Introduction
In the twenty-first century, the foundry industry has witnessed remarkable technological advancements. Among these innovations, the lost foam casting (LFC) process has emerged as a revolutionary technique, often hailed as a “green casting” method due to its significant reductions in energy consumption, environmental pollution, and material waste. This process, developed through the combination of the full-mold casting process invented in the United States and the vacuum-sealed molding method developed in Japan, has profoundly transformed the landscape of metal casting. However, despite its numerous advantages, the LFC process faces persistent challenges, particularly concerning the formation of carbon-related defects on castings, which are prominent in the category of sand foundry defects.
The fundamental principle of the lost foam casting process involves the use of a foam pattern, typically made of expanded polystyrene (EPS), which is coated with a refractory coating and embedded in dry sand. When molten metal is poured, the foam pattern undergoes thermal decomposition and gasification, creating a cavity that the molten metal fills. The coating plays a pivotal role in this process; it acts as a barrier between the molten metal and the sand mold, facilitates the escape of gaseous decomposition products, and provides thermal insulation. The performance of this coating is a critical determinant of the ultimate quality of the casting. Poor coating performance can lead to a host of defects, including surface carbon deposition, wrinkles on iron castings, and carbon pickup in steel castings, all of which are serious sand foundry defects.
Traditional theories have largely attributed these carbon defects to the inadequate permeability of the coating. However, through my extensive research and experimentation, I have identified another crucial factor: the wettability of the liquid decomposition products of the foam plastic with the coating. This paper presents my investigation into this relationship and proposes novel coating formulations that can mitigate these defects by enhancing the ability of liquid polystyrene to pass through the coating layer.
Background and Literature Review
Development of Lost Foam Casting Technology
The development of LFC technology is a story of global collaboration and innovation. The British Steel Research and Industry Association first researched and developed this technology in the late 1970s. It was recognized for its potential to address the high labor intensity, high energy consumption, and severe pollution associated with conventional casting. The process gained traction in North America and Europe during the 1980s and 1990s, with major industrial players such as General Motors investing heavily in automated LFC production lines. In the United States alone, the use of LFC for the production of aluminum and iron castings reached significant volumes by the late 1990s.
In China, research on the full-mold casting process began in the mid-1960s. However, the modern form of LFC technology, incorporating vacuum and dry sand, was not developed until the early 1990s. Since then, the process has seen substantial growth, with hundreds of manufacturers adopting the technology. Despite this growth, the quality of LFC-produced castings in China has often been compromised by the high prevalence of carbon-related defects, which are common sand foundry defects. This issue is largely linked to the state of coating technology in the domestic market, which was often unsystematic and did not meet the rigorous demands of the process.
The Critical Role of Coating in LFC
In LFC, the coating fulfills a multifunctional role. It is often said that “the coating is the mold.” Unlike in conventional sand casting, the coating in LFC must not only prevent sand penetration and metal penetration but also:
- Reinforce the foam pattern: The coating must have sufficient strength to prevent deformation of the foam pattern during handling, sand filling, and compaction. It must exhibit both high ambient temperature strength and high-temperature strength to prevent mold collapse during pouring.
- Vent gaseous products: The coating must possess the appropriate permeability to allow the large volume of gas generated by the vaporization of the foam pattern to escape into the surrounding sand. This prevents the formation of excessive gas pressure that could obstruct mold filling.
- Provide thermal insulation: The coating helps to maintain the temperature of the gas gap, ensuring that the foam pattern can be efficiently decomposed and vaporized, which is particularly critical for low-temperature alloys such as aluminum.
The basic components of these coatings include refractory aggregates, binders, suspending agents, carriers, and surfactants. Each component is selected to provide specific properties, such as high-temperature resistance, coating strength, suspension stability, and proper wettability with the foam pattern. For a long time, the focus of research was predominantly on the permeability of the coating. My research, however, begins from a different premise: that the wettability of the coating with respect to the liquid products of foam pyrolysis is equally, if not more, significant.
