Die Casting in China: Processes, Alloys, Tooling, and the Real Cost

Guide · Die Casting in China Die Casting in China: Processes, Alloys, Tooling, and the Real Cost Die casting forces molten aluminum, zinc, or magnesium into a precision steel die and ejects a near-net part in seconds — once you pay for the die. This guide walks the three processes, the four alloys that matter, what tooling honestly costs and how long it takes, the tolerances and porosity you must design around, and how to source it from China without getting burned. Request a quoteDFM rules What die casting is Die casting is a net-shape metal process: molten alloy is forced into a precision steel die under pressure, solidifies in seconds, and is ejected as a near-finished part. The economics define the process — a die is expensive and takes weeks to build, but once it exists each part costs little and cycles fast. That front-loaded trade is why die casting is a volume decision, not a capability decision. Two machine families cover the field. Aluminum and magnesium run a cold-chamber press: metal is ladled into a shot sleeve and a plunger drives it into the cavity, because molten aluminum would attack a submerged injection system. Zinc runs a hot-chamber press, where the injection mechanism sits inside the molten bath, so it cycles faster and fills thinner walls. Pressure pushes metal into every rib, boss, and corner, the shot freezes in seconds, ejector pins release the part, and the cycle repeats — a small aluminum part can run a 30–90 second cycle, and that speed is the whole economic point. The process shines on thin-wall, complex housings in high volume, and on consolidating several machined pieces plus fasteners into a single shot. It is the wrong call for very low volume, a design still in flux, or parts that must be pressure-tight or welded — both collide with the porosity that is inherent to the process. The three die casting processes: HPDC, gravity, and low-pressure "Die casting" is a family of three, and the method changes the density, cost, and wall section you can achieve. High-pressure die casting (HPDC). Metal is injected under high pressure — roughly 20–100+ MPa on aluminum cold-chamber presses — filling thin walls in milliseconds. It is the fastest and cheapest per part, but entraps the most gas, so it carries the most porosity. This is the default for high-volume, thin-wall housings. Gravity (permanent-mold) casting. Metal is poured into a reusable steel mold with no injection pressure. It fills slowly and suits thicker walls, but traps far less gas, so parts come out denser and pressure-tight. Tooling is cheaper than HPDC, cycles are slower. Low-pressure die casting. Metal is pushed up from a sealed furnace at low pressure, around 0.02–0.1 MPa, so fill is controlled and defects are fewer. It lands between the two — denser than HPDC, cheaper tooling than a high-pressure die — and is the standard for structural parts like wheels. The selection rule is simple: thin walls and lowest unit cost favor HPDC; density and leak-tightness favor gravity or low-pressure. If the part must seal against pressure, do not default to HPDC and hope — pick the process for the requirement. Alloys: A380, A360, Zamak, and AZ91D Four alloys cover most die-cast parts. The choice follows weight, thermal, corrosion, and tolerance needs — exactly as it would for machining. Cross-check any grade against the ASM Handbook before committing a drawing. AlloyBaseWhy choose itTypical use-case A380AluminumBest balance; the defaultHousings, brackets, engine parts A360AluminumMore pressure-tight and corrosion-resistant; better finishLeak-tight and cosmetic parts Zamak #3 / #5ZincThinnest walls, tightest as-cast tolerance, long tool lifeSmall precision parts, gears AZ91DMagnesiumLightest; needs corrosion protectionLightweight frames, electronics housings Two practical notes. First, aluminum vs zinc: zinc casts thinner walls, holds tighter as-cast tolerances, and needs less draft, but weighs more and costs more per kilogram; aluminum is lighter, conducts heat better, and is cheaper per part at volume. Second, magnesium wins the weight contest but demands a corrosion-protection plan — conversion coating and paint are the norm, and its machining and finishing must be specified up front. Tooling: the front-loaded bet A die is a machined steel assembly — typically H13 die steel — holding the cavities, cores, slides, ejectors, and cooling channels. Its price tracks cavity count and shot size more than part complexity, because the steel and the press both scale with the part envelope. More cavities raise the die price but cast several parts per shot, so cavity count is itself a breakeven decision. The cost is front-loaded and the lead time is long, and both are the reason casting is a volume decision. A simple single-cavity tool for a small part can run a few thousand dollars; a large multi-cavity die can run tens of thousands. Lead time commonly runs 4–10 weeks from design through steel, cavity cutting, fitting, and first-article trials — longer