Spring Back Effect In Sheet Metals

Sheet metal deformation is purely an implementation of Stress-Strain curve characteristics. 

Spring back in Sheet Metals

When a part is released from the forces of the forming tool at the end of the operation, springback is the geometric modification made to the part. Springback is the geometric change made to a part at the end of the forming process when the part has been released from the forces of the forming tool. Upon completion of sheet metal, deep-drawn and stretch-drawn parts spring back and thereby affect the dimensional accuracy of a finished part. The final form of a part is changed by springback, which makes it difficult to produce the part. 

As a result, the manufacturing industry is faced with some practical problems: Firstly, the prediction of the final part geometry after springback and secondly, appropriate tools must be designed to compensate for these effects.

When the force that caused the bend is released, the elastic region recovers, which causes a springback to occur. The partial recovery of the work from the bend to its original geometry is known as "spring back."

The magnitude of spring back depends largely on the modulus of elasticity and the yield strength of the material

Spring back Increases as (the R/T ratio & yield stress of material ) increase and as elastic modulus E decreases. Spring back effect happens as the thickness of sheet metals is very small and those are subjected to small strains during bending or forming processes. This is most commonly found where the ratio of bend radius to thickness is very high. 

Fig1: Sheet Metal under deformation representation

Fig2: Spring Back Representation 

Types of Sheet Metals:

Below are the major types of sheet metal materials:

• Alloy steel
• Stainless steel
• Carbon steel
• Tool steel
• Galvanized steel
• Aluminium

Mathematics for Spring Back effect in Sheet Metals: 

Degrees of spring back (D):  It’s  used to calculate bend allowances and bend deductions

D = [Ri / (Mt × 2.1)] × Material factor K

Where,      Ri is inside the bend radius

                Mt is material thickness

Spring back factor (Sf):   It’s a relationship between the beginning and ending angle

                 Sf = Bending angle/Bent angle. 

Actual radius(Rf):  

                  Rf = Sf × Ri

Types of Spring Back in Sheet Metals:

There are two types of spring back depending on the material properties

1. Positive spring back: In this, the bend angle or radius of curvature is increased, and the material will be expanded
2. Negative spring back: In this bend angle or radius of curvature is decreased, and the material will be contracted

 In general, the spring back effect is minimized by 

• Bottoming the punches or stretching the bends after forming. 
• The best way to minimize this is to add beads, darts, offsets, or other mechanical and geometrical stiffeners.
• Reducing the level of stresses that are applied to the part during the initial forming operation by altering the forming process or tooling.
• Sometimes over bending is also done to overcome the effect.