In the present work, I systematically studied the influence of coating wettability to liquid polystyrene (the decomposition product of foamed plastic patterns) on the formation of carbon-related sand foundry defects in lost foam casting. The research was motivated by the persistent industrial problems of surface carbon deposition, wrinkling defects in iron castings, and carburization defects in steel castings. I proposed that the key lies not only in the permeability of the coating but also in its wettability to the liquid intermediate products of the polystyrene pattern. Through theoretical analysis, orthogonal experiments, fluidity tests, practical pouring trials, and scanning electron microscopy (SEM) observations, I demonstrated that a coating with high wettability to liquid polystyrene can significantly reduce sand foundry defects such as surface carbon, cold laps, and internal carburization. This article summarizes my experimental methodology, results, and the underlying mechanisms.
1. Introduction and Background
Lost foam casting (LFC), also known as evaporative pattern casting, is a near-net-shape casting process that uses a polystyrene foam pattern embedded in dry sand. When molten metal is poured, the foam pattern decomposes and the metal fills the cavity. The coating applied on the foam pattern is critical: it must provide strength, thermal insulation, and controlled gas permeability. However, many foundries still suffer from defective castings due to carbon-related sand foundry defects. The defects originate from the incomplete removal of polystyrene decomposition products. When the foam pattern is heated, it first softens, then melts into a liquid state, and eventually vaporizes. If the liquid decomposition products cannot escape from the mold cavity quickly, they remain inside and decompose further into solid carbon black and hydrogen, leading to defects.
Traditional research emphasized coating permeability as the dominant factor. Nevertheless, I found that the wettability of the coating to the liquid polystyrene is equally important. A high wettability allows the liquid phase to penetrate the porous coating and be removed from the cavity under the vacuum negative pressure, thus preventing the liquid from pyrolyzing inside the mold. In contrast, if the wettability is poor, the liquid remains on the coating surface, eventually turning into carbonaceous residue and causing severe sand foundry defects.
2. Experimental Design
2.1 Materials and Coating Formulation
I selected magnesia olivine powder and high-alumina bauxite powder as refractory fillers. The organic binders were polyvinyl alcohol (PVA), sodium carboxymethyl cellulose (CMC), and a commercial organic binder “Guilin No.5” (a plant-based composite additive). Inorganic binders included lithium bentonite, kaolin, and silica sol. The carrier was water, and a nonionic surfactant (washing powder) was used to improve the coating application.
To investigate the influence of six factors at five levels, I designed a fractional orthogonal array. The six factors were:
- Refractory aggregate particle size
- PVA content
- CMC content
- Lithium bentonite content
- Kaolin content
- Guilin No.5 content
The levels are summarized in Table 1.
| Level | Aggregate size (mesh) | PVA (wt.%) | CMC (wt.%) | Li-bentonite (wt.%) | Kaolin (wt.%) | Guilin No.5 (wt.%) |
|---|---|---|---|---|---|---|
| 1 | 200 | 0.5 | 0.1 | 0.5 | 0 | 0.5 |
| 2 | 175 | 1.0 | 0.2 | 1.0 | 0.5 | 1.0 |
| 3 | 150 | 1.5 | 0.3 | 1.5 | 1.0 | 1.5 |
| 4 | 125 | 2.0 | 0.4 | 2.0 | 1.5 | 2.0 |
| 5 | 100 | 2.5 | 0.5 | 2.5 | 2.0 | 2.5 |
All percentages are based on the total mass of refractory aggregate. I prepared 25 coating compositions according to the orthogonal table. Each coating was tested for permeability, wettability (contact angle), and fluidity of molten aluminum in a spiral test.
2.2 Test Methods
Wettability measurement: I made spherical polystyrene balls of identical size and placed them on dried coating discs. The assembly was heated in a well-type resistance furnace at 300 °C for one minute. After cooling, the contact angle between the solidified polystyrene drop and the coating surface was measured from digital photographs. A smaller contact angle indicates better wettability.
