Lost foam casting has become one of the most important near-net-shape manufacturing processes in the foundry industry due to its high dimensional accuracy, excellent surface finish, and environmental friendliness. In this technology, a foam pattern is coated with a refractory coating layer, embedded in dry sand, and then poured with molten metal. The coating layer plays a decisive role in the success of the lost foam casting process. It not only protects the foam pattern from deformation but also regulates heat transfer, gas permeability, and the removal of decomposition products generated by the pyrolyzing foam pattern. For iron castings, the coating formulation must be carefully designed to meet the demanding requirements of high pouring temperatures, gas evolution, and the tendency for sand adhesion. In this study, I systematically investigated the preparation and performance of water-based lost foam casting coatings for iron castings. The main objective was to develop coating formulations with excellent comprehensive properties including high suspension, appropriate viscosity, good permeability, strong coating adhesion, low moisture absorption, and excellent crack resistance. Through single-factor experiments and orthogonal optimization, two optimized coating formulations were established, and their performance was thoroughly evaluated.
Experimental Materials and Methods
The raw materials used in this study were carefully selected based on the specific requirements of iron casting lost foam casting. The refractory aggregate system consisted of quartz powder and bauxite, both with a particle size range of 270 to 350 mesh. Quartz powder is inexpensive, chemically stable, and readily available, while bauxite offers high refractoriness and good sintering behavior. Talc powder was used as an auxiliary refractory filler to improve the leveling property and reduce the coefficient of thermal expansion of the coating. Sodium bentonite, xanthan gum, magnesium aluminum silicate, and sepiolite were investigated as suspending agents. Polyvinyl alcohol and silica sol were used as low-temperature and high-temperature binders, respectively. Water was used as the carrier liquid. Additional additives included OP-10 as a surfactant, n-butanol as a defoamer, and sodium benzoate as a preservative.
The coating preparation procedure involved several steps. First, the refractory aggregates were dried at 50 °C for 4 hours to remove absorbed moisture. Sodium bentonite was pre-hydrated in water at a mass ratio of 1:10 for 12 hours to form a gel. Magnesium aluminum silicate was dispersed in water at a concentration of 5 to 8 wt.% and allowed to hydrate for 12 hours. Xanthan gum was dissolved in water at a mass ratio of 1:40. PVA was dissolved in hot water to prepare a 5 wt.% aqueous solution and then mixed with silica sol at a stirring speed of 100 rpm for 30 minutes to form a composite binder. The coating was prepared by mixing all components in a high-speed stirrer. The mixing sequence was designed to ensure uniform dispersion: water and suspending agents were mixed first, then the refractory aggregates were added gradually, followed by the binder system. Finally, the coating was adjusted to a target density and subjected to a 24-hour aging treatment to stabilize its rheological properties.
The performance of the prepared coatings was evaluated using several standard test methods. The suspension property was measured by a graduated cylinder method. The coating was poured into a 100 mL cylinder and allowed to stand for 24 hours. The volume of the clear liquid layer at the top was recorded, and the suspension was calculated by the following formula:
$$S = \frac{V_0 – V}{V_0} \times 100\%$$
where \(V_0\) is the total volume of the coating (100 mL) and \(V\) is the volume of the clear liquid layer. The viscosity was measured using an LND-1 flow cup viscometer, recording the time for the coating to flow out of the cup. The normal-temperature permeability was measured using an STZ direct-reading permeability tester. A nylon gauze was fixed to the sample tube and coated with the test coating to a thickness of approximately 1 mm. After drying for 12 hours, the permeability was recorded. The rheological properties were measured with an NDJ-8S rotational viscometer. The shear stress and shear rate were calculated using the following equations:
$$\tau = Z \alpha$$
$$D = \frac{\pi n}{15}$$
where \(\tau\) is the shear stress, \(Z\) is the torsion constant, \(\alpha\) is the dial reading, \(D\) is the shear rate, and \(n\) is the rotational speed. The apparent viscosity was then calculated as:
$$\eta = \frac{\tau}{D}$$
The thixotropic index was determined by measuring the apparent viscosity at 30 rpm after 0.5 minutes and after 10 minutes of shearing, and then calculated as:
$$N = \frac{\eta_1 – \eta_2}{\eta_1} \times 100\%$$
where \(\eta_1\) is the apparent viscosity at 0.5 min and \(\eta_2\) is the apparent viscosity at 10 min. The coating strength was measured by a SUM coating surface strength tester. The coating was applied to a preheated mold and the amount of coating scraped off by the tester was recorded; a smaller scraped mass indicates higher coating strength. The moisture absorption rate was determined by applying the coating to a glass plate, drying it, weighing it, and then placing it in a desiccator at 15 °C with 95% relative humidity for 48 hours. The moisture absorption rate was calculated as:
$$M = \frac{m_2 – m_1}{m_1} \times 100\%$$
where \(m_1\) is the dry mass and \(m_2\) is the mass after moisture absorption. The applicability and crack resistance were evaluated by visual inspection and classified into grades I, II, III, and IV, with grade I being the best.
