BRIGHTSTAR
PROTOTYPE CNC CO., LTD
+86 137 5010 5351
EN
May. 22, 2026
Aluminum alloy is one of the most commonly used materials in CNC machining. However, aluminum has an obvious disadvantage: it oxidizes easily in air, causing the surface to gradually darken, turn black, and lose its metallic luster. To make matters worse, the naturally formed oxide film is loose and porous, unable to effectively protect the underlying metal.
This is why the vast majority of aluminum parts need to undergo anodizing after CNC machining. Anodizing not only prevents aluminum parts from oxidizing and discoloring but also provides color decoration, increases surface hardness, and improves wear resistance.
However, many people are confused when choosing an anodizing type. What is the difference between sandblasted anodizing and clear anodizing? When is hard anodizing used? Which type should I choose for my parts?
This article will comprehensively analyze the principles, types, processes, and selection methods of aluminum anodizing, helping you make the most suitable surface finishing decision for your CNC machined aluminum parts.
Anodizing is an electrochemical surface finishing process. The aluminum part is immersed in an electrolyte solution, and an electric current is applied to form a dense, uniform aluminum oxide film on the surface of the part.
This oxide film has several important characteristics:
First, the hardness of the oxide film is very high. The hardness of pure alumina is second only to diamond. After anodizing, the surface hardness can reach 300 to 500 HV, much higher than the aluminum alloy itself. This means anodizing can significantly improve the wear resistance of aluminum parts.
Second, the oxide film is porous. The newly formed oxide film surface has countless tiny pores that can absorb dyes. This is why anodized aluminum can display various colors.
Third, the oxide film can be sealed. After sealing, the pores are closed, the oxide film becomes dense, and corrosion resistance is greatly improved.
Fourth, the oxide film is electrically insulating. If your aluminum part needs to conduct electricity, anodizing is not suitable, or specific areas need to be masked.
According to purpose and performance requirements, anodizing is mainly divided into three types: sandblasted anodizing, clear anodizing, and hard anodizing.
Sandblasted anodizing involves first sandblasting the aluminum part and then anodizing it.
Sandblasting uses compressed air to propel fine abrasive particles at high speed onto the aluminum part surface, forming a uniform matte sandblasted texture. Sandblasting can hide the tool marks and minor imperfections left by CNC machining. Then anodizing is performed to form a transparent oxide film. If color is needed, dyeing can be done after anodizing.
The surface after sandblasted anodizing has a uniform matte texture with no reflection and a delicate feel. This surface is very resistant to dirt, and fingerprints do not easily leave marks.
Sandblasted anodizing is suitable for most consumer electronic product housings, medical device housings, industrial equipment panels, etc. Its cost is moderate, and it is one of the most commonly used surface finishing methods for aluminum parts.
Clear anodizing refers to anodizing directly on the machined surface without sandblasting pretreatment. The oxide film is transparent, retaining the original metallic luster and machining marks of the aluminum part.
The surface after clear anodizing retains machining marks such as turning marks and milling marks. For some products, these marks are actually a design element, reflecting the raw beauty of mechanical processing.
Clear anodizing is suitable for parts that need to retain metallic luster and machining marks, such as precision instrument parts, high-end audio panels, photographic equipment, etc. Its cost is similar to that of sandblasted anodizing.
One disadvantage of clear anodizing is that it amplifies imperfections. If the machined surface has scratches, uneven tool marks, etc., these problems will be more obvious after anodizing. Therefore, clear anodizing requires very high surface quality from previous processes.
Hard anodizing is a special anodizing process that uses low-temperature electrolyte and high current density to generate a thicker, harder oxide film.
The thickness of a hard anodized film is typically 25 to 100 microns, while standard anodizing is only 5 to 20 microns. The surface hardness of hard anodizing can reach 400 to 600 HV, with wear resistance far superior to standard anodizing.
The color of hard anodizing is typically dark gray, gray-black, or tan, and it is difficult to dye into bright colors. This is because the pore structure of the hard anodized film is different, making it difficult for dyes to penetrate.
Hard anodizing is suitable for parts that require high wear resistance and high corrosion resistance, such as cylinder walls, pistons, guide rails, food machinery parts, textile machinery parts, etc. The cost of hard anodizing is higher, approximately 2 to 3 times that of standard anodizing.
Sandblasted anodizing provides a uniform matte texture with no directionality, no reflection, and a delicate feel. The color can be silver or various dyed colors.
Clear anodizing retains the original metallic luster and machining marks of the aluminum part, with visible directional tool marks and a bright texture.
Hard anodizing provides a dark gray, gray-black, or tan color, with a relatively rough surface, no luster, and is not suitable for dyeing.