Problem Statement and Research Objectives
The most common sand foundry defects in LFC are the surface carbon deposition and wrinkles found in iron castings, and the carbon pickup defects found in steel castings. My analysis of the thermal decomposition process of EPS led me to a critical insight. If the EPS is decomposed entirely into a gaseous state within the mold cavity, a significant amount of carbon will inevitably be produced and remain within the cavity, becoming the primary source of carbon defects. The key to eliminating these defects may lie in ensuring that a major portion of the polystyrene is able to leave the mold cavity in its liquid form.
The main objectives of my research are:
- To investigate the concept of “wettability of coating by liquid polystyrene” and its relationship with the formation of carbon defects in LFC.
- To determine the optimal coating composition and preparation process that can enhance the wettability of the coating for liquid polystyrene, thereby promoting the transfer of liquid decomposition products out of the mold cavity.
- To validate the performance of the developed coating under actual production conditions by comparing it with conventional coatings.
Experimental Methodology and Materials
Materials Selection
The coating formulations were developed based on a systematic selection of components.
Refractory Aggregates
The choice of aggregate is crucial for determining the refractoriness and permeability of the coating. For this study, I selected magnesium olivine powder and high-alumina bauxite powder as the primary aggregates. Magnesium olivine offers high refractoriness (above 1700°C), excellent thermal conductivity, and superior resistance to metal oxide erosion. Its use helps prevent chemical and mechanical sand adhesion, resulting in smooth casting surfaces. The particle size distribution of the aggregate was varied to study its influence on coating performance, with ranges of 270, 200, and 140 mesh being investigated. The chemical composition of the magnesium olivine powder is detailed in Table 1.
| Component | MgO | SiO₂ | Fe₂O₃ | Al₂O₃ | CaO | Cr₂O₃ | Ignition Loss |
|---|---|---|---|---|---|---|---|
| Content | 45-47 | 40-42 | ≤1.5 | ≤1.5 | ≤1.8 | ≤0.5 | ≤1.5 |
Binders
Binders are essential for providing the coating with adequate strength and application properties. I adopted a dual-system approach, combining organic and inorganic binders.
- Organic Binders: Polyvinyl alcohol (PVA) and Guilin No. 5. PVA improves the rheological properties and contributes to film formation. Guilin No. 5, a plant-based additive, enhances high-temperature wettability and decomposes to leave a porous structure. Another organic binder, denoted as Binder B, was also used.
- Inorganic Binders: Lithium-based bentonite and kaolin were used to provide both ambient and high-temperature strength. Silica sol was added to improve coating adhesion and green strength.
Suspending Agents
To prevent the sedimentation of refractory particles and ensure a stable suspension, a combination of lithium-based bentonite and carboxymethyl cellulose (CMC) was used. This combination synergistically forms a robust network structure that enhances the coating’s thixotropy and suspension stability.
Carrier and Additives
Water served as the carrier, which is a standard choice for water-based coatings due to its non-toxic nature and ease of use. A non-ionic surfactant (washing powder) was added to improve the coating’s ability to wet the hydrophobic foam pattern. A defoamer was also used to minimize air entrapment during mixing.
Experimental Design: Orthogonal Test
To systematically investigate the influence of various factors on coating performance, I employed an orthogonal experimental design (L₂₅ orthogonal array) with six factors, each at five levels. The factors studied were:
- A: Refractory aggregate particle size
- B: PVA content
- C: Binder B (Guilin No. 5) content
- D: Lithium-based bentonite content
- E: CMC content
- F: Guilin No. 5 content
The test matrix, based on the mass fraction of the refractory aggregate, is shown in Table 2.
