{"id":7351,"date":"2026-11-04T09:00:00","date_gmt":"2026-11-04T01:00:00","guid":{"rendered":"https:\/\/belirollforming.com\/coating-peeling-roll-forming\/"},"modified":"2026-11-04T22:19:20","modified_gmt":"2026-11-04T14:19:20","slug":"coating-peeling-roll-forming","status":"publish","type":"post","link":"https:\/\/belirollforming.com\/ru\/coating-peeling-roll-forming\/","title":{"rendered":"\u043e\u0442\u0441\u043b\u0430\u0438\u0432\u0430\u043d\u0438\u044e \u043f\u043e\u043a\u0440\u044b\u0442\u0438\u044f \u043f\u0440\u0438 \u043f\u0440\u043e\u0444\u0438\u043b\u0438\u0440\u043e\u0432\u0430\u043d\u0438\u0438"},"content":{"rendered":"\t\t
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Coating peeling roll forming looks like a machine problem. A panel comes off the line with paint lifting at a rib, the operator blames the rolls, and the tooling gets polished or replaced. Two weeks later the same defect shows up on a different profile running the same coil.<\/p>\n

The defect usually starts before the strip reaches the first stand, or at a station nobody suspects. Seven root causes cover most coating peeling roll forming cases, and each one leaves a different signature on the failure surface.<\/p>\n\n\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t

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Why a Coating That Passes on Flat Sheet Peels at the Roll<\/h2>\n\n

Adhesion tests on flat sheet measure a bond at zero strain. Roll forming does not run at zero strain. At the outer radius of a bend, the steel elongates and the film bonded to it must elongate by the same amount. On a tight rib that strain reaches 10 to 20 percent, applied in a fraction of a second.<\/p>\n

That gap explains the most common contradiction in a defect report. The coil certificate shows a cross-hatch pass at 5B and a T-bend at 1T. The formed panel still shows paint lifting at the rib corner.<\/p>\n

ASTM D4145<\/a> describes the mechanism directly: coatings on precoated sheet are stressed during roll forming, brake bending and other deformation, and those stresses can exceed either the flexibility or the adhesive strength of the coating. The result is a fractured film that exposes steel, or a film that lets go of the steel.<\/p>\n

Strain rate matters too. A coating that survives a slow hand bend can crack when the same bend is applied at 25 m\/min, because a polymer’s effective modulus and elongation both shift with rate. That is why a bench test on a flat panel is evidence, not proof. For coating peeling roll forming, only a formed sample counts.<\/p>\n\n\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t

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Read the Failure Surface Before You Blame the Line<\/h2>\n\n

What sits under the flake tells you which interface failed. Strip a lifted edge with a knife and look at both faces at 10x.<\/p>\n\n
What you see under the flake<\/th>Interface that failed<\/th>First place to check<\/th><\/tr>\n
Bare grey steel, no zinc<\/td>Pretreatment or primer to substrate<\/td>Coil pretreatment route, primer PMT<\/td><\/tr>\n
Bright zinc, no primer residue<\/td>Primer to zinc<\/td>Primer cure record, primer DFT<\/td><\/tr>\n
Primer on the steel, topcoat on the flake<\/td>Topcoat to primer<\/td>Intercoat interval, second oven PMT<\/td><\/tr>\n
Coating intact on the flat, lifted only at ribs<\/td>Film elongation against bend strain<\/td>T-bend vs actual radius, roll gap<\/td><\/tr>\n
Lifted along one side, following roll contact<\/td>Friction and shear at the roll face<\/td>Roll Ra, polish direction, lubrication<\/td><\/tr>\n
Lifted from a cut edge inward<\/td>Shear burr or edge damage<\/td>Cut-off blade clearance, slitter burr<\/td><\/tr>\n
Lifted in patches on stacked sheets<\/td>Moisture or pressure in the pack<\/td>Stacking, banding, storage<\/td><\/tr>\n<\/table>\n

That last row matters more than it looks. Peeling at cut edges or under a stack is a handling story, not a tooling story, and coil storage practice<\/a> decides how much of it you get. Peeling in the middle of a flat face is almost never handling.<\/p>\n\n\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t

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Causes 1 to 3: The Coil and the Coating System<\/h2>\n\n

Cause 1: The primer never reached its peak metal temperature.<\/strong> Primer is where adhesion is engineered, and its process window is the narrowest on the coil line. If peak metal temperature (PMT) lands below the supplier’s range, the film stays under-crosslinked. It can pass a tape test on a flat coupon and still fail under forming stress, because cohesive strength inside the primer is lower than the stress the bend applies.<\/p>\n