Sandblasted anodizing has an oxide film hardness of approximately 250 to 350 HV, similar to clear anodizing.
Clear anodizing has an oxide film hardness of approximately 250 to 350 HV, suitable for general purposes.
Hard anodizing has an oxide film hardness of up to 400 to 600 HV, with excellent wear resistance.
Sandblasted anodizing typically has an oxide film thickness of 5 to 20 microns.
Clear anodizing typically has an oxide film thickness of 5 to 20 microns.
Hard anodizing typically has an oxide film thickness of 25 to 100 microns, which is 3 to 5 times that of standard anodizing.
Sandblasted anodizing has good corrosion resistance after sealing. However, because the sandblasted surface is rough, if sealing is insufficient, the corrosion resistance will be slightly lower than that of clear anodizing.
Clear anodizing has good corrosion resistance after sealing, and the smooth surface provides better sealing results.
Hard anodizing has excellent corrosion resistance, and the thick oxide film provides better protection.
Sandblasted anodizing has moderate cost, with the sandblasting process adding some cost.
Clear anodizing has the lowest cost and the simplest process.
Hard anodizing has the highest cost, requiring low-temperature electrolyte, higher current density, and longer processing time, approximately 2 to 3 times that of standard anodizing.
Recommend sandblasted anodizing.
Products such as mobile phone housings, laptop housings, and smartwatch housings require an understated, delicate appearance while being resistant to fingerprints and scratches. Sandblasted anodizing is the best choice. Colors can be selected according to brand style, with silver, black, blue, red, etc., being very common.
Recommend clear anodizing.
Optical instruments, measuring tools, high-end audio panels, etc., typically want to retain the original texture of the metal. Clear anodizing retains the CNC machining marks, reflecting the texture of precision manufacturing.
Recommend hard anodizing.
Parts such as cylinders, pistons, guide rails, and bearing housings that need to withstand friction can have their service life significantly extended by hard anodizing. Although appearance is not the focus, the dark gray surface also has an industrial feel.
Recommend sandblasted anodizing.
Surgical instruments and medical device housings require a matte surface to reduce reflection interference for operators. At the same time, the matte surface does not show fingerprints and is easy to keep clean. Sandblasted anodizing with clear sealing is a common choice.
Recommend hard anodizing.
Aluminum parts that come into contact with food in food machinery require extremely high wear resistance and corrosion resistance. Hard anodizing provides a thick protective layer to prevent aluminum ion precipitation.
Recommend sandblasted anodizing or clear anodizing.
In the prototype phase, choose standard anodizing with lower cost. Sandblasting can hide machining imperfections and is suitable for appearance prototypes; clear anodizing shows the true machining quality and is suitable for functional prototypes.
Yes. During anodizing, the oxide film grows in both directions. Approximately half of the film thickness grows outward and half inward. Therefore, the dimensions of the part will increase slightly after anodizing.
For standard anodizing with a film thickness of 5 to 20 microns, the dimensional change is approximately 2.5 to 10 microns. For hard anodizing with a film thickness of 25 to 100 microns, the dimensional change is approximately 12.5 to 50 microns.
If your part has precision fit requirements, stock needs to be reserved during machining.
No. Different series of aluminum alloys have very different anodizing results.
6xxx series aluminum (such as 6061, 6063) has the best anodizing results, with uniform oxide film and high transparency, and is the most commonly used material for anodizing.
5xxx series aluminum (such as 5052, 5083) has good anodizing results, with the oxide film slightly grayish.
7xxx series aluminum (such as 7075) has average anodizing results, with the oxide film easily appearing light yellow or gray, and uniformity is slightly poor.
2xxx series aluminum (such as 2024) has poor anodizing results, with the oxide film easily appearing gray or yellow-brown, and is not recommended for appearance parts.
Die-cast aluminum (such as ADC12) has high silicon content, and after anodizing, the surface appears dark gray and is not suitable for bright colors.
In theory, yes, but not all colors can be perfectly achieved. Anodizing dyeing uses the pores of the oxide film to absorb dyes. Almost any color can be achieved in theory, but the brightness, lightfastness, and cost of different colors vary.
Black, silver, and gold are relatively stable colors. Red, blue, and green are also very common. Light colors such as yellow and pink have poor lightfastness and are prone to fading.
Yes, but it is not recommended. The oxide film after anodizing is very hard, and secondary CNC machining will damage the oxide film. If secondary machining is necessary, the part needs to be anodized again after machining, or only non-critical areas can be machined.
Anodized surfaces are relatively easy to clean. Wipe with mild soapy water and a soft cloth. Avoid using strong acids, strong alkalis, or abrasive cleaners, as they can damage the oxide film.
