Surface Finishing (Coating): The Ultimate Guide

We’re at your disposal to answer any outstanding questions you still might have on the subject of surface finishing or surface coating. Here are all your answers!

What is surface finishing?

The term we’re searching for is “surface coating”, aka surface finishing, which is a process of perfecting the surface of a machined plastic or metal part in order to achieve a desired appearance, property, or performance.

Every machined part has its own surface roughness, which might not be appropriate for its application. To perfect the surface of the part, we use different types of surface coatings.

With surface finishing, you can achieve a smoother surface, better resistance, insulation, etc. It all depends on the material and type of coating or treatment used.

In their most general sense, there are two types of surface finishing processes:

• Processes that add a layer – for instance, spray painting and powder coating;
• Processes that remove a layer – typically abrasive processes like polishing, bead blasting, electropolishing, etc.;

What are the advantages of surface finishing?

These are some of the most significant benefits of surface finishing for your parts:

Decreasing roughness and a better smoothness

The first reason our clients ask for surface coating is to achieve an increased smoothness of their parts. Procedures like polishing, bead blasting, or electropolishing can help with this. Of course, the process you choose also depends on the material of the part and its application.

Removing surface layers and impurities

We also use a surface coating or surface treatments to remove an unnecessary surface layer or impurities. Often, we do this as a preparation for a further process. For instance, when polishing a part for spray paint to stick better. Or as a single process simply for better smoothness.

Increased water and corrosion resistance

With surface treatments like anodizing or electropolishing, your parts will become more water-resistant, making them less prone to corrosion as a result. Resistance to external elements and increased part quality is another reason why surface finishing is so important.

Decreasing friction and increasing durability

With high-performance parts exposed to movement and friction, the roughness of the surface can seriously affect their durability and lifespan. Rougher surfaces are exposed to greater friction, making them more prone to damage and less durable than smoother parts.

Changed or improved aesthetics

In some industries like automotive or consumerism, surface coating as a process can be a way to improve or alter the visual aesthetics of a specific part or machinery. Spray painting of external auto parts is an excellent example of this.

What surface finishing type is usually used in metal parts?

For metal parts, we typically use some of the following surface coatings, depending on the material and intended application:

Anodizing

With anodizing, we use an electrolytic passivation process which gives your parts a layer with an increased thickness, durability, and resistance to corrosion. We use anodizing as a surface coating treatment in the manufacturing of electronics, automotive industry, construction, etc.

Polishing

Another surface finishing we use for metal parts is polishing. By polishing the parts, they get a smooth and shiny surface, which is commonly needed in jewelry, dentistry, and similar applications.

Electroplating

If you want to achieve a golden, silver, or chrome finish of your part that’s esthetically pleasing, electroplating could be the surface finishing you need. Electroplating is an electrochemical treatment that isn’t abrasive but only adds a layer on top.

Electrophoretic Coating

With this type of electro-paint bath, we submerge the parts and give them an extra layer of protection no matter how complex or small their geometry is. An electrophoretic coating is also an excellent surface coating for parts that need a change of color.

Powder Coating

By spraying the parts with a powder coating, we can add a layer of protection to any metal or plastic part you need. Visual appearance can also be changed with powder coating, although it has its limitations when it comes to the thickness of the layer.

Bead Blasting

We use bead blasting as a surface treatment when we don’t need a change of color but simply to remove a layer or clean any impurities from the parts. Bead blasting is an abrasive process that aims to increase the smoothness and clean a surface thoroughly.

Plasma Electrolytic Oxidation (PEO)

If you need parts with increased longevity and resistance to corrosion and heat, PEO could be the surface coating you need. With PEO, we transform the surface of any metal part into ceramic oxide, giving them a longer lifespan as a result.

Black Oxide

For parts made of ferrous metals such as stainless steel, cast iron, or steel, black oxide is an excellent surface coating that offers increased protection when needed. By choosing black oxide for your parts, you boost their resistance and anti-corrosion properties.