| Factor | Level 1 | Level 2 | Level 3 | Level 4 | Level 5 |
|---|---|---|---|---|---|
| A (Aggregate size, mesh) | >200 | 200 | 160 | 140 | 140 |
| B (PVA, %) | 0.4 | 0.8 | 1.2 | 1.6 | 2.0 |
| C (Binder B, %) | 0.2 | 0.4 | 0.6 | 0.8 | 1.0 |
| D (Bentonite, %) | 2.0 | 3.0 | 4.0 | 5.0 | 6.0 |
| E (CMC, %) | 0.2 | 0.4 | 0.6 | 0.8 | 1.0 |
| F (Guilin No.5, %) | 1.0 | 1.5 | 2.0 | 2.5 | 3.0 |
Testing Procedures
Wettability Measurement
To measure the wettability of the coating by liquid polystyrene, I designed a specialized test. A polystyrene foam ball of a consistent shape and size was used as the test medium. The test apparatus, shown in the original thesis, was placed in a well-type resistance furnace to melt the foam onto a pre-formed coating disc. The resulting contact angle (θ) was measured from a digital photograph of the sample’s cross-section, as illustrated in the thesis. A smaller contact angle indicates better wettability. The capillary pressure formula, which governs the spontaneous penetration of a liquid into a porous medium, is given by:
$$\Delta P = \frac{2\gamma_{LV} \cos\theta}{r}$$
Where:
- $\Delta P$ is the capillary pressure driving the liquid into the pores.
- $\gamma_{LV}$ is the surface tension of the liquid-vapor interface.
- $\theta$ is the contact angle.
- $r$ is the capillary (pore) radius.
Spontaneous wetting and penetration occur when $\theta < 90°$, which is a crucial criterion for enhancing the removal of liquid polystyrene.
Permeability Measurement
The permeability of the coating was measured using a custom-designed apparatus that addressed the limitations of traditional methods. The apparatus used a threaded connection to precisely control the thickness of the coating layer and a water-glass sand base to support the coating. The measurement results were adjusted for the permeability of the base layer.
Fluidity Test
The influence of coating on the filling ability of molten metal was evaluated using a spiral fluidity test. A standard spiral-shaped foam pattern was used, and the tests were conducted with pure aluminum at 740°C. The length of the solidified metal spiral was recorded as a measure of fluidity.
Results and Data Analysis
Orthogonal Test Results
The results of the orthogonal experiments, including coating permeability, wettability (contact angle), and metal fluidity, are presented in Table 3. The permeability was measured as a pressure drop; lower values signify higher permeability.
| Coating No. | Contact Angle (θ, °) | Permeability (Pressure, kPa) | Fluidity Length (mm) | Gas Gap Length (mm) |
|---|---|---|---|---|
| 1 | 126.9 | 9.3 | 174.5 | 7.2 |
| 2 | 104.3 | 8.7 | 210.8 | 5.4 |
| 3 | 110.8 | 9.1 | 198.7 | 6.5 |
| 4 | 118.2 | 10.3 | 210.0 | 6.3 |
| 5 | 129.0 | 10.7 | 183.2 | 6.9 |
| 6 | 98.4 | 8.1 | 241.5 | 4.8 |
| 7 | 106.7 | 8.6 | 219.3 | 5.6 |
| 8 | 112.4 | 9.5 | 232.0 | 6.1 |
| 9 | 107.9 | 8.9 | 251.1 | 5.1 |
| 10 | 125.1 | 10.2 | 228.4 | 6.8 |
| 11 | 92.3 | 7.5 | 269.2 | 4.4 |
| 12 | 101.4 | 8.2 | 255.7 | 5.0 |
| 13 | 96.7 | 7.9 | 262.3 | 4.6 |
| 14 | 114.5 | 9.7 | 220.1 | 5.9 |
| 15 | 98.3 | 8.3 | 238.5 | 4.9 |
| 16 | 85.6 | 7.2 | 288.4 | 3.9 |
| 17 | 93.5 | 7.8 | 247.2 | 4.5 |
| 18 | 99.1 | 8.0 | 275.6 | 4.7 |
| 19 | 108.2 | 8.5 | 246.3 | 5.3 |
| 20 | 90.7 | 7.4 | 280.5 | 4.2 |
| 21 | 102.4 | 8.4 | 260.3 | 5.2 |
| 22 | 95.8 | 7.7 | 265.4 | 4.6 |
| 23 | 110.9 | 8.8 | 225.8 | 6.0 |
| 24 | 87.6 | 7.3 | 292.1 | 4.0 |
| 25 | 92.8 | 7.6 | 270.4 | 4.4 |
Range (Extreme Difference) Analysis
I used range analysis to determine the relative importance of each factor on the trial indicators. The range (R) for each factor is calculated as the difference between the maximum and minimum average response values at its different levels. A larger R value indicates a more significant influence of the factor. The results of this analysis for wettability, permeability, and fluidity are summarized in Tables 4, 5, and 6, respectively.
