Lost foam casting technology offers significant advantages, including wide applicability, high dimensional accuracy of castings, smooth surface finish, suitability for mechanized production, low production costs, design flexibility, high efficiency, clean production, and environmentally friendly manufacturing processes. In recent years, with deepening research into the principles and theories of lost foam casting by scientific institutions, the industry has experienced rapid growth. Statistics indicate that numerous research organizations in China are dedicated to advancing lost foam casting technology, with over 2,000 enterprises producing more than 3 million tons of castings annually, and this number continues to rise. However, to reduce costs, some companies use self-formulated coatings without optimizing the ingredient ratios, leading to reduced coating performance. Issues such as carbon deposition and elephant skin defects in cast iron parts produced via lost foam casting remain unresolved, resulting in low yield and high costs, which hinder the development of the lost foam casting industry. These problems primarily stem from improper selection of refractory aggregates and poor high-temperature permeability of the coatings.
To address these challenges, this study focuses on developing a water-based lost foam casting coating for cast iron. Bauxite is selected as the primary refractory aggregate due to its low thermal expansion, high refractoriness, chemical stability, and cost-effectiveness. Supplementary refractory aggregates include kaolin, wollastonite, and talcum powder, which enhance overall coating properties. A composite suspension system consisting of sepiolite and carboxymethyl cellulose (CMC) is employed, while silica sol and polyvinyl alcohol (PVA) serve as composite binders. Other additives, such as sodium dodecylbenzene sulfonate as a surfactant, n-octanol as a defoamer, and Fe2O3 as an auxiliary agent, are incorporated to optimize performance.
The coating preparation involves pre-treatment of raw materials: activation of silica sol and PVA by stirring at 1,200 rpm and adding surfactant, followed by 24-hour resting; dispersion of CMC in ethanol and water; and preparation of sepiolite slurry by mixing with water. The refractory aggregates and additives are blended in a roller-type sand mixer, followed by the addition of pre-treated solutions and grinding in a colloid mill. Key coating properties, including suspension rate, conditional viscosity, gas evolution, coating strength, adherence, and peelability, are evaluated using standardized methods.
An orthogonal experimental design based on the L9(34) matrix is implemented to investigate the effects of kaolin, wollastonite, and talcum powder on coating performance. The factors and levels are summarized in Table 1, with bauxite fixed as the base refractory aggregate. The experimental schemes and results are detailed in Tables 2 and 3, respectively.
| Level | A: Kaolin /% | B: Wollastonite /% | C: Talcum Powder /% |
|---|---|---|---|
| 1 | 20 | 10 | 10 |
| 2 | 30 | 20 | 15 |
| 3 | 40 | 30 | 20 |
| Component | Type | Proportion /% |
|---|---|---|
| Silica Sol | Binder | 4 |
| PVA | Binder | 1 |
| Sepiolite | Suspension Agent | 4 |
| CMC | Suspension Agent | 0.5 |
| Sodium Dodecylbenzene Sulfonate | Surfactant | 0.1 |
| n-Octanol | Defoamer | 0.1 |
| Fe2O3 | Auxiliary Agent | 0.5 |
| Water | Carrier Fluid | As Needed |
| Sample | Scheme | 24 h Suspension Rate /% | Conditional Viscosity /s | Coating Strength /g | Gas Evolution / (mL·g-1) | Adherence Grade |
|---|---|---|---|---|---|---|
| 1 | A1B1C1 | 90 | 6.63 | 145 | 21.2 | I |
| 2 | A1B2C2 | 91 | 9.14 | 298 | 27.7 | I |
| 3 | A1B3C3 | 95 | 7.24 | 281 | 33.5 | II |
| 4 | A2B1C2 | 90 | 11.06 | 313 | 19.9 | I |
| 5 | A2B2C3 | 94 | 13.32 | 344 | 29.5 | II |
| 6 | A2B3C1 | 97 | 10.11 | 236 | 21.7 | I |
| 7 | A3B1C3 | 95 | 16.28 | 278 | 32.9 | II |
| 8 | A3B2C1 | 99 | 12.77 | 364 | 24.5 | I |
| 9 | A3B3C2 | 99 | 10.65 | 290 | 31.7 | II |
The experimental data are analyzed using range analysis to determine the influence of each factor on coating properties. The results, presented in Table 4, show that kaolin has the most significant effect on suspension rate and conditional viscosity, wollastonite predominantly affects coating strength, and talcum powder is the key factor influencing gas evolution. The optimal levels for each factor are derived by considering the primary and secondary relationships among properties, leading to the formulation A3B2C1, which corresponds to 40% kaolin, 20% wollastonite, and 10% talcum powder relative to bauxite.
