Overview:
Having introduced anodising, today we turn to a more advanced surface treatment technique — two-colour anodising. Conco also possesses relevant expertise in this field. In this article, we’ll take an in-depth look at the principles, features, and applications of this technology.
Definition:
Engineers achieve two different coloured patterns or areas on the same metal surface through two separate anodising and dyeing processes in a surface treatment technique called two-colour anodising.
Purpose:
To achieve enhanced visual recognition, decorative appeal, or functional differentiation — such as indicators, button areas, or brand markings.
Applicable Substrates:
Primarily used for aluminium and its alloys.
Characteristics:
- Distinct colour contrast
- Relatively high process complexity
- Corrosion and wear resistance of the surface
- Capable of producing fine, localised patterns or text
Differences from Single-Colour Anodising:
- Additional process steps
- Requires masking techniques or laser-assisted methods
- Higher precision and process control requirements
Brief Introduction to the Principle
Two-colour anodising is a surface treatment process that achieves two distinct colour effects on the same metal surface. The core principle lies in the repeated application of the anodising process combined with controlled, localised dyeing. Engineers primarily use this technique on aluminium and its alloys, taking advantage of the porous structure of the anodised film, which allows dye to penetrate into the pores for colouring.
The basic principle of anodising is to utilise an electrochemical reaction to form a dense and uniform oxide film on the metal surface. This oxide film is inherently colourless and transparent, offering excellent corrosion and wear resistance. Additionally, the micro-porous structure of the surface allows it to absorb dyes, thereby achieving the colouring effect.
Technicians typically carry out two-colour anodising in two stages. First, the workpiece undergoes an initial anodising treatment followed by dyeing, creating the first colour on the surface. Next, technicians protect or pattern local areas using techniques such as physical masking (e.g., tape or photoresist paint) or laser removal. They then perform a second anodising and dyeing process, which allows the unmasked areas to absorb the second dye colour. Finally, they apply a sealing process to close the micro-pores of the oxide film, securing the dye and enhancing durability.
The key to this process lies in the effective control of localised areas. The precision of the masking material or laser directly affects the clarity of the colour boundaries. Furthermore, the control of timing between the two anodising processes, electrolyte parameters, and consistency in film thickness are all crucial factors in ensuring uniform colour and strong adhesion.
In conclusion, two-colour anodising is a composite surface treatment process that integrates chemical, electrochemical, optical, and precision machining technologies.Manufacturers widely use it in industrial and consumer products that require high standards of both appearance and functionality.
Process Flow
The process flow of two-colour anodising is more complex than that of traditional anodising, requiring two rounds of anodising and dyeing, along with high-precision masking or pattern control techniques. The typical process is as follows:
Pre-treatment
Before the anodising process begins, the aluminium components must undergo surface treatment, including degreasing, alkaline cleaning, acid cleaning, and neutralisation. The aim is to remove surface oils and any existing oxide layer to ensure uniformity of the anodised film. Subsequently, technicians can perform mechanical polishing or chemical polishing to enhance the final appearance quality.
First Anodising
Technicians place the cleaned aluminium components in an anodising electrolyte (usually an acidic solution, such as sulfuric acid). Under specific voltage, current density, and time conditions, they form a porous oxide film. They use the film created during this stage for the first dyeing process.
First Dyeing
Technicians immerse the anodised aluminium components in the first dye solution, allowing the dye to penetrate the micro-pores of the oxide film and form an initial colour layer. They must strictly control parameters such as dyeing time, temperature, and dye concentration to ensure colour consistency.
Localised Masking
After the first dyeing, technicians mask the areas that should not be affected by the second dyeing to preserve the first colour. They achieve this using corrosion-resistant tape, photoresist coatings, or laser engraving. The precision with which they perform the masking process directly determines the clarity of the colour boundary.
Second Anodising
Remove part of the masking or perform a second anodising on the unmasked areas. Technicians apply the newly formed oxide film to the surface of the uncoloured areas, allowing its porous structure to regain the ability to absorb dye. It is crucial to ensure that the first dyeing layer is not damaged during the second anodising process.
Second Dyeing
The newly formed oxide film is dyed for the second time, resulting in a colour different from the first. Due to the masking protection, the areas already dyed will not be affected by the second dye, achieving the two-colour effect.
Sealing
After all dyeing is complete, a sealing process is carried out, typically using hot water or a nickel salt solution. This step closes the pores of the oxide film, fixing the dye molecules in place, while also enhancing the film’s corrosion resistance and wear resistance.
This series of process steps requires extremely high operational precision and process control. Factors such as the quality of masking, anodising parameters, dyeing consistency, and sealing integrity are all crucial elements that determine the effectiveness and reliability of the two-colour anodising process.
