In sand casting processes, achieving optimal feeding efficiency while minimizing material waste remains a critical challenge. This study evaluates the application of exothermic pad technology as an alternative to traditional metallic pads for improving yield and reducing costs in duplex stainless steel (ASTM A890 5A) casting production.
1. Thermal Dynamics of Feeding Systems
The feeding efficiency in sand casting can be modeled using Chvorinov’s rule:
$$
t = B \left( \frac{V}{A} \right)^2
$$
where $t$ represents solidification time, $B$ the mold constant, $V$ the metal volume, and $A$ the cooling surface area. Exothermic pads modify this relationship by:
- Increasing localized temperature through exothermic reactions
- Reducing heat transfer coefficient at the pad-casting interface
- Extending the effective feeding time
| Parameter | Metallic Pad | EX5 BLEND Pad |
|---|---|---|
| Heat Generation (kJ/kg) | 0 | 3,200 |
| Insulation Factor | 1.0 | 2.8 |
| Effective Feeding Duration (min) | 2.25 | 5.70 |
2. Process Implementation in Sand Casting
The sand casting trial utilized a gate valve body (830mm × 500mm × 440mm) with the following process parameters:
| Feature | Specification |
|---|---|
| Flange Dimensions | Ø495mm (OD), Ø175mm (ID), 116mm thickness |
| Pad Configuration | 270mm × 260mm × 90mm (per flange) |
| Mold Material | Silica sand with 6-8% bentonite |
The thermal gradient across the sand casting mold was calculated using Fourier’s law:
$$
q = -k \frac{dT}{dx}
$$
where $q$ is heat flux (W/m²), $k$ thermal conductivity (W/m·K), and $\frac{dT}{dx}$ the temperature gradient.
3. Performance Comparison
Experimental results demonstrated significant improvements in sand casting efficiency:
| Metric | Metallic Pad | Exothermic Pad | Improvement |
|---|---|---|---|
| Metal Consumption (kg) | 1,050 | 980 | -6.7% |
| Yield Rate | 51.4% | 55.1% | +7.2% |
| Post-processing Energy (kWh) | 18.5 | 2.3 | -87.6% |
4. Quality Validation
Both sand casting methods achieved radiographic inspection compliance (ASTM E94):
- Zero shrinkage defects in critical flange sections
- Surface finish Ra ≤ 25μm
- Dimensional tolerance ±1.5mm/m
The improved thermal performance of exothermic pads in sand casting can be expressed through the modified Niyama criterion:
$$
N_i = \frac{G}{\sqrt{\dot{T}}}
$$
where $G$ is temperature gradient (°C/mm) and $\dot{T}$ cooling rate (°C/s). EX5 BLEND pads maintained $N_i$ > 1.2 throughout solidification.
5. Economic Analysis
Cost comparison for sand casting production of 1,000 units:
| Cost Component | Metallic Pad | Exothermic Pad |
|---|---|---|
| Material Cost ($) | 18,500 | 9,200 |
| Machining Cost ($) | 32,000 | 2,500 |
| Energy Cost ($) | 7,800 | 1,100 |
| Total Savings ($) | – | 45,500 |
The implementation of exothermic pad technology in sand casting demonstrates significant technical and economic advantages, particularly for high-alloy materials like duplex stainless steel. This approach aligns with sustainable manufacturing principles through reduced material usage and energy consumption.