Die Casting vs Injection Molding — Which 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, and the supplier that runs both processes keeps the metal-plastic interface honest.

This guide compares them on the numbers that decide the choice — tolerance, wall thickness, strength, thermal and EMI behavior, tooling cost, lead time — with every figure traced to a published source (RapidDirect, Xometry, FirstMold, HLH Rapid), an industry standard (NADCA, JIS), a typical published datasheet range, or a verified value from our own plant in Dongguan, marked [OUR PLANT]. Nothing is invented.

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.
• Die life: published die life runs ~100,000 cycles for aluminum, ~1,000,000 for zinc (RapidDirect) — a zinc die outlives an aluminum die tenfold.
• Tooling cost: die casting dies run roughly $7,000–75,000 (RapidDirect); plastic tools $3,000–6,000 simple to $7,000+ complex (HLH Rapid).

Table of Contents

1. The short answer
2. The Snapshot
3. Spec matrix
4. How the two processes work
5. Metal property reference
6. Tolerances, wall thickness and draft
7. Material and strength
8. Thermal, EMI and sealing
9. Lead time and tooling cost
10. Cost structure and volume crossover
11. Secondary operations and finishing
12. Metrology and quality control
13. When both run together
14. Choosing a supplier: what to audit
15. The final call
16. Frequently Asked Questions
17. Sources

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

Read the matrix as a screening tool, not a verdict: the wall thickness, tolerance and EMI rows are where most process decisions are actually won or lost. A part that needs 0.5 mm walls, ±0.05 mm on a locating feature and a sealed cosmetic housing is almost always plastic; a part that carries 50 N of vibration load at 80 °C in a dusty environment is almost always die-cast metal.

How the two processes work

Die casting forces molten metal into a hardened steel die at high pressure — typically 10–175 MPa for aluminum high-pressure die casting — where it solidifies in seconds into a near-net-shape metal part. The sequence is melt, inject, solidify, eject, trim:

1. Melting — aluminum alloys melt around 660 °C (zinc around 385 °C, magnesium around 600 °C).
2. Injection — molten metal is driven into the cavity by a plunger; fill times are milliseconds and the metal enters as a spray that must wet the steel before freezing.
3. Solidification — the die is water-cooled, so the metal freezes from the die wall inward; this is where shrinkage and porosity are decided.
4. Ejection and trimming — the die opens, ejector pins push the part off, and the runner system is trimmed.

Two process facts matter for buyers. First, aluminum must be cast on a cold-chamber machine — molten aluminum would destroy a hot-chamber gooseneck, so hot-chamber casting is limited to zinc and magnesium (RapidDirect, https://www.rapiddirect.com/blog/aluminum-die-casting/). Second, die life is alloy-dependent: about 100,000 cycles for aluminum and magnesium dies versus 1,000,000 for zinc (RapidDirect, https://www.rapiddirect.com/services/die-casting/).

Injection molding melts thermoplastic resin and forces it into a steel cavity; the tool is the capital cost, the cycle is cheap. The sequence is melt, inject, pack, cool, eject — on a 15–60 s cycle — and multi-cavity tools multiply output. Mold steel is machined to ±0.02 mm [OUR PLANT] and the cavity is cut oversized to compensate for resin shrinkage (amorphous 0.4–0.8%, semi-crystalline 1.0–2.5%).

The defining difference is melt temperature and what it does to the tool: a 660 °C aluminum melt erodes die steel and forces cold-chamber machines, while a 200–300 °C plastic melt is gentle on steel and permits fast, cheap cycles. That single fact drives most of the cost, tolerance and lead-time differences in the matrix above.

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. Zinc at ~6.6 g/cm³ is the heavyweight of the group — which is why it is chosen for small dense parts where weight is irrelevant and dimensional precision is everything.

Published die-life and tooling economics differ as much as the metals themselves: zinc dies last ~1,000,000 cycles and cast on fast hot-chamber machines, while aluminum dies last ~100,000 cycles on slower cold-chamber machines (RapidDirect). If your part could be either metal, zinc is the higher-volume, tighter-tolerance, lower-risk casting — aluminum is the choice when strength-to-weight and heat handling matter.

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. RapidDirect’s published capability confirms the ladder: ±0.1 mm general, ±0.05 mm on small critical features with steel-safe tooling, and ±0.01 mm only via secondary machining (https://www.rapiddirect.com/services/die-casting/). 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.

Wall thickness is where the two processes diverge most sharply:

• Die casting needs 1.5 mm+ minimum wall to fill without cold shut; RapidDirect’s recommendation is a 1.5 mm minimum with ~0.8 mm achievable in small areas using high-fluidity alloys, and a 1:3 ratio between adjacent walls to avoid uneven filling (https://www.rapiddirect.com/services/die-casting/, https://www.rapiddirect.com/blog/aluminum-die-casting/).
• Injection molding fills 0.5 mm walls in thin enclosures (PC, ABS) but thin walls raise fill pressure and cycle time.

