In the field of sand mould casting, large thin-walled cast iron components are widely used in machine tool manufacturing, automotive production, and heavy machinery because of their complex geometries and demanding service conditions. During solidification, geometric hot spots often experience uneven cooling rates, which leads to shrinkage porosity, shrinkage cavities, and other internal defects that degrade both mechanical performance and surface quality. External chills are a well-established process measure in sand mould casting: by placing metal blocks at specific locations of the mould, the local cooling rate is accelerated, the feeding condition is improved, and the solidification sequence can be controlled more precisely. The success of this external chill technique depends strongly on the performance of the coating applied between the chill and the casting. The coating acts as a critical barrier and must fulfil several functions simultaneously. It must possess sufficient refractoriness and thermal stability to resist the scouring and erosion of high-temperature molten iron, preventing sand adhesion and molten iron penetration. It must also provide excellent adhesion on both the sand mould surface and the metal chill surface, ensuring coating integrity under complex working conditions. At the same time, the coating should dry quickly and be easily stripped after casting, so that subsequent cleaning and chill recovery are efficient.
Traditional water-based coatings dry slowly, easily absorb moisture, increase the tendency for gas porosity, and exhibit poor storage stability. Although existing alcohol-based coatings offer the advantage of rapid drying, they often show an imbalanced adhesion between sand mould surfaces and metal surfaces. To make a coating originally designed for sand mould casting perform well on both sand and iron chill surfaces, the key is to adjust its adhesion, thermal stability, rheological behaviour, and sintering characteristics so that it can both “grab” sand particles and “bond” firmly to metal surfaces. In this study, I focused on improving the dual adaptability of the coating to sand moulds and metal chills, targeting the special working conditions of external chill sand mould casting. I innovatively adopted a two-step “intercalation-grafting” method to modify lithium-based bentonite and prepare a high-performance suspending agent. I then regulated the particle size distribution of a bauxite–white corundum composite refractory aggregate and optimized the ratio of a phenolic resin–polyvinyl butyral composite binder. Through a series of systematic investigations, I aimed to develop a new alcohol-based casting coating that combines fast drying, high strength, and easy stripping, providing a practical solution for the optimization of coatings in external chill sand mould casting and helping the foundry industry improve production efficiency and casting quality.
The heat transfer behaviour in external chill sand mould casting can be described by the transient heat conduction equation. For a simplified one-dimensional system, the temperature field satisfies
$$\frac{\partial T}{\partial t} = \alpha \frac{\partial^2 T}{\partial x^2}$$
where \(T\) is temperature, \(t\) is time, \(x\) is the spatial coordinate, and \(\alpha\) is the thermal diffusivity of the mould or chill material. The cooling rate at the casting–chill interface, \(\dot{T}\), is defined as
$$\dot{T} = \frac{\mathrm{d}T}{\mathrm{d}t}$$
and directly influences the local solidification time, \(t_f\), which can be approximated by
$$t_f = \frac{\rho L}{h (T_m – T_0)}$$
where \(\rho\) is the density of the casting alloy, \(L\) is the latent heat of fusion, \(h\) is the interfacial heat transfer coefficient, \(T_m\) is the melting temperature, and \(T_0\) is the initial mould temperature. The coating layer modifies \(h\) and therefore plays a decisive role in controlling the local cooling intensity. If the coating is too insulating, the external chill effect is weakened; if the coating is too conductive or discontinuous, sand adhesion and molten iron penetration may occur. Thus, the coating must balance thermal resistance and mechanical integrity in sand mould casting.
To evaluate the suspension stability of the coating, I used Stokes’ law as a theoretical guide. For a spherical particle settling in a viscous liquid, the terminal settling velocity \(v\) is
$$v = \frac{2 r^2 (\rho_p – \rho_f) g}{9 \eta}$$
where \(r\) is the particle radius, \(\rho_p\) is the particle density, \(\rho_f\) is the fluid density, \(g\) is gravitational acceleration, and \(\eta\) is the dynamic viscosity. A high suspension rate requires a low settling velocity, which can be achieved by reducing the effective particle size, increasing the viscosity of the continuous phase, or forming a three-dimensional network that supports the refractory particles. The modified bentonite suspending agent developed in this study was designed to create such a network in an alcohol-based system, which is essential for a reliable sand mould casting coating.