if the first shots reveal a fill or shrink problem that sends the die back for rework. You cannot compress it easily, because the die is a precision mechanical assembly, not a file you print. The amortization mathDie cost ÷ (machined per-part cost − die-cast per-part cost) = break-even quantity. Worked example: a CNC housing costs $12 at any quantity; the same part cast costs $3 after a $12,000 die. The lines cross near 1,300 parts — but add mold lead time, first-article revision risk, and the cost of a design change, and the practical crossover for a stable aluminum part usually sits in the 3,000–10,000+ range. Two hidden costs push the real crossover above the arithmetic one. The die wears and needs maintenance across its life — tens to hundreds of thousands of shots — and first articles frequently need a revision you pay for. Buyers who tool too early, on the formula alone and ignoring revision risk and time-to-market, are the most common over-spend in casting. Tolerances a cast part honestly holds For a quality lead. Die casting is a net-shape process, not a precision one. As-cast features hold looser tolerances than machining, and the range widens with part size. Shrinkage is the root cause: the metal contracts as it cools, and the contraction varies with wall thickness, gate location, and the shot itself, so a dimension that holds in one shot drifts in the next. A competent foundry compensates with gating and cooling design, but you are buying tenths of a millimeter of capability, not microns. The loosest number is not the foundry's fault — it is the process's. That is why a drawing full of ±0.05 mm callouts is a CNC drawing wearing a cast disguise, and why real cast parts move through a second step. The fix is the hybrid that defines production casting: cast the body, then CNC-machine the critical features. Sealing faces, bearing bores, threaded holes, and datums are machined into solid, porosity-free material, while the non-critical exterior stays as-cast. You keep the cheap net-shape blank and pay for cutting only on the few features that earn it. The CNC cost drivers. Porosity and the defects that come with it Porosity is the signature defect of die casting, and it is not cosmetic. Air entrapped during filling plus shrinkage as the metal cools leaves internal voids that concentrate near the gate and in thick sections. Those voids leak under pressure, make welding and heat treatment unreliable, and can show as blemishes under anodizing. Four fixes, in order of preference: Design it out. Uniform wall thickness and well-placed gates reduce the entrapment and shrinkage that cause voids in the first place. Specify vacuum-assist or squeeze casting. When tightness matters, evacuating the cavity before injection — or squeezing during solidification — cuts gas porosity for a denser part. Impregnate. Sealant fills the voids for low-pressure fluid parts where a pinhole would leak. Machine past the skin. Leave 0.5–1.5 mm of stock on sealing and bearing faces, then cut into solid metal. This is why casting and CNC are sequenced, not rivals. Beyond porosity, expect three surface artifacts on any cast part: a parting line where the die halves meet, ejector-pin marks, and a gate vestige where metal entered. All three are trimmed, blasted, or machined away in post-processing — they are expected, not defects, unless the design forgot to account for them. DFM: draft, wall thickness, and ribs For a manufacturing engineer. Most casting rework comes from a handful of rules broken at the drawing stage. They cost nothing to respect and a full die revision to ignore. Draft. Every surface parallel to the die opening needs a draft angle so the part releases cleanly — typically 1–2° per side on aluminum, 0.5–1° on zinc. Zero draft means the part drags, sticks, or tears on ejection. Wall thickness. Keep walls uniform, and hold a minimum near 0.8–1.0 mm in aluminum (zinc can go thinner, near 0.5 mm). Abrupt changes cause shrinkage porosity and sink marks as thick sections cool after thin ones. Ribs. A rib stiffens a wall without adding thickness — but keep it to roughly 50–60% of the wall it joins and add draft. A rib thicker than the wall leaves a sink mark on the opposite face. Corner radii. Add generous radii everywhere. Sharp corners concentrate stress, crack on ejection, and wear the die early. Bosses. A boss is a local thickening for a screw, so it sinks. Keep its wall thin and tie it to a side wall or rib for support. Undercuts. An undercut needs a slide or side action inside the die, which raises cost and complexity. Every undercut you add is money you add to the tool — or a signal to machine instead. None of these rules add cost; they remove it. A part drawn with draft, uniform walls, and no undercuts drops out of the die on cycle; the same part drawn without them spends a cycle being re-tooled. The complete DFM guide. Finishes: what as-cast gives you and what it needs As-cast aluminum typically lands around Ra 1.6–3.2 µm off a standard die, finer with a polished cavity — but you also get the