Something you need to know about electrostatic Powder Coating Technology for Aluminum Profiles
Something you need to know about electrostatic Powder Coating Technology for Aluminum Profiles
Brief information:
Aluminum alloy profile is one of the materials widely applied in many industries.
In practical applications, there are special requirements for its anti-corrosion performance and other functions due to different uses, which require surface treatment of aluminum materials.
The negative potential of aluminum materials makes it very easy to corrode, especially in contact with another type of metal, which will aggravate the corrosion of aluminum.
Therefore, the primary purpose of surface treatment is to prevent corrosion of aluminum profile to prolong its service life.
Electrostatic powder coating is a commonly used surface treatment for aluminum profiles.
This treatment method has good corrosion resistance, and the powder-coated aluminum profile has excellent acid, alkali and salt spray resistance.
Electrostatic powder coating technology for aluminum alloy materials has the characteristics of green environmental protection and economic space occupation, especially its simple process, easy operation and saving material loss.
Keywords: Aluminum profile electrostatic powder coating; powder coating; recovery system; powder supply system; coating gun control system; powder coating room
Electrostatic powder coating uses a coating machine with an atomizing nozzle (to atomize the powder) and a discharge stage (to generate electricity and current) to atomize the powder, apply a charge to it, and form an electric field between the electrode and the object to be coated, powder coated by electrostatic adsorption.
The electrostatic powder coating process appeared in the 1960s and is mainly used for metal surface coating.
In the late 1990s, with the development of powder coatings and coating equipment, it became possible to apply the electrostatic powder coating process to non-metallic surfaces.
The purpose of electrostatic coating is to spray the powder coating evenly on the surface of the workpiece.
Special workpieces (including the places where electrostatic shielding is easy to occur) should be sprayed with high-performance electrostatic coating machines.
The purpose of the curing process is to cure the sprayed powder onto the surface of the workpiece.
Process steps: Put the coated workpiece in a high-temperature furnace at about 200°C for 20 minutes (the curing temperature and time depend on the quality of the selected powder, and the curing temperature of special low-temperature powder is about 160°C, which saves energy more), so that the powder is leveling and curing.
1. The working principle of electrostatic powder coating
Use the corona discharge of the powder spray gun to make the powder coating adsorb on the workpiece.
After the workpiece is pre-treated and cleaned, it enters the powder spraying room.
The powder coating is fed into the powder spraying gun by the compressed air in the powder supply system.
The high-voltage electrostatic generator at the front of the spray gun generates high-voltage corona discharge.
Intensive charges are generated near it, and when the powder is sprayed from the nozzle, it forms charged paint particles (negatively charged). Under the action of electrostatic force, it is adsorbed to the workpiece with the opposite polarity (positively charged).
In this way, the entire workpiece is coated with a certain thickness of powder, and then the workpiece enters the curing oven, and the powder on the surface of the workpiece is melted, leveled and solidified after being heated at a high temperature, thereby forming a hard coating film on the surface of the workpiece.
2. Characteristics of electrostatic powder coating
Powder coating is a kind of powdery solvent-free coating, which is prepared by mixing special resin, pigment and filler, curing agent and other additives in a certain proportion, and then through hot extrusion, crushing and sieving.
Generally, it is divided into thermosetting and thermoplastic, the coating contains almost no VOC, has a high utilization rate, less solid waste and a high recovery rate.
3. Powder coating equipment
The spray gun is an important tool for powder coating.
The SJFP-2 portable spray gun developed by my country in the early years miniaturized the huge high-voltage electrostatic generator in the past and encapsulated it in the powder spray gun. It adopts the low-voltage input and high-voltage output mode, mainly for manual operation.
With the continuous deepening of the degree of automation, the spray gun for the mechanical arm installed on the mobile device has become the need of the situation.
The powder electrostatic spray gun for the mechanical arm invented by Mr Haihua Huang combines the electrostatic generator, nozzle, atomization device (electrode seat, conductive coil) ) into a straight gun body, the external connecting pipeline only needs to be connected to the powder supply pipe, power line (low voltage), and control line (low voltage) to use.
The overall performance is improved, the cost is reduced, and safety hazards such as static electricity leakage, cable breakage, and sparks that are easy to occur in existing spray guns are effectively avoided.
At this stage, a large number of intelligent Robot spraying equipment have been manufactured at home and abroad. However, most of these equipment require manual assistance and cannot complete more complicated operations.
The invention of Mr Nianzhen Han and others provides a five-axis robot spraying equipment, which includes a five-axis robot, a workbench, a quick connector, a spraying device and a control system.
The automatic spray gun is connected with the first-level head of the five-axis Robot by the quick connector, the spraying device and the five-axis robot are electrically connected to the control system respectively.
It can realize intelligent operation, save labor cost, and the product quality is stable and consistent.
In addition, Qiu Yunjie and others invented a five-degree-of-freedom hybrid spraying robot, including two parts: the basic serial motion unit and the terminal parallel spraying unit.
The basic tandem kinematic unit has 3 rotational degrees of freedom and 1 translational degree of freedom
4. Spray gun control system
The working state of the spray gun determines the quality of the product surface.
How to control the start and stop of the spray gun, operating parameters and avoid paint waste more effectively and conveniently is also the focus of research by relevant researchers.
The existing multi-spray gun spraying system cannot control the separate start and stop of a single spray gun, and can only automatically control the electrical parameters (electrostatic voltage and electrostatic current), but lacks automatic control of the air pressure parameters (atomization air pressure and fluidization air pressure).
