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Powder electrostatic spraying

Spraying process flow

Electrostatic spraying Utilize the principle of high-voltage electrostatic corona electric field. Airbrush The metal guide cup on the head is connected to the high-voltage negative electricity, and the coated workpiece is grounded to form a positive pole. A strong electrostatic field is formed between the spray gun and the workpiece. When the carrier gas (compressed air) will powder coating When the powder is delivered from the powder supply barrel to the guide cup of the spray gun through the powder conveying pipe, dense charge The powder is negatively charged. Under the action of electrostatic force and compressed air, the powder is evenly adsorbed on the workpiece melt solidification (or Plasticization ）Form a uniform, continuous, flat and smooth coating film.^[1]

Chinese name: Powder electrostatic spraying

Foreign name: Powder electrostatic spraying
Type: Spraying process flow

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Product features

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1. The workpiece can be coated at room temperature without preheating.

2. The utilization rate of powder is over 95%.

3. The coating is thin (50~100 μ m) and uniform, without Slung Phenomenon.

4. A continuous and smooth coating film can be formed on the tip and rough surface of the workpiece.

5. Easy to implement automation Production.

technological process

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Process flow of powder electrostatic spraying

Process flow chart of powder electrostatic spraying

workpiece Pretreatment To increase matrix And coating The binding force of. surface treatment The quality of film Quality and life. There are mechanical and chemical methods for its treatment. Pretreatment of powder coating generally includes rust removal Degrease , chemical phosphating or passivation And so on. Different materials have different pretreatment methods. See Table 1 below. The relationship between powder coating defects and pretreatment is shown in Table 2 below.

Table 1 Selection of Pretreatment of Various Metal Materials

Material Science	General situation	In case of surface corrosion	Material Science	General situation	In case of surface corrosion
Steel and iron	Degreasing, cleaning, phosphate treatment	Sand blasting and phosphate treatment	Hot dip galvanizing, steel, copper, copper zinc alloy	Degreasing, cleaning salt, etc	Phosphate treatment/chromate treatment, sandblasting
Aluminium (standard)	Degreasing and cleaning	Chromate treatment	Hot dip galvanizing, steel, copper, copper zinc alloy	Degreasing, cleaning salt, etc	Phosphate treatment/chromate treatment, sandblasting
Aluminum (pressed products)	Chromate treatment	Chromate treatment	Electrogalvanized iron sheet, galvanized steel	Degreasing and cleaning	Degreasing, cleaning, phosphoric acid treatment, chromate treatment
Magnesium (standard)	Degreasing and cleaning	Chromate treatment
Magnesium (pressed products, cast magnesium products)	Chromate treatment	Chromate treatment

Table 2 Relationship between powder coating defects and pretreatment

Film defect	Causes and methods		Film defect	Causes and methods
Film defect	Cause	resolvent	Film defect	Cause	resolvent
foreign body Oil residue	1. Foreign matter mixing 2. Foreign matters such as oil stain inside the casting	During operation, foreign matters shall not be mixed in and can be removed with screen. Preheating removal, serious oil stain, degreasing treatment by high-temperature boiler burning	pinhole	1. Improper phosphating treatment 2. Moisture not removed after pretreatment	Strengthen the inspection of surface treatment quality and control the adhesion amount of phosphating film 2~3g/m two Check the temperature of drying oven
Shrinkage cavity	1. Degreasing is not clean 2. Incomplete rust removal	The degreasing time, formula and construction method should be correct. Strengthen the inspection of surface treatment quality	Stain discoloration	1. Degreasing is not clean 2. Rust removal is not clean	Degreasing time, formula and construction method should be correct Check pretreatment appearance

During coating baking, the furnace temperature shall be uniform, and a certain distance shall be kept between workpieces to prevent collision. various powder coating The curing temperature and time vary with the formulation design.

influence factor

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The factors affecting the film performance in powder electrostatic spraying process are not only the spraying process parameters, but also the characteristics of powder coatings.

1. Powder size.

See Table 3-1 for the influence of powder particle size on coating process performance

2. Powder coating conductivity

The volume resistance of powder coating has a great impact on its construction performance. See Table 3-2 for details

Table 2-1 Influence of Powder Particle Size on Coating Process Performance

coating process	Grain size change
coating process	Impact on process (from large to small)
Powder fluidity	Gradual deterioration
Applicability	From difficulty to ease
Tendency to be blown away	From Difficult to Easy to Fly
Blockage of spray gun	Smooth to easily blocked
Film thickness	From thick to thin

Table 2-2 powder coating Influence of volume resistance of

Volume resistance/Ω· cm	Construction performance
ten nine following	It is easy to discharge charged charges, and the powder particles are easy to fall off
ten ten ~10 fourteen	Good construction performance, thick coating film can be obtained during construction within this range
ten fifteen above	The particle charge is not easy to release, and the charge accumulated on the surface of the workpiece can not get thick coating

It can be seen from Table 2-2 that the volume resistance of powder particles is required to be 10 during electrostatic spraying construction ten ~10 fourteen Ω · cm is good.

3. Spraying distance

The change of spraying distance will change the electric field strength, which will also affect the film thickness and the deposition efficiency of the powder.

See Table 3-1 for the relationship between spraying distance and deposition efficiency

Spraying distance/mm	Deposition efficiency (%)	Spraying conditions
two hundred and fifty	one hundred	Powder spraying amount: 60g/min
three hundred	ninety-one	Spraying time: 20s
four hundred	fifty-six	Spraying voltage: 90KV

It can be seen from the above table that the deposition efficiency is the highest when the spraying distance is 250mm.

4. Spraying voltage

With the increase of spraying voltage, the amount of powder adhesion increases, but when it exceeds 90Kv, the increase rate of adhesion decreases. The initial growth rate of film thickness increases with the increase of voltage, but the influence of voltage on the increase of film thickness decreases with the increase of spraying time. The influence of voltage on film thickness decreases with the increase of spraying distance. If the spraying voltage is too high, the powder will be broken down and the quality of the powder will be affected. Generally, the spraying voltage is controlled within 60~90kV.

5. Air supply pressure

The gas supply pressure of the powder feeder increases, and the deposition efficiency decreases. See Table 5-1 for the relationship^[1]

Supply pressure/Pa	Deposition efficiency (%)	Spraying conditions
four point nine	one hundred	Spraying distance: 250mm
six point eight nine	ninety-seven	Powder spraying amount: 60g/min
nine point eight	nine	Spraying time: 20s
fourteen point seven	eighty-eight	Spraying voltage: 90kV
nineteen point six	eighty-four	Epoxy powder

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