Shenzhen Alu Rapid Prototype Precision Co., Ltd.
Industry News
Controlling Porosity in Die Casting
Porosity is one of the most common defects in die casting, appearing as voids or pores inside the part. It compromises strength, pressure tightness, and surface finish. There are two main types:
Types of Porosity
Type | Cause | Location |
Gas porosity | Trapped air or gases during filling | Scattered, rounded pores |
Shrinkage porosity | Metal contracts during solidification without enough feed metal | Near thick sections, last-to-freeze areas |
1. Control Gas Porosity
Optimize venting
Place vents at the last points to fill (flow ends, deep pockets)
Vent cross-sectional area should be ~30–50% of gate area
Use overflow wells to collect the air-metal front
Vacuum-assisted die casting
Pull vacuum (50–100 mbar) before and during injection
Reduces trapped gas by 80–90%
Essential for heat-treatable or structural parts
Reduce turbulence during filling
Use slower first-phase plunger speed to push air out before fast injection
Optimize gate design (avoid jetting, use fan or tangential gates)
Avoid sharp corners that cause flow separation
Control lubrication
Excess die lubricant vaporizes → gas entrapment
Use minimum necessary lubricant; allow full evaporation before shot
Switch to water-based lubricants with lower residue
2. Control Shrinkage Porosity
Optimize thermal balance
Use die temperature control (oil or water cooling channels) near thick sections
Thicker sections should solidify last with feed metal available — or redesign to uniform wall thickness
Intensification pressure
Apply high pressure (intensification) after cavity fill to compress metal during solidification
Typical intensification: 500–1,500 bar
Must be timed correctly — too late and the gate freezes before pressure transmits
Gate and runner design
Gate into the thickest section where possible
Use multiple gates for large or complex parts
Avoid sharp thickness transitions in part design
Reduce wall thickness variation
Redesign parts with ribs instead of thick bosses
Blend transitions (e.g., taper from thick to thin gradually)
3. Process Parameter Controls
Parameter | Effect on Porosity | Recommended Action |
Injection speed (2nd phase) | Too fast → turbulence & gas trapping | Optimize per part geometry |
Metal temperature | Too high → more dissolved gas, more shrinkage | Use lowest viable pour temp |
Die temperature | Too cold → premature freeze; too hot → shrinkage | Balance with cooling channels |
Shot timing | Early intensification → better shrinkage control | Optimize with thermal simulation |
Plunger tip fit | Worn tip → air sucked into sleeve | Inspect and replace regularly |
4. Alloy & Melt Quality
Degas the melt using rotary degassing (nitrogen or argon purging) before casting
Avoid remelting scrap excessively — oxide inclusions worsen porosity
Monitor hydrogen content in aluminum alloys (target < 0.1 cc/100g)
Use clean, dry charge materials — moisture causes hydrogen pickup
5. Detection & Qualification
Method | What it finds |
X-ray / CT scan | Internal gas & shrinkage pores |
Pressure leak test | Connected porosity in pressure-tight parts |
Dye penetrant | Surface-breaking pores |
Metallographic cross-section | Size, distribution, type confirmation |
6. Remediation Options
If porosity exists in already-cast parts:
HIP (Hot Isostatic Pressing) — closes internal pores under high pressure/heat; works well for aluminum and magnesium
Impregnation (resin or sodium silicate) — seals connected porosity for pressure-tight applications
Both are common in automotive and hydraulic components