What is Spring back and Why It Occurs
Spring back is the elastic recovery of a material after a bending force is removed, causing the bend angle to open slightly. For Stainless Steel, which is known for its high strength and elasticity, spring back is more significant than with softer metals like mild steel. Understanding and compensating for it is critical to achieving a precise bend.
Why Stainless Steel Experiences Spring back
Elasticity: Stainless Steel’s high yield strength and inherent elasticity mean that more of its deformation is elastic and will recover after the force is removed.
Material Properties: High-strength Stainless Steel exhibits more spring back compared to softer metals like copper or low-carbon steel.
Forming Method: Air bending, which doesn’t fully press the metal into the die, leads to more spring back than bottoming or coining, where the metal is forced fully into a die.
Factors Influencing Spring back
Typical Spring back Ranges for Stainless Steel
The amount of spring back is highly dependent on the relationship between the bend’s inside radius (Ri) and the material thickness (t).
| Condition | Stainless Steel Spring Back (Approx. Angle) | Notes |
|---|---|---|
| Minimum (tight bend) | 2o - 4o | For a tight bend where the inside radius is equal to the material thickness (Ri = 1t). |
| Moderate | 4o - 15o | For medium radii, such as Ri = 6t to Ri =20t for annealed 304 or 302 Stainless Steel. |
| Profound/large radius | Exceeding to 30o - 60o | Occurs in air forming when the inside radius is very large relative to the material thickness, often Ri is eight times or more than t. |
For 304 Stainless Steel, specifically, in a tight Ri = 1t relationship, the spring back is often cited in the range of 2o – 3o.
The following ranges for spring back are generally true if there is a 1-to-1 relationship between the material thickness and inside radius:
- 304 Stainless Steel: 2-3o
- Mild aluminium: 1.5-2o
- Cold-rolled steel: 0.75-1.0o
- Hot-rolled steel: 0.5-1.0o
- Copper and brass: 0.00-0.5o
Several factors contribute to the degree of spring back in Stainless Steel:
Yield Strength: This is the most significant factor. Higher yield strength materials, like many Stainless Steel grades, have a greater tendency to “spring back” to their original shape, resulting in more spring back.
Elastic Modulus: A lower elastic modulus leads to greater spring back. Stainless Steel’s stiffness influences how much the angle opens after bending.
Bend Radius to Thickness Ratio (Ri/t): The greater the ratio of the inside bend radius to the material thickness, the greater the spring back. Tight, sharp bends (low Ri/t) result in a higher proportion of permanent (plastic) deformation and therefore less spring back.
Material Thickness: Thicker Stainless Steel sheets tend to experience less spring back than thinner ones, as thicker materials undergo more plastic deformation relative to their elastic recovery.
Temper/Condition: Stainless Steel in a “half-hard” or work-hardened condition will exhibit significantly more spring back than in the “annealed” (soft) condition. For example, half-hard 301 Stainless Steel can show 4-43o of spring back across the same radius range as annealed 304 Stainless Steel, which shows to 2-15o.
Grain Direction: Bending perpendicular to the grain direction can improve accuracy and reduce spring back.
Bending Method: Air bending typically results in more spring back than methods like bottoming or coining, which force the material into a tighter constraint to minimise elastic recovery.
Tooling Design: The punch and die geometry directly impacts how the material flows during bending and how much it rebounds afterward.
Calculation and Prediction of Spring back
Accurately predicting spring back requires careful consideration of material and bending parameters. Two common approaches are:
The most practical formula for calculating the spring back angle is to subtract the actual achieved bend angle from the intended bend angle, as spring back is the difference between the two:
If you intend to make a 90° bend, but the actual angle achieved is 85° after the bend, your spring back is 5° (90° – 85°). You would then program your bender to over-bend by 5° for future bends to compensate for this spring back.
Finite Element Analysis (FEA): Advanced simulation tools can accurately model material behaviour and predict spring back for complex parts, enabling more precise tool design and process planning.
Techniques to Compensate
To ensure accurate bends and folds, manufacturers use several compensation techniques:
Overbending: Bending the material slightly past the desired angle so it springs back into position.
Bottoming: Forcing the sheet to conform to the die angle, reducing spring back.
Coining: Applying very high force at the bend line to plastically thin the material, essentially eliminating spring back. This requires high tonnage and specific tooling.
Tooling Adjustments: Using smaller punch radii, tighter die openings, or adjusted tool geometries to minimise spring back.
Increased Force: Applying higher forming forces can reduce elastic recovery.
Active Angle Control Systems: Modern CNC press brakes can measure the bend angle in real time and adjust the ram position to compensate for spring back.
Post-Forming Techniques: Applying controlled tensile strain after bending can help lock in the shape and minimise recovery.
Tooling and Process Adjustments
Accurate bends depend not only on technique but also on the right tooling and process choices:
Adjustable Tooling: Dies that can be fine-tuned for different material grades and thicknesses provide flexibility and accuracy.
Process Monitoring: Incorporating sensors and digital feedback systems ensures consistency by allowing operators to make real-time adjustments.
Material Testing: Understanding the specific Stainless Steel batch helps predict and counteract spring back effectively.
Achieving Precise Bends and Folds
Spring back is a natural outcome of forming Stainless Steel, but it can be managed through careful planning, precise calculation, and the right methods. By considering material properties, calculating or simulating spring back, and applying techniques such as overbending, bottoming, coining, or real-time control, manufacturers can achieve accurate and repeatable results. Effective spring back management not only improves product quality but also reduces waste and rework, resulting in more efficient and cost-effective production.