In the field of heavy electrical equipment and power apparatus, the demand for large thin-walled aluminum alloy castings has grown significantly. These components, such as transformer tanks, switchgear housings, and engine crankcases, require high mechanical strength, excellent electrical conductivity, and absolute gas tightness to contain SF6 or other insulating media. Traditional gravity casting methods often lead to defects like porosity, shrinkage, and poor dimensional accuracy. To address these challenges, we have developed a comprehensive low pressure sand casting foundry technology that integrates optimized alloy chemistry, advanced melt purification, precision mold positioning, and specialized core making. Over the past decade, this technology has been implemented in multiple factories and has produced tens of thousands of castings exported to major international manufacturers. In this article, we present the key aspects of this sand casting foundry process, highlighting the critical role of sand casting foundry techniques in achieving high-quality large aluminum castings.
The fundamental principle of low pressure sand casting foundry involves using compressed air to force molten aluminum from a sealed crucible upward into a sand mold. This method offers superior filling control, reduced turbulence, and the ability to feed shrinkage during solidification. Unlike conventional low pressure die casting, our approach employs sand molds, which provide flexibility for large and complex geometries at lower tooling costs. The sand casting foundry system we designed uses a crucible sealing mechanism instead of the traditional furnace sealing, allowing heavy molds to be supported independently, thus extending furnace life. The combination of sand casting foundry with low pressure technology enables the production of parts weighing up to 620 kg with wall thicknesses as thin as 8 mm while maintaining tight tolerance and dense microstructure.
1. Optimized Alloy Chemistry and Melt Purification for Sand Casting Foundry
The base alloy for these applications is AlSi7Mg (equivalent to ZL101A, A356, or AC4CH). However, the electrical conductivity of aluminum decreases with increasing silicon content, while fluidity improves. To balance these conflicting requirements, we narrow the silicon range compared to standard specifications. Through extensive experimentation, we determined the optimal composition that maximizes both mechanical properties and conductivity. Table 1 summarizes the target composition limits adopted in our sand casting foundry practice.
| Element | Min (%) | Max (%) |
|---|---|---|
| Si | 6.8 | 7.2 |
| Mg | 0.30 | 0.50 |
| Ti | 0.15 | 0.25 |
| Zr | 0.10 | 0.15 |
| B | 0.02 | 0.05 |
| RE (La+Ce) | 0.05 | 0.10 |
| Fe | ≤0.12 | – |
| P | ≤0.0007 | – |
| Ca | ≤0.001 | – |
| Al | Balance | – |
The electrical conductivity (σ) of the aluminum alloy as a function of silicon content can be approximated by the following empirical relation derived from our measurements:
$$
\sigma = 35.0 – 1.2\,w_{\text{Si}} \quad (\text{in MS/m})
$$
where wSi is the weight percent of silicon. This equation is valid for the range wSi = 6.0–8.0%. The conductivity target for our sand casting foundry products is ≥ 28.5 MS/m, which corresponds to wSi ≤ 7.2%. Magnesium, while strengthening via Mg2Si precipitation, reduces conductivity. We found that keeping Mg between 0.30% and 0.50% yields an optimal combination of tensile strength (≥280 MPa), elongation (≥3%), and conductivity (≥28 MS/m).
Titanium addition at levels above 0.15% promotes grain refinement through Al3Ti particles, which act as heterogeneous nucleation sites. The nucleation rate increases dramatically at a critical undercooling, described by classical nucleation theory:
$$
I = I_0 \exp\left(-\frac{\Delta G^*}{kT}\right)
$$
where ΔG* is the activation energy for nucleation. The introduction of Ti also forms TiH compounds that remove hydrogen from grain boundaries. To further enhance refinement, we developed a secondary titanium addition technique: after melting, a salt-based grain refiner containing Ti-B-Zr is added (0.1% of melt weight), generating fresh Al3Ti, TiB2, and Al3Zr particles that are extremely effective in reducing grain size.
Impurity control is critical in sand casting foundry for electrical applications. Phosphorus, even at levels as low as 15×10−4%, forms AlP which coarsens eutectic silicon and degrades mechanical properties. Calcium promotes gas absorption and surface defects. Therefore, we strictly limit P to ≤0.0007% and Ca to ≤0.001%. The source of P is mainly the silicon raw material; we use only high-grade silicon (P ≤ 7×10−4%) for sand casting foundry production.
For melt purification, we developed a multifunctional flux that simultaneously refines, modifies, and degasses. The flux generates inert gas bubbles when added to the melt at 720 °C, which float to the surface and carry away hydrogen and oxide inclusions. The reaction product is non-wetting with aluminum, facilitating easy removal. Additionally, the rare earth elements (La, Ce) in the flux react with residual hydrogen to form stable hydrides (CeH3), preventing pore formation during solidification. For large centralized melting operations, we employ a rotary impeller degassing unit with pulsed gas injection to produce fine bubbles, achieving hydrogen levels below 0.10 mL/100g Al. This level of cleanliness is essential for sand casting foundry of pressure-tight components.
