1. Process Design
1.1 Structure Analysis
| Information | Details |
|---|---|
| Casting Material | QT400 – 15 |
| Casting Size | 430 mm × 620 mm × 684 mm |
| Casting Weight | 180 kg |
| Thick Parts | 183 mm × 40 mm × 59 mm (Easy to produce shrinkage area) |
| Oil Channel | Total length 510 mm, high requirements for airtight leak detection |
1.2 Process Schemes and Simulation Results
| Pouring System Scheme | Simulation Results (Shrinkage Porosity Risk Areas) |
|---|---|
| Side – Bottom Injection | Parallel plate and upper end face intersection area, “U” – shaped boss inside, bottom and boss dense area |
| Top Injection | Similar to side – bottom injection |
| Step Injection | Inside of “U” – shaped bosses on both sides of casting, parallel plate and upper end face intersection area |
| Bottom Injection | Upper end face hot spot area and inside of riser |
1.3 Process Experiment
| Experiment Content | Details |
|---|---|
| Selected Pouring System | Bottom injection with riser |
2. Defect Analysis
2.1 Main Defects
| Defect Type | Location |
|---|---|
| Shrinkage Hole | Upper end face of casting, at the intersection of parallel plate structure and upper end face |
2.2 Cause Analysis
2.2.1 Shrinkage Hole Formation Analysis
- Solidification Characteristics: Ductile iron casting solidifies in a pasty manner. During solidification, the thick area solidifies slower than the surrounding thin area. After the surrounding area solidifies, the thick area cannot be supplemented with liquid iron, resulting in shrinkage porosity or shrinkage hole defects.
- Effect of Eutectic Expansion: The eutectic expansion pressure causes the liquid iron volume to increase and may lead to the formation of shrinkage porosity or shrinkage hole. It also affects the mold wall, causing phenomena such as mold wall migration and box expansion.
2.2.2 Risers
- Function: The riser is mainly used to supplement liquid iron and control pressure.
- Pressure Change Stages: (Shown in Figure 8) After the gate solidifies, the casting and riser form a whole. When the liquid metal contracts, the pressure in the riser is the smallest. With the precipitation of graphite and the formation of austenite, the liquid metal expands and refills the riser.
3. Improvement Schemes
3.1 Improvement of Riser Parameters
| Riser Number | Riser Modulus (MR) | Riser Neck Modulus (MN) |
|---|---|---|
| 1# | MS | 0.8MR |
| 2# | 1.5MS | 0.6MR |
3.2 Improvement Effects
| Pouring System | Defect Situation |
|---|---|
| Bottom Injection with 1# Riser | Shrinkage hole defects in the hot spot area after rough turning, with a rejection rate of 37% |
| Bottom Injection with 2# Riser | No obvious defects on the upper end face after rough turning, no shrinkage porosity in the hot spot area, but with discrete small point defects on the end face. After full – process processing, the end face defects are removed |
4. Conclusions
- The optimal pouring process for ductile iron castings in evaporative pattern casting is bottom injection with a properly sized riser. Through comparative experiments based on the product structure, the best solution to solve shrinkage porosity defects is obtained.
- After determining the pouring system, strengthening the control of the whole process, including preventing collisions, controlling coating drying, pouring negative pressure, and pressure maintaining, is beneficial to improving the internal and external quality of the casting.