Permeability test: I designed a special holder consisting of a tube with a water-glass-sand backing layer at one end and a coating layer on top. The permeability was measured with a standard foundry sand permeability meter. The measured pressure drop was used as an inverse indicator of permeability: a higher pressure drop corresponds to lower permeability.
Fluidity test: Spiral-shaped polystyrene patterns with a cross-section of 10 mm × 10 mm were used. Pure aluminum was melted and superheated to 750 °C, then poured under a vacuum of −0.04 MPa. The resulting spiral length and gas gap length were recorded.
2.3 Practical Casting Trials
To verify the laboratory results, I conducted comparative pouring tests under actual production conditions for three alloys: high-chromium wear-resistant cast iron, heat-resistant steel (ZG40Cr25Ni20), and ductile iron. For each alloy, I used both a foundry’s original coating and my newly developed coating. The castings were inspected for surface carbon, wrinkles, internal shrinkage, and carburization. Carbon content was analyzed at different locations.
3. Results and Discussion
3.1 Orthogonal Test Results
The 25 coating formulations were evaluated for permeability (expressed as the pressure drop ΔP from the permeability meter), wettability (contact angle θ), and fluidity (spiral length L). The results were processed using range analysis and ANOVA. Table 2 shows a representative subset of the results.
| Coating No. | Permeability ΔP (Pa) | Contact angle θ (°) | Spiral length (mm) | Gas gap length (mm) |
|---|---|---|---|---|
| 1 | 180 | 105 | 560 | 45 |
| 2 | 150 | 92 | 610 | 38 |
| 3 | 120 | 80 | 680 | 30 |
| 4 | 135 | 88 | 635 | 34 |
| 5 | 95 | 65 | 760 | 22 |
| 6 | 110 | 70 | 720 | 26 |
| 7 | 200 | 115 | 500 | 55 |
| 8 | 170 | 98 | 580 | 42 |
| 9 | 130 | 75 | 700 | 28 |
| 10 | 145 | 85 | 650 | 32 |
| … | … | … | … | … |
Note: Lower ΔP means higher permeability; lower θ means better wettability.
3.2 Range Analysis
For each factor, I calculated the sum of results at each level. The range (R) indicates the influence strength. The trends are shown by plotting the level averages. The range analysis for permeability is presented in Table 3.
| Factor | R (permeability) | Optimal level |
|---|---|---|
| Aggregate size | 85 | 100 mesh (coarser) |
| PVA content | 42 | 1.0–1.5% |
| CMC content | 38 | 0.3% |
| Li-bentonite | 30 | 1.0% |
| Kaolin | 25 | 0.5% |
| Guilin No.5 | 48 | 1.5% |
For wettability, the range analysis is given in Table 4.
| Factor | R (contact angle) | Optimal level |
|---|---|---|
| Aggregate size | 40 | 100 mesh |
| PVA content | 30 | 1.0% |
| CMC content | 35 | 0.2% |
| Li-bentonite | 28 | 1.0% |
| Kaolin | 18 | 0% |
| Guilin No.5 | 45 | 1.5% |
3.3 ANOVA and Error Analysis
To quantitatively evaluate the significance of each factor, I performed analysis of variance. The results are summarized in Table 5 for the three indicators.
| Indicator | Significant factors (in order) | F-values |
|---|---|---|
| Permeability | Aggregate size, Guilin No.5, PVA | F0.05 = 2.9, Fcalc > 2.9 |
| Wettability | Guilin No.5, Aggregate size, CMC | F0.05 = 2.9, Fcalc > 2.9 |
| Fluidity | Aggregate size, Guilin No.5, PVA | F0.05 = 2.9, Fcalc > 2.9 |
The error estimate for each indicator was calculated as:
$$s_e = \sqrt{MS_{error}}$$
The coefficient of variation was:
$$C_v = \frac{s_e}{\bar{y}} \times 100\%$$
For permeability, I obtained \(C_v \approx 8.2\%\), indicating an “excellent” result. For wettability, \(C_v \approx 12.5\%\), considered “fair”. For fluidity, \(C_v \approx 6.8\%\), also “excellent”.
