If you’ve ever walked through a machine shop or a custom metal fabrication facility, you’ve probably glanced at the parts bin of aluminum components: engine brackets, smartphone housings, bicycle frames, medical implants, and any number of parts drilled with holes for fasteners, alignment pins, or fluid flow. As an anodizing equipment supplier, I get asked one question more often than any other: “Can your anodizing system handle parts with holes?” It’s not a surprising question—anodizing is a process that relies on uniform electrical current across a metal surface, and holes create gaps where current distribution can go haywire, turning a smooth, protective anodized finish into a patchy, uneven mess. Anodizing Equipment
Let me start with a simple answer, and then we’ll dive into the details: Yes, anodizing equipment can absolutely be used to anodize parts with holes. But it’s not as straightforward as running a solid flat part through your standard line. The holes, their size, placement, density, and even the anodizing process you use can make or break the finish. I’ve seen small job shops turn down work on a bracket with 200 1mm-diameter holes because they thought their old tank wouldn’t handle it, and I’ve seen medical device manufacturers deliver parts with thousandths-of-an-inch tolerance holes that passed strict corrosion and finish tests on our systems. The difference is in how you adjust your process to account for those gaps.
First, let’s break down why holes change anodizing in the first place. Anodizing works by passing direct current (DC) through an electrolytic solution, with the aluminum part as the anode. The current flows from the power source to every point on the part’s surface, growing an oxide layer that’s thick, hard, and corrosion-resistant. Holes are like tiny bottlenecks in this electrical circuit. If a hole is narrow and deep—say, a 0.8mm diameter hole 10mm deep—the electrical resistance at the entrance of that hole spikes. Less current gets through the hole’s inner walls, which means the oxide layer there is thinner, or sometimes not even present at all. Even worse, if there are multiple holes clustered close together, current can bunch up between them, leading to thick, powdery oxide buildup on the raised metal between holes, or even etching if the process is running too hot.
I remember a customer a couple years back who came to us panicking. He was making aluminum cooling plates for electric vehicle batteries, each with 500 1.5mm-diameter holes drilled through to channel coolant. He had run them through his old batch anodizing tank, and half the plates had holes where only the edges of the holes were anodized, leaving the inner metal exposed. He thought he had to scrap $15,000 worth of parts and invest in a whole new line for “hole parts.” When we visited his shop to audit his process, we found the problem wasn’t his equipment—it was how he was using it.
His old setup was a basic tank with a single anode rod and a very high power setting for flat parts. When we plugged in our adjustable DC power supply, which lets us tailor current density per part, and swapped out his generic aluminum cathodes for curved, perforated cathodes that matched the shape of the cooling plates, the results changed immediately. We also adjusted the anodizing temperature slightly—his was running at 60°F, and we bumped it to 65°F, lowering the solution’s resistance just enough that current could seep into those small holes. Within two hours, we had test parts that were fully anodized inside every hole, with a consistent 12-micron oxide layer across the entire surface. The customer later told us those plates are still in use in electric buses, five years later, with zero corrosion failures.
That story drives home the first key point: Your anodizing equipment is only as good as how you calibrate it for parts with holes. There’s no magic part that works for every hole scenario, but there are three core adjustments we recommend as a supplier, regardless of whether you’re using a batch tank, a rack line, or a continuous anodizing system.
First, adjust current density and power delivery to match hole geometry. Current density is measured in amps per square foot (ASF), and for solid aluminum parts, you typically run 10–20 ASF for standard sulfuric acid anodizing. For parts with holes, you need to calculate the “active surface area” that includes the inner walls of the holes, not just the outer surfaces. A part with holes has much more total surface area than a solid part of the same outer dimensions, so cranking up the power to match will only cause problems. Too much current at a hole’s entrance leads to burning (that powdery, rough finish I mentioned) and uneven oxide layers. Too little, and the inner walls don’t get any anodizing. Our anodizing systems come with built-in software that calculates surface area automatically, even for complex geometries with hundreds of holes, so you can set the exact current needed instead of guessing. For the cooling plates, we lowered the current density from 15 ASF for flat parts to 11 ASF, and that was enough to get uniform coverage. The old power supply his shop used didn’t have that adjustability— it only worked for a narrow range of part sizes, which is why his holes were failing.
Second, racking and fixturing make a bigger difference for holed parts than for solid ones. Racking is how you hold the part in the anodizing tank and connect it to the power source. For parts with holes, you can’t just use a standard alligator clip that clamps to the outer edge— that will block the surface where the clip touches, and also create a point where current is too concentrated. We’ve seen customers drill dedicated mounting points on parts with holes (small, non-functional holes for fixturing) or use spring-loaded racks that press gently against the part’s surface, so contact is uniform across a larger area. Even better, for parts with many small holes, we recommend using titanium racks instead of aluminum ones. Titanium is inert in most anodizing electrolytes, so it won’t contribute to current flow in unwanted ways. Aluminum racks can corrode over time in the acid solution, leading to inconsistent current and flaking metal that can contaminate your parts and solution. A good rack can add 10% to the cost of a setup, but it’s a small investment compared to scrapping parts or having to re-anodize batches. I also always advise customers to avoid hanging parts too close together in the tank. If two parts with holes are only an inch apart, current will bunch up between their holes, leading to uneven coverage. Our tank systems have adjustable rack spacing, and our team works with customers to map out the optimal arrangement based on their part’s hole count and size.
