Our research team has developed a novel alcohol-based coating specifically designed for sand casting foundry applications involving high-manganese steel castings. In the sand casting foundry, high-manganese steel components are widely used in mining, construction machinery, and railway transportation due to their excellent impact resistance and wear performance. However, the conventional coatings used in the sand casting foundry—such as zircon flour coatings and magnesia powder coatings—exhibit significant drawbacks. Zircon flour is weakly acidic, while high-manganese steel melt is alkaline, leading to severe metal penetration in heavy sections. Magnesia powder coatings, being alkaline, do not sinter adequately and fail to form a dense barrier against oxide infiltration, resulting in rough surfaces with fine burrs. To overcome these challenges, we formulated a coating based on forsterite (olivine) powder as the refractory aggregate, combined with lithium-modified bentonite as a suspending agent and other additives. This coating demonstrates excellent suspension stability, thixotropy, ease of application, high coating strength, low cost, and optimal sintering behavior under the high-temperature melt in the sand casting foundry, forming a dense, peelable isolation layer that yields a smooth casting surface.
Raw Materials
Refractory Aggregate
The anti-sticking performance of a coating in the sand casting foundry largely depends on the properties of the refractory aggregate. Olivine is a solid solution of forsterite (Mg₂SiO₄) and fayalite (Fe₂SiO₄). It has a low sintering point, no polymorphic transformations upon heating, uniform and slow thermal expansion, and a low coefficient of expansion. These characteristics make it an ideal choice for a sintered peelable refractory aggregate used in the sand casting foundry. We employed olivine sand sourced from Shangnan, with a density of 3.17 g/cm³, Mohs hardness of 6–7, and a refractoriness of 1690–1710°C. Its chemical composition is shown in Table 1.
| Component | SiO₂ | MgO | Fe₂O₃ | Al₂O₃ | CaO | Na₂O | Cr₂O₃ | Loss on ignition |
|---|---|---|---|---|---|---|---|---|
| Content (wt%) | 39.47 | 41.8 | 7.9 | 0.49 | 0.05 | 0.08 | 0.04 | 0.18 |
Binders
Room-temperature binder: A 2123 type phenolic resin from Xi’an Resin Factory was used. It is a pale yellow translucent solid with a softening point of 110°C, free phenol content below 6%, and is readily soluble in ethanol. The resin was crushed and dissolved in ethanol before incorporation into the coating. Our experiments showed that when the addition exceeded 1.5%, the coating would blister during ignition drying, causing surface roughness. Therefore, we limited the phenolic resin content to less than 1.5%.
High-temperature binder A: To enhance the coating’s resistance to molten metal erosion in the sand casting foundry, we added a phosphate-based high-temperature binder A. This binder polymerizes under heating to form a network structure that encapsulates the refractory particles, improving both the ambient and high-temperature bonding strength of the coating.
Suspending Agents
Polyvinyl butyral (PVB): PVB acts both as a suspending agent and a room-temperature binder. It dissolves in ethanol to form a viscous solution, significantly improving the suspension stability and yield value of the coating. However, the addition was controlled below 0.5% because excessive PVB forms a continuous film that hinders gas escape during ignition drying, leading to blistering on the coating surface and impairing surface quality and anti-sticking performance.
Lithium-modified bentonite: Lithium-modified bentonite exhibits excellent swelling and thickening ability in ethanol, making it a highly effective suspending agent and also serving as a high-temperature binder. It was prepared by cation exchange of calcium bentonite with lithium salts. The addition was kept under 3.5% to avoid cracking of the dried coating.
Solvent
The solvent used in alcohol-based coatings is typically isopropanol, methanol, or ethanol. Ethanol, with low cost and convenient handling, is widely adopted in the sand casting foundry. Our ethanol conformed to the specification: purity >95%, density 0.7939 g/cm³, and flame temperature 560°C.
Additive
Since the density of alcohol is relatively low, relying solely on lithium-modified bentonite and PVB as suspending agents is insufficient to prevent the sedimentation of the refractory aggregate. Therefore, we introduced a polymeric additive with active functional groups. This additive adsorbs onto the lithium bentonite particles, forming a three-dimensional network structure that enhances the suspension and thixotropy of the coating.
Coating Formulation and Preparation
Through orthogonal experiments, we determined the optimal coating formulation, which is listed in Table 2.
| Material | Ratio |
|---|---|
| Olivine sand | 100 |
| Lithium-modified bentonite | 2.5 |
| Phenolic resin | 1.0 – 1.5 |
| Polyvinyl butyral (PVB) | 0.2 – 0.5 |
| High-temperature binder A | 0.7 – 1.0 |
| Additive | Appropriate amount |
| Ethanol | Sufficient to achieve desired consistency |
The preparation process was as follows: First, the lithium-modified bentonite was mixed with a small amount of soft water and milled in a ball mill for 5 minutes to form a bentonite paste. Then, olivine sand, a pre-dissolved solution of PVB and phenolic resin in ethanol, the additive, and high-temperature binder A were added. The mixture was milled for 1–1.5 hours. Finally, the remaining ethanol was added, and milling continued for another 15–20 minutes before discharging the coating.
