Steel drum water-based paint electrostatic spraying solution (Yang Wenliang)
Steel drum water-based paint electrostatic spraying solution
Yang Wenliang
In the past two years, there have been more and more enterprises using water-based paints in the domestic steel drum manufacturing industry. Most enterprises adopt high-pressure airless spraying. Many enterprises have completely realized the switching of water-based paints and oil-based paints, and fully used water-based paints. Recently, several companies, such as Yantai Jinhai Packaging Co., Ltd., reported that the use of water-based paint for electrostatic spraying of steel drums has caused problems in spraying operations. We consider that many companies still use electrostatic spraying methods for steel drum coating. In order to match the use of water-based paint, this article gives a simple analysis of the electrostatic spray problem of water-based paint, hoping to solve the problems of these enterprises.
First, the basic situation of water-based paint
Water-based paint is mainly pure water due to the thinner of its paint. During the spraying process, most of the volatilization is water vapor, thus avoiding the evaporation of a large amount of organic solvent in the solvent-based paint (paint) during spraying. The VOC concentration and total amount of the exhaust gas, the solvent content of the water-based paint is only 1/7 to 1/10 of the solvent-based paint. The environmental protection advantage is obvious.
With the increasing understanding of water-based paints by domestic barrel-making enterprises, the improvement of the technical level of domestic equipment manufacturers, the development of domestic water-based paint companies and the enhancement of process application capabilities, the new water-based paint process is relatively easy to accept and gradually Promotion and application.
According to the survey, there are more and more steel drum manufacturers applying water-based paint abroad, especially in European countries, the application rate of water-based paint has reached more than 57%. At present, there are not many manufacturers of water-based paints in China, and there are only less than ten steel drum manufacturers.
Second, water-based paint electrostatic spraying process solutions
The water-based paint spraying system mainly involves the paint conveying system, the spraying system, the painting room system, the drying system and the air conditioning system.
1, paint system
Compared with solvent-based coatings, water-based coatings are pseudoplastic fluids, viscosity decreases with increasing shear rate, solvent-based coatings are Newtonian fluids, and viscosity does not change with shear rate. Therefore, in the design and production process of the paint conveying system, two points should be paid attention to: the pipeline resistance is large, and the shearing force should be avoided. Focus on and control the following parts during the selection process of the paint system:
(1) All parts of the paint must be selected in accordance with the requirements of water-based paints, such as stabilizer towers, four-way valves, pipe joints and other components.
(2) The paint pump for paint transfer is the best for electric type, the second is hydraulic type, the pressure of the pneumatic pump will be larger than the line pressure, and the voltage regulator needs to be added. The pump needs to be applied to the requirements of water-based paint (material, output pressure) , flow rate, etc.), due to the high viscosity of the water-based paint, the pipe resistance is large, and the power and output pressure of the paint supply pump need to be increased.
(3) The electric paint mixer is used in the paint system agitator to ensure low shear force on the paint. Especially when the paint tank level is different, the stirring speed will bring viscosity change. In order to prevent large changes in liquid level, it is best to use double tank mode, one circulating tank, one as feeding tank, automatic liquid level monitoring, automatic touch up and automatic speed control.
(4) All pipes, valves and related components of the piping system are made of stainless steel, preferably SUS316.
(5) Avoid pipeline design, construction and other details. There are some problems such as small pipe diameter, uneven pipe welding, ineffective passivation, and blind end of circulation, which will affect the cycle stability and coating quality of water-based paint in the future. .
2, automatic spraying system
Since water-based paint uses pure water as a diluent, it is highly corrosive. Therefore, all metal parts that come into contact with the paint should be made of stainless steel. It is best to use SUS316. Special parts and special functional parts are made of special stainless steel.
Because the water-based paint thinner is water, the conductivity is strong, and the conductivity of the solvent-based paint is weak. When the spray is electrostatically sprayed, the usual internal electricity connection method will cause the entire water-based paint delivery system to be fully turned on, losing the electrostatic spray. Features. If the grounding system of the paint transfer system is poorly grounded, the whole system will be charged with high-voltage static electricity, and there are safety hazards such as discharge and fire. For this reason, the welding equipment should be adjusted. At present, there are two main ways to supply electrostatic spray coatings for steel drums: one is external power-on mode; the other is the usual internal power-on mode, but it must be on the paint supply system. The circuit isolation device is added, and the piston type extraction and injection isolation system is adopted, which has a complicated structure, high cost and high cost. At present, most of the domestic electricity supply is the most economical and reliable solution, but the coating utilization rate will be slightly lower than 5%.
