In our sand casting foundry, we have been producing large and complex valve bodies made of silicon brass ZCuZn16Si4 for the chemical industry. This material is widely used for pressure-containing parts such as pump casings, impellers, and valves due to its excellent corrosion resistance and good casting properties. However, for large and intricate valve shells, leakage during hydrostatic testing is a common challenge. In this paper, we share our systematic approach to designing a robust sand casting foundry process that prevents cold shuts, shrinkage porosity, and oxide inclusions, ensuring reliable pressure tightness.
Material Characteristics of ZCuZn16Si4
Silicon brass ZCuZn16Si4 contains 79–81% copper, 2.5–4.5% silicon, and the balance zinc, with strict limits on impurities such as iron, aluminum, manganese, antimony, tin, and lead. The alloy has a body contraction smaller than that of manganese brass or iron brass, and its oxidation tendency is lower than other brasses. Nevertheless, due to its low zinc content, it has a high tendency to absorb gas during melting, which can cause porosity and swelling. In our sand casting foundry, we pay special attention to degassing and slag control to avoid these defects.
| Cu | Si | Zn | Fe | Al | Mn | Sb | Sn | Pb | Total Impurities |
|---|---|---|---|---|---|---|---|---|---|
| 79.0–81.0 | 2.5–4.5 | Balance | ≤0.60 | ≤0.10 | ≤0.50 | ≤0.10 | ≤0.30 | ≤0.50 | ≤2.00 |
| Tensile Strength Rm (MPa) | Yield Strength Rp0.2 (MPa) | Elongation A (%) | Hardness HBW |
|---|---|---|---|
| 345 | — | 15 | 885 |
Analysis of the Valve Shell Structure and Potential Defects
The selector valve shell has overall dimensions of 380 mm length, 319 mm width, and 335 mm height. It is a four-way structure with five flanges. The minimum wall thickness is 9 mm, and the thickest flange outer diameter is 240 mm with a thickness of 25 mm. The casting weight is about 70 kg. Based on our experience in this sand casting foundry, we identified three high-risk defects:
- Cold shut and lamination: The long flow paths and thin walls (9 mm) cause rapid temperature drop and difficulty in filling.
- Shrinkage cracks and porosity: Thick flanges (25 mm) and junctions where multiple walls intersect create hot spots that need effective feeding.
- Oxide inclusions: Complex internal cavities lead to turbulent flow, trapping oxides that compromise pressure tightness.
The hydrostatic test requirement is 0.9 MPa for 5 minutes with no leakage. This is challenging for a large complex valve shell produced in a sand casting foundry.
Countermeasures for Defect Prevention
We implemented the following targeted measures in our sand casting foundry:
| Defect Type | Countermeasure | Implementation Detail |
|---|---|---|
| Cold shut / Lamination | Increase filling ability | Optimize gating to shorten flow distance; use chills to avoid blocking; raise initial pouring speed. |
| Shrinkage / Porosity | Enhance feeding and chilling | Place chills on thick flanges and hot spots; locate risers at thick sections; apply green sand mold with dry cores to form a dense surface layer. |
| Oxide inclusions | Reduce turbulence and filtering | Install filters to trap primary oxides; avoid free fall of liquid metal; minimize gas pickup in melting. |
Casting Process Design
Machining Allowances and Shrinkage
For the thickest flange, a machining allowance of 6 mm was applied to the outer diameter, while other machined surfaces received 5 mm. The internal seal-ring grooves also had 6 mm allowance to accommodate any core shift. The pattern shrinkage for the outer mold (green sand) was set at 1.5%, and for the core boxes (dry sand) at 1.0% due to their lower collapsibility.
Molding Method and Casting Position
We adopted a horizontal parting line with green sand molds and dry sand cores. This configuration is typical in a sand casting foundry for complex valve bodies because it facilitates pattern withdrawal and core setting. The horizontal pouring position was chosen because silicon brass does not form heavy oxide films like aluminum bronze, so the need for vertical pouring is less critical.
Riser Design
Risers were placed on the thickest flange (top) and on two side flanges to provide both feeding and pressure head. A blind square riser was placed directly on the thick flange, while two side round risers were attached to the flanges at the ends of the length direction. This triangular arrangement shortens the feeding distance to thin sections and balances solidification.
Gating System
A partially pressurized (initially closed, then open) gating system was used. The downsprue diameter was 35 mm to ensure rapid initial filling. After passing through a filter to trap primary oxides, the metal flows into an open runner and then into the side risers. Two critical details: the ingate width into the side riser was limited to ≤20 mm to avoid inverse shrinkage, and the ingate height was tapered (shallower at top, deeper at bottom) to eliminate free fall of the liquid metal.
The total weight of the poured system is 134 kg, giving a casting yield of 56% (casting weight 75 kg after machining allowances). This yield is typical for pressure-tight valve castings in a sand casting foundry.
