In the production of high-quality steel castings, lost foam casting (LFC) technology plays a crucial role due to its ability to create complex shapes with minimal post-processing. However, the performance of the coating applied to the foam pattern is critical, as it directly impacts the surface quality and integrity of the final steel castings. Unlike traditional casting coatings, LFC coatings are applied directly to the foam pattern and must withstand high temperatures, facilitate gas evacuation, and prevent defects such as penetration and slag inclusion. This study focuses on optimizing the formulation of LFC coatings specifically for steel castings, utilizing an orthogonal experimental design to balance various additive components. The goal is to enhance both working and process properties, ensuring that the coatings contribute to defect-free steel castings with excellent surface finish.
The coating composition typically includes refractory aggregates, binders, carriers, suspending agents, and other additives. For steel castings, which require high-temperature resistance and stability, the selection of materials is paramount. In this work, the refractory base consists of 80% quartz powder and 20% zircon flour, with water as the carrier. The additives under investigation are sodium bentonite as a thickener, sodium carboxymethyl cellulose (CMC) as a suspending agent, silica sol as a high-temperature binder, phenolic resin as a low-temperature binder, and 801 glue as an oil-soluble binder to replace conventional water-soluble binders like white latex. The oil-soluble nature of 801 glue aids in the rapid expulsion of liquid decomposition products from the foam pattern, reducing carbon pick-up in steel castings—a common issue in LFC. Additionally, small amounts of surfactants, defoamers, and preservatives are included to improve coating uniformity and stability.
To systematically optimize the coating formulation, a five-factor, four-level orthogonal experiment was designed. The five factors are sodium bentonite, CMC, silica sol, phenolic resin, and 801 glue, each with four levels as specified in Table 1. Other components are held constant, with 200 g of water added per batch to maintain consistency. This orthogonal design allows for efficient analysis of the main effects and interactions, enabling the identification of optimal parameter combinations for enhancing coating performance in steel castings production.
| Factor | Level 1 | Level 2 | Level 3 | Level 4 |
|---|---|---|---|---|
| Sodium Bentonite (g) | 4 | 6 | 8 | 10 |
| CMC (g) | 4 | 6 | 8 | 10 |
| Silica Sol (g) | 6 | 8 | 10 | 12 |
| Phenolic Resin (g) | 2 | 4 | 6 | 8 |
| 801 Glue (g) | 2 | 4 | 6 | 8 |
The orthogonal array comprises 16 experimental runs, as derived from the combination of factors and levels. Each run produces a coating batch, which is then evaluated for both working and process properties. The working properties are essential for the coating’s performance during the casting process, particularly for steel castings that involve high pouring temperatures and rapid metal solidification. These include permeability (both at room temperature and high temperature), surface strength, and high-temperature strength. The process properties, on the other hand, affect the coating application and handling, such as coating ability, drip resistance, suspension rate, leveling, and viscosity. By analyzing these properties, we can determine the optimal formulation that ensures high-quality steel castings with minimal defects.
For the working properties, permeability is measured using a permeability tester on sand samples coated with the experimental coatings. The room-temperature permeability ($P_{rt}$) and high-temperature permeability ($P_{ht}$) are calculated based on the flow rate of air through the coating layer. The formulas for permeability can be expressed as:
$$ P = \frac{Q \cdot L}{A \cdot \Delta p \cdot t} $$
where $P$ is the permeability (in cm³·g⁻¹·min⁻¹), $Q$ is the volume of air passed, $L$ is the coating thickness, $A$ is the cross-sectional area, $\Delta p$ is the pressure difference, and $t$ is the time. High permeability is crucial for steel castings to allow gases from foam decomposition to escape, preventing porosity and blowholes.
Surface strength is assessed using a sand impact test, where standard quartz sand (0.212 mm粒径) is dropped from a height of 500 mm onto the dried coating until a 1 mm hole is formed. The mass of sand required ($M_s$) indicates the surface strength; a higher mass denotes better resistance to erosion during mold filling. High-temperature strength is evaluated by heating coated samples to 900°C for 2 minutes and observing surface cracks. A rating system assigns values: 3 for no cracks, 2 for minor cracks, and 1 for significant cracks, which correlates with the coating’s ability to maintain integrity under the thermal stresses of steel castings production.
