If you’ve ever sourced die casting parts, you know how critical it is to avoid cracking—whether it’s a tiny cosmetic flaw on a consumer electronics housing or a structural crack on an automotive transmission component, a single split can derail a production run, delay a delivery, and cost thousands of dollars in rework, scrap, or lost contracts. As a die casting supplier with over 18 years on the shop floor, I’ve seen my share of cracking issues: from cold cracks that show up right after ejection to hot cracks that emerge weeks or even months later due to residual stress. Today, I’m breaking down exactly how we prevent cracking in our parts—no vague industry jargon, just the actionable, shop-tested steps we use every single day. Die Casting Parts
First, let’s get clear on the two main types of die casting cracking, because fixing the wrong issue will never work. Cold cracks are sharp, linear breaks that form when the metal solidifies before the part is fully ejected, usually from high internal stress, uneven cooling, or poor material flow. You can spot them right after trimming or shotblasting, and they often run across thin walls or sharp corners where the metal had to stretch to fill the mold. Hot cracks (also called hot tears) are a bit sneakier—they form while the metal is still partially molten, when the mold is too cool or the part is pulled from the mold too early before it’s rigid enough to hold its shape. These cracks are usually jagged, not smooth, and often show up on the thickest sections or where metal has to flow around intricate cores or inserts. Over my career, I’ve found that 70% of cracking issues boil down to three areas: material selection, mold design, and process control.
Let’s start with material selection, because skimping here is the #1 mistake new customers make when they want to cut costs. I’ve had more than one client come to us asking, “Can you use a lower-grade aluminum alloy for this part? It’s cheaper.” The answer is almost always no, unless that part is going into a non-structural application like a decorative trim. All die casting alloys have different elongation rates (how much they can stretch before breaking) and tensile strength, and mismatching the alloy to the part’s design is a guaranteed way to get cracks. For example, A380 aluminum is the workhorse of die casting—great for automotive housings, power tool parts, and consumer goods—because it has a good balance of strength, fluidity, and elongation (around 3-5% elongation). But if you’re making a part with very thin walls (less than 1.5mm) or intricate undercuts, A380 might be too brittle; we’ll switch to A413, which has higher elongation (up to 8%) and better flow to fill tight spaces without stretching to the breaking point. For structural parts like engine mounts or load-bearing brackets, we use A356 aluminum, which has even higher elongation (10-12%) and better fatigue resistance. For zinc alloys, Zamak 3 is the standard for most parts, but Zamak 5 is better for higher-stress applications because it has a tighter chemical composition that reduces hot cracking risk. We never alter the alloy chemistry mid-run—we have a full-time materials technician who tests every batch of incoming metal with a spectrometer, checking for impurity levels (like iron, silicon, or lead) that can cause brittleness. Too much iron in aluminum, for example, forms hard intermetallic compounds that act like tiny cracks waiting to happen. We keep iron levels under 1.2% for all our aluminum alloys, which is the sweet spot for strength without brittleness.
Next up: mold design, and this is where the magic happens. A bad mold will turn even the best material into cracked parts. The biggest culprit in mold design for cracking is sharp corners and uneven wall thickness. When metal flows around a 90-degree corner, it has to speed up and slow down abruptly, creating turbulence and high stress at the bend. We always add a minimum of 0.5mm radius to all internal and external corners—even if the customer’s design drawing shows a sharp edge. I know, I know—customers hate when we have to adjust their design, but we explain it this way: a sharp corner is a crack waiting to form during ejection, and it’s way cheaper to add a radius in mold design than to scrap 10,000 parts. Uneven wall thickness is another huge issue. If one section of a part is 5mm thick and another is 1mm, the thick section will cool slower than the thin one, creating residual stress as the thin metal solidifies and pulls on the thick, still-molten section. That stress eventually leads to hot cracking. We work closely with every customer to adjust their wall thickness to within 10-15% of each other. If a customer needs a reinforced section, we add a gradual fillet or a gusset, not a sudden thick bump. For example, last year we had a customer coming to us with a bracket that had a 6mm thick mounting point and 1.5mm arms—their first run had a 25% scrap rate from hot cracks. We suggested thinning the mounting point to 4mm and adding a 2mm fillet between the arm and the mounting point, and their scrap rate dropped to less than 1%. We also pay close attention to core pins and ejector pin placement. Core pins are the metal rods that form holes in the part, and if they’re not cooled properly, they can create hot spots that cause hot cracking. We add cooling lines directly to every core pin over 10mm long to keep their temperature consistent with the rest of the mold. Ejector pins, meanwhile, need to be placed in areas of low stress—never across a sharp corner or a thin wall. We map stress points in every mold using flow simulation software (we use MAGMASoft, and it’s been a game-changer) to predict where stress will build up, then adjust ejector pin placement to avoid those areas. Early on in my career, I didn’t use flow simulation, and I lost an entire mold run to a part that cracked because ejector pins were placed right on a thin wall. Now, we run every design through flow simulation before machining the mold, and that single step has reduced our cracking-related scrap by 40%.
