How Colour-Coated Aluminium Coil Is Made
A colour-coated coil is a painted product, but it is not painted the way most people picture painting. A color-coated aluminum coil is made on a continuous line. The strip runs at 20 to 60 metres per minute, picks up primer, colour and clear coat on rollers, and dries all three in ovens in a single unbroken pass. Nothing is hung up, nothing is sprayed, and the line does not stop for a colour change mid-coil.
That single pass is why a coil-coated finish lasts and a sprayed one usually does not. The coating is cured before the coil is cut, so the film that ends up on a facade was formed on flat metal under controlled heat. This is a walk through the line, stage by stage, with the three settings that decide whether the finished panel still looks right in year ten.
Why coil coating is a line process, not a paint shop
Batch painting happens after the metal is formed. A fabricator folds a panel, then someone sprays it and hopes the edges and the reverse side got covered. Film thickness varies across the part, cure depends on how long the sprayer held the gun, and the shop produces nothing while a colour is changed.
Coil coating reverses the order. Paint goes on flat strip, wet film thickness is set by a roller gap, and cure is set by oven temperature and line speed. The result is a film that is uniform to a few micrometres across kilometres of coil, and a colour-coated aluminium coil manufacturer can then deliver a coil that a panel line slits, laminates and bends without the coating cracking at the fold.
The coating line, stage by stage
| Stage | What happens | What it controls |
|---|---|---|
| 1. Uncoil and splice | The bare coil is loaded on a mandrel; the tail of one coil is stitched to the head of the next | Continuous feed; the splice is scrapped later |
| 2. Accumulator | A loop tower stores strip so the line keeps moving while a new coil is loaded | No stop marks at coil joints |
| 3. Cleaning | Alkaline or solvent cleaning removes rolling oil and dust | Wetting; skip it and the pretreatment cannot bond |
| 4. Pretreatment | Chemical conversion coating, applied and rinsed in sequence | Corrosion resistance and coating adhesion |
| 5. Primer | Roller-applied primer coat | Adhesion between metal and topcoat |
| 6. Topcoat and clear | Colour coat, then optional clear coat, both roller-applied | Colour, gloss, UV resistance |
| 7. Cure, cool, rewind | Multi-zone ovens, then water or air cooling and rewinding | Film hardness, flexibility, colour |
Steps three and four are the ones buyers never ask about, and they are the ones that decide whether the finish survives a coastal site. Grease left under a coating does not show up on delivery. It shows up as a patch of flaking paint three or four years later.
Cleaning and pretreatment: the two stages that are invisible on delivery
Rolling oil has to come off completely, because the pretreatment chemistry cannot reach the aluminium through it. Lines use an alkaline cleaner or a solvent, usually with a brush or spray section, and the bath is topped up and titrated through the shift rather than dumped and replaced monthly.
After cleaning, the strip passes through a conversion coating. Two families are common: chrome-free and chrome-phosphate. The chrome-free route is preferred where panels go to markets that restrict hexavalent chromium, and it needs tighter bath control to reach the same adhesion result. Either way the strip is rinsed, sometimes twice, then dried before primer.
Three parameters are logged here, and a factory that cannot show you the log for your order is guessing:
- Bath concentration and pH, checked at intervals through the shift.
- Rinse water quality, because contaminated final rinse leaves salts that bloom under the coating.
- Coating weight of the conversion layer, the number that matters more than the brand of the chemical.
It is worth asking which pretreatment chemistry a factory runs. A named, documented system is a sign of process control; “we use good chemicals” is not.
Curing: the number that decides colour and flexibility
The ovens are the part of the line that most often limits what a plant can promise. A coil line cures in stages, with a short flash-off zone, a ramp, a peak and a hold, then cooling. The setting that matters is not air temperature but peak metal temperature: the temperature the strip itself reaches at the peak of the curve, measured with a contact probe and confirmed with a temperature strip on each run.
| Setting | Typical range | Why it matters |
|---|---|---|
| Peak metal temperature | Set by the coating system and the coating maker’s data sheet | Under-cure gives weak adhesion; over-cure gives a brittle film that cracks at the fold |
| Line speed | 20-60 m/min | Speed is set to achieve the required peak metal temperature, not the other way round |
| Wet film thickness | Set by roller gap and roller speed | Decides dry film build; measured wet on the line and dry on the sample |
| Dry film thickness | Commonly around 23-25 µm for architectural finishes | Thin film of the right chemistry still wears out early |
Two-coat and three-coat systems sell into different jobs. Two-coat applies primer and topcoat. Three-coat adds a clear coat, which is more common on metallic colours and where gloss has to be held for a long service life. The extra pass costs a second oven trip and reduces line speed for that run.
