In lost foam casting (LFC) production of QT450-10 ductile iron reducer housings, defects such as surface inclusions and shrinkage cavities were observed at geometric hot spots. Through systematic process optimization, we developed innovative solutions to improve casting quality while maintaining high process yield.
1. Formation Mechanisms and Countermeasures
1.1 Inclusion Formation and Mitigation
The decomposition of expandable polystyrene (EPS) and copolymer materials (STMMA) during casting generates gaseous and solid residues:
$$ \text{EPS: } C_8H_8(s) \rightarrow 8C(s) + 4H_2(g) $$
$$ \text{STMMA: } C_5O_2H_8(s) \rightarrow 3C(s) + 2CO_2(g) + 4H_2(g) $$
| Material | Gas Generation (mol) | Solid Carbon (mol) |
|---|---|---|
| EPS | 4 | 8 |
| STMMA | 6 | 3 |
By increasing machining allowance from 4mm to 8mm at critical surfaces, inclusion-related scrap rate decreased from 12% to 2.04%:
$$ \text{Process Yield Improvement} = \frac{(0.9796 – 0.88)}{0.88} \times 100\% = 11.36\% $$
1.2 Shrinkage Control Strategies
Two novel approaches were developed for shrinkage elimination:
1.2.1 Heat Dissipation Process
Attaching 12 polystyrene fins (50×30×7 mm) at hot spots enhanced localized cooling:
$$ \text{Cooling Efficiency} = k \cdot \frac{A_{fin}}{V_{hotspot}} \cdot \Delta T $$
Where k = thermal conductivity, Afin = fin surface area, Vhotspot = hot spot volume.
1.2.2 Flexible Chill Process
Steel shot (3-5 mm diameter) embedded near critical areas achieved directional solidification:
$$ G = \frac{\partial T}{\partial x} \geq \frac{T_{liquidus} – T_{solidus}}{L} $$
Where G = temperature gradient, L = characteristic length.
2. Process Validation
| Parameter | Original | Optimized |
|---|---|---|
| Shrinkage Defect Rate | 15.7% | 0.8% |
| Process Yield | 72% | 89% |
| Energy Consumption | 1.8 kWh/kg | 1.2 kWh/kg |
3. Critical Process Parameters
Key factors in lost foam casting of ductile iron components:
$$ \text{Negative Pressure} = -0.04 \sim -0.06 \text{ MPa} $$
$$ \text{Pouring Temperature} = 1370-1440^\circ C $$
$$ \text{Solidification Time} = \frac{(T_p – T_s)^2}{\pi k \rho c} $$
4. Conclusion
The optimized lost foam casting process demonstrates:
- 97.96% inclusion-free surface quality
- 0.8% shrinkage defect rate
- 11% improvement in process yield
Both heat dissipation fins and flexible chills effectively control shrinkage in lost foam casting while maintaining the inherent advantages of near-net shape forming. This research provides practical solutions for complex ductile iron components production using LFC technology.