With the development of industrial technologies, the automotive industry has penetrated many areas of the national economy. In the process of developing the automobile production line in China, it has gone from manual production line to automatic production line. Since the 1960s, due to the development of light industry in the bicycle industry, mechanized production lines and automated production lines first appeared, as well as CNC production lines formed by adding CNC machines based on the original chute production. line. It is mainly in the area of Shanghai and Tianjin. During this period, the main task of the Chinese auto industry was still corrosion protection. However, with the development of China's economy and the development of foreign surface treatment technology, through the introduction of technology and exchange with foreign technology, China's surface treatment technology began to develop rapidly. the promotion and application of powder coating technology, the development and promotion of powder coating technology, especially the active development of China's home appliance industry, household appliances, steel furniture, aluminum components, electrical products, automotive industry, etc., have made significant progress in the surface treatment industry.
1. Automotive surface treatment
There are many automotive surface treatment processes, classified by nature, as shown in the table
1.1 Electrochemical processing
The electroplating process is a method of coating a conductor with a layer of metal using the principle of electrolysis. Electroplating refers to a surface treatment method in which cations of a pre-plated metal in an electroplating solution are deposited on the surface of a base metal by electrolysis with the base metal to be plated as a cathode in a saline solution containing a pre-plating solution. metal to form the coating.
The characteristics of the coating are different from the characteristics of the base metal and have new characteristics. According to the purpose, the coating is divided into protective, decorative and other functional coatings.
During electroplating, coated metal or other insoluble materials are used as the anode,and the coated part is used as a cathode, and the coated metal cations are reduced on the surface of the coated part to form a coating. It can improve the corrosion resistance of metal (plating metal is mainly made of corrosion-resistant metal), improve hardness, prevent wear, improve electrical conductivity, smoothness, heat resistance and beautiful surface.
Automotive applications: Exterior trim: grille, decorative strips, signs, exterior mirrors, door handle bolts, electrical parts, engine mechanism parts, etc.
Galvanized decorative strip
The process of using aluminum or aluminum alloy products as anodes, placing them in an electrolyte solution for electrification, and using electrolysis to form an aluminum oxide film on the surface is called anodic oxidation treatment of aluminum and aluminum alloys. After anodic oxidation treatment, the surface of aluminum can form a micron-sized oxide film. Compared with natural aluminum alloy oxide film, its corrosion resistance, wear resistance and decorative effect have been greatly improved and enhanced.
Painting of automobiles: mainly for the surface treatment of automobile bodies, which is the main link in the production of automobiles.
Car body painting is generally divided into primer, intermediate paint and top paint. The primer is the first paint applied directly to the surface of the workpiece after surface treatment, and it is the basis of the entire coating. The intermediate coat is the last intermediate coat before the top coat, the paint content of it is between the primer and the top coat, the coating film is light and semi-gloss. Automotive topcoats are the last coat of paint in an automotive multicoat. It directly affects the decorative effect, weather resistance, gloss and color retention, chemical resistance of the car, and therefore makes very high demands on the quality of the car's finish. As a rule, acrylic polyurethane or acrylic polyurethane modified with polyester is used.
1.2.2 Cathodic electrophoresis
Travel electrophoretic coating is a special method of coating film formation. The object being coated is used as the cathode and the electrophoretic coating used is cationic (positively charged). Tank for electrophoretic paint, diluted with water and relatively. A low concentration is used as a cathode, and an anode corresponding to it is installed in the tank, and a direct current is passed between the two electrodes, and a uniform, water-insoluble coating film is applied to the object to be coated. method.
Compared to traditional solvent-based coatings and other water-based dip and spray coatings for baking, cathodic electrophoretic coatings have incomparable advantages: the primer application process can be fully automated and is suitable for high-volume production; uniform film thickness; good buoyancy. penetration, which improves the corrosion resistance of the inner cavity of the workpiece, especially the corrosion resistance of the cathode electrophoretic coating film; high paint utilization rate; relatively high safety, it is a low pollution paint; appearance of the coating filmi Yes, has better adhesion and flatness when dry.
Cathode electrophoresis in a car
1.3 Chemical treatment
Phosphating is a widely used pretreatment technology. In principle, it should be considered as a chemical conversion film. It is mainly used for phosphating steel surfaces. Non-ferrous metals (such as aluminum and zinc) can also be used for phosphating.
The purpose of phosphating is mainly to protect the base metal and prevent its corrosion to a certain extent; it is used as a primer before painting to improve the adhesion and anti-corrosion ability of the paint film layer. ; plays a role in the process of cold working of metals Anti-friction lubricant.
Phosphating film is a layer of inorganic salt crystals between the metal substrate (metal) and electrophoretic ink (organic matter), which plays an important role in bonding the previous and the next. Specific functions of the phosphating film include: improving the corrosion resistance of the base and anti-corrosion ability; improving the adhesion force between the metal and the coating film; improving the electrical insulation of the car body.
