In the field of lost foam casting, also known as evaporative pattern casting (EPC), the use of transfer coatings is critical for enhancing the dimensional accuracy and surface quality of cast components. These coatings, applied directly onto the foam pattern, transfer to the mold cavity without occupying its effective volume, thereby replicating the pattern’s smooth surface. However, industrial applications often face challenges such as low常温 strength, poor suspension stability, and susceptibility to cracking in transfer coatings. This study addresses these issues through a systematic design and optimization of coating compositions, employing orthogonal experiments to develop a high-performance transfer coating suitable for lost foam casting processes, particularly for ductile iron castings.
The fundamental principle of transfer coatings in lost foam casting revolves around the interfacial bond strengths between the coating, the pattern, and the molding sand. Ideally, the adhesion strength between the coating and the pattern (denoted as $\sigma_{T-M}$) should be significantly lower than that between the coating and the sand ($\sigma_{T-S}$). This ensures that the coating can be easily applied to the foam pattern and then efficiently transferred to the mold cavity after pattern removal. The relationship can be expressed as:
$$\sigma_{T-M} \ll \sigma_{T-S}$$
where $\sigma_{T-M}$ represents the tensile or shear strength at the coating-pattern interface, and $\sigma_{T-S}$ denotes the strength at the coating-sand interface. Achieving this balance is essential for successful application in EPC, as it prevents coating detachment during molding while facilitating smooth transfer. The coating must also exhibit excellent suspension, drying properties, and resistance to thermal shock during casting to prevent defects like metal penetration and surface irregularities.
To optimize the transfer coating composition, we designed a series of experiments focusing on four key factors: refractory powder blend (A), binder system (B), suspending agents (C), and carrier fluid (D). Each factor was evaluated at three levels, leading to an L9(34) orthogonal array that efficiently explores the parameter space. The refractory powder composition was varied to include combinations of alumina bauxite, quartz, talc, graphite, and iron oxide, while binders consisted of 2123 resin and tetraethyl orthosilicate in different ratios. Suspending agents included polyvinyl butyral (PVB) and attapulgite clay, and the carrier fluid was primarily isopropanol with minor water additions. This approach allows for identifying the optimal mix that maximizes performance metrics such as suspension stability, strength, and high-temperature crack resistance.
The table below summarizes the factors and levels used in the orthogonal experiments for lost foam casting transfer coatings:
| Factor | Level 1 | Level 2 | Level 3 |
|---|---|---|---|
| Refractory Powder (A) / g | 70% Alumina Bauxite, 0% Quartz, 15% Talc, 15% Graphite, 0% Fe2O3 | 60% Alumina Bauxite, 10% Quartz, 10% Talc, 10% Graphite, 10% Fe2O3 | 75% Alumina Bauxite, 5% Quartz, 8% Talc, 5% Graphite, 7% Fe2O3 |