Spray Painting

One of the most commonly used surface coatings for both metal and non-metal parts is spray painting. With spray painting as a surface finishing, you can get any color and property desired. That’s why it’s used in many different industries such as marine, decorative, automotive, etc.

Electropolishing

Electropolishing can be a great option for durable and corrosion-resistant metal parts that also need to be esthetically pleasing and smoother. With electropolishing, we can achieve a surface that’s 50% smoother than before!

What surface finishing type is usually used in plastic parts?

Plastic parts are typically manufactured with CNC machining, injection molding, or 3D printing. Depending on their manufacturing technology and material, they all have different recommended surface coatings. These are some of the most commonly used ones:

Sanding (sandpaper)

Sanding or sandblasting is the first surface treatment you can choose for your plastic parts. Typically, sanding will remove all the impurities from the surface and result in a matte surface since it’s still a rougher surface treatment.

Bead Blasting

Same as for metal parts, you can choose bead blasting as a surface treatment for your plastic components as well. With bead blasting, you’ll remove any larger deformities and impurities on the surface of your parts.

Plating

With plating, we can easily add a metal layer to the surface of any plastic parts. We can use either an electrical or a chemical process with a conductor paint that will transform into a metal film. As a result, your parts will enjoy increased resistance and durability.

Painting

Painting is another excellent surface finishing option for your plastic parts. Depending on the need and application, we can spray paint or submerge the parts in dye solutions. Typically, painting plastic parts is for decorative purposes but can also add other properties.

Diamond Buff

We use diamond buffing when you need plastic parts that are esthetically pleasing and have a high degree of smoothness, shine, and gloss. A diamond buff will give you the smoothest plastic surface you can get.

Which surface coating has the best hardness?

Depending on the industry, certain parts might need an increased toughness, hardness, or resistance to heat or corrosion. For instance, anodizing, PEO, and the black oxide surface coatings all have great hardness and can be used when you need strong parts.

Which surface coatings can insulate?

Typically, we use surface coating treatments like PEO when you want insulation against electricity or heat. For heat-only insulation, we can also use different thermal insulation paints.

Which surface coatings are commonly used in mold components?

Here are some common surface coatings for in-mold components or parts:

Diamond-Like Carbon (DLC)

DLC will give your in-mold components a beautiful and naturally-rich black color and a visually attractive appearance. Additionally, it will provide strength and resistance to your parts and protect them from different external exposures.

Titanium Carbonitride (TiCN)

With TiCN as a surface coating, we can give your parts an increased hardness and boost their resistance to wear and tear. Parts treated with TiCN have a high mechanical stress threshold, making them great for milling, punching, and forming tools.

Titanium Nitride (TiN)

The TiN surface coating entails using a physical vapor deposition (PVD) process to get a bright gold ceramic layer on your parts. TiN-treated parts have a high hardness and resistance, although slightly less than parts treated with a TiCN surface finish.

Which surface coatings are commonly used in machinery metal components?

For machinery parts, we use surface finishings like bead blasting, anodizing, PVD, electrophoretic coating, and similar. Often, machinery parts need a higher level of resistance, so the surface coatings are there to strengthen the component instead of giving it a better visual appearance.

Which surface coatings are commonly used in medical device machined parts?

In the medical industry, our clients typically use these surface finishing treatments:

• Anodizing
• Bead blasting
• Powder coating
• Electroplating
• Polishing
• Brushing
• Painting

Which surface coatings are commonly used in automotive metal machined parts?

We use most surface coatings in the automotive industry, including those meant for decorative and esthetic parts like spray painting, powder coating, or polishing.

Depending on the application of the part, the automotive industry can use the following surface coatings or treatments:

• Spray painting
• Powder coating
• Polishing
• As machined
• Bead blasting
• Anodizing
• Black oxide
• Electropolishing

Which surface coatings are commonly used in aerospace metal machined parts?

The aerospace industry uses different surface coatings for various purposes related to safety, durability, and heat resistance. Typically, we use Plasma Electrolytic Oxidation (PEO), anodizing, bead blasting, electropolishing, and more in aerospace metal machined parts.

Does DEK offer surface finishing services for CNC machined parts?