How to make color coated steel plate

1. Substrate preparation
1. Substrate type selection
Cold-rolled steel sheet: The surface is flat, suitable for coatings with high precision requirements, but the corrosion resistance is poor and requires subsequent coating treatment.
Hot-dip galvanized steel sheet (GI sheet): The surface of the substrate is galvanized (the thickness of the zinc layer varies from 5-100μm), and the corrosion resistance is significantly improved. It is the most commonly used color-coated substrate.
Hot-dip aluminum-zinc steel sheet (GL sheet): zinc-aluminum coating (55% aluminum + 43.5% zinc + 1.5% silicon), the corrosion resistance is better than galvanized sheet, suitable for high humidity or corrosive environment.
Electrogalvanized steel sheet: The zinc layer is uniform but thin (1-5μm), mostly used in scenes with medium anti-corrosion requirements such as home appliances.
2. Substrate pretreatment
Kaiping: Unroll the steel coil and cut it into flat steel sheets (or keep it in coils for continuous production lines).
Cleaning: Remove surface oil, iron powder and other impurities through alkaline solution, emulsion or electrolytic cleaning to ensure coating adhesion.
Annealing (if necessary): Cold-rolled substrates need to be annealed to improve mechanical properties (such as hardness and ductility).
2. Surface treatment (enhancing adhesion)
1. Chemical conversion treatment
Passivation treatment: Form a nano-scale passivation film on the surface of the substrate through chromate (such as hexavalent chromium, trivalent chromium) or chromium-free passivator (such as zirconium salt, titanium salt) to enhance corrosion resistance and coating adhesion.
Phosphating treatment: Generate a zinc phosphate or iron phosphate crystalline film on the surface of the substrate to increase surface roughness and enhance the mechanical bite ability of the coating.
2. Roller coating primer (or plating)
Primer type: Commonly used epoxy primer, polyester primer or polyurethane primer, with a thickness of about 5-10μm.
Function:
Enhance the adhesion between the substrate and the topcoat;
Provide additional anti-corrosion protection (such as primer containing zinc powder);
Improve surface flatness and prepare for topcoat.
3. Coating (core process)
1. Selection of coating type
Organic coating:
Polyester (PE): cost-effective, rich in color, moderate weather resistance, used in construction and home appliances.
Silicon modified polyester (SMP): better weather resistance than PE, suitable for long-term outdoor use (such as roofs).
High durability polyester (HDP): stronger UV resistance, good color retention, used in high-end buildings.
Polyvinylidene fluoride (PVDF): excellent weather resistance (service life of more than 20 years), but high cost, used in high-end scenes such as curtain walls.
Thin film coating: such as polyvinyl chloride (PVC) film, polyolefin (PO) film, providing high wear resistance and decorative properties.
2. Coating process
Roller coating: the most commonly used, the coating is evenly applied to the surface of the substrate by a coating roller, which is divided into single coating and single baking (one coating on one side) or multiple coating and multiple baking (such as two coatings and two baking on both sides).
Coating thickness: topcoat is usually 5-25μm, adjusted according to the purpose (such as thicker coating for building exterior).
Spraying method: used for complex shapes or special effects (such as sand grain, wood grain), the coating utilization rate is low.
Lamination method: the prefabricated film is laminated on the substrate by hot pressing, which is suitable for products with high surface requirements (such as home appliance panels).
3. Drying and curing
After coating, it enters the drying oven and is heated at 150-250℃ for several minutes to evaporate the solvent in the coating and cause a cross-linking reaction to form a hard coating.
IV. Post-processing and processing
1. Surface inspection
Online detection of coating thickness, surface flatness, glossiness, adhesion (such as cross-cutting method) and corrosion resistance (such as salt spray test), and rejection of unqualified products.
2. Finishing treatment
Laminating: a protective film (such as PE film) is laminated on the coating surface to prevent scratches during transportation or processing.
Embossed/printed: patterns (such as orange peel grain, wood grain) or printed patterns are pressed out by rollers to enhance decorativeness.
Slitting/rewinding: Cut the steel plate into specified sizes (such as coils or flat sheets) according to customer needs.
3. Special functional treatment (optional)
Antibacterial coating: Add antibacterial agents such as silver ions for medical and food equipment.
Self-cleaning coating: Contains photocatalysts (such as TiO₂), decomposes pollutants through light, and is used for exterior walls.
Fingerprint-resistant coating: Prevents fingerprint residue and is suitable for home appliance panels.
V. Application areas
Color-coated steel plates are widely used in:
Construction industry: roofs, walls, rolling doors, etc. (high weather-resistant coatings are required).
Home appliance industry: refrigerators, washing machines, air conditioner shells (need to be beautiful and scratch-resistant).
Automotive industry: car panels, parts (need to be impact-resistant and chemical-resistant).
Furniture and decoration: cabinets, ceilings, decorative panels (need diversified surface effects).
Key technical points
Coating adhesion: depends on the cleanliness of the substrate, the quality of chemical treatment and the coating formula.
Weather resistance: Outdoor panels need to choose highly weather-resistant coatings (such as PVDF, HDP) and increase the coating thickness.
Environmental protection requirements: In recent years, chromium-containing passivation processes and solvent-based coatings have been gradually eliminated, and chromium-free treatments and water-based coatings have been promoted.