Traditional sand casting methods for ductile iron water meter shells often involve complex procedures, high labor intensity, and environmental pollution. Lost foam casting (LFC) offers a sustainable alternative by simplifying production, reducing machining allowances, and enhancing yield rates. This study focuses on optimizing the LFC process for manufacturing large-diameter (150 mm) thin-walled (6.5 mm) water meter shells, achieving lightweight design and cost efficiency.
1. Process Design and Material Selection
The water meter shell structure features curved internal and external surfaces with distributed flange holes. Compared to sand casting, lost foam casting reduces wall thickness from 8 mm to 6.5 mm and machining allowances from 3 mm to 1.5 mm. Key parameters include:
| Parameter | Value |
|---|---|
| Foam Material | EPS-4S (Density: 23–25 g/L) |
| Coating Layers | 3 layers (1st: graphite-based; 2nd/3rd: Al-Si refractory) |
| Coating Thickness | 1.2–1.6 mm |
The foam density is optimized to minimize carbon residue during decomposition. The relationship between foam density ($\rho$) and gasification efficiency ($\eta$) can be expressed as:
$$
\eta = k \cdot \frac{1}{\rho} \cdot T^{1.5}
$$
where $k$ is a material constant, and $T$ is the pouring temperature.
2. Gating System Design
Three bottom-gating systems were tested (Figure 2). Scheme I demonstrated superior performance due to its effective slag-trapping capability and minimized turbulence. Critical parameters include:
| Scheme | Yield Rate (%) | Defect Rate (%) |
|---|---|---|
| I | 79.8 | 0 |
| II | 58.0 | 50 |
| III | 84.2 | 100 |
The optimal pouring temperature range was determined as 1,500–1,520°C. The thermal balance equation for foam decomposition is:
$$
Q_{\text{total}} = Q_{\text{metal}} – Q_{\text{foam}} – Q_{\text{coating}}
$$
where $Q_{\text{metal}}$ is the liquid metal’s enthalpy, $Q_{\text{foam}}$ is the foam’s gasification energy, and $Q_{\text{coating}}$ is the coating’s thermal resistance.
3. Defect Analysis and Mitigation
Common defects in lost foam casting include misruns, slag inclusions, and shrinkage porosity. Key countermeasures include:
| Defect | Root Cause | Solution |
|---|---|---|
| Metal Splashing | Incomplete foam drying | Extend drying time (>48 hrs at 45°C) |
| Misruns | Low pouring temperature | Maintain $T_{\text{pour}} > 1,480°C$ |
| Slag Inclusions | Residual foam decomposition | Optimize gating ratio: $A_{\text{sprue}}:A_{\text{runner}}:A_{\text{gate}} = 1:1.2:1.5$ |
The vacuum pressure profile during pouring significantly affects defect formation. The recommended sequence is:
$$
P(t) = \begin{cases}
0.06–0.07\ \text{MPa} & \text{during pouring} \\
0.03–0.035\ \text{MPa} & \text{post-pouring (5–8 mins)}
\end{cases}
$$
4. Metallurgical Control
The chemical composition of ductile iron was optimized for lost foam casting:
| Element | Composition (wt%) |
|---|---|
| C | 3.8–3.9 |
| Si | 2.6–2.8 |
| Mn | <0.15 |
| S | <0.05 |
The nodularity ($N$) and nodule count ($N_c$) meet ISO 945 standards:
$$
N = \frac{\pi d^2}{4A} \geq 80\%,\quad N_c > 100\ \text{nodules/mm}^2
$$
5. Economic and Environmental Benefits
Compared to sand casting, lost foam casting demonstrates:
- 40% reduction in machining costs
- 15–20% material savings
- Zero waste sand generation
The process yield improvement ($\Delta Y$) follows:
$$
\Delta Y = \frac{Y_{\text{LFC}} – Y_{\text{sand}}}{Y_{\text{sand}}} \times 100\% = 22.4\%
$$
This study confirms that lost foam casting enables sustainable production of high-performance water meter shells while addressing environmental concerns associated with traditional foundry processes.