Painting materials, painting process, painting equipment, and painting management are the four major elements of automobile painting. They complement each other and promote the progress and development of painting process and technology. The 21st century is known as the environment-oriented new century. Environmental protection has attracted worldwide attention and has become the most urgent research topic for human beings. The painting of automobiles and their parts is one of the most energy-intensive and waste-producing parts of the automobile manufacturing process. Therefore, reducing paint pollution, reducing coating costs, and improving coating quality have always been the subject of the development of coating technology.
Application of new coating materials
The application of new coating materials is the forerunner in the progress of coating technology. Under the premise of continuously satisfying the coating performance requirements, the main development goal is to apply materials that can reduce pollution and reduce coating costs.
Degreasing agent formulated with biodegradable active agent, nickel-free phosphating solution, nitrite-free phosphating solution, chromium-free deactivator, low-temperature degreasing agent (processing temperature 43°C), and performance with the same low temperature as conventional (35 °C) Less slag (10 to 30% lower than normal) Phosphating solution, lead-free and tin-free cathodic electrodeposition coating and low temperature curing (160°C for 10 minutes), low heating loss (4% or less), low VOC evaporation (0.4% The ~0.8%) cathodic electrodeposition coating has been used for many years in Europe, America and Japan. In North America and Europe, secondary electrocoats that can replace conventional midcoats have begun to be applied.
In Europe, some auto companies have adopted water-based paints in newly-built paint lines in recent years, and VOC emissions have fallen below the regulatory requirement of 35 g/m2 (Germany TA-Luft, 1995). Since the 1990s, all new coating line primers have used electrophoretic primers or powder coatings, while middle coatings use waterborne coatings or high solids materials, and topcoats use waterborne basecoats and high solids varnish. At present, powder varnish has begun to be applied to car body painting of cars. Following the industrial application of powder varnish, powder metal background has also been commercialized. In North America, powder coating has been used industrially for many years. At the same time, the background color of water-based topcoats has become very popular in recent years. High solids intermediate coating and topcoat applications are also quite common. The application technology of ultraviolet (UV) curable coatings in Europe and the United States in automotive painting has reached near maturity. Japan is also actively developing and promoting waterborne coatings, high solids and ultra-high solids varnish.
As the global environment continues to deteriorate, we must significantly improve the acid rain resistance and scratch resistance of automotive coatings. In recent years, it has become common to reduce the thickness of the electrophoretic primer film on the inner and outer surfaces of the vehicle body, such as high throwing power, low pigmented electrophoretic coating, acid rain and scratch-resistant top coats, multi-color coating, and viscosity control techniques. application. The major automotive companies in China have little difference from the international level in the application of paint pretreatment materials and electrophoretic paints. However, due to the use of special equipment for waterborne intermediate and waterborne topcoats, coating costs are increased. Although wholly foreign-owned and joint-venture companies have the ability to produce aqueous mid-coats and water-based topcoats in the domestic market, automotive coatings for middle and topcoats still use traditional medium-low solids solvent-based paints, and coating VOC emission lags behind Europe.
Coating process and equipment
In the past decade or so, the progress of painting processes and equipment has been mainly reflected in the application of environmentally friendly coating materials, reduction of waste water and waste residue emissions, reduction of costs, and optimization of automobile production processes. Thanks to the advancement of coating materials, the design of body coating systems has also undergone revolutionary progress. Several typical new coating systems and new technologies have been or will be used in industrial production. The current coating technology and equipment in our country is equivalent to that of Europe and the United States 10 years ago. Some companies have adopted some new advanced equipment of the current international level in the new coating line.
1. Water saving and waste recycling technology
Pretreatment and electrophoresis are the biggest links in the water consumption and waste water discharge for automotive painting. With the continuous maturation of membrane technology, the use of membrane separation technology (UF and RO) to recover degreasing liquid, regenerating cleaning water and pre-treatment wastewater, makes it possible to achieve true electrophoretic closed-circuit cleaning. At present, membrane separation technology has begun to be applied.
In recent years, with the industrialized application of environmental protection coatings in developed countries, some of the coating recycling technology (such as powder slurry recycling technology, powder color and varnish recycling technology, waste paint flocculating dryer technology, ultrafiltration, cooling method The use of water-based lacquer technology for electrostatic adsorption, and water-based lacquer recovery technology for overspray, etc., have also been applied to further increase the coating utilization rate of the coating line and minimize the emission of spent paint slag.
At present, our country pays less attention to the comprehensive utilization of water-saving coating and three wastes, and it is relatively backward in the application of new technologies.
2. Several new body coating processes
Reverse process: Spray the powder coating on the outer surface of the vehicle body, and perform the electrophoretic coating after hot melting, and then dry the powder/electrophoretic coating together. The use of this process can reduce the amount of electrophoretic coating by about 60%, replace the electrophoretic primer and middle coat on the outer surface of the car body with a powder coating with a thickness of 70μm, and eliminate the middle coating and drying process, thereby saving material and energy costs. Reduce VOC emissions.
Secondary Electrophoresis Process: Two coats of electrophoretic material were used and a second layer of electrophoresis (35-40 μm) was substituted for the middle coat. The automation of electrophoresis process construction has high stability and reliability, high primary qualification rate, high material utilization rate, and low equipment investment (without air conditioning system), which can save 48% of the cost, and reduce the frequency of maintenance and traditional paint slag and VOC. emission.
Integral painting process (three-layer concept): The functional layer (15μm) in the same color as the topcoat is used instead of the middle coating. No drying is required between the functional layer and the background color of the topcoat. The middle coating line is cancelled, and the production efficiency is improved. At the same time, VOC emissions have been substantially reduced.