A multi-spray gun electrostatic powder spraying control system invented by Mr Xiangyang Li, etc. can realize the independent configuration of the working parameters of each spray gun through network technology, realize the independent control of high-speed start-stop sequence through direct I/O control technology, and adapt to uniform powder spraying of different spray patterns.
5. Powder recovery system
The powder recovered from the existing aluminum profile spraying equipment may be mixed with different colors, and can only be treated as waste powder in the end.
To solve this problem, several powder recovery pipelines are set in the powder spraying room, and a powder cyclone recovery cylinder and a recovery powder storage box connected to each powder recovery pipeline are installed.
The larger powder is recycled to the recovery powder storage box through the action of the cyclone, and the fine powder (waste powder) is pumped to the final filter, which retains the fine powder, and the clean air is discharged out of the factory building through the exhaust fan, energy saving and environmental protection, superior performance.
In addition, if the over-capacity powder is suctioned by the fan and directly recovered, it will inevitably be mixed with dust in the air, resulting in insufficient powder purity and easy to reduce the quality of secondary processing.
Mr Xiaofeng Xi invented a powder recycling device, which uses powder to dissolve crystals and extract crystals to ensure powder purity.
6. Powder supply system
In order to improve the spraying efficiency, most of the existing technologies use multi-spray gun spraying equipment, so the powder supply system usually arranges multiple discharge holes in parallel on the upper end of the powder supply barrel, and connects the corresponding powder delivery pump and powder delivery pipe, and finally delivers to each spray gun, supplemented by the spray gun controller, changes the powder output and the atomization state of the powder to achieve different coating thicknesses to meet the needs of different products.
According to different working conditions and environments, there are still researchers who continue to improve and innovate the powder supply system.
The existing powder supply center generally controls the movement and positioning of the powder pump carrying device by the expansion and contraction of the cylinder.
If the maintenance is not proper, the powder is easy to enter the cylinder, and the normal expansion and contraction cannot work, and the powder supply center will be no workable.
Mr Hongbing Chen invention replaces the original cylinder telescopic method.
The powder pump bearing mechanism is installed on the moving plate, and connected to two parallel positioning shafts from top to bottom through pin holes.
On the fixed column of the hole, manually push the powder pump bearing device to move up and down along the positioning axis and fix it.
7. Powder coating spraying room
Aiming at the problem that small and medium-sized enterprises cannot afford large-scale spraying line equipment and can only spray in the open environment of the workshop, an electrostatic powder spray room with low price, small footprint, complete prevention of powder diffusion and powder recovery is proposed.
The spraying room is equipped with a powder primary recovery device and a powder secondary recovery device, and the powder primary recovery device is located on the side of the spraying room; the air channel of the powder secondary recovery device is connected with the powder secondary recovery room through an air filter pipe.
8. Main Factors Affecting Electrostatic Powder Coating
It is necessary to strictly control the volume resistance of powder particles in electrostatic spraying.
Usually, controlling the volume resistance within the range of 1010-1014Ω·cm, which can ensure the construction performance and obtain film thickness coating.
The charged amount of the powder particles is proportional to the size of the particle radius.
If the particle size of the powder is smaller, the charged amount of the particle is smaller, and its electrostatic attraction is also smaller.
Particles with a small electrostatic attraction are easily thrown out of the electrostatic field or suspended in the electrostatic field, resulting in a lower spraying powder rate.
When the particle size of the powder is too large, although the charged amount is large, the particle itself is heavy, and the coating particles also have a large repulsion, which reduces the powder feeding rate.
The coating with too large particle size is difficult to obtain a thinner coating.
In practical applications, the particle size of powders usually used is in the range of 30-50 μm.
8.3 Powder coating voltage
At low voltage, if the powder coating voltage continues to increase, the charge amount of the coating powder will increase accordingly, and the adhesion amount will also increase accordingly.
However, the electrostatic voltage has a saturation value.
Once it is greater than the saturation value, electrostatic displacement will occur and the adhesion amount will increase.
The amount becomes less, and the high voltage increases the possibility of powder breakdown, which endangers the coating quality.
The commonly used powder coating voltage is controlled at 60-80kV
8.4 Powder coating distance
Usually, the controlled coating distance is 200-300mm, which has better powder deposition efficiency.
If the spraying distance is too small, it is easy to break down the powder and affect the coating quality.
If the coating distance is too large, the deposition efficiency will be too low, the powder adhesion will be reduced, and the work of recovering powder will be increased.
The high gas supply pressure makes the powder obtain greater kinetic energy, making the powder easy to rebound on the surface of the aluminum alloy part and difficult to adsorb.
In practical application, there are three types of air supply pressure parameters, namely, powder supply air pressure, atomization air pressure and fluidization air pressure. Changes in these three types of pressure parameter values will lead to significant changes in spraying efficiency and quality.
9. Conclusion
Electrostatic powder coating technology is used in the surface treatment of aluminum profiles to make it have good anti-corrosion performance, and its ability to resist acid, alkali and salt spray is also relatively superior.
Electrostatic powder coating technology has: a relatively simple process, high yield, significantly reduced energy consumption, reduced water and air pollution, significantly reduced labor intensity of workers, significantly reduced standards for mill finish aluminum profile quality, and some coating film Compared with other surface treatment films, the physical index is significantly improved.
In addition to the necessary professional knowledge, powder coating process technicians should also have basic knowledge related to spraying equipment. Have the spirit of hard work and be good at summing up experience and lessons in practice.
Brightstar Aluminum Machinery offers not only horizontal and vertical aluminum profiles powder coating production line with the subcontractor, but also aluminum profiles surface defects brushing and cleaning machine, aluminum profile shape corrector and aluminum profile packaging machine, etc, turnkey project and one package deal!
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February 2, 2023