Chill Application
We used graphite chills extensively because they can be easily shaped and prevent moisture pickup by simply coating with a thin layer of oil. The chills were placed as follows:
- On the thickest flange: two quarter-circle chills (30 mm thick) on the lower half of the outer circumference.
- On the bottom flange (in the lower mold): two half-circle chills (25 mm thick).
- Inside the valve shell at two seal-ring grooves: four quarter-circle chills (18 mm thick).
- At three locations where multiple walls intersect (hot spots): rectangular chills (25 mm thick).
The use of chills accelerates solidification locally, promoting a dense microstructure and reducing shrinkage porosity. This is a common practice in any well-run sand casting foundry producing pressure-tight components.
Mathematical Model for Solidification and Feeding
To quantitatively design the riser and chill dimensions, we applied the modulus method. The modulus $M$ of a casting section is defined as the ratio of volume $V$ to cooling surface area $A$:
$$
M = \frac{V}{A}
$$
For a flat plate of thickness $t$, the modulus simplifies to $M = t/2$. For a cylinder (riser) of diameter $d$ and height $h$, the modulus is:
$$
M_{\text{riser}} = \frac{\pi d^2 h /4}{\pi d h + \pi d^2/4} = \frac{d h}{4h + d}
$$
We ensured that the riser modulus is at least 1.2 times the modulus of the casting section it feeds:
$$
M_{\text{riser}} \ge 1.2 \cdot M_{\text{casting}}
$$
The feeding distance $L$ for a plate with a chill at one end and a riser at the other can be approximated by:
$$
L = 2.5 \sqrt{t} \quad \text{(in cm)}
$$
Where $t$ is the plate thickness in cm. For our 9 mm (0.9 cm) wall, $L \approx 2.5 \sqrt{0.9} \approx 2.37 \text{ cm}$, which is very short. Thus, without chills, feeding would be impossible. By placing chills at strategic locations, we effectively reduced the local solidification time and extended the effective feeding range of the risers.
The volume contraction of silicon brass during solidification is approximately $\beta = 4.5\%$. The required riser volume $V_r$ to compensate for shrinkage is:
$$
V_r = \beta \cdot V_c \cdot \frac{1}{\eta}
$$
Where $V_c$ is the casting volume to be fed, and $\eta$ is the riser efficiency (typically 0.14–0.20 for open risers in sand casting foundry). For the thick flange (volume approximately 500 cm³), the required riser volume is:
$$
V_r = 0.045 \times 500 / 0.15 = 150 \text{ cm}^3
$$
Our designed square riser had a volume of ~200 cm³, providing a safety margin.
Production Experience and Results
We manufactured five batches (total ~30 pieces) of this valve shell in our sand casting foundry. The first trial cast two pieces to validate the process. Both passed hydrostatic testing at 0.9 MPa without any leakage. In subsequent batches, we optimized the shakeout time: we opened the mold when the casting was still dark red (~500°C) and allowed it to air cool, which improved the density of the surface layer. No water quenching was used to avoid distortion.
All production castings passed the pressure test. No cold shuts, shrinkage cracks, or oxide inclusions were observed. The yield and quality were consistently high, confirming the effectiveness of our sand casting foundry process.
Summary of Key Process Parameters
| Parameter | Value | Remarks |
|---|---|---|
| Alloy | ZCuZn16Si4 | See Table 1 |
| Casting weight (with allowance) | 75 kg | Machined weight ~70 kg |
| Pouring weight | 134 kg | Yield 56% |
| Downsprue diameter | 35 mm | Initial closed system |
| Ingate width | ≤20 mm | To avoid inverse shrinkage |
| Riser type | Blind square + side round | Modulus check applied |
| Chill material | Graphite | Coated with oil to prevent moisture |
| Mold process | Green sand + dry sand cores | Dense surface layer |
| Shakeout temperature | ~500°C (dark red) | Air cool |
Conclusions
Through systematic process design in our sand casting foundry, we successfully produced large silicon brass selector valve shells without defects. The key conclusions are:
- Cold shuts are prevented by optimizing gating layout, using chills to avoid flow blockages, and increasing initial pouring speed. These measures are standard in any sand casting foundry dealing with thin-walled castings.
- Shrinkage porosity and cracks are eliminated by placing risers on thick sections and applying chills at hot spots. The use of graphite chills in this sand casting foundry proved effective and easy to implement.
- Oxide inclusions are minimized by installing filters to capture primary oxides, avoiding free fall of liquid metal, and controlling melt quality. This is a critical step for pressure-tight requirements in a sand casting foundry.
- All production castings passed the 0.9 MPa hydrostatic test, demonstrating the robustness of the process.
The methodology described here can be adapted to other complex valve castings in a sand casting foundry, providing a reliable reference for similar applications.