Die casting and injection molding are siblings: both force molten material into a steel tool under high pressure, both produce precision parts at volume, both punish sloppy design with defects that surface only in production. The sibling resemblance ends at the material (one runs metal, the other runs plastic), and that single difference decides almost everything downstream.
This guide gives you the decision framework with real numbers, not slogans.
1. The One-Page Comparison
| Factor | Die Casting | Injection Molding |
|---|---|---|
| Material | Aluminum, zinc, magnesium alloys | Thermoplastics and filled resins |
| Tooling cost | USD 15,000 – 150,000 | USD 10,000 – 100,000 |
| Cycle time | 20 – 90 seconds | 15 – 60 seconds |
| Typical wall thickness | 1 – 6 mm | 0.5 – 4 mm |
| Break-even volume | 5,000 – 50,000+ parts | 1,000 – 10,000+ parts |
| Part strength | High, metal structural parts | Lower, polymer dependent |
| Heat resistance | Excellent | Limited by resin (typically 80–120°C) |
| Electrical insulation | No (conductive) | Yes (inherent) |
| Weight | Heavier (metal density) | Light |
| Surface finish | Good as-cast; often machined or coated | Excellent as-molded |
| EMI shielding | Yes (metal) | No (requires coating) |
2. The Wall-Thickness Rule That Trips Everyone Up
Here is the difference that most design guides bury: the two processes demand opposite things from wall thickness.
- Injection molding demands uniform walls to control cooling. When walls vary significantly, thick areas retain heat longer than thin ones, creating internal stress that causes warping after ejection, sink marks above thick sections, and structural weakness that only appears under load. Where a section needs strength, use ribs and gussets (or core out the thick area), rather than adding solid mass.
- Die casting demands adequate wall thickness to ensure complete metal fill. Molten metal must travel through the entire die cavity and reach every corner before it solidifies. Walls too thin restrict flow, causing incomplete fills and surface defects. Walls too thick trap gas as the metal cools, creating internal porosity. The minimum achievable wall depends on the alloy, zinc flows more freely than aluminum and can achieve thinner walls.
Both processes punish poor wall-thickness decisions with defects that are difficult and costly to correct once tooling is in production. Get this decision right at the design stage and both processes behave; get it wrong and neither does.
3. Cost Reality: It Is Not Just Per-Part
Buyers consistently compare per-part price and stop. The full picture includes tooling amortization, cycle time, and secondary operations:
| Cost Element | Die Casting | Injection Molding |
|---|---|---|
| Material | Aluminum ingot ~USD 2–5/kg | Resin ~USD 1–4/kg (varies widely by grade) |
| Machine hour cost | Higher, die cast cells cost more than molding presses of similar tonnage | Lower |
| Secondary operations | Often trim dies plus machining, adds unit cost until secondary ops replace separate metal parts | Usually minimal for as-molded parts |
| Small housing example | ~USD 1.50 die cast | ~USD 0.40 molded |
The verdict on cost: per part, injection molding is usually cheaper when the part can be plastic. Die casting is the economical choice for high-volume metal parts, where no plastic process can deliver the required strength. And remember the break-even: die casting tooling is roughly 1.5x the cost of injection tooling and needs roughly 5x the volume to amortize.
4. When to Choose Die Casting
- The part must carry structural load
- Heat dissipation matters (heat sinks, thermal components)
- The product needs a metal housing or frame
- EMI shielding is required
- Dimensional stability under load or elevated temperature is a requirement
- Production volume is high enough (5,000–50,000+) to justify tooling
- The application is industrial, automotive, or power-related
5. When to Choose Injection Molding
- The part is primarily plastic and does not require metal properties
- Weight reduction is a design requirement
- Electrical insulation is needed
- Cosmetic appearance matters (as-molded finish, color integration)
- The part includes snap-fits, clips, ribs, living hinges or other integrated plastic features
- Tooling lead time and investment need to be minimized relative to die casting
- Production volume is medium (1,000–10,000+) with per-part cost efficiency
6. Design Rules Each Process Demands
| Die Casting Design Rules | Injection Molding Design Rules |
|---|---|
| Keep wall thickness adequate for complete fill | Keep wall thickness uniform (±25%) |
| Avoid sharp corners (stress + flow interruption) | Use ribs instead of thick walls |
| Add draft angles for ejection | Add proper draft (1°/side + texture allowance) |
| Plan for trimming and finishing | Position gates carefully (weld lines off critical zones) |
| Design uniform walls where possible (shrink control) | Select resin by use environment, not only price |
7. The Verdict
There is no universal winner, only the right process for the part in front of you:
- Plastic part, weight and unit cost matter, medium-to-high volume? → Injection molding.
- Metal part for strength, heat or shielding, high volume? → Die casting.
- Still deciding? → Evaluate the part with a manufacturing partner that can review material, structure and volume requirements before tooling starts. That review costs nothing and routinely saves a tooling mistake.
One practical note: most hardware products eventually contain both metal and plastic parts. Choosing a supplier with die casting, injection molding and CNC machining under one roof means the metal-plastic boundary in your BOM stops being a supplier boundary. Send us the part function, volume and performance requirements, we will tell you which process the physics favors, including when the answer is the one that earns us less.