Key Process Control Points
Two-tone anodising is a surface treatment technique that demands exceptionally high precision and process stability. Its successful implementation relies not only on the fundamental principles of anodising but also on meticulous control at various stages to ensure clear colour separation, strong adhesion, and excellent surface quality. Below are several key control points within this process:
Voltage and Current Control
The voltage and current density applied during the anodising process directly determine the thickness and pore structure of the oxide layer. Excessively high voltage may lead to a rough or burnt surface, while too low a voltage may result in an incomplete or overly thin coating, affecting dye absorption and colour saturation. In two-tone anodising, as the process involves two separate anodising stages, it is essential to ensure consistency and compatibility of the electrical parameters in both stages to avoid damaging the previously formed oxide layer.
Anodising Time Control
Anodising time affects the formation rate and thickness of the oxide layer. If the duration is too short, the film may be insufficiently dense, compromising dye absorption; if too long, the layer may become overly thick, affecting surface smoothness and increasing the difficulty of sealing. In the two-tone process, the anodising time for each stage must be finely adjusted according to the proportion of masked and exposed areas to maintain overall balance.
Masking Precision
Masking is a core step in the two-tone anodising process. The precision of masking determines the clarity of the colour boundaries and the completeness of the pattern. Common masking methods include corrosion-resistant tapes, photosensitive materials, or laser engraving, all of which must offer strong adhesion and excellent corrosion resistance. Any issues during masking, such as edge leakage, lifting, or shifting, can easily lead to dye bleed during the second colouring stage, contaminating the first dyed area and severely affecting the overall appearance quality.
Dyeing Uniformity
The dyeing process must ensure stability in parameters such as dye concentration, temperature, and immersion time. Uneven dye distribution or aged solutions can result in inconsistent colouring and noticeable colour variation, which undermines the contrast effect of two-tone finishes. Furthermore, different dyes vary in their ability to adhere to the anodised layer, so compatibility testing should be conducted in advance.
Coating Compatibility and Protection
During the second anodising and dyeing stages, it is essential to ensure that the oxide layer and dye absorbed during the first process are not damaged. This requires the previously treated areas to possess a certain level of protection after sealing, or to be masked to prevent direct exposure to the electrolyte or dye solution.
Sealing Process Control
Sealing is the final step in the two-tone anodising process and plays a crucial role in determining the durability and colour stability of the surface treatment. Common methods include hot water sealing, steam sealing, or nickel salt sealing. Incomplete sealing may result in dye fading or surface blotching, while over-sealing can reduce the gloss of the oxide layer. After two-tone treatment, the sealing process must ensure effective closure of both coloured areas, avoiding uneven results that could lead to dullness or surface roughness.
In summary, two-tone anodising imposes more stringent requirements on the synchronised control of multiple process parameters. Only through stable and high-precision management across all stages—including electrochemical treatment, masking, dyeing, and sealing—can the final two-tone finish achieve excellent visual quality, durability, and consistency.
Common Issues and Solutions
Due to the complexity of the two-tone anodising process and its multiple treatment stages, it is highly susceptible to various interferences that may result in defects. Below, we outline common issues encountered in actual production along with their corresponding solutions, aimed at optimising the process and enhancing product quality.
Noticeable Colour Variation
- Issue Description:
Inconsistency in colour between the two-tone areas or across different batches, with varying shades that affect both aesthetics and uniformity.
- Possible Causes:
Unstable dye concentration or dye degradation.
Inconsistent control of dyeing temperature and duration.
Variations in oxide layer thickness leading to significant differences in dye absorption capacity.
- Solution:
Maintain strict control over dye concentration and replace dye baths regularly.
Set standardised dyeing time and temperature, using thermostatically controlled dye tanks.
Standardise anodising time and current density to ensure a uniform and consistent oxide layer.
Colour Bleeding / Dye Contamination
- Issue Description:
During the second dyeing stage, the first coloured area becomes contaminated, resulting in colour mixing or blurred boundaries.
- Possible Causes:
Masking materials did not fully cover the surface or lacked proper adhesion.
Edges of the masked areas were not cleanly treated, leaving burrs or gaps.
Inadequate sealing allowed dye to be penetrated by subsequent liquids.
- Solution:
Use professional masking materials with strong adhesion and corrosion resistance.
Apply laser precision engraving or high-accuracy graphic transfer during masking.
Perform thorough sealing immediately after the first dyeing stage to enhance resistance to penetration.
Blurry Colour Boundaries
- Issue Description:
The boundary between the two coloured areas appears blurred, compromising the clarity and definition of the design.
- Possible Causes:
Irregular masking lines and high manual operation errors.
Poor contact between the masking material and substrate leads to edge seepage.
Improper laser masking parameters result in incomplete ablation.
- Solution:
Give priority to laser engraving or photoresist masking to improve boundary sharpness.
Select suitable masking materials and ensure the surface is thoroughly cleaned and pre-treated.
Adjust laser power and speed settings to ensure complete and accurate transfer of the masking pattern.
