If you’ve ever worked in industries like automotive, aerospace, heavy machinery, or energy, you’ve almost certainly encountered closed die forgings. As a closed die forging supplier with over 15 years in the business, I’ve sold thousands of tons of these components to clients ranging from small fabrication shops to global aerospace primes. They’re beloved for their unmatched strength, consistent grain structure, and ability to produce complex, high-load parts that outperform cast or machined alternatives. But for all their advantages, closed die forgings aren’t a one-size-fits-all solution. Over the years, I’ve turned away more potential projects than I’ve won simply because the parts they needed would never work well (or at all) with this process. Today, I want to pull back the curtain—no sales pitch, no hyperbole—on the real limitations of closed die forgings, from the technical constraints that our engineers grapple with every day to the business realities that make this process a bad fit for certain jobs. Closed Die Forgings
Let’s start with the basics for anyone who might be new to this side of manufacturing. Closed die forging works by placing a heated billet of metal between two shaped dies (one fixed, one moving) that slam together with thousands of tons of pressure, forcing the metal to fill the die’s cavity. The grain structure of the metal is compressed and oriented along the part’s shape, which is why closed die forgings are so strong. But that same process is also what creates its biggest downsides. The first and most immediate limitation is size. You can’t forge a component larger than what your die and press can handle. I’ve worked with presses as small as 200 tons for tiny, precision parts like valve stems, and as large as 12,000 tons for big parts like axle beams or turbine shafts. But even with the biggest presses, there’s a hard upper limit. Last year, a wind turbine manufacturer came to us needing a 12-foot diameter rotor hub for a new offshore model. We couldn’t do it—not because we didn’t have the space, but because no commercially available closed die press on the planet can apply enough uniform pressure to forge a part that big without the metal cracking or developing weak, inconsistent grain flow. For parts over, say, 10 feet in diameter, most suppliers switch to open die forging, which uses simpler dies to shape large parts by hammering the metal incrementally. Closed die just can’t scale to those sizes without sacrificing quality.
Next up is complexity. Wait, hold on—didn’t I just say closed die is great for complex parts? That’s partially true, but it’s a nuance that a lot of first-time buyers miss. Closed die forgings can’t handle undercuts, internal cavities, or hollow features in the same way casting or machining can. Let me explain: when the two dies come together, the metal has to flow smoothly into every detail of the die. If you have an undercut—a groove that’s narrower at the bottom than the opening, like a dovetail slot cut into the middle of a part—the metal would never fill that space, or it would create a fold (a common defect called a “lap”) that makes the part unusable. Internal cavities? Forget it. To make a hollow part with closed die forging, you’d have to use a loose core inside the billet, but the high pressure of the forging process would crush that core long before the metal takes shape. Last quarter, a medical device company asked us to make a small bone screw with a hollow core to fit a sensor. We ran simulations and test forgings three times, and every single one developed a lap in the hollow section that would have caused the screw to break under load. We had to refer them to a CNC-machined titanium part, which was more expensive, but it was the only way to get the hollow feature without defects. That’s not a failure on our part—that’s a fundamental limit of the process.
Then there’s material waste, or what we call “flash.” Flash is the thin, excess metal that squeezes out between the two dies as they close around the billet. It’s necessary to ensure the entire die cavity fills properly, but it’s also almost always scrapped. For every part you forge, you end up with 10 to 30% more metal than what’s in the finished component, depending on its complexity. That’s not a big deal for common, low-cost materials like carbon steel, but when you’re working with high-performance alloys like titanium, Inconel, or high-strength stainless steel, that waste adds up fast. A titanium aerospace part might have 25% of its original billet end up as flash, and titanium is $20 to $50 a pound—so that’s a $500 to $1,250 loss per part just from scrap. I’ve had clients choose a machined billet part over a closed die forging for exactly this reason, even though the forging is stronger. The flash waste makes the forging too expensive for low-volume runs. Speaking of low volume, that’s another key limitation. Closed die forging requires custom dies, and those dies aren’t cheap. A single set of dies for a small automotive engine part can run $5,000 to $10,000, and dies for larger, more complex parts can hit $50,000 or more. If you’re only making 10 or 20 parts, the die cost alone makes the forging way more expensive than machining from a solid block of metal. I see this all the time with startups or companies testing new products: they come to us needing 50 prototype parts, and when they get the die quote, they realize it’s cheaper to machine the prototypes and switch to forging once they hit 500+ units. The fixed cost of dies is a barrier that closed die forging can’t overcome for small batch runs.
