Hey everyone, Sand Castings
If you’ve ever ordered sand castings from a supplier, you probably know that “yield” is one of those terms that gets thrown around a lot—like, “We hit 82% yield on that last run” or “This alloy has a lower yield than expected.” But when I first started in the sand casting game, I had no clue what that actually meant, let alone how to calculate it. I thought it was just some random number we make up to sound like we know what we’re doing (don’t judge—new guy nerves are real). Now, 7 years later as a sand castings supplier, I can tell you it’s way more than that. It’s the backbone of our production, the thing that keeps our quotes fair, and even the reason we can pass lower costs to our clients. Today, I’m breaking down exactly how we calculate sand casting yield—no fancy engineering degree required, just the stuff we actually use on the shop floor.
First off, let’s get the basics right. What is casting yield, anyway? At its core, it’s the percentage of molten metal that actually turns into a good, useable casting (you know, the ones you ship to your line, not the ones we scrap). Wait, but hold on—there’s two types of yield people mix up all the time: numerical yield and process yield. Let’s start with the one everyone means when they talk shop floor stuff, because that’s the one that affects your order.
Numerical yield (sometimes just called “casting yield”) is the big one. That’s (weight of good, finished casting ÷ total weight of molten metal we pour into the mold) × 100. Sounds simple, right? But if it were that easy, we wouldn’t have scrap bins full of metal that went wrong. Let’s use a real example from last month to make this concrete. We had a client order 50 aluminum pump housings, each 2.3 kg. So the total needed castings weight was 50 × 2.3 = 115 kg. But when we poured the mold, we had to account for more than just the part. There’s the gating system—those channels we run the molten metal through to fill the mold, the risers that feed extra metal to prevent shrinkage, the sprues that connect the ladle to the mold. That extra metal doesn’t become part of the part, so it’s scraped off and recycled. For that pump housing run, the gating and risers alone weighed 148 kg. Wait, so total molten metal we poured was 115 + 148 = 263 kg? Then our numerical yield would be (115 ÷ 263) × 100 = ~43.7%. That seems low, but that’s normal for sand casting, especially for complex parts. I’ve seen yields as low as 35% for intricate stainless steel parts and as high as 60% for super simple, blocky castings.
But wait—there’s another layer here, process yield. That’s the percentage of poured metal that actually ends up as good castings, not just the math of the gating. Because even if you pour 263 kg, 115 kg should be good, but sometimes you get defects: a cold shut where the metal didn’t fuse right, a shrinkage pore in the part, a sand inclusion, something that makes it scrap. So process yield is (weight of good castings ÷ total poured metal) × 100. For that pump housings run, we had 5 parts that had tiny surface defects we fixed with grinding, and 2 that were scrapped because of internal porosity. So good castings were 50 – 7 = 43 parts, 43 × 2.3 = 98.9 kg. So process yield was (98.9 ÷ 263) × 100 = ~37.6%. That’s why when a client asks for a quote, we don’t just price for 115 kg of metal—we price for enough to cover scrap, right? Because if we quoted only the part weight, we’d lose money every time we had a defect.
Now, let’s talk about what actually impacts this yield, because it’s not just random. As a supplier, we tweak every variable here to bump yield up (or at least keep it consistent) because that’s good for both of us. First, part complexity. Simple, flat plates? Easy to fill, minimal gating needed, yield around 55-60%. The pump housing? Curved walls, internal passages, that needs a lot more gating to make sure metal gets everywhere, so yield is lower. Alloy type matters too. Aluminum is way more forgiving than, say, carbon steel, which has higher shrinkage rates, so we need bigger risers for steel, dropping yield a few points. Sand type? Green sand is cheaper but can cause more sand inclusions, leading to lower process yield. We use resin sand for most custom parts because it’s more consistent, which keeps defects down.
Then there’s the gating design, which is like 50% of the battle. Back when I first started, our lead machinist, Joe, made fun of me for drawing a gating system with too narrow a sprue for a big casting. “Narrow sprue = turbulence = more defects,” he said, and he was right. Turbulent flow of molten metal sucks in air, creates porosity, ruins parts. So we design gating that’s tapered—wider at the bottom, slower flow—to keep metal smooth. That cuts down on defects, so process yield goes up. We also use “bottom gating” for most parts, where metal enters the mold at the bottom instead of the top, so it fills evenly, no cold shuts. For that pump housing run, switching to bottom gating bumped process yield from 34% the last time we made them to 37.6% this time—saved us like 15 kg of metal per batch, which adds up over 1000 parts.
Scrap rate tracking is another big part of this. We log every single part that gets scrapped, and why: is it sand inclusions? Shrinkage? Wrong alloy? Last quarter, we noticed 12% of our stainless steel valve castings were scrapped for shrinkage, so we tweaked the riser size, and that dropped shrinkage scrap to 4%—yield went up 5 points overnight. That’s why we keep a running spreadsheet (not some fancy ERP that I still half don’t get, just a Google Sheet we update every shift) of every run: part name, alloy, total poured weight, good parts weight, scrap reasons. If a new guy makes a gating mistake, we fix it, log it, so no one repeats it. That consistency is why our clients come back—they know we’re not guessing, we’re optimizing.
