Springback of Bent Parts and Its Prevention Methods

Springback of Bent Parts and Its Prevention Methods

In the Metal Stamping process, plastic bending of metal sheets is always accompanied by elastic deformation. When the bending deformation concludes and the load is removed, elastic recovery causes changes in the bending angle and bending radius of the workpiece, making them inconsistent with the mold’s shape. This phenomenon is known as springback (or elastic recovery). Typically, springback greater than the mold dimensions is referred to as positive springback, while springback smaller than the mold dimensions is termed negative springback.

1 Forms of Springback

The springback of bent parts manifests as changes in the bending radius and the bending angle, as illustrated in Figure 5-5.
The springback value of the bending radius, $\Delta r$, represents the change in the bending radius before and after springback:

Δr=r0​−r

where r0​ is the actual radius of the workpiece after bending (in mm), and r is the radius of the bending punch (in mm).

The springback value of the bending angle, $\Delta a$, denotes the change in the bending angle before and after springback:

Δa=a0​−a

where a0​ is the actual angle of the workpiece after springback, and $a$ is the angle of the bending punch.

2 Determining Springback Values

Due to the numerous factors influencing springback, theoretical calculation of springback values is challenging. In mold design, data summarized from experiments are typically used in the form of tables or charts. Adjustments are then made to the mold working parts after trial tests.

(1) Calculation Method

This method is suitable for workpieces with a relatively large bending radius. When the relative bending radius is large ($r/t>10$), not only does the angular springback increase, but the bending radius also undergoes significant changes. In such cases, the following formulas can be used to calculate springback values, with further adjustments made during mold testing based on analysis of the workpiece’s condition.

① For sheet metal bending:
Punch radius

rc1​=1​/r+3σs/Et​​​​

Let K=E3σs​​, then

​​

Punch angle

where rc2​ is the punch radius (in mm), $r$ is the workpiece radius (in mm), $\alpha$ is the bending angle of the workpiece (in degrees), αc2​ is the punch angle (in degrees), $t$ is the material thickness (in mm), $E$ is the material’s elastic modulus (in MPa), σs​ is the material’s yield strength (in MPa), and $K$ is the simplification coefficient (see Table 5-3).

② For round cross-section bar bending:
Punch radius

​​

where $d$ is the diameter of the round bar (in mm), and other symbols are as previously defined.

Table 5-3 Simplification Coefficient $K$ Values

(2) Chart Method
This method is suitable for workpieces with a relatively small bending radius. When the relative bending radius is small ($r/t<5$), the change in bending radius after bending is minimal, and only angular springback needs to be considered. The values can be referenced from following Tables

Table 5-4 Springback Angle for Single-Angle 90° Free Bending

Table 5-5 Springback Angle for Single-Angle 90° Corrective Bending

Table 5-6 Springback Angle $\Delta \alpha$ for U-Shaped Bending

Prevention Methods for Springback

To mitigate springback in bending operations, several strategies can be employed:

1. Overbending Technique: Compensate for springback by intentionally overbending the workpiece during the bending process.

2. Bottoming or Coining: Apply additional pressure at the end of the bending stroke to plastically deform the material and reduce elastic recovery.

3. Material Selection: Use materials with lower yield strength or higher elasticity to minimize springback.

4. Adjusting Bend Allowance: Modify the bend allowance calculations to account for anticipated springback.

5. Use of Specialized Dies: Implement dies with specific features, such as springback compensation angles, to counteract the effects of springback.

Conclusion

Springback is a critical consideration in metal bending processes, impacting the dimensional accuracy of finished parts. By understanding its mechanisms and employing appropriate calculation or chart methods, manufacturers can effectively predict and compensate for springback. Additionally, utilizing preventive measures ensures higher precision and quality in bent components. For professional metal stamping and Deep Drawing services, China Custom Stamping is a trusted manufacturer in China, offering expertise in producing high-quality, precision-engineered parts.