Spring back

During the bending process ① an elastic-plastic load occurs. When the workpiece is then unloaded ②, the remaining internal stresses in the cross section lead to a restoring moment, which causes the workpiece to spring back.

Elastic spring back is a central phenomenon when bending workpieces and plays a crucial role in the planning and execution of bending processes. To achieve the desired end product, it is essential to accurately predict the effects of elastic spring back and compensate accordingly. This can be achieved either through initial excessive bending or through an adjusted post-bending process.

Spring back torque — Fig. 1
Stresses in the bending zone
① Tensions under load ② Tensions relieved
ε_i internal strains ε_a external strains e elastic region p plastic region R_e yield strength

The phenomenon of spring back is a ubiquitous factor in all bending processes and must be taken into account for precise and error-free execution of the process. In order to ensure effective compensation for spring back , it is essential to precisely calculate the springback angle or radius in advance and incorporate it into the bending process.

Spring back sheet metal — Fig. 2
Spring back bending
_α1 Bending angle under load α₂ Workpiece angle unloaded Δα Springback angle r_m1 mean bending radius under load r_m2 mean bending radius unloaded r_i1 Inner bending radius under load r_i2 Inner bending radius unloaded s₀ Sheet thickness

Eqn. 1

Eqn. 2

Eqn. 3

Eqn. 4

Springback factor	k	=	0.981
Bending factor	c	=	5
Inner radius	r_i1	=	4.9	mm
Bending angle	α₁	=	91.7	°
Change angle	Δα	=	1.697	°

Yield strength	R_{p 0,2}	=	80	MPa
Young's modulus	E	=	70	GPa
Workpiece inner radius	r_i2	=	5	mm
Sheet thickness	s₀	=	1	mm
Workpiece angle	α₂	=	90	°

Calc 1

Springback factor k after21

The bending factor c is material-specific and must not fall below minimum values.

Material parameters eg from the table for sample materials.

Spring back Diagram — Fig. 3
Springback factor k depending on the bending factor c for different materials

Leaving the bending punch in the bending die for a longer period of time does not change the springback behavior. By pressing further, a reduction in spring back can be achieved via the follow-up effect.8

Oehler; Kaiser / Schnitt-, Stanz- und Ziehwerkzeuge / Springer Verlag / Berlin / 1993 / 7. Auflage

Kluge, Siegfried / Prozesse der Blechumformung / Carl Hanser Verlag / München / 2020 / …