General knowledge of coatings

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11.1 Functions of coating The functions of coating are mainly two: one is corrosion prevention, and the other is decoration. In addition, there are some special functions, such as: regulating the conductivity of heat, electricity, and other forms of energy, **as well as the protective color of the target. 1.2 The three key elements of coating The essentials of a coating project are: coating materials, coating processes and equipment, and coating management. The quality of coating materials is a fundamental requirement for obtaining high-quality coatings. Painting processes and equipment are essential for fully leveraging the properties of painting materials, achieving high-quality coatings, reducing production costs, and improving economic efficiency. Coating management is an important condition for ensuring the implementation of the established processes, maintaining stable coating quality, achieving the objectives of coating, and obtaining optimal economic benefits. 2 Painting Process Flow: Remove local rust spots → Clean by hand → Hot water washing → Pre-degreasing → Degreasing → Water washing → Water washing → Surface preparation → Phosphating → Water washing → Water washing → Passivation → Washing with recycled deionized water → Washing with clean deionized water → Drying to remove moisture → Cooling → Coating application (primer spraying, powder coating) → Drying → Anodic (cathodic) electrophoresis → Rinsing in tank → Washing with recycled UF water → Washing with recycled UF water → Washing with fresh UF water → Washing with recycled deionized water → Washing with clean deionized water → Drying → Cooling → Metal shaping and trimming → Applying putty → Drying → Cooling → Grinding → Applying sealant to welds → Applying cover coat → Spraying PVC coating → Removing the cover coat → Drying → Cooling → Grinding → Cleaning → Spraying intermediate coat → Drying → Cooling → Applying putty → Drying → Cooling → Grinding → Cleaning → Spraying topcoat → Drying → Cooling → Applying cover coat → Spraying color strips → Removing the cover coat → Drying → Cooling → Inspection and refinishing → Polishing → Waxing → Cleaning. 3 Main painting equipment The main painting equipment includes surface pre-treatment equipment before painting, painting equipment, equipment for drying and curing the paint film, and mechanical conveying equipment. 3.1 Surface pretreatment equipment before painting 3.1.1 The purpose of pretreatment is to remove any foreign substances on the surface of the part to be painted, such as oxide scales, rust spots, and grease, thereby creating a suitable substrate that meets the requirements for painting. Methods such as phosphating, oxidation, and passivation are used to ensure that the coating has good corrosion resistance and decorative properties. 3.1.2 Pretreatment equipment is mainly divided into immersion type and spray type. It mainly consists of a tank body, a tank fluid heating system, a ventilation system, a tank fluid stirring system, a phosphating slag removal system, an oil-water separation system, etc. Among them, the phosphating slag removal and oil-water separation systems are key systems. Common methods for phosphating slag removal include sedimentation, hydrocyclone separation, inclined plate sedimentation, bag filtration, and plate and frame filter pressing. Oil-water separation mainly includes: adsorption method, ultrafiltration method, hot oil separation method, and centrifugation method, etc. Currently, most domestic and foreign automobile manufacturers use a pretreatment method that combines spraying, dipping, and spraying. 3.2 Painting Equipment 3.2.1 Painting Methods There are many methods of painting, including brushing, dipping, air spraying, high-pressure airless spraying, electrostatic spraying, electrophoretic coating, and pouring paint. 3.2.2 Painting Environment The ideal painting environment should provide good lighting, air with appropriate temperature, humidity, and cleanliness, good ventilation, as well as meet fire and explosion prevention requirements. 3.2.3 Painting Equipment The function of painting equipment is to contain the excess paint mist within a certain area and filter it, thereby providing operators with a working environment that meets health and safety standards. In addition, construction conditions with specific temperature, humidity, and cleanliness levels should also be provided. 3.2.3.1 Common classifications of painting booths 1) Dry painting booth: Suitable for painting small batches of workpieces ； 2) Spray painting booth: Suitable for painting small workpieces ； 3) Water curtain painting booth: suitable for painting medium-sized workpieces ； 4) Rotary spray painting booth: Suitable for spraying large workpieces with high decorative requirements. 