Introduction to the characteristics and methods of powder painting
First, the characteristics of powder coating
Compared with the construction of solvent-based paint, powder coating has the following characteristics due to the use of low-pollution paint and the installation of a powder recovery system:
(1) The utilization rate of paint is high, reaching more than 90%, and it is a low-pollution painting method;
(2) It is suitable for automatic production and has high production efficiency;
(3) Coating film thickness, one coating film thickness can reach 100~300µm
Powder coating also has certain defects:
①The baking temperature is high (≥200℃), and the coating film is easy to change color;
②The coating equipment is dedicated, and it is inconvenient to change the color; ③The coating film defects after baking are not easy to repair;
④ The decorative appearance of the coating film is poor, and the leveling and gloss are not as good as those sprayed with solvent-based paint;
⑤The adhesion of the coating is poor. In many cases, heat treatment or humidity control treatment is required. The coatings used in powder coating are divided into thermoplastic and thermosetting. The special varieties of powder coatings are polyvinyl chloride, polyethylene, Polypropylene, polyamide, fluororesin, chlorinated polystyrene, polyphenylene sulfide, etc.; thermosetting powder coatings include epoxy, polyester, acrylic acid, polyurethane, etc. The melting temperature and melt viscosity of thermosetting powder coatings are lower than those of thermoplastics. The adhesion and flatness of the film are better than thermoplastic powder coating. Many resins of thermoplastic powder coatings are crystalline, and should be quenched after baking to ensure sufficient adhesion of the coating.
2. Powder coating method
Powder coating methods include flame spraying, fluidized bed method, electrostatic fluidized bed method, electrostatic spraying method and powder electrophoresis method.
The flame spray method is rarely used because the resin is susceptible to pyrolysis and the quality of the coating film is poor.
The fluidized bed method is to preheat the workpiece to a temperature higher than the melting temperature of the powder by 20°C, and then immerse it in the ebullating bed to partially melt the powder and adhere to the surface, and form a complete coating by heating and fusion. The fluidized bed method is only suitable for small workpieces with thick walls and large heat capacity, and it is not suitable for thin plate parts with small heat capacity. The higher the workpiece preheating temperature, the thicker the coating, generally between 100 and 300 µm. To form a thin coating below 100µm, the fluidized bed method is not suitable. In addition, when the fluidized bed method is used for coating, because the resin is heated at a high temperature and takes a long time, it is often forced to cool with water after baking to reduce thermal decomposition.
Electrostatic fluidized bed is to put the cold workpiece in the fluidized bed through electrostatic adsorption of powder, so the workpiece does not need to be preheated, and can form a complete thin coating, but it is only suitable for the coating of small pieces.
The electrostatic spraying method can form a complete coating of 50-200 μm, and the coating has good appearance quality and high production efficiency. It is the most widely used method of powder coating. The current powder electrostatic spraying equipment is equipped with a set of automatic powder cleaning f1 device, so it is more convenient to change the color.
The powder electrophoresis method is to disperse the resin powder in the electrophoretic paint, and attach it to the surface of the workpiece according to the method of electrophoretic coating. When baking, the resin powder and the electrophoretic paint base material are integrated to form a coating. It has the advantages of electrophoretic coating. For example, the deposition time is short, the production efficiency is high, the film thickness is uniform, and it is easy to adjust the thickness by voltage. In addition, the dust problem common to powder coating is avoided. The disadvantage of this method is that due to the effect of moisture, the coating will produce pores during baking, and the baking temperature is high; in addition, the coating is thicker than ordinary electrophoretic paint, generally 40~100μm.