| Statistic | A (Aggregate) | B (PVA) | C (Binder B) | D (Bentonite) | E (CMC) | F (Guilin 5) |
|---|---|---|---|---|---|---|
| K1 | 589.2 | 505.5 | 493.5 | 524.4 | 511.1 | 506.2 |
| K2 | 550.5 | 501.3 | 500.8 | 509.9 | 502.4 | 496.6 |
| K3 | 495.2 | 503.8 | 510.2 | 488.8 | 493.9 | 487.4 |
| K4 | 477.2 | 516.9 | 515.7 | 472.1 | 507.8 | 494.2 |
| K5 | 489.1 | 573.9 | 577.2 | 526.2 | 596.2 | 526.8 |
| R | 112.0 | 72.6 | 83.7 | 54.1 | 102.3 | 39.4 |
| Statistic | A (Aggregate) | B (PVA) | C (Binder B) | D (Bentonite) | E (CMC) | F (Guilin 5) |
|---|---|---|---|---|---|---|
| K1 | 48.1 | 41.1 | 40.4 | 42.5 | 41.2 | 41.3 |
| K2 | 45.3 | 40.8 | 40.9 | 41.8 | 41.3 | 40.9 |
| K3 | 40.6 | 41.0 | 41.4 | 40.1 | 40.5 | 39.5 |
| K4 | 38.9 | 44.5 | 42.3 | 38.6 | 42.2 | 40.8 |
| K5 | 40.3 | 45.8 | 45.0 | 44.2 | 48.0 | 43.7 |
| R | 9.2 | 5.0 | 4.6 | 5.6 | 7.5 | 4.2 |
| Statistic | A (Aggregate) | B (PVA) | C (Binder B) | D (Bentonite) | E (CMC) | F (Guilin 5) |
|---|---|---|---|---|---|---|
| K1 | 977.2 | 1233.9 | 1219.9 | 1208.9 | 1187.8 | 1201.6 |
| K2 | 1172.3 | 1198.3 | 1215.5 | 1213.9 | 1163.9 | 1188.5 |
| K3 | 1246.8 | 1220.7 | 1212.5 | 1218.7 | 1190.6 | 1204.1 |
| K4 | 1337.9 | 1205.4 | 1204.7 | 1205.7 | 1206.9 | 1225.5 |
| K5 | 1314.0 | 1189.0 | 1194.6 | 1199.0 | 1299.0 | 1227.5 |
| R | 360.7 | 44.9 | 25.3 | 19.7 | 135.1 | 39.0 |
The range analysis shows that for wettability (contact angle), the aggregate particle size (A) has the most significant effect, followed by CMC (E) and binder B (C). For permeability, the aggregate size (A) and CMC (E) are the most critical factors. Similarly, for metal fluidity, the aggregate size (A) and CMC (E) play dominant roles.