The specific plan is as follows:
(1) Spraying system: color change valve, gear pump, metering pump and air motor are stainless steel, and the rotating cup is made of titanium alloy.
(2) Atomization system: equipped with an air bearing atomization system and a spray cup for external power supply, the compressed air pressure is generally required to be 0.6 to 0.8 MPa, and the rotary cup speed is controlled at 3 to 60,000 rpm.
(3) High-voltage electrostatic generator: The high-voltage electrostatic generator is used for water-based paint. The general setting voltage is 40~50kV.
(4) The equipment grounding method and other related connection components are all water-based.
(5) Equipment man-machine interface and spray control program, designed according to water-based paint.
(6) There is no direct relationship between the main body of the equipment and the type of paint. For example, the water-based paint has high anti-corrosion performance on the material of the body. For example, solvent-based paint is used, and the explosion-proof level of the electrical control system of the robot is required to be high.
It is understood that many domestic electrostatic spray equipment manufacturers have internal electric spray cups suitable for water-based paint production. The utilization rate of paint is about 5% higher than that of external electric spray cups. In other industries, the water-based paint coating production line adopts the method of cutting the paint passage during the spraying process, and adopts the internal EFI cup to spray the water-based paint.
Water-based paint electrostatic spray system configuration diagram
1-spray disc; 2-gear pump; 3-control system; 4-axial flow exhaust system;
5- electric lift; 6-insulated table; 7-air pipe, paint pipe and low-voltage cable; 8-spray room
3, manual spraying system
Since the thinner of the water-based paint is pure water and the conductivity of the water-based paint is high, the conventional method cannot produce high pressure between the workpiece and the spray gun, and artificial water-based electrostatic spray is difficult to achieve. In order to establish a high pressure between the steel drum and the spray gun, two methods are adopted: one is to establish an insulated transmission and paint system around the spray booth, and to establish a high pressure between the spray gun and the paint transfer system by means of insulation; An externally applied power is applied to the outlet of the gun to establish an electrostatic field between the gun and the workpiece.
At present, there are equipments specially used for electrostatic spraying of water-based paints. The system is installed around the spray booth. The system comes with a built-in high-voltage generator. The high-voltage static electricity is transmitted to the spray gun needle through the paint, and is insulated from the surrounding facilities through the feed supply and insulated supply pipeline, and is equipped with an automatic ground discharge device. . When the gun is released, the sliding door is opened, or the compressed air source is accidentally disconnected, the system automatically performs the grounding protection action. The system is mainly applied to air pressure of 0.2 to 0.4 MPa, and the spraying efficiency is close to solvent-type electrostatic spraying, about 50% to 60%.
The other is an external power-on method, which is a water-based paint electrostatic spray gun with an externally powered air turbine generator. The maximum fluid pressure of the system does not exceed 0.7 MPa, the maximum compressed air pressure does not exceed 0.7 MPa, and the maximum output voltage does not exceed 60 kV. The system uses an external power-on method that is less efficient than the internal power-on mode and is more efficient than an air gun.
Water-based paint special electrostatic spraying manual painting equipment
4, drying system
Water-based paints use pure water as a diluent. Water does not volatilize easily after spraying. No volatile water will boil at 100 °C, so that coatings without surface drying will have defects such as foaming and shrinkage. In order to ensure the quality of the paint film of the water-based paint, it is necessary to increase the pre-baking system so that the moisture in the coating is sufficiently volatilized at a temperature of 50 to 90 °C. In general, when the water-based paint enters the middle high temperature section in the drying furnace, the dewatering rate of the water-based paint should reach 80% to 90%. To this end, it is necessary to increase the water-based paint pre-drying of the appropriate station before entering the high temperature section of the drying chamber. Pre-drying generally adopts two-stage heating mode. The first stage temperature is between 50 and 70 °C, the drying time is 1-2 minutes, the second stage is between 70 and 90 °C, and the drying time is 2 to 3 minutes.
5, spray booth system
Due to the large humidity in the air caused by the water-based paint spraying, the water content of the paint is large, such as the coating adheres to the wall of the chamber body to bring certain corrosion to the wall panel.
Therefore, the spraying chamber is generally made of stainless steel plate, the bottom of the chamber body and the skeleton. The air duct is preferably made of stainless steel. In order to save investment, the dynamic pressure and static pressure chamber wall in the spray booth The plate can be galvanized.
The water-based paint spraying process involves many aspects of the entire spray system, is a system engineering, and puts new and higher requirements on each system. It is better to learn from the successful domestic production enterprises, and to cooperate with some units with water-based paint process design, equipment design, manufacturing and debugging experience, which will greatly help to ensure the success of the whole project.
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Food and beverage packaging
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