Quick Answer
Permanent mold casting — permanent mold casting (also called gravity die casting) uses a reusable steel die (mold) to produce aluminum, magnesium, copper alloy, and iron parts at moderate-to-high volume. The die is not consumed — it is used repeatedly like a die casting die, but without high-pressure injection. It delivers better surface finish and tolerance than sand casting, lower porosity than high pressure die casting, and handles alloys that HPDC cannot.
Permanent Mold Casting vs Other Casting Processes
| Factor | Permanent Mold (Gravity) | Sand Casting | HPDC | Investment Casting |
|---|---|---|---|---|
| Mold material | Steel (reusable) | Sand (consumable) | Steel (reusable) | Ceramic shell (consumable) |
| Fill method | Gravity or low pressure | Gravity only | High pressure injection | Gravity (lost wax) |
| Tooling cost | $10,000–$50,000 | $500–$15,000 | $50,000–$150,000 | $5,000–$30,000 |
| 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 | Higher (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 |
| Undercuts | With slides (expensive) | Natural with cores | With slides | With ceramic cores |
Permanent Mold Casting Processes
Gravity Die Casting
Molten metal is poured from a ladle into the open die, filling by gravity alone:
- Simplest permanent mold process
- Lower tooling cost than low pressure
- Higher labor content: Manual pour, manual die handling
- Best for: Aluminum and copper alloy parts at moderate volume
Low Pressure Die Casting (LPDC)
Molten metal is pressurized in a crucible (typically 0.1–0.3 bar) and pushed up through a tube into the die cavity:
- More controlled filling: Less turbulence, lower porosity than gravity pour
- Better surface: Smoother fill, less oxide inclusion
- Higher tooling cost: Pressurized system, more complex die design
- Better for: Larger parts, pressure-tight parts, higher quality requirements
Squeeze Casting (Solid Die Casting)
Metal is poured into the die and high pressure (50–100 MPa) is applied during solidification:
- Combines gravity fill + pressure compaction
- Very low porosity: Pressure eliminates shrinkage porosity
- Excellent mechanical properties: Near-forging quality
- Higher tooling cost: Hydraulic press required
- Best for: High-strength structural aluminum parts
Materials for Permanent Mold Casting
Aluminum Alloys (Most Common)
| Alloy | Properties | Applications |
|---|---|---|
| A356 | Good strength, excellent castability, weldable | Automotive wheels, structural parts |
| A357 | Higher strength (T6 heat treatment), pressure-tight | Automotive engine, aerospace |
| 319 | Good castability, moderate strength | Automotive engine brackets, housings |
| B390 | High wear resistance, low thermal expansion | Compressor housings, pistons |
| A413 | Excellent castability, thin walls | Complex thin-wall housings |
Magnesium Alloys
AZ91D and AM60B are cast in permanent molds for automotive and electronics:
- Extremely light: 35% lighter than aluminum
- Good strength: Comparable to aluminum
- Corrosion: Requires protective treatment
- Applications: Automotive instrument panels, seat frames, electronics housings
Copper Alloys (Brass and Bronze)
Permanent mold is the preferred process for copper alloy castings:
- C93200 (bearing bronze): Bearing surfaces, wear components
- C95400 (aluminum bronze): High strength, corrosion resistance
- C83600 (red brass): Valves, fittings, plumbing hardware
Design Rules for Permanent Mold Casting
Wall Thickness
| Part Size | Minimum Wall (mm) | Recommended Wall (mm) | Uniformity |
|---|---|---|---|
| Small (< 100mm) | 2.0 mm | 3–4 mm | Max 2:1 transition |
| Medium (100–300mm) | 2.5 mm | 4–6 mm | Max 2.5:1 transition |
| Large (> 300mm) | 3.0 mm | 5–8 mm | Max 3:1 transition |
Draft Angle
| Surface Type | Minimum Draft | Recommended Draft |
|---|---|---|
| External surfaces | 1° | 1.5–2° |
| Internal surfaces (cores) | 2° | 2–3° |
| Deep sections (> 50mm) | 2° | 2.5–3° |
Other Design Requirements
- Fillet radii: Minimum 1–2 mm internal radius; larger is better
- Bosses: Reinforced around holes to prevent sink marks
- Gating: Gate location critical for surface quality; away from appearance surfaces
- Cooling: Die cooling channels are critical for cycle time and quality
Permanent Mold vs HPDC: When to Choose Each
| Requirement | Permanent Mold (Gravity/LP) | HPDC |
|---|---|---|
| Pressure-tight parts | ✅ Natural (gravity fill, low porosity) | ⚠️ Needs vacuum assist (VAD) |
| Non-aluminum alloys (Cu, Brass) | ✅ Yes | ❌ No (Al, Zn, Mg only) |
