All About Surface Finishes

Now that you have your casting design, you’re starting to think about surface finishes options. If you don’t know what questions to ask, choosing a surface finish or plating could get tricky. We have compiled a run-through of factors to take into consideration when selecting a surface finish as well as different surface finish characteristics.

Do I need a surface finish?

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

There have been advancements in alloys that have allowed us to provide raw castings where a finish would normally have been required. For instance, when casting an aluminum 380 alloy, traditionally you would normally have wanted a chem film or anodizing for corrosion resistance. With the advent of K-Alloy, we’ve been able to provide parts without an additional finish that meet or exceed the requirements of the customer’s original intent, therefore saving them a lot of money. There have been advancements in zinc alloys as well.

While these advancements have greatly improved our casting processes, they’re  not necessarily applicable to each particular application. We suggest talking with your die cast manufacturer or asking an engineer for more information on raw castings.

Discovery phase

After you have decided that your part does need a surface finish, it is important to walk through what we call the “discovery phase.” Here we highlight important aspects that you need to consider in terms of your part design, the functionality of the part, and some price considerations. These are some useful questions to ask yourself 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 you’ve spoken with your design and engineering team to determine what surface finish is best for your project, your next consideration is process control. In many cases, the performance of your chosen finish is dependent on the quality of the substrate or the casting. Over the years, we have found that there are key process parameters that need to be monitored during the die cast process. When choosing a die casting company, you need to have a conversation about how they plan on monitoring process parameters. If you have control of your process and you’re monitoring it appropriately, you’re manufacturing quality into the component. 
It is important to note that the process is more than just die casting. The process begins with a tool design based on the part manufacturability and tool construction. Because of this, you want to get your die caster involved as early as possible so that the part is designed with things like mold flow analysis and construction in mind.

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

Popular die casting surface finishes

While there are many different surface finishes available and more that are invented daily, we cover the most popular surface finishes and some notable takeaways below:

Anodizing:

Non-conductive protective coating that seals the part. Comes in a variety of colors including red, blue and black. This is a very affordable option to create durability and corrosion resistance.

Chromate:

Non-conductive protective coating that seals the part. Comes in a variety of colors including red, blue and black. This is a very affordable option to create durability and corrosion resistance.

E-Coat:

Racked process, adding more cost in the racking and un-racking process. You get really great coverage with e-coat. It is often used on its own but can also be used as an undercoat for subsequent coatings like a powder coat. Traditionally more functional than decorative, though Dynacast has been successful in providing decorative components using e-coat.

Black Oxide:

Primarily used on ferrous metals—also copper. You see this a lot on firearms because of its uniform black finish. There is no dimensional change with this coating and it is resistant to peeling and chipping. It provides some corrosion resistance and acts as an excellent vehicle to absorb oils and waxes.

Powder Coat:

One of the most popular finishes. It’s cured at higher temperatures so it is a tougher finish. Generally, scratch and ding resistant. Available in different colors, gloss levels, and textures. Given that is it cured at higher temperatures, it is important to have a very controlled die casting process.

Chrome Plated:

One of the more expensive finishes due to the amount of labor involved in chrome plating. Provides a mirror-like finish. There is bright chrome that is used quite a bit in the automotive industry. As well as satin chrome that creates a pearlescent look. 1,000+ salt spray hours so it is great 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 aluminum die castings, this finish differs from anodizing by being electrically conductive. As a conversion finish, it does not result in plating buildup. It can be applied through a dipped process, spray, or brush, with dipping being the most common method.

Copper-nickel-tin:

Utilized to provide solderability to the base metal substrate. There is a matte tin that generally has better solderability, but bright tin is specified more because of its appearance.

Cobalt tin:

Not quite as expensive as chrome finishes, but it is a racked process. Instead of getting the copper-nickel-chrome, it is a bright nickel with a flash of cobalt-tin. It actually looks similar to bright chrome, but it’s less expensive and it has very good corrosion resistance and wear properties.

Electroless nickel: 

Unique because the nickel is not applied via electrolysis but is instead submerged in a bath, resulting in very 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, though the finish tends to be brittle, while higher phosphate content results in a more ductile finish.

Gold plating:

Doesn’t oxidize and it retains its connectivity and solderability at normal temperatures. Used primarily in the electronics industry for connectors, printed circuits, transistors, and integrated circuits—anywhere where contact resistance, solderability, or wire bonding is crucial. The excellent physical and chemical properties of it can offset the whole price of the gold. More expensive, but depending on the application it’s money well spent.

Silver: 

Low cost but it’s susceptible to tarnish when exposed to the atmosphere. It is somewhat of a decorative finish but it has the highest electrical and thermal connectivity of any metal, so it’s highly ductile, malleable, and solderable.

Nickel-Free Coating:

Hypoallergenic finish. Great for consumer electronics and wearables. We have a whole 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 its 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. Additionally, it is 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. Utilizes 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.