parting line, ejector marks, and gate vestige described above. Real parts therefore move through post-processing: gates and flash are trimmed, parts are deburred and bead- or shot-blasted, then finished. The finish options map to the alloy and the surface. Powder coat is the most forgiving choice on as-cast aluminum, hiding minor surface texture and sealing the part. Anodizing is possible but uneven — the silicon in cast alloys and the surface porosity make finish patchy and prone to pitting — so a cosmetic anodized look usually means machining the visible face first or specifying a fine-grained alloy with a polished cavity. Plating and paint round out the set, and magnesium parts always carry a conversion coating plus paint for corrosion protection. Specify the finish before the die cavity is polished, because the cavity surface transfers directly to the part and changing it later means re-polishing steel. The surface-finish guide. Die casting vs CNC vs injection molding Three processes compete for the same enclosure or housing, and the decision comes down to three questions: what volume, what material, what tolerance. Die casting and injection molding both carry a front-loaded tool; CNC carries none but costs more per part. The table maps the trade. FactorDie castingCNC machiningInjection molding MaterialAluminum, zinc, magnesiumAny metal or plasticThermoplastics ToolingSteel die, $k–$$k, 4–10 wkNoneSteel mold, $k–$$k, 4–8 wk Minimum orderThousands (to pay the die)One (MOQ 1)Thousands (to pay the mold) First-part lead timeWeeksDaysWeeks Per-part cost at volumeLowHigher, flatLowest (cents of resin) ToleranceLooser; porosity risk±0.005 mm±0.1–0.5 mm Best forMetal housings at volumePrototypes, spares, complex, low volumePlastic housings at volume The practical pattern is a sequence, not a single choice. Machine prototypes and pilot runs through EVT → DVT → PVT while the design settles, because CNC absorbs every change for free and ships in days. Then tool the die — or the mold, if the part is plastic — once the geometry freezes and the forecast justifies it, and machine the critical faces when the cast or molded parts land. A partner that runs CNC in-house and casting through vetted partners executes that whole handoff under one quality system. The full casting-vs-CNC crossover. Why China for die casting China's manufacturing belts run the densest clusters of foundries, die shops, and finishers anywhere. In the Pearl River Delta — Dongguan and its surrounding towns — the die-steel suppliers, EDM and grinding shops, the foundries themselves, and the platers and powder coaters all sit minutes apart, so a die can be designed, cut, and first-articled inside one district without importing a single component. That density changes the economics three ways. First, tooling is fast and cheap: a die that costs weeks and a premium in a Western toolroom is a same-district order in the Delta, where the steel, machining, and tryout press are next door. Second, labor depth: the operators and die makers have run your exact part family thousands of times, turning a good die into a great one. Third, shared finishing: the plater, painter, and blaster are specialists serving every foundry, so a small shop delivers a finished part without owning any of those lines. The honest caveat applies to every process in China: the cluster holds world-class shops and shops that cut corners, often quoting similar prices. In casting the risk concentrates in die steel — a soft substitute that wears early — and in alloy substitution, where a cheaper grade replaces the one on the drawing. The fix: specify the alloy grade and the steel, request a measured first article, and hold the partner to documented quality. Vetting a supplier without flying out. Selecting a Chinese foundry: the six checks that catch the bad half For a sourcing engineer. The cluster holds world-class foundries and shops that cut corners, often at similar prices. The difference is caught by six checks you can run before the die is cut, none requiring a plane ticket. Die steel. Name the steel — H13 for aluminum is the norm — and ask for the material cert. A soft substitute is the single most common cause of an early-worn die. Alloy grade. Require the grade on the drawing (A380, A360, and so on) and a measured first article, not a look-alike sample. Substitution is the other classic corner. DFM review. The foundry should flag draft, wall-thickness, and undercut problems before steel is cut — a shop that quotes without a DFM pass is pricing a part it has not read. Vacuum and impregnation. If the part must seal, confirm the foundry runs vacuum-assist and can source impregnation — do not discover after the leak test that neither is available. Post-processing. Confirm the trimming, blasting, and machining path, and who owns each step. A foundry with no machining partner will hand you a part that still needs a second shop. Documentation. Ask for a first-article dimensional report, a PPAP or control plan, and lot-level material traceability — not a wall of