Influence of Refractory Aggregate Composition
The refractory aggregate composition is the most critical factor determining the performance of the lost foam casting coating. In the initial stage, I prepared three coating formulations with different quartz powder to bauxite ratios while keeping other components constant. The formulations are listed in Table 1.
| Group | Quartz powder (%) | Bauxite (%) | Sodium bentonite (%) | Xanthan gum (%) | PVA (%) | Silica sol (%) |
|---|---|---|---|---|---|---|
| 1 | 30 | 70 | 3 | 0.5 | 2 | 3 |
| 2 | 50 | 50 | 3 | 0.5 | 2 | 3 |
| 3 | 70 | 30 | 3 | 0.5 | 2 | 3 |
The performance test results of these three groups are presented in Table 2. The density of all coatings was controlled at approximately 1.22 g/cm³.
| Group | Suspension (%) | Permeability | Applicability | Flow cup viscosity (s) | Coating strength (g) | Yield value (Pa) | Thixotropic index | Moisture absorption (%) |
|---|---|---|---|---|---|---|---|---|
| 1 | 88 | 12.8 | I | 11.97 | 111.6 | 3.476 | 1.431 | 2.78 |
| 2 | 90 | 12.0 | II | 10.12 | 123.2 | 1.344 | 1.316 | 3.23 |
| 3 | 93 | 11.2 | II | 11.15 | 129.6 | 1.195 | 1.352 | 3.66 |
The experimental data in Table 2 indicate that the refractory aggregate ratio significantly influences the coating performance. The suspension property increased as the quartz powder content increased, mainly because quartz powder has a lower density than bauxite, which reduces the overall density of the coating and slows the settling of solid particles. However, the moisture absorption rate also increased with increasing quartz content, since SiO₂ is more prone to moisture absorption than Al₂O₃ under high-humidity conditions. In contrast, the permeability and coating strength decreased as the bauxite content decreased. Group 1, with 30% quartz powder and 70% bauxite, exhibited the best overall performance: it had the highest permeability (12.8), the best applicability (grade I), the highest flow cup viscosity (11.97 s), the lowest scraped mass (111.6 g, indicating the highest strength), the highest yield value (3.476 Pa), and the highest thixotropic index (1.431). Additionally, it showed the lowest moisture absorption rate (2.78%) among the three groups. Based on the comprehensive comparison, I selected the combination of 30% quartz powder and 70% bauxite as the optimal refractory aggregate ratio for iron casting lost foam casting coatings.
Influence of Talc Powder Content
On the basis of the fixed refractory aggregate ratio (30% quartz + 70% bauxite), I further investigated the effect of talc powder as an auxiliary refractory filler. Five levels of talc powder addition were tested: 0%, 2.5%, 5%, 7.5%, and 10% relative to the mass of the refractory aggregate. The performance results are presented in Table 3.
| Talc content (%) | Suspension (%) | Permeability | Applicability | Flow cup viscosity (s) | Scraped mass (g) | Moisture absorption (%) |
|---|---|---|---|---|---|---|
| 0 | 88.0 | 12.8 | I | 11.97 | 144.2 | 2.78 |
| 2.5 | 90.3 | 13.2 | I | 11.15 | 138.0 | 2.83 |
| 5 | 91.4 | 13.4 | I | 10.61 | 133.5 | 2.96 |
| 7.5 | 91.8 | 13.6 | I | 10.17 | 131.2 | 3.11 |
| 10 | 92.1 | 13.8 | II | 9.71 | 129.6 | 3.35 |
The results show that the addition of talc powder gradually improved the suspension property and normal-temperature permeability of the coating. The suspension increased from 88.0% to 92.1% as the talc content increased from 0 to 10%. The permeability also increased steadily from 12.8 to 13.8. The scraped mass decreased from 144.2 g to 129.6 g, indicating that the coating strength improved with increasing talc content. However, the applicability deteriorated from grade I to grade II when the talc content reached 10%, and the flow cup viscosity decreased continuously from 11.97 s to 9.71 s, which may deteriorate the brushing performance. The moisture absorption rate also increased from 2.78% to 3.35%, which is undesirable for long-term coating stability. Considering all factors, the addition of 7.5% talc powder provided the best balance among suspension, permeability, applicability, viscosity, strength, and moisture resistance. Therefore, I fixed the talc content at 7.5% for the subsequent optimization.
