yvonne@nydcasting.com 
Shrinkage cavities and porosity are common defects in lost foam casting of ductile iron parts, which are mainly caused by volume shrinkage and insufficient shrinkage compensation during solidification. The following is a detailed analysis of its formation mechanism and prevention and control measures:
1. Causes of shrinkage cavities and porosity
1.1 Solidification characteristics of ductile iron
When ductile iron solidifies, graphitization expansion (about 4~5%) can partially offset the shrinkage of molten metal (about 1~2%), but if the process is improper, insufficient shrinkage compensation will still lead to shrinkage cavities/shrinkage porosity.
Graphite floating: When the carbon equivalent is too high, graphite floats in the early stage of solidification, resulting in increased local shrinkage.
1.2 Limitations of lost foam process
Gasification residues hinder shrinkage compensation: Gas or residues generated by gasification of foam models may block the shrinkage compensation channel.
Negative pressure and cooling rate: Insufficient negative pressure leads to poor exhaust, or uneven cooling rate (such as poor thermal conductivity of coating) causes local overheating areas.
1.3 Process design defects
Improper design of the pouring and rising system: unreasonable riser position and size, or too high/too low pouring temperature.
Alloy composition fluctuation: Too high carbon equivalent (CE) (>4.6) is easy to cause graphite floating, and too low CE (<3.8) will increase the shrinkage tendency.
Poor inoculation and spheroidization: Insufficient inoculation leads to low graphite spheroidization rate, weakening the shrinkage compensation effect of graphite expansion.
1.4 Uncontrolled solidification sequence
Isolated liquid phase area is easy to form at the junction of thin and thick walls, and the shrinkage compensation path is cut off, forming shrinkage.
2.Key measures to prevent shrinkage
2.1. Optimize alloy composition and metallurgical quality
Control carbon equivalent (CE): It is recommended to have CE=3.8~4.3 (carbon 3.6~3.9%, silicon 2.0~2.5%) to balance graphitization expansion and shrinkage.
Strengthen inoculation and spheroidization: Use multiple inoculations (such as flow inoculation) and high-efficiency spheroidizers (such as MgRE alloy) to ensure that the graphite spheroidization rate is ≥90%.
Reduce sulfur content: sulfur <0.02% to reduce slag phase interference.
2.2. Improve process design
Cooperation between riser and chill:
The riser is placed in the thick and large part, and the insulation riser or heating riser is used to extend the shrinkage compensation time.
The chill accelerates local cooling and forces sequential solidification (such as placing it opposite the thick wall).
Optimize the pouring system:
Stepped pouring or bottom pouring design to reduce turbulence and local overheating.
The pouring temperature is controlled at 1350~1420℃ to avoid being too high (aggravated shrinkage) or too low (poor fluidity).
2.3 Lost foam process control
Coating permeability: coating thickness ≤1mm, permeability >100cm³/(min·cm²), to ensure timely discharge of gas.
Negative pressure parameters: Negative pressure 0.03~0.06MPa, maintained until the casting is completely solidified.
Model material selection: Use low-gassing EPS or STMMA foam to reduce gasification residue.
2.4 Solidification process control
Simulation-assisted design: Use ProCAST or MAGMA software to simulate the solidification process and identify shrinkage risk areas.
Local pressurization technology: Apply mechanical pressure (such as extrusion risers) to thick and large parts to compensate for shrinkage.
Rapid cooling: Spray water-based paint or place chilling materials in shrinkage-prone areas.
2.5 Production monitoring
Real-time monitoring of casting parameters: Use infrared thermometers to monitor molten iron temperature to ensure stability.
Metallographic and non-destructive testing: Regularly slice to observe graphite morphology, supplemented by ultrasonic testing to check internal defects.
3. Typical case analysis
Problem: A ductile iron part (wall thickness 10~50mm) has concentrated shrinkage holes in the thick wall.
Solution:
Add a chiller opposite the thick wall and adjust the riser position to above the hot node.
Reduce CE from 4.5 to 4.2 and increase the flow inoculation amount (0.1%→0.15%).
The air permeability of the coating is increased to 150cm³/(min·cm²), and the negative pressure is adjusted to 0.05MPa.
Effect: The shrinkage rate is reduced from 8% to <1%.
4. Summary
The prevention and treatment of shrinkage and porosity requires systematic consideration of the synergy of materials, processes and designs. The core lies in controlling the solidification sequence, optimizing the shrinkage feeding capacity and reducing gas interference. The compactness of ductile iron parts can be significantly improved through precise composition control, simulation-assisted design and refinement of process parameters.