In the production of large and medium-sized resin-bonded sand casting parts, burn-on defects remain a critical challenge. These defects primarily manifest as mechanical and chemical burn-on, both stemming from molten metal penetration into sand molds. This article systematically analyzes the mechanisms of burn-on and proposes practical solutions to enhance casting quality.
1. Mechanisms of Burn-on Defects
1.1 Mechanical Burn-on
Mechanical burn-on occurs when molten metal infiltrates sand mold pores and solidifies, mechanically anchoring sand particles to the casting surface. The penetration depth depends on the equilibrium between driving and resisting forces. The critical infiltration pressure \( P \) is calculated as:
$$ P = \frac{2\delta \cos\theta}{r} $$
Where:
\(\delta\) = Surface tension of molten metal (N/m)
\(\theta\) = Wetting angle between metal and sand
\(r\) = Radius of sand pore channels (m)
| Factor | Effect on Mechanical Burn-on |
|---|---|
| Pouring temperature | Higher temperatures increase metal fluidity and penetration depth |
| Sand grain size | Finer grains reduce pore size but require proper distribution |
| Resin content | Optimal 0.8-1.2% ensures adequate mold strength |
1.2 Chemical Burn-on
Chemical reactions between molten steel oxides (FeO) and silica sand form low-melting-point silicates (FeSiO₃, m.p. 1205°C). The reaction intensity follows:
$$ \text{FeO} + \text{SiO}_2 \rightarrow \text{FeSiO}_3 $$
Alkaline additives in sand molds accelerate this reaction, creating complex eutectics with melting points below 1000°C.
2. Key Control Strategies
2.1 Sand Selection and Preparation
Optimal sand characteristics for sand casting parts:
| Parameter | Specification |
|---|---|
| AFS Grain Fineness | 45-65 |
| Clay Content | <0.5% |
| Thermal Stability | Recycled sand preferred |
| Hardener Ratio | 0.35-0.40% of sand weight |
The relationship between resin/hardener ratios and tensile strength shows distinct optimization zones:
$$ \text{Peak Strength} = f(\text{Resin}\%, \text{Hardener}\%) $$
2.2 Coating System Optimization
Multilayer coating application significantly reduces burn-on in sand casting parts:
| Layer | Function | Thickness (mm) |
|---|---|---|
| Base | Penetration barrier | 1.5-2.0 |
| Intermediate | Thermal insulation | 0.5-1.0 |
| Surface | Chemical resistance | 0.3-0.5 |
Coating performance parameters:
$$ \text{Coating Effectiveness} \propto \frac{\text{Refractoriness} \times \text{Adhesion Strength}}{\text{Surface Tension}} $$
2.3 Process Parameter Control
For carbon steel sand casting parts:
$$ T_{\text{pour}} = 1550 \pm 10^\circ\text{C} $$
Temperature effects on defect formation:
| Temperature Range (°C) | Defect Risk |
|---|---|
| <1540 | Cold shut, misrun |
| 1540-1560 | Optimal zone |
| >1560 | Severe burn-on |
3. Case Implementation
In our foundry trials with 5-ton steel sand casting parts:
- Adopted four-sieve sand mixture (40/50/70 mesh)
- Implemented zircon-based base coating + chromite facing coating
- Controlled pouring temperature at 1550-1570°C
Results showed 78% reduction in burn-on defects compared to conventional methods, with final surface roughness improved from Ra 25μm to Ra 12.5μm.
4. Conclusion
The prevention of burn-on in resin-bonded sand casting parts requires comprehensive control of:
$$ \text{Total Effectiveness} = \prod_{i=1}^{n} \left(1 – \frac{D_i}{D_{\text{max}}}\right) $$
Where \( D_i \) represents defect contribution factors (sand quality, coating performance, thermal parameters). Continuous monitoring and adaptive process adjustments remain essential for maintaining high-quality sand casting parts production.