All about surface finishes

Now that you have your casting design, you may be considering surface finish options. If you do not know which questions to ask, choosing a surface finish or plating can be challenging. We have compiled an overview of factors to consider when selecting a surface finish, as well as the characteristics of different surface finishes.

Do I need a surface finish?

While most castings require a surface finish, it is not always necessary.

Advancements in alloys have enabled us to provide raw castings where a finish would normally have been required. For instance, when casting an aluminium 380 alloy, you would traditionally have wanted a chem film or anodising for corrosion resistance. With the advent of K-Alloy, we have been able to provide parts without an additional finish that meet or exceed the customer's original intent, thereby saving them a significant amount of money. There have also been advancements in zinc alloys.

While these advancements have greatly improved our casting processes, they are not necessarily applicable to every application. We suggest speaking with your die casting manufacturer or asking an engineer for more information on raw castings.

Discovery phase

After deciding that your part requires a surface finish, it is important to go through what we call the "discovery phase". Here, we highlight important aspects to consider regarding your part design, the functionality of the part, and cost considerations. These are useful questions to ask when narrowing down your finish options:

• Do I need a decorative finish?
• Do I need enhanced corrosion protection?
• Do I need enhanced wear properties on the substrate?
• Do I need a 100% leak-proof casting?

Die casting process control

After speaking with your design and engineering team to determine the best surface finish for your project, your next consideration is process control. In many cases, the performance of your chosen finish depends on the quality of the substrate or casting. Over the years, we have found that key process parameters need to be monitored during the die casting process. When choosing a die casting company, you need to discuss how it plans to monitor process parameters. When you control and monitor your process appropriately, you build quality into the component. 

It is important to note that the process involves more than just die casting. The process begins with a die casting die design based on part manufacturability and die construction. For this reason, you should involve your die caster as early as possible so that the part is designed with factors such as mould flow analysis and construction in mind.

For more details on surface finish preparation, including surface cleanliness, deburring and vibratory shot blasting, download our full on-demand seminar.

Popular die casting surface finishes

While there are many different surface finishes available, with more being developed every day, we cover the most popular surface finishes and key considerations below:

Anodising:

Non-conductive protective coating that seals the part. Available in a variety of colours, including red, blue and black. This is a very affordable option for improving durability and corrosion resistance.

Chromate:

Non-conductive protective coating that seals the part. Available in a variety of colours, including red, blue and black. This is a very affordable option for improving durability and corrosion resistance.

E-coat:

Rack process that adds cost during racking and unracking. E-coat provides excellent coverage. It is often used on its own, but can also be used as an undercoat for subsequent coatings such as powder coat. Traditionally more functional than decorative, Dynacast has nevertheless successfully provided decorative components using e-coat.

Black oxide:

Primarily used on ferrous metals and copper. It is widely used on firearms because of its uniform black finish. This coating does not cause dimensional change and is resistant to peeling and chipping. It provides some corrosion resistance and acts as an excellent medium for absorbing oils and waxes.

Powder coat:

One of the most popular finishes. It is cured at higher temperatures, resulting in a tougher finish. Generally resistant to scratches and dents. Available in different colours, gloss levels and textures. As it is cured at higher temperatures, a highly controlled die casting process is important.

Chrome plated:

One of the more expensive finishes due to the labour involved in chrome plating. Provides a mirror-like finish. Bright chrome is widely used in the automotive industry, while satin chrome creates a pearlescent appearance. Offers 1,000+ salt spray hours, making it well suited for exterior parts.

Bright nickel:

Applied over copper and under chrome for a decorative finish, this plating is fairly brittle, making it unsuitable for parts that may be bent or crimped after plating.

Chem film: 

Used on aluminium die castings, this finish differs from anodising because it is electrically conductive. As a conversion finish, it does not result in plating build-up. It can be applied by dipping, spraying or brushing, with dipping being the most common method.

Copper-nickel-tin:

Used to provide solderability to the base metal substrate. Matte tin generally has better solderability, but bright tin is specified more often because of its appearance.

Cobalt tin:

Not as expensive as chrome finishes, but it is a rack process. Instead of copper-nickel-chrome, it uses bright nickel with a flash of cobalt-tin. It has a similar appearance to bright chrome, but is less expensive and offers very good corrosion resistance and wear properties.

Electroless nickel: 

Unique because nickel is not applied through electrolysis, but instead by immersion in a bath, resulting in highly uniform plating thickness. The process offers low-phos, mid-phos and high-phos options, with the phosphorus content in the bath determining the nickel's density. Lower phosphate content provides excellent wear properties, although the finish tends to be brittle, while higher phosphate content results in a more ductile finish.

Gold plating:

Does not oxidise and retains its electrical conductivity and solderability at normal temperatures. Used primarily in the electronics industry for connectors, printed circuits, transistors and integrated circuits—anywhere contact resistance, solderability or wire bonding is crucial. Its excellent physical and chemical properties can offset the overall cost of gold. More expensive, but depending on the application, it is money well spent.

Silver: 

Low cost, but susceptible to tarnishing when exposed to the atmosphere. It is somewhat decorative, but has the highest electrical and thermal conductivity of any metal, making it highly ductile, malleable and solderable.

Nickel-free coating:

Hypoallergenic finish. Ideal for consumer electronics and wearables. A hypoallergenic finish that is ideal for consumer electronics and wearables.

Polyurethane paint:

Long-lasting and intended for exterior use, this paint is slightly thicker, requiring consideration of mating surfaces. Wet-process and water-borne paints are generally very durable once cured.

Impregnation: 

Seals porosity to create watertight components, offering a highly viable option for improving yields and reducing scrap. It is also an effective solution after machining, particularly when the "skin" of the casting is removed, to ensure a leak-free component.

Teflon:

Thermally cured solid-film lubricant. Excellent corrosion resistance. Uses a rack process.

For a more in-depth overview of surface finishes, visit the full library of surface finish and plating options—filter by benefit to explore available options for specific project needs. Register below for access to the full on-demand webinar, Surface Finish 101.