Variance Analysis
To further validate the significance of these factors and estimate the experimental error, I performed a variance analysis. The variance ratio (F-value) for each factor was calculated and compared with critical values from the F-distribution to assess significance. The results are summarized in Table 7.
| Trial Indicator | Order of Significance (from most to least significant) |
|---|---|
| Permeability | A (High), E (Medium), D (Medium), B (Low), C (Low), F (Low) |
| Wettability | A (High), E (Medium), C (Low), B (Low), D (Low), F (None) |
| Metal Fluidity | A (High), E (Medium), B (Low), F (Low), C (None), D (None) |
The error analysis indicated that the coefficient of variation (CV) for permeability and fluidity was below 10%, classifying the test results as “excellent.” For wettability, the CV was between 10% and 20%, indicating “good” reliability. This statistical analysis confirms the robustness of my experimental findings.
Carbon Pickup Test in Steel Castings
To quantitatively assess the impact of coating wettability on sand foundry defects, I conducted a dedicated carbon pickup test on steel castings. Four different coatings with varying wettability and permeability were selected. Rectangular test samples were cast using both top and bottom gating systems. The carbon content was measured at different depths from the surface (2mm, 5mm, and 10mm) in the upper, middle, and lower sections of the castings. The original carbon content of the steel was 0.25%, and the pouring temperature was 1580°C. The results are presented in Table 8.
| Coating Type | Wettability (Contact Angle θ) | Gating System | Sample Location | Carbon Content (%) | Carbon Pickup (%) |
|---|---|---|---|---|---|
| Coating I (Poor wettability) |
~125° | Bottom | Bottom | 0.34 | 0.09 |
| Middle | 0.31 | 0.06 | |||
| Top | 0.28 | 0.03 | |||
| Top | Bottom | 0.41 | 0.16 | ||
| Middle | 0.32 | 0.07 | |||
| Top | 0.27 | 0.02 | |||
| Coating II (Good wettability) |
~90° | Bottom | Bottom | 0.29 | 0.04 |
| Middle | 0.27 | 0.02 | |||
| Top | 0.25 | 0.00 | |||
| Top | Bottom | 0.33 | 0.08 | ||
| Middle | 0.28 | 0.03 | |||
| Top | 0.25 | 0.00 |
The results clearly show that the casting produced with the coating having better wettability (Coating II) exhibited significantly lower carbon pickup. The maximum carbon pickup was reduced from 0.16% to 0.08%. This experiment unequivocally demonstrates the direct relationship between coating wettability and the severity of carbon-related sand foundry defects in steel castings.
Production Trial Verifications
To validate my findings in real-world scenarios, I performed comparison casting trials under actual production conditions. These trials involved different metal alloys and were conducted in collaboration with industrial partners.
High-Chromium Wear-Resistant Cast Iron
Trials were conducted for a high-chromium white cast iron plate. The original carbon content was 2.4%, and the pouring temperature was 1450°C. Two castings were produced: one using the factory’s standard coating and the other using my newly developed coating. The results, shown in Table 9, demonstrated a dramatic difference.
| Property | Factory Coating | Developed Coating |
|---|---|---|
| Wettability (θ) | ~118° | ~88° |
| Permeability | Low | High |
| Surface Quality | Poor, black carbon deposits | Excellent, clean surface |
| Internal Quality | Large internal shrinkage cavity | No internal defects |
| Carbon Content at Surface (2mm) | 3.5% | 2.9% |
The casting produced with the factory’s coating exhibited significant surface carbon deposition and a severe internal shrinkage defect, rendering it scrap. In contrast, the casting from my developed coating had a clean surface and excellent internal integrity, meeting all quality standards. The carbon content tests confirmed the superior performance of the new coating in reducing carbon pickup.
Heat-Resistant Steel Cylinder
Similar comparative trials were performed on a heat-resistant steel cylinder (ZGCr25Ni20). The original carbon content was 0.20%. The results, as detailed in the thesis, further confirmed the benefits of the developed coating. The casting from the factory’s coating displayed a rough surface with significant carbon accumulation at the coating-casting interface and suffered from mold collapse. In contrast, the casting from my developed coating had a smooth, clean surface with no carbon defects. Carbon analysis showed a maximum content of 0.24% in my casting, well within acceptable limits, while the factory’s casting had carbon contents up to 0.34%.
