Anodizing is one of the most common surface treatment options for CNC aluminum. It occupies a large proportion in the market share of anodized parts. This process is very suitable for aluminum parts made by various manufacturing processes, such as CNC machining, casting and plate forming.
This article will guide you to the design considerations of anodizing.
Introduction to anodic oxidation
Anodic oxidation is the process of converting metal surface into oxide layer through electrolytic process. Through this process, the thickness of this natural oxide layer is increased to improve the durability of parts, paint adhesion, component appearance and corrosion resistance. The following figure shows some parts that have been anodized and then dyed into different colors.
The process uses an acid bath and current to form an anode layer on the base metal. In short, it is to create a controlled and durable oxide layer on the component, instead of relying on the thin oxide layer formed by the material itself. It is similar to bluing, phosphating, passivation and other surface treatments of steels used for corrosion resistance and surface hardening.
Type of anodizing
In this paper, anodic oxidation is divided into three categories and two categories. The three types are as follows:
Type I and IB – chromic acid anodizing
Type IC – non chromic acid anodizing instead of type I and IB
Type II - conventional coating in sulfuric acid bath
Type IIB - non chromate alternatives to type I and IB coatings
Type III - hard anodizing
There are specific reasons for each type of anodization. Some of these reasons are:
1. Type I, IB and II are used for corrosion resistance and a certain degree of wear resistance. For fatigue critical applications, type I and type Ib are used because they are thin coatings. One example is the highly fatigued structural components of aircraft.
2. When I and IB need non chromate alternatives, type IC and IIB shall be used. This is usually the result of environmental regulations or requirements.
3. Type III is mainly used to increase wear resistance and wear resistance. This is a thicker coating, so it will be superior to other types of wear. But the coating may reduce the fatigue life. Type III anodizing is commonly used for firearm parts, gears, valves and many other relatively sliding parts.
Compared with bare aluminum, all types of adhesives contribute to the adhesion of paint and other adhesives. In addition to the anodizing process, some parts may need to be dyed, sealed or treated with other materials, such as dry film lubricants. If a part is to be dyed, it is considered to be class 2, while an unstained part is class .
So far, you may have been prompted to consider some key factors when designing anodized parts. These are easily (and often) overlooked in the design world.
The first factor we need to consider is the dimensional changes associated with anodized components. On the drawings, the engineer or designer may specify to apply the size after processing to compensate for this change, but for rapid prototyping, we rarely have drawings, especially if we use the fast turning service that relies on solid models.
When parts are anodized, the surface will "grow". When I say "growth", I mean that the outer diameter will become larger and the hole will become smaller. This is because the anode layer grows inward and outward from the surface of the part when the aluminum oxide is formed.
It can be estimated that the size increase is about 50% of the total thickness of the anode layer. The following table details the thickness range of different types of coatings according to Mil-A-8625.
These thicknesses may vary depending on the specific alloy and process control used. Shielding may be required if the designer is concerned with controlling the growth of high-precision features. In some cases, such as thicker type III coating, the parts can be lapped or polished to the final size, but this will increase the cost.
Another dimensional consideration is the radius of the edges and inner corners because the anodic coating cannot be formed on the sharp corners. This is particularly true for type III coatings, where the following corner radii for a given type III thickness are recommended in accordance with Mil-A-8625:
For thinner coatings, edge fracture in the range of 0.01-0.02 is sufficient, but it is better to consult the process engineer of speedup to verify this.
2. Wear resistance
Considering the increase in the hardness of the anode layer, we know that the surface hardness increases. The hardness of the actually specified coating is not typical due to the interaction between the softer base metal and the hard anode layer. Mil-A-8625 specifies wear resistance tests to meet these challenges.
As a reference frame, the hardness of 2024 aluminum base material is in the range of 60-70 Rockwell B, wherein the hardness of type III anodizing is 60-70 Rockwell C. The following figure shows one of my CNC clamping clamps, which has been anodized and dyed red.
Although hardwood, engineering plastics and non ferritic metals have been difficult to apply in high vibration environment, the surface has hardly worn.
3. Coloring with dye
As described above, the anodized film can be stained. This may be done for a variety of reasons, such as aesthetics, reduction of stray light in the optical system, and part contrast / identification in the assembly.
When it comes to anodizing, some challenges to discuss with your suppliers are:
Color matching: it is difficult to obtain true color matching with anodized parts, especially if they are not processed in the same batch. If an assembly consists of several anodized parts of the same color, a special control device is required.
Fading: anodized film exposed to UV or high temperature may fade. Organic dyes are more affected than inorganic dyes, but many colors need organic dyes.
Dye responsiveness: not all anodizing types and coatings can use dyes well. Type I anodizing will be difficult to achieve true black because the coating is very thin. In general, although black dyes are used, the parts will still appear gray, so color dyes may not be practical without special treatment. When the coating thickness is high, the type III hard coating may also appear dark gray or black on some alloys, and the color selection will be limited. Some thinner type III coatings may accept multiple colors, but if aesthetics is the main driving force, type II coatings are the best choice for color options.
These are not comprehensive, but they will give you a good start when making the required parts for the first time.
The anode layer is a good insulator, although the base metal has conductivity. Therefore, if the chassis or components need to be grounded, it may be necessary to apply a transparent chemical conversion coating and cover some areas.
A common method to determine whether aluminum parts have been anodized is to use a digital multimeter to test the surface conductivity. If the parts are not anodized, they may be conductive and have very low resistance.
5. Composite coating
The anodized part may also be subjected to secondary processing to coat or treat the anodized surface to improve performance. Some common additives for anodic coatings are:
Paint: the anodic coating can be painted to obtain a specific color that the dye cannot achieve, or further improve the corrosion resistance.
Teflon impregnation: type III hard coating can be impregnated by Teflon to reduce the friction coefficient of bare anodizing. This can be done in the mold cavity as well as in the sliding / contact parts.
There are other processes that can be used to change the performance of the anode coating, but they are less common and may require specialized suppliers.
1. The thick anode coating may reduce the fatigue life of components, especially when they use type III process.
2. Geometric changes of any part to be anodized need to be considered. This is critical for type II and III processes, but may not be required for some type I processes.
3. When processing multiple batches, color matching may be very difficult. When cooperating with different suppliers, color matching may be very difficult.
4. For adequate corrosion protection, it may be necessary to seal the holes of the anode layer.
5. When the thickness approaches and exceeds 0.003 inch, the wear resistance of type III hard coat may decrease.
Different alloys may respond to the anodic oxidation process in different ways. For example, compared with other alloys, alloys with copper content of more than 2% or higher generally have poor wear resistance when subjected to mil specification tests for class III coatings. In other words, the type III hard coating on the 2000 series aluminum and some 7000 series aluminum will not be as wear-resistant as the 6061 hard coating.