| Property | Range for Kaolin (A) | Range for Wollastonite (B) | Range for Talcum Powder (C) |
|---|---|---|---|
| 24 h Suspension Rate /% | 5.7 | 4.3 | 2.0 |
| Conditional Viscosity /s | 5.56 | 2.41 | 2.44 |
| Coating Strength /g | 69.4 | 90.0 | 52.7 |
| Gas Evolution / (mL·g-1) | 6.0 | 4.2 | 9.4 |
The optimized coating formulation is summarized in Table 5. The performance of this coating is evaluated, and the results are listed in Table 6. The coating exhibits a density of 1.5 g/cm³, suspension rate of 93%, conditional viscosity of 9.65 s, coating strength of 145 g, gas evolution of 21.2 mL/g, and Grade I adherence, meeting the requirements for lost foam casting applications.
| Component | Proportion /% |
|---|---|
| Bauxite | 100 |
| Kaolin | 40 |
| Wollastonite | 20 |
| Talcum Powder | 10 |
| Silica Sol | 4 |
| PVA | 1 |
| Sepiolite | 4 |
| CMC | 0.5 |
| Sodium Dodecylbenzene Sulfonate | 0.1 |
| n-Octanol | 0.1 |
| Fe2O3 | 0.5 |
| Water | As Needed |
| Property | Value |
|---|---|
| Density / (g·cm-3) | 1.5 |
| 24 h Suspension Rate /% | 93 |
| Conditional Viscosity /s | 9.65 |
| Coating Strength /g | 145 |
| Gas Evolution / (mL·g-1) | 21.2 |
| Adherence Grade | I |
To validate the coating’s performance, production trials are conducted. The coating is applied to expandable polystyrene (EPS) patterns via dipping, dried at 40°C, and subjected to vacuum-assisted casting. The patterns show smooth and uniform coating layers with excellent adherence. After casting, the resulting iron components exhibit smooth surfaces without defects such as burning-on, blowholes, carbon deposition, or elephant skin. The coating demonstrates good peelability, with most areas disintegrating freely and only minor sections requiring light tapping for removal. This is attributed to the optimized refractory aggregate combination, which enhances sintering and strength, while the organic binders create a porous structure at high temperatures, improving permeability and facilitating the removal of pyrolysis products during casting.
The relationship between coating composition and properties can be modeled mathematically. For instance, the coating strength (S) is influenced by the mass fractions of wollastonite (W) and other aggregates. A linear approximation can be expressed as: $$ S = k_1 \cdot W + k_2 \cdot K + k_3 \cdot T + C $$ where K is kaolin content, T is talcum powder content, and k1, k2, k3, and C are constants derived from experimental data. Similarly, gas evolution (G) relates to the decomposition of hydrated minerals: $$ G = \alpha \cdot T + \beta \cdot K + \gamma $$ where α and β are coefficients representing the gas generation potential of talcum powder and kaolin, respectively, and γ is a constant. These models help in understanding the synergistic effects of ingredients in lost foam casting coatings.
In conclusion, this study successfully develops a water-based lost foam casting coating for cast iron using bauxite as the primary refractory aggregate, supplemented by kaolin, wollastonite, and talcum powder. The orthogonal experiments and range analysis identify the optimal ratios, with kaolin significantly affecting suspension and viscosity, wollastonite enhancing coating strength, and talcum powder increasing gas evolution due to its crystalline water content. The finalized formulation ensures excellent performance in terms of suspension, adherence, strength, and low gas evolution. Production trials confirm that the coating prevents common defects like carbon deposition and elephant skin, with good peelability and surface quality. This coating formulation provides a reliable solution for improving the efficiency and quality of lost foam casting processes in cast iron production, contributing to the advancement of lost foam casting technology.