3.4 Effect of Coating Wettability on Carburization of Steel Castings
I conducted carburization tests using four coatings with different wettability and permeability. Rectangular steel specimens were cast under identical conditions. The carbon content was measured at three depths (1 mm, 3 mm, 5 mm) below the surface, for both top-pouring and bottom-pouring. Table 6 shows the results for two coatings: one with poor wettability (θ = 110°) and one with good wettability (θ = 62°).
| Coating | Pouring method | Location | Carbon content (%) | Increase in carbon (%) |
|---|---|---|---|---|
| Poor wettability (θ=110°) | Bottom | Bottom | 0.45 | +0.20 |
| Middle | 0.40 | +0.15 | ||
| Top | 0.38 | +0.13 | ||
| Top | Bottom | 0.48 | +0.23 | |
| Middle | 0.42 | +0.17 | ||
| Top | 0.35 | +0.10 | ||
| Good wettability (θ=62°) | Bottom | Bottom | 0.28 | +0.03 |
| Middle | 0.26 | +0.01 | ||
| Top | 0.25 | 0.00 | ||
| Top | Bottom | 0.30 | +0.05 | |
| Middle | 0.27 | +0.02 | ||
| Top | 0.24 | −0.01 |
These results clearly indicate that the coating with high wettability drastically reduced the carburization of the steel castings. The carbon content increase dropped from around +0.20% to nearly zero. Therefore, wettability plays a decisive role in controlling sand foundry defect of carburization.
3.5 Practical Casting Trials
High-chromium wear-resistant cast iron: When using the foundry’s original coating, the casting surface was covered with black carbon and the interior had a shrinkage crack of about 2 mm width, causing rejection. With my coating, the surface was clean, and no shrinkage defects were found. The carbon content remained within the acceptable range.
Heat-resistant steel ZG40Cr25Ni20: The original coating produced a rough surface with carbon black at the interface and even caused mold collapse due to excessive gas pressure. My coating produced a smooth casting without any visible carbon residue. Carburization was minimal.
Ductile iron: The original coating led to heavy surface carbon (lustrous carbon films) and internal slag inclusions, along with cold shuts. My coating eliminated these defects entirely, and the casting quality was excellent.
Table 7 summarizes the results of the practical trials.
| Alloy | Coating | Wettability (θ, °) | Permeability (ΔP, Pa) | Casting quality |
|---|---|---|---|---|
| High-Cr cast iron | Original | 108 | 160 | Surface carbon, shrinkage crack |
| Developed | 58 | 70 | No defects | |
| Heat-resistant steel | Original | 115 | 90 | Carbon, rough surface, mold collapse |
| Developed | 60 | 65 | Smooth, no carburization | |
| Ductile iron | Original | 105 | 120 | Lustrous carbon, cold shuts, slag |
| Developed | 55 | 70 | Excellent surface and internal quality |
4. Mechanism of Carbon Defect Formation and the Role of Wettability
The formation of carbon defects in lost foam casting can be traced to the decomposition behavior of polystyrene. Polystyrene has the formula \([C_8H_8]_n\), with a carbon-to-hydrogen atomic ratio of 1:1. Upon heating, it undergoes two major decomposition pathways that generate carbon black:
Pathway 1: At lower temperatures, polystyrene releases small-molecule gases such as CO, CO₂, and light hydrocarbons. These gases have a higher H/C ratio than the original polymer, meaning that hydrogen is preferentially removed, leaving behind carbon-enriched residues.
Pathway 2: At temperatures above 800 °C, the aromatic rings and C–C bonds in styrene and lighter intermediates crack deeply, producing solid carbon (soot) and hydrogen gas.
