Porosity is a common defect in casting production that can significantly compromise the mechanical strength, sealing performance, and service life of components. Effective prevention requires systematic control of pouring temperature, optimization of mold and gating design, ensuring proper venting, and appropriate use of riser feeding or vacuum sealing technologies. Through scientific process design and strict control, porosity defects can be greatly reduced, leading to higher overall casting quality.
Different casting processes require tailored strategies. Below, we explore porosity prevention in sand casting, die casting, investment casting, and iron castings, with a focus on solidification control, venting design, and process management.
The following table shows the main causes of porosity and prevention measures for the four major casting processes:
|
Casting Process |
Main Causes of Porosity | Core Prevention Measures |
|
Sand Casting |
Poor mold permeability, excessive moisture | Control mold humidity, optimize gating, and the riser system |
| Die Casting | Trapped cavity air, gas entrapment |
Vacuum assistance, optimize vents, control process parameters |
|
Investment Casting |
Low shell permeability, difficult gas escape during solidification |
Design auxiliary vents, control solidification sequence |
| Iron Casting | Gas in molten metal, solidification shrinkage |
Molten metal purification, enhance the feeding system |
Preventing Porosity in Sand Casting
Sand casting is widely used due to its low cost and flexibility, but it is also prone to porosity caused by poor venting and improper metal flow.
Temperature Control
Pouring temperature must be tightly controlled within a suitable range—typically 680–750°C for aluminum alloys, 1350–1420°C for cast iron, and 1550–1600°C for cast steel.
Excessive temperature increases gas solubility in the molten metal, leading to gas release during solidification; insufficient temperature reduces fluidity, preventing gas escape. Identifying the optimal pouring window is essential.
Humidity Control
Sand mold moisture should be maintained between 4.5–5.5%. High moisture (>6%) generates excessive steam when contacting molten metal, which can cause porosity if not vented.
Low moisture (<4%) reduces mold strength and permeability, hindering gas escape. Sand compactness should be 70–85 units, with permeability between 80–120—key factors in porosity prevention.
The following table shows the range of key control parameters for sand casting:
|
Control Parameter |
Aluminum Alloy | Cast Iron | Cast Steel |
|
Pouring Temperature |
680-750°C | 1350-1420°C | 1550-1600°C |
| Mold Humidity | 4.5-5.5% | 4.5-5.5% |
4.5-5.5% |
|
Sand Compaction |
70-85 units | 70-85 units |
70-85 units |
| Permeability | 80-120 | 80-120 |
80-120 |
Melt Treatment
Effective refining during melting is critical. Refining temperature is usually 100–150°C above the melting point.
For aluminum alloys, argon refining for 10–15 minutes at 2–3 L/min is typical; cast iron may be desulfurized with 0.3–0.5% sodium carbonate.
A clean melt reduces gas formation and improves mechanical properties and density.
Gating and Riser Design
The gating system should be scientifically designed, with a typical ratio of sprue ingate = 1.2:1.1:1.0.
Riser size is usually 1.2–1.5 times the hot spot diameter, with a height of 1.5–2.0 times the diameter.
A well-designed system supports feeding and provides an escape path for gas, reducing shrinkage and gas-related defects.
Preventing Porosity in Die Casting
Die casting involves high filling speeds and pressures, which can trap air in the cavity and cause internal porosity.
Vacuum Assist
For high-integrity castings, vacuum die casting is recommended, with vacuum levels of 80–90 kPa and an evacuation time of 2–3 seconds.
Existing micro-porosity can be sealed using vacuum impregnation at 0.5–0.7 MPa for 15–20 minutes.
The sealant penetrates 0.15–0.25 mm, permanently sealing leaks and improving pressure tightness.
The following figure shows the aluminum die casting equipment at CEX Casting with an integrated vacuum system:
Vacuum Assist Die Casting Machine
Lubricant Management
Excessive release agent vaporizes instantly, generating gas that becomes trapped.
Spraying should be controlled to 0.5–1.5 seconds, from a distance of 200–300 mm, with a dilution ratio of 1:80–1:100.
Coating thickness should be 10–20 µm, with no more than 3–5 ml per shot, ensuring a thin film without pooling.
Venting Design
The gating system should promote laminar flow. Ingate speeds are typically 20–40 m/s for aluminum and 30–50 m/s for zinc alloys.
Venting channels at the end of flow paths or trapped air zones should be 0.05–0.15 mm deep and 5–10 mm wide, with a total area ≥20–30% of the ingate area, to allow efficient gas escape.
Process Parameters Control
Control Slow shot speed is set at 0.2–0.5 m/s, fast shot at 4–6 m/s, with intensification pressure of 80–120 MPa and response time ≤30 ms.
Proper parameter setting reduces turbulence and supports feeding during solidification.
