Discussion on quality control of electrostatic spraying coatings for household appliances

Home appliances not only bring great convenience to people's lives, but also play a role in beautifying and decorating people's living environment. The development of the market puts forward higher and higher requirements for home appliance decoration. There are a variety of coating methods for household appliances. At present, the industry mainly uses flat coating and electrostatic powder coating. This paper mainly discusses the quality problems in electrostatic powder spraying. On the premise of correct equipment selection and good condition, coating quality is mainly controlled from three aspects: coating selection, powder performance and process execution.

1 Select the required coating according to the different requirements of home appliance decoration and protection

When selecting coating materials, the coating shall be selected according to the use environment and appearance requirements of household appliances.

When washing machines are used indoors, they have low requirements for atmospheric resistance, ultraviolet resistance, and humidity sudden change resistance, but they are often eroded by moisture, moisture, and alkali. The performance of coatings is mainly determined by the following aspects:

-The coating has strong adhesion to the surface of metal parts of the washing machine, and the coating film is firm and durable. It does not peel off after long-term use, has good wear resistance, and is not easy to be damaged;

-Good water resistance, alkali resistance, damp heat resistance;

-It has good decorative property and is in harmony with the color inside the family. The coating film is flat, plump and not easy to change color. The commonly used coatings are amino baking paint, thermoplastic acrylic paint and epoxy powder paint, most of which are powder coatings.

When the refrigerator is used indoors and comes into contact with food, it is required to have a sense of cleanness and cooling. Coating requirements:

-Soft color, high requirements for gloss and other appearance quality;

-The film has a certain hardness and is resistant to the impact of food containers, glass, etc;

-Good pollution resistance, no effect with edible oils;

-Good corrosion resistance, resistant to moisture, fruit juice, soda, alkaline water erosion;

-Odorless and non irritating substances are released.

Acrylic paint is used because of its good pollution resistance, high film hardness, color retention, corrosion resistance and film adhesion.

The TV shell is mainly made of ABS HPS, PS and other plastics are often used indoors and have high decorative requirements, so most of them are coated with acrylic paint, and some of them are coated with polyurethane. In this kind of paint, silicone resin is usually added as a surfactant, aliphatic or polyethylene wax, aliphatic ammonium salt and other additives can eliminate the defects of the plastic blank itself, so as to improve the appearance of the product, Increase scratch resistance, and often add NC fiber modifier to improve the coating's resistance, alkali resistance and adhesion.

The indoor water heater does not contact with the atmosphere and has a certain operating temperature. It is often exposed to water vapor and alkaline liquid and generally uses epoxy polyester resin.

For household appliances used outdoors, it is mainly required that the coating should be air resistant, UV resistant, and temperature resistant resins and additives.

2 Powder performance control

Good powder coatings are the prerequisite for obtaining high-quality coatings. However, various environmental factors may cause changes in the performance of powder coatings during storage and transportation. The coating quality can be controlled only when the coating performance is tested before use and the bad factors of the coating itself are eliminated. There are many testing items and methods for coating performance, and the performance closely related to the process can be selected for testing. Such as appearance, apparent density, fluidity, nonvolatile content, etc.

3 Process control

The electrostatic powder spraying process can be divided into four processes: surface treatment, drying, spraying and curing. The surface treatment process is to remove oxides, grease, dust and other dirt, ensure the durability of the film, enhance the adhesion between the coating and the surface of the object, and improve the appearance quality of the coating. The spraying process is under the action of high-voltage static electricity, the powder is sprayed to the surface of the workpiece through the spray gun, and the finished product is formed after the film is formed in the curing stage. In the production process, the operation shall be carried out in strict accordance with the process parameters of each process. If the process of each process is not fully implemented, the final quality of the coating may be affected.

Shrinkage and pore degreasing and degreasing are not complete. Increase the temperature of degreasing tank and extend the processing time

Phosphorus containing degreaser and metal ion production

Learn the reaction, produce easy decomposition precipitation, improve the surface treatment method, and select the surface treatment agent with higher efficiency;

The phosphating temperature is too high in the process of particle surface treatment, resulting in impurities that are difficult to clean. Improve the phosphating process, and adopt appropriate temperature

If the acidity is too high, it is easy to produce an impurity between the phosphating film and precipitation. Adjust the phosphating process and select the appropriate total acidity and free acidity

Poor mechanical performance Phosphating treatment does not form a phosphating film, or the phosphating film is rough and uneven (the workpiece sampling is less than 8 s after passing the copper sulfate titration test) Adjust the phosphating process or reach a more effective phosphating agent