Adhesion between the coating and the substrate can be discussed using the thermodynamic work of adhesion:
$$W_{adh} = \gamma_s + \gamma_l – \gamma_{sl}$$
where \(\gamma_s\) is the surface energy of the substrate, \(\gamma_l\) is the surface tension of the coating liquid, and \(\gamma_{sl}\) is the interfacial energy. For sand mould casting, the substrate may be a sand grain or a metal chill surface, and these two surfaces have very different surface energies and roughness. Therefore, a single adhesion mechanism is often insufficient. The composite binder and modified suspending agent were expected to provide both polar and nonpolar interactions, improving wetting and bonding on both types of surfaces.
The gas evolution of a coating, \(V_g\), is another critical parameter. It can be expressed as
$$V_g = \frac{V_{gas}}{m_c}$$
where \(V_{gas}\) is the volume of gas released under standard conditions and \(m_c\) is the mass of the coating sample. Excessive gas evolution during pouring can cause gas porosity, surface defects, and dimensional inaccuracy. In sand mould casting, the coating must therefore maintain a low gas evolution while still providing adequate binding strength.
The wear resistance of the dried coating can be quantified by the mass loss per unit area:
$$W = \frac{m_0 – m_1}{A}$$
where \(m_0\) is the initial mass, \(m_1\) is the mass after abrasion, and \(A\) is the abraded area. A lower \(W\) indicates better wear resistance, which is necessary for withstanding handling, drying, dust removal, and mould assembly operations in sand mould casting.
For the orthogonal experiment analysis, I used range analysis. For a given factor \(j\) at level \(i\), the mean response \(k_{ij}\) is
$$k_{ij} = \frac{1}{n_i} \sum_{p=1}^{n_i} y_{ijp}$$
where \(y_{ijp}\) is the response value of the \(p\)-th trial at level \(i\) of factor \(j\), and \(n_i\) is the number of trials at that level. The range \(R_j\) is
$$R_j = \max_i (k_{ij}) – \min_i (k_{ij})$$
A larger \(R_j\) indicates a stronger influence of factor \(j\) on the performance index. For the final optimization, I applied a weight normalization method. For a positive indicator such as 24 h suspension rate, the normalized value \(x’_{ij}\) is
$$x’_{ij} = \frac{x_{ij} – \min(x_j)}{\max(x_j) – \min(x_j)}$$
For a negative indicator such as viscosity, gas evolution, wear resistance, and brushability grade, the normalized value is
$$x’_{ij} = \frac{\max(x_j) – x_{ij}}{\max(x_j) – \min(x_j)}$$
The comprehensive score \(S_i\) for the \(i\)-th formulation is then
$$S_i = \sum_{j=1}^{m} w_j x’_{ij}$$
where \(w_j\) is the weight assigned to the \(j\)-th performance index and \(\sum w_j = 1\). This approach allowed me to rank the formulations objectively and select the optimal one for external chill sand mould casting.
The raw materials used in this study are summarized in Table 1. Bauxite with 250 mesh and 400 mesh grades was selected as the main refractory aggregate because of its high refractoriness and low thermal expansion. White corundum with 250 mesh was used as a complementary aggregate to improve acid and alkali resistance. The bauxite had an Al₂O₃ content of approximately 86% and SiO₂ content of about 5.7%, with the remainder being minor impurities. The white corundum had a specified oxide composition and was used to enhance the thermal and chemical stability of the coating. Calcium-based bentonite with a montmorillonite content of at least 70% and a cation exchange capacity (CEC) of 92 mmol/100 g was used as the precursor for the modified suspending agent. A thermosetting phenolic resin with a residual carbon rate of 55% and free phenol content below 1% was used as one component of the composite binder. Polyvinyl butyral (PVB) with a viscosity of 210 s in a涂-1# cup was used as the second binder component. Industrial ethanol with a mass fraction of 95% was used as the solvent.
