With the wide application of aluminum alloy parts, CNC machining of aluminum alloy is becoming more and more popular. But aluminum alloy is easy to cause a chemical reaction with oxygen, so generally needs to do metal surface treatment, and anodized aluminum parts are the most common kind of metal surface treatment.
When the manufacturing process of a part is complete, the surface finish of the part to be processed is the next process that the factory needs to deal with. There are many ways to deal with the surface of parts and components in the industrial field.
Part manufacturers often choose the way of surface treatment according to the technical level and the actual demands of customers, such as anodizing process. The anodizing process is a popular method in the industry, it has good durability, and corrosion resistance and also increases the aesthetics of the aluminum product surface. To color aluminum products, the relevant operator f needs to understand the concept of anodizing, the common colors of anodized aluminum, and how to color aluminum products with anodizing.
Why Do Aluminum Alloy CNC Products Often Do Oxidation Surface Treatment?
- The aluminum alloy itself is a kind of raw material with strong corrosion resistance, but when it encounters oxygen, it generates oxidation on the surface of the aluminum alloy, and gradually eliminates its corrosion resistance and bran resistance for a longer period of time. In addition, aluminum alloys are very sensitive to acids. When it encounters acidity, it destroys the surface layers of aluminum. Then we need to oxidize again and add another layer to the surface layer to improve the surface corrosion resistance of the anti-corrosion protective film.
- Aluminum alloy anodized membrane surface porous, very easy to absorb dyes and colored components, so can be dyed, improve its ornamental. Oxidation film after boiling water, high-temperature water steam or nickel salt closed processing, can further improve its corrosion resistance and wear resistance.
Basic knowledge of Anodizing Process
From the surface point of view, the anodizing process is similar to many surface treatment processes and is a treatment method applied to the surface of metal objects. The significance of this anodizing process is to improve the aesthetics and corrosion resistance of the processed parts, so the anodizing process is recognized by many parts machining industries. Anodic oxidation is an electrochemical cell reaction, the aluminum part is the anode, and the cathode is mostly inert material or acidic electrolyte. As an electrochemical process, anodizing is generally welcomed by the industry because it can improve the surface properties of processed aluminum products.
Types of Anodizing Processes
There are three types of anodizing processes, and there are obvious differences between the different types of anodizing processes: different coating strengths, and different electrodes, electrolytes, and energy used by each type of anodizing process.
Type 1 Anodizing Process: The current is slowly passed through the electrolyte, and the + ions on the anode will be ejected, forming tiny grooves on the metal surface. Anodizing Type 1 increases the heat and corrosion resistance of the aluminum metal compared to untreated aluminum.
Type II Anodizing Process: The Type II anodizing process changes the electrolyte feedstock from butyric acid to sulfuric acid. Sulfuric acid can eject + ions better, forming deeper grooves on the metal surface, and the oxide layer will be thicker than the type 1 anodizing process. The Type II anodizing process results in good paint retention properties for aluminum metal parts.
Type III Anodizing Process: The type III anodizing process is a commonly used process in the manufacture of important aluminum metal products. Compared with the previous two, the type III anodizing process uses a combination of voltage and strong acid on the electrolyte. The strong acid is the sulfuric acid used in the Type II anodizing process.
Anodized Aluminum Coloring Steps
Using anodizing to add color to aluminum products is a more rigorous process. Unlike traditional paint or powder coating, in terms of achieving color consistency, since anodizing involves many variables, it puts forward higher requirements on the level of practitioners. Generally speaking, the anodizing process can provide a variety of colors to the metal, but the color effect also depends on the size of the metal, grade, and other factors.
1. Cleaning and Etching of the Aluminum Parts
Coloring aluminum products by anodizing requires a very rigorous process. At the beginning of the anodizing process, the operator needs to clean the aluminum parts first, carefully clean the surface of the aluminum parts with detergent, clean and ventilate and dry them before etching. This process is particularly important. If the aluminum parts are not handled cleanly, the subsequent process will cause uneven coloring and fading. Etching is also an indispensable step in the coloring of aluminum products because etching can remove trace metals.
2. Build the Oxide Layer
After cleaning and etching, the aluminum parts can be anodized. Operators can choose one of three anodizing processes to color aluminum products based on customer needs.
3. Adding Color
①Electrolytic Coloring: Electrolytic coloring is the basic operation method, and it is also the most common coloring method for aluminum parts processing. The method is to soak aluminum products in a solution containing metal salts. When filling micropores on metal surfaces, the salt solution provides a coating that is sufficiently resistant to attack by the sun’s UV rays. However, it should be noted that this method of coloring aluminum products has many restrictions on bronze and black.
②Pigment Coloring: In this method, the aluminum part needs to be further processed, that is, the aluminum part is placed into a container containing dye. The dye will slowly fill all the micropores on the surface of the aluminum part and boil in subsequent deionized water, at which point the coloring of the aluminum product is complete. When dealing with complex aluminum parts, operators often pour different pigments into the tank to increase color diversity and achieve gradients.
③Overall Coloring: The overall coloring is relatively simple. This method is to perform anodizing and coloring at the same time and dye the aluminum product black or bronze. The advantages of the overall coloring process can make aluminum products more wear-resistant.
④Sand Blast: Overcoming defects, deburring and brightening the surface, oxidation (coloring), polishing.
Sealing is the final stage of the anodizing process, where the dye molecules in the gaps of the metal part are firmly held in place, and sealing prevents unwanted molecules from being absorbed in the gaps. The temperature of sealing is generally maintained at 180-200, that is, in hot water around 93°.
Notes for Color Matching
Unlike paint, etc., anodized color is subtractive. About the Color Matching of aluminum products, below are what you should look out for:
The Number of Dyes That Make Up the Colors and Layers
The type of dye used tends to play a larger role in the change in color. When an aluminum part needs more dye to match the color, it can have multiple color changes on its surface. In addition, aluminum parts manufacturers also need to take into account the layering, each layer will also have a certain color change.
Crystal Structure of The Product
Will the metal crystal structure of the product affect the coloring of aluminum products? The answer is probably yes. From a certain Angle, the colors seem to match perfectly. However, due to the different Angles of reflection, the result is often surprising. This phenomenon is called “color inversion” in anodic oxidation coloration.
Mismatch Solution for Anodized Coloring
Color mismatch is a very vexing problem in dealing with anodic oxidation coloring. The removal of color depends largely on the nature of the dye used and the condition of the anodized product. It is generally recommended to use crushes or phosphorus to remove dyes from the outer layer of anodized metal. Anodic oxidation is a very important electrolysis process, which can improve the appearance of aluminum products.
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After the aluminum alloy shell is processed by CNC, it can carry out metal surface treatment processes such as sandblasting, oxidation, grinding and polishing, powder coating, oil injection, laser engraving, metal drawing, etc.
YIJIN can customize the professional post-surface treatment process according to customers’ requirements, in line with industry standards in various industries.