The surface oxide film of extruded profiles is relatively soft and thin, generally ranging from 0.3 to 4 μm. After proper surface treatment, the film thickness can be increased to 60-2500 μm, greatly improving the mechanical properties and appearance quality of aluminum profiles, thereby extending the service life of the product. Surface treatment methods include chemical treatment and mechanical treatment. Chemical treatment methods include anodizing, electrophoretic coating, and electrostatic powder coating. Mechanical treatment methods include sandblasting oxidation, grinding, polishing, and brushing. Today, we will focus on the chemical treatment methods for aluminum profile surface treatment.
Currently, the most widely used method is still chemical treatment. Each surface treatment method has different process flows and parameter requirements. The process flows of these three chemical treatment methods are listed below.
1. Anodizing
Anodizing is an electrochemical oxidation process of aluminum alloy, where aluminum profiles are used as anodes and lead plates as cathodes. Under the influence of direct current or alternating current, a dense and porous oxide film is formed on the aluminum profiles, enhancing their surface oxidation resistance and mechanical properties. Based on different types of electrolytic solutions, anodizing can be categorized into sulfuric acid anodizing, oxalic acid anodizing, and chromic acid anodizing. The process flow is illustrated in the figure below:
Mechanical polishing → Degreasing → Water washing → Chemical polishing → Water washing → Anodizing → Water washing → Sealing treatment → Mechanical brightening
Since the chemical reagent used is sulfuric acid, which is relatively easy to obtain in daily production and not too expensive, sulfuric acid anodizing is a relatively energy-efficient surface treatment method. Coupled with its relatively simple process flow, it can also achieve excellent oxide films. The oxide film on the surface of aluminum profiles after anodizing can reach 5-20μm. Therefore, low-cost and high-efficiency sulfuric acid anodizing is the most widely used method in aluminum profile surface treatment. Generally, unless otherwise specified, anodizing refers to sulfuric acid anodizing.
It should be noted that after anodizing, aluminum profiles form a porous oxide film with strong adsorption capacity, which is prone to corrosion by atmospheric and other media. Therefore, the anodized aluminum profiles must undergo sealing treatment to eliminate pollution sources and enhance their corrosion resistance and wear resistance.
2. Electrophoretic coating
Electrophoretic coating employs various electrophoretic coatings, using water as the dispersion medium. These coatings dissolve in water and undergo dissociation, generating charged particles. Under the influence of an electric field, these charged particles move towards the opposite polarity and deposit onto the surface of aluminum profiles, evenly covering every part of the profiles. This process not only forms a uniform, dense, and highly corrosion-resistant coating, but also ensures that the water content in the coating is very low, preventing sagging. Furthermore, the coating can be reused multiple times, thanks to a unique recyclable system that eliminates environmental pollution issues after water washing. Compared to traditional solvent-based coating methods, electrophoretic coating offers significant advantages.
Loading → degreasing → water washing → alkaline etching → water washing → neutralization → anodizing → water washing → electrolytic coloring → water washing → hot pure water washing → pure water washing → electrophoresis coating → RO circulating water washing tank 1 → RO circulating water washing tank 2 → drip drying → baking → cooling → unloading → inspection → packaging → storage
3. Electrostatic powder coating
Electrostatic powder coating utilizes the adsorption principle of static electricity to make powder particles uniformly adhere to the surface of aluminum profiles. After curing and leveling, a uniform film layer is formed. Its main process flow is shown in the figure below:
Pre-treatment (degreasing → water washing → chemical conversion → water washing → drying) → electrostatic powder coating → leveling and curing
Compared to the previous two coating methods, electrostatic powder coating has unique advantages: it can be carried out at room temperature without preheating the aluminum profiles; powder coatings can be recycled; the coating can be easily controlled from thin to thick, with a film thickness of 50-300μm achieved in one go, greatly reducing the number of coating applications, improving spraying efficiency, and eliminating sagging; it can be automated for production; the film is strong and durable; it can meet customers' different color requirements, making it very popular in modern aluminum profile surface treatment technology.