If the shrinkage cavity workpiece is placed for too long, the placing time of the workpiece shall be controlled within 16 h

The compressed air contains liquid oil and water particles, which causes the powder coating to be polluted and increases the oil and water separation equipment. The drying tower and two-stage or three-stage oil and water separator are used; It is better to be the nearest to the final gas consumption point

The environment around the powder room contains volatile silicone oil and other solvents. Strictly control the environment of the powder room, and use isolated air purification powder room if possible

If there are impurities in the environment, the environment shall be strictly controlled, and environmental construction is prohibited during spraying

There are volatile matter and other quality problems in the powder itself. Strictly control the quality of powder coating

In the recovery system of electrostatic spraying equipment, some organic materials are mixed into the powder and the recovery system is cleaned regularly to prevent the invasion of organic substances

Powder mixing of different brands, different production dates and different manufacturers is prohibited. Powder room and powder spraying system should be thoroughly cleaned before replacing powder

Poor dispersion of pigment or curing agent in the production of granular powder Strictly control the particle size of pigment and filler

Ultra fine powder is easy to accumulate at the head of the spray gun, the powder pump or the part where the air flow inside the pipe changes. When the accumulation reaches a certain level, it will be sprayed out in solid form and attached to the surface of the workpiece. During curing, it is not allowed to become particles (0.8~3.5mm) due to leveling. Select appropriate powder, and control the particle size distribution of the powder at (16~80 μ m)

Deposited powder at the top of powder room or spray gun does not fall on the surface of the workpiece through catenary vibration to produce particles (2.0~4.5mm) to keep the workpiece environment clean; Clean the equipment in time after repair and maintenance

When the coating is too thin, the melting flow is easy to produce fine particles, and the coating thickness should be controlled At 40~85 μ m

Replace the powder coating when the pigment of the powder coating itself is not temperature resistant or the resin is prone to yellowing

Discoloration and tarnishing powder coatings of different manufacturers or different brands shall not be mixed. Powder room and powder spraying system shall be thoroughly cleaned before replacing powder

If the coating film is too thick, it will cause light loss and control the thickness of the coating film to be in the proper range

When the proportion of recovered powder is too large, it is also easy to lose light. The mixing ratio of recovered powder and new powder should be 2:1 generally

The surface quality of the workpiece with air holes is poor. The workpiece with air holes should be manually polished to improve the surface quality of the workpiece

If the spray gun is too close to the workpiece, or the spray gun voltage is too high, breakdown will occur to adjust the electrostatic spraying process parameters, and the voltage should be 70kV~100kV; The distance between the workpiece and the spray gun is about 24cm

There are volatile impurities in orange peel coating during the curing process. Strictly control the incoming quality of powder. The powder is stored at 25 ℃

The surface of the workpiece is rough, resulting in uneven coating surface and improving the smoothness of the workpiece surface, such as polishing

The powder particles are too coarse, the curing speed of the powder coating is too fast, and the particle size is controlled at 16~80 μ m due to poor automatic leveling, so high-quality powder coating is selected

The coating film is too thick, and the surface of the coating film presents a striped orange peel. Adjust the process parameters of electrostatic spraying, and control the voltage, current, compressed air flow, atomizing gas, and the distance between the workpiece and the spray gun

Electrostatic shielding, uneven film thickness

Poor powder atomization during powder spraying

The flakes, dust, etc. on the particle drying furnace equipment are attached to the surface of the workpiece through catenary vibration. Clean the drying furnace regularly with a vacuum cleaner, and clean the filter screen of the hot air circulation system

The color change and gloss loss of the coating film caused by too long baking time or too high temperature shall be subject to appropriate baking time and temperature. Process parameters are adjusted according to the model and manufacturer of powder coating

Too low curing temperature and insufficient curing time will cause too bright. Appropriate baking conditions should be formulated and the operation should be carried out according to the process requirements

Check the exhaust system and maintain it to keep it unblocked

The roasting temperature of orange peel is too low, and the melting fluidity is poor. Increase the roasting temperature, and adjust the temperature rise curve

Poor mechanical properties The powder is not fully cured or excessively baked According to the technical conditions of the powder coating, use the temperature measurement signal to detect the temperature rise curve of the oven, and readjust the curing temperature and time

4 Performance test of forming coating film

Before each batch of production, small batch trial production shall be conducted for the determined powder and production process. Formal production can be started only after the film performance test is qualified. The film performance test focuses on different purposes.

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