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Aluminum is one of the most popular, and affordable materials used in the CNC machining industry because of its strength and resistance to corrosion and its relatively low cost. Aluminum is a versatile and high-performing material and this makes it popular among architects and designers in CNC manufacturing.
Aluminum does not rust but it corrodes when exposed to oxygen for a long period. To CNC machining businesses, this is a challenge in ensuring the quality of machined aluminum components. To this effect, several layers of protection are placed on aluminum to ensure that it is not compromised in its usage, thereby making it suitable for OEM use and long-term use. In this article, you will be able to learn about the different types of coatings and why they are crucial for CNC-machined aluminum parts. Let’s begin!
Why Bother Coating Aluminum in CNC Machining?
You may ask yourself—since aluminum is an element that forms a passive oxide layer in the air, why would you use a coating for it?
In today’s CNC machining, the aluminum parts standard is higher than before, and the above are some of the reasons why this is the case. As much as the metal does develop a self-healing oxide layer, it does not always afford enough protection for the metal in such conditions. Consider aerospace applications for argument’s sake. When you are machining aluminum parts for rockets, the parts undergo intense temperatures of heat close to the melting point of the oxide layer. Such temperature changes can result in cracks in the oxide film and thus decrease the protective nature of aluminum. Likewise, if you are using aluminum parts for marine purposes, the corrosive seawater has the potential to gradually degrade that thin oxide layer.
That’s why you find areas like automotive and aerospace using special coatings to protect aluminum parts. These coatings help you retain the strength, durability, and corrosion protection of the aluminum no matter the conditions to which it is exposed. For a CNC machining business owner, it is essential to use the right coating to ensure that the products serve the various requirements in the industry and meet the satisfaction of the clients.
Benefits & Drawbacks of Aluminum Coating
There are advantages and disadvantages to using aluminum coatings for your CNC machining processes which should also be considered. Aluminum coatings have wide applicability in various fields such as construction, transportation, and manufacturing industries. They are much sought after due to their ability to resist rusting, being very hard-wearing, and offering protection against wear. But, as it has been said, there are strengths and weaknesses of every material that you have to evaluate.
Advantages:
Strength is one of the most important properties of aluminum coatings and due to this, their usage is quite widespread. Since aluminum is heat resistant it has a high melting point and can be used in the temperature range of 200°C to 650°C depending on the type of coating then you can be assured that it can be used in heat resistant areas. Besides, it has great chemical and rust protection characteristics, which makes the coatings appropriate for usage in aggressive environments.
Moreover, it’s possible to give it any desired form and use it in construction and CNC machining operations where it is necessary to have strength and load-carrying capacity.
Disadvantages:
However, there are some drawbacks to using aluminum coatings as follows: One significant disadvantage is that it has low electrical conductivity especially when compared to copper. This is to mean that it is not suitable for use where electricity is involved, as it does not efficiently transfer current. When using electrical systems in areas where there are aluminum-coated surfaces ensure that there is adequate airflow to avoid overheating.
The other thing that should be looked at is the cost of the entire process. Aluminum is costly especially when it is coated. This can be a problem when you are working on a project that requires you to work on a tight budget. Second of all, aluminum can be somewhat problematic to use as a material because it is rather soft and elastic. If you’re attempting to machine complex or intricate designs the chances are high that you will not get the right degree of precision.
Finally, aluminum has a high coefficient of thermal expansion which implies that if exposed to heat it can expand or contract. This indicates that if not well managed it is likely to crack or otherwise be damaged during the machining process hence requiring a lot of precaution.
Types of Aluminum Coatings Employed In Industrial Use Cases
Anodizing is among the common surface finishes used by manufacturers and designers to protect aluminum alloys from high temperatures, friction, and chemical and moisture attacks. Below are some common types of aluminum coatings suitable for various projects.
Anodizing
Anodizing is an electrochemical process that forms a thicker natural oxide layer, ranging from 5 to 25 microns in aluminum parts, which can improve their durability in terms of wear and corrosion. The thickness of the coating depends on the time the aluminum takes in the anodizing tank, which is an electrolyte. Other benefits; dyne levels of anodized aluminum surfaces are more vibrant, and machined aluminum has superior heat dissipation. Here below are the types of anodizing:
Clear Anodizing
Clear anodizing does not incorporate any dye, and as such, the final appearance of the product is dictated by the base metal. The issue with the clear anodizing type of coating is that it is much more uniform than the darker anodized colors, which could have imperfections or uneven colors when coated.
Dyed Anodizing
Post-treatment anodizing includes adding color to the anodic film after anodizing has been performed. When in a dye solution, aluminum absorbs the dye through its outer oxide layer. This makes it possible to achieve a very wide array of bright and saturated colors that can be fine-tuned depending on the design requirements.
