Contents
One of the biggest decisions in product development is picking the right manufacturing process. Two of the most common methods for making high-volume parts are injection molding and die casting.
They look similar, but they serve different needs. Picking the wrong one can cost you time and money. This guide compares both processes on materials, costs, tolerances, and applications. Use it to make the right choice.
What Is Injection Molding?
Injection molding makes plastic parts. Melted plastic is injected into a metal mold under high pressure. It cools and hardens into the final shape.
How It Works
- Plastic pellets go into a heated barrel.
- A screw melts and mixes the plastic.
- The liquid plastic is shot into a closed mold.
- The part cools and solidifies.
- The mold opens and ejector pins push the part out.
Each cycle takes seconds to minutes, depending on part size.
Materials
- ABS: Tough impact resistance. Used for consumer goods, car interiors.
- Polypropylene (PP) : Chemical resistant, flexible. Used for containers, car parts.
- Polycarbonate (PC) : Strong and clear. Used for lenses, electronics housings.
- Nylon (PA) : Great wear resistance. Used for gears, bearings.
- Acrylic (PMMA) : Optical clarity. Used for displays, lights.
- TPE/TPU: Soft, rubber-like. Used for grips, seals.
What Is Die Casting?
Die casting is the metal version of injection molding. Molten metal is forced into a steel die under high pressure.
How It Works
- The die is clamped and lubed.
- Molten metal is shot in at high pressure (up to 25,000 PSI).
- The metal cools fast against the die.
- The die opens and the part is ejected.
- Extra material (flash, runners) is trimmed off.
Materials
- Aluminum (A380, A383, ADC12) : Light, corrosion resistant. The most common choice.
- Zinc (Zamak 3, Zamak 5) : Very accurate, high ductility. Good for thin-wall parts.
- Magnesium (AZ91D) : Ultra-light. Used in cars and aerospace.
- Copper alloys: High strength. For special uses.
Side-by-Side Comparison
1. Materials
| Factor | Injection Molding | Die Casting |
| Material | Plastic | Metal (Al, Zn, Mg, Cu) |
| Weight | Light | Heavy |
| Strength | Moderate | High |
| Heat resistance | Low to moderate | High |
| UV resistance | Limited | Excellent |
| Recyclable | Good (thermoplastics) | 100% recyclable |
2. Tooling Costs
Injection molding: $3,000 to $100,000+. Molds last 100,000 to 1M+ cycles. Can make very complex shapes.
Die casting: $5,000 to $150,000+. Dies last 50,000 to 500,000+ shots. Good for complex shapes but limited by die opening direction.
Bottom line: Injection molding tooling is usually cheaper for small parts. The gap narrows for large parts.
3. Part Cost
Injection molding: Very low per-part price at high volume. Fast cycles (5-60 seconds). No extra work needed for most parts.
Die casting: Low to moderate per-part price. Slower cycles (30-120 seconds). May need extra steps like machining or coating.
Bottom line: At 10,000+ parts, injection molding is usually cheaper. Die casting wins when you would need to assemble many plastic parts into one metal part.
4. Tolerances
| Factor | Injection Molding | Die Casting |
| Standard tolerance | ±0.005 inches | ±0.002 inches |
| Precision tolerance | ±0.001 inches | ±0.001 inches |
| Wall thickness | 0.030-0.250 inches | 0.040-0.200 inches (Al) |
| Draft angle needed | 0.5-2 degrees | 1-3 degrees |
Die casting usually gives tighter tolerances, especially with zinc.
5. Volume Break-Even
- Under 1,000 parts: Neither is ideal. Try 3D printing (plastic) or machining (metal).
- 1,000-10,000 parts: Injection molding works with aluminum tooling. Die casting is borderline.
- 10,000-100,000+ parts: Both work well. Tooling cost is spread across enough parts.
- 100,000+ parts: Both excel. Pick based on material needs.
When to Choose Each
Choose Injection Molding When:
- You need plastic (lightweight, insulates, flexible, clear).
- You need complex shapes with undercuts or snap-fits.
- The part won’t face extreme heat.
- Weight matters.
- You want color molded in without painting.
Choose Die Casting When:
- You need metal strength or heat conductivity.
- The part will face high temperatures.
- You need electromagnetic shielding.
- You need stable dimensions over time.
- The part is for automotive, aerospace, or industrial use.
Surface Finishing
Injection Molding
- As-molded: Textured, glossy, or matte.
- Painting: For matching colors.
- Pad / screen printing: For labels and graphics.
- Laser etching: For permanent marks.
- Plating: For a metallic look.
Die Casting
- As-cast: Clean but may have small pores.
- Machining: For precise surfaces.
- Powder coating: For color and corrosion protection.
- Anodizing: For aluminum — wear and color.
- Electroplating: For chrome or nickel.
- Vibratory finishing: For smoothing and deburring.
Conclusion
Both processes are reliable and produce quality parts at scale. The choice comes down to materials — plastic for light, flexible, insulating parts, and metal for strong, heat-resistant parts.
For many products, the best answer is both: die-cast metal parts for structure or heat, plus injection-molded plastic for housings and handles.