roll-forming

name	roll-forming
description	Roll forming process design — pass schedule (number of passes, bend angle per pass), flower pattern, springback compensation (overbend), roll tooling (material, hardness, surface finish), material selection (HSLA, stainless, coated steel), speed and lubrication, strip width calculation (developed length), side edge conditions, bow and camber, forming limits (r/t ratio, crack avoidance), tolerance capabilities, and applications (structural sections, gutters, automotive door frames).
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Roll Forming Process Design — Complete Skill

Process Overview

Roll Forming Principle

Roll forming: continuous bending of a flat strip through successive pairs of rolls (passes); each pass adds a small incremental bend to form a complex cross-section profile Advantage over press braking: continuous (high speed); complex profiles; long lengths; consistent Limitation: high tooling cost; not economical for short runs (< 5,000 m typically)

Applications: Structural: C-channel, Z-section, hat section, square/rectangular tube, roof decking Automotive: door frames, window tracks, seat tracks, bumper reinforcements Building: rain gutters, downspouts, standing seam roofing, siding Material: low carbon steel, HSLA (High Strength Low Alloy), stainless, aluminum, copper, pre-coated steel

Production speeds: Mild steel: 30–120 m/min; HSLA: 20–80 m/min; aluminum: 60–150 m/min; stainless: 15–60 m/min Higher speed → more heat from friction → lubricant critical

Pass Schedule Design

Number of Passes (N_passes)

Empirical rules for estimating N_passes: N ≥ θ_total / (3 × ∛(r/t)) [rough estimate; θ_total = total bend angle; r = bend radius; t = thickness] More practically: use experience tables or forming simulation software

General guidelines:

Total angle	Material yield strength	Min passes
90° single bend	< 400 MPa	3–5
90° single bend	400–700 MPa (HSLA)	5–8
90° single bend	> 700 MPa (AHSS)	8–12
180° fold	Any	6–10
Complex (multiple bends, > 270° total)	Any	15–30+

Angle per pass: Δθ_pass = θ_total / N_passes [uniform distribution; but often non-uniform: larger increments early, smaller near final] Too large Δθ: edge cracking, tool wear; too small: unnecessary passes, high tooling cost

Flower Pattern (Unfolding Diagram)

Flower pattern: shows cross-sections at each pass (from final profile going backward to flat strip) Constructed by:

1. Start with final cross-section
2. At each prior pass: open bends by Δθ (going in reverse from pass N to pass 1)
3. Each intermediate profile = flower petal

Software: COPRA RF (Data M), RollForm Design (Bradbury), QUINT Rollform (Matsushita) Manual: draw each intermediate cross-section in CAD; check clearances and material flow

Strip width (developed length): L_strip = Σ(straight segments) + Σ(arc lengths) Arc length = (π/180) × θ_bend × (r + k × t) [k = neutral axis factor; k ≈ 0.33 for r/t < 2; k ≈ 0.5 for r/t > 4] Simplified: k = r/(r + t) × t for transition

Springback and Overbend

Springback in Roll Forming

Springback: elastic recovery after forming; bend angle recovers partially after leaving roll gap Springback angle: Δθ_spring = θ_formed - θ_final = (M / (E × I / r)) × (1 - σ_y × (r + t/2) / (E × t/2))

Simplified springback ratio: θ_final/θ_formed ≈ 1 - (3 × r_neutral × σ_y)/(E × t) [for thin sheet; r_neutral = neutral axis radius] For steel (E = 200 GPa, σ_y = 300 MPa, r = 3 mm, t = 1 mm): Springback ≈ 1 - 3 × 3.5 × 300,000 / (200×10⁶ × 0.001) = 1 - 1.575/200 ≈ 0.992 (0.8% recovery per 90° bend) Negligible for mild steel; significant for HSLA/AHSS: up to 5–15° on 90° bends

HSLA springback: σ_y_HSLA = 500–700 MPa → springback ratio ≈ 1 - 3rσ_y/(Et) ≈ 0.97–0.94 → 6–9° underbend on 90° Must overbend by springback amount in final pass

Overbend compensation: Roll gap set to θ_overbend = θ_design + Δθ_springback Tool radius r_tool = r_final × E / (E - 3σ_y/t × r_final) → back-calculate required tool radius to achieve final r

Roll Tooling Design

Roll Materials and Hardness

Tool steel materials:

Material	Hardness (HRC)	Application
D2 (Cr/Mo/V cold work)	58–62	Standard; mild to HSLA steel; good wear
A2 (medium alloy)	56–60	Better toughness than D2; HSLA; slightly lower wear resistance
M2 (high-speed steel)	60–65	High-speed forming; stainless steel; best wear; more expensive
H13 (hot work)	44–48	High-temperature applications; soft materials with heat
Carbide inserts	HV 1,500+	Ultra-hard; narrow contact widths; long runs