Polyester topcoats are typically cured to a PMT of 232 to 243 \u00b0C (450 to 470 \u00b0F), with primers running lower, around 204 to 232 \u00b0C. A supplier quoting oven air temperature instead of PMT is not giving you a verifiable cure record. Ask for the PMT log for that coil.<\/p>\n

Over-cure fails the other way. Excess PMT or long dwell embrittles the film, and a brittle primer that passes flat adhesion will fracture at the first tight rib. A line stop with strip sitting in the oven produces the same result without anyone changing a setpoint.<\/p>\n

Cause 2: Pretreatment and primer are mismatched.<\/strong> A chromate or chrome-free conversion coating exists to give the primer something to bond to. Change the pretreatment route without telling the coating supplier and adhesion can drop while every visual check still passes. On galvanized substrate the zinc surface is the weak link: a primer that keys well to bare steel may key poorly to a passivated zinc layer.<\/p>\n

Cause 3: The specified T-bend does not match the real bend radius.<\/strong> T-bend ratings run from 0T, where the panel is folded flat on itself, upward in sheet thicknesses. A lower number means a more flexible coating. Architectural panels often specify 1T or 2T. A sharp rib or hem can demand 0T, and buying 2T coil for a 0T profile is a purchase specification error that shows up as peeling weeks into production.<\/p>\n

Translate the drawing into the tightest inside radius the tooling actually produces, then convert it to a strain. A 1.5 mm sheet bent to a 1.5 mm inside radius puts roughly 33 percent strain on the outer film surface. No standard architectural coating survives that without cracking. This is where most coating peeling roll forming specifications fail, because the drawing radius is rarely the radius the last stand delivers.<\/p>\n\n\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t

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Coating and zinc weight belong in the RFQ, because both decide what the profile can survive at a tight rib.<\/figcaption>\n\t\t\t\t\t\t\t\t\t\t<\/figure>\n\t\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t
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Causes 4 to 6: What the Line Does to a Good Coil<\/h2>\n\n

Cause 4: Roll gap set too tight.<\/strong> Squeezing the film is a common attempt to control profile tolerance, and it converts a forming operation into a compression test on the coating. Gap<\/a> should be set by material thickness plus the clearance the profile needs, typically 5 to 10 percent of thickness per side for the finishing passes, not by closing until the angle comes right.<\/p>\n

Cause 5: Roll surface finish and polish direction.<\/strong> A roll with a rough face drags the film instead of rolling over it. Friction raises local temperature, softens the film, and shears it at the contact line. Polishing across the strip direction leaves a scratch pattern<\/a> that acts as a crack starter. Rolls that contact prepainted faces should be finished finer than the ones running bare steel, and the polish should run along the strip travel. Worn radii are the other half of this cause: a worn roll edge<\/a> that has opened by 0.3 mm no longer matches the radius the coating was specified for.<\/p>\n

Cause 6: Shear and cut-off burr.<\/strong> A dull cut-off blade or wrong clearance tears rather than shears. The burr lifts the film at the cut edge, moisture gets under it, and the lift grows across the pack. Notching and piercing tools do the same at holes. Blade clearance on thin prepainted sheet is usually set at 5 to 8 percent of thickness, and a blade that has run millions of cycles on bare steel is not automatically sharp enough for coated stock. Shear and cut-off practice<\/a> on coated stock differs from bare steel in exactly this detail.<\/p>\n\n\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t

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Edge condition and burr height follow the coil and the slitting, not the forming stands.<\/figcaption>\n\t\t\t\t\t\t\t\t\t\t<\/figure>\n\t\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t
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Cause 7: Lubricant, Stacking and Site Handling<\/h2>\n\n

Two failure modes live outside the machine.<\/p>\n

The first is lubricant. A drawing oil that works on bare steel may not be compatible with the coating, and residue trapped under the film at a cut edge becomes an unbonded path. Water-based lubricants left to dry can leave surfactant residue that lifts the film in storage.<\/p>\n

The second is the pack. Sheets stacked warm, banded tight, and stored where humidity swings will show edge lift within weeks. Water between sheets attacks the cut edge, the film lifts inward, and by the time the bundle is opened the defect looks like a mill failure.<\/p>\n\n\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t

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Coils stored on their side and banded against a damp floor show edge lift before the first pass.<\/figcaption>\n\t\t\t\t\t\t\t\t\t\t<\/figure>\n\t\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t
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Acceptance Checks Before You Sign Off<\/h2>\n\n