logos. Run these six against any two quotes and the real difference usually appears before you spend either. The cheaper tool is only cheaper if it survives the first-article revision. Nex-G: casting via vetted partners, QA-managed Nex-G's own floor is EMS and CNC under one roof in Dongguan Hengli — a 6,800 m² facility staffed by more than 100 people and running since 2006. Die casting is sourced through a vetted partner network and QA-managed by Nex-G, which matters: the cast part comes back to a shop that can machine to the same drawing, inspect to the same standard, and integrate it into the same build the electronics live in. The anchor is the quality system behind the sourcing. Nex-G holds ISO 9001, IATF 16949 (build-to-print — design excluded under clause 8.3), and ISO 14001, with URS certification to 2027. SPC-controlled processes hold a real Cpk above 1.67, not a spreadsheet promise. That discipline is applied to sourced casting: the die design is reviewed before steel is cut, the tool is approved from a measured first article, and incoming parts are inspected against the drawing before they enter your build. In-house, 80+ CNC machines hold ±0.005 mm on milled and turned features and ±0.002 mm on grinding — the capacity that picks up any cast feature needing a machined tolerance, and that machines the prototypes that de-risk the die. The service runs no MOQ — a minimum of one part — carries programs from EVT → DVT → PVT, and quotes lead times of 3, 7, and 30 days by scope. One partner manages the casting, the machining, and the board, so the fit is checked at the bench, not at your dock. See the full capability. On the floor the checks are concrete rather than aspirational. Every BOM line is cross-checked against the drawing and the purchase spec before release, so an A380 callout cannot silently swap to a softer substitute alloy. Critical alloys are bought through a cross-checked second source with a matching mill test certificate, and each incoming lot is confirmed before it enters the build. For aerospace programs we run FAI per AS9102 so the first article is dimensionally signed off before the run begins. Frequently asked questions When does die casting beat CNC machining?Above the break-even volume, where the die cost amortizes across enough parts that the low per-part cost beats flat CNC machine time. For a small aluminum part that crossover is often in the low thousands, but die lead time and revision risk push the practical number higher — model it before committing to a die. How much does a die casting tool cost, and how long does it take?A simple single-cavity die for a small part can run a few thousand dollars; a large multi-cavity die can run tens of thousands. Lead time commonly runs 4–10 weeks from design through steel, cavity cutting, and first-article trials, longer if the first shots need rework. What tolerances can a die-cast part hold?Looser than machining, and widening with part size. The critical, mating, and sealing features are CNC-machined after casting into solid material — that hybrid is the usual production answer, not a tighter cast. Is porosity a real problem in die casting?Yes, and it is expected. Entrapped gas and shrinkage leave internal voids that leak under pressure, resist welding and heat treatment, and show under anodizing. The standard fixes are to design uniform walls, specify vacuum-assist or squeeze casting when tightness matters, impregnate, or machine the sealing faces into solid metal. What draft angle does die casting need?Typically 1–2° per side on aluminum and 0.5–1° on zinc, so the part releases without dragging. If your design cannot carry draft at all, that is a signal CNC or a different process is the better fit. Which alloy should I choose: A380, A360, Zamak, or AZ91D?Default to A380 for general aluminum housings. Use A360 for pressure-tight or cosmetic parts, Zamak for thin-walled small precision parts, and AZ91D when weight is the driver and a corrosion-protection plan is acceptable. Confirm the grade against the ASM Handbook. Do you cast in-house?Nex-G sources die casting through a vetted partner network and QA-manages it in-house — die-design review, tool approval from a measured first article, and incoming inspection under the same ISO 9001 / IATF 16949 / ISO 14001 system that runs the CNC floor. CNC and EMS are in-house, so casting, machining, and electronics integrate under one quality gate. How do I know the die steel is what the quote says? Require the steel grade on the tool drawing and the material certificate, and ask for a hardness check on the cavity after cutting. A soft die shows up as early wear and drifting dimensions within the first few thousand shots. Sourcing die-cast parts for your build?Send the drawing, alloy, and annual volume — we will review the die design, confirm the tool cost and the break-even, and quote the casting with the machining and electronics under one quality gate.Request a quote Related articlesDie Casting vs CNC MachiningMetal Stamping in ChinaThe Complete Guide to Materials Selection