Roll surface finish: Ra = 0.4–0.8 μm for coated materials (avoid scratching coating) Ra = 0.8–1.6 μm for uncoated; sufficient to hold lubricant

Roll dimensions: Roll diameter D_r: typically 150–250 mm for light gauge (<2 mm); 250–400 mm for heavy gauge (>3 mm) Roll face width: cover strip + side clearance (5–10 mm each side) Roll gap: set to t_material + 5–15% (slight compression assists forming and edge control)

Material Forming Limits

Minimum Bend Radius

Minimum inside bend radius without cracking (r_min/t):

Material	r_min/t
AISI 1010 (soft steel)	0.5–1.0
AISI 1050 (medium carbon)	1.0–2.0
HSLA 50 (340 MPa)	1.0–1.5
HSLA 80 (550 MPa)	2.0–3.0
DP590 (dual phase)	1.5–2.5
DP980	3.0–5.0
Stainless 316	1.0–1.5
Aluminum 5052-H32	1.5–2.5
Aluminum 6061-T6	4.0–6.0

Edge cracking: occurs at strip edge; edge quality critical (cut vs. slitted; edge condition: smooth vs. burr) Edge crack avoidance: dull or smooth slitting; edge chamfer; use pre-notched corners for sharp features

Lubrication

Functions: reduce friction (tool wear, heat), prevent pickup/galling, protect coated surfaces

Lubricants: Light mineral oil (ISO 32–100): mild steel; good rust protection; easily removed (detergent wash) Vanishing oil: evaporates after forming; no cleaning needed; pre-coated or precision parts Dry lubricant (wax, PTFE): pre-applied to coil; no oil in forming zone; food grade or corrosion applications Cutting fluid with EP additives: for HSLA/stainless; prevents galling

Application: flood, spray, or pre-applied film on coil; apply 0.5–2.0 g/m² typical

Common Defects and Causes

Defect	Cause	Remedy
Edge wave	Excessive forming speed or large angle per pass	Reduce speed; add passes
Bow (longitudinal)	Asymmetric forming; unequal tension	Straightener rolls; adjust pass angles
Camber (lateral curve)	Strip tension imbalance; edge elongation	Adjust roll alignment; reduce strip width variation
Springback variance	Material yield stress variation (lot-to-lot)	Closed-loop angle control; more passes
Galling (tool pickup)	Inadequate lubrication; surface hardness	Increase lubrication; use harder roll material
Edge cracking	r/t too small; sharp burrs on strip edge	Increase radius; condition strip edge
Twist	Asymmetric geometry; roll misalignment	Anti-twist guides; precision alignment

Dimensional Tolerances

Achievable tolerances (strip thickness 0.5–3 mm): Width of profile: ±0.5–1.5 mm; angle: ±0.5–1°; length (flying cutoff): ±0.5–2.0 mm Straightness: ≤ 1 mm/m bow; ≤ 0.5 mm/m camber Coating preservation: Pre-coated (galvanized, pre-painted) — bend radius ≥ 1.5t to avoid coating fracture

Standards and References

Standard	Scope
AISI Design Manual for Cold-Formed Steel	Roll-formed structural sections design
ASTM A1008	Cold-rolled steel strip for roll forming
ASTM A1011	Hot-rolled strip; higher strength for structural
AWS D1.3	Structural welding of sheet steel (for welded roll-formed assemblies)
Halmos "Roll Forming Handbook" (2006)	Definitive roll forming reference

Output

Provide: profile description (cross-section sketch or description; key dimensions; total bend angle θ_total [°]; thickness t [mm]), material (grade; σ_y [MPa]; elongation [%]; r_min/t check for each bend), developed strip width L_strip [mm] (with neutral axis k; arc + straight segments), pass schedule (N_passes; angle per pass Δθ [°]; intermediate cross-sections from flower pattern), springback (Δθ_spring [°] at last pass; overbend angle in final pass; critical for HSLA?), roll tooling (material D2/M2; hardness HRC; D_roll [mm]; roll face width [mm]; surface finish Ra [μm]), lubrication (type; application rate [g/m²]; coated steel precautions), forming speed [m/min] (recommendation for material/thickness), defect risk assessment (edge wave, bow, galling: likelihood and mitigation), achievable tolerances (width [mm]; angle [°]; straightness [mm/m]), and applicable standard (AISI Design Manual, Halmos Roll Forming Handbook).