The third big area is process control, which is where we make sure the mold and material work as intended. Even a perfect mold and the right alloy can produce cracked parts if the process is off. Let’s start with shot parameters. The die casting process has two main shot phases: slow shot, which fills the mold’s gating and runners to avoid air entrapment, and fast shot, which fills the cavity to get the part’s shape. If the slow shot speed is too fast, the metal will push air into the cavity, creating porosity that weakens the part and leads to cracks during machining. If the fast shot speed is too slow, the metal will solidify before it fully fills the mold, leading to cold cracks. We test shot speeds for every new part—we do 5-10 trial shots, then check the parts for porosity and flow to find the perfect speed. We also control the mold temperature tightly. Mold temperature affects how fast the metal solidifies; if the mold is too cold, the metal will solidify too quickly, leading to cold cracks. If the mold is too hot, the metal will stay molten too long, leading to hot cracks. For aluminum parts, we keep mold temperature between 180°C and 240°C, depending on the part size. For small, intricate parts, we run the mold a bit hotter (220°C) so the metal flows evenly, and for large, thick parts, we run it a bit cooler (190°C) to reduce cycle time and prevent hot spots. We use temperature sensors in every mold, and our operators check the temperature every hour during runs—no exceptions. Ejection timing is another critical factor. We’ve all heard the old joke about pulling a part too early and it warping, but what many people don’t know is that pulling a part too late is just as bad. If you wait too long to eject the part, it cools and shrinks, and can weld itself to the mold, leading to big cracks when you yank it out. We set ejection timing based on part thickness: for parts thinner than 2mm, we eject at 10 seconds after fill; for parts between 2mm and 5mm, we eject at 15 seconds; for parts thicker than 5mm, we eject at 20 seconds. We also apply a uniform mold release agent every cycle, and we make sure to apply it only in areas that don’t affect material flow—over-spraying mold release can create a barrier that causes uneven cooling, leading to cracks.
Wait, let’s not forget post-casting processes, because cracking doesn’t always show up right away. Even if a part looks perfect after ejection, it could develop a crack during trimming, machining, or even assembly. We use CNC trimming tools with sharp, well-maintained blades—dull blades tear the metal instead of cutting it, leading to micro-cracks that grow over time. When we machine holes or threads, we use slow spindle speeds and high pressure coolant to reduce friction and heat, which can cause thermal cracking. For parts that go through heat treatment (like for hardness or stress relief), we control the heating and cooling rates: heating too fast can create thermal stress, and cooling too fast can cause cracks. For example, when we stress relief aluminum parts, we heat them to 290°C slowly over 2 hours, hold them for 2 hours, then cool them slowly over 4 hours—no quenching, which is a common mistake that leads to cracks. We also do a 100% visual inspection on every part, and we use magnetic particle inspection for high-stress parts to catch micro-cracks that are too small to see with the naked eye.
Over the years, we’ve learned that cracking prevention isn’t about a single fix—it’s a holistic approach that starts with the customer’s design and ends with the finished part. I know that when you’re sourcing die casting parts, you’re looking for consistency and reliability, and cracking is one of the biggest headaches that can ruin that. We don’t see our role as just a supplier—we see ourselves as a partner, which is why we spend time walking through every step of the process with our customers, from material selection to mold design, to make sure their parts are built to last. If you’re dealing with cracking issues in your current die casting parts, or if you have a new project coming up and you want to avoid those issues altogether, we’d love to chat. We can review your design, run a quick flow simulation, and put together a plan that meets your requirements without the risk of cracks. Don’t let preventable setbacks delay your production—reach out to us today to discuss your next die casting project.
Die Casting Parts References
- John R. Davis, Aluminum and Aluminum Alloys, ASM International, 1993.
- Donald J. Schaefer, Die Casting Handbook, Society of Manufacturing Engineers, 2007.
- MAGMASoft Corporation, Die Casting Process Simulation: Best Practices, 2021.
- T. A. Engh, Principles of Metal Rolling and Casting, Elsevier, 2006.
Suzhou Ruide Die-Casting Mold Co., Ltd.
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