Quick Answer
Gravity die casting — gravity die casting (permanent mold casting) uses a reusable steel die filled by gravity to produce aluminum, magnesium, and copper alloy parts at moderate volume (500–10,000/year). It delivers better surface finish and tolerance than sand casting, lower porosity than HPDC, and lower tooling cost than high pressure die casting. It is the preferred process for pressure-tight aluminum parts and non-aluminum alloys.
What Is Gravity Die Casting?
Gravity die casting pours molten metal into a steel die (mold) using only gravity — no pressure injection. The die is typically made from tool steel or alloy steel and is reused for many production cycles.
Process steps:
- Prepare the die: coat with release agent, preheat to 150–300°C
- Close the die
- Pour molten metal from ladle or pour cup into the die cavity (gravity fills the cavity)
- Metal cools and solidifies inside the die
- Open the die, eject the casting
- Remove the casting, inspect, clean up flash
Gravity Die Casting vs Other Processes
| Factor | Gravity Die | Sand Casting | HPDC | Investment |
|---|---|---|---|---|
| Tooling cost | $10,000–$50,000 | $500–$15,000 | $50,000–$150,000 | $5,000–$30,000 |
| Per-part cost | Moderate | Low (high labor) | Low (at volume) | Moderate-High |
| Volume range | 500–10,000/year | 10–500/year | 10,000+/year | 50–5,000/year |
| Surface finish | Ra 3.2–12.5 µm | Ra 12.5–25 µm | Ra 0.8–3.2 µm | Ra 3.2–6.3 µm |
| Tolerance | ±0.2–0.5 mm | ±0.5–1.0 mm | ±0.1–0.25 mm | ±0.1–0.3 mm |
| Porosity | Low | Low | High (needs VAD) | Low |
| Minimum wall | 2–3 mm | 3–5 mm | 0.8–1.5 mm | 1.5–2 mm |
| Alloys | Al, Mg, Cu, Brass | Any metal | Al, Zn, Mg | Any metal |
Alloys Used in Gravity Die Casting
Aluminum Alloys
Most common gravity die casting alloys:
| Alloy | Properties | Applications |
|---|---|---|
| A356 | Good strength, excellent castability, weldable | Automotive, general |
| A357 | Higher strength (T6), pressure-tight | Automotive engine, aerospace |
| 319 | Good castability, moderate strength | Automotive, general |
| B390 | High wear resistance, low thermal expansion | Pistons, compressor housings |
Copper Alloys
Gravity die casting handles copper alloys that HPDC cannot:
- C93200 (bearing bronze): Bearing surfaces, wear components
- C95400 (aluminum bronze): High strength, corrosion resistance
- C83600 (red brass): Valves, fittings, plumbing
Tolerance and Surface Finish
Typical Tolerances
| Dimension Type | Tolerance | Notes |
|---|---|---|
| Standard dimensions | ±0.2–0.5 mm per 100 mm | Better than sand casting |
| Best achievable | ±0.1 mm per 100 mm | Precision tooling required |
| Wall thickness | ±0.2–0.4 mm | Good uniformity |
| Flatness | 0.1–0.3 mm per 100 mm | Good as-cast flatness |
Surface Finish
Gravity die casting produces smooth surfaces:
- As-cast: Ra 3.2–12.5 µm (depends on die surface finish and coating)
- Improved: Ra 1.6–3.2 µm achievable with polished die and proper coating
- After machining: Ra 0.8–3.2 µm (machine critical surfaces)
Porosity in Gravity Die Casting
Gravity die casting has significantly lower porosity than HPDC because:
- Gravity fill does not trap gas in the molten metal
- Slower fill speed means better metal quality
- Parts are naturally pressure-tight in most cases
Pressure-tight applications: Gravity die casting is the preferred process for parts that must hold pressure — hydraulic valve bodies, pump housings, engine components — without the cost of vacuum-assisted HPDC.
When Porosity Can Occur
- Very thick sections (shrinkage porosity)
- Improper gating (turbulent filling)
- Die temperature too cold (premature solidification)
Die Design and Life
Die Material
- H13 tool steel: Most common; good wear resistance and thermal fatigue resistance
- Die life: 50,000–200,000 shots depending on alloy, part design, and die material
Die Cooling
Proper die cooling is critical for cycle time and quality:
- Water cooling channels: Remove heat from die between shots
- Air cooling: For smaller parts or between-cycle cooling
- Proper cooling: Reduces cycle time, improves surface finish, reduces热 stress in die
Design Rules for Gravity Die Casting
- Minimum wall thickness: 2–3 mm for aluminum (thinner possible with A356)
- Draft angle: 1–2° minimum on external, 2–3° on internal
- Uniform walls: Graduated transitions for thickness changes
- Fillet radii: Minimum 1–2 mm internal radius
- Parting line: Flat or simple is preferred
- Undercuts: Possible with slides but adds cost
Cost Factors
| Factor | Cost Impact |
|---|---|
| Die material (H13 steel) | $10,000–$50,000 depending on size and complexity |
| Die size (part weight) | Larger parts need larger, more expensive dies |
| Part complexity | Cores and slides add cost |
| Surface finish requirement | Polished die adds cost |
| Volume | Higher volume justifies higher tooling investment |
Before finalizing the sourcing decision, many OEM buyers also compare Aluminum Casting Buyer Guide, HPDC vs Gravity Die, Casting Tolerances, and Surface Finish Guide to clarify process fit, cost trade-offs, tolerance expectations, and supplier risk.
If you need application-specific guidance, drawing review, or a quotation, you can Contact YCUMETAL.
Buyer Checklist
- Is gravity die casting the right process for your volume (500–10,000/year)?
- Does the part need to be pressure-tight?
- What surface finish is required? Is Ra 3.2–12.5 µm acceptable?
- What tolerance is needed on each feature?
- Is the material aluminum, or does it require copper alloy (gravity die can handle it)?
- Who owns the die? Can you transfer it?
- What is the die life guarantee?
FAQ
What is the difference between gravity die casting and low pressure die casting?
Gravity die casting fills the die by gravity from a ladle. Low pressure die casting (LPDC) pressurizes a crucible of molten metal (typically 0.1–0.3 bar) to push metal up through a tube into the die — this gives more controlled filling and less turbulence than gravity pour. LPDC has slightly higher tooling cost and is used for very large parts (up to 100 kg) and higher quality requirements.
Can gravity die casting produce thin walls?
Thinner than sand casting (2–3 mm vs 3–5 mm minimum), but not as thin as HPDC (0.8–1.5 mm). For very thin walls, HPDC is the better choice at high volume.