Dip coating is also a commonly used coating application method. The dip coating methods currently used include not only the original manual dip coating, but also new dip coating methods with a high degree of mechanization such as batch station transmission dip coating, rotary dip coating, centrifugal dip coating, and vacuum dip coating.
Dip coating is to immerse the object to be coated in a liquid tank filled with paint, after a short period of immersion, the surface of the object to be coated is adhered to the paint, and then the workpiece is taken out of the liquid tank and the remaining paint is dripped to form a paint film. the process of.
(1) Advantages and disadvantages of dip coating
1. Advantages
The advantages of dip coating are: the equipment used is simple, the operation is simple, the production efficiency is high, the coating loss is small, and it is easy to realize mechanized operation, and it is more suitable for coating workpieces with complex shapes.
2. Disadvantages
The disadvantages of the dip coating method are: the solvent volatilization loss in the coating is large during the dip coating process; the formed coating film is not easy to be uniform; the phenomenon of liquid hanging is easy to occur, and the appearance of the dip coating product is poor.
(2) Dip coating process and equipment
The automatic line using the dip coating process usually includes a dip coating tank, a dripping device for removing excess paint, a stirring device, a heating and cooling device, a fire prevention and ventilation device, and a conveying suspension device.
Since the dip coating process is mainly completed by two steps of dipping and paint removal, the paint dipping tank and the paint removal dripping device are key equipment.
1. Dipping tank and dipping operation
The dipping tanks are generally made of steel plates, and there are two shapes: boat-shaped for continuous operation and rectangular for station operation. The shape and size of the tank depends on the shape and size of the object to be coated. It is necessary to meet the premise that the object to be coated does not collide with the tank wall during dip coating, and minimize the volume and open surface of the tank. A stirring device is installed at the bottom of the dipping tank to ensure the uniformity of the paint. In order to prevent the workpiece from falling into the tank and colliding with the stirring device, a horizontal grid partition is installed at the bottom of the tank.
During the dip coating operation, the coating is put into the dipping tank. Since the paint needs to be used repeatedly for a long period of time, it is required that the performance of the coating can be stable for a long time, so that it does not deteriorate and the components are evenly distributed. Since thermosetting coatings and water-based coatings are more stable at room temperature and the solvents contained in them evaporate slowly, these two types of coatings are suitable for dip coating. Moreover, the type of paint used for dip coating and its viscosity will directly affect the appearance and thickness of the paint film. At present, dip coating is mainly used in the coating of baking coatings, but it is not suitable to use dip coating for fast-drying paints with strong volatility, because of the large amount of solvent volatilization, which is not only easy to pollute the environment, but also the viscosity of the paint liquid changes quickly, which makes the coating Membrane performance is not easy to guarantee.
The viscosity of the paint has a great influence on the quality of the dip coating. The paint film formed by the paint with too low viscosity will be too thin; while the paint with too high viscosity will flow slowly on the surface of the object to be coated, the appearance of the paint film will be poor, and it will easily form flow marks. Therefore, the viscosity of the coating is generally controlled at 20~30s by dipping (coating -4 Viscosity Cup). During the dip coating process, the viscosity of the paint should be checked frequently, and the solvent should be added in time if the viscosity is found to increase.
In order to ensure a satisfactory coating film thickness, in addition to controlling the viscosity of the coating, the lifting speed of the workpiece after dipping should also be controlled. Uneven, so it is necessary to control the lifting speed of the workpiece after passing the test according to the specific conditions of the equipment.
The dipping workpiece can be sent into the dipping tank by a hanging chain or in a wire basket (small workpiece) to complete the dipping.
2. Residual paint removal device and process
The methods of removing excess paint mainly include natural dripping and electrostatic removal.
① Remove residual paint by natural dripping method. It uses gravity to make the excess paint adhering to the surface of the workpiece naturally leveled and then dripped and removed. When using this device to remove excess paint, it should be noted that the workpiece should be lifted slowly and evenly during operation, so as to ensure that the coating film is fully leveled. Keep the thickness even.
② Electrostatic method to remove residual paint. It is to ground the metal workpiece to be coated (connected to the positive pole of the electrode), and when it passes through the mesh plate electrode area connected to the negative pole of the electrode, the electrostatic field formed will cause the residual paint to be accelerated by the force of the electric field and drip, and Improves film thickness and uniformity.
For small workpieces that are dip-coated in metal mesh baskets, they can also be sent to the centrifugal drum, and the excess paint can be removed by high-speed rotation for a short time.