Draft is mandatory in both, but the details differ. RapidDirect specifies a minimum 0.5° for die castings, with deeper textures and cores needing 1–3° (https://www.rapiddirect.com/services/die-casting/); injection molding typically needs 1–2° (Protolabs, https://www.protolabs.com/resources/blog/injection-molding-tolerances/). Die-cast parts additionally 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.

Material	Tensile (typical)	Density	Service temp	Role
Die-cast Al A380	~320 MPa	~2.7 g/cm³	~200 °C	Structural, heat-shedding
Die-cast Zamak zinc	~280 MPa	~6.6 g/cm³	—	Small precise castings
Molded PPS-GF40	~150–200 MPa	~1.55–1.65 g/cm³	200–240 °C	High-temp plastic
Molded PA66-GF30	~170–200 MPa	~1.35–1.40 g/cm³	120–140 °C	Load-bearing plastic
Molded PP	—	0.90 g/cm³	100–120 °C	Enclosures, living hinges

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. The honest framing: die-cast aluminum at ~320 MPa tensile outmuscles every unreinforced plastic by an order of magnitude, but glass-filled PPS at ~150–200 MPa with 200–240 °C service covers a surprisingly wide load-bearing territory — at one-third the density.

Thermal, EMI and sealing

• Heat dissipation / EMI: choose die casting. Die-cast aluminum conducts heat and shields EMI inherently; molded plastic insulates and needs a conductive coating or metal insert for shielding. For a motor housing, a 5G box or a power module enclosure, this single row decides the process.
• Sealing surfaces: molded TPU or LSR gaskets seal better than cast metal flanges without extra machining. Die-cast flanges can be machined flat and gasketed, but that adds a secondary operation; a molded-in gasket bead seals at zero extra cost.
• Weight: molded PP at 0.90 g/cm³ beats cast aluminum at ~2.7 g/cm³ — decisive in automotive and handheld electronics, where every gram is budgeted.
• Corrosion: as-cast aluminum does not self-passivate as reliably as zinc; Xometry states plainly that aluminum parts need a coating to achieve corrosion resistance, while zinc is self-passivating and easily plated (https://www.xometry.com/capabilities/die-casting-services).

The 2026 pattern is clear: heat and EMI push parts toward metal; weight, sealing and cost push them toward plastic. When both forces act on one product, the design splits — metal where the heat is, plastic where the hand touches.

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.

• Die casting tooling cost: RapidDirect publishes USD 7,000–75,000 depending on die size, cavity count and complexity (https://www.rapiddirect.com/blog/aluminum-die-casting/).
• Plastic tooling cost: HLH Rapid quotes $3,000–6,000 for simple molds, $7,000+ for complex steel/multi-cavity tools, with total programs $10,000–100,000 (https://www.hlhrapid.com/capabilities/injection-molding/).
• Production lead time: RapidDirect publishes a 20–25 day production lead for die casting after tooling (https://www.rapiddirect.com/services/die-casting/), with T1 samples approved before production begins (Xometry, https://www.xometry.com/capabilities/die-casting-services). Molding runs at 15–60 s cycles per shot and ships in 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 structure and volume crossover

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.

Annual volume	Die casting	Injection molding
< 100K units/yr	Tool cost dominates; quote carefully	Soft tool 3–5 weeks; lower entry cost
100K–500K	Tool amortizes; metal cost visible per part	Thin-wall plastic wins on per-part cost
> 500K units/yr	Both amortize; metal for load/heat/EMI	Plastic wins enclosures on cost and weight

Two more published anchors complete the picture. FirstMold’s molding cost structure splits raw material 40–60%, processing 20–35%, mold 15–25%, post-processing 5–20% (https://firstmold.com/materials/injection-molding-materials/) — material dominates, which is why thin-wall design and high-MFI resins matter. And die casting is fundamentally a volume process: RapidDirect’s published MOQ band is 500–1,000 pieces (https://www.rapiddirect.com/services/die-casting/). Below that, CNC machining or 3D printing usually wins on total cost — the same logic as the molding side.

Rule of thumb: tooling drives the decision to die cast; per-part price drives the volume decision. If the die amortization doesn’t pencil out against machining or molding at your annual volume, die casting is the wrong process — no matter how good the quote looks.

Secondary operations and finishing

Neither process ships a finished part straight off the press.