2. Low Pressure Sand Casting Foundry Equipment and Process
The low pressure sand casting foundry apparatus we designed uses a crucible sealing method, which is fundamentally different from conventional furnace-sealed machines. In typical low pressure die casting, the entire furnace body is pressurized, requiring that the mold weight be borne by the furnace shell. For large sand molds weighing up to 13 tons, this is impractical. Our patented crucible seal design, illustrated conceptually in the following figure (see Figure 1), isolates the pressure chamber to the crucible only. The mold rests on a large support plate that transfers its weight directly to the ground via steel columns, while the furnace shell and refractory lining experience no mechanical load. This extends furnace life and allows for larger mold sizes.
Table 2 provides the specifications of two sizes of low pressure sand casting foundry units developed for this technology.
| Parameter | Unit 500 kg | Unit 800 kg |
|---|---|---|
| Melt capacity (Al) | 500 kg | 800 kg |
| Max mold weight | 8 tons | 15 tons |
| Max mold size (L×W×H) | 2500×1500×2000 mm | 3000×1800×2500 mm |
| Operating pressure | 0 – 0.15 MPa | 0 – 0.15 MPa |
| Crucible material | Cr-Si-Ni heat-resistant steel | Cr-Si-Ni heat-resistant steel |
| Crucible life | >150 cycles | >150 cycles |
| Sealing method | Crucible seal (patented) | Crucible seal (patented) |
The crucible is made of a specially developed chromium-silicon-nickel heat-resistant steel, which withstands both high temperature (∼750 °C) and internal pressure (up to 0.13 MPa) for over 150 cycles. This is a significant improvement over conventional cast iron crucibles used in sand casting foundry, which typically last only 50–100 cycles even without pressure. The filling pressure is controlled by a throttle valve and pressure transducer, ensuring a smooth and repeatable rise rate. This avoids turbulence and cold shuts, common problems in gravity sand casting of large thin-walled parts.
The low pressure sand casting foundry process offers several advantages:
- Clean melt: Only metal from below the surface enters the mold, leaving dross and oxides in the crucible.
- Directional solidification: The pressure gradient allows efficient feeding of shrinkage, especially in thick sections.
- Reduced gas entrapment: The quiescent filling minimizes air entrainment.
- Versatility: Molds can be dry clay sand, resin-bonded sand, or water-glass sand, depending on production scale and complexity.
The pressure profile during casting is typically approximated by:
$$
P(t) = P_0 + \frac{P_f – P_0}{t_f} t \quad \text{for } 0 \leq t \leq t_f
$$
where P0 is the initial pressure (atmospheric + metal head), Pf is the final holding pressure, and tf is the filling time. Holding pressure is maintained at 0.05–0.10 MPa during solidification to feed shrinkage. The optimal parameters depend on casting geometry; for a typical 500 kV transformer tank (ϕ1280 mm × 1480 mm, 420 kg), the filling time is 30–40 seconds and holding time is 5–8 minutes.
3. Precision Mold Positioning and Chill Technology in Sand Casting Foundry
One of the major challenges in sand casting foundry is achieving accurate wall thickness, especially when the casting has large flanges (50–80 mm thick) adjacent to thin walls (8–15 mm). Traditional sand core prints allow a clearance of 1.0–1.5 mm between core and mold, leading to wall thickness deviations of 2–3 mm. To overcome this, we developed a precise positioning system that integrates metal chills with metal core prints.
Table 3 compares the traditional and improved core seating methods used in our sand casting foundry.
| Feature | Traditional method | Improved method (our technology) |
|---|---|---|
| Core print material | Sand (formed in mold) | Machined ductile iron ring |
| Chill material | Steel or cast iron without treatment | Ductile iron with nitriding |
| Chill surface | Rough, often coated with oil and sand | Machined, with vent grooves every 20 mm |
| Core mold assembly | Loose fit (1.0–1.5 mm gap) | Tight fit (0.5 mm gap) |
| Typical wall thickness deviation | 2–3 mm | ≤1 mm |
| Core construction | Solid core with straw rope and coke | Hollow core using steel tube and special tooling |
The chills are made of ductile cast iron, which has excellent thermal conductivity and resistance to oxidation. Before use, each chill is sandblasted and then salt-bath nitrided to form a dense, corrosion-resistant surface layer. This prevents rust formation (Fe3O4) that would otherwise release moisture upon contact with molten aluminum, causing porosity. The machined surface includes grooves that allow gases to escape during filling, further improving casting quality. The core print ring is also machined from ductile iron, and when combined with the chill ring, creates a metal-to-metal interface with only 0.5 mm clearance. This dramatically improves positioning accuracy in sand casting foundry.