Ductile Iron Casting
Trials for a ductile iron casting also highlighted the superiority of the developed coating. The factory’s coating produced castings with severe surface carbon and wrinkles, internal slag inclusions, cold shuts, and misruns. My coating, however, yielded castings with excellent surface quality and sound internal structure. These production trials conclusively demonstrate the practical applicability and superior performance of my newly developed coating in mitigating carbon-related sand foundry defects across different alloy systems.
Mechanism of Defect Formation and the Role of Wettability
Formation of Carbon Black
The formation of carbon defects is intrinsically linked to the thermal decomposition pathway of the EPS pattern. Polystyrene (PS) decomposes through two primary pathways:
- Initial decomposition: At temperatures above 300°C, PS depolymerizes to release smaller molecules (e.g., styrene, benzene, toluene). These molecules have a higher H/C ratio than the original polymer, leading to the formation of some carbon-rich residues.
- Deep cracking: At temperatures exceeding 600°C, these gaseous intermediates undergo further deep cracking. The C-H and C-C bonds break, leading to the formation of significant amounts of solid carbon black and hydrogen gas.
The overall decomposition process is:
$$\text{Polystyrene} \xrightarrow{\text{Heat}} \text{Liquid products} \xrightarrow{\text{Further Heat}} \text{Gaseous products} + \text{Carbon black (Solid)}$$
If this entire process occurs within the mold cavity, the solid carbon black remains trapped, leading to defects. My key insight is that if the liquid intermediate products can be rapidly transported out of the mold cavity through the coating, the formation of carbon black can be significantly reduced at its source.
A Model for Liquid Product Escape
The escape of liquid polystyrene via the coating layer is a process of capillary-driven infiltration. The dynamics of this process are governed by the Lucas-Washburn equation for the penetration of a liquid into a horizontal capillary:
$$\frac{dl}{dt} = \frac{r^2 \Delta P}{8 \eta l} = \frac{r \gamma_{LV} \cos\theta}{4 \eta l}$$
Where:
- $l$ is the penetration depth of the liquid into the coating at time $t$.
- $r$ is the capillary radius (proportional to pore size in the coating).
- $\eta$ is the viscosity of the liquid polystyrene.
This equation highlights the critical parameters for enhancing liquid outflow:
- Coating pore radius (r): A larger pore size, achieved by using coarser and more spherical aggregate particles, directly increases the penetration depth of the liquid.
- Contact angle (θ): A smaller contact angle (i.e., better wettability) results in a larger value for $\cos\theta$, which reduces capillary resistance and enhances penetration.
- Viscosity of liquid products (η): Lower viscosity liquids penetrate more easily. The coating composition can influence the temperature in the gas gap, thereby affecting the viscosity of the liquid decomposition products.
This model confirms that enhancing the wettability of the coating for liquid polystyrene is a scientifically sound strategy to promote liquid-phase transport and suppress carbon black formation, thereby minimizing carbon-related sand foundry defects.
Influence of Coating Components on Wettability
Refractory Aggregate
The aggregate is the primary structural component. Coarser aggregate particles lead to larger pores and higher porosity in the dried coating. This increased porosity facilitates the infiltration of liquid polystyrene. My experiments confirmed that increasing the aggregate particle size from 270 mesh to 140 mesh significantly improved the wettability. However, excessively coarse particles can impair the suspension stability and application properties of the coating. Therefore, an optimal particle size range (140-200 mesh) provides the best compromise.
Binders and Suspending Agents
- PVA: As a water-soluble polymer, PVA decomposes completely at high temperatures, leaving behind a porous structure that enhances permeability. However, PVA has a low surface energy. Since a liquid will only wet a solid if the liquid’s surface tension is less than the solid’s critical surface tension, an excessive amount of PVA could potentially reduce the coating’s wettability. This explains the non-monotonic trend observed in the wettability with increasing PVA content.
