The surface treatment of metals before painting mainly includes degreasing, rust removal and phosphating (oxidation).
The oil removal methods can be divided into physical mechanical method and physical chemical method, the former includes wiping method, combustion method and injection method. The main degreasing methods used before automobile painting are physical and chemical methods, including alkali degreasing, emulsion degreasing and solvent degreasing Liquor degreasing process removes grease mainly through saponification. When removing unsaponifiable mineral oils, surfactants are used to help emulsify and degrease. This method is widely used and has better degreasing effect and is more economical. Emulsion degreasing is a mixture of one or more surfactants Or weak trisexual detergents in organic solvents. When Lou impregnates or sprays the cleaned product with the emulsion, the solvent soaks the grease to make the grease particulate, while the surfactant emulsifies and disperses the grease particulate in the cleaning solution, thus removing the grease. There are usually two methods of solvent degrea Sing: impregnation and trichloroethylene cleaning. Impregnation is a method of degreasing by immersing the cleaned substance in organic solvents, stirring or rinsing manually. The commonly used solvents are gasoline, xylene, acetone, etc. Trichloroethylene cleaning method, also known as dry cleaning method , usually heating trichloroethylene, using trichloroethylene vapor to degrease the cleaned products.
Rust removal is the removal of oxides and hydroxides from metal surfaces. The method of rust removal is usually selected according to the rust color, formation state and procedure. Rust removal methods can be divided into physical and chemical rust removal. The rust removal usually uses the tools such as spade, scraper, hammer, wire brush and abrasive cloth, so it is not suitable for automobile manufacturing because of its high The usual used tools for mechanical rust removal are hand-held electric grinding wheel, electric brushing wheel, Pneumatic brush and pneumatic rust removal wheel with needle bundle. At present, most of the mechanical rust removal methods are sand bl The metal immersed in acid solution, so that oxides and hydroxides attached to metal surface are chemically reacted and dissolved in acid solution to remove Rust.
Phosphating treatment is a method by which the metal surface is chemically treated to form an insoluble and phosphating double salt protective film. Phosphating treatment is widely used in automotive industry for panels, cabs, body panels, etc. As a surface treatment before painting, it Zinc phosphate film is thin, compact, smooth, corrosion-resistant and can with stand about desire deformation, but the bath is not stable enough and Maintenance is complex. The manganese phosphate type has the characteristics of stable tank liquid and convenient maintenance.
Precautions for spraying metal paint
(1) After spraying metal paint, dust must be removed before the next operation.
(2) Metal paint mainly plays a covering role, do not spray too thick, otherwise the paint color will be too dark and too dark.
(3) Stir evenly. Metal particles such as aluminium powder in metal paint are easy to precipitate at the bottom of paint tank, so metal paint needs to be thoroughly stirred before spraying.
(4) The appropriate diluent and dilution ratio must be selected according to the instructions provided by the paint manufacturer, and the viscometer should be used correctly to master the required viscosity to ensure that the paint color meets the requirements.
(5) Use of spray guns. When spraying metal paint, uniform film thickness is very important. The nozzle should aim at the spraying surface at a vertical angle, and the distance between them should be kept between 5 and 20 cm (Fig. 5 The spraying gun should be moved in a straight line at a uniform speed to avoid moving the spraying gun in an arc shape. Each reciprocating spray requires 50% of the overlapping area, so that a fixed color can be obtained. When it is The necessary to adjust the amount of paint sprayed from the spray gun, the spray gun trigger can be pulled to the bottom, and the moving speed of the spray gun can be used to increase or reduce the thickness and the dryness of the paint film.