Influence of Suspending Agents
Suspending agents are essential components of lost foam casting coatings. They impart thixotropy, increase the low-shear viscosity, and prevent the sedimentation of refractory particles. In this study, I systematically evaluated the effects of sodium bentonite, xanthan gum, magnesium aluminum silicate, and sepiolite on the coating performance.
Sodium Bentonite
Sodium bentonite is the most widely used suspending agent in water-based foundry coatings. I conducted single-factor experiments with sodium bentonite contents of 1%, 2%, 3%, 4%, and 5%, while the PVA content was fixed at 2% and the silica sol content at 3%. The test results are summarized in Table 4.
| Content (%) | Suspension (%) | Permeability | Applicability | Flow cup viscosity (s) | Crack resistance | Moisture absorption (%) |
|---|---|---|---|---|---|---|
| 1 | 89 | 13.2 | I | 10.01 | II | 3.05 |
| 2 | 92 | 13.8 | I | 10.23 | I | 3.26 |
| 3 | 93 | 14.3 | I | 11.78 | I | 3.37 |
| 4 | 91 | 14.6 | II | 13.84 | II | 3.45 |
| 5 | 88 | 14.7 | II | 14.23 | III | 3.52 |
As shown in Table 4, the suspension property first increased from 89% to 93% when the bentonite content increased from 1% to 3%, and then decreased to 88% at 5%. This behavior can be attributed to the formation and subsequent destruction of the network structure. At an appropriate concentration, bentonite particles form a three-dimensional network that effectively suspends the refractory particles. Excessive bentonite disrupts this pseudo-plastic fluid structure and reduces the suspension stability. The permeability increased continuously with increasing bentonite content, but the increasing rate gradually slowed down. The flow cup viscosity increased from 10.01 s to 14.23 s over the entire range, with a particularly sharp increase above 2% content. The applicability remained grade I up to 3% and then deteriorated to grade II. The moisture absorption rate increased from 3.05% to 3.52% with increasing bentonite content. The crack resistance was grade I at 2% to 3%, but degraded at both lower and higher contents. Based on these results, I determined the optimal sodium bentonite content to be approximately 3%.
Xanthan Gum
Xanthan gum is a biopolysaccharide that provides excellent thickening and stabilizing effects in water-based coatings. In this experiment, the PVA content was fixed at 2% and the silica sol content at 3%, and xanthan gum was varied at 0.1%, 0.3%, 0.5%, and 0.7%. The results are listed in Table 5.
| Content (%) | Suspension (%) | Permeability | Applicability | Flow cup viscosity (s) | Crack resistance | Moisture absorption (%) |
|---|---|---|---|---|---|---|
| 0.1 | 93 | 14.7 | II | 10.13 | II | 3.25 |
| 0.3 | 93 | 14.3 | I | 11.78 | I | 3.37 |
| 0.5 | 92 | 13.2 | I | 13.55 | I | 3.47 |
| 0.7 | 90 | 11.6 | I | 15.84 | II | 3.53 |
The suspension property remained high (93%) at low xanthan gum contents but decreased to 90% at 0.7%. The permeability showed a continuous decrease from 14.7 to 11.6 as the xanthan gum content increased, because the polymeric chains of xanthan gum filled the pores between refractory particles. The applicability improved from grade II at 0.1% to grade I at 0.3% and above, indicating that xanthan gum enhances the adhesion and film-forming properties of the coating. The flow cup viscosity increased dramatically from 10.13 s to 15.84 s, which could make the coating too thick for proper brushing when the xanthan gum content exceeds 0.3%. The crack resistance was grade I at 0.3% to 0.5% but degraded at higher contents. The moisture absorption increased from 3.25% to 3.53% with increasing xanthan gum content. From the comprehensive analysis, I identified 0.3% as the optimal xanthan gum content for this coating system.
Magnesium Aluminum Silicate
Magnesium aluminum silicate is an inorganic layered silicate that forms a gel structure in water, providing excellent thixotropy and suspension stability. I tested contents of 0.5%, 1%, 2%, and 3% while keeping PVA at 2% and silica sol at 3%. The performance data are presented in Table 6.
