I. Basic Principles


A high-voltage corona discharge electric field is formed between the spray gun and the workpiece. When the powder particles are ejected from the nozzle and pass through the discharge area, a large number of electrons are added and become negatively charged particles, which are adsorbed to the workpiece with positive charge under the action of electrostatic attraction. When the powder adheres to a certain thickness, it will have the effect of "homo-repellent" and can no longer adsorb the powder, so that the thickness of each part of the powder layer is uniform, and then the powder layer is leveled into a uniform film after heating, baking and curing.


Powder Electrostatic Spraying Process

The typical process of powder electrostatic spraying is as follows: upper part degreasing cleaning rusting cleaning phosphating cleaning passivation powder electrostatic spraying solidification cooling lower part.


Main factors affecting the quality of powder electrostatic spraying


In powder electrostatic spraying, the factors affecting spraying quality include spraying time, spraying gun form, spraying voltage, powder quantity, powder conductivity, powder particle size, powder particle size, velocity gradient of powder and air mixture, etc.


1. The resistivity of powder


The resistivity of the powder is between 1010 and 1016 ohms/cm, too low resistivity can easily lead to powder dispersion, and too high resistivity will affect the thickness of the coating.


2. Powder Spraying Quantity


In the initial stage of spraying, the amount of powder spraying has a certain effect on film thickness. Generally, the amount of powder spraying is small and the deposition rate is high. The amount of powder spraying is generally controlled in the range of 50 g/min to 1000 g/min.


3. Velocity and gradient of mixtures of powder and air


Velocity gradient is the ratio of the velocity of powder air mixture at the exit of spraying gun to the spraying distance. In a certain spraying time, the film thickness will decrease with the increase of spraying gradient.


4. Spraying distance


Spraying distance is a main parameter of the thickness of the rejection film. Generally, it is controlled at 10-25 cm away from the workpiece, which is mostly determined by the form of spraying gun.


5. Spraying time


The influence of spraying time and voltage, distance and quantity of spraying on film thickness limit value decreases with the increase of spraying time and distance. With the increase of spraying time, the effect of spraying amount on the growth rate of film thickness decreases significantly.


Water Dispersible Powder Coating


Water dispersible powder coatings are stable dispersions of powder coatings in aqueous media. They have the advantages of both water-based coatings and powder coatings. All conventional means including dipping, brushing, spraying and electrostatic coating can be used in the process.
Powder Electrophoretic Coating


It is the product of powder coating and current coating, and has both characteristics. Its basic principle is to disperse powder particles (generally less than 40 micron) into aqueous solution containing electrophoretic resin, take water-based electrophoretic resin as carrier, take powder particles as film-forming material, make powder particles charged, electrophoretic deposition in DC electric field. It is suitable for complex workpiece construction.








Composition and Structure of Powder Electrostatic Spraying Equipment


Powder electrostatic spraying equipment mainly includes powder spraying chamber, high voltage electrostatic generator, electrostatic spraying gun, powder feeder, powder recovery device, workpiece rotating mechanism, etc.


1. Powder Spraying Chamber


Powder spraying chamber is one of the main equipments for powder electrostatic coating. Stable air flow is the cleanliness of powder room, providing a clean working environment for operators. Control the dust content in the spray chamber to make it lower than the explosion limit (generally set at 10g/m3). In addition, the powder spraying chamber should be easy to clean, so that the powder can not easily deposit in the room, so as to change the color of the powder, and there should be enough light in the room to facilitate the coating work.


2. Electrostatic spray gun


The spray gun can be divided into portable spraying gun, fixed automatic spraying gun, disc spraying gun, etc. According to its use, the spray gun can be divided into internal and external charged gun according to its charged form, and according to its diffusion mechanism, it can be divided into conflict gun, rebound gun, secondary air inlet gun, centrifugal rotary cup gun, etc.



The electric mechanism of powder spraying gun is the key factor to improve the spraying efficiency.


Generally speaking, the core of powder spraying equipment is spraying gun and charging system. As far as the current market is concerned, corona spraying gun accounts for a large proportion. This is the greatest advantage of charging powder by high voltage corona discharge. It can spray all kinds of thermosetting powder coatings and obtain very good results. Its advantages mainly lie in its excellent stability, flour feeding rate and flour feeding speed.


