A prepainted or metallic-coated coil is bought for its surface, and the surface is the first thing a workshop destroys. Bending, slitting, punching and roll forming all drive strain into a layer a few tens of microns thick, and the base metal survives a bend that the coating does not. Most forming complaints on coated steel are tooling complaints.

The Coating Fails Before the Steel Does

A Z275 galvanized coil carries 275 g/m² of zinc over two faces, against 40, 60 or 80 g/m² on electro-galvanized SECC, and a DX51D+AZ150 sheet carries 150 g/m² of the 55% aluminium, 43.4% zinc, 1.6% silicon alloy used across the hot-dip galvanized and aluminium-zinc range. Under tension the substrate yields; the intermetallic coating is brittle and crazes at a strain the steel takes in its stride. The ST02Z galvanized coil is quoted at 200-550 N/mm² tensile with 16-30% elongation, and the aluzinc service-life order of rural through severe marine assumes an intact film. Once the coating cracks, that crack starts the rust.

T-Bend Is the Number to Order Against

One forming figure appears on the coating pages: cold rolled aluzinc galvalume coil is specified at T-bending of 3T on the top face and 4T on the back. On a 0.5 mm coil that means wrapping a 1.5 mm former without coating failure, with 2.0 mm allowed on the back. A coil that cannot pass its own T-bend figure will craze in service, and the inside of the fold becomes a rust trap. Quote this on the enquiry: "we brake 0.6 mm AZ150 over a 2 mm inside radius, 90 degrees, both faces exposed" gets a usable answer; a bare thickness and coating designation does not.

Heavier coatings resist corrosion longer but survive less bending. AZ150 or Z275 forms harder on a tight radius than AZ30 or Z40, so where a part has both a sharp fold and a corrosive environment, the answer is a moderate coating with a smaller radius, or a heavy coating with a larger radius and a painted finish.

Choose the Base Metal by Forming Grade

Coating and base metal are separate decisions. The Z275 page lists SGCC, SGCD and SGCE beside DX51D+Z to DX56D+Z and the structural S250GD+Z to S550GD+Z grades, and the DX51D+Z coil lists soft, hard and full hard tempers in the G300-G550 band. DX51D is commercial and lock-forming quality, DX52D is a drawing grade — the DX52D+AZ coil page backs that with C 0.12, Si 0.50, Mn 0.60, P 0.10, S 0.045 and Ti 0.30 maxima — and DX53D or DX54D are deep drawing. Profiling material says so on its own page: TDC51D+Z prepainted coil is described as conducive to deep processing such as corrugated sheet, and DC01 cold rolled sheet is quoted with good cold bending. The trap is temper: the same grade can arrive soft or full hard, and full hard will not take a tight radius. Specify tensile, elongation and hardness, not just the grade.

Roll Forming: Spread the Strain Across Many Stands

A corrugated panel is a sequence of small bends, so each pass should ask for a small angular increment, and the coating rather than the drive motor sets the limit. Set the roll gap on measured thickness, not nominal: both the aluminium-zinc sheet and the galvalume coil carry ±0.01 mm thickness tolerance, so a nominal gap pinches or releases the strip and marks the coating permanently. Keep rolls polished and, for prepainted and anti-fingerprint material, hard-chromed; guides that touch the coated face scratch it within a shift. Order flat width from the developed profile: corrugated aluzinc sheet is supplied 0.151-1.2 mm thick in 914, 1000, 1200, 1220 and 1250 mm flat widths, and profiling consumes width, so a 1000 mm coil covers roughly 875-915 mm. Classic rib galvalume panel runs 0.14-4.0 mm in widths from 20 to 1500 mm.

Press Brake: Radius, Die Opening and Which Face Touches the Dies

For air bending the V-die opening is normally six to eight times the material thickness, and the punch nose sets the inside radius, so the T-bend figure becomes a working floor: a 0.5 mm AZ150 coil specified at 3T cannot be bent over a 1 mm nose and keep its coating. Do not copy the die table used for structural plate, because sheet at 0.4-1.2 mm is far softer than 6 mm plate. Air bending is the right process; coining compresses the metal at the bend line and shears the paint film. Decide which face ends up outside before the first part runs. Prepainted panels are usually bent with the painted face inside, putting the paint in compression and leaving bare substrate at worst on the outside radius; if the painted face must be outside, increase the radius and plan on touch-up. The plate bending service covers knife bending, 180 degree and double-layer bends with unfolding calculations, which is where a bend allowance error turns into an oversize part.

Shear, Slit and Punch: Clearance and Burr Direction

Knife clearance is a percentage of thickness, and coated material belongs at the tight end of the range: too wide and the cut tears, pulling coating back from the edge; too tight and the blade wears while cutting force rises. The shear service and the cutting overview give the workshop ladder — mechanical cutting to about 10 mm, hydraulic shear above, CNC or turret punching under 2 mm, laser for complex outlines, plasma and waterjet for thick plate. Coated sheet under 2 mm sits in the CNC and laser zone, with the caveat that laser and plasma burn the coating back, so that edge is bare however clean the cut looks. Every sheared, slit or punched edge is unprotected steel, and the aluminium-rich phase in a galvalume coating reduces the microcrack width at cut and deformed areas, which is why aluzinc roofing holds cut edges better than plain zinc. Put burr direction on the drawing, burrs down on visible faces, deburred after punching, because a burr on a galvanized edge holds water and rusts first. Buying processed rather than raw removes handling damage from the buyer's shop, and the galvanized processing, cut-to-length cold rolled coil and custom processing routes exist for exactly that reason.

A Forming Checklist Before the Coil Is Unstrapped

• Quote the forming severity: thickness, coating designation, inside radius, bend angle and number of bends.
• Confirm grade, temper, tensile and elongation, not just the coating.
• Match coating mass to radius — light coatings where bends are tight, heavy coatings for aggressive environments.
• Set roll gap and die opening on measured thickness, and re-check at every coil change.
• Put burr direction, deburring and edge touch-up on the drawing.
• Strip transit film on schedule and keep prepainted parts out of direct sun.

None of this needs new equipment. It needs the coating figure, the T-bend limit and the temper to travel with the part number from enquiry through roll former, brake and shear, so the shop works to the numbers the mill quoted against.