| Material | Specification / Key Property | Role in Coating |
|---|---|---|
| Bauxite | 250 mesh, 400 mesh; Al₂O₃ ≈ 86%, SiO₂ ≈ 5.7% | Main refractory aggregate; high refractoriness, low thermal expansion |
| White corundum | 250 mesh; specified oxide composition | Complementary aggregate; acid and alkali resistance |
| Calcium-based bentonite | 200 mesh; montmorillonite ≥ 70%; CEC = 92 mmol/100 g | Precursor for modified suspending agent |
| Thermosetting phenolic resin | Residual carbon rate 55%; free phenol < 1% | Composite binder; high-temperature strength |
| Polyvinyl butyral (PVB) | Viscosity 210 s (涂-1# cup) | Composite binder; flexibility and adhesion |
| Industrial ethanol | Mass fraction 95% | Solvent; rapid drying |
| Li₂CO₃ | Battery grade | Lithiation modifier |
| CTAB | Purity 99% | Intercalation modifier |
| OTAC | Solid content 29%–31% | Intercalation modifier |
| MNTA | Purity > 98% | Intercalation modifier |
| CMC | Purity 99% | Intercalation modifier |
| KH-550 | Silane coupling agent | Grafting modifier |
| BTFA | Purity 95% | Grafting modifier |
The preparation of the composite modified bentonite was carried out using a two-step “intercalation-grafting” method. First, the calcium-based bentonite was lithiated with Li₂CO₃ to convert it partially to a lithium-based form. Then, intercalation modification was performed using MNTA, CMC, and CTAB. Finally, grafting modification was conducted with KH-550 and BTFA. The modified bentonite was dried and then dispersed in water at 400–600 r/min for 20 min, followed by standing to obtain a modified bentonite slurry. This slurry was used as the suspending agent in the coating formulation.
The composite binder was prepared separately. The thermosetting phenolic resin was dissolved in ethanol and stirred at 400–600 r/min for 2 h, then left to stand. The PVB solution was prepared in the same manner. The two solutions were then combined at a phenolic resin to PVB mass ratio of 2.5:1 to form the composite binder solution.
For the coating preparation, the bauxite and white corundum were first dry-mixed in a wheel-type sand mixer. The mass ratio of bauxite to white corundum was fixed at 4:1, and the total refractory aggregate mass was set to 100 parts. The composite modified suspending agent was added at 3%–5% based on the refractory aggregate mass. The composite binder was added at 2%–6% based on the refractory aggregate mass. Ethanol was used at 50%–70% based on the refractory aggregate mass. The pre-treated modified bentonite slurry, binder solution, and appropriate amount of ethanol were added sequentially. The mixture was milled for 2.5–3 h until fully homogenized. The resulting paste was discharged and then diluted with ethanol under stirring to adjust the coating density to (1.60 ± 0.02) g/cm³. This final coating was used for all subsequent tests in sand mould casting.
For specimen preparation, the sand mould substrate was made of furan resin sand. Cylindrical specimens with a diameter of 50 mm and a height of 55 mm were prepared on a hammer-type specimen preparation machine with three impacts and then hardened according to the standard sand mould casting process. Iron blocks with the same material as the external chill were machined into rectangular specimens of 100 mm × 50 mm × 20 mm. These specimens were used to evaluate coating adhesion, brushing performance, and drying behaviour on both sand and metal surfaces.
Performance testing was conducted with reference to JB/T 9226—2008 “Coatings for sand mould casting” and the corresponding test methods. The following properties were measured: conditional viscosity (涂-6# cup viscosity), suspension rate after 24 h, gas evolution, and coating wear resistance. The brushing performance was evaluated by applying the coating twice consecutively on both sand and iron specimens. After standing for 2 min, the flow behaviour on vertical surfaces was observed and graded as follows. Grade I: the coating on both specimens was uniform, without brush marks, and the coating thickness was 0.5–1.2 mm. Grade II: the coating on both specimens was basically uniform, with slight brush marks, and the coating thickness was 0.5–1.2 mm. Grade III: the coating on either specimen was uneven, or the thickness at measured points was too thin or too thick (< 0.5 mm or > 1.3 mm), or obvious brush marks were present.