3.2.3.2 Main structure of the painting booth 1) Booth body: Its function is to contain the overspray mist within a certain range. 2) Paint mist filtration device: available in dry and wet types. In the dry method, paint mist is filtered out using baffles or filter paper. The wet method separates paint mist particles from the air through the mixing of air and water. 3) Water supply system: Ensure the normal circulation of the water volume required by the wet painting booth. 4) Ventilation system: It consists of air supply and exhaust systems, which provide air with specific temperature, humidity, and cleanliness levels to the equipment, ensuring that the indoor air remains in a laminar flow state. 5) Lighting fixture: Ensure the equipment has adequate natural light. 3.2.4 Dip-coating equipment: Submerges the workpiece in the paint solution, using the adhesive properties of the paint to apply a layer of coating to the surface of the workpiece. The dipping equipment mainly consists of a tank, a residual paint removal device, a stirring device, a heating device, a ventilation device, and fire safety devices. 3.2.5 Electrostatic spraying equipment Electrostatic painting is achieved based on the principle of the effect of electric fields on charges. It generally consists of an electrostatic painting booth, a high-voltage electrostatic generator, an electrostatic spray gun unit, a paint supply system, and safety devices. 3.2.6 Electrophoretic coating equipment: Electrophoretic coating involves applying an ionized coating powder dispersed in water, through electrophoretic movement, to a work surface under the influence of an external electric field, thereby forming a protective coating. Electrophoresis equipment generally consists of a tank, a stirring and circulation system, electrode assemblies, a paint temperature control device, a paint supply system, a ventilation system, a power supply unit, and a rinsing system after electrophoresis. 3.2.7 Powder electrostatic spraying equipment: Powder coating offers advantages such as cost-effectiveness, no environmental pollution, high efficiency, and the ability to achieve a coating thickness of over 50 μm with a single application. Powder electrostatic spraying equipment mainly consists of a powder spraying chamber, a high-voltage electrostatic generator, an electrostatic powder spraying gun, a powder supply device, and a powder recovery system. 3.3 Drying Equipment 3.3.1 Main classifications and components of drying equipment Classified by heating method: convection drying, radiation drying, convection + radiation drying ； Based on their structural form, they are classified into: straight-through type, bridge type, П-shaped type, etc. Main components: 1) Chamber body: Prevents the circulating hot air from escaping, thus maintaining the indoor temperature within a certain range ； 2) Heating system: It includes radiant heating and convection heating to ensure that the indoor air temperature remains within the requirements specified by the process ； 3) Wind curtain system: Prevents hot air from escaping and improves thermal efficiency ； 4) Temperature control system: A device that ensures the temperatures in various areas of the room meet the process requirements. 3.4 Mechanized conveying equipment Mechanized conveying equipment is mainly divided into two categories: aerial conveying and ground conveying. Aerial conveying methods include: conventional suspended conveyors, double-chain suspended conveyors, lightweight suspended conveyors, stackable suspended conveyors, and self-propelled hook lifts. Ground conveyors include: reverse stacking ground conveyors, sled conveyors, tank chain conveyors, and ordinary ground conveyors. The role of mechanical equipment is to play a coordinating function throughout the painting production line. 4 Main testing requirements for painting equipment: 1) For drying equipment, the main thing to test is the uniformity of the furnace temperature, which should generally be maintained within ±5 ℃; a temperature sensor attached to the furnace can be used to measure up to 6 different locations at once. 2) For painting equipment, the main parameters to be tested are illuminance, temperature, humidity, and average wind speed: For painting equipment with higher requirements, the illuminance should be between 800 and 1000 Lx, the temperature should be 23 ± 2 °C, the relative humidity should be 55 ± 5%, and the average wind speed should be 0.45 to 0.55 m/s. 3) The noise level in the operation area for all equipment is ≤ 85db.