Are You Paying for the Wrong Process?

Every procurement manager has stared at two competing quotes and wondered why one is 30 % cheaper. When the project involves injection molding vs die casting, the price gap usually traces back to subtle differences in material utilization, cycle time, and secondary finishing. Ignore these details and you’ll either overpay up-front or swallow hidden costs later.

Quick Recap: What Each Method Actually Does

• Injection molding melts polymer pellets and ram-rods them into a steel cavity at 10 000–30 000 psi. The plastic cools in seconds, the tool opens, and a robot flicks the part onto a conveyor.
• Die casting injects molten metal—typically aluminum, zinc, or magnesium—into a hardened die at 15 000–40 000 psi. A water-cooled channel chills the alloy in under a minute, the platens part, and an ejector plate shoots the casting out.

Same vocabulary—“injection,” “tool,” “cavity”—but the thermodynamics and economics could not be more different.

Tooling Cost Showdown: Steel Price Shock

Plastic molds generally use P20 or H13 tool steel, heat-treated to 48–52 HRC. A 12 × 16 in. multi-cavity tool lands in the US$30k–80k range, depending on slides and lifters. Die-cast dies must survive 1 200 °F aluminum splatter, so they’re carved from premium H13 or even maraging grades and vacuum-hardened to 44–48 HRC. Result: the same envelope costs US$60k–150k. If your annual volume is <50k shots, the mold amortization alone can flip the business case toward plastic. No surprises here—except for buyers who forget to ask for the tooling spreadsheet.

Material Unit Economics: Pennies That Multiply

Raw aluminum (A380) runs ≈ US$1.60/lb, while ABS pellets hover around US$0.90/lb. “Only 70 ¢ difference,” you shrug—until you multiply by 0.8 million phones or 300k dishwasher racks. On a 50 g part, that delta snowballs into a six-digit annual hit. Factor in the buy-to-fly ratio: die casting yields 95 % net shape, whereas injection molding can hit 99 % because cold runners are reground. In short, if weight is mission-critical, metal tax adds up fast.

Cycle Time Reality Check: 18 s vs 45 s

A four-cavity ABS housing ejects every 18 s on a 300 t electric press. The equivalent aluminum cover, trimmed to equal stiffness, cycles in 45 s on a 600 t cold-chamber machine. Multiply the 27 s gap across 500k parts and you need a second casting cell—or a night shift—to keep up. Yet the aluminum part can ship straight to e-coat, while the plastic shell still waits for conductive primer. Which bottleneck hurts more? Depends on your takt-time contract.

Dimensional Tolerance: Can You Skip Machining?

Both processes hold ±0.1 mm on critical features, but aluminum shrinks 0.6 % compared with 0.4 % for ABS. Worse, die-cast gates leave witness marks that may interfere with bearing bores, forcing a CNC finish pass. Injection molded bosses can be molded to net, avoiding secondary ops. So, if the hub must mate with a pressed-in brass insert, plastic often wins the tolerance chess game.

Heat Deflection & Strength: When Plastic Melts at 120 °C

Under-bonnet electronics pushed engineers toward glass-filled PPA (deflection temp 280 °C) to compete with ADC12 aluminum (320 °C). The plastic part saved 1.2 kg, but the customer demanded salt-spray corrosion resistance. After 2 000 h of testing, the PPA swelled and cracked. Lesson: light-weighting is seductive until environmental validation kills the schedule. A quick rule-of-thumb: if service temperature >150 °C, aluminum rarely loses.

Environmental Scorecard: CO₂ per Part

Aluminum primary production belches 11 kg CO₂ per kg of metal, but 60 % is today from recycled scrap, dropping the burden to ≈ 2 kg. Virgin PP or ABS averages 1.8 kg CO₂ per kg, and regrind can shave another 20 %. Factor in cycle energy: a 600 t die-caster draws 45 kWh/h, while an all-electric 300 t injection press sips 18 kWh/h. Crunch the numbers and the carbon gap narrows to 5–8 %—close enough that regional electricity mix decides the winner.

Transition Time: From PO to First Article

Plastic tooling lead time averages 8–10 weeks stateside; die-cast dies need 12–14 weeks because conformal cooling channels are milled deeper and slides are nitride-coated. Got a panic program? Some Chinese shops quote 5-week T1 for molds, but die-cast tools rarely beat 8 weeks even with 24/7 lights-out machining. Translation: if your CEO promised the board a Q2 launch, start the tool RFQ yesterday.

Hidden Gotchas: Trim, Polish, Impregnation

Die castings flash like a Halloween pumpkin. Manual trim adds US$0.15–0.40 per part; porosity leaks may require vacuum impregnation at US$0.08–0.12. Injection molded parts pop off the sprue cleanly, and sink marks get ironed out with a quick heat-stake. Buyers who model only the metal unit price miss these tail costs and then panic when the P&L turns crimson.

Decision Matrix: Injection Molding vs Die Casting

Criterion	Injection Molding	Die Casting
Tooling cost	Low–Medium	High
Material cost	Low	High
Cycle time	Fast	Moderate
Strength at 150 °C	Poor	Excellent
Minimum wall	0.4 mm	0.8 mm
Post-machining	Rare	Often

Score each line 1–5 for your program, multiply by your internal weighting, and the math will shout the winner.

Real-World Example: EV Battery Housing

Startup “VoltaPulse” needed a 400 × 200 mm enclosure, IP67, 200k annual volume. Aluminum (A383) delivered best EMI shielding, but the die-cast tool quote of US$110k and 38 s cycle scared the CFO. A 40 % glass-filled PPE+PS blend passed flammability UL94 V-0 and cost US$55k for tooling, 22 s cycle. Even after adding a copper-nickel conductive paint for EMC, plastic shaved US$1.40 per part and 0.9 kg. They went with injection molding and banked the savings for UL certification.

Bottom Line: Let the Numbers Speak, Not the Sales Rep

Still asking “which is cheaper?” is like asking “car or truck?” without payload or miles. Build a cost-of-ownership spreadsheet that folds in tooling amortization, variable cycle cost, secondary finishing, and end-of-life recycling. When you do, the injection molding vs die casting debate stops being tribal and starts being empirical—and your purchasing team finally gets the last laugh.