Colour-coated aluminium coil is pre-painted coil in which a paint system is applied and cured on a continuous coil coating line before the metal is formed. The finish is uniform, formable and far more durable than site-applied paint, which is why the product dominates architectural cladding, roofing, signage and appliance panel work.
Substrate, Coating Build-Up and Cure
The substrate is normally 1xxx, 3xxx or 5xxx series aluminium, chosen for formability, corrosion resistance or stiffness as required, supplied in widths up to about 1600 mm and thickness from roughly 0.2 mm to 1.5 mm. The coating system is applied in layers on a continuous line: a conversion or pretreatment layer for adhesion and corrosion resistance, a primer, then a top coat that carries the colour and the weathering performance. Coating thickness is expressed as dry film thickness and is controlled across the coil width and along the length.
| Coating system | Typical top-coat DFT | Main use |
|---|---|---|
| Polyester | 20 - 25 micron | Interior panels, signage, general cladding |
| Polyurethane | 20 - 25 micron | Exterior cladding with strong colour retention |
| Fluoropolymer | 23 - 27 micron | Long-life architectural facades |
| Epoxy primer under all systems | 5 - 8 micron | Adhesion and corrosion protection |
Cure is controlled by peak metal temperature rather than by oven air temperature. Polyester systems are typically cured in the region of 216 to 224 C, while fluoropolymer systems need a higher peak metal temperature, around 232 to 249 C, to develop full film properties. Under-cure shows up as poor solvent resistance and delamination during forming; over-cure produces a brittle film that cracks at the bend.
Colour, Texture and Surface Options
Solid colours, metallic effects, wood grain and stone textures are all produced by printing or by pigmentation of the top coat. Textured finishes give a surface that hides handling marks and diffusion of light, and matt finishes are common where glare must be controlled. Colour is specified numerically, using measured coordinates and a maximum permitted colour difference, so that panels from different production runs can be matched in one elevation.
Mechanical and Weathering Performance
The finished coil must survive roll forming, bending, profiling and shearing without loss of adhesion. Flexibility is assessed by bend tests and by T-bend evaluation, impact resistance is measured by falling-weight test, film hardness by pencil test, and gloss by specular gloss measurement. Weathering is assessed by accelerated exposure and by salt-spray testing, with the acceptance criteria written as a maximum permitted colour change and a maximum loss of gloss. Interior applications rarely require the same exposure data as external facades.
Forming, Cutting and Jointing
Forming is done with protective film in place where the visible face must remain unmarked, and the film is removed soon after forming to avoid adhesive transfer. Shearing and punching are carried out with tooling that neither chips the coating nor leaves burrs that cut the film. Cut edges expose bare aluminium and are normally hidden in a folded joint, covered with a matching trim, or sealed, because edge exposure is the usual origin of both corrosion and visible colour drift on an elevation.
Specification and Test Methods
A complete specification states substrate alloy and temper, coating chemistry, dry film thickness, colour coordinates with tolerance, gloss level, and the test methods and acceptance limits for adhesion, flexibility, impact, hardness, salt spray and weathering. Recognised methods include ASTM D523 for gloss, ASTM D2794 for impact, ASTM D3363 for pencil hardness, ASTM D4145 for coating flexibility of prepainted sheet, ASTM D2244 for colour difference calculation and ASTM B117 for neutral salt spray. Requiring the substrate to ASTM B209/B209M closes the loop on the metal underneath the paint.
Frequently Asked Questions
Q: How thick is the paint on colour-coated aluminium coil?
A: A typical two-coat system runs about 25 to 35 micron total, made up of a 5 to 8 micron primer and a 20 to 27 micron top coat, measured as dry film thickness.
Q: Why is peak metal temperature controlled instead of oven temperature?
A: Cure depends on the temperature reached by the metal, not the air around it. The line controls peak metal temperature to keep adhesion, flexibility and colour consistent along the coil.
Q: Can coated coil be bent to a sharp radius?
A: Only within the flexibility limits of the coating system. T-bend testing establishes the minimum radius a given film can take without cracking, and pre-coated grades differ in this respect.
Q: Does the coating protect cut edges?
A: No. Shearing exposes bare aluminium on the edge, which is why folded, trimmed or sealed edges are used on exposed elevations.
Q: How is colour consistency between batches controlled?
A: By instrumentally measuring colour coordinates and applying a maximum permitted colour difference, assessed with the method described in ASTM D2244.
Q: Which substrate alloys are common?
A: 1xxx for formability and conductivity, 3xxx where moderate strength is needed, and 5xxx for higher strength and marine-grade corrosion resistance.