| Content (%) | Suspension (%) | Flow cup viscosity (s) | Yield value (Pa) | Thixotropic index |
|---|---|---|---|---|
| 0.5 | 83 | 9.95 | 1.772 | 2.550 |
| 1 | 88 | 10.21 | 1.873 | 2.572 |
| 2 | 94 | 12.46 | 2.485 | 2.580 |
| 3 | 98 | 14.74 | 3.370 | 2.600 |
Magnesium aluminum silicate exhibited a strong influence on both suspension and rheological properties. As the content increased from 0.5% to 3%, the suspension property improved significantly from 83% to 98%, and the yield value increased from 1.772 Pa to 3.370 Pa. This is because the silicate particles form a card-house network structure through electrostatic interactions, which impedes the settling of refractory particles. The flow cup viscosity increased from 9.95 s to 14.74 s. The thixotropic index also increased gradually from 2.550 to 2.600, indicating improved shear-thinning behavior. Figure 1 shows the relationship between apparent viscosity and shear rate for coatings with different magnesium aluminum silicate contents. All coatings exhibited typical shear-thinning behavior characteristic of yield-pseudoplastic fluids. The apparent viscosity decreased sharply with increasing shear rate at low shear rates and gradually leveled off at higher shear rates. The coating containing 3% magnesium aluminum silicate maintained higher apparent viscosity across the entire shear rate range, which is beneficial for both suspension stability and vertical surface application. The coating with 0.5% content showed the most rapid viscosity drop, indicating poor structural recovery. Based on these results, the optimal addition range of magnesium aluminum silicate was determined to be 2% to 3%.
Sepiolite
Sepiolite is a fibrous clay mineral with a unique chain-layer structure. I investigated the effect of sepiolite at contents of 5%, 10%, and 15%, with fixed amounts of 0.3% xanthan gum, 2% PVA, and 3% silica sol. The results are shown in Table 7.
| Content (%) | Suspension (%) | Flow cup viscosity (s) | Permeability | Scraped mass (g) |
|---|---|---|---|---|
| 5 | 87 | 11.35 | 13.7 | 1.35 |
| 10 | 93 | 13.31 | 15.1 | 1.29 |
| 15 | 97 | 16.04 | 10.3 | 1.26 |
The suspension property increased from 87% to 97% as the sepiolite content increased from 5% to 15%. This is attributed to the fibrous network formed by sepiolite particles in water, which physically hinders the settling of coarse refractory particles. The flow cup viscosity increased from 11.35 s to 16.04 s, showing that sepiolite has a strong thickening effect. The permeability first increased to a maximum of 15.1 at 10% sepiolite and then sharply decreased to 10.3 at 15%. The initial increase may be related to the formation of a more open structure, while excessive sepiolite caused the coating to become too dense, blocking the gas escape channels. The coating strength, indicated by the scraped mass, improved with increasing sepiolite content. Importantly, sepiolite also contributes to moisture resistance. In addition to these performance aspects, the unique fibrous morphology of sepiolite enhances the high-temperature stability and anti-cracking properties of the coating. Based on these experimental results, the optimal sepiolite content was determined to be around 10%.
Influence of Binders
Binders are necessary to provide sufficient strength to the coating layer at both ambient and elevated temperatures. In this study, I used a combination of PVA as a low-temperature binder and silica sol as a high-temperature binder.
Polyvinyl Alcohol
PVA was investigated at contents of 0.5%, 1.0%, 1.5%, 2.0%, and 2.5%, with a fixed addition of 10% sepiolite and 3% silica sol. The performance results are listed in Table 8. Note that the scraped mass value inversely reflects the coating strength: a lower scraped mass indicates higher strength.
| Content (%) | Suspension (%) | Permeability | Applicability | Flow cup viscosity (s) | Crack resistance | Scraped mass (g) | Moisture absorption (%) |
|---|---|---|---|---|---|---|---|
| 0.5 | 87 | 15.9 | III | 11.07 | III | 1.30 | 3.52 |
| 1.0 | 90 | 15.6 | III | 11.43 | II | 1.28 | 3.43 |
| 1.5 | 92 | 15.3 | II | 11.97 | I | 1.26 | 3.35 |
| 2.0 | 93 | 14.9 | I | 13.27 | I | 1.29 | 3.28 |
| 2.5 | 92 | 14.6 | II | 15.98 | III | 1.31 | 3.24 |
The suspension property increased from 87% to 93% as the PVA content increased from 0.5% to 2.0%, but slightly decreased at 2.5%. The permeability continuously decreased from 15.9 to 14.6, indicating that PVA fills the interstitial pores among refractory particles. The applicability and crack resistance both exhibited a peak performance at 2.0% and 1.5% to 2.0%, respectively. The flow cup viscosity increased from 11.07 s to 15.98 s, with a sharp increase above 2.0% content. The coating strength reached its maximum at 1.5% PVA, as evidenced by the minimum scraped mass of 1.26 g. The moisture absorption rate decreased linearly from 3.52% to 3.24% with increasing PVA content, suggesting that PVA helps to reduce the hygroscopicity of the coating. Based on the comprehensive comparison, the optimal PVA content was determined to be approximately 1.5%.
