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Powder Coating
2019-04-28

Powder coating is the use of corona discharge to cause the powder coating to adhere to the workpiece. The process of dusting is: the powder gun is connected to the negative pole, the workpiece is grounded (positive), the powder coating is fed into the spray gun by the powder supply system by compressed air gas, and the high pressure generated by the high-voltage electrostatic generator is added at the front end of the spray gun, due to corona discharge, A dense charge is generated in the vicinity thereof, and when the powder is ejected from the nozzle, a loop is formed to form charged paint particles, which are attracted to the workpiece having the opposite polarity by the electrostatic force, and the electric charge increases as the sprayed powder increases. The more accumulated, when a certain thickness is reached, due to the electrostatic repulsion, the adsorption is not continued, so that the entire workpiece obtains a certain thickness of the powder coating, and then the heat is melted, leveled, and solidified, that is, on the surface of the workpiece. Form a hard coating film.

 

Wide range of uses

Dusting is an indispensable process in modern high-speed multi-color offset printing. Its main function is to prevent the back of the printed matter from getting dirty, improve the printing quality and efficiency. The powder is made from pure natural plants and refined by modern science. Printing powder is an indispensable tool in offset printing! Its main function is to prevent the printed matter from being printed. The back of the printing process is sticky and quick to dry. Good spray powder particles are smooth, excellent in assimilation with ink, and evenly distributed after spraying, which is very helpful for improving printing and printing quality and printing efficiency! It is widely used; in addition to color printing, film, rubber, nylon, composite can be used. Use this product. The powder particles are spherical; the surface is smooth; the particle size is 10-25 μm (mesh number is 300-450); the powder particles are uniform; the fluidity and the dispersibility are excellent; and the assimilation and lipophilicity with the ink are particularly good. It can be completely and evenly distributed on the surface of the printed product, so that the printing quality can be guaranteed.

 

Powder type

Plastic dusting: spraying on plastic with low-temperature materials, similar to paint-like coatings, but domestic powder suppliers have not developed them.

Chemical fiber dusting: Powder spraying, also known as powder coating, is a new type of coating process that has been rapidly developed in recent decades. The raw material used is plastic powder. As early as the 1940s, some countries began to study experiments, but progress was slow. In 1954, James of Germany successfully applied polyethylene to the fluidized bed method. After the electrostatic spraying of the powder in France in 1962, the powder coating began to be formally adopted in production. In recent years, due to the environmental protection of countries. The importance of protection, powder coatings that are not polluted by water and the atmosphere, have developed rapidly.

 

Work safety

1. The scope of the dusting area should generally include:

a) Powder spraying chamber, powder feeding device (including powder conveying device of circulating powder feeding device, powder storage bin and discharging device thereof), recycling device, fan, purification device and powder conveying pipeline connected thereto.

b) the area at the opening of the powder chamber is 3m horizontally and 1m vertically;

c) where the powder coating is stored or deposited on the aluminum coating site;

d) The inside of the exhaust duct, the air circulation filter and its maintenance structure, and other areas where it is possible to produce explosive suspended dust or accumulated dust.

2, dust spray area fireproof explosion-proof grade

2.1 The fire danger zone in the dusting area is zoned 22.

2.2 The dusting area shall be classified as Zone 11 according to the hazardous area of the explosive dust environment. Those who meet the requirements of GB 50058 can be classified as non-explosive hazardous areas.

3, design

3.1 Powder electrostatic spraying process design, powder electrostatic spraying equipment and equipment development, design and manufacture should comply with the provisions of GB 7691.

3.2 The safety and hygiene indicators of the dusting room should meet the following requirements:

a) Except for the local area such as the outlet of the spray gun, the average concentration of suspended powder in the spray chamber (ie, the concentration in the exhaust duct of the spray chamber) should be less than half of the minimum explosive concentration of the powder, and the lowest explosive concentration (MEC) is unknown. The maximum concentration is not allowed to exceed 15 g/m3. If there is a blasting device in the system, the concentration of suspended powder in the outlet duct of the dusting chamber is allowed to exceed 50% of the minimum explosive concentration;

b) The use of electrostatic powder guns and their auxiliary devices shall comply with the requirements of GB 14773;

c) the allowable concentration of total dust in the workplace air is 8 mg/m3;

d) The wind speed of the opening surface of the powder spraying chamber should be 0.3~0.6 m/s.

4, the place

4.1 Powder electrostatic spraying and painting operations should not be set in the same working area. If it is set in the same working area, its explosion danger area and fire danger area should be divided according to the painting area.

4.2 The dusting operation area should be arranged in a single-storey factory building; if it is arranged in a multi-storey factory building, it should be arranged on the top floor of the building. If it is arranged in a multi-span factory building, it should be arranged in the side span and comply with the relevant provisions of GB 6514.

4.3 The dusting operation shall be carried out in a dusting room complying with the provisions of Chapter 5.

4.4 The dusting chamber should be placed in a position where no disturbing airflow is generated, and should be avoided in combination with the generation or dissipation of water vapor, acid mist and other devices with adhesive, corrosive, flammable, explosive and other media. It should be isolated from the area where the above media is produced.

4.5 The dusting room should not double as a spray booth.

5, fire, explosion-proof

5.1 The surface of the workpiece entering the powder room shall be 28 °C lower than the ignition temperature of the powder used.