If the liquid intermediate can be transported through the coating before reaching high temperatures, the generation of solid carbon can be greatly reduced. Thus, the ability of the coating to wet the liquid polystyrene is crucial. This can be understood through capillary wetting theory:
$$\Delta P = \frac{2\gamma_{LV} \cos\theta}{r}$$
where \(\gamma_{LV}\) is the surface tension of the liquid, \(\theta\) is the contact angle, and \(r\) is the effective capillary radius of the porous coating. If \(\theta < 90^\circ\), the capillary pressure is positive and drives the liquid into the pores. The smaller the contact angle, the stronger the driving force. Therefore, a coating with a small contact angle promptly absorbs the liquid decomposition products and transfers them under vacuum to the sand, preventing the liquid from accumulating inside the cavity.
4.1 Physical Model of Liquid Polystyrene Escaping Through the Coating
I established a physical model as shown in the concepts discussed in my work. The molten metal front heats the foam pattern, creating a layer of liquid polystyrene ahead of the gas gap. Partial liquid vaporizes immediately, while the rest contacts the inner surface of the coating. If the coating is well-wetted, the liquid is drawn into the capillary network by surface tension and the externally applied vacuum. Inside the coating, the liquid may still decompose, but its solid residue remains trapped in the coating or is exhausted as gas through the open pores. The metal front then advances with minimal gas pressure buildup.
In contrast, a poorly wetted coating causes the liquid to bead up on the surface. This liquid layer blocks gas permeation, increases the gas pressure in the gap, and eventually pyrolyzes to carbon black. The carbon deposits on the casting surface or diffuses into the metal, resulting in surface carbon, cold laps, and carburization.
4.2 Effect of Coating Components on Wettability
Refractory aggregate: Larger particles increase the pore size and porosity, providing larger capillary channels and enhancing the penetration of the liquid. However, too coarse an aggregate deteriorates suspension and application properties. A compromise is necessary.
PVA: PVA improves the green strength and forms a film on the pattern. During pouring, it burns out, increasing porosity. But because PVA has a low surface energy, an excessive amount reduces the overall surface energy of the coating, making wetting by the liquid polymer more difficult. This explains the observed peak in wettability at intermediate PVA content.
CMC: CMC is an anionic polyelectrolyte. It strongly reduces the surface tension of water, which may hinder wetting of the liquid polymer. However, in combination with bentonite, CMC forms a strong three-dimensional network that keeps the particles dispersed and creates a uniform porous structure, thereby facilitating liquid penetration. The final effect depends on the balance between these two mechanisms.
Lithium bentonite: Bentonite swells in water and shrinks when heated, increasing pore volume. But its layered structure can orient with nonpolar groups outward, creating a low-energy surface that is difficult to wet by the nonpolar polystyrene liquid. Thus, excessive bentonite reduces wettability.
Guilin No.5: This plant-based composite additive is particularly effective. It decomposes at high temperatures, leaving a carbonaceous residue that has a high affinity for liquid polystyrene. It also creates many micro-pores due to the volatilization of organic matter. It significantly enhances the wettability and permeability simultaneously.
4.3 Relationship Between Wettability and Permeability
I observed that in most cases, coatings with better wettability also showed higher permeability. This is because both properties depend on the pore structure and surface chemistry. However, high permeability does not guarantee high wettability. Some coatings with coarse particles and high porosity had large contact angles because their surface energy was lowered by hydrophilic binders or because the pore walls were covered by low-energy films. Therefore, wettability must be considered independently.
Figure 1 shows a typical image of an automatic pouring line used in my industrial validation trials.
4.4 SEM Analysis of Coating Microstructure
Scanning electron microscopy of the coating cross-sections before and after pouring revealed important differences. Before pouring, the coatings were relatively dense with a lamellar structure. After pouring, low-melting-point components had volatilized, leaving numerous pores. In coatings with good wettability, I observed glassy regions inside the pores, indicating that the liquid polystyrene had penetrated deeply into the coating before solidifying or decomposing. In coatings with poor wettability, the pores remained blocked, and carbon black accumulated at the coating–metal interface.
These SEM observations directly support the wettability theory: a well-wetting coating allows the liquid decomposition products to be absorbed and removed, thereby eliminating the source of carbon defects.
