Coating is divided into dip coating, spraying, drenching, rolling and brushing, etc., of which dip coating and spraying are the main ones. Dip coating is also known as electrophoretic coating, which employs electrochemical methods to deposit organic resin particles on the surface of workpieces to form transparent or organic coatings of various colors. Among them, cathodic electrophoresis is a new process developed in the seventies and is the main process in the dip coating industry. Advantages of good corrosion resistance, strong anti-color change performance, good bonding characteristics. Spraying is divided into powder coating and liquid coating, of which electrostatic coating is the more popular process in the industry.
Ⅰ The scope of painting process
Due to good corrosion resistance, low price, color system (can do any different colors) it is suitable for all aluminum materials, but the material structure is too loose such as sand products, need to scrape the gray work, the relative quality is also to be poor.
1. The working principle of electrostatic coating
Under the action of the pump, the paint enters the electric field, and the paint particles are negatively charged.
Under the action of atomizing air pressure, the paint becomes finer and uniform and then under the action of working air pressure, the paint is evenly sprayed on the surface of the workpiece under the action of rotating cup or nozzle.
As in the same vertical plane, the distance between the spray gun and the workpiece is equal, the same interface electric field strength is also the same, so the positive and negative charges are attracted to each other, and the surface of the workpiece is coated with a layer of uniform organic resin coating.
Then under the condition of high temperature baking, the resin reaches the state of leveling and curing. Firmly set on the surface of the workpiece to achieve the role of protecting the workpiece.
2. Aluminum coating process
Mechanical grinding → Degreasing → Removing oxide film → Chromating → Spraying powder or liquid → Baking → Hanging down → Full inspection → Packing → Storage
1）Common abnormalities in powder and liquid operations and handling methods
| Appearance | Cause | Corrective Preventive Measures | Remark |
| Orange peel | 1, Powder spray too thick; 2, Oven temperature mismatch; 3, Powder quality is too bad. | 1, Adjust the gun spit out; 2, Adjust the temperature; 3, Change the quality of powder. | Liquid spraying when out of the skin: 1, Paint performance matching; 2, Work with splash to the work; 3, Paint viscosity. |
| Paint chipping | 1, Pre-treatment is too poor; 2, Temperature is not enough, did not bake dry; 3, Powder itself is not a good combination of power. | 1, Strengthen the pretreatment; 2, Adjust the temperature; 3, Adjust the powder quality. | Liquid paint chipping: 1, Improper pretreatment; 2, Improper temperature; 3, The difference between primer and topcoat is too large. |
| Expose | 1, Powder sprayed too thin or not sprayed; 2, Mechanical pre-treatment is not thorough. | Change of mode of operation | Same as before |
| Aberration | 1, Powder itself; 2, Substandard operation methods. | Change of mode of operation | When liquid: 1, Primer and topcoat itself; 2, Paint viscosity, spray thickness. |
| Hanging | 1, Powder quality is not good; 2, Powder spray is too thick. | Change of mode of operation | Adjust the gun spacing and adjust the spit. |
| Pinhole | 1, Pre-treatment is not good, there is oil on the workpiece; 2, Spraying is too thin. | 1, Adjust the processing time of each pre-treatment process, laboratory pre-treatment of each liquid content is standard; 2, Replacement of the relevant process water. | Liquid identical |
2）Quality standard reference
| Event | Quality Standards | Test Basis |
| Color | Take the sample as the standard, color difference is less than 1.5E | Depending on customer requirements |
| Hardness | >2H | GB6739-86 |
| Impact resistance | 50kg/cm2 | GB/T1732 |
| Bending test | Workpiece folded at 45 degrees | ASTMD522 |
| Adhesion | Baige | ASTMD3002 |
| Salt spray experiment | 500 hours | Resistant to corrosion for 10 years |
| Thermo-cold test | Cold, hot 168 times each | Ten Years of denaturation |
| Appearances | According to customer requirements | According to customer requirements |
| Film thickness | Powder 80-120um, liquid 30-40um | Resistant to discoloration for 10 years |
| Boiling water resistance | Boil-resistant treatment 1.5h | GB/T17748 |
