The Complete Coating Line in One Pass
Colour-coated galvanized steel coil is produced on a continuous coil coating line, where an already galvanized strip is painted, cured and recoiled without stopping. The sequence is fixed and every step exists to protect the quality of the next one:
Uncoiling and stitching of the strip ends, followed by an entry accumulator that lets the line run while coils are changed.
Pretreatment, normally alkaline degreasing followed by a conversion coating such as chrome-free passivation or a thin pretreatment film.
Coating of the front and back faces by roller, usually primer first and then topcoat, each with its own oven pass.
Baking and curing in a multi-zone forced-air oven, then post-treatment, cooling, exit accumulator, recoiling and final inspection.
Strip thickness typically ranges from 0.2 mm to 1.2 mm and line speed from 80 to 200 m/min, which is what makes coil coating one of the most efficient ways to apply an organic finish to steel.
Roller Coating: The Core Application Step
Roller coating is the mainstream technique for applying liquid paint to a moving strip. A pick-up roller lifts paint from the pan, a metering roller controls how much paint stays on the surface, and the applicator roller transfers it to the strip. Coating weight is set by the roller gap, the roller hardness and the speed ratio between rollers and strip, so it can be adjusted without stopping the line.
Forward coating. The applicator roller rotates in the same direction as the strip. More paint is transferred, the film looks fuller and hides the substrate better, but roller marks or a slight ribbed pattern can remain on the surface.
Reverse coating. The applicator roller rotates against the strip direction. Transfer is lower and more even, the film is thinner and smoother, and thickness control is more precise. Reverse coating is the preferred method for appearance-critical topcoats and is used for most automotive and appliance panels.
Baking and Curing: Peak Metal Temperature
The oven does two jobs: it drives off solvent and it triggers the cross-linking reaction that turns the resin into a tough solid film. Ovens are divided into a preheat zone, which raises the strip gently so solvent leaves without forming blisters or pinholes, a reaction zone that brings the strip to the peak metal temperature required by the resin, and a cooling zone that stops the reaction and stabilises the film.
Peak metal temperature (PMT) is the highest temperature reached by the strip itself, not by the oven air, and it is the single most important curing parameter. Too low and the film stays under-cured and soft; too high and the resin degrades and the colour shifts.
| Resin system | Typical peak metal temperature | Dwell in the curing zone |
|---|---|---|
| Polyester primer | 210-224 °C | 20-40 s |
| Standard polyester topcoat | 224-232 °C | 20-40 s |
| Silicone-modified polyester topcoat | 232-241 °C | 25-45 s |
| Fluorocarbon (PVDF) topcoat | 241-249 °C | 30-60 s |
Post-Treatment, Cooling and Coiling
After curing, the strip is cooled by air and sometimes by water quenching to the recoiling temperature, then given a thin post-treatment. Typical options are a passivation rinse that improves resistance to finger marking and storage staining, a thin wax or oil film that reduces friction during pressing, or a removable protective film for panels that will be formed or transported. The strip is then recoiled under controlled tension so the coil does not telescope.
Parameters That Control Finished Quality
Substrate quality. Zinc coating uniformity, roughness and cleanliness determine how well the pretreatment and paint adhere.
Pretreatment. A complete, defect-free conversion film is the foundation of long-term adhesion and corrosion resistance.
Paint formulation. Resin type, pigment, additives and solvent balance must be stable from batch to batch.
Coating weight accuracy. Film thickness should be uniform across the width and along the length of the coil.
Curing accuracy. PMT and dwell time together determine cross-linking, weather resistance and formability.
Environment control. Cleanliness, temperature and humidity around the coating heads and ovens must be kept within set limits.
Dry film thickness for a colour-coated coil is normally 5-8 µm of primer plus 15-25 µm of topcoat, giving a total of about 25-35 µm. Material utilisation approaches 100 percent because the coating is applied to a continuous strip rather than sprayed, which is one reason coil coating remains economical at high volume.
Frequently Asked Questions
Q: What is the difference between forward and reverse roller coating?
Forward coating rotates the applicator roller with the strip and transfers more paint, giving a fuller film with possible roller marks. Reverse coating rotates against the strip, giving a thinner, smoother and more precisely controlled film for topcoats.
Q: Why is peak metal temperature more important than oven temperature?
The resin reacts according to the temperature of the steel it sits on, not the air around it. Strip thickness, line speed and oven load all change how much heat reaches the strip, so PMT is measured directly and used as the control set point.
Q: How thick is the paint on a colour-coated galvanized coil?
A typical product carries 5-8 µm of primer and 15-25 µm of topcoat on the exposed face, so total dry film thickness is usually between 25 and 35 µm. Heavier builds are used for exterior architectural panels.
Q: Can both faces be coated at the same time?
Yes. Most lines coat the two faces with separate heads in the same pass, and the faces can carry different films or different colours. A common configuration applies a thick topcoat on the exposed face and a thin primer or a low-gloss backing coat on the reverse.
Q: What happens if the coating is under-cured?
Cross-linking stays incomplete, so the film is soft and solvent-sensitive, adhesion is poor and corrosion resistance drops sharply. Under-cured coil can also block or stick when it is stacked, because residual solvent remains trapped in the film.
Q: Is the process environmentally acceptable?
Solvent and waste gas from the ovens are collected and treated centrally, so emissions are easier to control than in open spray painting. Chrome-free pretreatment is now widely used, further reducing the environmental load of the line.