The development course of this kind of gun is as follows: voltage control, current control and total energy control. Total energy control is a component with the distance from the spray gun to the workpiece. Its voltage and current are constantly adjusted to the ideal state to achieve the best coating effect.


3. Powder feeding system


1) The powder feeding system is to continuously and uniformly transport the coated powder from the powder container to the powder spraying gun for spraying.


The powder feeding system consists of air compressor, oil-water separator, air dryer, regulating valve, compressed air pipeline, electromagnetic control valve, powder feeder, powder conveying pipeline, etc.


2) Form of powder feeder


In powder electrostatic spraying system, there are many kinds of powder feeders, which can be divided into pressure vessel type, screw or turntable mechanical conveying type, Venturi air suction type.


3) Powder Recovery Device


The recovery of powder can be divided into wet method and dry method.


The wet method is to let the air flow with powder filter through the container of liquid to achieve purification. The powder with liquid is dried and reused.


Dry powder recovery is the collection of powder particles in the powder flow discharged from the powder injection chamber. The types of dry powder recovery are gravity sedimentation, inertia separation, cyclone separation, sintering plate separation and so on. In actual production, multi-stage recovery device is often used to achieve better separation effect.

Here is a brief introduction of powder spraying technology and equipment. Here is a brief introduction of paint technology and equipment.


3. Brief Introduction of Paint Technology and Equipment


1. Purpose of painting


Firstly, it is protective to prolong the life of the workpiece, secondly, it is installable to achieve beautiful and pleasant. The third is special use to achieve special performance. Such as: sound insulation, heat insulation, fire prevention and so on.


According to the different purposes and requirements of painting, there are several layers of coating, including primer, putty, finish, etc.

Primer: The lowest layer of coating which is in direct contact with the body of the workpiece to be coated. The function of primer is to strengthen the adhesion between the coating and the body and to enhance the protective performance of the coating. Ferrous metals should be phosphated before removal and non-ferrous metals should be oxidized before painting.


Putty layer: For rough and uneven body, putty layer has many shortcomings, such as troublesome construction, reducing the binding force between coating and body, etc.


The main purpose of the topcoat is to increase the gloss of the product and to be used as the outermost layer of the coating.

2. Painting Method


There are many methods of painting, such as brushing, dipping, air spraying, high pressure airless spraying, electrostatic spraying and electrophoretic coating.

3. Coating assembly and requirements for product materials are seldom used in coating construction, because such a coating is not uniform and porous. Primers and topcoats are often used. Intermediate and closed topcoats can also be added as needed. The primer layer should have good adhesion to the workpiece to be painted, good adhesion to the facing paint layer, and anti-rust ability. The matching principles of primer and topcoat are as follows:

1) It is better to match the drying primer with the drying topcoat, the self-drying primer with the self-drying topcoat, and the primer with the same base paint with the topcoat.


2) When selecting topcoats with strong solvents, such as nitro paint, perchloroethylene paint, etc., the primer must be resistant to strong solvents, such as alkyd paint, epoxy paint, etc.


3) Primers and topcoats should have roughly the same hardness and extensibility.


4) The heat and temperature resistance of the display paint is poor when it is matched with the solidified primer.


5) Primer paints have a shorter degree of paint than topcoats. Otherwise, the weather resistance of topcoats is poor, and the drying shrinkage of primer topcoats is different, which can easily cause cracks in each layer.


6) When using multi-layer heterogeneous coatings, the intermediate layer is often used to make the base transition and achieve a good combination of the bottom layer and the surface layer.


The adhesion of organic coatings is related to the nature of product materials. According to the adhesion, metals can be arranged as follows:


Nickel Steel Copper Brass Aluminum Tin Lead


Ferrous metals are suitable for almost all types of primers, while magnesium-aluminium parts and their alloys are usually passivated with zinc chromate as the body primer. Effective pretreatment is preferred on the surface of the workpiece to produce a layer of phosphating film or oxide film to improve the adhesion between the body and the primer. Then choose the primer with strong adhesion. For aluminium and galvanized parts, red-red pigments must not be used as primers, otherwise it will cause electrochemical effect and reduce adhesion.