5. Mathematical Description of the Wetting and Permeation Process
To quantify the infiltration of liquid polystyrene into a porous coating, I used the Washburn equation modified for a porous medium:
$$\frac{dl}{dt} = \frac{r^2}{8\mu l} \left( \Delta P_{vac} + \frac{2\gamma_{LV}\cos\theta}{r} \right)$$
where \(l\) is the penetration depth at time \(t\), \(r\) is the average capillary radius, \(\mu\) is the dynamic viscosity of the liquid polystyrene, \(\Delta P_{vac}\) is the vacuum pressure, \(\gamma_{LV}\) is the surface tension of the liquid, and \(\theta\) is the contact angle. This equation demonstrates that the penetration velocity increases with decreasing \(\theta\), increasing effective pore radius \(r\), and higher vacuum.
The equilibrium condition for complete removal of liquid before decomposition is:
$$t_{penetration} < t_{decomposition}$$
where \(t_{decomposition}\) depends on the heating rate and temperature history. By tailoring the coating to have a low \(\theta\), the penetration time is shortened, and more liquid escapes the cavity.
Additionally, the total amount of carbon residue \(m_C\) formed inside the cavity is proportional to the fraction of liquid that remains:
$$m_C \propto (1 – f_{wetted}) \cdot m_{pattern}$$
where \(f_{wetted}\) is the fraction of liquid pattern material that successfully penetrates the coating. This simple model explains why coatings with high wettability dramatically reduce sand foundry defects.
6. Optimum Coating Composition
Based on the orthogonal experiments and verification trials, I determined the following optimum coating formulation (percentages based on the total refractory aggregate mass):
| Component | Content (wt.%) |
|---|---|
| Magnesia olivine powder (100–150 mesh) | 70 |
| High-alumina bauxite powder (150 mesh) | 30 |
| PVA | 1.0–1.5 |
| CMC | 0.2–0.3 |
| Lithium bentonite | 1.0 |
| Kaolin | 0.5 |
| Guilin No.5 | 1.5 |
| Silica sol | 2.0 (added after mixing) |
| Surfactant | 0.1 |
This coating achieved a contact angle below 60°, a pressure drop below 80 Pa (high permeability), sufficient strength, and excellent suspension. It proved effective for both iron and steel castings in actual production.
7. Conclusions
From my research, I draw the following conclusions:
- Carbon defects in lost foam castings originate from the incomplete removal of liquid decomposition products. There are two major pathways for carbon black formation: preferential hydrogen release during early pyrolysis, and deep cracking of hydrocarbon fragments at temperatures above 800 °C.
- The wettability of the coating to liquid polystyrene is a critical parameter controlling sand foundry defects. A smaller contact angle allows the liquid to quickly penetrate the coating and be evacuated under vacuum, thus preventing carbon formation inside the mold cavity.
- There is a close correlation between wettability and permeability, but they are not interchangeable. Some coatings with high permeability still produce defects due to poor wettability. Therefore, both properties must be optimized.
- Increasing the refractory aggregate particle size improves both wettability and permeability. Guilin No.5 and moderate amounts of PVA and CMC are beneficial. Excessive bentonite or CMC can degrade wettability due to the formation of low-energy surfaces.
- Practical casting trials on high-chromium cast iron, heat-resistant steel, and ductile iron confirmed that the developed coating effectively eliminates surface carbon, lustrous carbon, cold shuts, shrinkage defects, and carburization.
- The optimum coating composition is: magnesia olivine/bauxite aggregate (100–150 mesh), 1.0–1.5% PVA, 0.2–0.3% CMC, 1.0% lithium bentonite, 0.5% kaolin, 1.5% Guilin No.5, 2.0% silica sol, and 0.1% surfactant. This formulation yields a contact angle below 60°, high permeability, and excellent casting quality.
In summary, the wettability of the coating to liquid polystyrene is a key parameter that has been largely overlooked in previous studies. By emphasizing this property, I have developed a coating that greatly reduces sand foundry defects in lost foam casting, providing a practical solution for foundries that suffer from carbon related quality problems.