3. Membrane technology replaces plastic cover coating
The film coating technology is a kind of prefabricated top-coating film suitable for thermoforming, and the top-coat paint performance and appearance of the thermoformed product are very similar to the traditional baking and spraying coating film. This technology is mainly applied to the production of plastic parts. The "inclusion molding" or "inner mold" process is used to form the prefabricated composite coating at the same time as the plastic part is poured into shape and melted into one with the plastic part to obtain a defect-free coating. Fit the cover. The body frame is manufactured using the traditional stamping and welding process. The painting shop only paints the body frame, and the top coat is made of powder coating technology. Because the exposed area of ​​the body frame is small, the color of the top coat does not have to be the same as that of the cover. Large-area coverings are plastic parts that are manufactured using a film coating technique. There are thousands of colors. This greatly simplifies the body coating process, while reducing the coating costs, while the coating VOC emissions to about 7g/m2, far below the European emissions regulations.
4. Body painting P2ZERO concept
The so-called P2Zero concept is a zero discharge paint shop. Under the premise of meeting stringent environmental protection requirements and user quality requirements, the cost of the three wastes treatment is reduced, the paint shop operation cost is reduced, and the paint process is simplified. Anti-corrosion primer for the body steel plate is applied before the parts are made. The body that enters the paint shop does not need to be primed. Only a powder background color and a powder coating are sprayed on, so that the process waiting time can be minimized. Eliminate the traditional paint booths, adjust the process more flexibly, eliminate the anti-rust process from the production process of the steel plate to the painted body, completely eliminate the problem of poor corrosion protection of the traditional coating welds and cavity structures, save the paint shop area, and reduce Three waste treatment costs, no paint slag system and waste paint treatment system, no painting room exhaust, air pollution and solid waste tends to zero, no liquid discharge, paint manufacturing and use efficiency is greater than 95%, odorless and non-hazardous.
5. Painting of chassis parts
In developed countries, phosphating, cathodic electrophoresis, or powder coating processes are commonly used for the coating of chassis parts. Generally, parts are first electrophoretically or powder-coated and then assembled. If necessary, the assembly is then sprayed with a low-temperature or room-temperature curing finish. . Due to the good mechanical properties of cathodic electrophoresis and powder coating, machining of some parts can be performed after painting, which can prevent parts from rusting due to long processing time before painting. Frames and chassis parts are mostly hot-rolled plates and castings. Shot or sandblasting is used before or before coating. Pickling is gradually eliminated.
At present, the production scale of parts and components in China is generally small, and it is generally relatively backward. Passenger cars and other small passenger vehicles are relatively good, and the painting process and the level of paint used for trucks, passenger cars and agricultural vehicles are not high.
6. New coating transporter
At present, the typical conveyors used in pre-processing and electrophoresis of body coating lines at home are push rods, catenary chains, and program-controlled hoists. Each of them has its own advantages and disadvantages, and the common disadvantage is that it cannot solve the problem of effective exhaust in many cavity structures inside the vehicle body, especially the airbag inside the roof cover. These sites cannot be phosphated and electrophoresed. The birth of the new Vario-Shuttle and RoDip not only solved these problems, but also inherited all the advantages of the aforementioned transport aircraft.
The multi-functional shuttle also has the ability to realize multiple types of different processes on a single production line, and fully meets the requirements for automated flexible coating production. These two kinds of transport aircraft have already begun to be applied in the new domestic car body painting line.
7. Improvement of the structure and function of other coating equipment
In Europe, the structural materials of painting equipment are mainly stainless steel. The electrical circuits of the equipment are designed in the equipment structure. The electrical piping, the modular design of the equipment and the three-dimensional partitioning arrangement of the workshop are eliminated to ensure the maximum High cleanliness requirements and safety fire protection requirements. The new generation of drying chambers with no external air ducts and internal radiation-enhanced convection significantly improves the thermal efficiency and the uniformity of the drying temperature. With the advancement of robot technology, the auto body coating machine has gradually been replaced by a multi-degrees-of-freedom painting robot. Spraying and sealing have been largely automated by the robot and are more flexible than the coating machine. Non-metallic materials are commonly used in mechanized transmission chains to significantly reduce transmission noise.
Coating management
Coating management includes the ordering of coating materials, material quality control, and construction process control. Due to the variety of materials used in automobile painting, the deterioration in storage and transportation, the long process flow, and the many parameters that need to be controlled, the complexity of automotive production management is the highest. In the developed countries of the automotive industry, the development of automotive coating materials has promoted the progress of automotive coating technology. In order to reduce coating costs, the supply mode has also transitioned from a single material supply to a system supply. The system supply method started in the early 1990s and has now been basically popularized in Europe and the United States. The world's major automobile manufacturers have always attached great importance to coating management. The paint manufacturers are directly responsible for the technical management of coating production, which greatly simplifies the coating management of automobile production plants, increases the qualification rate of coating once, and reduces production costs. At present, this type of management is still at an experimental stage for domestic auto companies.
In recent years, some of the most powerful coating material companies and coating equipment companies have also launched the BOT service model, that is, the paint shop of the automobile company is invested by the coating material company or the coating equipment company and is responsible for production management. The automotive company's entire vehicle production plan is painted, and the supply to the car company is a qualified coating film. Automobile companies only need to formulate technical standards and acceptance inspections to monitor the quality of finished products. This will become a major trend in the development of coating management.
The gap between China's auto painting technology and the international level is continuously narrowing, but the development is still unbalanced. In terms of guaranteeing coating quality, several major car manufacturers have reached international standards, but the overall comparison still has a gap of about 10 years. Mainly reflected in the clean production technology; in terms of automotive coating production key equipment technology, China may rely mainly on imports for a long time in the future. It is expected that in the next 10 to 15 years in China, the level of car painting will be fully in line with international standards, accelerating the application of resource conservation and environmental protection technologies.
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