Shenzhen Alu Rapid Prototype Precision Co., Ltd.
Industry News
Porosity is one of the most common defects in die casting — appearing as voids, holes, or pits inside or on the surface of cast parts. Here's a complete guide to controlling it.
Types of Porosity
Type | Cause | Location |
Gas porosity | Trapped air or gas during filling | Scattered throughout part |
Shrinkage porosity | Metal contracts during solidification | Thick sections, last to solidify |
Cold shut porosity | Two metal fronts meet and don't fuse | Along flow paths |
Hydrogen porosity | Hydrogen dissolved in molten metal | Fine, dispersed voids |
1. Control Metal Temperature
Too hot = more gas dissolution and shrinkageToo cold = premature solidification and cold shuts
Alloy | Recommended Pouring Temp |
Aluminum (ADC12) | 650 – 700°C |
Zinc (Zamak 3) | 415 – 430°C |
Magnesium (AZ91D) | 640 – 680°C |
Copper/Brass | 900 – 980°C |
Best Practices:
✅ Maintain consistent melt temperature
✅ Use temperature-controlled holding furnace
✅ Monitor with calibrated thermocouples
✅ Minimize metal sitting time in ladle
2. Optimize Injection Parameters
Injection speed and pressure directly affect porosity
Shot Phases:
Phase 1 (Slow shot) → Phase 2 (Fast shot) → Phase 3 (Intensification)
Fill sleeve Fill cavity Pack and compensate
Parameter | Effect on Porosity |
Slow shot speed too fast | Traps air in sleeve → gas porosity |
Fast shot speed too slow | Cold shuts, incomplete fill |
Intensification pressure too low | Shrinkage porosity in thick sections |
Switch point wrong | Air entrapment at transition |
Guidelines:
✅ Slow shot: keep metal below 0.5 m/s until sleeve 40–60% full
✅ Fast shot: 30–60 m/s gate velocity for aluminum
✅ Intensification: apply immediately after cavity fills
✅ Hold intensification pressure until gate freezes
3. Improve Venting Design
Trapped air must escape — if it can't, it becomes porosity
Vent placement:
Place vents at last points to fill in cavity
Add vents opposite to gates
Vent at all dead-end flow areas
Use overflow wells to capture first cold metal
Vent specifications:
Alloy | Vent Thickness | Vent Width |
Aluminum | 0.08 – 0.12 mm | 10 – 25 mm |
Zinc | 0.05 – 0.08 mm | 8 – 20 mm |
Magnesium | 0.08 – 0.15 mm | 10 – 25 mm |
Advanced venting options:
Vacuum venting — actively pulls air out before injection
Chill vents — freeze metal in vent to prevent flash
Overflow wells — collect cold slugs and trapped air
4. Vacuum Die Casting
Most effective method for reducing gas porosity
Vacuum system evacuates air from cavity before injection
Reduces cavity pressure from atmospheric to 50–100 mbar
Dramatically reduces trapped air porosity
Allows heat treatment of parts (not possible with standard die cast)
Benefits:
✅ Porosity reduced by 50–80%
✅ Parts can be welded and heat treated
✅ Better mechanical properties
✅ Improved surface finish
Cost: Adds $15,000 – $50,000 to mold cost
5. Gate and Runner Design
Poor gating = turbulent flow = air entrapment
Key principles:
✅ Gate velocity: 30–60 m/s (aluminum) — too fast causes turbulence
✅ Use fan gates for wide thin parts
✅ Avoid sharp corners in runners — causes turbulence
✅ Runner should fill before gate opens to cavity
✅ Multiple gates for large complex parts
✅ Balance runner system for multi-cavity molds
Gate thickness guidelines:
Wall Thickness | Gate Thickness |
1.0 – 2.0 mm | 0.8 – 1.2 mm |
2.0 – 3.0 mm | 1.2 – 1.8 mm |
3.0 – 4.0 mm | 1.5 – 2.5 mm |
4.0+ mm | 2.0 – 3.0 mm |
6. Control Die Temperature