The results for the working properties across the 16 orthogonal runs are summarized in Table 2. These data provide a basis for analyzing the influence of each factor on the coating performance for steel castings.
| Run | Room-Temp Permeability (cm³·g⁻¹·min⁻¹) | High-Temp Permeability (cm³·g⁻¹·min⁻¹) | Surface Strength (g) | High-Temp Strength (Rating) |
|---|---|---|---|---|
| 1 | 1285 | 3216 | 407 | 1 |
| 2 | 1680 | 2821 | 745 | 2 |
| 3 | 1556 | 2215 | 1865 | 3 |
| 4 | 1048 | 3062 | 2512 | 1 |
| 5 | 955 | 1994 | 565 | 2 |
| 6 | 853 | 1452 | 803 | 2 |
| 7 | 1041 | 1917 | 1803 | 1 |
| 8 | 662 | 1802 | 2480 | 3 |
| 9 | 1893 | 2811 | 457 | 2 |
| 10 | 2146 | 3506 | 817 | 1 |
| 11 | 863 | 2498 | 2057 | 2 |
| 12 | 1105 | 3495 | 3250 | 2 |
| 13 | 1231 | 3651 | 343 | 1 |
| 14 | 859 | 3157 | 1315 | 3 |
| 15 | 967 | 3753 | 1965 | 2 |
| 16 | 1392 | 4048 | 2996 | 1 |
To quantify the impact of each factor, we calculate the range (R) for each property across the levels of the five factors. The range analysis helps identify the primary and secondary factors influencing the coating performance for steel castings. The formula for range is:
$$ R_j = \max(\bar{Y}_{j1}, \bar{Y}_{j2}, \bar{Y}_{j3}, \bar{Y}_{j4}) – \min(\bar{Y}_{j1}, \bar{Y}_{j2}, \bar{Y}_{j3}, \bar{Y}_{j4}) $$
where $R_j$ is the range for factor $j$, and $\bar{Y}_{jk}$ is the average response for factor $j$ at level $k$. The results of this analysis for working properties are presented in Table 3, highlighting the dominant factors that ensure optimal performance in steel castings.
| Factor | Room-Temp Permeability Range (cm³·g⁻¹·min⁻¹) | High-Temp Permeability Range (cm³·g⁻¹·min⁻¹) | Surface Strength Range (g) | High-Temp Strength Range (Rating) |
|---|---|---|---|---|
| Sodium Bentonite | 624.00 | 1861.00 | 272.5 | 0.25 |
| CMC | 377.75 | 506.00 | 2366.5 | 0.50 |
| Silica Sol | 268.25 | 537.75 | 262.5 | 1.75 |
| Phenolic Resin | 439.75 | 253.25 | 287.5 | 0.25 |
| 801 Glue | 570.25 | 391.50 | 195.0 | 0.50 |
From Table 3, it is evident that sodium bentonite has the most significant effect on both room-temperature and high-temperature permeability, with ranges of 624.00 and 1861.00, respectively. This underscores its role in controlling gas flow during the casting of steel castings. For surface strength, CMC exhibits the largest range (2366.5), indicating its critical influence on coating durability. High-temperature strength is primarily governed by silica sol, with a range of 1.75, emphasizing its importance as a high-temperature binder for steel castings that experience extreme thermal conditions. These insights guide the optimization process to enhance coating reliability for steel castings.
In addition to working properties, the process properties are vital for practical application in steel castings production. Coating ability is evaluated by immersing foam plastic sheets into the coating and assessing uniformity; ratings are assigned: 3 for excellent, 2 for good, and 1 for poor. Drip resistance is measured using a drip rate test, where the percentage of coating that drips off a substrate over 30 seconds is calculated. The drip rate ($\eta$) is given by:
$$ \eta = \frac{G_3}{G_3 + G} \times 100\% $$
where $G_3$ is the mass of dripped coating, and $G$ is the mass of coating adhered to the substrate. A lower $\eta$ indicates better drip resistance, which is essential for achieving uniform coating thickness on complex patterns for steel castings. Suspension rate is determined by letting the coating sit in a graduated cylinder for 24 hours and measuring the settled volume; all formulations showed 100% suspension, indicating excellent stability. Leveling is assessed by allowing coating to flow from a funnel onto a marked paper; the diameter of the spread indicates flowability, with larger diameters denoting better leveling. Viscosity is measured using a rotational viscometer, with all batches averaging around 667 Pa·s, ensuring consistent application properties for steel castings.
