The roll-coated coil production process is shown in Figure 2_7_9, including four major parts: introduction section, pretreatment section, coating section and lead-out section.
(1) Introduction section
Including uncoiling, cutting, seam welding, storage looper, etc. Loosen the raw material coils and connect them, so as to continuously and uniformly supply the metal sheet to be coated to the unit.
(2) Preprocessing section
It is the substrate to be coated and surface treated to improve corrosion resistance and enhance the adhesion of the coating.
① During the rolling process of metal coils, there is hole lubricant on the surface, and some of them are coated with a layer of anti-rust grease before leaving the factory. During pretreatment, the grease and other adherents should be removed first, and then the coated surface should be cleaned Chemical treatment is carried out to make a stable conversion film. The degreasing method is generally to use a 60°C aqueous solution containing a small amount of sodium polyphosphate and a mass fraction of 0.5% to 1.5% sodium hydrogenate, and spray degreasing at a pressure of 0.3MPa.
②Chemical treatment is to form a conversion film on the surface of the coated substrate through pressurized spraying or dipping. In order to obtain the most satisfactory conversion film, different conversion solutions should be selected for metal coils of different materials.
A. Iron oxide-phosphate conversion solution is selected for cold-rolled steel sheets, which is an acidic solution containing phosphoric acid, alkali metal phosphates and oxidants. The acid etches the surface of the steel to form an iron oxide-phosphate amorphous conversion film. The chemical reaction formula is
The ratio of iron oxide to iron phosphate in the conversion film is determined by the pH value of the conversion solution and the operating temperature. The film weight is controlled at about 0.3 s/m2. If oxides are added to the conversion solution as a promoter, it can be used for aluminum and zinc. metal surface treatment,
B. Chromate-oxide conversion solution for aluminum plates, which is a chromic acid solution containing fluoride and molybdate as accelerators, forms a yellow amorphous conversion film on the aluminum surface:
The weight of the film is generally controlled at 0.25g/m2. This conversion liquid contains hexavalent chromium ions. The aluminum plate with this conversion film cannot be used to make food utensils and beverage cans. If phosphoric acid is added to the above conversion liquid, the The conversion of hexavalent chromium into trivalent chromium can solve the above problems and form a chromium salt monophosphate conversion film from white to green:
The film weight is generally controlled at about 0.3g/m2,
c. Two kinds of conversion coatings are commonly used for galvanized sheets, composite oxide type and zinc phosphate type. The composite oxide type conversion solution is an alkaline solution containing heavy metal chelates such as iron, cobalt or nickel. It is corroded by alkali and forms a conversion coating composed of chelates of cobalt oxide, zinc oxide and iron oxide. It is very stable and has good adhesion to the coating.
Zinc phosphate conversion solution is an acidic solution containing phosphoric acid, zinc phosphate and accelerators, which forms crystalline zinc phosphate conversion on the surface of zinc:
The film weight is generally controlled at 0.2g/m2
The pretreatment section should control the indicators of the treatment solution according to the process requirements, and it needs to be continuously tested and adjusted during use.
(3) Coating section bottom
① Topcoat construction. Coatings for rate Kun coating coils should meet the requirements of production technology and processing and use:
The production process is fast rate coating construction. In order to ensure the thickness and leveling of the coating, a certain construction viscosity is required. The 1-volume coating takes 40~50s (coating_4 cups) as the declaration, and the water-based coating takes 28~ 35s (lacquer_4 cups) is declared.
During rate coating, the moving speed of the coating rate and the base plate is different, and the speed of each rate is also different. Taking the full reverse three rate coating machine as an example (see Figure 2_7_10), if the base plate travel speed is +100% , then the painting rate is _120%, the paint extraction rate is _140%, and the control rate is _10%. Due to the large speed difference, the paint between them will be subjected to a large shear force, which requires that the pseudoplasticity of the paint is not or very small, otherwise the viscosity of the paint will decrease significantly with the increase of the shear force received, and it will Make the rate Kun adhere to the required amount of paint.
During roller coating, the traveling speed is very fast, the oven should not be too long (50m), and the drying time of the coating is very short. In order to prevent the coating from producing bubbles, pinholes and good leveling, it is necessary to choose alcohols, ketones, etc. Alcohols, high-boiling aromatics solvents with suitable volatilization speed.
Starting from the performance of the coating film, since only the primer and topcoat are combined, it is required that the primer should have good corrosion resistance and adhesion to the substrate and topcoat, and the topcoat should have good hiding power. And decorative, the coating film does not crack or fall off during processing and forming, and it also has good toughness and hardness.
② layer pressure film. The laminated film is a processed synthetic resin film that is pressure-bonded to a metal tape by an adhesive or other methods. The film can be decoratively treated during manufacture, or printed or embossed after lamination to obtain products with different decorative effects.
In recent years, a pre-coated coil material laminated with a composite film has been developed, that is, a composite film made of Western species or a variety of synthetic resins with different properties, and products with better performance can be obtained.
