Hot-dip galvanized steel coils are widely used in the field of home appliances, and the surface treatment processes for hot-dip galvanized household appliances for different purposes are also different. Hot-dip galvanized home appliances are mainly divided into appearance parts and functional parts. Among them, the surface of the appearance part is generally painted, mainly including spraying and color coating; the surface of the functional part is generally only chemically treated, and the most widely used is passivation treatment.
The purpose of surface treatment is primarily to improve the life of the product in harsh environments and to achieve the desired surface effects or functions. Hot-dip galvanized household appliances are mainly classified into chemically treated steel sheets and coated steel sheets according to surface treatment methods. The chemically treated steel sheet is treated with chromate or phosphate on the surface of the galvanized steel sheet to improve the corrosion resistance, coating adhesion, fingerprint resistance and lubricity of the galvanized steel sheet. Among them, chromate passivation is generally used as a single surface treatment process and is the most widely used. The coated steel plate is processed by coating and laminating the surface of the galvanized steel sheet, and the main processes are divided into spraying and color coating.
Passivation treatment. If the galvanized steel coil is exposed to the air for a long time, the surface of the steel sheet will be white rust due to oxidation, which will affect the appearance. Therefore, the surface needs to be passivated for rust prevention. At present, the passivation treatment mainly uses chromate passivation treatment, including environmentally friendly passivation, full chromium-free passivation, and fingerprint-resistant passivation. Among them, the environmental protection passivation is ordinary passivation, the passivation film contains trivalent chromium; the whole chromium-free passivation film is completely free of chromium, more environmentally friendly, and the corrosion resistance is better than the environmentally friendly passivation film; Developed to solve the problem of rusting of home appliances due to fingerprints during the assembly process, the passivation film is completely free of chromium and meets high environmental standards. The passivation treatment process is generally roller coating, the environmentally friendly passivation is generally smooth coating, and all chromium-free passivation and fingerprint-resistant passivation are generally reverse coatings. The smooth coating can provide a thinner passivation film with low roll consumption; the reverse coating can provide a thicker passivation film, but the roll consumption is large. The galvanized sheet is also dried and cooled after chemical passivation.
The surface treatment of the outer panel of the appliance is often carried out by a spraying process. By spraying, the outer panel of the appliance can be obtained with desired color and color, and its corrosion resistance can be enhanced. The household electrical appliances produced by the spraying process mainly include an air conditioner outdoor casing, a vertical air conditioner indoor panel, a microwave bottom case, a refrigerator panel and a side panel, a cooktop bottom plate, a pulsator washing machine panel, and a small appliance casing. The typical process flow of spraying is the upper part – pre-de-oiling – degreasing – washing – surface conditioning – phosphating – washing – blowing – drying – pre-spray inspection – dusting – curing – the next piece.
The color coated board is also called a pre-coated steel sheet. It is usually a product obtained by surface-treating a galvanized sheet by roll coating or bonding an organic film and baking it, which can be further processed into a final product. The color coating can be divided into one coating one drying, two coatings two baking and three coating three baking according to different coating processes and different baking times. The color coated board can replace most of the spraying boards, and compared with the spraying board, the customer can eliminate the degreasing and coating when using the color coating board, and has the advantages of environmental protection and convenient use. The panels, side panels and back panels of household appliances such as refrigerators, washing machines, and microwave ovens can be produced by color coated panels.
The requirements for the shape of the outer panel of the appliance are very demanding, and some of the outer panels of the appliance are even higher than the requirements of the automobile panel. Because the outer panel of the appliance is directly used after being simply bent and formed, the surface of the appliance is mostly flat, so the shape of the outer panel of the appliance directly affects the shape of the appliance and the reflection of the plane. In addition, some functional parts are also more demanding on the shape of the board, such as a fingerprint-resistant backboard for making a backlight module of a liquid crystal display, which supports the weight of the backlight module, improves the rigidity of the backlight module, and provides good heat dissipation. At the same time, it plays the role of auxiliary reflection. If the display backplane is deformed, the rigidity of the backlight module is reduced, especially the reflection angle of the light is greatly affected, and the reflection sheet is equally deformed, so that the anti-scatter sheet cannot completely reflect the light to the light guide plate.
In summary, the outer cover produced by the spraying and color coating process has higher requirements on the shape and surface quality of the functional parts produced by the passivation process. Since the surface coating of the color coated plate is thin and has a magnifying effect on the surface defects, the color coated plate requires higher surface defects than the sprayed plate.