Do I need a Surface Finish?
There have been advancements in alloys that have allowed us to provide raw castings where a finish would normally have been required. So for instance, an aluminum 380 alloy, perhaps you would normally have wanted a chem film or anodizing for corrosion resistance, but with the advent of K-Alloy, we’ve been able to provide parts without the finish that have met or exceeded the requirements of the customer’s original intent, therefore saving them a lot of money. There have been advancements in zinc alloys as well. There are new alloys that are available that allow you to get the wear properties you need without having to do a surface finish. It’s not necessarily applicable to every particular application, but again those are important considerations. So while most castings do require a surface finish, it is not always necessary. We suggest talking with your die cast manufacturer 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. It is very important to start with the design and function of the part before deciding what surface finish is best. Here 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 you’ll utilize, process control is the next important thing to consider. Really, the performance of your chosen finish is going to be dependent in part on the quality of the substrate or the casting. What we have found over the years is that there are key process parameters that need to be monitored during the die cast process. When you partner with your die caster you really need to have a conversation about how they plan on monitoring those process parameters because if you have control of your process and you’re monitoring it appropriately, you’re actually manufacturing quality into the component.
It is also important to note that the process is more than just die casting. The process begins with a tool design based on the part manufacturability. So, you want to get your die caster involved very early on so that the part is designed properly and mold flow analysis is tested to ensure that the tool is constructed properly. Tool construction comes into play on the surface condition of the substrate casting itself.
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 during the webinar. Listed below are a few notable takeaways:
Anodizing: a 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: very 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: this is a racked process so you’re adding a little bit 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 we 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: usually 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: normally 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: relatively expensive finish. It’s 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: this is 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: relatively 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.
Polyurethane paint: very long lasting. 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. Creates 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.
Surface Finish Q&A
At the end of each of our webinars, attendees have time to ask our presenters questions. I think this is one of the best parts of the seminar and thought we’d share a few of those with you:
What material does E-coat apply to?
E-coat can be applied to almost anything. It can be applied to aluminum, zinc, magnesium, steel, and more. It is a very robust finish.
What does PVD stand for?
Physical Vapor Deposition. PVD is a batch process where there is a controlled explosion and the target metal is vaporized and transferred to the surface of the component. To use this process, the part must first be chrome-plated and any and all liquid within the part must be removed. It is a very expensive process but has a lot of different colors available. Its physical properties are tremendous, it is extremely durable, but everything has to be just right to get the desired effect and avoid damaged parts.
What is the best pre-treatment for prepping aluminum castings?
Shot blasting aluminum castings prior to finish is most common. E-coating would be a great economical operation. Chemical film is another treatment. It acts as an excellent primer for wet coat finishes on aluminum.
What do salt spray hours mean in real-world applications?
There is little evidence indicating a positive correlation of salt spray hours to what’s going to happen to any given part over a period of time in the real world. Typically, with automotive interior parts, you’re looking at around 48 hours. However, for exterior applications in service condition, it can be up to a thousand hours. There is also something called copper-accelerated acetic acid salt spray (CASS), which simulates a lifetime of use in a short period of time. CASS is different from natural salt spray.
How much material is removed during the shot blast process?
During the shot blast process, the surface of a part is peened off, no part material is removed or lost at all. This process is typically used to soften parting lines or eliminate any sharp edges that could impact your line workers in the assembly process. It can also be used to give a uniform finish.
Is black plating different than chrome plating?
No. Chemical companies have come up with a variety of ways and chemicals to make what otherwise would be the typical shiny chrome finish into what is called black chrome or black nickel. You can get that black chrome look with chrome plating. You can also get other colors, such as an indigo chrome. However, not every chrome plater offers black or colored chrome, so as a result, it’s generally more expensive than bright chrome.
How does E-coat differ from powder coating?
E-coating is a dip process; powder coating is a spray process. E-coating is much more fluid and is able to reach all corners and pockets of a part, whereas powder coating can sometimes lack in this regard. E-coating and powder coating are both rack processes. With e-coating, the entire rack is dipped, whereas a rack design and placement is more important on a powder coat to ensure equal coverage on all sides of a part. Oftentimes, especially if it is an exterior application, a part will be e-coated then followed by a powder coat