| Very high volume (50,000+/year) | ⚠️ 10,000 max practical | ✅ Best |
| Moderate volume (1,000–10,000/year) | ✅ Best fit | ⚠️ Tooling may not amortize |
| Very thin walls (0.8–1.5 mm) | ❌ Minimum 2–3 mm | ✅ Best |
| Excellent surface finish | ⚠️ Ra 3.2–12.5 µm (good) | ✅ Ra 0.8–3.2 µm (excellent) |
| Lower tooling cost | ✅ $10,000–$50,000 | ⚠️ $50,000–$150,000 |
Cost and Volume Considerations
Tooling Cost Breakdown
| Component | Cost Range | Notes |
|---|---|---|
| Steel die (H13) | $8,000–$40,000 | Core and cavity, depending on size |
| Die cooling system | $2,000–$10,000 | Water channels, fittings |
| Ejector system | $1,000–$5,000 | Ejector pins, sleeves |
| Pattern plate/fixture | $500–$2,000 | For molding machine |
| Total tooling | $10,000–$50,000 | Typical range |
Per-Part Cost by Volume
| Volume | Typical Per-kg Cost | Notes |
|---|---|---|
| 500 parts/year | $8–$15/kg | Tooling amortized over 500 units |
| 2,000 parts/year | $4–$8/kg | Good amortization |
| 5,000 parts/year | $3–$6/kg | Approaching HPDC territory |
| 10,000 parts/year | $2–$5/kg | HPDC becomes competitive |
Real Buyer Scenarios
Here are three real-world buyer decisions involving permanent mold casting:
Scenario 1: Automotive Hydraulic Pump Housing
Requirement: 5,000 aluminum pump housings per year, must hold 200 bar hydraulic pressure, Ra 6.3 µm surface finish on sealing areas.
Decision: Gravity die casting with A357 alloy. A357 provides the strength and pressure-tightness needed. Gravity die casting naturally produces low-porosity parts — no vacuum assist required. The moderate volume (5,000/year) justifies the tooling cost. Machining allowance of 1.5 mm on sealing surfaces allows finish machining to Ra 1.6 µm.
Result: Pressure-tight without VAD, cost per part within budget, tooling amortized over 3 years.
Scenario 2: Copper Alloy Valve Body
Requirement: 500 brass valve bodies per year, C83600 alloy, DN50 size, pressure-tight to 16 bar.
Decision: Gravity die casting. HPDC cannot cast copper alloys. Sand casting would work but with rough surface and looser tolerance. Gravity die casting provides the right combination of pressure-tightness, surface quality, and dimensional accuracy for a valve body at this volume.
Result: Best process fit, pressure-tight at 16 bar, surface suitable for valve finishing.
Scenario 3: Large Aluminum Manifold (30 kg)
Requirement: 200 large manifolds per year, 30 kg each, complex internal passages.
Decision: Low pressure die casting (LPDC). At 30 kg per part, gravity die casting is too slow and inconsistent. HPDC can’t cast this size (practical limit ~20 kg). LPDC provides controlled filling for the large cavity and good pressure-tightness for fluid handling.
Result: LPDC is the only process that can reliably produce this size at this quality level.
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 permanent mold the right process for your volume (500–10,000/year)?
- Gravity pour vs low pressure — which filling method suits your part?
- Does the part need to be pressure-tight? (Gravity/low pressure naturally lower porosity)
- Is the minimum wall thickness achievable (2–3 mm minimum)?
- What surface finish is required — Ra 3.2–12.5 µm acceptable?
- What alloy is required? (Al, Mg, or Cu alloy?)
- Who owns the die? What is the die life guarantee?
- Has low pressure die casting been considered for larger or higher-quality parts?
FAQ
What is the difference between permanent mold casting and die casting?
The key difference is filling method. Die casting (HPDC) injects metal at high pressure (1,000–10,000 bar) into the die. Permanent mold casting (gravity die casting) fills the die by gravity only (or low pressure of 0.1–0.3 bar). The lower pressure means lower porosity and lower tooling cost, but also lower production rate and thicker minimum walls.
Can permanent mold casting produce pressure-tight parts without vacuum assist?
Yes — this is one of its key advantages over HPDC. Gravity filling does not trap gas, so the inherent porosity is very low. Permanent mold (gravity and low pressure) castings are naturally pressure-tight in most cases, making them the preferred process for hydraulic components, pump housings, and engine parts that must hold pressure.
What is the maximum part size for permanent mold casting?
Practical maximum is approximately 50–100 kg for aluminum (larger for copper alloys with lower melting points). Above this size, the die becomes too large and expensive, and sand casting becomes more economical. The actual limit depends on the foundry’s largest available machine.