The process property results are summarized in Table 4, focusing on coating ability, drip rate, and leveling, as suspension rate and viscosity were constant across trials. This data helps in fine-tuning the formulation for ease of use in steel castings foundries.
| Run | Coating Ability (Rating) | Drip Rate (%) | Leveling (mm) |
|---|---|---|---|
| 1 | 1 | 55.95 | 100.53 |
| 2 | 3 | 47.18 | 92.15 |
| 3 | 2 | 37.81 | 87.03 |
| 4 | 1 | 31.39 | 84.26 |
| 5 | 2 | 46.28 | 94.03 |
| 6 | 2 | 58.00 | 103.88 |
| 7 | 1 | 53.06 | 99.38 |
| 8 | 1 | 41.01 | 103.13 |
| 9 | 3 | 21.48 | 89.16 |
| 10 | 3 | 15.74 | 88.19 |
| 11 | 2 | 45.99 | 88.44 |
| 12 | 1 | 35.94 | 80.81 |
| 13 | 3 | 39.11 | 92.13 |
| 14 | 3 | 60.23 | 89.56 |
| 15 | 2 | 49.36 | 82.06 |
| 16 | 2 | 27.49 | 84.19 |
The range analysis for process properties, shown in Table 5, reveals the key factors affecting coating application for steel castings. Coating ability is most influenced by CMC, with a range of 1.5, highlighting its role in ensuring uniform coverage. Drip rate is predominantly controlled by 801 glue, with a range of 8.67%, indicating its effectiveness in reducing dripping and improving coating efficiency for steel castings. Leveling is mainly affected by sodium bentonite, with a range of 13.46 mm, suggesting its impact on flow behavior during application. These findings are integrated with the working property analysis to derive an optimal formulation.
| Factor | Coating Ability Range (Rating) | Drip Rate Range (%) | Leveling Range (mm) |
|---|---|---|---|
| Sodium Bentonite | 1.0 | 19.8 | 13.46 |
| CMC | 1.5 | 14.77 | 5.87 |
| Silica Sol | 0.75 | 12.03 | 7.00 |
| Phenolic Resin | 0.75 | 19.47 | 5.60 |
| 801 Glue | 0.5 | 8.67 | 4.41 |
Synthesizing the results from both working and process properties, the optimal combination of factors for steel castings is determined. For sodium bentonite, level 4 (10 g) is selected to maximize permeability and leveling. For CMC, level 3 (8 g) is chosen to balance surface strength and coating ability. Silica sol at level 3 (10 g) provides the best high-temperature strength. Phenolic resin at level 3 (6 g) offers a good compromise between permeability and drip resistance. 801 glue at level 4 (8 g) minimizes drip rate and enhances gas evacuation. Thus, the optimized formulation per 200 g of water is: 10 g sodium bentonite, 8 g CMC, 10 g silica sol, 6 g phenolic resin, and 8 g 801 glue. This combination aims to deliver superior performance for steel castings production.
To validate the optimized coating, a new batch was prepared and tested. The measured working properties are: room-temperature permeability of 1106 cm³·g⁻¹·min⁻¹, high-temperature permeability of 3364 cm³·g⁻¹·min⁻¹, surface strength of 3057 g, and high-temperature strength rated as excellent. The process properties include: coating ability rated excellent, drip rate of 25.19%, suspension rate of 100%, leveling of 95.32 mm, and viscosity of 668 Pa·s. These values confirm that the optimization successfully enhanced both sets of properties, making the coating ideal for steel castings applications.
The optimized coating was then applied in a production setting for manufacturing high-manganese steel liner plates, a common type of steel castings. The coating demonstrated excellent wettability and adhesion to foam patterns, forming a uniform layer that dried with sufficient flexibility and room-temperature strength. During casting, the coating facilitated the escape of decomposition gases, preventing defects such as porosity and slag inclusion. The resulting steel castings exhibited smooth surfaces and high dimensional accuracy, meeting the stringent quality requirements for industrial steel castings. The success of this trial underscores the importance of tailored coating formulations in achieving defect-free steel castings through lost foam casting.
In conclusion, this study utilized an orthogonal experimental design to optimize the formulation of lost foam casting coatings specifically for steel castings. By analyzing five key additives—sodium bentonite, CMC, silica sol, phenolic resin, and 801 glue—across four levels, we identified the primary factors influencing both working and process properties. The optimal combination was determined to be: 10 g sodium bentonite, 8 g CMC, 10 g silica sol, 6 g phenolic resin, and 8 g 801 glue per 200 g of water. This formulation ensures high permeability, strength, and application performance, leading to improved quality in steel castings. The practical validation in producing high-manganese steel components confirmed that the optimized coating eliminates common defects like pores and slag, resulting in superior surface finish and integrity for steel castings. This research provides a systematic approach for coating optimization in lost foam casting, with direct benefits for the manufacturing of high-performance steel castings in various industrial applications.