Silica Sol
Silica sol is a colloidal silica solution that acts as an inorganic binder, providing high-temperature strength to the coating. I varied the silica sol content from 2% to 6% while keeping 10% sepiolite and 2% PVA fixed. The test results are summarized in Table 9.
| Content (%) | Suspension (%) | Permeability | Applicability | Flow cup viscosity (s) | Crack resistance | High-temperature scraped mass (g) | Moisture absorption (%) |
|---|---|---|---|---|---|---|---|
| 2 | 91 | 15.9 | III | 14.44 | III | 1.34 | 3.43 |
| 3 | 93 | 15.1 | II | 13.31 | II | 1.29 | 3.26 |
| 4 | 95 | 14.5 | I | 12.42 | II | 1.27 | 3.06 |
| 5 | 96 | 14.0 | II | 13.15 | I | 1.23 | 2.88 |
| 6 | 97 | 13.5 | III | 14.66 | I | 1.19 | 2.75 |
The suspension property increased from 91% to 97% with increasing silica sol content. However, the permeability decreased from 15.9 to 13.5, because the colloidal silica particles filled the gaps between refractory particles and reduced the pore size. The flow cup viscosity exhibited a minimum value of 12.42 s at 4% silica sol. The applicability was best at 4%, reaching grade I. The high-temperature coating strength increased continuously with increasing silica sol content, as indicated by the decreasing scraped mass from 1.34 g to 1.19 g. The crack resistance improved from grade III at 2% to grade I at 5% and 6%. The moisture absorption rate decreased significantly from 3.43% to 2.75% with increasing silica sol content. Considering the balance among all performance indicators, the optimal silica sol content was determined to be approximately 4%.
Orthogonal Optimization of Coating Formulations
Based on the single-factor experiments, I identified the suitable ranges for each component. To determine the optimal combination of these components, I conducted two separate orthogonal experiments using the L9(3⁴) design. The first orthogonal experiment (Formulation I) involved sodium bentonite, xanthan gum, PVA, and silica sol. The second orthogonal experiment (Formulation II) involved magnesium aluminum silicate, xanthan gum, PVA, and silica sol.
Orthogonal Experiment I
The factor levels for Formulation I are shown in Table 10. The refractory aggregate was fixed at 30% quartz powder, 70% bauxite, and 7.5% talc powder.
| Level | Sodium bentonite (A) | Xanthan gum (B) | PVA (C) | Silica sol (D) |
|---|---|---|---|---|
| 1 | 2 | 0.2 | 1 | 3 |
| 2 | 3 | 0.3 | 1.5 | 4 |
| 3 | 4 | 0.4 | 2 | 5 |
The nine experiments and the corresponding performance test results are listed in Table 11.
| No. | A | B | C | D | Suspension (%) | Permeability | Flow cup viscosity (s) | Moisture absorption (%) |
|---|---|---|---|---|---|---|---|---|
| 1 | 1 | 1 | 1 | 1 | 86 | 15.2 | 11.15 | 3.37 |
| 2 | 1 | 2 | 2 | 2 | 90 | 13.3 | 11.43 | 3.23 |
| 3 | 1 | 3 | 3 | 3 | 93 | 11.0 | 11.84 | 2.95 |
| 4 | 2 | 1 | 2 | 3 | 92 | 12.9 | 12.18 | 2.67 |
| 5 | 2 | 2 | 3 | 1 | 97 | 14.8 | 12.79 | 3.44 |
| 6 | 2 | 3 | 1 | 2 | 88 | 13.0 | 11.76 | 3.29 |
| 7 | 3 | 1 | 3 | 2 | 95 | 13.7 | 13.25 | 3.18 |
| 8 | 3 | 2 | 1 | 3 | 87 | 12.2 | 12.33 | 2.84 |
| 9 | 3 | 3 | 2 | 1 | 88 | 14.3 | 12.87 | 3.51 |
The range analysis results are presented in Table 12. The range R for each factor was calculated using the following formula:
$$R = \max(K_1, K_2, K_3) – \min(K_1, K_2, K_3)$$
where \(K_i\) is the sum of the test results at level i for each factor.
| Range | Suspension (%) | Permeability | Flow cup viscosity (s) | Moisture absorption (%) |
|---|---|---|---|---|
| R(A) | 8 | 1.2 | 4.03 | 0.15 |
| R(B) | 5 | 3.5 | 0.11 | 0.53 |
| R(C) | 24 | 1.0 | 2.64 | 0.16 |
| R(D) | 2 | 8.2 | 0.46 | 1.86 |
From the range analysis, I determined the influence ranking of each factor on different performance indicators. For suspension property, the order was: PVA > sodium bentonite > xanthan gum > silica sol. For permeability, the order was: silica sol > xanthan gum > sodium bentonite > PVA. For flow cup viscosity, the order was: sodium bentonite > PVA > silica sol > xanthan gum. For moisture absorption, the order was: silica sol > xanthan gum > PVA > sodium bentonite.
