BotswanaWhat Are the Requirements for the Surface Finish of Rolls for Cold Forming Profile

1. Fundamental Purpose of Standardized Roll Surface Finish

Roll surface finish is the decisive factor controlling product surface quality, forming friction, roll wear resistance and long-term dimensional stability during continuous cold forming. Any tiny imperfection on roll surfaces will be copied onto metal strips, leading to scratches, galling, coating peeling, indentations and mass rejected profiles. Uniform, defect-free and roughness-controlled surface finish is mandatory for all contact forming surfaces of cold forming rolls.

2. Quantitative Roughness (Ra) Requirements by Application Scenarios

Surface roughness is measured by Ra (arithmetic mean deviation), with clear classified standards for different forming materials and product grades:

1. General unpainted carbon steel, galvanized steel common profiles (purlin, track, solar brackets)Standard Ra: 0.8 μm ~ 1.6 μmAchieved by precision cylindrical grinding + fine polishing; balances anti-wear performance and low friction to avoid strip scratching.

2. Pre-painted steel, color coated aluminum, decorative exposed profiles (door frames, wall panels, automotive exterior parts)Standard Ra: 0.4 μm ~ 0.8 μmHigh-gloss mirror polishing is required; super-smooth surface prevents paint layer abrasion and color fading during continuous bending.

3. Aluminum alloy, stainless steel bright decorative profilesStandard Ra: ≤ 0.4 μm, partial mirror areas reach Ra 0.1–0.2 μmStrictly eliminate grinding grain marks and micro pits to guarantee bright strip surface without visible drawing lines.

4. High-strength steel, thick structural profiles with high forming pressureStandard Ra: 1.0 μm ~ 1.6 μmModerate roughness improves strip gripping to prevent slipping during heavy deformation, without obvious surface marks on finished parts.

Tolerance control: The Ra deviation of the same roll forming surface shall not exceed ±0.1 μm, and roughness must be consistent across full roll contour including fillets, straight sections and transition arcs.

3. Visual Surface Defect Restrictions (Mandatory Acceptance Standard)

All working contact surfaces of cold forming rolls must be 100% free of the following visible defects under uniform lighting inspection:

• Linear defects: Longitudinal/transverse grinding lines, tool chatter marks, scratch grooves, polishing streaks

• Local damage: Dents, pits, chipping, edge burrs, micro-cracks at fillet radii

• Heterogeneous texture: Uneven polishing brightness, discoloration, residual abrasive particles, rust spots

• Attachment risks: Material galling adhesion, plating peeling, tiny blisters on coated rolls

Transition arcs, corners and narrow forming grooves are key inspection zones; micro-defects at these positions tend to concentrate stress and cause continuous strip surface defects in mass production.

4. Surface Hardness & Wear Resistance Matching Finish Standards

Smooth finish must be supported by high-hardness surface layers to maintain stable roughness after long-hour production:

1. Base tool steel rolls (Cr12MoV, D2, SKD11): Surface hardness HRC 58–62 after quenching, fine grinding and polishing; the hard layer avoids rapid roughness loss from abrasion.

2. Hard chrome plated rolls (mainstream for coated/aluminum materials): Chrome layer thickness ≥0.08 mm, surface hardness ≥850 HV. Plating surface must be polished to target Ra without pinholes or shedding risks; effectively suppress aluminum galling and steel adhesion.

3. WC thermal spray coating rolls for ultra-high strength steel: Coating surface is finely ground to Ra 0.8–1.2 μm, with superior anti-adhesion performance to extend roll service life.

5. Contour Uniformity & Geometric Finish Requirements

Surface finish is not limited to roughness; the whole forming contour must maintain continuous smooth transition:

1. No abrupt surface step difference between grinding and polishing zones; fillet transition curves are free of micro-flats or uneven grinding sag.

2. After finish processing, roll profile dimensional tolerance within ±0.01 mm; surface waviness is strictly controlled to prevent periodic strip ripples.

3. Edge chamfers of roll forming zones are smoothly polished without sharp edges that cut metal strip edges.

6. Post-Finish Cleanliness & Anti-Corrosion Requirements

1. All grinding paste, abrasive dust, oil residue must be thoroughly cleaned after polishing; residual foreign particles will cause indentations during trial forming.

2. Bare steel rolls shall be coated with anti-rust oil after finishing; chrome-plated rolls are naturally corrosion-resistant for workshop storage without extra protection.

3. No oxidation discoloration on roll surfaces after polishing; acid pickling residues are forbidden to avoid later corrosion pitting.

7. Common Failures Caused by Unqualified Surface Finish

• Overly rough surface (Ra >1.6 μm for coated strip): Permanent linear scratches on paint/aluminum surface, low product yield

• Over-polished mirror surface for thick high-strength steel: Strip slipping, unstable forming size, edge wave defects

• Residual grinding marks: Periodic straight lines on full-length profiles, unable to be removed by post-processing

• Micro pits or chrome peeling: Recurring dot indentations on each formed piece

• Uneven roughness: Unstable friction force leading to inconsistent profile straightness

Summary

Cold forming profile rolls adopt graded surface roughness standards according to forming substrates and product appearance demands, with strict limits on surface defects, hardness, contour smoothness and post-processing cleanliness. Matching proper polishing and surface coating technology can stabilize forming quality, reduce reject rate and prolong roll service cycle. All surface finish indicators shall be tested with roughness testers and visual inspection before roll delivery.