5.2 The following regulations should be followed in the dusting area:

a) Fire-fighting sources, open flames and equipment and apparatus that generate sparks are not allowed;

b) prohibiting the impact or friction from generating sparks;

c) heating equipment that does not ignite powder or powder gas mixture should be used;

d) Fire extinguishing fire extinguisher according to GB 50140, but it is not suitable to use fire extinguisher which is easy to make powder coating fly or pollute.

5.3 Reliable alarm devices and automatic fire suppression systems should be installed in the automatic dusting room. In the event of a fire, the gas supply system and power supply can be automatically shut off.

6, the ground

The ground of the dusting area should be laid with non-combustible or flame-retardant anti-static materials. The floor should be smooth and seamless, with grooves to facilitate cleaning of the powder.

7, lighting

The spray zone shall be illuminated by dust-proof cold light source lamps, and the illumination shall comply with the provisions of Article 4.3.1 of GB 12367-2006. When using transparent materials for partition lighting, the following requirements should be met:

a) using a fixed luminaire as the light source;

b) Separate the luminaire from the dusting area with a partition, and the installation and sealing shall ensure that the dust does not enter the luminaire;

c) The partitions shall be of non-flammable or non-combustible materials that are not easily damaged;

d) the thickness of the deposit on the partition is not allowed to affect the specified illuminance;

e) The surface temperature of the separator does not exceed 93 °C.

 

Inspection of dusting products

1. Test conditions

l Arctic dawn or indoor high-efficiency two-source fluorescent lamp (illuminance 1000 lumens)

l Visual distance: 300mm for A-level surface, 500MM for B-side, 1000mm for C-plane, and 3m/min for gaze scanning.

2. Inspection criteria:

l All surfaces should not be exposed, peeled, scratched, blistering, wrinkled, pinhole, powder

l Film thickness: outdoor powder 60-120; indoor powder 50-100; spray paint generally 40-70 (film thickness meter 1 point five times, average, the whole face takes four corners and the middle five points and then average).

3. Production of paint swatches

l A: When baking, make two color plates per furnace for testing (one test, one archive). After the furnace is baked, the powder number, curing condition and time are marked and signed by QE.

l B: The swatch is stored for two years, the humidity at room temperature is 70±15%, and no direct sunlight is applied.

4. Performance requirements

1) Non-destructive testing

Film thickness test

Luminance test: The gloss meter has an incident angle of 60 degrees and an error of ± 5% is acceptable. Some customers require gloss ≥90%.

Color difference test: A surface color difference of 0.8, the remaining side color difference of 1 or less, or swatches.

2) Destructive testing

Hundreds of tests: in the area of 100mm2, draw a line every 1mm with a utility knife (the depth must see the substrate), cross-cut 100 squares, and paste it with 3M#600 Scotch tape 10~13mm or compatible with it. The test area is characterized, and the tape is instantly and quickly peeled off the paint on the test surface in the vertical direction, and cannot be peeled off by 5%.

Bending test: the swatch is bent 180 degrees with r=t or repeated 90 degrees, and the coating does not fall off as qualified;

Hardness test: 2H pencil is applied with a force of 1KG at a 45-degree angle to push forward 15-30MM along the ruler. After the eraser is erased, no leakage is allowed, allowing slight marks; or pencil hardness H or more, cutting the tip of the pencil into a right angle. For the test surface in the direction of 45 degrees, apply a force of 1KG to draw a straight line of 8CM on the surface of the paint, the surface can not be damaged, scratched, allowing a slight trace of paint. .

Reference room temperature hardness: 3H or more.

Resistance test: Dip 99.8% of anhydrous alcohol cotton sticks, 50 times back and forth with 1KG force, except that the gloss can be changed a little, the coating should not change color, rise, peel off; or clean cotton cloth, wet clean solvent (industrial alcohol 95%); apply 50g force to the test surface for 50 times. The test surface should not change color, peel paint, float, or lose luster, and the gloss can be changed a little.

Impact test: 500 gram weight with a cross-sectional area of 1/4 is free to fall from 500 meters, and the coating film does not fall off and crack is qualified. It is also useful for the ball drop test: the ball drop test (using a 1.5 cm diameter solid ball and a 1.5 M length hollow raft to test the strength and adhesion of the paint). Or on the QCJ type film impactor, (1Kg weight hammer free fall impact at 500mm height, no cracks and spray layer peeling phenomenon).

Abrasion resistance test: brush with a sponge brush, 5000 times, can not expose the material with a sponge brush hard side, plus a width of 30mm, 500g load, 55mm distance, 60 times / min back and forth brush; or Great Wall rubber 500g Force back and forth 50 times without leaking bottom.

Moisture resistance: 240H in 60°C relative humidity 90%, the coating surface is not abnormal

Boiling water resistance, boiling water, 1hr soaking, no abnormality;

Resistance to fading: Use a wavelength of 2800A ° ~ 3000A °, a 15W UV lamp. The lens was continuously irradiated for 72 hours at a distance of 25 cm, and compared with the comparative sample, the test surface could not have leg color, peeling paint, floating or tarnishing.

Alkali resistance 5% NaOH, no change in 240hr;

Acid resistance 3% HCL, no change in 240hr;

Solvent resistance xylene, no change in 24hr;

Pollution-resistant lipstick, no abnormality at 24 hr at room temperature;

Heat resistance: 180 ° C x l hr color difference E < 0.8, excellent gloss retention;

Corrosion resistance: salt spray resistance No change for 1000hr; no heat or humidity resistance 500hr; or salt spray should not have bubbles or rust (5% salt water 35 degrees Celsius, continuous spray for 400 hours).


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