Electrostatic Powder Spraying Process


Principle of Electrostatic Spraying Technology:


Powder coatings are sprayed onto the surface of workpieces by electrostatic spraying equipment. Under the action of electrostatic force, powder will evenly adsorb on the surface of workpieces and form powder coatings. Powder coatings are cured levelly at high temperature and become final coatings with different effects. Spraying effect is superior to spraying process in mechanical strength, adhesion, corrosion resistance and aging resistance.








1. Surface pretreatment.


The quality of pretreatment process directly affects the quality of powder coating, and poor pretreatment results in the phenomenon of film falling off and bubbling. Therefore, the pretreatment work must be paid attention to.


In order to remove the dirt and defects on the surface of aluminium products, such as dust, metal oxides, residual oil, fingerprints,








For minor scratches (scratches) and other injuries, the necessary surface pretreatment must be carried out before surface treatment. Its general process is: degreasing washing alkali etching one washing second washing neutralization washing.


(1) Degreasing and degreasing treatment is also known as degreasing. Its purpose is to remove process lubricants, rust-proof oils and other contaminants on the surface of aluminium materials, so as to ensure uniform corrosion on the surface of aluminium materials and cleanliness of bath fluid in alkali corrosion process.








(2) Alkali washing and alkali washing corrosion generally uses 35-100g/L NaOH as the treatment solution. Its purpose is to remove the dirt on the surface of aluminium material and expose the base metal under the natural oxide film on the surface of aluminium material, so as to facilitate the smooth oxidation. This process is the most aluminium soluble process in the whole surface treatment process. Its corrosion amount is about 20 micron, and the soluble aluminium is 50 g per square meter. When the amount of aluminium dissolved in alkali washing solution reaches 30g/L, aluminium hydroxide precipitates and accumulates at the bottom of the tank to form hard solids. In order to prevent precipitation, sodium citrate or sodium gluconate (1.5g/L) is usually added to the production.








(3) The main purpose of neutralization is to get rid of the black corrosion spots on the surface of aluminium after alkali corrosion, so as to obtain a bright metal surface, and at the same time neutralize the alkali solution. The commonly used neutralizing liquids are HNO 3 (relative density 1.42) 100-400 g/L or H2SO4 (relative density 1.84) 100-200 g/L.








(4) The purpose of water cleaning is to completely remove the residual liquid on the surface of the product and the reaction products soluble in water, prevent the contamination of the tank liquid in the next process, and ensure the efficiency and quality of the treatment.








2. Spraying:


The workpiece enters the spraying gun position of the powder spraying room through the conveying chain to prepare for spraying operation. The electrostatic generator releases high-voltage electrostatic (negative) electricity to the workpiece space by the needle of the electrodes at the nozzle muzzle, which ionizes the mixture of powder and compressed air ejected from the nozzle muzzle and the air around the electrodes (with negative charge). The workpiece passes through the hanging device and is connected to the ground (grounding pole), so that an electric field powder is formed between the spray gun and the workpiece under the dual impetus of electric field and compressed air pressure to reach the workpiece surface, and a uniform coating is formed on the workpiece surface by electrostatic attraction.








3. Baking and curing:


The sprayed workpiece is heated in the drying room at 180-200 C through the conveyor chain, and the corresponding time (15-20 minutes) is kept to melt, leveling and solidify the workpiece, so as to obtain the desired surface effect of the workpiece. Different powders have different baking temperature and time. This should be noted in the curing process.








IV. Inspection:


After curing, the appearance (flatness, brightness, granularity, shrinkage and other defects) and thickness (controlled at 55-90 um) of the workpiece are checked daily. Repair or re-spray the workpieces with defects such as leakage, pinholes, bruises and bubbles detected.








5. Packaging: After inspection, the finished products are classified and placed in transport vehicles and turnover boxes, separated from each other by soft packaging buffer materials such as foam paper and bubble film, in order to prevent scratch and abrasion (can be packed according to customer's requirements).