An In-depth Guide to metal finishing: Everything you need to know
Once you’ve laser cut, bent, welded, or CNC machined a metal component, there is one vital, final step—finishing. The term metal finishing covers a broad spectrum of operations, each with specific applications depending on the material or end-use.
What are the benefits of metal finishing?
The most common reason for metal finishing is to increase the lifespan of the metal component through wear or corrosion.
Without getting too technical, corrosion happens when the metal reacts with elements in the environment, like oxygen. A coating on the metal creates a barrier to prevent that chemical reaction.
Similarly, metals wear when they are in contact with other materials, so a surface coating barrier will help the metal component last longer. However, there are many other benefits to metal finishing, including:
- Applying a coating to improve or reduce the electrical conductivity of the material.
- Enhancing the aesthetics of the finished part.
- Reducing reflectivity.
- Providing a substrate for bonding other materials to the metal.
- Creating chemical resistance.
What are the different types of metal finishing processes?
Wet painting is the traditional process we all know about, although we use specialised spray equipment to provide an even level of coating in an industrial setting. The metal has to be thoroughly cleaned of surface debris and degreased beforehand. Then the paint is sprayed on in layers, dried then repeated until the desired thickness is achieved. The components can be dried naturally in the air or in an oven to speed up the process.
Benefits of wet paint:
- Low cost.
- Suitability for components that can’t be heated.
- Versatility—possible to create custom colours.
- Wet paint can be applied in thinner layers than powder coating
Disadvantages of wet paint:
- Not as long-wearing as other finishes and will require periodic touching up.
- Usually requires multiple coats to achieve a uniform finish.
- Bright colours fade quicker.
Electrostatic powder coating is the main alternative to wet paint spraying and is widely used across all industries. As with wet painting, the component has to be cleaned and degreased before the powder is applied. Next, an earthing connector is attached to the component then the powder is sprayed through a gun that charges the particles. The electrostatically charged particles of powder then bond to the component. Finally, the piece is placed in an oven, allowing the powder to form a tight bond onto the part.
Advantages of powder coating:
- Durability due to the thicker application of powder.
- Speed, as powder coating is usually a one-coat process.
- Uniform surface finish, as the powder is thick enough to smooth out surface defects in the base material.
- The powder can be reclaimed and reused, making for a cost-effective process.
Disadvantages of powder coating:
- As the final coating is quite thick, it may not be suitable for fine-tolerance components.
- Changeover times between colours are more significant than with wet spray systems.
- Initial cost of the equipment is high.
Electroplating is a chemical process that bonds a metal such as zinc, chromium, or nickel to the surface of a component. The process involves passing an electrical current through a tank of electrolytic solution of the material to be applied. The part requiring plating acts as the electrode in this circuit and attracts the metal atoms to form on its surface.
Advantages of electroplating:
- Applying a metal to another metal provides long-lasting corrosion protection.
- Increased cosmetic appeal.
- Nickel plating reduces friction on the surface of the parts, which reduces wear.
- Electroplating provides a robust substrate for additional finishing operations such as powder coating or wet spraying.
Disadvantages of electroplating:
- Time-consuming process.
- Harmful waste products produced.
- High set-up costs
Anodising may appear to be similar to electroplating but uses a different chemical process. Electroplating coats a metal with a layer of another metal, but anodising uses the base metal to create its own hard-wearing oxide layer. Anodising is commonly used with aluminium and its alloys, although it is used on certain types of ferrous metal as well.
The component is used as an anode and is dipped into a tank of acid which causes oxidation on the surface of the metal. The result is a thick, hard-wearing surface coating. Dyes can also be added to the process for cosmetic appearance.
Advantages of anodising:
- Extremely long-lasting surface protection.
- High corrosion resistance.
- High wear resistance.
- Colours don’t fade.
- Uniform and visually appealing finish.
Disadvantages of anodising:
- Only suitable for certain materials, like aluminium.
- Not appropriate for low volumes due to the set-up costs.
- Anodising reduces the thermal conductivity of aluminium.
When stainless steel has been fabricated, the finished part is usually visible in its raw form, i.e. it’s not painted. Although stainless steel sheet is generally processed with a protective plastic film, it’s impossible to avoid any damage to the surface finish. Laser cutting, punching, bending, and welding all leave their marks which need removing and restoring to the required finish. Grinding is the basic level of stainless steel finishing, where a coarse finish is applied. Then, using gradually finer abrasives, the stainless steel can be polished to a uniform ‘brushed’ finish and right up to a bright mirror finish.
Advantages of polishing stainless steel:
- Surface contaminants are removed.
- A high-quality and uniform finish can be achieved.
- Finished components are visually and cosmetically appealing.
Disadvantages of polishing stainless steel:
- It is a lengthy and costly process.
- Highly skilled labour is needed.
- Polishing equipment is expensive.
Metal finishing is a crucial factor in the longevity, quality, and performance of the finished component. Therefore, serious consideration of all the available options should be given to ensure the final part functions correctly for as long as it is required.
At Pegasus Precision, our sales engineers can advise you on the most appropriate finishing process for your components. Please contact them on 01233 801649 or email firstname.lastname@example.org to discuss your requirements.