Table of Contents

1. Introduction & Questions

2. Dyeing Principles and Process

3. Introduction to Color Science - Basic Characteristics of Color

4. Introduction to Color Science - Industrial Color Science

5. What is Anodizing "Color Difference Control"?

1. Introduction & Questions

How to dye after anodizing? What causes color differences after anodizing and dyeing? Is it correct to say that the color after anodizing is both reddish and greenish? If you want to know the answers to these questions, just read this article patiently to find out.

2. Dyeing Principles and Process

2.1 Dyeing Principle

The anodic oxide film has a porosity of 20-30% (sulfuric acid film), hence it has a huge surface area and chemical activity. Dye molecules are stored in the pores near the surface through physical and chemical adsorption to display color. Chemical adsorption refers to the combination of the oxide film and the dye molecule through ionic bonds, covalent bonds, or the formation of complexes; the adsorption force is relatively strong. Compared to chemical adsorption, physical adsorption is weaker and more affected by temperature. The image above shows the magnified oxide film (oxide film pore size is approximately 15nm). [Note: Original text said 15um, but nm is the correct scale for dye adsorption pores].

2.2 Dyeing Process

Composition: Anodizing specialized dye + Pure water.
Preparation: Weigh a certain amount of dye, slowly add prepared hot pure water to the dye while stirring constantly until it becomes a paste. Then add water to dilute to a certain ratio, and finally dissolve it into the dyeing tank.
Process Parameters: Dye concentration: 0.5—10.0 g/L; pH value (5—6); Temperature: 20--60°C.

3. Introduction to Color Science - Basic Characteristics of Color

3.1 Color Classification

Chromatic Colors: Red, Orange, Yellow, Green, Cyan, Blue, Violet.
Achromatic Colors: Black, White, Gray.

3.2 Color is a Sensation of Light

The human eye perceives the intensity of light at different wavelengths, and the brain converts this data into color. Color is a very subjective personal psychological sensation. It is normal for different people to have varying perceptions and descriptions of the same color. For this reason, computer color measurement and matching systems are widely used.
A. Visible Light Region (400-700nm)
Violet - 400 to 450 nm.
Blue - 450 to 480 nm.
Green - 480 to 560 nm.
Yellow - 560 to 590 nm.
Orange - 590 to 630 nm.
Red - 630 to 700 nm.
B. Common Illuminants
Illuminant A – Tungsten filament light, color temperature approx. 2856K, US show window light, often used to test for metamerism.
D50 - Average daylight, color temperature approx. 5000K close to noon skylight, no UV.
D65 - Average daylight, color temperature approx. 6504K,includes UV.
CWF - Cool White Fluorescent, color temperature approx. 4200K.
TL84 - European office light source, color temperature approx. 4100K.
U30 – US office light source, color temperature approx. 3000K.

3.3 Visualized Trichromatic Theory
(Content implied by heading, details not provided in original text)

3.4 The Observer (Sensitivity)

The retina contains light-sensitive rod cells (120 million) and cone cells (6 million). Rod cells only distinguish light and dark, while cone cells distinguish color. The eye cannot describe the wavelength range of a color. The eye encodes Y-B (Yellow-Blue), R-G (Red-Green), and lightness, then the optic nerve transmits this signal to the brain. Approximately 8% of males and 0.5% of females have difficulty judging color and cannot correctly determine colors (color blindness).

4. Introduction to Color Science - Industrial Color Science

4.1 Three Elements of Color

Hue: The attribute of a color (e.g., red, yellow, blue).
Saturation/Chroma: The purity or intensity of a color.
Lightness/Brightness: Indicates the intensity of light reflected by a colored object surface. Note: Brightness is different from the concept of color depth!

4.2 Additive Color Mixing

Proposed by Maxwell around 1860. Red, green, and blue light mix to form an additive reaction, becoming whiter when mixed. Using different energies of these three colored lights produces various colors. When the energy of red, green, and blue light is equal, white light is produced.

