So far, there have been numerous aesthetic techniques available for electronic cigarettes, including anodizing, dual-color injection molding, spraying, stickers, UV printing, skins, 3D glass, and more. In terms of appearance effects, options include frosted finishes, tactile coatings, polishing, gradient coloring, patterns, and others.
Among these, it’s understood that one of the most common aesthetic effects is gradient coloring, with two main stable techniques used: anodizing and spraying. Today, we will focus on explaining the principles and processes of anodizing.
Basic Principles
The process of using aluminum or aluminum alloy products as anodes in an electrolyte solution and using electrolysis to form an aluminum oxide film on the surface is called anodizing of aluminum and aluminum alloys.
The cathode in the device is a material with high chemical stability in the electrolytic solution, such as lead, stainless steel, aluminum, etc. The principle of aluminum anodization is essentially the principle of water electrolysis. When current passes, hydrogen gas is released on the cathode; on the anode, the oxygen released is not only molecular oxygen, but also atomic oxygen (O) and ion oxygen, which are usually expressed as molecular oxygen in the reaction. The aluminum used as the anode is oxidized by the oxygen precipitated on it, forming an anhydrous aluminum oxide film. Not all of the generated oxygen interacts with the aluminum, and part of it is precipitated in the form of gas.
Anodizing Classification
- Based on the type of current: direct current anodizing, alternating current anodizing, and pulse current anodizing.
- Based on the electrolyte solution: sulfuric acid, oxalic acid, chromic acid, mixed acid, and organic acid with sulfonic acid as the main solvent for natural color anodizing.
- Based on the properties of the film layer: regular film, hard film (thick film), porcelain film, bright decoration layer, semiconductor blocking layer, etc.
Process Flow
- Direct current sulfuric acid anodizing: Initially, the aluminum product as the anode undergoes uniform oxidation to form an extremely thin yet dense film. Due to the action of sulfuric acid solution, weak points in the film (such as grain boundaries, impurity-rich points, lattice defects, or structural deformations) experience local dissolution, resulting in numerous pores or primary oxidation centers. This allows the base metal to come into contact with the electrolyte, enabling continued current conduction. The newly generated oxygen ions are used to oxidize fresh metal and expand from the bottom of the pores, ultimately converging to form a new film layer between the old film and the metal, effectively “repairing” the locally dissolved old film. With prolonged oxidation time, the continuous dissolution or repair of the film allows the oxidation reaction to proceed deeper, resulting in the formation of an oxide film composed of a thin and dense inner layer and a thick and porous outer layer on the product’s surface. The thickness of the inner layer (blocking layer, dielectric layer, active layer) remains unchanged until the end of oxidation, but its position continuously shifts deeper. In contrast, the outer layer thickens over time within a certain oxidation period.
- Activation: After anodizing, before coloring, an acid treatment can be applied to activate the oxide film. This dissolves some substances in the anodized film, increasing the porosity of the oxide film and thereby enhancing its ability to adsorb coloring agents.
- Dyeing: There are chemical dyeing methods and electrolytic dyeing methods.
Chemical dyeing is characterized by its simple process, easy control, high efficiency, low cost, minimal equipment investment, wide color range, and bright colors.
Requirements for Film Thickness:
1. Sufficient thickness is necessary, with the specific thickness depending on the desired color tone; darker colors require thicker films, while lighter colors require thinner films.
2. The oxide film must have sufficient porosity and adsorption capacity.
3. The oxide film should be uniform in color, and the color of the film layer itself should be suitable for coloring treatment. - Sealing: To improve the quality of aluminum parts and enhance the durability of coloring, the microfine pores in the oxide film layer must be sealed after coloring. After sealing treatment, the surface becomes uniform and pore-free, forming a dense oxide film. The dye deposits within the oxide film can no longer be rubbed off, and the sealed oxide film no longer has adsorption properties, thus avoiding contamination or early corrosion caused by adsorption of harmful substances. Common methods for sealing after coloring include hydration sealing, inorganic salt solution sealing, and transparent organic coating sealing.
Effect After Anodizing
If coloring metal is required, it needs to undergo dyeing and sealing to achieve the desired coloration.
Gradient coloring is achieved by controlling the coloring time of the dye in the dyeing tank. The dye molecules enter the interstices of the oxide film during the dyeing process. The longer the dyeing time, the more dye enters, resulting in a deeper color. By controlling the time that the workpiece stays in the dye with a computer, such as gradually removing the workpiece from the dye, a uniform gradient color effect can be achieved.
Of course, this is just one method. There are many other methods for achieving gradient coloring after anodizing, such as silver salt dyeing, pattern dyeing, and heat transfer dyeing. Because the mechanism of anodizing has been well studied, many coloring processes have been developed for anodizing.