Run three checks and keep the records with the coil number.<\/p>\n\n
Check<\/th>Method<\/th>What it catches<\/th><\/tr>\n
Coil cure<\/td>PMT log, MEK double rub<\/td>Under-cure, over-cure<\/td><\/tr>\n
Flexibility against the real radius<\/td>T-bend to ASTM D4145 at the tightest radius on the profile<\/td>Flexibility mismatch<\/td><\/tr>\n
Adhesion after forming<\/td>Cross-hatch to ASTM D3359<\/a> on a formed sample, plus impact to ASTM D2794<\/a><\/td>Intercoat and interface failure<\/td><\/tr>\n<\/table>\n

A formed-sample test is the only one that reproduces the strain your line imposes. Flat coupons cannot. Test at the tightest rib, outside face, at line speed if a running sample can be cut.<\/p>\n\n\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t

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FAQ<\/h2>\n\n

1. Why does my prepainted coil pass adhesion tests but peel after roll forming?<\/strong> Because flat-panel tests measure adhesion at zero strain. Roll forming applies 10 to 20 percent elongation at the outer radius of a tight bend, which exceeds what the flat test proves. Test a formed sample instead.<\/p>\n

2. What T-bend do I need for a roll formed profile?<\/strong> Match the T-bend to the tightest inside radius in the profile, not to the product family. Architectural panels commonly run 1T or 2T, while sharp ribs and hems can need 0T. Confirm the edition and acceptance criteria in the contract.<\/p>\n

3. Is peeling always the coil supplier’s fault?<\/strong> No. Peeling concentrated at tight ribs, following roll contact, or at a cut edge points to the line. Peeling on a flat, unformed face points to the coil. Read the failure surface first.<\/p>\n

4. What is PMT and why does it decide adhesion?<\/strong> Peak metal temperature is the highest temperature the steel itself reaches in the cure oven. It controls crosslink density. A film that never reaches its PMT stays soft and weak, even when oven air temperature looks correct.<\/p>\n

5. Can over-curing cause peeling?<\/strong> Yes. Excess PMT or long dwell embrittles the film, so a brittle primer passes flat adhesion and fractures at the first tight bend. A line stop with strip in the oven does it too.<\/p>\n

6. How do I tell primer failure from topcoat failure?<\/strong> Look at both faces of the flake. Primer on the steel with topcoat on the flake means intercoat failure. Bright zinc with no primer residue means the primer never bonded.<\/p>\n

7. Does roll gap affect coating adhesion?<\/strong> It does. Closing the gap to fix an angle compresses the film and shears it at the contact line. Set gap from thickness plus clearance, and fix the angle with the forming sequence.<\/p>\n

8. Will thicker paint give better protection?<\/strong> Not for formability. A thicker film cracks at a looser radius than a thin film of the same chemistry, because there is more material to stretch. Higher DFT buys corrosion margin at the cost of bend performance, which is the trade most coating peeling roll forming projects miss.<\/p>\n

9. Can peeling appear months after delivery?<\/strong> Yes, if it starts at a cut edge or under a banded pack. Moisture trapped between sheets lifts the film inward, and the defect shows only when the bundle is opened.<\/p>\n

10. What should I write into the purchase order?<\/strong> Specify T-bend at the tightest radius on your profile, PMT ranges with a log per coil, adhesion after forming to ASTM D3359, and impact to ASTM D2794. A certificate without those four items is a promise, not a specification.<\/p>\n\n\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t

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Conclusion<\/h2>\n\n

Coating peeling roll forming is a chain of small margins: primer cure, pretreatment, film flexibility against real bend strain, roll gap, roll finish, shear quality and handling. The chain breaks at the weakest link, and the failure surface tells you which one it was.<\/p>\n

Beli Rollforming builds roll forming machines<\/a> and has commissioned lines for prepainted and galvanized stock for over 15 years, in more than 20 countries. When a coating defect appears on a new profile, the first job is to separate coil from tooling with a formed sample and a bend test, not to polish the rolls.<\/p>\n

Last updated:<\/strong> 2026-11-04<\/p>\n\n\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t

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Article Changelog<\/h2>\n\n
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  • 2026-11-04: First publication. Seven root causes ordered from coil to finished pack, failure-surface table, T-bend and PMT thresholds, three acceptance checks.<\/li>\n<\/ul>\n\n\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t
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    Next Review Triggers<\/h2>\n\n
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    • ASTM D4145 or D3359 revision that changes acceptance criteria<\/li>\n
    • A coating supplier changes its PMT window for polyester or PVDF systems<\/li>\n
    • New project data on peeling traced to a specific station<\/li>\n<\/ul>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t
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