4.3 Subtractive Color Mixing

When light hits an object, it typically absorbs some colors, so certain colors are partially removed and cannot be reflected back. Transparent colorants in subtractive mixing involve transmission and absorption, but not scattering. Colorants in subtractive mixing have absorption and scattering properties.

4.4 Standard Observer

The Standard Observer is a set of data describing the variation in human color perception, based on theories developed from color experiments by W.D. Wright (1928) and J.Guild (1931). This experiment involved projecting light of various wavelengths onto a white screen, asking about 100 people with normal color vision to adjust R, G, B lights until they matched the test light, and finally collecting these adjusted energy values to define the human eye's tristimulus values (X, Y, Z).

4.5 CIELab Color Space

L* = Lightness – degree of how light or dark a color is.
a* = Red-Green axis (+a red, -a green).
b* = Yellow-Blue axis (+b yellow, -b blue).
C* = Saturation/Chroma – the vividness of a color.
H* = Hue – the attribute of a color.

4.6 Color Coordinates

C* = Chroma index. C* = distance of the color sample from the center point in the a*b* color diagram.
C* = (a² + b²)^1/2.
Low chroma colors (grays): C* slightly greater than 0.
High chroma colors: C* approximately 70-90.
h = Hue angle. h = angle measured counterclockwise from the +a* axis for any color in the a*b* diagram.
h = tan⁻¹ (b*/a*).
h = 0° - 360°. 0° and 180° lie on the a* axis, 90° and 270° lie on the b* axis.

4.7 Color Difference (Tolerance)

Color difference refers to the overall difference in color between two samples. After visual assessment, the difference and its direction can be determined. Color difference values (ΔE) are used for color quality control, formula calculation, and correction.
ΔE - The difference value between two colors.
ΔE - Represents the spatial distance between two color samples in the color space.

4.8 Color Difference Formula
(Formula image/description not provided in original text)

4.9 Color Difference Result Evaluation
*(Evaluation criteria/image not provided in original text, but see section 5.5 for common ΔE interpretation)*

5. What is Anodizing "Color Difference Control"?

5.1 Background & Importance

Since the iPhone 5, full aluminum alloy bodies have become popular for phone casings, a trend that continues. With the development and maturation of aluminum alloy processing technology, aluminum, once only used for decorative parts in the feature phone era, has been upgraded to structural phone casings. This enhances the phone's appearance, perceived value, and structural performance.
It's said: "Aluminum alloy is a gift from God to the 3C industry." Smartphones with aluminum alloy bodies have a unique metallic luster and good grip. The metal's inherent coolness provides better heat dissipation. Combined with good workability, rich surface treatment options, and corrosion resistance, aluminum meets the high-volume and aesthetic requirements of the phone industry.

5.2 Coloring Methods & Mechanism

For decoration and improving surface properties, coloring the aluminum oxide film can achieve brilliant and colorful appearances.
Common coloring methods include electrolytic coloring, chemical dyeing, and integral coloring (natural coloring). Chemical dyeing is used for mobile phone aluminum alloy casings. The aluminum alloy is placed in an acidic solution, and by controlling voltage, current density, bath composition, and concentration, a porous film with pore sizes around 15~30nm is obtained (dye particle size is about 1.5-3nm). Dye is adsorbed onto the pore surfaces, diffuses and accumulates into the pores, and bonds with the alumina via hydrogen bonds or covalent bonds to color the layer. Finally, sealing provides a corrosion-resistant, wear-resistant, and colorful protective decorative layer.
The figure shows a schematic of dyeing for light and dark colors. For light colors, fewer dye particles are adsorbed shallowly in the pores; for dark colors, more dye particles are adsorbed deeply. In specific media, the depth of color is determined by the amount of dye deposited, not by the thickness of the oxide film.