Based on the K values, the optimal combination for maximum suspension was A2B2C3D2, for maximum permeability was A2B1C2D1, for maximum flow cup viscosity was A3B1C3D1, and for minimum moisture absorption (maximum resistance) was A2B1C2D3. To determine the formulation with the best overall performance, I compared these candidate formulations through additional verification tests. The results are shown in Table 13.
| No. | Combination | Suspension (%) | Permeability | Flow cup viscosity (s) | Moisture absorption (%) |
|---|---|---|---|---|---|
| 1 | A2B2C3D2 | 97 | 14.5 | 12.72 | 3.39 |
| 2 | A2B1C2D1 | 91 | 13.6 | 12.23 | 2.84 |
| 3 | A3B1C3D1 | 95 | 14.2 | 13.27 | 3.23 |
| 4 | A2B1C2D3 | 92 | 12.9 | 12.18 | 2.67 |
Comparing the four candidate formulations, combination A3B1C3D1 exhibited excellent overall performance: a high suspension of 95%, a permeability of 14.2, a suitable flow cup viscosity of 13.27 s, and a relatively low moisture absorption of 3.23%. Although its suspension was slightly lower than combination 1, the overall balance of properties was deemed superior for the practical requirements of iron casting lost foam casting. Therefore, the optimal formulation of Formulation I was determined as follows:
| Quartz powder | Bauxite | Talc powder | Sodium bentonite | Xanthan gum | PVA | Silica sol | OP-10 | n-Butanol | Water |
|---|---|---|---|---|---|---|---|---|---|
| 30 | 70 | 7.5 | 4 | 0.2 | 2 | 3 | appropriate | appropriate | appropriate |
The final performance of the optimized Formulation I is listed in Table 15.
| Suspension (%) | Permeability | Flow cup viscosity (s) | Moisture absorption (%) | Applicability | Crack resistance | Density (g/cm³) |
|---|---|---|---|---|---|---|
| 95 | 14.2 | 13.27 | 3.23 | I | I | 1.29 |
Orthogonal Experiment II
The second orthogonal experiment was designed to optimize a coating system using magnesium aluminum silicate as the primary suspending agent instead of sodium bentonite. The factor levels are shown in Table 16.
| Level | Mg-Al silicate (A) | Xanthan gum (B) | PVA (C) | Silica sol (D) |
|---|---|---|---|---|
| 1 | 2 | 0.2 | 1 | 3 |
| 2 | 2.5 | 0.3 | 1.5 | 4 |
| 3 | 3 | 0.4 | 2 | 5 |
The orthogonal experiment scheme and the test results are summarized in Table 17.
| No. | A | B | C | D | Suspension (%) | Permeability | Flow cup viscosity (s) | Moisture absorption (%) |
|---|---|---|---|---|---|---|---|---|
| 1 | 1 | 1 | 1 | 1 | 88 | 15.6 | 12.38 | 3.24 |
| 2 | 1 | 2 | 2 | 2 | 89 | 14.4 | 12.54 | 2.94 |
| 3 | 1 | 3 | 3 | 3 | 90 | 12.7 | 12.71 | 2.48 |
| 4 | 2 | 1 | 2 | 3 | 92 | 12.4 | 12.91 | 2.63 |
| 5 | 2 | 2 | 3 | 1 | 93 | 15.3 | 13.03 | 3.35 |
| 6 | 2 | 3 | 1 | 2 | 91 | 13.8 | 12.83 | 3.02 |
| 7 | 3 | 1 | 3 | 2 | 97 | 13.4 | 13.33 | 3.11 |
| 8 | 3 | 2 | 1 | 3 | 94 | 12.0 | 13.15 | 2.77 |
| 9 | 3 | 3 | 2 | 1 | 95 | 14.8 | 13.25 | 3.42 |
The range analysis results are presented in Table 18.
| Range | Suspension (%) | Permeability | Flow cup viscosity (s) | Moisture absorption (%) |
|---|---|---|---|---|
| R(A) | 19 | 2.5 | 2.1 | 0.64 |
| R(B) | 1 | 0.4 | 0.17 | 0.14 |
| R(C) | 8 | 0.2 | 0.71 | 0.09 |
| R(D) | 1 | 8.6 | 0.11 | 2.13 |
For Formulation II, the influence ranking for suspension was: magnesium aluminum silicate > PVA > xanthan gum = silica sol. For permeability, the order was: silica sol > magnesium aluminum silicate > xanthan gum > PVA. For flow cup viscosity, the order was: magnesium aluminum silicate > PVA > xanthan gum > silica sol. For moisture absorption, the order was: silica sol > magnesium aluminum silicate > xanthan gum > PVA.