Oxide Film Peeling or Poor Coating Adhesion
- Issue Description:
The oxide layer is prone to scratching or peeling, compromising durability and decorative appearance.
- Possible Causes:
Inadequate pre-treatment resulting in poor adhesion between the oxide layer and the substrate.
Excessive electrolytic parameters or prolonged anodising time causing high internal stress in the coating.
The second anodising process damaged the oxide layer formed during the first stage.
- Solution:
Enhance pre-treatment steps such as degreasing, de-oiling, and alkaline cleaning.
Control voltage, current, and anodising time within reasonable limits.
Properly protect dyed areas to prevent direct exposure during the second anodising stage.
Poor Sealing Quality
- Issue Description:
Post-dyeing, the colour fades quickly, is prone to contamination, and the surface appears hazy or rough.
- Possible Causes:
Inadequate sealing temperature or time, resulting in incomplete pore closure.
Impure water or sealing solution containing contaminants.
Inconsistent sealing performance across the two-tone areas, leading to localised variations.
- Solution:
Use purified water or standard nickel salt solution, and maintain proper sealing temperature and duration.
Clean the sealing tank regularly to prevent impurity build-up.
Apply zoned or cyclic sealing techniques for different areas to improve uniformity.
Key Points for Quality Inspection
Two-tone anodised products must undergo rigorous quality inspection before leaving the factory to ensure their appearance, functional performance, and consistency meet technical standards. The inspection covers not only the basic characteristics of the anodised coating but also focuses on the specific requirements of the two-tone process. The main inspection points are as follows:
Appearance Quality Inspection
Appearance is the most immediate evaluation criterion for two-tone anodising, especially for products such as consumer electronics and decorative components. Key inspection aspects include:
- Colour Consistency:Check whether the saturation, brightness, and purity of both coloured areas are uniform, and whether there is any colour variation between batches.
- Boundary Clarity:The junction between the two colours should have a well-defined outline, free from bleeding, blurring, colour mixing, or jagged edges.
- Surface Flatness:The surface should be free from anodising burn marks, bubbling, scratches, wrinkles, particles, or impurity deposits.
- Pattern Integrity:If there are logos, graphics, or text, check whether their outlines are complete and the edges are clear and sharp.
Coating Thickness Measurement
The thickness of the anodised coating directly affects its corrosion and wear resistance. It is typically measured using an eddy current thickness gauge or by the electrolyte stripping method:
- Check whether the coating thickness in different areas (both colour zones) meets the design specifications.
- The coating thickness must fall within the acceptable tolerance range (e.g. ±2 μm) to ensure consistent product performance.
Adhesion Testing
Good coating adhesion is essential for ensuring dye durability and wear resistance. Common testing methods include:
- Tape Peel Test: Apply standard 3M tape to the coated surface and peel it off quickly to check for any delamination or colour loss.
- Coin Scratch Test (for non-critical functional parts): Simulates light mechanical contact to assess the firmness of the dyed coating.
Colour Fastness Testing
To verify the durability of the dyed areas, the following dye performance tests should be conducted:
- Light Ageing Test: Expose the sample to ultraviolet light for a specified period to observe any fading or discolouration.
- Sweat / Acid-Alkali Resistance Test: Simulate everyday usage conditions to assess the chemical resistance of the dyed areas.
- Boiling Water Test: Check the effectiveness of the sealing process by determining whether the dye leaches or transfers under high-temperature conditions.
Sealing Quality Inspection
Sealing is a crucial step in fixing the dye and enhancing the performance of the oxide layer. The main inspection methods include:
- Dye Bleed Test: Wipe the surface with alcohol or a neutral solution to check for colour transfer, indicating the effectiveness of sealing.
- Dye Absorption Test: Re-dye the sealed sample; if no colour is absorbed, it suggests sufficient sealing.
- Gloss Measurement: The surface should retain a certain level of gloss after sealing, with no signs of chalking or hazing.
Corrosion and Wear Resistance Testing
Two-tone anodising serves not only decorative purposes but also provides functional protection. Common performance tests include:
- Salt Spray Test (ASTM B117): Assesses the sample’s corrosion resistance under a controlled corrosive environment.
- Wear Resistance Test: Uses rubber wheels, abrasive cloths, or specialised wear testing equipment to evaluate the durability of the anodised layer against abrasion.
Dimensional Tolerance and Masking Precision
As anodising slightly increases the surface dimensions of a part, it is necessary to check whether the final dimensions remain within the specified tolerance range. Additionally, for two-tone areas, the masking precision must be inspected to ensure it meets the tolerance requirements of the design layout.
Summarize
Two-tone anodising requires exceptionally high precision in process control, with a need to ensure stable electrochemical parameters, accurate masking, and uniform dyeing. Common issues such as colour variation, dye bleeding, and film delamination can be improved through the optimisation of process details and careful material selection. A stringent quality inspection system helps guarantee that the final product offers outstanding decorative and functional properties, meeting the demands of high-end applications.
Conco currently has experience in two-tone anodising and operates a small-scale manual workshop of around 100 square metres dedicated to this process. Several standard colour options are available for customers to choose from.