Understanding the anodizing process flow helps you understand the impact of each step on final quality.
Immerse the aluminum part in a degreasing solution to remove surface oil and cutting fluid residue. If degreasing is incomplete, the oxide film will be uneven.
Immerse the aluminum part in an alkaline solution to slightly corrode the surface, removing the natural oxide film and minor scratches. Alkaline etching gives the surface a uniform matte texture.
Immerse the aluminum part in an acidic solution to neutralize residual alkali and remove surface impurities.
Use the aluminum part as the anode, immerse it in a sulfuric acid electrolyte, and apply direct current. Parameters such as current density, temperature, and time determine the thickness and properties of the oxide film.
For standard anodizing, the electrolyte temperature is controlled at 18 to 22 degrees Celsius. For hard anodizing, the electrolyte temperature is controlled at 0 to 5 degrees Celsius, with higher current density.
If color is needed, immerse the aluminum part in an organic dye solution. Dye molecules adsorb into the micropores of the oxide film. The longer the dyeing time, the deeper the color.
Immerse the aluminum part in hot water or a sealing agent to close the micropores of the oxide film. After sealing, the dye is trapped inside the pores, and lightfastness and corrosion resistance are greatly improved.
Clean and dry the aluminum part.
The part shows different shades of color or different levels of gloss in different areas.
The cause is usually incomplete degreasing, residual oil on the part surface, or poor contact of the hanging fixture during anodizing, resulting in uneven current distribution.
There is a layer of white powdery substance on the surface of the oxide film that falls off when wiped.
The cause is usually excessively high electrolyte temperature or excessive current density. This defect is called "powdering" and in severe cases affects appearance and performance.
The color shade is inconsistent, or some areas cannot be colored.
The cause is usually uneven pore structure of the oxide film, improper dye solution temperature or concentration, or unclean part surface before sealing.
There are fine cracks on the surface of the oxide film.
The cause is usually that the oxide film is too thick or too hard, with insufficient toughness, or the part is subjected to thermal shock or mechanical shock during use.
Q: Which is more expensive, sandblasted anodizing or clear anodizing?
Sandblasted anodizing is slightly more expensive than clear anodizing because of the added sandblasting process. The difference is typically around 10% to 20%.
Q: Can hard anodizing be done in colors other than black?
It is very difficult. The pore structure of the hard anodized film is different from that of standard anodizing, making it difficult for dyes to penetrate. It can typically only appear dark gray, gray-black, or tan. If bright colors are needed, standard anodizing with a hard coating is recommended.
Q: Can my part be partially anodized?
Yes. If certain areas need to remain conductive or require secondary machining, these areas can be masked. The masking material protects the specified areas from being oxidized during the anodizing process.
Q: Will anodized parts fade?
Good anodized dyeing with adequate sealing has good lightfastness. However, with long-term exposure to strong sunlight, some colors (especially light colors) will gradually fade. If extremely high lightfastness is required, it is recommended to choose silver (no dyeing) or use more lightfast dyes.
Q: Do you offer anodizing services?
Yes. Brightstar offers sandblasted anodizing, clear anodizing, and hard anodizing services. We can recommend the most suitable type based on your part material and application requirements.
Anodizing is one of the most important surface finishing methods for aluminum parts after CNC machining. Choosing the right type of anodizing can significantly improve the appearance quality and service life of your products.
Sandblasted anodizing provides a uniform matte texture, has strong imperfection hiding capability, and is suitable for consumer electronic products and medical device housings.
Clear anodizing retains metallic luster and machining marks, has a bright texture, and is suitable for precision instrument parts and high-end decorative parts.
Hard anodizing provides excellent wear resistance and corrosion resistance and is suitable for high-wear parts, but the appearance color is limited and the cost is higher.
Choosing the right type of anodizing depends on your part material, usage environment, appearance requirements, and budget constraints.
Brightstar provides one-stop services from CNC machining to anodizing. Whether you need sandblasted, clear, or hard anodizing, we can complete it for you, ensuring that both the appearance and performance of your aluminum parts meet your requirements.
Ready to Choose Anodizing for Your Aluminum Parts?
Whether you need matte and delicate sandblasted anodizing, clear anodizing that retains metallic luster, or wear-resistant and durable hard anodizing, Brightstar can provide professional services.
Email Amy: amy@brightstarprototype.com
Call or WhatsApp: +86 13750105351
Send us your CAD files and drawings for a free anodizing recommendation and quote.
Brightstar – Precision CNC Machining. Anodizing, the Best Protection for Aluminum Parts.
