1. Introduction to CNC Materials and Related Application Examples

CNC materials are required to possess high strength, good heat resistance, electromagnetic shielding capability, dimensional stability, and an appealing appearance. As technology advances toward lightweight and thin-wall designs, these materials must also fulfill functions such as protection, heat dissipation, and aesthetics.

Common examples include: engineering plastics like PC/ABS/PMMA for mobile phone housings, metallic alloys such as magnesium, aluminum, and stainless steel, as well as composite materials combining alloys with glass fiber.

For instance, the first-generation iPhone released in 2007 already employed aluminum anodizing technology for its casing. Later, Apple’s Mac series, Apple Watch, iPhone 6S, iPhone 7, and others largely adopted 7000-series aluminum alloys—specifically 7075 aluminum alloy. This alloy, primarily containing zinc, is an aluminum‑magnesium‑zinc‑copper alloy and a heat‑treatable alloy. It belongs to the high‑pressure die‑casting aluminum category and offers excellent wear resistance.

2. General Anodizing Process for Aluminum Mobile Phone Housings

In the manufacturing of mobile phone housings, the aluminum anodizing process involves an electrochemical reaction on the surface of the aluminum substrate to form an anodic oxide film. This film enhances the housing’s wear resistance, corrosion resistance, reflectivity, and aesthetic appearance, among other functions.

When an electric current passes through the electrolytic cell, hydrogen gas evolves at the cathode; oxygen (including molecular, atomic, and ionic oxygen) evolved at the anode is typically denoted as O₂ in the reaction. The aluminum acting as the anode is oxidized by this evolved oxygen, forming a non‑hydrated anodic aluminum oxide film.

Functions of Each Process Step

(1) Degreasing – Used to remove grease and oils from the surface of the aluminum alloy housing. If organic contaminants such as oil stains remain on the metal surface, they will adversely affect the levelness, adhesion, and corrosion resistance of the subsequent plating layer, and may even cause discontinuous, loose, or peeling deposits. Therefore, degreasing before electroplating is a critical step.

(2) Alkaline Etching (Stripping) – This step removes the old oxide layer from the aluminum housing surface, exposing the bare base metal. There are two stripping methods: chemical and electrochemical. The most commonly used chemical method is alkaline etching.

(3) Acid Pickling (Desmutting) – This procedure removes the black water film and dirt from the aluminum housing surface, helping to adjust the uniformity, thickness, and brightness of the surface material. (Note: The film removed here mainly originates from NaAlO₂ generated by the reaction Al + NaOH → NaAlO₂ + H₂. NaAlO₂ is a black soluble salt that forms a black film adhering to the aluminum housing surface.)

(4) Polishing – The purpose of this step is to eliminate mechanical defects on the aluminum housing surface, reduce surface roughness, and improve surface gloss. Phosphoric acid is typically used for this treatment.

(5) Activation – Used to enlarge the pores and increase porosity, thereby enhancing the adsorption effect of dyes in the subsequent step.

After anodizing, the oxide film can be activated with an acid solution before coloring. Due to the chemical dissolution of the anodic oxide film by the acid solution, the pore walls dissolve from the outside inward, enlarging the pore diameter. Meanwhile, by controlling the activation time, oxide films with different pore sizes can be obtained.

(6) Coloring (Dyeing) – Based on customer requirements, housings with attractive colors and appearances can be produced. The dye is adsorbed onto the pore surfaces, then diffuses and accumulates within the pores. It forms ionic and hydrogen bonds with the anodic oxide film, imparting color to the film.

(7) Sealing – Improves the quality and color fastness of the housing, prevents deformation of the oxide film appearance, and enables the housing to resist fingerprints, stains, and grease. Sealing methods include hydration sealing, inorganic salt solution sealing, organic coating sealing, and others.