Dark colours and thick films pull the achievable line speed down, because the strip takes longer to reach peak metal temperature. A supplier who promises the same lead time on a metallic at full speed and on a stock white is either running a much longer oven or not measuring peak metal temperature at all.
Cooling comes straight after the last oven, and the strip is quenched or air-cooled before the rewind. Winding a coil while the film is still soft presses roller marks into the surface that never come out.
What comes off the line, and what happens to the first metres
Every coil joint, startup and shutdown produces material outside the specification. A well-run line marks those transition zones and cuts them out rather than blending them into finished goods. Ask how a factory handles startup metres: the honest answer is that they are scrapped.
Before the coil is released, samples from head, middle and tail are tested:
- Dry film thickness, measured with an eddy-current gauge across the coil width.
- Adhesion, by cross-hatch and by tape pull, and on some specifications by a boiling water test.
- Bend or T-bend, folding the coated strip over itself to check that the film does not crack at the radius. This is the test that predicts how the coil behaves on your panel line.
- Colour, measured as a difference against the approved standard, with a tolerance agreed in writing before the run.
- Gloss and pencil hardness, for the finishes where those are specified.
Adhesion after bending is the one to watch. A coating can pass a tape test on a flat sample and still fail the moment a panel line folds it 90 degrees.
The back coat, and why a coil is coated on both sides
Every coil carries a back coat. It is thinner than the topcoat and it is not decorative, so buyers assume it is optional. It is not: the back coat stops the reverse face corroding and keeps the coil from sticking to itself in the stack when the metal gets warm in transit. Unroll a coil that has been sitting in a hot container and hear it tear, and you are hearing a back coat that was too thin or under-cured.
Back coats are usually a light grey service colour, specified separately from the topcoat. Ask for both, because they are cured on the same pass. A color-coated aluminum coil specification that names only the topcoat is incomplete.
What a buyer should confirm before the line is booked
Most of what goes wrong with a coated coil order was decided before the run started. Four items, all fixed in writing on the order confirmation, remove nearly all of it:
- Substrate grade and temper, not just the alloy number. AA1100, AA3003 and AA5005 behave differently under the same coating, and the temper decides how the strip forms.
- Coating system and dry film thickness, with primer and topcoat named separately, and the colour tolerance stated as a number.
- Coil dimensions and coil weight, matched to what your own line can decoil. A width that fits our line but not your slitter is a problem you inherit.
- A sample agreed before the run. The approved sample is the reference production is measured against, and the only fair way to settle a colour argument later.
If the coating has to hold its colour on an exterior facade for two decades, the chemistry behind that promise is a separate decision from the process above, and it is worth reading how a PVDF coating is specified for exterior work before choosing the finish. The line described here is the same line for a wood-look finish: a wood grain panel is a printed and coated coil, run after a print station instead of straight after the topcoat. Same pretreatment, same ovens, same tests at the end.
About Jinxiang
Zhejiang Jinxiang Panel Industry Co., Ltd. has manufactured aluminium composite panel and colour-coated aluminium coil in Jinhua, Zhejiang since 1998, from an 18,000 m² factory. We run 5 coil coating lines and a dedicated 2-metre-wide A2 composite line, so the coil and the coating are made under one roof, and we export to more than 70 countries. MOQ is one standard container for stock colours, with a 500 sqm minimum per colour for custom colours, and lead time is 10-15 days.
Send us the specification you are working to, the exposure the building will see and the destination market, and we will tell you which coating system the line should run and what the coil will measure when it comes off. See the colour-coated aluminium coil range, read why buyers visit our factory before they order, or contact us for a quote, a sample kit or a factory tour.