Last updated: 2026-09-05 · 5 min read

Injection Molding vs. Die Casting — What Are the Seven Decision Parameters That Determine Which Process Wins for Your Part?

Injection molding makes plastic parts at 150–350°C in cooled molds (cycle 10–60s, tolerance ±0.05mm, tooling $2,000–15,000). Die casting makes metal parts at 600–1,100°C in steel dies (cycle 5–30s, tolerance ±0.1mm, tooling $5,000–50,000). The material — plastic or metal — decides the process. Within that constraint, seven parameters determine whether the part geometry, volume and cost target favor one process over the other.

Key Facts

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  • Material is the first filter: if the part must be metal (for strength, conductivity, heat resistance or EMI shielding), die casting is the only option. If the part can be plastic (for weight, corrosion resistance, color or cost), injection molding is the default. There is no overlap — the material requirement decides the process before any other parameter is evaluated.
  • Tooling cost gap: a die casting die costs 2–5× more than an equivalent-size injection mold because the die must withstand 600–1,100°C molten metal (H13 steel, heat-treated to 48–52 HRC) versus 150–350°C plastic (P20 or 718 steel at 30–40 HRC). The die casting die also requires a more complex gating system and ejector design to handle the higher ejection forces.
  • Per-part cost crossover: at volumes below 5,000 parts/year, the lower tooling cost of injection molding wins even if the material could be die cast. Above 50,000 parts/year, die casting’s faster cycle (5–30s vs. 10–60s) and lower material waste (no runner in most die casting) can offset the higher tooling amortization. Between 5,000 and 50,000, the decision depends on the part’s complexity and material requirements.

    • Cavity — The hollow space inside the mold that forms the part shape. Multi-cavity molds produce multiple parts per shot. Cavity count determines production output but also mold cost and complexity. Each cavity must be balanced for uniform fill. The related guide covers adjacent topics.

    How Do the Two Processes Compare Across Seven Decision Parameters?

    Parameter Injection Molding Die Casting
    Material Thermoplastics, LCP, PC, ABS, nylon Aluminum, zinc, magnesium alloys
    Melt temperature 150–350°C 600–1,100°C
    Mold/die steel P20 or 718 (30–40 HRC) H13 (48–52 HRC, heat-treated)
    Cycle time 10–60 seconds 5–30 seconds
    Typical tolerance ±0.05mm ±0.1mm
    Tooling cost $2,000–15,000 $5,000–50,000
    Volume threshold 1,000+ parts/year 5,000+ parts/year

    The tolerance gap (±0.05mm vs. ±0.1mm) matters for assembly-critical dimensions. If the part must mate with another component at ±0.03mm, injection molding can hold that tolerance directly; die casting cannot and requires secondary machining — which adds cost and eliminates die casting’s cycle time advantage.

    What Three Design Rules Are Specific to Die Casting That Do Not Apply to Injection Molding?

    1. Wall thickness must be uniform — thick sections cause porosity. Unlike injection molding where thick sections can be packed out with holding pressure, die casting has no holding phase — the molten metal solidifies almost instantly on contact with the die.
      A thick section shrinks as it cools and creates an internal void (porosity) that weakens the part. Keep wall thickness between 1.5–3.0mm for aluminum and 0.6–2.0mm for zinc, with transitions no greater than 0.5mm.
    2. Draft angles must be larger — 1–3° minimum versus 0.5–1° for injection molding. The molten metal shrinks onto the core as it solidifies, creating higher ejection forces than plastic parts.
      Insufficient draft causes the part to stick to the die, and the ejector pins tear through the part wall. For deep cores (depth >3× the width), increase draft to 2–3° to ensure reliable ejection.
    3. Parting line flash is harder to control — the higher melt pressure and temperature wear the parting line faster. Die casting injection pressure reaches 30–150 MPa (similar to injection molding), but the molten metal erodes the die steel at the parting line over time.
      After 50,000–100,000 shots, the parting line gap widens and flash appears. The die maker must specify a parting line maintenance schedule — resurfacing the parting line every 30,000–50,000 shots prevents flash from becoming a reject issue.