After die casting:

• Trimming — runner, flash and gate removal; gate vestige placement is a tooling decision.
• Machining — the ±0.01 mm features, tapped holes and precision bores are machined, never cast; each machined feature adds handling and a fixture.
• Surface finishing — powder coating for durable color, chem film (Alodine) for a thin conductive conversion coating, anodizing for corrosion and wear. Two cautions from the published sources: anodizing is non-conductive, so choose chem film when grounding or EMI matters (RapidDirect), and die-cast surfaces do not produce a show-grade cosmetic anodize (Xometry, https://www.xometry.com/capabilities/die-casting-services).
• Porosity treatment — pressure-tight parts can be vacuum-impregnated with sealing resin to close interconnected porosity; or specify vacuum-assisted casting up front.
• Zinc advantage — zinc self-passivates and plates easily; aluminum needs coating for corrosion (Xometry).

After molding:

• Gate and runner trimming — automatic gates minimize the vestige.
• Molded-in color and texture — a plastic part’s color and texture are free, molded in the cycle; a metal part’s color is a coating step.
• Graphics — pad printing ($$) and silk screen ($$) for labels; laser engraving ($$) for permanent marks; painting rates ★★ and electroplating ★★★★ (electroplating-grade ABS only) on FirstMold’s finish coefficients (https://firstmold.com/materials/injection-molding-materials/, https://firstmold.com/surface-finishing/).

The planning rule is the same for both: list every secondary operation in the RFQ. Unlisted machining on a die casting — or unlisted pad printing on a molded part — is where budgets die.

Metrology and quality control

Tolerance claims are only as good as the measurement behind them.

• First article inspection (FAI) — CMM measurement of critical features against the print at T1 samples, before production runs.
• Our plant holds ±0.02 mm on controlled dimensions [OUR PLANT] and runs CMM, height gauges, moisture analyzers, pressure gauges and color controllers — the same QA equipment list FirstMold publishes (https://firstmold.com/pbt-injection-molding/).
• Porosity verification — for castings, specify the porosity acceptance standard up front (X-ray or sectioning criteria), so “acceptable porosity” is a written agreement, not a surprise at the first pressure test.
• Process capability — automotive programs hold Cpk ≥ 1.33 as the standard acceptance bar (industry practice under PPAP); our connector program shipped under IATF 16949 [OUR PLANT].
• Material traceability — metal heats ship with material certs; plastic resins with lot certs.

The deeper point is the same in both processes: tolerance is controlled through DFM, tool design and process control, not through inspection alone (Xometry’s tolerance guidance: https://www.xometry.com/resources/injection-molding/injection-molding-tolerances/). A stable die temperature on the casting side and a stable melt profile on the molding side hold tolerance far better than any end-of-line measurement.

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 die-cast frame and the molded shell are designed, quoted and measured against the same assembly print — one DFM, one CMM report, one accountable supplier. Our plant runs die casting, injection molding, mold making, CNC and stamping as in-house processes [OUR PLANT], which is exactly how the programs above shipped.

Choosing a supplier: what to audit

When the program spans metal and plastic, audit the supplier on five things:

• Certifications — IATF 16949 (automotive), ISO 13485 (medical) and ISO 9001; all three are held by our plant [OUR PLANT]. FirstMold’s automotive page cites the same IATF 16949 + ISO 9001 pairing (https://firstmold.com/industries/automotive/).
• Process breadth — does one supplier actually run die casting, injection molding, tooling and machining? Our plant: 10,000 m², 280 people, 18+3 Sodick molding machines, 9+4 wire EDM machines, 20 Aida presses [OUR PLANT]. A broker adds a middleman and a second DFM.
• In-house tooling — a shop that cuts its own dies and molds controls the DFM loop; H13 is the standard die steel for aluminum casting (RapidDirect), and the cooling-channel layout decides both cycle time and part quality.
• Metrology — CMM on site, FAI included, porosity acceptance defined, material certs on file.
• Track record — FirstMold’s published scale is 500+ global automotive projects with 30%+ client cost reduction (https://firstmold.com/automotive-injection-molding/). Ask any candidate for comparable mixed metal-plastic programs in your industry.

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.

• Injection molding service — production molding with certified resins
• Mold making service — dies and molds cut in-house
• Rapid tooling — first molded parts in 3–5 weeks
• Materials — metal and plastic selection hub
• Get a quote — upload the print for a combined DFM review

Frequently Asked Questions

1. Die casting vs injection molding — which should I choose? Metal when the part must carry load, shed heat, shield EMI or survive harsh environments; plastic when it must be light, insulating, low-cost and organically shaped. Operating temperature, load case and annual volume decide faster than any brand preference.

2. What tolerance can die casting hold? ±0.1 mm on general dimensions, ±0.05 mm on small critical features with steel-safe tooling, and ±0.01 mm only via secondary machining (RapidDirect). Injection molding holds ±0.05–0.2 mm general, with ±0.005 mm demonstrated on critical features in engineered tooling.