Hard, Clear, and Dyed Anodizing
Hard, clear anodizing is more corrosion-resistant and wear-resistant than other surface treatments. It is the same as anodizing but develops a thicker and more abrasion-resistant oxide layer on the surface of the aluminum. Hard clear anodizing is ideal for parts that are clear and anodized in a hard material.
Moreover, hard anodizing provides a denser and thicker layer than soft anodizing, while dyeing makes the anodizing different colors. This process also strengthens the aluminum part and allows it to be colored to suit the client’s preference.
Blasting + Anodizing
Blasting and anodizing are two significant processes, commonly employed in the preparation of aluminum parts. Initially, blasting is carried out, in which a material is propelled at high velocity, typically ranging from 50 to 100 meters/sec, towards the surface of the aluminum part. In the blasting process, normally, abrasive media, such as glass beads or sand, is applied to abrade off surface roughness or unnecessary material. After blasting, anodizing is carried out so as to acquire a high-standard oxide skin on the aluminum surface.
Powder Coating
What is Powder Coating? Powder coating is an electrostatic process of applying finishes. It provides rich and long-lasting colors with excellent UV stability for different types of metals. The process starts with cleaning and preparing aluminum parts, which are washed thoroughly to remove any contaminants. After this, a dry paint powder is blown by an electrostatic gun on the surface of the glass. This powder tends to stick to the metal surface through static electricity(charge). The coated parts are then baked to melt and cure the powder, and the end product is a uniform and hard layer.
While powder coating is not as hard or as durable as anodized aluminum, it has a better look and superior anti-corrosive properties. Powder coating is not a liquid paint, but a dry material that is not hazardous like liquid paint, as it contains solvents and VOCs.
Chemical Conversion Coating
Chemical conversion coatings, also known as alodine finishes, are cheaper than anodizing. The process refineries the oxide layer on aluminum parts to improve their corrosion characteristics while giving them a nice appearance on the outer surface. Alodine coatings are especially useful for applications that need high electrical conductivity as they guarantee the good conductivity of aluminum parts.
Environmental Implications: Although chemical conversion coatings have several benefits, they are relatively more harmful to the environment than most finishing processes. The chemicals used in alodine solutions are toxic, and hazardous to the environment. Therefore, the usage should be balanced according to the benefit it brings in the specific use that it is intended for.
Chrome Plating
Chrome plating is a common process in the manufacture of consumer products and industrial products. This process involves immersing aluminum parts in an electrolytic bath to deposit a chromium layer on the surface, thus giving it a shiny, lustrous appearance. Chrome plating is widely used for ornamental applications and to improve the mechanical characteristics of components. Here are common types of chrome plating described below;
Hard Chrome Plating
Industrial chrome plating, or Hard chrome plating, is a denser layer of chrome than decorative chrome. This type of plating is suitable for use in industries where materials require high strength, wear, and corrosion resistance. The hard chrome is long-lasting and minimizes the wear and tear between the working surfaces of the machines, thus increasing their working life. Moreover, it reduces the time of equipment operation and the frequency of maintenance.
Decorative Chrome Plating
Bright chrome plating deposits a thin layer of chromium on aluminum parts, mostly for beautification. It does not offer the same corrosion and wear resistance as hard chrome. Although, it provides some degree of it. Decorative platings are normally, easy to polish and are used where appearance matters, such as in kitchen utensils, automobile accessories, or tools.
Polyvinylidene Fluoride (PVDF) Coating
PVDF is a resin-based coating on aluminum parts. These use color pigments and allow for a wide range of matte-finished aluminum parts. PVDF coatings are normally done in a factory setting, and they are extremely hard and prevent rusting on parts.
Advantages of PVDF:
Coating PVDF protects the metal from fading, chemicals, and corrosive agents, making it perfect for exterior applications such as aluminum wall cladding. It is worth noting that PVDF does not need oven curing and does not contain toxic VOCs, making it a safer and environmentally friendly coating solution for metals.
Teflon Coating
Teflon refers to PTFE, a versatile coating. Typically, Teflon coatings are employed in car manufacturing, electronics, pharmaceuticals, aerospace, optics, and cookware production, among other applications. Due to its high molecular weight, this material has excellent water resistance.
Why Choose Teflon Coating?
Design manufacturers and Engineers prefer Teflon because it offers several benefits. It provides good electrical insulation, does not degrade when exposed to ultraviolet light, and does not swell when exposed to water. Additionally, it can resist high temperatures, so it is suitable for most applications of the technology.
