Same Part, Two Price Tags
An aluminum enclosure and a glass-filled nylon enclosure can look identical in CAD and perform similarly on the bench. The cost math behind them is not similar, though — and picking the wrong process shows up either as a 30–50% premium per part or a tooling bill you never needed. Here's how the numbers actually break down for low-to-mid volume programs, based on the quotes we build every week.
Tooling: Where the Programs Diverge First
Injection molds scale with ambition. A soft prototype tool — aluminum or P20 pre-hardened steel, single cavity, hand-loaded inserts — typically lands in the low thousands of dollars and survives a few hundred to a few thousand shots. A production tool in hardened steel with hot runners and multiple cavities is a different animal: five figures is normal, and complex multi-cavity tools climb well beyond that.
Die casting dies don't have a cheap tier. Because the die meets 600+ °C molten aluminum every cycle, it lives in thermal-fatigue country: H13-class hot-work steel, engineered cooling lines, vacuum assist if porosity matters. Expect production-tool pricing even for modest parts — nobody builds a serious casting die in aluminum.
The practical consequence: below a few thousand parts, soft tooling for plastic injection can beat any casting die on upfront cost. For metal parts at low volume, machining usually beats building a casting die you'll underuse.
Cycle Time: The Silent Cost Multiplier
Per-part machine time is where high volumes get decided:
- Injection molding: 20–60 s is typical for parts with 2–3 mm walls. Thin-wall parts cycle faster; thick sections with deep bosses can stretch past 60 s on cooling alone.
- Die casting: commonly 45–120 s. The metal shot itself is quick, but die spray, lubrication and a controlled first-stage fill add up. Thick sections cool slowly, and hot spots shorten die life.
Even at identical machine-hour rates, the molded plastic part often cycles 1.5–2.5× faster. That gap compounds mercilessly at 50,000+ parts a year.
Material Economics: Closer Than You'd Expect
Commodity resins and casting alloys sit surprisingly close in raw price — both often land in the low single digits per kilogram, though engineering resins like PC or PA66-GF30 push higher. The real differences are elsewhere:
- Plastic cold-runner scrap (sprues and runners) is reground and reused in-house; casting gates, biscuits and overflows are harder to recycle at the machine.
- Mass adds up: aluminum is about 2.7 g/cm³ versus roughly 1.2–1.4 for engineered plastics, so the metal part consumes around twice the material mass for the same volume.
- Resins are hygroscopic. PA66 and PC must be dried before molding or you get splay and brittle parts — budget the drying time and keep bins sealed.
Hidden Costs That Quotes Sometimes Miss
Die casting: trim dies, machining for critical bores and sealing faces, and porosity-sensitive finishing. High-silicon alloys like ADC12 anodize poorly — expect grayish, blotchy results — so if appearance matters, powder coating or plating is the realistic route. Impregnation to seal porosity on pressure-tight parts is a real line item, not a rounding error.
Injection molding: degating, occasional annealing for dimensional stability, and the permanent war on sink marks and warp. Glass-filled resins shrink differently along and across flow, so a flat part can come out like a potato chip if the gate location is wrong — and fixing that means mold steel work, not a parameter tweak on the floor.
So Where's the Break-Even?
There is no universal crossover number — wall thickness, projected area and required tonnage move everything. But the pattern repeats often enough to be useful:
- Under ~1,000 parts: machining (metal) or 3D printing and soft-tool molding (plastic) wins; production tooling can't amortize.
- ~1,000–10,000 parts: the murky middle. Single-cavity tools on both sides get weighed case by case; cycle time and secondary operations usually decide it.
- 10,000+ parts: both processes hit their stride, per-part prices drop hard, and the decision shifts from cost to material properties.
Choosing on Properties, Not Just Price
Cost per part should be the tiebreaker, not the decider. Die casting brings EMI shielding, stiffness and heat resistance — it's the default for housings near motors or RF circuitry. Injection molding brings weight savings, snap-fits and living hinges, corrosion immunity, and consistent walls down to around 1 mm that casting struggles to fill reliably. If your product sits in both worlds, it's worth talking to our plastic injection molding and die casting teams before the CAD freezes.
Bottom Line
Tooling is a sunk bet, cycle time is a running tax, and material mass is a quiet multiplier. Price both processes at your real annual volume — not the volume you hope for — and the cheaper path usually becomes obvious within a quote or two.