The optimal combination for maximum suspension was A3B1C3D2, for maximum permeability was A1B2C2D1, for maximum flow cup viscosity was A3B3C3D3, and for minimum moisture absorption was A1B3C3D3. I then compared the four candidate formulations through verification tests, as shown in Table 19.
| No. | Combination | Suspension (%) | Permeability | Flow cup viscosity (s) | Moisture absorption (%) |
|---|---|---|---|---|---|
| 1 | A3B1C3D2 | 97 | 13.4 | 13.33 | 3.11 |
| 2 | A1B2C2D1 | 89 | 15.2 | 12.63 | 3.17 |
| 3 | A3B3C3D3 | 96 | 13.1 | 13.35 | 2.73 |
| 4 | A1B3C3D3 | 90 | 12.7 | 12.71 | 2.48 |
Among these, combination A3B1C3D2 exhibited the highest suspension (97%), a good permeability of 13.4, and a suitable viscosity of 13.33 s. Its moisture absorption of 3.11% was acceptable. This combination was selected as the optimal Formulation II, as listed in Table 20.
| Quartz powder | Bauxite | Talc powder | Mg-Al silicate | Xanthan gum | PVA | Silica sol | OP-10 | n-Butanol | Water |
|---|---|---|---|---|---|---|---|---|---|
| 30 | 70 | 7.5 | 3 | 0.2 | 2 | 4 | appropriate | appropriate | appropriate |
The final performance of the optimized Formulation II is listed in Table 21.
| Suspension (%) | Permeability | Flow cup viscosity (s) | Moisture absorption (%) | Applicability | Crack resistance | Thixotropic index | Density (g/cm³) |
|---|---|---|---|---|---|---|---|
| 97 | 13.4 | 13.33 | 3.11 | I | I | 2.612 | 1.28 |
Microstructure Analysis of the Optimized Coatings
After determining the optimized formulations, I examined the microstructure of the dried coating layers using scanning electron microscopy. The coating prepared from Formulation I exhibited a dense and uniform microstructure. The refractory aggregate particles were clearly visible and tightly packed, while the finer binder and suspending agent phases were distributed uniformly in the interstitial spaces without obvious agglomeration. At higher magnification, some uniformly distributed microcavities and connected channels were observed on the coating surface. These pores are beneficial for the gas permeability of the coating, as they provide escape passages for the gaseous decomposition products of the foam pattern during the lost foam casting process. This connected pore network is essential to prevent gas-related defects such as blowholes and incomplete filling in iron castings.
The microstructure of the Formulation II coating was similarly dense and uniform, with the refractory particles well bonded by the binder system. The talc powder, characterized by its platy morphology, could be observed interspersed among the larger quartz and bauxite particles. This platy structure contributes to the improved leveling property and reduced thermal expansion of the coating. The presence of interconnected pores was also evident, confirming that Formulation II possesses a favorable microstructure for gas permeability. The uniform distribution of the components in both formulations ensures that the coating can provide consistent protection for the foam pattern during handling, vibration compaction, and pouring. The microstructural observations are consistent with the excellent permeability, crack resistance, and applicability measured for both optimized coatings.
Comparison and Discussion
The two optimized coating formulations developed in this study exhibit distinct advantages for iron casting lost foam casting applications. Formulation I, which uses sodium bentonite as the primary suspending agent, achieves a higher permeability of 14.2 compared to 13.4 for Formulation II. This suggests that Formulation I is more suitable for applications where gas evacuation is critical, such as thick-walled iron castings or complex geometries where a large amount of foam pattern needs to be pyrolyzed. Formulation II, which uses magnesium aluminum silicate, achieves a higher suspension of 97% and a higher thixotropic index of 2.612, indicating better long-term storage stability and more pronounced shear-thinning behavior. This formulation also exhibits a slightly lower moisture absorption rate of 3.11%, making it more resistant to humidity during storage and application.
Both formulations contain 30% quartz powder and 70% bauxite as the composite refractory aggregate, with 7.5% talc powder as an auxiliary filler. The selection of this aggregate combination provides a good balance among refractoriness, thermal expansion control, and cost. Quartz powder is inexpensive and abundant, while bauxite offers high refractoriness and good sintering characteristics. Talc powder contributes to the leveling property and helps to reduce the thermal expansion coefficient of the coating layer.