Defect sensitivity is another big one. Closed die forgings are prone to a set of defects that are hard to catch, and even harder to fix, if they’re not designed correctly. We already talked about laps, but there’s also porosity, especially if the billet isn’t heated evenly. If the center of the billet is cooler than the edges, the metal won’t flow smoothly, leaving small gaps that weaken the part. Or we get “grain direction” issues: if the grain flow is misaligned with the load the part will see, it can crack under stress. Unlike casting, where you can often rework small defects with heat treatment, most forging defects are permanent. If a part fails our ultrasonic testing (the standard way we check for internal flaws), we have to scrap it. Last year, we had a job for a tractor manufacturer where 8% of the parts came back with a small lap in the flange. It was a design flaw—their die engineer had specified an undercut that was too tight—but by the time we caught it, we’d already forged 200 parts. We ended up losing money on that order, because we couldn’t pass defective parts to the client. That’s a risk that comes with closed die forging: defects are not just costly, they can cause project delays if they’re not caught early in the design phase.
Wait, let’s not forget material compatibility. Closed die forging works best with metals that are ductile enough to be shaped when hot. Some materials are too brittle at forging temperatures, or require extremely precise heating and pressure that makes the process uneconomical (or impossible). For example, cast irons—especially grey cast iron—are way too brittle to be forged. Try slamming a grey iron billet with 10,000 tons of pressure, and it will shatter like glass. Even some high-strength aluminum alloys, like 7075, can be tricky to forge because they have a narrow forging temperature window. Get the metal too hot, and it’ll crack; too cold, and it won’t fill the die. I’ve had clients bring us 7075 aluminum parts that they tried to forge themselves at an in-house shop, and every one had tiny surface cracks that made the part unsafe for use in aircraft. We now require third-party testing for any 7075 forging we make, which adds to the cost, but it’s non-negotiable. For materials like ceramics, or certain ultra-high-strength steels, closed die forging isn’t a feasible option at all—those are better suited for hot isostatic pressing or powder metallurgy.
Surface finish is the last major limitation I want to cover. Closed die forgings have a rough, textured surface right out of the die, because the metal flows against the die’s walls under high pressure. To get a smooth finish, you have to machine the part, which adds time and cost. For parts that will be painted or coated, that’s not a big deal—you just have to do a light grind. But for parts that need precision fits, like gears or shaft sleeves, you have to machine away a significant amount of material. I once worked on a set of hydraulic pump shafts that needed a mirror-smooth surface for sealing. The closed die forging had a 0.5mm layer of rough surface that had to be machined off, adding 20% to the total cost of the part. If we had machined them from a solid bar, we could have skipped that extra step. For applications where surface finish is a top priority, closed die forging might not be the most efficient process.
Now, I’m not here to say closed die forgings are bad—they’re still the gold standard for high-load, high-volume parts like automotive crankshafts, aircraft landing gear, and wind turbine gear components. But knowing their limitations is critical for anyone choosing a manufacturing process. If you’re making a small batch of prototype parts, need a part with an undercut or internal cavity, or are working with a material that’s hard to forge, closed die might not be the right call.
As someone who runs a closed die forging shop, I’ve learned that the best part of my job isn’t winning every contract—it’s guiding clients to the best solution, even if it means sending them to a competitor or a different manufacturing process. If you’re working on a project and wondering if closed die forgings make sense for your needs, or if you have questions about process limits, our engineering team is here to help. We can run simulations, suggest alternative approaches, or walk you through the pros and cons of different methods to get you the best part at the best price. Just reach out to start a conversation about your next project.
Brass Upset Forgings References
- Forging Industry Association. (2022). Closed Die Forging: Process Capabilities and Limitations. FIA Technical Report.
- Campbell, J. (2012). Manufacturing Processes for Advanced Components. Elsevier.
- Dieter, G. E. (1986). Mechanical Metallurgy (3rd ed.). McGraw-Hill.
- Aerospace Industries Association. (2021). Forging Standards for Aircraft Structural Components. AIA Technical Bulletin.
- Open Die Forging Institute. (2020). Size and Scale Limitations in Closed Die Forging. ODIF White Paper.
Hebei Shata Machinery Co., Ltd.
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