elastic-plastic bending, in fact, there will always be a zone of elastic deformation where the stress is lower than the yield stress during the transition from tensile stress to compressive stress.
Due to the presence of the elastic zone, the workpiece will inevitably bounce back after unloading.
When the relative curvature radius is larger, the proportion of the elastic deformation zone is larger, which makes this type of springback more significant.
To explain springback more intuitively, a formula for the amount of springback is introduced.
Springback is a reverse elastic deformation that occurs during unloading after bending. The classic calculation formula for sheet metal springback is:
Where: Δρ is the change in curvature; ρ is the radius of curvature before unloading; ρ' is the radius of curvature after unloading; M is the bending moment; E is the modulus of elasticity; I is the moment of inertia of the gross folded section; you are Poisson's ratio; t is the internal bending moment of the sheet before springback.
Rearranging the above formula, we can obtain the relationship between the radius of curvature before and after unloading:
From the relationships between the parameters in the above equation, we can see that the difference in the radius of curvature of the bent part before and after unloading, that is, the amount of springback, is determined by the bending moment M, the moment of inertia L the shape of the cross-section of the blank, the modulus of elasticity E of the material and the radius of curvature ρ of the bending deformation.
The greater the bending moment M applied to the blank before unloading, the greater the radius of curvature ρ of the bending deformation.
The lower the modulus of elasticity E of the material, the greater the amount of springback.
If there are two straight parts of the arm on both sides of the bent part, the springback phenomenon that occurs during unloading will also manifest itself as a change in the angle between the two straight arms.
When elastic recovery occurs during unloading, the length of the neutral layer in the bent part does not change.
Therefore,
where ρ and ρ' are the radii of curvature before and after unloading, and θ and θ' are the angles before and after unloading.
In practical operations, to ensure the angle of the bent part, the angle of the upper and lower dies must be considered in the design of the compression mold.
Since there are many factors that affect the size of the springback angle, it is very difficult to accurately calculate its size. Generally, some empirical data is used as a reference.
3. Measures to resolve Springback
(1) Choose the appropriate material.
Under the premise of meeting the requirements, materials with lower yield strengths and higher elastic moduli should be used as much as possible to reduce or eliminate springback and achieve higher bending quality.
Furthermore, the thickness tolerance of the blank, the quality of the surface finish and the flatness have a great influence on the elastic return to bending. For parts with high bending accuracy requirements, it is particularly important to strengthen the selection of raw part quality.
(2) Design a reasonable parts structure.
Choose a smaller relative bend radius. A smaller relative bend radius is beneficial to reduce springback.
Generally, when the bending radius is ≤3-5 times the thickness of the sheet metal, the entire bending zone of the sheet metal is considered to have entered the plastic state. However, too small a bending radius may cause cracks in the bending zone.
The minimum bending radius of the material given in current literature is mainly based on empirical data and can be used as a reference to design the bending radius of the part.
Change the shape of the product without changing the original function of the product, perform flanging or bending on the bent part, or press suitable reinforcing ribs at the bending point.
The springback deformation will be restricted, which can not only reduce the springback after bending, but also improve the rigidity of parts.
(3) Design a reasonable training process.
Correct the curve.
The springback angle of corrected flexion is significantly smaller than that of free flexion, and the greater the correction force, the lower the springback.
The correction force will concentrate the punching force in the flexural deformation zone, forcing the extrusion of the inner metal layer.
After the sheet is straightened, both the inner and outer layers are stretched, and the springback tendencies of the tension and compression zones after unloading compensate each other, thereby reducing the springback. This method is suitable for small rounded corners with a small deformation zone.
Heat treatment.
For some hard materials and materials that have been cold worked and hardened, annealing before bending can reduce their hardness and yield stress, thereby reducing springback. At the same time, it can also reduce the bending force and then harden after bending.
Annealing generally uses recrystallization, normal and bright annealing. Local tempering of the bent part of the metal sheet can reduce the yield strength and achieve the objective of eliminating springback.
Excessive bending.
During the bending production process, due to the elastic recovery of the sheet metal, the deformation angle and bending radius of the sheet metal will increase.
Therefore, a method to make the degree of deformation of the sheet metal in the mold exceed the theoretical degree of deformation can be used to reduce springback.
Hot bending.
Heating and bending can be used, and appropriate temperatures can be selected according to different types of sheet metal. Due to sufficient softening time, the amount of springback can be reduced.
Pull up push-up.
Parts with relatively large relative curvature radii can use the cupping and bending method. This method applies tangential stress while bending the sheet metal to change the stress state and distribution within the sheet metal.
The magnitude of the applied tensile stress should cause the total stress at each point in the flexural deformation zone to be slightly greater than the yield stress of the material, allowing the entire section to be in the plastic tensile deformation range.
In this way, the stress-strain direction of the inner and outer zones is consistent and, after unloading, the springback tendencies of the inner and outer layers compensate each other, reducing springback.
Hardening of the inner corner.
Compression is applied from within the flexure area to eliminate springback. This method is most effective when there are symmetrical bends on both sides of the sheet metal in a U-shaped bend.
L-shaped curves sometimes produce dimensional deviations, so this method is not suitable for forming products that require strength and elasticity.
Control residual stress.
When drawing and forming, add local convex shapes (circular protrusions) on the tool surface, and then eliminate the added shape in the subsequent process to change the balance of residual stress in the material to eliminate springback.
(4) Design a reasonable mold clearance.
For U-shaped bending, the springback decreases as the concave mold opening depth increases and the mold gap decreases. Mold clearance should be maintained between 110% to 115% of the sheet metal thickness for optimal forming and springback control effects.
For high bending accuracy requirements, the bend side clearance value can be set according to the sheet thickness, using slightly thinner bends to reduce springback.
Tensile bending technology or adjustable gap molds can also be used to reduce springback. For V-shaped bending, pay attention to controlling the height of the closed mold.
(5) Choose a reasonable mold structure.
Use molds to bend polyurethane rubber.
Polyurethane rubber molds can be used to bend parts with thin parts.
Because polyurethane rubber bending molds can transmit pressure evenly in all directions and achieve gap-free bending, the bent part fits snugly to the convex surface of the mold, placing the part in a state of three-way compressive stress. It can even achieve a bend similar to the drawing, reducing springback and achieving high bending quality.
Use inclined wedge bend molds.
Inclined wedge bending molds use the extrusion correction bending method, which can generally obtain higher quality bent parts.
For workpieces with high requirements for blank accuracy, the mold shoulder can be used to longitudinally press the end of the bent part, allowing both the inner and outer sides of the bending deformation zone to receive compressive stress to reduce the elastic return.
Compensation method.
Based on the direction and magnitude of springback of the bent part, the geometric shape and size of the functional part of the mold can be controlled to compensate for springback after bending. In single-angle bending, the convex mold is reduced by a springback angle.
In double-angle bending, a slope equal to the springback angle is made on the wall of the convex mold so that corresponding compensation can be made for the springback angle after bending, or the top plate and the bottom of the convex mold are made in an arc shape, so that the curved surface at the bottom of the part becomes straight again after springback, and the springback on both sides is compensated.
Sheet metal springback compensation based on CAE technology can also be used to process the CAD mathematical model after springback compensation, reducing the number of actual springback corrections.
