Under the background of global energy conservation and emission reduction, people's requirements for environmental protection are getting higher and higher
China's environmental protection law has increasingly strict
regulations on the content of organic matter (VOC) in the atmosphere.
In addition, people have also put forward new requirements for product appearance quality coating online coatingol.
On this basis, Changde Tobacco Machinery Co.
changed the original air spraying process to powder electrostatic spraying process, so that the parts have been improved
in terms of wear resistance, corrosion resistance and surface adhesion.
Powder electrostatic spraying process
1 Principle of powder electrostatic spraying
The workpiece is transported via a suspension chain to the spray gun in the powder booth ready for spraying
The electrostatic generator relies on the electrode needle to discharge high-voltage static electricity in the direction of the workpiece, so that the powder coating sprayed from the electrostatic spray gun is negatively charged, and the coated object is grounded as a positive electrode
The powder particles reach the surface of the workpiece under the drive of electric field force and compressed air pressure, rely on the action of electrostatic field electrostatic attraction, coated to the surface of the grounded coated, and solidified into a film
after heating and melting.
2 Process characteristics of powder electrostatic spraying
The comparison of powder electrostatic spraying and air spraying processes is shown in Table 1
By comparison, the characteristics of powder electrostatic spraying: (1) high adhesion of the coating film; (2) High coating hardness; (3) The coating has good
corrosion resistance and oil resistance.
Application of powder electrostatic spraying on smoke machine products
1 Pre-coating treatment
In order to ensure the quality of the powder coating, the workpiece must be prepared for spraying
At present, most of the company's sheet metal parts are Q235 steel, and the pretreatment process used is phosphating
After removing the oil, rust and dust on the surface of the workpiece, a dense and uniform phosphating film is formed on the surface of the workpiece to enhance the adhesion
after powder spraying.
The phosphating process flow is shown in Table 2
Solution ratio and related precautions for each process:
(1) The concentration of PA30-IM2 (A) used for chemical oil removal is 3%~5%, the concentration of PA30-IM2 (B) is 10%, and the free alkalinity (point) should be > 6
(2) PA51-A content used in oil removal and rust removal is 90 g/L, and sulfuric acid is industrial-grade sulfuric acid, content.
(3) The sulfuric acid used for activation is industrial-grade sulfuric acid, with a concentration of 0.
(4) The concentration of PA66-SC used in the table adjustment is 1%, and the pH value is controlled at 8~10
(5) The content of phosphating solution PA177-RUT2 is 280 g/L, the total acidity is ≥60, and the free acidity is pH value 4~7
2 Powder electrostatic spraying process (Figure 1)
In order to meet the characteristics of the company's large variety of parts, small quantity, irregular size and shape, a separate manual spray booth
is set up.
When the parts are phosphated and dried, as the conveyor chain enters the automatic spray room, the automatic spray gun evenly sprays the powder to the surface of the workpiece, and the edges and corners that are prone to electrostatic shielding are completed
by the front and rear hands.
For sheet metal parts with particularly complex shapes, they are transported to a manual spray booth for manual spraying
via a accumulation chain.
In the spraying process, the powder is evenly adsorbed on the surface of the workpiece by electrostatic attraction, and the falling powder is recovered through the large cyclone three-stage filtration recovery system, and then reused
After spraying, the workpiece is transported to (15~20)min in a high-temperature curing oven at (180~200)°C (curing temperature and time are determined according to the quality of the selected powder) through a suspension chain, and finally the required powder coating
is obtained after heating, concentration, leveling and curing.
3 Coating selection for powder electrostatic spraying
Although there are not as many types of powder coatings as solvent-based coatings, there are many polymeric resins as powder coatings, which can be divided into two categories
(1) Thermosetting powder coating
Thermosetting powder coating is made of epoxy, polyester, polyacrylic acid and other resins as film-forming substances, adding cross-linking reaction pigments, fillers and additives, etc.
, and forming an insoluble texture hard coating
Because of the low degree of polymerization and small molecular weight of the resin used, powder coatings have good leveling and good decorative properties
And after curing, the low molecular weight prepolymer can form a network of cross-linked macromolecules, so that the coating has good corrosion resistance and mechanical properties
(2) Thermoplastic powder coating.
Thermoplastic powder coating is made of polyethylene, polypropylene, polyester, polyvinyl chloride and other resins as film-forming substances, thermoplastic powder coatings melt at spraying temperature, solidify into a film when cooling, and do not require complex curing devices
The resin in thermoplastic powder coatings has high polymerization, large molecular weight, strong toughness of resin, cannot be simply mechanically crushed, the leveling of the coating is not good, and the adhesion to the substrate is poor
Considering the requirements of the company's products for the decorative and mechanical properties of the coating, epoxy polyester powder coating
was selected according to the actual situation.
4 Economic benefit analysis
The cost comparison of powder electrostatic spraying and air spraying (painting) is shown in Table 3 (taking 2017 products as an example).
Powder electrostatic spraying process is already a mature coating process, but the application of smoke machine products is a new attempt
In the future, we should actively explore the feasibility of powder spraying process in casting spraying applications and continuously improve the market competitiveness