Injection pressure is typically 50–80 MPa for aluminum and 30–50 MPa for zinc.
CEX Casting’s Approach to Aluminum Die Casting Porosity Control
Based on the above principles, we have developed a specialized process to ensure high-integrity aluminum die castings:
Vacuum and Monitoring: Vacuum level is maintained at 85–92 kPa, with response time ≤2.5 seconds. Real-time monitoring and alarms ensure consistent cavity evacuation.
Mold and Venting: Vent grooves are 0.08–0.12 mm deep, with total vent area accounting for 18–22% of the parting line area. CAE simulation validates vent layout to avoid turbulence and trapped air.
Process Control: Slow shot 0.3–0.4 m/s, fast shot 4.5–5.5 m/s, switchover accuracy ±3 mm, intensification pressure 90–110 MPa, and response time ≤30 ms ensure smooth filling and effective feeding.
Melt Treatment: Combined argon fluxing and refining at 2.5–3.0 L/min for 12–15 minutes reduces hydrogen content to ≤0.12 ml/100g Al.
Squeeze Casting: For high-density parts, squeeze casting can achieve a forging-like effect by maintaining intense pressure during solidification, virtually eliminating porosity and shrinkage, and resulting in exceptional part density, airtightness, and structural integrity.
Request the Squeeze Casting Case Study
The following video shows the squeeze casting workshop and workflow at CEX Casting:
Squeeze Casting Workshop at CEX Casting
Preventing Porosity in Investment Casting
Investment casting is used for complex, precise parts, but the ceramic shell’s low permeability and solidification control challenges make porosity a common issue.
Gating Design
Gates should be located at thick sections or hot spots, with a cross-sectional area 2–3 times the maximum wall thickness.
The gating ratio ∑sprue:∑runner:∑ingate = 1.2:1.1:1.0 is common.
Pouring angle of 15–30° and rate of 2–4 kg/s promote sequential filling from thick to thin areas, aiding feeding and venting.
Solidification Control
Shell preheat temperature of 900–1100°C, along with chills (thermal conductivity ≥200 W/m·K) or insulation (≤0.5 W/m·K), helps establish a gradient of 20–50°C/cm.
Solidification rate of 1–3 mm/min ensures directional solidification, pushing shrinkage and gas to the riser.
Structure Optimization
Flow guides (0.6–0.8 times adjacent wall thickness) and adjusted angles improve metal flow.
Vent wax, pins (0.8–1.2 mm diameter, 20–30 mm spacing), or auxiliary gates (20–30% of main gate area) at non-critical areas or last-to-solidify zones help collect and remove gas.
Simulation
Casting simulation software predicts filling, solidification, temperature fields, and defect risks.
With mesh size 0.5–2 mm, time step 0.001–0.01 s, filling speed error ≤5%, and temperature error ≤3%, the simulation accurately identifies defects and optimizes the process, reducing trial costs and scrap rates. Prediction accuracy reaches 85–90%.
The following figure shows a screenshot of the investment casting simulation software:
Simulation Software for Investment Casting Parts
Preventing Porosity in Iron Castings
Porosity in iron castings is often related to dissolved gas and solidification shrinkage, requiring controls in melting and process design.
Process Optimization
Pouring temperature is set at 1300–1380°C for gray iron and 1320–1400°C for ductile iron.
Pouring time t = K√G (K=1.8–2.2). Cooling rate of 30–100°C/min promotes uniform solidification, reducing shrinkage stress and gas evolution.
Melt Control
Vacuum degassing, inert gas purging, or alloy additives reduce hydrogen (≤2 ppm) and nitrogen (≤80 ppm).
Carbon equivalent is kept at 3.9–4.3%. Melt oxidation is minimized; sulfur after desulfurization is ≤0.02%, and residual magnesium after nodularization is 0.03–0.05%.
Feeding Enhancement
Modulus calculation or solidification simulation identifies hot spots.
Risers (neck modulus 0.6–0.8 times casting modulus), chills (0.8–1.2 times casting thickness), and insulating covers (30–50 mm thick, ≥1.5 times solidification time) ensure directional solidification and feeding.
Learn More About How to Check Casting Porosity?
After implementing the above preventative measures, a rigorous porosity check is crucial to ensuring final quality.
For a deeper understanding of various non-destructive testing techniques, please refer to our guide, “How to Check Porosity in Castings: NDT Methods Explained“.
Conclusion
Porosity prevention requires a systematic approach covering process design, parameter control, and material management.
Optimized gating, venting, and feeding systems greatly reduce defects, improving mechanical properties and sealing performance.
As a specialized aluminum casting supplier, CEX Casting utilizes advanced process technology and quality control to provide high-integrity, low-porosity aluminum die casting products.
Contact us today to learn more about our defect-free casting solutions for your next aluminum die casting project.