With cathodic electrophoresis, the general body of the car needs a passivating treatment. The main component is a solution of hexavalent chromate. On the other hand, calcium and magnesium ions introduced by washing with water are removed, and the surface becomes slightly acidic for subsequent electrophoretic painting, thereby improving corrosion resistance and paint adhesion.
Dacromet is a zinc-chromium coating, a new type of anti-corrosion coating, the main components of which are zinc powder, aluminum powder, chromic acid and deionized water.
Main application on trucks: anti-corrosion treatment of various high-strength medium and small chassis brackets, connectors, exposed parts and fasteners, such as various special-shaped steel brackets, bolts (including seat bolts, wheel bolts, etc.). ), nuts, etc. Anti-corrosion protection of metal parts in high temperature environments, such as around the engine, such as heat shields, exhaust pipes, radiators, engine cylinder heads and other parts. Anti-corrosion treatment of various elastic parts of trucks, such as hoops, semi-circular hoops, various springs, leaf springs, etc.
Blackening is a common chemical surface treatment method. Its principle is to create an oxide film on the metal surface to insulate air and prevent rust. When the requirements for appearance are not high, blackening can be used. The surface of steel parts is blackened, and some are also called bluish.
Processing the connector with dacromet
1.4 Heat treatment
1.4.1 Hot dip coating
Pre-treatment before coating and post-treatment during hot dip coating are also important processes. Pre-treatment is to clean the surface of the coated parts and remove or restore the surface oxide to obtain a clean surface of the base metal, which is the key to ensure the quality of the coating and prevent the absence of coating defects. In a batch or low speed continuous hot dip plating process, steps such as caustic cleaning, pickling and fluxing are typically included. In the HSS continuous hot plating production line, the reducing gas annealing method is mainly used for surface pretreatment. Post-treatment typically includes passivation, phosphating, and oiling of the surface of the coating to improve corrosion resistance, paintability, and other characteristics of the coating.
Main automotive applications: muffler heat shields, truck body frames, car compartments and exhaust pipes, etc.
1.4.2 Thermal Spray
Thermal spraying is a surface hardening technology and an important part of surface treatment technology. It has always been a new technology project that has been promoted in my country. It uses a specific heat source to heat powdered or fibrous metal or non-metal. metal materials to a molten or semi-molten state, and then sprayed onto the surface of the pre-treated substrate at a certain speed using the flame itself or compressed air, and deposited to form a surface coating with various functions.
Main automotive applications: engine pistons, timing rings on gearboxes, etc.
1.4.3 Hot Stamping
Hot stamping consists in transferring a coating previously applied to a foil base film onto a decorated article by heat and pressure. This transfer process is divided into three processing processes depending on the shape of the article to be decorated, namely, traditional hot stamping, in-mould decoration (IMD), and insert molding (INS). Traditional hot ironing and due to the shape of the surface, you can use the flat ironing method or the rolling ironing method. Both of these processes use a heated ironing plate or roller to transfer the coating to molded parts mounted on a fixture. Today's different standards Hot stamping foils can already be market, and the equipment and fixtures are relatively common and easy to deploy. With clear cost advantages and environmental factorsenvironment, the effect of metal hot stamping is better than that of electroplating.
Automotive applications: Heat dissipation grills, chrome trims, engine top covers, signs and buttons are more suitable for this process.
1.4.4 Chemical-thermal treatment
Chemico-thermal treatment is a metal heat treatment process that uses chemical reactions, and sometimes physical methods, to change the chemical composition and surface structure of steel in order to obtain better technical and economic advantages than homogeneous materials. Since most of the failure and damage of mechanical parts occurs in the surface layer, especially for parts that can cause wear, fatigue, metal corrosion, oxidation and other conditions, the characteristics of the surface layer are especially important. Steel parts after chemical-thermal treatment can essentially be considered as a special composite material. The core is steel of the original composition, and the surface layer is a material impregnated with alloying elements. There is a strong crystalline bond between the core and the surface, which is much stronger than the bond between the heart and the surface obtained with surface repair methods such as electroplating.
1.5 vacuum method
1.5.1 Vacuum evaporation
Vacuum evaporation (vacuum evaporation) coating is to use an evaporator to heat the evaporated material under vacuum to make it sublimate, and the stream of evaporated particles is sent directly to the substrate and deposited on the substrate to form a solid film. , or heat and vaporize the coating material by the vacuum coating method.
Used in car: car headlights
1.5.2 Ion sputtering (PVD)
Ion coating deposition is a glow discharge in a partially evacuated spray chamber to generate positive gas ions; when the voltage between the cathode (target) and the anode (sample) is accelerated, positively charged ions bombard the cathode surface, sputter the material on the cathode surface, the resulting neutral atoms are sprayed from all sides and fall on the sample surface, forming a uniform film on the sample surface.
Automotive applications: automotive glass, engine piston rings, etc.