What is springback and how is it reduced?

What springback is

While a sheet is formed, it deforms both elastically and plastically. When the tools open and the forming load is removed, the elastic part is released and the part relaxes to a new shape. That change is springback. It shows up as an opened or closed bend angle, a curled sidewall or a twisted part, and it makes the part miss the shape the die was cut to.

Springback is not a defect of the press or the die. It is physics, so it cannot be removed by better forming alone. It has to be predicted and compensated.

Why high-strength steel springs back more

The elastic strain stored in the sheet is roughly the stress divided by the elastic modulus. Steel has a modulus near 200 GPa whatever its strength, but a stronger steel is stressed higher. At 300 MPa the elastic strain is about 0.15%. At 1,000 MPa it is about 0.5%, more than three times as much, and that larger elastic recovery shows up as more shape change.

This is why springback moved from a minor correction to a main design problem as automakers switched to 590, 780 and 980 MPa steels.

What controls it

• Material: yield strength, hardening and the Bauschinger effect, which is the way the steel softens when the load direction reverses, as it does when the sheet is bent over a radius and straightened.
• Geometry: thickness, bend radius and the part shape. Open sections and long sidewalls curl and twist the most.
• Process: blank holder force and draw beads. More stretching of the sheet raises tension through the thickness and generally reduces sidewall curl.
• Tooling: how stiff the die and press are, and whether the part is fully bottomed at the end of the stroke.

Ways to reduce it

1. Change the process. Add stretch with beads or holder force, add a restrike or bottoming step.
2. Change the part. Add stiffening beads or ribs, or change radii, where the part design allows.
3. Compensate the die. Predict the springback by simulation, then cut the die surface so that the part springs back into the target shape. This is the method that scales to high-strength steel.

Where simulation fits

Compensation depends on predicting the springback accurately, and that depends on the material model. A model that includes the Bauschinger effect and the change in elastic modulus with strain gives much better results for high-strength steel than a simple model.

Two cases on this site show it: springback compensation of a 980 MPa pillar part, where 97.97% of the tryout part ended within 0.50 mm of the CAD target, and calibrating the Yoshida-Uemori model for a fuel tank. See also JSTAMP springback tools.