The modification effect was characterised by X-ray diffraction (XRD) and scanning electron microscopy (SEM), as well as by physical and chemical property comparisons. The XRD results showed that after modification, the first diffraction peak of bentonite changed in intensity and shifted to a lower angle. According to the Bragg equation,
$$2d \sin \theta = n \lambda$$
where \(d\) is the interlayer spacing, \(\theta\) is the incident angle, \(\lambda\) is the wavelength of the incident X-ray, and \(n\) is an integer, the interlayer spacing of the modified bentonite can be calculated. The \(d_{001}\) spacing of the lithiated bentonite decreased slightly from 1.26 nm to 1.22 nm. After intercalation with MNTA and CTAB, the spacing increased to 3.12 nm. After grafting, it stabilised at 3.085 nm. This indicates that the organic reagents entered the interlayer space of the original clay and enlarged the interlayer distance. The SEM images showed that natural bentonite had a smooth surface, dense agglomerated state, and obvious edge-curled sheet structure. The modified bentonite retained the layered silicate structure overall but was more loosely packed, with a rougher surface and increased surface area and pore gaps. These observations are consistent with the XRD calculations. The swelling index of the bentonite, measured according to GB/T 20973—2020, increased from 5.8 mL/g before modification to 20.1 mL/g after modification. This demonstrates that the modification process significantly improved the interlayer spacing and interlayer interactions. Taken together, these results confirm the success of the “intercalation-grafting” composite modification. The modified bentonite properties are summarised in Table 2.
| Property | Natural Calcium-Based Bentonite | Modified Bentonite |
|---|---|---|
| d₀₀₁ interlayer spacing (nm) | 1.26 | 3.085 |
| After lithiation (nm) | — | 1.22 |
| After intercalation (nm) | — | 3.12 |
| Swelling index (mL/g) | 5.8 | 20.1 |
| Surface morphology | Smooth, dense agglomerates | Rough, loose, increased porosity |
| Dispersion in alcohol system | Poor | Significantly improved |
To investigate the optimal aggregate grading and the optimal addition amounts of suspending agent and binder, I designed an L9(3³) orthogonal experiment. The factors and levels are shown in Table 3. Factor A was the mass ratio of 250-mesh bauxite to 400-mesh bauxite, with levels of 60:20, 53:27, and 40:40. Factor B was the suspending agent addition, with levels of 3%, 4%, and 5%. Factor C was the binder addition, with levels of 2%, 4%, and 6%. The total refractory aggregate mass was 100 parts, and the bauxite-to-white-corundum ratio was fixed at 4:1. The 250-mesh bauxite particles are larger and can improve the permeability and scouring resistance of the coating while reducing the risk of cracking at high temperature. The 400-mesh bauxite particles are finer and can fill the gaps between the 250-mesh particles, making the coating structure denser, reducing the probability of molten iron penetration, and improving surface finish. Using two or more refractory aggregates in combination can improve the overall performance of the coating. A multi-component particle packing typically yields a denser coating than a single-particle system. This is particularly important in sand mould casting, where the coating must withstand both thermal shock and mechanical erosion.
| Factor | Level 1 | Level 2 | Level 3 |
|---|---|---|---|
| A: m(250-mesh bauxite) : m(400-mesh bauxite) | 60:20 | 53:27 | 40:40 |
| B: Suspending agent / % | 3 | 4 | 5 |
| C: Binder / % | 2 | 4 | 6 |
The orthogonal experimental results are presented in Table 4. Each formulation was evaluated by conditional viscosity, 24 h suspension rate, gas evolution, coating wear resistance, and brushing performance. For data analysis, the brushing performance grades III, II, and I were assigned numerical values of 3, 2, and 1, respectively. The range analysis results are shown in Table 5, where \(k_1\), \(k_2\), and \(k_3\) are the mean responses at each level, and \(R\) is the range. The order of influence of each factor on each performance index is also given.