- We have also provided a comparison table with anodising for easier comparison.
| Project | Standard Anodising | Two-Colour Anodising |
|---|---|---|
| Definition | Formation of a single-colour oxide film on the metal surface | Achieving two different colours on the same surface |
| Number of Colours | Single colour | Two colours |
| Process Complexity | Low, simple process | High, requires multiple anodising, dyeing, and masking steps |
| Processing Steps | One anodising + one dyeing + sealing | Two anodising + two dyeing + masking + sealing |
| Masking or Pattern Control | Not required | Required, precise masking or laser pattern control needed |
| Application Scenarios | General industrial parts, structural components, corrosion protection | High-end consumer electronics, customised parts, brand decorations, etc. |
| Design Capability | Limited, no area differentiation | Enables complex designs such as patterns, sections, and logos |
| Cost and Cycle | Low | High, longer production cycle |
| Technical Requirements | Standard | High, requires coordination of multiple processes |
| Common Issues | Colour differences, uneven film thickness | Masking failure, edge dye bleeding, difficulty in dual-dye control |
Commonly Used Materials
In the two-colour anodising process, selecting the right materials is crucial for ensuring the effectiveness of the process and improving product quality. The main materials include substrates, anodising electrolytes, dyes, masking materials, sealing solutions, and electrode materials, which will be introduced individually below.
Substrate: Aluminium and Aluminium Alloys
Two-colour anodising typically uses aluminium and its alloys as the substrate materials. Aluminium offers excellent conductivity, plasticity, and corrosion resistance, making it the most commonly used metal in anodising. Aluminium alloys provide advantages in terms of strength, corrosion resistance, and workability, and are widely used in industries such as consumer electronics (e.g., mobile phone cases, computer accessories), aerospace, and automotive. The oxide film formed on the surface of aluminium after anodising not only provides an attractive decorative effect but also enhances wear resistance and corrosion protection.
Electrolyte (Acidic Solution)
The electrolyte is a crucial medium for the anodising reaction, with sulfuric acid solution commonly used as the electrolyte. The concentration of the sulfuric acid solution is typically maintained between 10% and 20%, providing sufficient conductivity and stable reaction conditions. The temperature, current, and acidity of the electrolyte directly affect the quality and performance of the oxide film, so precise control is required during the anodising process.
Dyes and Pigments
In the two-colour anodising process, dyes are used to impart colour to the aluminium surface. Dyes can be classified into organic dyes and inorganic pigments. Organic dyes are commonly used for colouring aluminium surfaces due to their vibrant colours and ability to provide rich visual effects. Inorganic pigments, on the other hand, focus more on durability, offering strong weather resistance and corrosion resistance. In two-colour anodising, two different dyes are typically required to achieve distinct coloured areas, thereby fulfilling the desired design objectives.
Masking Materials
A key step in two-colour anodising is the masking of the surface to ensure that different areas present distinct colour effects. Common masking materials include corrosion-resistant tape, photoresist coatings, and laser engraving techniques. Technicians often use corrosion-resistant tape for manual masking, physically blocking dye penetration in specific areas.Technicians can expose and control photoresist coatings to create patterns or regional protection on the surface.while laser engraving precisely etches patterns onto the surface, providing high-precision masking. Regardless of the masking method used, the materials must have sufficient corrosion resistance and accuracy to ensure the final result is precise.
Sealing Solution
After completing the two-colour anodising and dyeing process, technicians apply a sealing treatment to the oxide film, typically using hot water or a nickel salt solution as the sealing solution. Sealing not only effectively fixes the dye, preventing it from washing out, but also enhances the corrosion resistance and wear resistance of the oxide film. Technicians usually carry out the sealing process at high temperatures to ensure that the pores of the oxide film are fully closed, thereby improving its performance.
Electrode Materials
Technicians use electrode materials in the electrolyte bath during the anodising process to conduct current.Common electrode materials include lead, aluminium, or stainless steel. These materials offer good conductivity, ensuring the smooth progression of the anodising process, and are resistant to reacting with the electrolyte. The selection and maintenance of electrode materials directly affect the quality and efficiency of the anodising process.
These materials play a crucial role in the two-colour anodising process. Proper selection and control of these materials not only ensure the stability and consistency of the colour effect but also enhance the durability and functionality of the product.
Summary
In conclusion, technicians use two-colour anodising as a sophisticated surface treatment process to create distinct colour effects on the same metal surface. The process involves multiple stages, including anodising, dyeing, masking, and sealing, with precise control over materials and parameters. The choice of substrates, electrolytes, dyes, and masking materials plays a crucial role in achieving consistent, high-quality results. Industries like consumer electronics, automotive, and aerospace widely apply this process, where both aesthetic and functional properties are required. Mastery of this technique ensures the durability, corrosion resistance, and enhanced visual appeal of the final product.
In the next article, we will continue to explain the key process control points of two-colour anodising and the common issues encountered.