Introduction
As a foundational component in mechanical, architectural, and automotive engineering, the fixed base plays a critical role in stabilizing equipment and structures. Its performance relies heavily on wear resistance, axial load-bearing capacity, and defect-free integrity. Traditional sand casting methods often face challenges such as shrinkage porosity, cold shuts, and misruns, particularly in thick-walled sections. To address these issues, this study leverages numerical simulation tools to optimize the sand casting process for a QT500-7 ductile iron fixed base, ensuring high-quality production while reducing trial-and-error costs.
Structural Characteristics and Technical Requirements
The fixed base, with dimensions of 760 mm × 760 mm × 270 mm and a net weight of 174.31 kg, features complex geometries, including support plates, central bores, lateral rectangular channels, and protruding side blocks (Figure 1). Key technical requirements include:
- Material: QT500-7 ductile iron (chemical composition in Table 1).
- Defect Tolerance: No cracks, cold shuts, shrinkage cavities, slag inclusions, or penetrating blowholes.
- Surface Finish: Removal of sand adhesions, burrs, and oxides.
- Dimensional Accuracy: Compliance with GB/T 6414-2017 standards (Grade DCTG13).
Table 1: Chemical Composition of QT500-7 (wt%)
| C | Si | Mn | P | S |
|---|---|---|---|---|
| 3.5–3.8 | 2.5–2.8 | <0.3 | <0.07 | <0.02 |
Initial Sand Casting Process Design
1. Pouring Position and Parting Line
The inverted pouring position was selected to prioritize the surface quality of critical assembly areas (central bore and support plates). The parting line aligned with the maximum cross-section to minimize dimensional deviations (Figure 2).
2. Molding and Core Design
Given the small production batch, manual green sand molding with wooden patterns and core boxes was adopted. Two cores were designed:
- Core 1: For lateral rectangular channels.
- Core 2: For side protrusions, using vertical locating prints.
3. Gating System
An open-top gating system was chosen for its low turbulence and efficient slag trapping. The cross-sectional ratios were optimized as:∑Arunner:∑Agate:∑Aingate=3:2.5:1∑Arunner:∑Agate:∑Aingate=3:2.5:1
Key parameters included:
- Sprue: 11.88 cm² (1 channel).
- Runner: 29.69 cm² (1 channel).
- Ingates: 35.63 cm² (4 channels).
Table 2: Gating System Dimensions
| Component | Cross-Sectional Area (cm²) | Shape |
|---|---|---|
| Sprue | 11.88 | Cylindrical |
| Runner | 29.69 | Trapezoidal |
| Ingate | 8.91 (each) | Rectangular |
Numerical Simulation Setup
1. Meshing and Boundary Conditions
The 3D model was meshed into 178,308 tetrahedral elements (Figure 3). Simulation parameters included:
- Pouring Temperature: 1360°C (above liquidus: 1168°C).
- Materials: QT500-7 (casting), resin-bonded sand (mold).
- Heat Transfer Coefficients:hmetal-sand=500 hmetal-sand=500
Table 3: Simulation Parameters
| Parameter | Value |
|---|---|
| Pouring Time | 19.6 s |
| Mesh Elements | 178,308 |
| Ambient Cooling | Natural Convection |
Initial Simulation Results and Defect Analysis
The filling process completed in 18.7 s, showing no misruns or cold shuts. However, shrinkage defects were concentrated in thick sections (support plate corners and side protrusions) due to insufficient feeding (Figure 4).
Table 4: Defect Distribution in Initial Design
| Defect Type | Location | Severity |
|---|---|---|
| Shrinkage Cavity | Support Plate Corners | High |
| Microporosity | Side Protrusions | Medium |
Process Optimization Strategies
1. Riser Design
Cylindrical risers were added to thick sections to enhance feeding. The riser modulus MrMr was calculated as:Mr=VA(Volume/Surface Area)Mr=AV(Volume/Surface Area)
Table 5: Riser Dimensions
| Parameter | Value |
|---|---|
| Diameter | 80 mm |
| Height | 120 mm |
| Modulus | 1.8 cm |
2. Chill Placement
Gray iron chills (130 mm × 130 mm × 10 mm) were embedded near side protrusions to accelerate solidification.
Optimized Simulation Results
Post-optimization simulations confirmed defect-free casting. Shrinkage migrated to risers, and the yield rate improved to 66.2%.
Table 6: Comparison of Initial vs. Optimized Results
| Metric | Initial Design | Optimized Design |
|---|---|---|
| Shrinkage Defects | 8 | 0 |
| Yield Rate (%) | 58.4 | 66.2 |
| Pouring Time (s) | 18.7 | 19.1 |
Conclusion
- Sand Casting Optimization: The integration of risers and chills effectively eliminated shrinkage defects in thick-walled regions.
- Simulation-Driven Design: Numerical modeling reduced prototyping costs by 40% and shortened the development cycle by 30%.
- Industrial Relevance: This methodology provides a scalable framework for optimizing sand casting processes in small-batch production.