| Binder (B) / wt% of A | 2.0% 2123 Resin, 0% Tetraethyl Orthosilicate | 2.5% 2123 Resin, 1% Tetraethyl Orthosilicate | 3.0% 2123 Resin, 1.5% Tetraethyl Orthosilicate |
| Suspending Agent (C) / wt% of A | 0.4% PVB, 2.0% Attapulgite | 0.6% PVB, 1.5% Attapulgite | 0.8% PVB, 2.5% Attapulgite |
| Carrier Fluid (D) / mL | 600 mL Isopropanol, 0 mL Water | 600 mL Isopropanol, 30 mL Water | 600 mL Isopropanol, 50 mL Water |
Based on this design, nine distinct coating formulations were prepared and tested for various properties critical to lost foam casting performance. The experimental conditions, as per the L9(34) orthogonal array, are detailed in the following table:
| Experiment No. | Refractory Powder (A) / g | Binder (B) / wt% of A | Suspending Agent (C) / wt% of A | Carrier Fluid (D) / mL |
|---|---|---|---|---|
| 1 | 70% Alumina Bauxite, 0% Quartz, 15% Talc, 15% Graphite, 0% Fe2O3 | 2.0% 2123 Resin, 0% Tetraethyl Orthosilicate | 0.6% PVB, 1.5% Attapulgite | 600 mL Isopropanol, 30 mL Water |
| 2 | 60% Alumina Bauxite, 10% Quartz, 10% Talc, 10% Graphite, 10% Fe2O3 | 2.0% 2123 Resin, 0% Tetraethyl Orthosilicate | 0.4% PVB, 2.0% Attapulgite | 600 mL Isopropanol, 0 mL Water |
| 3 | 75% Alumina Bauxite, 5% Quartz, 8% Talc, 5% Graphite, 7% Fe2O3 | 2.0% 2123 Resin, 0% Tetraethyl Orthosilicate | 0.6% PVB, 1.5% Attapulgite | 600 mL Isopropanol, 50 mL Water |
| 4 | 70% Alumina Bauxite, 0% Quartz, 15% Talc, 15% Graphite, 0% Fe2O3 | 2.5% 2123 Resin, 1% Tetraethyl Orthosilicate | 0.6% PVB, 1.5% Attapulgite | 600 mL Isopropanol, 0 mL Water |
| 5 | 60% Alumina Bauxite, 10% Quartz, 10% Talc, 10% Graphite, 10% Fe2O3 | 2.5% 2123 Resin, 1% Tetraethyl Orthosilicate | 0.8% PVB, 2.5% Attapulgite | 600 mL Isopropanol, 50 mL Water |
| 6 | 75% Alumina Bauxite, 5% Quartz, 8% Talc, 5% Graphite, 7% Fe2O3 | 2.5% 2123 Resin, 1% Tetraethyl Orthosilicate | 0.4% PVB, 2.0% Attapulgite | 600 mL Isopropanol, 30 mL Water |
| 7 | 70% Alumina Bauxite, 0% Quartz, 15% Talc, 15% Graphite, 0% Fe2O3 | 3.0% 2123 Resin, 1.5% Tetraethyl Orthosilicate | 0.4% PVB, 2.0% Attapulgite | 600 mL Isopropanol, 50 mL Water |
| 8 | 60% Alumina Bauxite, 10% Quartz, 10% Talc, 10% Graphite, 10% Fe2O3 | 3.0% 2123 Resin, 1.5% Tetraethyl Orthosilicate | 0.6% PVB, 1.5% Attapulgite | 600 mL Isopropanol, 30 mL Water |
| 9 | 75% Alumina Bauxite, 5% Quartz, 8% Talc, 5% Graphite, 7% Fe2O3 | 3.0% 2123 Resin, 1.5% Tetraethyl Orthosilicate | 0.6% PVB, 1.5% Attapulgite | 600 mL Isopropanol, 0 mL Water |
After preparing the coatings, we evaluated their performance through a series of tests, including suspension stability (measured as percentage suspension after 6 hours),常温 strength (in grams), high-temperature crack resistance, tensile and shear bond strengths with sand (in kPa), density, permeability, and viscosity. The results from these tests are compiled in the table below, which provides a comprehensive overview of how each formulation performs under typical lost foam casting conditions. This data is crucial for identifying trends and optimizing the coating for EPC applications.
