Die Casting vs Injection Molding — Which Process?
Process
The short answer
Die casting makes metal parts; injection molding makes plastic parts. They are not competitors — the question is whether your part should be metal or plastic, and that decision is driven by load, heat, EMI and the environment the part lives in.
The most common error is treating them as alternatives when they are complements: a molded plastic housing with a die-cast internal frame is a standard mixed program.
The Snapshot
- Wall thickness: die casting runs 1.5–5 mm; injection molding reaches down to 0.5 mm for thin enclosures.
- Tolerance: die casting holds ±0.1 mm typical; injection molding lands ±0.05–0.2 mm, with ±0.005 mm achievable on critical features in engineered tooling.
- Tooling lead time: die casting 6–12 weeks; injection molding 3–6 weeks for soft tool, 8–12 weeks for hardened production.
- Strength: die-cast Al/Zn carries structural load and sheds heat; molded thermoplastic insulates and encloses at 0.90–1.4 g/cm³.
Spec matrix
| Factor | Die casting (Al / Zn / Mg) | Injection molding (thermoplastic) |
|---|---|---|
| Material | Metal (Al, Zn, Mg) | Thermoplastic resin |
| Tooling cost | High | Medium–High |
| Per-part cost at volume | Low | Very low |
| Wall thickness | 1.5–5 mm | 0.5–4 mm |
| Tolerances | ±0.1 mm | ±0.05–0.2 mm |
| Strength | High (structural) | Moderate (resin-dependent) |
| Heat dissipation | Excellent | Poor (insulator) |
| EMI shielding | Inherent (conductive) | Needs coating / insert |
| Lead time (tooling) | 6–12 weeks | 3–6 weeks (soft) / 8–12 (hard) |
| Best for | Brackets, load-bearing housings | Enclosures, non-structural, cosmetic |
Metal property reference
| Die-cast metal | Density (g/cm³) | Molten temp (°C) | Mold temp (°C) | Use |
|---|---|---|---|---|
| Aluminum (A380-class) | ~2.7 | ~660 | 200–300 | Brackets, housings |
| Zinc (Zamak) | ~6.6 | ~385 | 150–250 | Small precise parts |
| Magnesium (AZ91) | ~1.8 | ~600 | 200–280 | Lightweight structural |
Metal density is the weight penalty and the strength dividend: cast aluminum at ~2.7 g/cm³ is 3× the weight of molded PP at 0.90 g/cm³, but carries load plastic cannot.
Tolerances, wall thickness and draft
Die casting tolerances cluster around ±0.1 mm on features, tightening to ±0.05 mm only with premium tooling and secondary machining. Injection molding spans a wider band: ±0.1–0.2 mm general, ±0.05 mm on critical locating surfaces, and ±0.005 mm on a connector program we ran in glass-filled PPS / PA66 at 2M units/year under IATF 16949.
If the print demands ±0.05 mm on a feature that also must carry load, die casting may be the only path without insert molding.
Die casting needs 1.5 mm+ minimum wall to fill without cold shut; very thin sections starve. Injection molding fills 0.5 mm walls in thin enclosures (PC, ABS) but thin walls raise fill pressure and cycle time.
Both need draft — typically 1–3° per side — but die-cast parts need more generous fillets to avoid hot-tear at ejection. A VR headset front cover molded in ABS+PC held ±0.05 mm on 0.8 mm walls; that geometry would be uncastable in aluminum.
Material and strength
Die-cast aluminum (A380-class) and zinc (Zamak) deliver structural stiffness and thermal conductivity plastic cannot match. For a bracket under vibration or a heatsink-adjacent housing, metal wins.
Injection molding offers a resin for nearly every non-structural need: PP for living hinges, PC-ABS for impact enclosures, PPS for 200–240 °C environments, PEEK for 250–260 °C. A HV busbar program combined C11000 copper (conductive insert) with PA6-GF30 overmolding at 250,000+ units/year — metal where current flows, plastic where it insulates.
Quick metal-vs-plastic reference:
- Die-cast Al A380: tensile ~320 MPa, density ~2.7 g/cm³, service to ~200 °C.
- Die-cast Zamak zinc: tensile ~280 MPa, density ~6.6 g/cm³, small precise castings.
- Molded PPS-GF40: tensile ~150–200 MPa, density ~1.55–1.65 g/cm³, service 200–240 °C.
- Molded PA66-GF30: tensile ~170–200 MPa, density ~1.35–1.40 g/cm³, service 120–140 °C.
Strength is not the only column — molded PPS or PA66-GF30 carries real load, which is why the automotive connector program used glass-filled PPS at ±0.005 mm.
Lead time and tooling cost
Die-cast tooling is heavier steel and longer to machine: 6–12 weeks, higher upfront cost. Injection molding soft tool lands in 3–5 weeks for validation; hardened production tool in 8–12 weeks.
The crossover is volume. Below 100K units/year, die casting’s tool cost dominates. Above 500K units/year, both amortize well, and the per-part material saving of thin-wall plastic often wins for enclosures.
Cost crossover math: die-cast tooling is heavier and longer to machine, so its upfront cost is generally the higher of the two. A 50 g molded PP enclosure uses about $0.10 of resin per part at ~$2/kg; a 150 g die-cast aluminum part uses roughly $0.45 of metal at ~$3/kg before machining, finishing and secondary operations. The gap is small at low volume and decisive at 500K+ units/year — which is why thin-wall plastic wins enclosures at scale while metal keeps load-bearing and heat-shedding roles.
Thermal, EMI and sealing
- Heat dissipation / EMI: choose die casting. Molded plastic insulates and needs a conductive coating or metal insert for shielding.
- Sealing surfaces: molded TPU or LSR gaskets seal better than cast metal flanges without extra machining.
- Weight: molded PP at 0.90 g/cm³ beats cast aluminum at ~2.7 g/cm³ — decisive in automotive and handheld electronics.
When both run together
Many programs combine both — a molded plastic enclosure with a die-cast internal frame or heatsink. An automotive electrical connector used glass-filled PPS molding over metal terminals; a consumer speaker ran PP / ABS / ABS+PC molded shells over die-cast or stamped internals at 150+ mold sets / 30 models.
Real mixed-program data points:
- Automotive connector: glass-filled PPS / PA66, ±0.005 mm, 2M / yr, 10 weeks.
- HV busbar: PA6-GF30 + C11000 Cu, 250K+ / yr, 8 weeks.
- NEV dust cover: FR-TPU (UL94 V-0), 500K+ / yr, 6 weeks.
Running both under one roof avoids supplier finger-pointing when a fit issue spans the metal and plastic interface.
The final call
If the part carries load or sheds heat or needs EMI shielding, die cast it. If it encloses, insulates or must be light and low-cost at volume, mold it. We run both processes, so a mixed program gets one accountable supplier.
Bring the operating temperature, the load case and the annual volume — those three inputs decide the process faster than any brand preference.
Send us your drawing for a side-by-side quote in both processes.