Wait, but a common question: do you recycle that gating and riser metal? Hell yeah, that’s how we stay in business. We send all the sprue, runners, risers, and even scrap castings back to the furnace. But when you recycle, metal loses a tiny bit of purity each time, so we have to add fresh metal to each pour. That affects yield too, because some of that recycled metal isn’t usable for the next run’s high-quality parts. So we balance recycling to cut costs without messing up part quality, which in turn keeps process yield high.
Now, how does this translate to what you, the client, care about? Let’s say you need 100 bronze valve bodies for your irrigation system. First, we ask for your part drawing so we can get the exact part weight, figure out the complexity, estimate yield. For bronze valve bodies, we usually hit ~40% numerical yield and ~35% process yield. So to get 100 good parts (each 0.8 kg, so 80 kg total good), we’d need to pour (80 kg ÷ 0.35) = ~228.5 kg of molten metal. We quote you for that amount of metal, plus machining, finishing, etc. If we estimated wrong, say, actual process yield is 33% instead of 35%, we’d have extra metal, but we don’t charge you more. If it’s higher, that’s our win, not yours—so we’ve got skin in the game to calculate that estimate as accurately as possible.
I’ve seen other suppliers lowball quotes by underestimating yield, then hit you with extra charges because they poured more metal than they planned. That’s a garbage move. We never do that. We’ve been burned before—once, we took a quick quote job for a client, guessed 42% process yield, but the part had a hidden internal passage we didn’t catch, so actual yield was 31%. We had to pour 280 kg instead of our estimated 190 kg, ate the extra cost, and the client still got a fair price. That taught us to never cut corners on calculating yield, because it builds trust.
Another thing: don’t confuse “yield” with “scrap rate.” A lot of new clients mix these up. Scrap rate is (total scrapped weight ÷ total poured weight) × 100. If process yield is 35%, scrap rate is 65%. That makes sense, right? Because most of the poured metal is either gating/risers or scrapped parts. Numerical yield is (good parts ÷ (good parts + gating/risers)) × 100, so that’s 35% in our example, meaning 65% is gating/risers. That’s why sand casting isn’t as “efficient” as, say, die casting—but die casting has higher upfront tooling costs, so for low volumes, sand casting is still the way to go, even with lower yield.
Wait, let’s test this with another example to make sure I’m not messing up the math. Last week, we had a run of 200 gray iron engine brackets, each 1.1 kg. Total good parts needed: 200 × 1.1 = 220 kg. Numerical yield for these simple brackets was 52%, so total poured metal: 220 ÷ 0.52 ≈ 423 kg. Process yield was 48%, so good parts were 423 × 0.48 ≈ 203 kg, so we scrapped 7 parts (200 target, 203 good—we always overproduce a little to account for scrap, so you get exactly what you ordered). That’s why when you place an order, we usually send a few extra parts too—just in case, no extra charge.
Common myths I want to bust: first, yield is just about metal. No, it’s 100% about process. We’ve had the exact same part, same alloy, same metal, just different shift workers, and yield varies by 3 points. Because the night shift guys are usually more careful with furnace temperature, mold prep, pouring speed. Temperature is huge—if metal is too cold, it doesn’t fill the mold, causes defects; too hot, it shrinks more, needs bigger risers, lowers yield. We check metal temp with a pyrometer every pour, no exceptions.
Another myth: higher yield means better casting. Not always. If we bump yield by cutting corners on risers, we get more good parts in the short term, but those parts have hidden shrinkage that fails in the client’s machine. That’s a way bigger problem than a 2% lower yield. We prioritize part quality over yield every single time. A casting that fails down the line costs both of us way more than a few extra kilograms of metal.
So if you’re a buyer looking for sand castings, what should you ask your supplier about yield? Don’t be shy. Ask how they calculate it, how they track scrap, what their typical yield is for your part type. If they can’t give you a straight answer, or they act like yield is some secret, run—because they’re either cutting corners or don’t know their own shop. We always walk new clients through our process, show them our tracking sheets, so they see we’re transparent.
At the end of the day, yield is a numbers game, but it’s also about care. We don’t just punch numbers into a calculator—we adjust for every part, every alloy, every shift. We’ve been refining our yield calculations for years, and that’s why we’ve kept the same core clients for 5+ years. If you need custom sand castings, whether it’s a small batch of brackets or a big order of complex components, we can give you a clear, accurate yield calculation for your order, no hidden fees, no guesswork. Hit us up to chat through your project, and we’ll make sure you get castings that are on spec, on time, and priced fairly, with full transparency on how we get the numbers.
Stainless Steel Closed Die Forgings References:
- Campbell, J. (2015). Castings Practice: The Ten Rules of Castings. Butterworth-Heinemann.
- ASM International. (2008). Sand Casting Design and Practices. ASM Handbook, Volume 15: Casting.
- Steel Founders’ Society of America. (2020). Yield Calculation Guidelines for Steel Sand Castings.
- American Foundry Society. (2019). Aluminum Sand Casting Process Optimization: Yield and Defect Reduction.
Hebei Shata Machinery Co., Ltd.
Address: No.11 Cangshun Rd., Shijiazhuang, Hebei, China
E-mail: Info@shatamachinery.com
WebSite: https://www.shatamachinery.com/
