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.
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: Cost-effective bulk process conversion coasting. Typical salt spray hours are 150 hours per trivalent chrome but you can increase that considerably with sealers. This coating also comes in a variety of different colors.
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. It is a fairly brittle plating so if you have a part that may be bent or crimped after plating this would not be something you’d want to consider.
Chem Film: Used on aluminum die castings. It differs from anodizing in that is electrically conducted. It is a conversion finish, so there’s really no plating buildup. You can apply it either by a dipped process, spray, or even brush—dip being the most common.
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 in that the nickel is not applied via electrolysis, it’s submerged in a bath. Provides very uniform plating thickness. There’s low-phos, mid-phos, high-phos. Phos pertaining to the amount of phosphorus content in the bath. So the lower phosphate content leads to higher density nickel. The low-phos, you’re going to have excellent wear properties, but the finish is going to be brittle. Then high-phos is going to be more ductile.
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 blog post on nickel-free coatings.
Polyurethane paint: Long lasting and intended for exterior use. It is a little thicker so you’ll want to keep in mind what it mates to. Wet process and water-borne paints, in general, are very durable once they’ve cured.
Impregnation: Seals porosity, creating watertight components. Very viable option to improve your yields and reduce your scrap. Also, a great option to use after machining when you remove the “skin” of the casting to create a leak-free component.
Teflon: Thermally cured solid film lubricant. Excellent corrosion resistance. Utilizes a rack process.