Commonly used metal materials include stainless steel, aluminum alloy, pure aluminum profiles, zinc alloy, brass, etc. This article mainly focuses on aluminum and its alloys, introducing several common surface treatment processes used on them.
Aluminum and its alloys have the characteristics of easy processing, rich surface treatment methods, and good visual effects, and are widely used in many products. I once saw a video introducing how the shell of an Apple laptop is processed from a single piece of aluminum alloy using CNC machining equipment and subjected to multiple surface treatments, involving multiple main processes such as CNC milling, polishing, high gloss milling, and wire drawing.
For aluminum and aluminum alloys, surface treatment mainly includes high gloss milling/high gloss cutting, sandblasting, polishing, wire drawing, anodizing, spraying, etc.
1. High gloss milling/high gloss cutting
Using high-precision CNC machining equipment to cut some details of aluminum or aluminum alloy parts, resulting in local bright areas on the surface of the product. For example, some mobile phone metal shells are milled with a circle of bright chamfers, while some small pieces of metal appearance are milled with one or several bright shallow straight grooves to increase the brightness of the product surface. Some high-end TV metal frames also apply this high gloss milling process. During high gloss milling/high gloss cutting, the speed of the milling cutter is quite particular. The faster the speed, the brighter the cutting highlights. Conversely, it does not produce any highlight effect and is prone to tool lines.
2. Sandblasting
Sandblasting process refers to the use of high-speed sand flow to treat metal surfaces, including cleaning and roughening of metal surfaces, in order to achieve a certain degree of cleanliness and roughness on the surface of aluminum and aluminum alloy parts. It can not only improve the mechanical properties of the part surface, improve the fatigue resistance of the part, but also increase the adhesion between the original surface of the part and the coating, which is more beneficial for the durability of the coating film and the leveling and decoration of the coating. It has been found that on some products, the effect of forming a matte pearl silver surface through sandblasting is still very attractive, as sandblasting gives the metal material surface a more subtle matte texture.
3. Polishing
Polishing refers to the process of using mechanical, chemical, or electrochemical effects to reduce the surface roughness of a workpiece to obtain a bright and flat surface. The polishing on the product shell is mainly not used to improve the dimensional accuracy or geometric shape accuracy of the workpiece (as the purpose is not to consider assembly), but to obtain a smooth surface or mirror gloss appearance effect.
Polishing processes mainly include mechanical polishing, chemical polishing, electrolytic polishing, ultrasonic polishing, fluid polishing, and magnetic abrasive polishing. In many consumer products, aluminum and aluminum alloy parts are often polished using mechanical polishing and electrolytic polishing, or a combination of these two methods. After mechanical polishing and electrolytic polishing, the surface of aluminum and aluminum alloy parts can achieve an appearance similar to the mirror surface of stainless steel. Metal mirrors usually give people a feeling of simplicity, fashion, and high-end, giving them a feeling of love for products at all costs. The metal mirror needs to solve the problem of fingerprint printing.
4. Anodizing
In most cases, aluminum parts (including aluminum and aluminum alloys) are not suitable for electroplating and are not electroplated. Instead, chemical methods such as anodizing are used for surface treatment. Electroplating on aluminum parts is much more difficult and complex than electroplating on metal materials such as steel, zinc alloy, and copper. The main reason is that aluminum parts are prone to forming an oxide film on oxygen, which seriously affects the adhesion of the electroplating coating; When immersed in the electrolyte, the negative electrode potential of aluminum is prone to displacement with metal ions with a relatively positive potential, thereby affecting the adhesion of the electroplating layer; The expansion coefficient of aluminum parts is larger than that of other metals, which will affect the bonding force between the coating and aluminum parts; Aluminum is an amphoteric metal that is not very stable in acidic and alkaline electroplating solutions.
Anodic oxidation refers to the electrochemical oxidation of metals or alloys. Taking aluminum and aluminum alloy products (referred to as aluminum products) as examples, aluminum products are placed in the corresponding electrolyte as anodes. Under specific conditions and external current, a layer of aluminum oxide film is formed on the surface of the aluminum products. This layer of aluminum oxide film improves the surface hardness and wear resistance of the aluminum products, enhances the corrosion resistance of the aluminum products, and also utilizes the adsorption capacity of a large number of micropores in the thin layer of the oxide film, Coloring the surface of aluminum products into various beautiful and vibrant colors, enriching the color expression of aluminum products and increasing their aesthetics. Anodizing is widely used in aluminum alloys.
Anodizing can also endow a specific area with different colors on a product, such as dual color anodizing. This way, the metal appearance of the product can reflect the comparison of dual colors and better reflect the unique nobility of the product. However, the process of dual color anodizing is complex and costly.
5. Wire drawing
Surface wire drawing process is a relatively mature process that forms regular lines on the surface of metal workpieces through grinding to achieve decorative effects. Metal surface wire drawing can effectively reflect the texture of metal materials and is widely used in many products. It is a common metal surface treatment method and is loved by many users. For example, metal wire drawing effects are commonly used on product parts such as the end face of desk lamp metal joint pins, door handles, lock trim panels, small home appliance control panels, stainless steel stoves, laptop panels, projector covers, etc. Wire drawing can form a satin like effect, as well as other effects that are ready for wire drawing.
According to different surface effects, metal wire drawing can be divided into straight wire, disordered wire, spiral wire drawing, etc. The line effect of wire drawing can vary greatly. Fine wire marks can be clearly displayed on the surface of metal parts using wire drawing technology. Visually, it can be described as a fine hair luster shining in a matte metal, giving the product a sense of technology and fashion.
6. Spraying
The purpose of surface spraying on aluminum parts is not only to protect the surface, but also to enhance the appearance effect of aluminum parts. The spraying treatment of aluminum parts mainly includes electrophoretic coating, electrostatic powder spraying, electrostatic liquid phase spraying, and fluorocarbon spraying.
For electrophoretic spraying, it can be combined with anodizing. The purpose of anodizing pretreatment is to remove grease, impurities, and natural oxide film from the surface of aluminum parts, and to form a uniform and high-quality anodizing film on a clean surface. After anodizing and electrolytic coloring of aluminum parts, electrophoretic coating is applied. The coating formed by electrophoretic coating is uniform and thin, with high transparency, corrosion resistance, high weather resistance, and affinity for metal texture.
Electrostatic powder spraying is the process of spraying powder coating onto the surface of aluminum parts through a powder spraying gun, forming a layer of organic polymer film, which mainly plays a protective and decorative role. The working principle of electrostatic powder spraying is briefly described as applying a negative high voltage to the powder spraying gun, grounding the coated workpiece, forming a high-voltage electrostatic field between the gun and the workpiece, which is beneficial for powder spraying.
Electrostatic liquid phase spraying refers to the surface treatment process of applying liquid coatings to the surface of aluminum alloy profiles through an electrostatic spraying gun to form a protective and decorative organic polymer film.
Fluorocarbon spraying, also known as “curium oil”, is a high-end spraying process with high prices. The parts using this spraying process have excellent resistance to fading, frost, acid rain and other corrosion, strong crack resistance and UV resistance, and can withstand harsh weather environments. High quality fluorocarbon coatings have metallic luster, bright colors, and a clear three-dimensional sense. The fluorocarbon spraying process is relatively complex and generally requires multiple spraying treatments. Before spraying, a series of pre-treatment processes need to be carried out, which is relatively complex and requires high requirements.
Post time: May-07-2024