All about surface finishes

Now that you have your casting design, you're starting to think about surface finish options. If you don't know what questions to ask, choosing a surface finish or plating could be complex. We have compiled a run-through of factors to consider when selecting a surface finish as well as different surface finish characteristics.

Do I need a surface finish?

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

Advancements in alloys have allowed us to provide raw castings where a finish would normally have been required. For example, when casting an aluminum 380 alloy, you would traditionally have needed 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 original intent requirements, therefore saving them a lot of money. There have been advancements in zinc alloys as well.

While these advancements have greatly enhanced our casting processes, they're not necessarily applicable to every application. We suggest consulting with your die-cast manufacturer or asking an engineer for more information on raw castings.

Discovery phase

Once you have decided 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 for your part design, the functionality of the part, and some cost considerations. Here are 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 consulting with the design and engineering team to determine the best surface finish for your project, the next consideration is process control. Often, the performance of the chosen finish depends on the substrate's or casting's quality. Over the years, we have identified key process parameters that need to be monitored during the die-cast process. When choosing a die casting company, you need to discuss how they plan to monitor process parameters. If you control and monitor your process appropriately, you manufacture quality into the component. 

It is essential to note that the process is more than just die casting. The process starts with die cast tool design based on part manufacturability and tool construction. Thus, engaging your die caster early is advisable so the part is designed with considerations like mold flow analysis and construction.

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 numerous surface finishes are available and more are developed daily, we cover the most popular ones below with notable takeaways:

Anodizing:

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

Chromate:

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

E-Coat:

Racked process, adding more cost in racking and un-racking. E-coat provides great coverage. Often used alone but can also be an undercoat for other coatings like powder coat. Traditionally more functional than decorative, though Dynacast has produced decorative components using e-coat.

Black Oxide:

Mainly used on ferrous metals and copper. Commonly seen on firearms for its uniform black finish. Provides no dimensional change and resists peeling and chipping. Offers some corrosion resistance and excellently absorbs oils and waxes.

Powder Coat:

One of the most popular finishes. Cured at high temperatures, resulting in a tougher finish. Generally, scratch and ding resistant. Available in various colors, gloss levels, and textures. Since it is cured under high temperatures, a controlled die casting process is crucial.

Chrome Plated:

One of the higher-cost finishes due to the labor involved in chrome plating. Provides a mirror-like finish. Bright chrome frequently used in the automotive industry, along with satin chrome for a pearlescent appearance. Over 1,000+ salt spray hours, ideal for exterior parts.

Bright Nickel:

Applied over copper and under chrome for a decorative finish but is fairly brittle, making it unsuitable for bending or crimping post-plating.

Chem Film: 

Used on aluminum die castings. Differing from anodizing, this finish is electrically conductive. Being a conversion finish, it doesn't cause plating buildup. It is applied via 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 often has better solderability, while bright tin is preferred for its appearance.

Cobalt tin:

Not as expensive as chrome finishes but a racked process instead of copper-nickel-chrome. It appears similar to bright chrome being bright nickel with a cobalt-tin flash. It has excellent corrosion resistance and wear properties.

Electroless nickel: 

Unique as the nickel is not applied via electrolysis but by submerging in a bath, leading to uniform plating thickness. The process offers low-phos, mid-phos, and high-phos options. Low-phosphate content offers great wear properties, though brittle, while high-phosphate results in a ductile finish.

Gold plating:

Doesn't oxidize, maintaining connectivity and solderability at normal temperatures. Primarily used in electronics for connectors, printed circuits, transistors, and integrated circuits where contact resistance, solderability, or wire bonding are vital. Its excellent physical and chemical properties can justify its cost, depending on the application.

Silver: 

Low cost but prone to tarnish in the atmosphere. Somewhat more decorative but boasts the highest electrical and thermal connectivity of any metal, making it highly ductile, malleable, and solderable.

Nickel-Free Coating:

A hypoallergenic finish suitable for consumer electronics and wearables, providing an ideal solution for product applications.

Polyurethane paint:

Long-lasting and designed for exterior use. The paint is thicker, necessitating consideration of mating surfaces. Generally durable when cured, this wet process and water-borne paint offers excellent resilience.

Impregnation: 

Seals porosity to create watertight components, providing a viable option for improving yields and reducing scrap. Effective post-machining, particularly when removing the casting's "skin" to ensure leak-free components.

Teflon:

A thermally cured solid film lubricant offering excellent corrosion resistance, utilizing a rack process.

For more detailed information on surface finishes, explore the library of surface finish and plating options—filter by benefit to explore options meeting specific project needs. Register below for full access to our on-demand webinar, Surface Finish 101.