| Formulation | Conditional viscosity / s | 24 h suspension rate / % | Gas evolution / (mL·g⁻¹) | Wear resistance / g | Brushing performance grade |
|---|---|---|---|---|---|
| A1B1C1 | 5.0 | 88 | 11.0 | 0.067 | III |
| A1B2C2 | 6.5 | 94 | 13.4 | 0.056 | I |
| A1B3C3 | 7.3 | 91 | 15.9 | 0.052 | II |
| A2B1C2 | 6.2 | 90 | 13.8 | 0.063 | I |
| A2B2C3 | 8.3 | 93 | 14.2 | 0.049 | III |
| A2B3C1 | 5.9 | 95 | 12.6 | 0.069 | I |
| A3B1C3 | 8.0 | 89 | 13.5 | 0.050 | III |
| A3B2C1 | 6.0 | 94 | 12.0 | 0.070 | II |
| A3B3C2 | 7.1 | 93 | 13.2 | 0.058 | II |
| Index | Level mean / factor | A | B | C | Range R | Order of influence |
|---|---|---|---|---|---|---|
| Conditional viscosity / s | k1 | 6.27 | 6.40 | 5.63 | A: 0.76 B: 0.53 C: 2.24 |
C > B > A |
| k2 | 6.80 | 6.93 | 6.60 | |||
| k3 | 7.03 | 6.77 | 7.87 | |||
| R | 0.76 | 0.53 | 2.24 | |||
| 24 h suspension rate / % | k1 | 91.0 | 89.0 | 92.3 | A: 1.7 B: 4.7 C: 1.3 |
B > A > C |
| k2 | 92.7 | 93.7 | 92.3 | |||
| k3 | 92.0 | 93.0 | 91.0 | |||
| R | 1.7 | 4.7 | 1.3 | |||
| Gas evolution / (mL·g⁻¹) | k1 | 13.43 | 12.77 | 11.87 | A: 0.63 B: 1.13 C: 2.66 |
C > B > A |
| k2 | 13.53 | 13.20 | 13.47 | |||
| k3 | 12.90 | 13.90 | 14.53 | |||
| R | 0.63 | 1.13 | 2.66 | |||
| Wear resistance / g | k1 | 0.0583 | 0.0600 | 0.0687 | A: 0.0020 B: 0.0017 C: 0.0184 |
C > A > B |
| k2 | 0.0603 | 0.0583 | 0.0590 | |||
| k3 | 0.0593 | 0.0597 | 0.0503 | |||
| R | 0.0020 | 0.0017 | 0.0184 | |||
| Brushing performance grade | k1 | 2.00 | 2.33 | 2.00 | A: 0.66 B: 0.66 C: 1.34 |
C > A ≈ B |
| k2 | 1.67 | 2.00 | 1.33 | |||
| k3 | 2.33 | 1.67 | 2.67 | |||
| R | 0.66 | 0.66 | 1.34 |
The range analysis shows that the factors have different effects on different performance indices. For conditional viscosity, which directly reflects the rheological behaviour of the coating, the most significant factor is the binder content. The order of influence is binder content > suspending agent content > refractory aggregate grading. The main reason is that the phenolic resin–PVB composite binder system forms a “microgel” in ethanol, which increases the viscosity of the coating. For conditional viscosity, the optimal combination is A1B1C1, because it provides the lowest viscosity among the tested levels. However, viscosity must be considered together with other properties, as a coating that is too viscous may be difficult to apply uniformly in sand mould casting.
For the 24 h suspension rate, which reflects the dispersion stability of the coating system, the most significant factor is the suspending agent content. The order of influence is suspending agent content > refractory aggregate grading > binder content. After the “intercalation-grafting” modification, the bentonite layers are fully exfoliated and form a dense “card-house” network. This network creates a good spatial structure within the system and effectively supports the aggregate particles. For the suspension rate, the preferred combinations are A2B2C1 or A2B2C2. The higher suspending agent content improves the stability of the coating, which is crucial for storage and application in sand mould casting.
