In the modern aerospace industry, the fuel housing serves as a critical component within the engine fuel system, demanding exceptional structural integrity, dimensional precision, and long-term reliability. The geometric complexity of these housings has increased substantially, rendering conventional sand casting methods inadequate due to issues such as porosity, shrinkage, and poor surface finish in intricate internal passages. We have adopted a low-pressure sand casting process combined with 3D printing technology to fabricate these high-performance components. This study systematically investigates the influence of resin content and coating parameters on the key properties of 3D printed sand molds, including compressive strength, gas evolution, and permeability, specifically for sand casting applications. Our objective was to optimize the sand casting process to eliminate gas porosity defects in ZL114A aluminum alloy fuel housings produced by low-pressure casting. The integration of 3D printing provides unprecedented design freedom for complex core geometries, while low-pressure casting ensures smooth and controlled mold filling, making the combination highly suitable for advanced sand casting production. Through a series of controlled experiments, we analyzed how the resin fraction in the binder system affects the mechanical and thermal behavior of the sand mold. Furthermore, we evaluated the impact of coating parameters, such as immersion time and Baumé degree, on the mold’s gas evolution and permeability. Our findings demonstrate that a careful balance between strength and gas-related properties is essential for successful sand casting. By optimizing these parameters, we successfully mitigated gas porosity defects in the final castings, confirming the efficacy of our approach for the low-pressure sand casting of complex aerospace components.
The 3D printing of sand molds for sand casting relies on a binder system, typically a furan resin, to bond the silica sand particles. The resin content is a crucial variable that directly affects both the mechanical integrity of the mold and its thermal behavior during pouring. In our study, we prepared specimens using 70–140 mesh silica sand with resin mass fractions ranging from 0.8% to 2.0%. These specimens were printed using a commercial 3D sand printer and subsequently tested for tensile strength, compressive strength, gas evolution, and permeability. The mechanical tests were conducted using an intelligent sand strength tester, while the gas evolution was measured using a gas evolution tester at a decomposition temperature of 710°C for 100 seconds. Permeability was assessed using a standard permeability tester with a 981 Pa air pressure differential. Our experimental data, summarized in Tables 1 and 2, reveal a clear trade-off between strength and gas evolution as the resin content increases.
| Resin Content (wt.%) | Tensile Strength (MPa) | Compressive Strength (MPa) |
|---|---|---|
| 0.8 | 0.21 | 2.10 |
| 1.2 | 0.38 | 3.60 |
| 1.6 | 0.52 | 4.30 |
| 2.0 | 0.65 | 4.55 |
Table 1 shows that both tensile and compressive strengths increase monotonically with resin content. For instance, the compressive strength rises from 2.10 MPa at 0.8% resin to 4.55 MPa at 2.0% resin. This enhancement is attributed to the increased formation of resin bridges between sand particles, which strengthens the mold structure. However, as illustrated in Table 2, this gain in mechanical performance is accompanied by a significant increase in gas evolution. The gas evolution rises from 2.83 mL/g at 0.8% resin to 10.70 mL/g at 2.0% resin. This is because a higher resin content provides more organic material that decomposes under the heat of the molten metal, generating volatile gases. If these gases cannot escape through the mold pores, they may become trapped in the solidifying metal, forming gas porosity defects. The permeability data in Table 2 indicate a slight decreasing trend with increasing resin content, from 34 cm⁴/(g·min) at 0.8% to 30 cm⁴/(g·min) at 2.0%. This reduction is due to the resin filling some of the inter-particle voids, thereby restricting gas flow paths. The relationship between resin content and compressive strength can be approximated by a linear regression model, as shown in Equation (1).
$$ C_s = 2.34 + 1.14 \times R_c $$
where \(C_s\) is the compressive strength in MPa and \(R_c\) is the resin content in wt.%. This model indicates that each 0.5% increase in resin content yields approximately a 0.57 MPa increase in compressive strength. Considering the balance between strength, gas evolution, and permeability, we identified 1.2% resin content as the optimal value for our sand casting application. At this level, the compressive strength reaches 3.6 MPa, which is sufficient to withstand the static and dynamic pressures during low-pressure casting. The gas evolution of 8.6 mL/g at 1.2% resin is manageable, and the permeability of 32 cm⁴/(g·min) remains adequate for gas evacuation. Excessive resin, such as 2.0%, leads to gas evolution exceeding 10.7 mL/g, which poses a high risk for gas porosity, especially in complex internal channels of the fuel housing. Therefore, for the low-pressure sand casting of these components, we selected 1.2% resin as the baseline for subsequent coating experiments.
| Resin Content (wt.%) | Gas Evolution (mL/g) | Permeability (cm⁴/(g·min)) |
|---|---|---|
| 0.8 | 2.83 | 34 |
| 1.2 | 8.60 | 32 |
| 1.6 | 9.80 | 31 |
| 2.0 | 10.70 | 30 |
Having established the optimal resin content, we focused on the coating process, which is essential for improving the surface quality of the casting, particularly in the intricate oil passages of the fuel housing. However, the application of a coating can significantly alter the gas evolution and permeability of the sand mold. We investigated the effects of coating dilution (water addition) and immersion time on these properties. A water-based coating was diluted with different amounts of water (0 mL to 400 mL per unit volume of coating) and applied to standard sand mold specimens by immersion for durations ranging from 2 seconds to 8 seconds. The gas evolution and permeability of the coated specimens were measured and are summarized in Tables 3 and 4. Our results show that immersion time significantly influences gas evolution, as shown in Table 3. At a water addition of 200 mL, the gas evolution decreases from 10.5 mL/g at 2 s immersion to a minimum of 9.1 mL/g at 4 s, but then increases again to 10.0 mL/g at 8 s. This trend suggests that a short immersion time does not allow sufficient coating penetration to form a uniform barrier, while excessive immersion leads to a thick coating layer that itself contributes to gas generation during thermal decomposition. The optimal immersion time appears to be 4 seconds, which provides a uniform coating that seals the sand surface without adding excessive organic material that would later decompose.