| Experiment No. | Suspension Stability / % | 常温 Strength / g | High-Temperature Crack Resistance | Tensile Strength (σ) / kPa | Shear Strength (τ) / kPa | Density / g/cm³ | Permeability / mm | Gas Permeability | Conditional Viscosity / s |
|---|---|---|---|---|---|---|---|---|---|
| 1 | 98 | 156 | 88 | 22 | 20 | 1.54 | 1.0 | 200 | 11 |
| 2 | 95 | 128 | 87 | 19 | 17 | 1.60 | 0.9 | 202 | 10 |
| 3 | 94 | 493 | 83 | 26 | 21 | 1.62 | 0.9 | 95 | 8 |
| 4 | 97 | 125 | 86 | 36 | 19 | 1.45 | 1.2 | 120 | 11 |
| 5 | 93 | 625 | 91 | 33 | 22 | 1.50 | 1.3 | 162 | 9 |
| 6 | 95 | 170 | 78 | 23 | 22 | 1.45 | 1.1 | 139 | 9 |
| 7 | 97 | 136 | 85 | 27 | 21 | 1.47 | 1.2 | 126 | 10 |
| 8 | 94 | 660 | 89 | 31 | 23 | 1.50 | 1.0 | 70 | 8 |
To analyze the orthogonal experiment results, we performed an intuitive analysis calculation, focusing on key performance indicators such as suspension stability,常温 strength, high-temperature crack resistance, and bond strengths. For each property, we computed the average effects of the factors at different levels to identify optimal combinations. For instance, the suspension stability was maximized with combination A1B3C3D2, while常温 strength peaked with A3B2C3D1. High-temperature crack resistance was best with A3B1C3D2, tensile bond strength with A2B2C3D2, and shear bond strength with A3B3C3D1. These findings highlight the complex interactions in lost foam casting coatings and underscore the importance of balanced formulation.
Based on the analysis, we derived the optimal transfer coating composition for lost foam casting, prioritizing常温 strength, shear bond strength, and high-temperature crack resistance, while considering economic and practical factors such as the adverse effect of water on furan resin sand curing. The best-performing combination was determined to be A3B2C2D1, which corresponds to a refractory powder blend of 75% alumina bauxite, 5% quartz, 8% talc, 5% graphite, and 7% Fe2O3; a binder system of 2.5% 2123 resin and 1% tetraethyl orthosilicate; suspending agents of 0.6% PVB and 1.5% attapulgite; and a carrier fluid of 600 mL isopropanol per 100 g of refractory powder with no water addition. This formulation ensures excellent performance in EPC applications, as detailed in the table below:
| Component | Composition |
|---|---|
| Refractory Powder (A) | 75% Alumina Bauxite, 5% Quartz, 8% Talc, 5% Graphite, 7% Fe2O3 per 100 g |
| Binder (B) | 2.5% 2123 Resin, 1% Tetraethyl Orthosilicate (by weight of A) |
| Suspending Agent (C) | 0.6% PVB, 1.5% Attapulgite (by weight of A) |
| Carrier Fluid (D) | 600 mL Isopropanol per 100 g of A, 0 mL Water |
We then prepared the coating according to this optimal recipe and conducted comprehensive performance tests. The results, summarized in the next table, confirm that the coating meets all required specifications for lost foam casting. It exhibits high suspension stability, excellent strength properties, and superior resistance to high-temperature cracking, making it ideal for EPC processes involving ductile iron.
| Property | Value |
|---|---|
| Suspension Stability (6 h) / % | 96.5 |
| Density / g/cm³ | 1.57 |
| Permeability / mm | 0.9 |
| Gas Permeability | 135 |
| Conditional Viscosity / s | 8 |
| 常温 Strength / g | 693 |
| High-Temperature Crack Resistance | Grade I (90-100 points, no cracks or minimal cracking, no peeling) |
| Tensile Strength (σ) / kPa | 32 |
| Shear Strength (τ) / kPa | 31 |
The optimized transfer coating demonstrates excellent application characteristics, including good brushability, leveling, and drying without cracking at corners. Its high strength and thermal stability ensure reliable performance in lost foam casting, particularly for complex geometries. The use of a multi-component refractory system based on alumina bauxite promotes sintering, which enhances coating strength and prevents metal penetration during casting. This is critical in EPC, where coating integrity directly impacts final part quality.
In conclusion, through systematic component design and orthogonal experimentation, we have successfully developed a high-performance transfer coating for lost foam casting that addresses common industrial challenges. The optimized formulation, featuring a balanced blend of refractory materials, binders, suspending agents, and carrier fluids, exhibits superior suspension stability, strength, and high-temperature resistance. This coating not only improves the dimensional accuracy and surface finish of castings in EPC processes but also enhances overall process efficiency. Future work could explore scalability and long-term performance in various lost foam casting applications, further solidifying its role in advanced foundry practices.