In alcohol-based casting coatings, gas evolution is an important indicator of the amount of gas released at high temperature. Excessive gas evolution can lead to gas porosity on the casting surface during pouring, affecting surface finish and dimensional accuracy. The most significant factor affecting gas evolution is the binder content. The order of influence is binder content > suspending agent content > refractory aggregate grading. The reason is that the binder decomposes at high temperature. Phenolic resin decomposes above 600 °C to produce CO, CO₂, and light hydrocarbons, while PVB also releases butyraldehyde and other organic species. Therefore, the binder content is the most important factor controlling gas evolution. For gas evolution, the preferred combination is A3B1C1, which minimises the amount of gas released.
The wear resistance of the coating represents the surface strength of the cured coating. After the sand mould is coated, it must undergo handling, drying, dust removal, and mould assembly. Therefore, the cured coating must have sufficient surface strength. For wear resistance, the order of influence is binder content > refractory aggregate grading > suspending agent content. Increasing the binder content makes the resin–ceramic skeleton denser, while a reasonable combination of coarse and fine particles forms a “skeleton–filler” structure that further improves wear resistance. For wear resistance, the preferred combination is A1B2C3. This formulation provides the highest surface strength among the tested combinations.
The most critical requirement for an external chill sand mould casting coating is to improve adhesion. Whether on the iron chill surface or on the sand mould surface, the coating must be uniform in thickness and must not “strike through” or leave bare areas. The most significant factor affecting brushing performance is the binder content. The order of influence is binder content > refractory aggregate grading ≈ suspending agent content. Based on the range analysis, the preferred combination for brushing performance is A2B3C2. This combination provides good adhesion and uniform coverage on both sand and metal surfaces.
To determine the optimal formulation, I used a data analysis weight normalization method. The weights were assigned as follows: conditional viscosity 10%, 24 h suspension rate 25%, gas evolution 20%, wear resistance 10%, and brushing performance 35%. The 24 h suspension rate was treated as a positive indicator, while brushing performance, conditional viscosity, gas evolution, and wear resistance were treated as negative indicators. The data were standardised, and the comprehensive scores are shown in Table 6. The final selected formulation was A2B3C1. This formulation achieves “low viscosity–high suspension–good brushing” performance. Although its gas evolution and wear resistance are not the absolute best among all trials, they are within the safe threshold. According to the results in Table 4, this formulation satisfies the requirements of JB/T 9226—2008 for water-based paste bauxite powder coatings (SJ-LF): conditional viscosity 5.5–12 s, 24 h suspension rate ≥ 93%, gas evolution < 20 mL/g, and coating wear resistance < 0.5 g. The comprehensive score of A2B3C1 was the highest, confirming its suitability for external chill sand mould casting.
| Formulation | Weighted Normalized Score |
|---|---|
| A1B1C1 | 18.24 |
| A1B2C2 | 19.81 |
| A1B3C3 | 18.13 |
| A2B1C2 | 18.76 |
| A2B2C3 | 18.52 |
| A2B3C1 | 20.28 |
| A3B1C3 | 17.70 |
| A3B2C1 | 19.79 |
| A3B3C2 | 19.44 |
The optimal formulation A2B3C1 was prepared with a density of 1.60 g/cm³ and tested in a production pouring experiment at a large foundry. The sand mould was a furan resin sand mould. The exposed area of a single chill block was 25 cm × 15 cm, and the total surface area was approximately 1.2 m². The casting was an HT300 machine tool bed. The coating was applied twice by brushing, with a total thickness of 0.7 mm. The coating was uniform, and the brushing performance was good. After ignition, the coating surface solidified within 5 s. The pouring temperature was 1 420 °C. After pouring and holding, the mould was cooled and knocked out. During casting cleaning, the coating exhibited good stripping behaviour, and the casting surface became smooth. The cleaning time was greatly shortened, and the iron blocks were easily separated from the mould, making external chill recovery convenient. The production verification results are compared in Table 7. The improved coating effectively enhanced the adhesion, brushing performance, and sintering behaviour on both the iron block surface and the sand mould surface. The actual feedback shows that the coating meets production requirements well, reduces production cost, and improves casting quality.
