For mild steel (yield strength ~250 MPa), springback is typically 0.5–2° per 90° bend. For high-strength steel (yield strength >600 MPa), springback can reach 3–6° per 90° bend. For aluminum and stainless steel, springback is even higher.

1.2 Why Springback Is More Complex in Roll Forming Than in Press Braking

In press braking, bending occurs at a single point. Springback is relatively uniform across the bend line.

In roll forming, bending happens gradually over many stations. Each station adds incremental bending. Springback at early stations interacts with subsequent bends. The final profile’s springback is the accumulated result of all bending steps. This makes compensation more challenging, especially for asymmetric profiles.

Part 2: Analytical Springback Compensation Methods

Analytical methods use mathematical formulas based on material properties and bend geometry. They are fast, require no software, and work well for simple, symmetric profiles with uniform material properties.

2.1 Basic Springback Formula for Air Bending

For a single 90° bend (simplified), the springback angle Δθ can be estimated by:

Δθ = θ × (3 × σᵧ × R) / (E × t)

Where:

• θ = bend angle (radians)
• σᵧ = yield strength of material (MPa)
• R = inside bend radius (mm)
• E = Young’s modulus (200,000 MPa for steel)
• t = material thickness (mm)

Example (mild steel):
θ = 90° = 1.57 rad, σᵧ = 250 MPa, R = 3 mm, t = 2 mm
Δθ = 1.57 × (3 × 250 × 3) / (200,000 × 2) = 1.57 × (2250 / 400,000) = 1.57 × 0.005625 = 0.00883 rad ≈ 0.5°

So a 90° bend will spring back to about 89.5°.

To compensate, you over-bend by the calculated springback angle. For the example above, bend to 90.5° to achieve 90° after springback.

2.2 Limitations of Analytical Methods

• They assume uniform material properties (no variation along the coil).
• They do not account for work hardening or strain gradient.
• They are inaccurate for complex profiles (multiple bends interacting).
• They ignore friction and roller contact conditions.
• They cannot predict twist or bow, only angular springback.

2.3 When Analytical Methods Are Sufficient

Analytical methods work well for:

• Simple C- or U-channels
• Low-strength materials (mild steel)
• Loose tolerances (±1–2°)
• Quick estimates during initial design

For tighter tolerances or higher-strength materials, analytical methods are not enough.

2.4 Practical Analytical Compensation by LOTOS Engineers

LOTOS uses analytical formulas as a starting point for tooling design. For standard profiles (e.g., C-channel, strut channel) in mild steel, the calculated over-bend is applied in the last 2–3 forming stations. A final sizing station then calibrates the profile to nominal dimensions. This approach consistently achieves ±0.5° angular accuracy.

Part 3: FEA-Based Springback Compensation Methods

Finite Element Analysis (FEA) simulates the entire roll forming process digitally. It predicts springback with high accuracy by modeling material behavior, friction, roller geometry, and bending sequence.

3.1 How FEA for Springback Works

A typical FEA workflow for roll forming springback:

1. Create CAD model of the strip, rollers, and forming stations.
2. Define material properties – including stress-strain curve (elastic and plastic), Young’s modulus, Poisson’s ratio, and hardening rule (isotropic or kinematic).
3. Apply boundary conditions – strip velocity, roller rotations, friction coefficients.
4. Run the simulation – the software calculates the incremental forming and the final residual stresses.
5. Extract the formed shape – including springback deformation.
6. Compare to target geometry – calculate the error.
7. Modify roll pass design – adjust bend angles or add over-bend stations.
8. Re-simulate until the final profile matches the target within tolerance.