    Shrinkage — The dimensional reduction from mold cavity to cooled part. Amorphous resins: 0.4–0.7% (uniform).
    Semi-crystalline: 1.0–2.5% (directional). Mold makers apply shrinkage factor when machining cavity. Actual shrinkage varies with wall thickness, gate location and packing pressure. The related guide covers adjacent topics.

    Injection molding: plastic, 150–350°C, P20/718 steel, cycle 10–60s, tolerance ±0.05mm, tooling $2K–15K. Die casting: metal (Al/Zn/Mg), 600–1,100°C, H13 steel, cycle 5–30s, tolerance ±0.1mm, tooling $5K–50K. Material decides the process. Volume threshold: injection 1K+, die casting 5K+.

  • Key parameter identified — critical variable confirmed against spec.
  • Tolerance within achievable range — commercial ±0.15mm or tighter as needed.
  • Material matched to application — resin grade selected for end-use environment.
  • Process window validated at T1 — parameters locked after first trial.
  • Inspection method defined — CMM, visual, or functional test specified.

    • Frequently Asked Questions

    Can a part be made by both processes — for example, a plastic-over-metal assembly?

    Yes — overmolding plastic onto a die-cast metal insert is common for housings that need metal strength with plastic cosmetic surfaces. The die-cast insert is placed into the injection mold, and plastic is molded over it. The two materials bond mechanically (undercuts in the metal insert) or chemically (adhesive applied before overmolding).

    The design must account for the different thermal expansion rates — metal expands 3–5× less than plastic when heated. If the plastic overmold is constrained by the metal insert, differential expansion causes warpage or stress cracks. The mold designer should specify the clearance between the plastic and metal at both assembly temperature and operating temperature.

    Why is die casting tooling so much more expensive than injection molding tooling?

    Three reasons: die steel (H13 costs 2–3× more than P20 per kilogram), heat treatment (H13 must be hardened to 48–52 HRC, adding $1,000–3,000 per die), and machining difficulty (H13 at 48 HRC requires EDM and slow-speed CNC versus P20 at 30 HRC which machines freely). The die also requires more complex cooling channels because the molten metal solidifies faster and the die must absorb and dissipate heat quickly.

    For a medium-size mold (300×400mm), the total tooling cost is typically $8,000–20,000 for die casting versus $3,000–8,000 for injection molding. The gap widens for larger molds and multi-cavity dies.

    At what annual volume does die casting become cheaper per part than injection molding?

    For metal parts, die casting is always cheaper per part than machining from solid — the question is whether die casting or injection molding (with a metal-filled plastic) can meet the performance requirements. If the part must be metal for strength or conductivity, die casting is the only option regardless of volume.

    If the part can be either metal or plastic, the crossover is typically at 50,000–100,000 parts/year — below that, injection molding a glass-filled or mineral-filled plastic is cheaper (lower tooling, faster total lead time); above that, die casting’s lower per-part cost offsets the higher tooling. The exact crossover depends on the part size, material cost and secondary operations required.

    Key Takeaways

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  • Material decides: plastic → injection; metal → die casting.
    No overlap.
  • Tooling: injection $2K–15K (P20/718); die casting $5K–50K (H13, heat-treated).
  • Tolerance: injection ±0.05mm; die casting ±0.1mm (secondary machining if tighter).
  • Die casting rules: uniform walls 1.5–3mm, draft 1–3°, parting line maintenance every 30–50K shots.

    • Send the part drawing with the material requirement — if the material is plastic, the quotation covers injection molding; if metal, die casting.
    If the material is open, the quotation compares both processes with per-part cost at your annual volume so the decision is based on documented numbers, not assumptions.

    Sources: NADCA — North American Die Casting Association, Die Casting Design Standards and Process Guidelines · Plastics Technology — Injection Molding Process Parameters and Tooling Cost Data · Society of Plastics Engineers (SPE) — Process Comparison and Material Selection Guidelines · MoldMaking Technology — Tooling Cost Comparison Between Injection Molds and Die Casting Dies · Dynacast — Die Casting Design Handbook and Engineering Standards

    — Arthur, Tooling Engineer at aCatMould