| Item | Before Improvement | After Improvement |
|---|---|---|
| Surface roughness Ra / μm | 25.0 | 12.5 |
| Sand adhesion area / % | 8.5 | < 2 |
| Cleaning time / min | 45 | 25 |
| Chill recovery rate / % | 65 | > 90 |
| Gas porosity defects (Φ > 2 mm) | Present | None |
The results of this study demonstrate that the two-step “intercalation-grafting” modification of bentonite significantly improves the dispersion and compatibility of the suspending agent in an alcohol-based system. The modified bentonite forms a robust network in ethanol, which supports the refractory particles and prevents settling. This is essential for the storage stability and application uniformity of the coating in sand mould casting. The XRD and swelling index results confirm that the organic modifiers enter the interlayer space and increase the interlayer spacing, while the SEM observations show a more open and rougher structure that provides better interaction with the solvent and binder.
The orthogonal experiment revealed that the binder content is the most influential factor for conditional viscosity, gas evolution, wear resistance, and brushing performance. The suspending agent content is the most influential factor for the 24 h suspension rate. The refractory aggregate grading has a moderate effect on most properties but is particularly important for packing density and surface finish. The optimal formulation A2B3C1 balances these factors and achieves a 24 h suspension rate of 95%, a gas evolution of 12.6 mL/g, a brushing performance grade of I, and excellent coating wear resistance. The conditional viscosity is 5.9 s, which is within the acceptable range for brushing application in sand mould casting.
The production verification confirms that the coating is suitable for external chill sand mould casting of large cast iron components. The coating provides good adhesion on both sand and metal surfaces, which solves the problem of insufficient coating adhesion on chill surfaces. The improved chill recovery rate reduces material waste and production cost. The coating also strips easily after casting, which shortens the cleaning time and reduces labour intensity. The absence of gas porosity defects and the low sand adhesion area indicate that the coating effectively prevents molten iron penetration and sand burn-on. The surface roughness is reduced from 25.0 μm to 12.5 μm, which is a significant improvement in casting quality.
In summary, I developed an alcohol-based casting coating specifically for external chill sand mould casting. The coating uses a bauxite–white corundum composite refractory aggregate, a composite modified bentonite suspending agent prepared by a two-step “intercalation-grafting” method, and a phenolic resin–PVB composite binder in an industrial ethanol solvent. The optimal formulation was determined by orthogonal experiment and weight normalization analysis. The resulting coating exhibits fast drying, high suspension stability, low gas evolution, good brushing performance, and excellent wear resistance. Production trials show that the coating improves casting surface quality, reduces sand adhesion and gas porosity, and enhances chill recovery. This study provides a practical and effective solution for the optimization of coatings in external chill sand mould casting and contributes to the improvement of production efficiency and casting quality in the foundry industry.
For future work, I would consider further optimising the ratio of organic modifiers to reduce the cost of the suspending agent while maintaining its performance. I would also investigate the effect of coating thickness on interfacial heat transfer in sand mould casting using a heat transfer model that couples the coating layer, the chill, and the casting. The interfacial heat transfer coefficient \(h\) can be expressed as
$$h = \frac{1}{\frac{1}{h_c} + \frac{\delta_c}{k_c} + \frac{1}{h_m}}$$
where \(h_c\) is the contact conductance between the casting and the coating, \(\delta_c\) is the coating thickness, \(k_c\) is the thermal conductivity of the coating, and \(h_m\) is the contact conductance between the coating and the mould or chill. A better understanding of this relationship would allow more precise control of solidification in external chill sand mould casting. I would also explore the use of alternative bio-based solvents or binders to reduce environmental impact without sacrificing performance. These directions could further advance the development of high-performance coatings for sand mould casting.