[China Aluminum Network] Aluminum alloy material not only has a low density, but also has suitable mechanical properties, excellent corrosion resistance, weldability, formability, and good surface treatment properties. It also has unique reproducibility and is an automotive realization. The preferred material for weight reduction, energy saving, environmental protection and safety. With the automotive industry entering the stage of environmental protection, energy saving and light weight, aluminum and aluminum alloy parts have been extensively used in automobiles, of which aluminum alloy wheels are widely used. In this article, the design of aluminum wheel coating process is discussed to promote the continuous improvement of domestic aluminum wheel coating technology.
Aluminum wheel coating line process design 1 process design analysis 1.1 coating process mode The use of aluminum alloy wheels in large quantities today, according to the different requirements of the product, using different methods. The current development trend of the main aluminum wheel hubs in the world is painting hubs, bright-surfaced hubs, polishing hubs, and plating hubs. The main coating modes are as follows:
1) Low-end product pre-treatment → Metallic paint coating → Finishing varnish coating 2) Mid-range product pretreatment → base powder coating → metallic paint coating → varnish coating 3) High-end product pretreatment → base powder coating Equipment → Metallic Coating → Coating → Light Powder Coating 1.2 Mechanized Conveying Modes The internationally popular aluminum hub transportation method generally adopts two types, ie, ordinary type suspension conveyor and ordinary annular ground conveyor. In the middle, the robots must be used in large quantities and the small batches must be manually.
1) Ordinary suspension conveyor is suitable for painting or dusting, and automatic spraying can ensure the quality of product spraying. The process layout is flexible and convenient. Drying can be done with multiple-stroke drying chambers, or a drying chamber where the hubs run in parallel rows.
2) Ordinary annular ground conveyor is suitable for pre-treatment cleaning process, and 2-3 workpieces can be hung according to different output, and the cleaning effect is good. The spreader is simple and suitable.
1.3 Process Layout Features As with the body coating line, the aluminum hub coating line also uses a regional approach to ensure the different levels of cleanliness needed in each area of ​​the coating shop. Generally, spray booths and powder spray booths should be arranged in the same area. Drying chambers should be arranged in the same area. Drying chambers should be multi-stroke drying. Such an arrangement of aluminum hub painting workshops is conducive to production organization, management, and line-to-line conversion.
2 Introduction to the main process 2.1 Pre-treatment process 1) Processes According to the different products, 9-sequence or 13-sequence processes can be used, all of which use spray treatment. The details are as follows: Pre-degreasing → Degreasing → Washing → Washing → Excluding oxide film treatment → Removing oxide film treatment → DI water wash → DI water wash → DI water wash → titanium/zirconium treatment → DI water wash → Chromium-free passivation → DI water wash → moisture drying →Cooling 2) Process The pre-degreasing and degreasing process is mainly responsible for removing oil on the surface of the workpiece; the purpose of pickling is to remove the oxide on the surface of the workpiece; the purpose of passivation is to regenerate a dense oxide film on the surface of the workpiece.
2.2 Powder spraying and painting process 1) Powder spraying process Base powder spraying and glitter powder spraying can be used according to the different requirements of the product. Spraying is generally electrostatic spraying in the dusting room. When the workpiece reaches the spray station, it starts to rotate and the paint is sprayed from the positioned automatic spray gun to complete the spraying work. Powder spraying equipment (spraying equipment) purchase complete sets of equipment with high voltage electrostatic generator and paint recovery device. An electrostatic precipitator (automated electrostatic precipitator sprays air from 12 nozzles) is required before spraying to ensure the quality of the product.
2) Spray painting process Some products use spray painting process after spraying the bottom powder. Spray booths can be water-rotated or water curtain structures. Before the painting, an electrostatic precipitator (automatic dust-free electrostatic spray from 12 nozzles) is required to ensure the quality of the product. Spray painting or dusting are all automatically sprayed and the workpiece can be raised, rotated, etc. in the spray booth or dusting chamber.
