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Paint coating is a construction process that uses certain equipment and process to evenly apply paint on the surface of the coated object to form an anti-corrosion coating. Common coating methods include brushing, scraping, spraying, sprinkling, air spraying, and high-pressure airless spraying.

1. Brush coating

Brushing is the earliest, simplest, and most traditional manual painting method used, which is convenient and flexible to apply to objects of any shape. Except for coatings with fast drying and poor leveling, it can be applied to various coatings. The brushing method can allow the coating to penetrate the fine pores on the metal surface, enhancing the adhesion of the coating film to the metal.

Generally speaking, brushing can be divided into three steps: coating, smoothing, and trimming. The common defects of brush coating are sagging, brush marks, bubbles, pinholes, uneven thickness, high labor intensity, and low work efficiency.

2. Scraping and coating

Scraping is a method of painting using a scraper. Scraping is also a commonly used coating method, mainly used in the process of scraping putty. Scrapers can be made of wood, steel, ox horns, rubber, etc. At present, some coatings with high solid components or solvent-free, high viscosity, and high solid content are coated by scraping, such as glass flakes.

The common defects of the coating in scraping operation are cracking, peeling, and flipping, and the thickness of the coating is also difficult to be uniform. Cracking of the scratch coating film is generally caused by the thickness of a single scratch exceeding the allowable thickness of the coating, or by mismatch with the substrate.

3. Dip coating

Dip coating is also a traditional coating method. This method involves immersing the coated object in a tank containing paint, then removing it and allowing excess paint to drip back into the tank, or using mechanical methods to shake off the excess paint. This method is applicable to the coating of equipment or workpieces with complex structures. The equipment is simple and the degree of mechanization is high. But it is not suitable for coatings with high volatility. This type of coating causes significant solvent loss during immersion, which can easily lead to air pollution.

The disadvantage of immersion coating is that the thickness of the coating is not easy to be uniform, which can cause sagging and foaming.

4. Spray coating

Spray coating is a coating process that uses a pressure or gravity nozzle to spray a liquid coating from the nozzle onto the surface of the coated object. The coating flows from top to bottom, completely covering the surface of the coated object and forming a coating film.

Spray coating is a coating method developed from the immersion coating method, which relies on excessive coating to form a coating film on the surface of the coated object. The only difference is the way the coating is applied to the surface of the object being coated. The equipment used for coating is simple, easy to achieve mechanized production, easy to operate, and has high production efficiency.

The solvent consumption of spray coating is higher than that of dip coating, but the amount of paint input in one go is less than that of dip coating. The workpiece suitable for spray coating is generally larger than that of dip coating, such as large plates. However, the shape of the workpiece itself should not be too complex, and there should be no pits, grooves, etc., which are not easy to produce paint retention. The operation method of spray coating can be divided into intermittent and continuous types, just like immersion coating; The most typical spray coating equipment is a through type spray coating equipment suitable for large-scale and continuous production, mainly composed of a chamber, paint tank, solvent tank, spray device, and ventilation and fire prevention device. The spray device consists of a nozzle, a spray pipe, and a paint pump. The nozzle is installed in the paint spraying area, 300-400mm away from the coated workpiece. The nozzle shape is circular or fan-shaped, and the spray pipe should be designed according to the shape of the coated object.

The spray coating process can also cause safety and pollution issues. To prevent solvent evaporation and diffusion, suction type air curtains should be installed on both sides of the inlet and outlet of the spray chamber. The suction wind speed of the wind curtain should be greater than the solvent evaporation speed. To prevent fires, there should be automatic fire extinguishing devices.

The common coating faults in spray coating are uneven or incomplete coating, uneven coating thickness, or excessively thick or thin coating.

5. Spray painting

Spraying can be divided into two methods: air spraying and airless spraying. Its advantages are that the coating film thickness obtained by spraying construction is uniform, the surface is flat, and the production efficiency is high. The disadvantage is that the material loss is much greater than that of brushing and spraying methods, and the use of solvent based coatings can cause environmental pollution. Spray painting is suitable for various coatings and objects, and is the most widely used painting process method.

5.1 Air spraying principle:

Air spraying is a coating process that utilizes the negative pressure generated when compressed air is sprayed from the nozzle of the spray gun to suck the coating out of the coating tank and immediately enter a high-speed compressed air flow, causing the coating liquid gas phase to rapidly diffuse and be dispersed into mist like particles, which are shot towards the coated object. The mist like particles are evenly coated on the surface of the coated object to form a continuous coating film.

A brief summary of spray painting process issues

1. High air pressure, fine paint mist particles, more dispersed paint mist, and large paint loss;

2. Low air pressure, coarse paint mist particles, rough paint film surface, producing defects such as orange lines and pinholes.

The amount of paint sprayed is limited by the amount of air. Increasing the spraying amount will make the paint mist particles thicker, affecting the smoothness of the paint film surface.

4 The width of the spray pattern shall be adjusted according to the condition of the object to be painted. A small width will affect the spraying efficiency. If the width is too large, the paint mist will disperse more and the paint loss will be large.

When spraying, overlap should be taken as 1/3 or 1/2

5.2 Characteristics of air spraying

The characteristics of air spraying are as follows:

(1) High painting efficiency, with an hourly spraying area of approximately 150-200 square meters, which is 10 times higher than brushing;

(2) The coating quality is good, uniform, and the adhesion with the coated surface is improved;

(3) Strong applicability, almost applicable to various coatings and coated materials;

(4) Paint loss is high, the utilization rate of paint is low (50% or less), and it pollutes the environment.

5.3. Main equipment and types of spray guns for air spraying

The main process equipment for air spraying consists of spray guns, air compressors, oil-water separators, air storage tanks, and coating supply equipment. According to the supply method of coatings, they are divided into gravity type, suction type, and pressure delivery type.

5.4 Introduction to Airless Spraying Machine

5.4.1 Principle of high-pressure airless spraying machine

High pressure airless spraying continuously delivers paint into a closed pipeline through a pressure pump, forming high pressure in the enclosed space. Then, the trigger of the spray gun connected to the end of the paint pipe is released, causing it to collide with the air at a speed of 100m/s at the moment of spraying, while rapidly expanding and atomizing.

5.4.2 Characteristics of Airless Spraying Process

(1) The one-time coating has a large thickness and can spray various high solid components and thick slurry coatings, resulting in a thicker coating film;

(2) The spraying efficiency is high, which means that the air spraying source can reach about 3 letters and can reach 400-1000 square meters/h;

(3) The utilization rate of coatings is high (no air atomization is used, resulting in less paint scattering), reducing environmental pollution;

(4) Can be used for spraying various two-component heavy-duty anti-corrosion coatings such as epoxy, polyester, polyurethane, and polyester.