Applications in Different Industries
Teflon’s chemical stability enables it to be used to coat aluminum parts in the medical line. Moreover, It’s widely adopted for machined components like coat tubes, pipes, and vessels and is known as the ASME code. Due to its low friction coefficient, it is also suitable for automotive and industrial applications where smooth motion is desirable.
Electrophoretic Coating
Electrophoretic coating, commonly known as E-coat, involves submerging the aluminum parts in a coating bath. An electric current is then passed to produce a steady layer of the coating material on the surface. The electrophoretic coating process has several technical advantages, as discussed.d below.
E-coat is said to be very efficient, especially when it comes to cost. It is most commonly used in automotive applications because of its capacity to offer a durable shield. The process is very flexible, and other coatings can be incorporated to improve other performance characteristics, like surface hardness and appearance.
Coating Options for CNC Machined Aluminum Parts
| Coating Option | Coating Thickness | Surface Finish | Durability | Corrosion Resistance | Heat Resistance | Wear Resistance |
| Anodizing | 5 – 25 µm | Smooth, matte, or glossy | High | High | Moderate to High | High |
| Powder Coating | 30 – 100 µm | Smooth, glossy, or textured | Very High | Very High | High | High |
| Plasma Electrolytic Oxidation (PEO) | 10 – 150 µm | Rough texture; porous or sealed | Extremely High | Excellent | Very High | Extremely High |
| Hard Anodizing | 25 – 150 µm | Matte, rough texture | Very High | Excellent | High | Extremely High |
| Liquid Painting | 15 – 30 µm | Smooth, glossy finish | Moderate | Moderate | Low | Moderate |
| Chrome Plating | 0.5 – 10 µm (decorative) 25 – 500 µm (hard chrome) | Mirror-like finish | High (decorative), Extremely High (hard chrome) | Very High (hard chrome) | High | Extremely High |
| Ceramic Coating | 50 – 500 µm | Matte, rough finish | Extremely High | Extremely High | Very High | Extremely High |
| Nickel Plating | 10 – 30 µm (electroless) | Smooth, reflective | High | High | Moderate | High |
Possible use of Plasma Electrolytic Oxidation (PEO) for Aluminium Alloys
Plasma Electrolytic Oxidation (PEO) is the surface treatment technique that can improve the properties of aluminum alloys through the formation of a dense and hard layer by plasma chemical activity. Though it has many similarities with hard anodizing, PEO offers enhanced advantages, which make it ideal for CNC-machined aluminum parts.
Advantages of PEO for CNC Machining
PEO coatings exhibit high hardness, and therefore they have good wear and corrosion-resistant properties. This makes them suitable to be used in CNC applications where the durability and protection of the product are paramount. The process leaves the surface with a rougher texture, which enhances the surface’s ability to bond with the next layer needed or another layer of protection.
Flexibility and Applications in CNC Machining
PEO can be applied to any kind of industry such as automotive, aerospace, and electronics. You can also control the electrochemical bath to achieve the necessary industry standards to meet the need for CNC machining versatility in the coated surface.
Environmental Benefits
The last but not the least, PEO is environmentally friendly, which is a great advantage in today’s world. In this process, no toxic chemicals are used and the solutions used are slightly toxic hence disposal is not a problem. When it comes to CNC machining, if you are seeking a replacement for toxic chemicals such as hexavalent chromium, PEO is a green solution that will also assist you in compliance with regulations.
Final Thoughts
In CNC machining, liquid painting, powder coating, and anodizing are all very important for the protection of aluminum parts. Anodizing and powder coating have better appearance and corrosion protection than other coatings. If one is to anodize and subsequently powder coat the aluminum surface, the protection offered is even better, though the price may be a little higher. These are the best coating solutions for CNC machined aluminum parts and are recommended for use where you have a large budget to meet high-end and long-lasting durability. For such unique colors or isolated regions, painting and chrome plating are ideal since they will give you the right appearance while offering the necessary performance.
Richconn offers all in-house secondary operations that can be completed for CNC-machined aluminum parts as a one-stop solution. From anodizing to powder coating, we provide the best service of surface treatments to cater to the various needs of your projects. To learn more or to get a quote, you may go to our official website and complete the quote request form.
FAQs-Aluminum Coating
Q1: How much does anodizing increase aluminum’s corrosion resistance?
Anodizing can enhance the corrosion protection of aluminum by as much as 1000 hr in salt spray tests thus making parts last longer in severe conditions.
Q2: Regarding the PEO process, what does the hardness of levealuminuminium become?
PEO coatings can attain a hardness of up to 2000 HV which is an improvement to the untreated aluminum in terms of wear resistance.
Q3: How much thickness does powder coating typically add to aluminum parts?
Powder coatings generally apply a layer of 50 to 150 microns thickness. Normally, this results in better looks and better protection to the surface.