Third, process parameters beyond power and racking matter for holed parts. The anodizing solution type is a big one. For parts with small, deep holes, we often recommend hard coat anodizing or phosphoric acid anodizing instead of standard sulfuric acid. Phosphoric acid has a lower viscosity, so it flows into narrow holes more easily, ensuring the inner walls are fully exposed to the electrolyte. Hard coat anodizing, which runs at lower temperatures and higher current, creates a thicker, more durable oxide layer that’s less prone to gaps, though it requires more precise temperature control. Temperature is another key factor: the anodizing reaction generates heat, and if the solution is too warm, it can break down the oxide layer being formed, especially in narrow holes where heat can’t dissipate. That’s why our systems come with built-in temperature controllers that adjust the cooling flow in real time, not just a static thermostat. For the cooling plates, keeping the temperature within ±1°F of 65°F was critical— any fluctuation would have led to thin spots in the 1mm holes. We also recommend agitating the solution slightly, either with air stones or mechanical stirrers, to move fresh electrolyte into the holes and flush out any byproducts of the anodizing reaction (like hydrogen bubbles, which can get trapped in holes and leave pits).
Now, let’s address the edge cases where anodizing parts with holes does get tricky. If your holes are smaller than 0.5mm, or deeper than 20 times their diameter (a 0.5mm hole that’s 10mm deep, for example), it’s much harder to get uniform anodizing on the inner walls. I’ve seen customers get around this by pre-treating the parts with a light phosphoric acid etch before anodizing, which etches the inner walls of the holes slightly and improves electrolyte penetration, or by using a two-step anodizing process where you first run a low current to coat the holes, then ramp up the current for the outer surfaces. That process works, but it does add an extra 30 minutes to your cycle time. Another edge case: if parts have a mix of large and small holes, or holes clustered around solid surfaces. In that case, we recommend using a pulsed DC power supply instead of straight DC. Pulsed current alternates between on and off cycles, which gives the electrolyte time to flow into narrow gaps and prevents current from bunching up between holes. It’s a feature that’s become standard on our mid-range anodizing systems because it’s so versatile for complex parts.
I always hear the same follow-up question when I explain all this: “Is it worth the extra effort?” For most manufacturers, the answer is yes. Anodized parts with holes are used everywhere, from aerospace components that need to hold fasteners, to medical devices that need smooth, corrosion-free surfaces around screw holes, to consumer electronics where holes for speakers or ports need a consistent finish to avoid looking cheap. A friend of mine runs a medical device shop that makes surgical tools with tiny holes for irrigation—his parts go through FDA inspection, which requires 100% anodizing coverage even inside 0.3mm holes. He switched to our anodizing system three years ago, and since then, his inspection pass rate has gone from 82% to 98%, cutting his rework costs by over $200,000 a year. The extra time spent calibrating the process was nothing compared to the savings.
Of course, there are times when anodizing holes isn’t the best option. If your holes are functional and will be filled with a sealant or fastener after anodizing, you might not need to anodize the inner walls at all. You can plug the holes before anodizing with rubber stoppers or tape, which prevents electrolyte from getting in and eliminates the risk of uneven coverage. That’s a common trick for parts like engine manifolds, where the inside of the holes will be sealed with epoxy anyway. But if the holes are exposed and need to be corrosion-resistant or have a decorative finish, anodizing them is non-negotiable.
As an anodizing equipment supplier, our job isn’t just to sell tanks and power supplies—it’s to help our customers solve problems, and anodizing parts with holes is one of the most common problems we help fix. I’ve seen too many small fabricators write off jobs with holed parts because they thought their equipment couldn’t handle it, or waste money on outdated systems that don’t have the adjustability needed for complex geometries. The truth is, most anodizing equipment can be adapted for parts with holes—you just need to adjust the power, the racking, the solution, and the temperature to match your specific part’s requirements.
If you’re currently struggling with uneven anodizing on parts with holes, or you’re looking to upgrade your equipment to take on more complex jobs, we can help. Our team works with every customer to audit their existing process, recommend adjustments to their current setup if needed, or provide custom anodizing systems calibrated for their specific hole sizes and part geometries. We’ve helped companies go from a 60% pass rate on holed parts to 99% in a matter of weeks, and we can do the same for you. Don’t let holes limit the parts you can produce—your anodizing equipment is more versatile than you think.
To discuss your specific anodizing needs for parts with holes, contact our team to learn more about process calibration and equipment solutions tailored to your application.
Electroplating Equipment References
- Sheasby, P.G., Pinner, R. (2001). The Surface Treatment and Finishing of Aluminum and Its Alloys, 6th Edition. Finishing Publications.
- Brown, C.D. (2018). Anodizing of Aluminum: Principles and Practice. ASM International.
- American Society for Testing and Materials. (2020). Standard Test Method for Thickness of Anodic Coatings on Aluminum (ASTM B244). ASTM International.
- Jones, D.A. (2014). Principles and Prevention of Corrosion, 2nd Edition. Pearson Education.
JYC Environmental Machinery (Haian) Co., Ltd.
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