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Can Lin Classroom: surface treatment
  • Release date:2020-03-09
  • Browse times:612

Surface treatment


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To ensure that the coating film is durable, meets the requirements for use, and gives full play to the protective and decorative role of the coating, in addition to the quality of the coating itself, construction is also an important link. The surface treatment of the object to be painted is the preparation work before painting, it is related to the adhesion and service life of the coating film, and directly affects the quality of the coating project, so it must be paid sufficient attention.


The method of surface treatment varies with the material, application, construction method, and type of coating. Good surface treatment can prolong the service life of the coating film and give full play to the role of the coating.


The surface treatment has the following purposes:

-Enhance the adhesion of the coating film to the surface of the object

-Preparing conditions for apparent leveling of the coating film

-Increase surface corrosion resistance


 1.Surface treatment of zinc parts

The surface of the newly processed zinc product is due to its smooth surface and the chemical reaction between zinc and the coating, which will cause the coating adhesion on the zinc material to decrease. .


Atmospheric erosion, mechanical sanding, or simply etching the zinc surface can improve the adhesion of the coating. However, in order to improve the corrosion resistance, a chemical treatment can be adopted, and a zinc salt phosphating treatment that generates a crystalline phosphating film can obtain a very good effect. However, amorphous iron salt phosphating is also widely used. Galvanized steels of various brands sold in the market generally do not need to be added with additional cleaning or processing steps after phosphating (unless they are contaminated during the production process). After its surface coating, the coating film performance is good.


When cleaning zinc products, since zinc reacts with alkali and acid, it is necessary to carefully select a suitable cleaning chemical to prevent excessive metal corrosion.


2.Surface treatment of aluminum parts


The corrosion resistance of aluminum is very good, because when it is exposed to the atmosphere, it can form a strong oxide film on the surface to prevent the substrate from further corrosion. Because there is no need for anticorrosive treatment except in extremely humid or salt spray environments.


However, most coatings do not adhere well to aluminum. In order to improve adhesion and prevent the coating film from becoming brittle, surface treatment must be performed before painting.


In many cases, treatment with a phosphoric acid-based solution can give the desired effect. Wipe method, spray method or dip method may be adopted. Similar effects can be obtained with hot dilute chromic acid. When the applicable environment of the workpiece is more severe, using anodizing treatment or alkali dichromate treatment can get better results. After the above treatment, a protective film is formed, and the amount of this film and paint adhesion oil. However, both of these treatment methods are relatively slow and require soaking for 30 minutes. Because alkali can quickly attack aluminum, care must be taken in alkaline washing while complying with the requirements of the manufacturer's production instructions. And check the adhesion.


 

3. Surface treatment of magnesium alloy parts

Coatings applied to magnesium alloy parts exhibit the same "alkaline peeling" phenomenon as zinc parts.


Most magnesium alloy products are treated with a chemical protective film before the final coating. This layer of chemical protection film protects the metal during the storage, transportation and manufacturing of magnesium alloy products. The treatment method is to immerse the magnesium alloy for a short time in a mixed solution of nitric acid and sodium dichromate. This protective film is suitable for use as a base for paint, but it cannot be used to process parts that require precise tolerances because the treatment liquid will etch metal. After dichromate treatment, it can obtain very good corrosion resistance, so it is widely used as the base of paint. Due to the danger of this treatment, it is absolutely necessary to follow the operating specifications proposed by the manufacturer.


 4, plastic surface treatment

Like the surface of other materials, sludge, grease, oil or other pollutants adhering to the surface of the plastic must be clear and clean in order for the coating to form a good adhesion to the plastic surface.


During the manufacturing of foam plastic, the surface of the plastic is often contaminated with a release agent. This release agent is used to prevent plastic from sticking to the mold. It reduces the adhesion between the plastic surface and the coating.


The following alcohol solvents are generally recommended for removing contaminants from plastic surfaces: ethanol, propanol, isopropanol, or butanol. When encountering difficult-to-remove mold release agents, you can also use 10% methyl ethyl ketone in water to treat plastic surfaces.


For thermosetting plastics, it is recommended to use a stronger solvent for cleaning, such as aromatic amidines (such as xylene, toluene), or ketones (such as methyl ethyl ketone or methyl isobutyl ketone).


 5. Surface treatment of wood

Store all wood in a dry, warm room to prevent moisture absorption. The amount of water contained in a piece of wood is called "water content", and this amount is expressed as a percentage of the weight of water contained in the wood to the weight of the dry wood itself. Wood swells when it absorbs moisture and shrinks when it loses water. The water in the wood changes with the humidity in the atmosphere. When painting wood, its water content should be about 5% -8%, which can be detected by special instruments. All wooden surfaces should be sanded with sandpaper and sanded along the lines. The sandpaper used for each sanding is finer than the previous one. The conventional sanding process is first sanded with 100 sandpaper and then with 150, 180 and 220 sandpaper. The particle size difference between the two adjacent sanding papers should be moderate. The principle is that the last sanding just removes the wear marks from the previous sanding paper. All dust must be clear and can be cleaned with a cloth or blown through the spray gun with clean air. If there is any contamination or grease on the surface before spraying, it must be removed with a solvent.

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