The suspending agent system significantly affects the rheological behavior of the coatings. Both optimized coatings behave as yield-pseudoplastic fluids, which can be described by the Herschel-Bulkley model:
$$\tau = \tau_y + K D^n \quad (0 < n < 1)$$
where \(\tau_y\) is the yield stress, \(K\) is the consistency index, and \(n\) is the flow behavior index. This rheological behavior is highly desirable for lost foam casting coatings because it allows the coating to flow easily under the shear force applied during brushing or dipping, while maintaining sufficient viscosity to prevent sagging and dripping on vertical surfaces. The yield stress prevents the settling of refractory particles under quiescent conditions, ensuring long-term suspension stability.
The binder system of PVA and silica sol provides dual strengthening mechanisms. PVA forms a film at ambient temperature, binding the refractory particles together and providing the green strength required for handling and transporting the coated foam patterns. At high temperatures, PVA decomposes, and the silica sol undergoes gelation and sintering, forming ceramic bonds that maintain the integrity of the coating layer during pouring. This dual mechanism ensures that the coating retains adequate strength throughout the entire lost foam casting process. The silica sol also contributes to reducing the moisture absorption of the coating, as evidenced by the lower moisture absorption rates at higher silica sol contents.
One of the key challenges in iron casting lost foam casting is the control of coating moisture absorption. Excessive moisture absorption can lead to steam generation during pouring, which may cause defects such as pinholes, blowholes, and surface roughness in the castings. The moisture absorption rates of the optimized formulations (3.23% and 3.11%) are considered acceptable for practical applications, representing a significant improvement over traditional formulations. The combination of inorganic suspending agents with the hydrophobic nature of the organic xanthan gum and PVA contributes to the reduction of water uptake by the dried coating layer.
Conclusions
In this study, I developed and optimized two water-based lost foam casting coatings specifically designed for iron castings. Based on the extensive single-factor experiments and orthogonal optimization, the following conclusions can be drawn:
(1) The optimal composite refractory aggregate for iron castings lost foam casting coating was determined to be 30% quartz powder and 70% bauxite. This combination provides an excellent balance among permeability, applicability, coating strength, moisture resistance, and thixotropic properties, all of which meet the requirements of the lost foam casting process.
(2) The addition of 7.5% talc powder as an auxiliary refractory filler was found to be optimal. Talc powder improves the suspension property, permeability, and coating strength while maintaining adequate applicability and viscosity characteristics.
(3) Single-factor experiments demonstrated that the optimal contents of the suspending agents and binders are approximately: sodium bentonite 3%, xanthan gum 0.3%, magnesium aluminum silicate 2% to 3%, PVA 1.5%, and silica sol 4%. These agents each play specific roles in controlling the suspension, rheology, strength, and moisture resistance of the coating.
(4) The addition of 10% sepiolite was found to provide superior overall performance in terms of suspension, viscosity, permeability, and coating strength. Moreover, sepiolite contributes to the moisture resistance of the coating, offering a potential solution to the problem of excessive hygroscopicity frequently encountered in practical lost foam casting production.
(5) Through orthogonal experiments and range analysis, two optimized coating formulations were established. Formulation I consists of 30% quartz powder, 70% bauxite, 7.5% talc powder, 4% sodium bentonite, 0.2% xanthan gum, 2% PVA, 3% silica sol, and appropriate amounts of additives and water. Formulation II consists of 30% quartz powder, 70% bauxite, 7.5% talc powder, 3% magnesium aluminum silicate, 0.2% xanthan gum, 2% PVA, 4% silica sol, and appropriate amounts of additives and water.
(6) The performance of Formulation I was determined to be: suspension 95%, normal-temperature permeability 14.2, flow cup viscosity 13.27 s, moisture absorption 3.23%, applicability grade I, crack resistance grade I, and density 1.29 g/cm³. The performance of Formulation II was: suspension 97%, normal-temperature permeability 13.4, flow cup viscosity 13.33 s, moisture absorption 3.11%, applicability grade I, crack resistance grade I, thixotropic index 2.612, and density 1.29 g/cm³.
(7) Scanning electron microscopy analysis revealed that both optimized coatings exhibit a dense and uniform microstructure with interconnected pores that facilitate gas permeability. The well-distributed refractory particles and the dual binder system ensure the mechanical integrity of the coating layer, which is essential for the production of high-quality iron castings by the lost foam casting process.
Overall, the two coating formulations developed in this work provide practical solutions for the production of iron castings through the lost foam casting process. The coatings exhibit excellent suspension stability, appropriate viscosity, good permeability, strong adhesion, low moisture absorption, and high crack resistance, all of which are critical for achieving high-quality casting surfaces and defect-free iron castings. Future work may focus on further enhancing the high-temperature performance of the coatings and evaluating their behavior under various casting conditions in industrial production.