In our work on high-precision impeller and rotor-type castings for export, we encountered a challenging brake rotor component. The casting had an overall dimension of 285 mm × 122 mm, contained 24 hollow blades each 8 mm thick, weighed 13.5 kg, and was made of alloy cast iron with the composition shown below. The requirements were stringent: tight dimensional tolerances, high surface finish, and zero allowance for sand casting defects such as shrinkage porosity, gas holes, or slag inclusions. Tensile strength had to exceed 276 MPa. Initial attempts by other foundries resulted in less than 50% yield. Through systematic refinement of our casting process, we achieved a consistent yield of 98%–100%, ensuring on-time delivery for the export order. This article summarizes our approach, emphasizing the elimination of sand casting defects through special molding techniques, strategic use of chills, and optimized gating and risering.
Table 1: Required Chemical Composition of Alloy Cast Iron (wt%)
| C | Si | Mn | Ni | Cr | Mo | Fe |
|---|---|---|---|---|---|---|
| ≥3.40 | 1.0–1.7 | 0.6–0.9 | 0.9–1.2 | 0.25–0.35 | 0.25–0.35 | Balance |
1. Casting Process Analysis and Solution Selection
Given the high quality demands, conventional green sand molding could not satisfy the requirements. We adopted a combination of self-hardening resin sand molds and chills with an air gap to control solidification. The core challenge was the complex cavity shape with 24 thin blades, which made the metal flow unstable during pouring. Early trials using top gating with a pressure-side riser led to gas holes and iron shots in the upper rim area of the blades. These sand casting defects originated from turbulence and splashing. Switching to a bottom-gating system eliminated those defects but introduced shrinkage porosity at the ingate vicinity (thermal center). This is a classic example of sand casting defects caused by localized overheating.
2. Iterative Elimination of Sand Casting Defects
We systematically addressed each defect type. The following table summarizes the evolution of our process modifications:
| Defect Type | Root Cause | Initial Solution | Result | Final Solution |
|---|---|---|---|---|
| Gas holes & iron shots (upper rim) | Turbulence in top gating | Changed to bottom gating | Gas defects eliminated; shrinkage appeared near ingates | Used chills with air gap to create controlled directional solidification |
| Shrinkage porosity (lower wheel rim) | Local overheating at ingate | Added chills with air gap near ingates | Porosity shifted to other locations | Combined a circumferential chill ring on the lower rim with localized chills on upper hot spots |
| Shrinkage in upper riser region | Inadequate riser feeding | Increased riser size | Larger machining allowance and cost | Applied ring chill on lower part to promote uniform temperature gradient and used smaller, optimized risers |
The final production setup is illustrated conceptually (see the image link below).
3. Chill and Gating/Riser System Design
Chills were made of cast iron with multiple holes (12–18 mm diameter) drilled or cast into them. These holes served dual purposes: they allowed self-hardening sand to pass through to secure the chill in the mold, and they enhanced gas venting. The chills were placed with a 3–3.5 mm air gap (sand layer) between the chill and the casting surface, positioned using templates. This gap prevented a sharp change in cooling rate and avoided cold shut defects.
The gating system was designed using the A-san (or equivalent) formula to determine the minimum choke area. The calculated minimum ingate area was 287 mm². The actual ingates in the tooling were 19 mm × 2 mm × 8 gates = 304 mm². The ratio of sprue : runner : ingate was 2.3 : 2 : 1. Below are the cross-sectional areas:
Table 2: Gating System Cross-Sectional Areas
| Component | Cross-Sectional Area (mm²) | Ratio |
|---|---|---|
| Sprue (vertical) | Computed from A-san formula | 2.3 |
| Runner (horizontal) | Typically 2 × ingate area | 2 |
| Ingate | 304 | 1 |
The riser was designed to have a modulus (volume/cooling surface area) equal to 1.4 times the modulus of the hot spot (thermal center). The hot spot was identified as the junction of the blade root with the rim. Using modulus calculations:
$$ M_{\text{riser}} = 1.4 \times M_{\text{hot spot}} $$
For the casting, the hot spot modulus was approximately 1.2 cm, so the riser modulus was required to be 1.68 cm. We achieved this by a cylindrical top riser with dimensions of about 60 mm diameter and 80 mm height, placed directly above the thickest section.
4. Alloy Melting and Pouring
The alloy was melted in a medium-frequency induction furnace. At the furnace front, a triple inoculant (Si-RE-Al) was added for grain refinement and deoxidation. The tapping temperature was 1500–1580°C, and the pouring temperature was 1380–1400°C. These temperatures were critical to control fluidity and minimize sand casting defects like misruns or cold shuts, especially in the thin blades.
5. Process Advantages and Quantitative Comparison
Compared to conventional sand casting, our special process significantly reduced machining allowance and casting weight. The following table quantifies the improvements:
| Parameter | Conventional Sand Casting | Our Special Process (Self-hardening Resin Sand + Chills) |
|---|---|---|
| Machining allowance (mm) | 4–8 | 2–2.5 |
| Cast weight per piece (kg) | ~24 | ~15 |
| Yield (first pass) | < 50% | 98–100% |
| Defect types encountered | Gas holes, shrinkage, iron shots | None after optimization |
6. Mathematical Models Applied
We used the A-san formula for the choke area calculation:
$$ A_{\text{ingate}} = \frac{W}{\rho \cdot t \cdot v \cdot \sqrt{2gH}} $$
where:
– W = casting weight (13.5 kg)
– ρ = density of liquid iron (~7.0 g/cm³)
– t = pouring time (estimated as 8–10 seconds for this small casting)
– v = velocity coefficient (0.5–0.6)
– g = 981 cm/s²
– H = effective metallostatic head (average head during pour, ~15 cm)
This gave an ingate area of approximately 3.0 cm², consistent with our 304 mm².
For modulus calculation of the hot spot, we approximated the largest thermal center as a cylinder of diameter 25 mm and height 30 mm:
$$ M_{\text{hot spot}} = \frac{V}{A} = \frac{\pi r^2 h}{2\pi r h + 2\pi r^2} = \frac{25^2 \times 30 \times \pi}{2\times 25\times 30\pi + 2\times 25^2\pi} \approx 1.2\ \text{cm} $$
Riser volume was determined by the modulus ratio of 1.4.
7. Practical Observations and Quality Control
During production of nearly 200 pieces in the first half of 1997, we achieved 100% acceptance. The use of self-hardening resin sand allowed easy mold assembly and dismantling, while the chills with air gap eliminated all sand casting defects. The visual and radiographic inspection showed no shrinkage, no gas porosity, and no slag inclusions. The dimensional accuracy was within ±0.5 mm on all critical features, meeting the customer’s specifications.
8. Conclusion
Our experience demonstrates that for complex impeller and rotor-type castings, a hybrid approach combining self-hardening resin sand with strategically placed chills (with air gap) and a carefully designed gating system can completely eliminate common sand casting defects. The process is robust, reproducible, and cost-effective for mass production. Key factors include: (1) bottom gating to avoid turbulence, (2) use of chills to control solidification gradients, (3) proper riser modulus design, and (4) strict control of melt temperature and inoculation. This methodology has since been applied to similar castings with equally favorable results.