Cold dies cause premature solidification and misrunsHot dies cause longer cycle times and shrinkage
Alloy | Recommended Die Temp |
Aluminum | 180 – 250°C |
Zinc | 150 – 200°C |
Magnesium | 200 – 280°C |
Best Practices:
✅ Use temperature-controlled water/oil cooling circuits
✅ Preheat die before production starts
✅ Monitor die temp with thermal camera regularly
✅ Balance cooling channels for uniform temperature
✅ Avoid hot spots near thick sections
7. Part Design for Low Porosity
Design is the most cost-effective porosity control
Wall thickness:
Keep walls as uniform as possible
Avoid sudden thick-to-thin transitions
Thick sections = last to solidify = shrinkage porosity
Recommended max wall thickness:
Alloy | Preferred Wall | Max Wall |
Aluminum | 2.0 – 3.0 mm | 5.0 mm |
Zinc | 1.0 – 2.5 mm | 4.0 mm |
Magnesium | 1.5 – 3.0 mm | 4.5 mm |
Design tips:
✅ Core out thick sections to reduce mass
✅ Use ribs instead of thick walls for strength
✅ Add generous fillets to improve flow
✅ Avoid isolated thick bosses
8. Metal Quality Control
Clean metal = less porosity
Common contaminants:
Contaminant | Effect |
Moisture/water | Steam → hydrogen porosity |
Oxides | Inclusions → weak spots |
Hydrogen gas | Fine dispersed porosity |
Scrap contamination | Unpredictable alloy chemistry |
Control measures:
✅ Degas molten metal with nitrogen or argon purging
✅ Flux treatment to remove oxides
✅ Use quality ingots from certified suppliers
✅ Keep scrap ratio under 30% of melt
✅ Skim dross before ladling
✅ Dry all tools and ingots before contact with melt
9. Release Agent Control
Excess die lubricant causes gas porosity
Over-spraying = lubricant burns = gas trapped in cavity
Use minimal effective amount of release agent
Allow adequate blow-off time before closing die
Use air blow to remove excess lubricant
Monitor lubricant dilution ratio carefully
10. Porosity Detection Methods
Identify and measure porosity in parts
Method | What It Detects | Cost |
Visual inspection | Surface porosity only | Low |
Pressure testing | Leak paths through part | Low-Medium |
X-ray / radiography | Internal porosity | Medium |
CT scanning | Full 3D porosity map | High |
Dye penetrant | Surface cracks and pores | Low |
Sectioning | Internal porosity directly | Destructive |
Porosity Control Priority Order
1. ✅ Good part design (uniform walls, cored thick sections)
2. ✅ Proper venting (last-fill areas, overflow wells)
3. ✅ Correct injection parameters (slow shot, fast shot, intensification)
4. ✅ Consistent metal temperature
5. ✅ Die temperature control
6. ✅ Clean metal (degassing, fluxing)
7. ✅ Minimize release agent
8. ✅ Consider vacuum die casting for critical parts
Quick Troubleshooting Guide
Porosity Type | Location | Most Likely Cause | Fix |
Large voids | Near gate | Turbulent fill | Reduce fast shot speed |
Scattered small pores | Throughout | Trapped air | Improve venting |
Pores in thick areas | Bosses, ribs | Shrinkage | Increase intensification |
Surface pits | Anywhere | Excess lubricant | Reduce release agent |
Fine dispersed pores | Throughout | Hydrogen | Degas metal |
Pores at flow ends | Far from gate | Cold shut | Increase metal/die temp |
Bottom Line
Controlling porosity requires a systematic approach covering design, process parameters, tooling, and metal quality — no single fix solves all porosity problems. The most effective strategy combines good part design + proper venting + optimized injection parameters + vacuum assistance for critical applications.