3. What is the minimum wall thickness for each process? Die casting: 1.5 mm recommended minimum, ~0.8 mm achievable in small areas with high-fluidity alloys (RapidDirect). Injection molding: down to 0.5 mm in thin enclosures, with 1–4 mm the comfortable design band.

4. Is die-cast metal stronger than molded plastic? Yes, by an order of magnitude on raw tensile: die-cast Al A380 at ~320 MPa and Zamak zinc at ~280 MPa versus unreinforced plastics in the tens of MPa. But glass-filled PPS (~150–200 MPa) and PA66-GF30 (~170–200 MPa) carry real load at one-third the density.

5. Does die casting cost more than injection molding? Up front, yes — dies run $7,000–75,000 versus plastic tools $3,000–6,000 simple to $7,000+ complex. At volume both amortize; below 100K units/year the die’s tool cost dominates, and thin-wall plastic wins enclosures on per-part cost above 500K.

6. How long does a die casting die last? About 100,000 cycles for aluminum and magnesium dies; about 1,000,000 cycles for zinc dies (RapidDirect). Zinc also casts faster on hot-chamber machines — worth knowing when the part could be either metal.

7. Why is aluminum die cast on cold-chamber machines? Molten aluminum at ~660 °C would destroy a hot-chamber gooseneck, so aluminum is cast on cold-chamber machines; hot-chamber casting is limited to low-melting-point zinc and magnesium (RapidDirect).

8. Can die-cast aluminum be anodized? Yes — Type II and Type III anodizing are standard, but die-cast surfaces do not produce a show-grade cosmetic anodize (Xometry), and anodize is non-conductive. For conductive corrosion protection choose chem film (Alodine); for cosmetics, machine the visible faces or specify a low-iron alloy.

9. What causes porosity in die castings and how do you prevent it? Turbulent fill traps air (gas porosity) and solidification shrinkage creates voids in thick sections. Countermeasures: uniform walls, optimized shot profile, vacuum-assisted casting, semi-solid casting, and resin impregnation for pressure-tight parts. Specify the porosity acceptance standard up front.

10. Can a product use both die casting and injection molding? Yes — and many should. A molded plastic housing over a die-cast internal frame or heatsink is standard: our HV busbar (PA6-GF30 over C11000 copper, 250K+ units/year) and automotive connector (PPS/PA66 at ±0.005 mm, 2M units/year) both mix metal and plastic in one program.

11. What is the typical MOQ for die casting? RapidDirect’s published band is 500–1,000 pieces (https://www.rapiddirect.com/services/die-casting/). Below that, CNC machining or 3D printing usually wins on total cost; above it, die casting amortizes the die.

12. How long does a die casting project take? Tooling 6–12 weeks plus a published 20–25 day production lead (RapidDirect), with T1 samples approved before production (Xometry). Injection molding is faster to first parts: 3–5 weeks soft tool, 8–12 weeks hardened production tool.

Sources

• RapidDirect — Aluminum Die Casting: A Comprehensive Guide: https://www.rapiddirect.com/blog/aluminum-die-casting/
• RapidDirect — Custom Die Casting Services (tolerance ladder, wall 1.5 mm / 0.8 mm, 1:3 ratio, MOQ 500–1000, 20–25 day lead, die life): https://www.rapiddirect.com/services/die-casting/
• Xometry — Custom Die Casting Services (aluminum needs coating, zinc self-passivates, T1 approval): https://www.xometry.com/capabilities/die-casting-services
• Xometry — Injection Molding Tolerances (DFM + process control): https://www.xometry.com/resources/injection-molding/injection-molding-tolerances/
• Protolabs — Injection Molding Tolerances (tool ±0.003 in, resin ±0.002 in/in, draft 1–2°): https://www.protolabs.com/resources/blog/injection-molding-tolerances/
• FirstMold — Automotive Injection Molding (IATF 16949, 500+ projects): https://firstmold.com/industries/automotive/
• FirstMold — PBT Injection Molding (QA equipment list): https://firstmold.com/pbt-injection-molding/
• FirstMold — Injection Molding Materials (cost structure 40-60/20-35/15-25/5-20%, finish coefficients): https://firstmold.com/materials/injection-molding-materials/
• FirstMold — Surface Finishing (price tiers): https://firstmold.com/surface-finishing/
• HLH Rapid — Injection Molding (mold cost $3,000–6,000 / $7,000+ / $10,000–100,000): https://www.hlhrapid.com/capabilities/injection-molding/
• Alloy and material values (tensile, density, service temperature): typical published values per NADCA standards, JIS H 5302, and alloy/resin supplier datasheets; exact values vary by heat and grade. Plant capability values marked [OUR PLANT] are MOLDITQUICK verified internal values.