3 Some points worthy of attention 3.1 Selection of coating equipment 1) Pretreatment equipment The pretreatment equipment adopts full spray cleaning method and consists of 9-13 processes (according to different conditions). All the inner walls of the process tank, the inner wall of the chamber, and the circulation pipeline are made of stainless steel. 316L stainless steel or 304 stainless steel is used for the inner wall of the process tank according to the operating conditions. In order to prevent the draining time of the workpieces in the bath or in the spray zone, it takes 0.5 to 1 minute (the interval length is 2m to 4m). Washing should be counter-current cleaning. A drainage section and an automatic water blowing device should be provided at the exit of the pre-processing equipment.
2) Water Drying Room The moisture drying room adopts the convection heating method of large air volume and hot air, and the air inlet and outlet devices are installed to save energy. The ternary body type direct-fired gas hot blast stove, fan, hot blast stove and high temperature filter are combined, and the overall assembly area is small to reduce the pollution link.
3) Before the electrostatic precipitator is sprayed, the electrostatic precipitator is used to blow ionized air to the workpiece to remove the dust adsorbed on the workpiece due to electrostatic charges. Electrostatic fan filter dust removal device, need to be regularly cleaned. The electrostatic precipitator room is equipped with an exhaust system, and the suction outlet is provided with a filtering device.
4) Spraying room and painting room Spraying room cleanliness of 10,000, the illumination range 800lx-1000lx; spray room into the exhaust air can be adjusted to ensure that the paint chamber positive pressure and energy saving requirements. Powder room illumination is not less than 300Lx. Using variable frequency air supply, the temperature control is: 20+2°C in winter, 26+2°C in summer, and the air supply temperature in the leveling room is 60°C. The amount of dust in the air supplied to the dust room is <1.5 mg/m3, and the size of the dust particles is less than 3 μm.
5) Paint Drying Furnace and Powder Curing Furnace The paint drying oven and the powder curing oven adopt a bridge type drying structure, which has the effect of preventing the hot gas from overflowing and the heat insulation effect to be good to save energy. The inner wall of the room should be provided with good heat insulation measures and no thermal bridge can be produced.
3.2 Application of Mechanized Conveying Systems Transportation equipment usually employs suspension conveyor systems and ground conveyor systems.
The front processing line adopts ordinary type suspension conveyor. Transmission speed adjustment adopts variable frequency method, stepless speed regulation and emergency stop button at manual operation place. The pre-treatment line is equipped with an oil pan.
Powder spray booths, spray booths and drying equipment use ground conveying systems. The ground conveyor should be protected against paint fog and high temperature, the same transmission speed adjustment using variable frequency method, should be stepless speed regulation, in the upper and lower parts and workers at the operation of the emergency stop button.
Rotating device (positive and reverse function) is set in the powder spraying room and painting room to ensure that the workpiece can be sprayed in forward and reverse directions.
The orbit in the drying oven has a telescopic function that can absorb temperature-induced changes. If the drying chamber with parallel running of the wheel hubs is used, automatic transfer by the robot arm shall be used to ensure safe and reliable reloading.
3.3 Determine the process plan 1) Flexible production of aluminum wheel coating line construction should be able to meet the needs of flexible mass production, the general economic output of a single coating line is suitable for the conveyor speed of about 7-10m/min.
2) Robot spraying robot spraying is a painting trend and should be sprayed by a robot. The coating production can save energy and paint, improve the spraying quality and the first-time qualification rate of products, and achieve the purpose of recovering investment as soon as possible.
3) Energy-saving equipment must develop new painting energy-saving equipment. Research and development of energy-saving paint spray booths for robotic spraying. Application of circulating air supply to reduce the amount of air intake and exhaust, so that the solvent content in the paint booth can be maintained below the lower explosive concentration. Paint booths can save a lot of heating energy during winter production.
4) Environmental protection coating The continuous development of China's coating technology will make the application of environmentally friendly coatings inevitable. Water-based paints, high-solids paints and other environmentally friendly coatings will be applied to the aluminum wheel coating. The newly designed aluminum wheel coating line should consider the application of water-based paint technology to meet the needs of future trends.
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