Coating Properties
Thixotropy
We measured the apparent viscosity of the coating using an NXH-1 rotational viscometer with a No. 3 rotor at 6 r/min. The results are shown in Table 3.
| Shearing time (min) | Apparent viscosity (Pa·s) |
|---|---|
| 2 | 5.0 |
| 5 | 3.0 |
| 7 | 1.8 |
| 10 | 1.5 |
The thixotropic ratio (TR) was calculated using the formula:
$$
\text{Thixotropic ratio} = \frac{\eta_{0.5} – \eta_{10}}{\eta_{0.5}}
$$
Where η₀.₅ is the viscosity at the beginning (t=0.5 min, approximated as 5.0 Pa·s) and η₁₀ is the viscosity after 10 minutes (1.5 Pa·s). Thus, TR = (5.0 – 1.5)/5.0 = 0.70 = 70%. This high value indicates excellent shear-thinning behavior.
We further characterized the rheological behavior using an NXS-11 rotational viscometer with a rotor outer diameter of 3.17 cm and a cup inner diameter of 3.74 cm. The measured shear stress at various shear rates is given in Table 4.
| Shear rate (s⁻¹) | Shear stress (upward curve) (×10⁻¹ Pa) | Shear stress (downward curve) (×10⁻¹ Pa) |
|---|---|---|
| 0 | 5.5 | – |
| 5 | 7.05 | 3.05 |
| 10 | 8.20 | 6.0 |
| 20 | 9.40 | 6.20 |
| 40 | 10.50 | 8.02 |
| 60 | 11.50 | 7.20 |
| 80 | 12.30 | – |
| 100 | 13.00 | – |
The upward and downward curves formed a large thixotropic loop, confirming the coating is a pseudoplastic fluid with a yield value of 5.5 Pa. The flow curve can be described by the Herschel-Bulkley model:
$$
\tau = \tau_0 + K \dot{\gamma}^n
$$
Where τ is shear stress (×10⁻¹ Pa), τ₀ is yield value (5.5 Pa), K is consistency coefficient, \dot{\gamma} is shear rate (s⁻¹), and n is flow index. Substituting two data points from the upward curve (e.g., \dot{\gamma}=5, τ=7.05; \dot{\gamma}=10, τ=8.20) into the equation yields K ≈ 21 and n ≈ 0.21. Therefore, the shear stress relationship is approximately:
$$
\tau = 5.5 + 21 \dot{\gamma}^{0.21}
$$
This low n value indicates strong shear-thinning, meaning the coating becomes significantly thinner under shear, which is ideal for brushing or dipping in the sand casting foundry.
Other Properties
The coating exhibited a pH of 7.5, density of 1.42 g/cm³, viscosity of 58 seconds (measured with a φ6 standard viscosity cup), solid content of approximately 60%, 24-hour suspension stability of 91%, gas evolution less than 20 mL/g, and no cracking after being heated rapidly at 1000°C for 2 minutes.
Anti-Sticking Mechanism of the Olivine Coating in Sand Casting Foundry
The working principle of the olivine coating in the sand casting foundry for high-manganese steel can be explained by the sintering-oxidation mechanism. High-manganese steel is typically poured at 1360–1380°C. Since olivine contains fayalite (Fe₂SiO₄, melting point 1205°C), its sintering temperature lies between 1250°C and 1350°C. During pouring, fayalite oxidizes to Fe₂O₃, which precipitates onto the particle surfaces, forming a viscous glassy phase that fills the interstices among refractory particles. This leads to partial melting and sintering, creating a dense barrier that prevents molten metal from penetrating the coating. Moreover, the FeO in fayalite further oxidizes to Fe₂O₃ or Fe₃O₄ at high temperature, accumulating an iron oxide isolation layer thicker than 0.1 mm (the critical thickness) at the casting-coating interface. This oxide layer expands during crystallization and bonds weakly with the casting surface. High-manganese steel has the highest linear shrinkage among cast steels; during solidification and cooling, relative slip at the interface generates significant shear stress, causing the sintered coating shell to peel off from the casting spontaneously, leaving a clean, smooth surface.
Production Application in Sand Casting Foundry
We applied the olivine alcohol-based coating in several sand casting foundries for high-manganese steel components. The molds used included sodium silicate-lime-sand CO₂ hardened molds, sodium silicate-silica sand CO₂ hardened molds, and dried molds. The castings produced included tooth plates, grid plates, grate bars, liners, mill door liners, and railway crossings, with individual weights up to 730 kg. The resulting high-manganese steel castings exhibited clean, smooth surfaces with sharp edges and corners. The coating shell automatically peeled off, appearing purplish-black and well-sintered, with a smooth interface that was easy to clean. Workers reported that the coating “does not agglomerate during storage, is easy to stir,” “feels smooth, applies easily on vertical surfaces without running,” and “the shell self-peels during shakeout, leaving a bright surface.”
Conclusion
We have developed an olivine-based alcohol coating specifically formulated for the sand casting foundry of high-manganese steel castings. This coating behaves as a pseudoplastic fluid with excellent thixotropy and a yield value, conforming to the rheological equation τ = 5.5 + 21\dot{\gamma}^{0.21}. It offers high suspension stability, does not blister upon ignition, does not crack under rapid heating, and possesses high coating strength. During pouring, the coating sinters appropriately to form a dense isolation layer that prevents metal penetration and automatically peels off after cooling, yielding a smooth casting surface. Moreover, the cost of olivine powder is only about 10% of that of zircon flour, and the use of inexpensive lithium-modified bentonite as a suspending agent further reduces the overall cost, providing significant economic benefits for the sand casting foundry industry.