Buyers often compare injection molding and die casting as if they were interchangeable, choosing purely on price. They are not interchangeable. The materials behave differently in ways that decide the project long before cost-per-part does. Here is the honest comparison.
The Fundamental Difference: Polymer vs Metal
Plastic injection pushes a molten polymer (typically 200–300 °C) into a steel mold. Die casting forces a molten aluminum or zinc alloy (roughly 580–660 °C for A380/ADC12 aluminum, ~400 °C for Zamak 3) into a tool steel die. That temperature gap drives everything: die casting tools live a brutal thermal-cycling life, plastics mold faster, and the finished parts occupy different strength classes entirely.
Strength and Stiffness
- A380 aluminum: tensile strength ~320 MPa, elastic modulus ~71 GPa.
- ADC12: similar, slightly better castability, commonly used in Asia-sourced parts.
- ABS: ~40 MPa tensile, modulus ~2.3 GPa. PC: ~60–65 MPa, ~2.4 GPa. Glass-filled nylon 6 at 30% fiber reaches ~160 MPa but becomes anisotropic and warps.
Rule of thumb: no engineering polymer comes close to aluminum stiffness. If a bracket must not flex under a few hundred newtons, the decision is made. But stiffness can mislead — for a housing that just needs to survive a drop, PC or ABS absorbs impact better than a cast aluminum corner, which tends to crack at sharp internal radii.
Weight and Thermal Behavior
Aluminum die castings are roughly 2.7 g/cm³ versus 1.05–1.2 for ABS/PC — parts feel substantially heavier, which sometimes matters in handheld products. On the thermal side there is no contest: aluminum conducts ~100 W/m·K versus ~0.2 for polymers. Heat-sink housings, LED fixtures, and motor end bells are die casting territory, full stop. Plastics win where electrical insulation is required and where metallic EMI characteristics are unwanted.
Surface Finish, Detail, and Post-Processing
Draft and radii: die castings need 1–3° draft (more on deep cores) and fillets of at least 1 mm to avoid hot-tearing. Injection molded parts can tolerate 0.5° on shallow features and capture finer texture directly from the mold. Coating behavior differs sharply too — anodizing works on aluminum but tends to look blotchy on high-silicon casting alloys like ADC12, while plastics accept painting, plating (with etch pretreatment), and in-mold texture with less drama.
Tooling and Volume Economics
Both processes need hard tooling, but the tool costs differ in character. A simple aluminum-alloy die cast tool often starts lower than a fully hardened plastic injection mold because castings tolerate fewer cavities and simpler gating; however, shot cycles of 45–120 s versus 20–60 s for plastics eat into per-part cost at volume. Typical break-even thinking:
- Under ~1,000 parts: both tooling routes are painful — consider vacuum casting or 3D printing first.
- 1,000–50,000: decide on material requirement, not price. If aluminum's stiffness/thermal performance is needed, casting pays for itself; if not, plastic usually molds cheaper per part at equal volume.
- Above ~50,000: multi-cavity tools amortize fast in either process; cycle time and secondary machining decide the winner.
Common Selection Mistakes
- Choosing plastic to save tooling cost, then adding a metal stiffener plate — the assembly ends up costlier than one casting.
- Specifying ADC12 and expecting cosmetic anodizing — porosity and silicon make consistent color nearly impossible.
- Ignoring draft on cast ribs, then paying for secondary CNC machining on every face that should have been cast clean.
How We Decide
Start with the requirement that cannot be negotiated: temperature exposure, stiffness per gram, insulation, or drop survival. Then price both routes with honest tooling quotes. Protom's die casting services and plastic injection molding teams quote side by side, so you can compare real numbers — and if you are still at prototype stage, our vacuum casting line bridges the gap while tools are cut.