| Immersion Time (s) | Gas Evolution (mL/g) |
|---|---|
| 2 | 10.5 |
| 4 | 9.1 |
| 6 | 9.4 |
| 8 | 10.0 |
Table 4 presents the influence of immersion time on permeability for the same coating dilution. The permeability decreases from 28 cm⁴/(g·min) at 2 s immersion to 22 cm⁴/(g·min) at 8 s immersion. This reduction is expected, as a thicker coating layer fills more of the surface pores, thereby restricting gas flow. For the sand casting process of a fuel housing, a balance must be struck. A permeability of 28-32 cm⁴/(g·min) is desirable, as it allows evolved gases to escape without excessive resistance. At an immersion time of 4 seconds, we achieved a permeability of 26 cm⁴/(g·min), which, combined with a gas evolution of 9.1 mL/g, provides a favorable condition for casting. The coating also acts as a thermal barrier, slightly reducing the rate of resin decomposition. The effect of water addition on permeability is also shown in Table 4 for an immersion time of 4 seconds. As the water addition increases from 0 mL to 400 mL, the coating viscosity decreases, leading to a thinner coating layer and consequently higher permeability. However, a very dilute coating may not provide sufficient surface protection, especially in high-temperature casting of aluminum alloys. The optimal water addition was found to be around 200 mL, corresponding to a coating Baumé degree of 20°Bé, which balances permeability and protective capacity.
| Immersion Time (s) | Water Addition (mL) | Permeability (cm⁴/(g·min)) |
|---|---|---|
| 2 | 200 | 28 |
| 4 | 0 | 18 |
| 4 | 200 | 26 |
| 4 | 400 | 35 |
| 8 | 200 | 22 |
Coating viscosity, measured by Baumé degree, is another critical parameter in the sand casting process. We tested coatings with Baumé degrees of 15°Bé, 20°Bé, 25°Bé, and 30°Bé. The results in Table 5 indicate that permeability decreases as the Baumé degree increases, from 30 cm⁴/(g·min) at 15°Bé to 14 cm⁴/(g·min) at 30°Bé. This is because a higher Baumé degree corresponds to a thicker, more concentrated coating, which fills the sand surface pores more effectively. However, we observed that at Baumé degrees below 20°Bé, the coating was too thin to provide adequate coverage, leading to poor surface finish on the casting. Conversely, at Baumé degrees above 25°Bé, the coating became excessively thick. During drying, such thick coatings often caused peeling and spalling, as illustrated by the physical observations from our experiments. This would introduce loose coating fragments into the mold cavity, potentially causing inclusions in the final casting. Therefore, the optimal Baumé degree was determined to be 20°Bé, which yields a permeability of 24 cm⁴/(g·min) and provides sufficient surface quality without the risk of peeling. At 20°Bé, the coating forms a robust, crack-free layer that adheres well to the sand substrate and withstands the thermal shock during pouring.
| Baumé Degree (°Bé) | Permeability (cm⁴/(g·min)) |
|---|---|
| 15 | 30 |
| 20 | 24 |
| 25 | 18 |
| 30 | 14 |
Based on our comprehensive analysis, we established the optimal process parameters for the 3D printed sand mold in low-pressure sand casting: a resin content of 1.2%, a coating Baumé degree of 20°Bé, and an immersion time of 4 seconds. These parameters were applied in a validation production run for a ZL114A aluminum alloy fuel housing. The 3D printed sand molds were designed with integrated gating and riser systems, which were also printed simultaneously. The molds were coated with the optimized process, dried, and then assembled for low-pressure casting. The casting process parameters included a pouring temperature of 720°C, a mold preheat temperature of 200°C, and a pressurization rate of 0.01 MPa/s. After casting and solidification, the fuel housings were inspected using X-ray radiography and dye penetrant testing. The results showed a complete elimination of gas porosity defects. Previously, with non-optimized parameters, gas porosity was a common defect, particularly in the thick-walled sections and at the junctions of internal passages. The optimized resin content reduced the total gas generation, while the optimized coating ensured that these gases could be efficiently vented through the mold. The permeability of the 3D printed sand mold, even after coating, was sufficient to prevent gas entrapment. The surface quality of the internal oil passages was excellent, meeting the stringent roughness requirements for fuel system components. This validation confirms that the synergistic optimization of 3D printing parameters and coating technology is key to successful sand casting of complex aerospace parts.
In conclusion, our study demonstrates that for low-pressure sand casting of complex fuel housing components using 3D printed sand molds, the resin content and coating process parameters must be carefully optimized. A resin content of 1.2% provides a good balance between mechanical strength and gas evolution, with a compressive strength of 3.6 MPa and a gas evolution of 8.6 mL/g. The coating process, particularly immersion time and Baumé degree, has a profound effect on the mold’s gas evolution and permeability. An immersion time of 4 seconds and a Baumé degree of 20°Bé were found to be optimal, reducing the coating-induced gas evolution to 9.1 mL/g while maintaining a permeability of 24-26 cm⁴/(g·min). These optimized parameters successfully eliminated gas porosity defects in ZL114A aluminum alloy fuel housings, as verified by industrial production trials. The findings highlight the critical role of process stability in sand casting, where the interplay between the sand mold’s innate properties and the applied coating can dictate the quality of the final casting. Our work provides a practical guideline for implementing 3D printed sand molds in low-pressure sand casting for demanding aerospace applications, ensuring that the benefits of design freedom and process control are fully realized.