For cores, we replaced solid sand cores with hollow cylindrical cores made using a steel tube as the armature. The core sand (water-glass bonded) is compacted around the tube using a special jig, ensuring uniform density. The core is then baked or CO2 hardened. After casting, the sand collapses easily due to the hollow design, and the remaining sand is removed by vibration and high-pressure water jets. This method is particularly effective for closed cavities like the connector tubes (ϕ280 mm, wall thickness 4 mm) that require smooth internal surfaces.
4. Simulation and Core Manufacturing for Sand Casting Foundry
To reduce development time and cost, we employ three-dimensional CAD modeling and solidification simulation for all new sand casting foundry projects. The simulation uses finite element analysis to predict shrinkage porosity, hot spots, and mold filling patterns. The governing heat transfer equation during solidification is:
$$
\rho c_p \frac{\partial T}{\partial t} = \nabla \cdot (k \nabla T) + \dot{q}
$$
where ρ is density, cp is specific heat, k is thermal conductivity, and \dot{q} is the latent heat release rate. The latent heat is modeled using the enthalpy method. The simulation allows us to optimize the placement of chills, risers, and gating system before building the mold. For example, in the 500 kV transformer housing shown earlier, the simulation revealed a hot spot at the thick flange junction, which was eliminated by adding external chills and adjusting the pressure profile.
A critical innovation in our sand casting foundry is the “female-male” core making technique for high-precision internal cavities. The process steps are:
- Machine a metal master core (usually steel) to the exact final dimensions and surface finish of the required cavity.
- Cast a silicone rubber mold around the metal master; the rubber has zero shrinkage and reproduces the master’s surface faithfully.
- Use the rubber mold as a pattern to produce a sand core with a low-dust, high-collapsibility water-glass sand that contains a special binder (patented). Before filling, the rubber mold cavity is sprayed with a thin coating.
- After curing, the sand core is removed from the rubber mold; its dimensions and surface finish are identical to the metal master.
- The core is assembled into the sand mold, and the casting is poured using low pressure.
Table 4 summarizes the properties of the sand system used in this technique.
| Property | Value |
|---|---|
| Grain fineness (AFS) | 50–60 |
| Water glass content | 3–4% by weight |
| Collapsibility after casting | >95% |
| Gas evolution | ≤15 mL/g |
| Surface finish (Ra) | ≤3.2 μm (on core) |
| Binder type | Modified sodium silicate with organic additives |
This technique has allowed us to produce connector tubes that were previously made abroad by plaster vacuum casting. Our sand casting foundry method not only achieves the required internal surface finish (Ra ≤ 3.2 μm) but also ensures dense microstructure free of leakage, outperforming the plaster process. The cycle time is 5 times faster, and the cost is only 30% of the imported parts.
5. Production Experience and Results
Over the past decade, our low pressure sand casting foundry technology has been transferred to multiple factories in China. More than 200 different types of large thin-walled housings have been produced, totaling over 50,000 pieces. The annual production capacity of the licensed foundries exceeds 4,000 tons of aluminum castings. The products are exported to Germany, Italy, Switzerland, France, the United States, and Japan, supplying major electrical equipment manufacturers such as ABB, Siemens, Areva, Hitachi, and Alstom.
Table 5 lists some typical castings produced using our sand casting foundry process and their key parameters.
| Casting name | Dimensions (mm) | Weight (kg) | Wall thickness range (mm) | Application |
|---|---|---|---|---|
| 500 kV transformer housing | ϕ1280 × 1480 | 420 | 8–50 | SF6 insulated transformer |
| 345 kV switchgear tank | ϕ870 × 2350 | 620 | 10–60 | Gas insulated switchgear |
| Diesel engine cylinder block | 1500 × 600 × 500 | 280 | 12–40 | Heavy truck engine |
| Connector tube | ϕ280 × 800, wall 4 mm | 45 | 4 (uniform) | Electrical connector |
All castings undergo rigorous quality control: X-ray or CT inspection for internal porosity, helium leak testing for gas tightness (leak rate < 0.5% per year), and tensile testing. The consistent results demonstrate that our sand casting foundry process reliably meets or exceeds international standards.
6. Conclusion
We have developed a comprehensive low pressure sand casting foundry technology tailored for large, thin-walled aluminum alloy castings used in high-voltage electrical equipment and heavy machinery. By optimizing the AlSiMg alloy composition with controlled impurities and trace additions (Ti, Zr, B, RE), and employing advanced melt purification, we achieve a balance of mechanical properties and electrical conductivity. The specialty designed low pressure equipment with crucible sealing enables the production of castings up to 620 kg with excellent dimensional accuracy. The combination of machined metal chills and core prints reduces wall thickness variation to less than 1 mm. Furthermore, the innovative “female-male” core making method yields intricate internal cavities with high surface finish and productivity. Over ten years of industrial application and thousands of successful export orders confirm that this sand casting foundry technology is robust, cost-effective, and capable of meeting the most demanding specifications.