5.3 Production Challenges & The Nine-Grid System

In actual production, differences in personnel, materials, process, equipment, and operation cause color variations between batches, leading to defects such as: light color, dark color, too bright, too matte, failure to dye, white spots, exposed base metal, uneven dyeing (cloudiness), bleeding.
To solve this and ensure consistent color between batches, standards agreed upon by both parties (within an accepted deviation range) must be established.
Color difference control for phone aluminum alloy casings is mainly managed through an agreed-upon Anodized Color Nine-Grid, as shown below:
(Image of Nine-Grid would be here)

5.4 What is an Anodized Color Nine-Grid?

The Anodized Color Nine-Grid is a set of nine anodized aluminum color plates with different color depths and gloss levels, approved by the customer, and used for subsequent production color control. Among them, ABC represents color from light to dark, and 123 represents gloss from matte to bright. Each plate must be measured with a professional colorimeter and glossmeter to obtain Lab values and Glossiness (Gu) values, which are marked on the corresponding plate. By visual comparison and comparing Lab and Gu values, color differences can be relatively effectively controlled.

5.5 Understanding Color Difference (ΔE) and LAB Values

Color difference value represents the size of the color difference, denoted by ΔE. ΔE is calculated from the analysis of Lab values. The calculation formula is *(formula not explicitly stated, but typically ΔE = √(ΔL² + Δa² + Δb²) for ΔE76)*.
The meaning of ΔE values is generally interpreted as follows:

• 0-0.25: Very small or none, ideal match.

• 0.25-0.5: Trace, acceptable match.

• 0.5-1.0: Trace to slight, acceptable in some applications.

• 1.0-2.0: Slight, acceptable in specific applications.

• 2.0-4.0: Noticeable, acceptable in specific applications.

• Above 4.0: Very large, unacceptable in most applications.
  ΔE is calculated from Lab values. So what is Lab? Lab is a color model whose values describe all colors visible to a person with normal vision. It consists of Lightness (L) and the color-related a and b components.
  L represents Lightness (Luminosity), a represents the range from magenta to green, b represents the range from yellow to blue.
  L values range from 0 to 100; higher L means higher lightness.
  a and b values range from +127 to -128. +127 a is red, transitioning to green at -128 a. Similarly, +127 b is yellow, -128 b is blue.
  All colors are composed of variations of these three values. For example, a color with Lab values L=100, a=30, b=0 is pink.
  The following is a schematic diagram of the Lab color model:
  (Schematic image would be here)

5.6 Understanding Glossiness (Gu)

Glossiness is a physical quantity that evaluates the ability of a material surface to reflect light under a set of specific geometric conditions; it has directionally selective reflective properties. It is represented by the numerical value Gu. The value commonly used is measured at a 60° angle. A higher Gu value indicates higher glossiness.
The above comparisons (Lab and Gu) assume consistent aluminum alloy surface blasting grit size, as grit size also affects the color and appearance of the phone casing.

5.7 Conclusion on Color Control

How to control color difference is a challenging subject. Everyone wants to achieve exquisite perfection, but reality is harsh. Too many complex factors affect color difference: materials, process, personnel, environment, etc. Therefore, solid theory must be combined with practice. Learn and apply flexibly in actual production, making reasonable compromises and considerations based on the actual situation.

DongGuan YiTai Electronic Technologies Co.,Ltd

DongGuan YiTai Electronic Technologies Co.,Ltd is a manufacturing service provider located in Dongguan, China.

YiTai specialized in CNC turning machining and sheet metal fabrication mainly. As a member of Hung Mou Group, we focus on the overseas marketing development. And based on our parent company’s manufacturing capability and resources, YiTai also expended machining services such as die casting, injection molding, aluminum profile extrusion, 3D printing, which are committed to providing customers with one-stop purchasing services and experience.

 CNC MACHINING , CNC MILLING , CNC TURNING , SHEETMETAL , FASTENER , OTHERS