- Bentonite: The addition of bentonite can initially improve wettability by increasing the porosity of the coating upon loss of structural water. However, since bentonite is a hydrous aluminosilicate with a layered structure, its high specific surface area and affinity for water could otherwise hinder the wetting process. At higher concentrations, it tends to form a low-energy surface (through the formation of a molecular layer of adsorbed water or the exposure of non-polar faces), which is difficult to wet by the liquid polystyrene, thereby reducing the wettability. This dual effect leads to an optimal concentration range.
- CMC: Similar to PVA, carboxymethyl cellulose is a high-molecular-weight polymer that forms a low-energy surface. On the one hand, at low concentrations, it might reduce wettability. But as a key suspending agent, it plays a crucial role in interacting with bentonite to form a strong, stable network structure. This uniform suspension prevents segregation of components and creates a consistently porous microstructure, which is beneficial for liquid infiltration. This explains the complex influence of CMC content, with an optimal balance point.
- Guilin No. 5: This organic plant-based binder serves a unique dual purpose. In the low-to-moderate temperature range, it provides excellent bonding strength. More importantly, it is designed to decompose and leave behind very little residue. This decomposition creates microporosity and, most crucially, its char is designed to be more easily wetted by non-polar liquid hydrocarbons like molten polystyrene, thereby actively improving the wettability of the coating, particularly at high temperatures.
Mechanism of Permeability Enhancement
The permeability of the coating is determined by its pore structure. The role of binders is multifaceted. Inorganic binders like bentonite provide strength but can block pores. Organic binders, when burned off during pouring, create new pore channels. The thermal decomposition of all organic components (PVA, CMC, Guilin No. 5) at high temperatures contributes to a more porous structure, as shown below:
$$\text{Organic Binder} \xrightarrow{\text{High Temperature}} \text{Gaseous Products} \uparrow + \text{Porous Residue}$$
The addition of silica sol acts as a high-temperature binder, maintaining coating strength and preventing cracking. It also potentially contributes to the microstructure by forming a network that bridges the aggregate particles while leaving interconnected pores open.
Microstructural Analysis and Physical Model
Scanning electron microscopy (SEM) analysis of the coating cross-sections provided direct visual evidence supporting my theoretical framework. Before pouring, the coating layer appeared dense and compact. After pouring, the coating that had good wettability displayed a porous structure with spherical pores. Interestingly, in some cases, vitreous or glass-like regions were observed within the coating. These regions are formed by the liquid polystyrene infiltrating the coating and then resolidifying. This is a direct confirmation that the liquid product was indeed being transported through the coating, a process that is enhanced by good wettability. In contrast, coatings with poor wettability showed fewer such internal features and more carbon build-up at the surface, confirming that the liquid products were trapped and subsequently decomposed inside the mold cavity, creating the carbon defects.
The physical model for this process, as depicted in the thesis, shows the sequence of events: the leading edge of the molten metal heats and melts the foam pattern. The resulting liquid film, if the coating’s wettability is high, is immediately wicked into the porous coating by capillary forces and the applied vacuum. Once inside the coating, it can vaporize and escape. This efficient removal mechanism prevents the accumulation of liquid at the metal/foam interface, thereby reducing gas pressure, preventing thermal degradation, and ultimately eliminating carbon defects.
I further observed that the fluidity of the molten metal was directly correlated with the wettability of the coating. Castings made with coatings that had poor wettability exhibited short flow lengths and larger gas gap lengths. The metal front was impeded by the high gas pressure and the thermal insulation effect of trapped liquid and gas, leading to premature solidification. This confirms the “pulse-like” flow behavior often observed in LFC. In contrast, with a high-wettability coating, the metal flowed more smoothly and for a longer distance, with a minimal gas gap, which resulted in a “goose-head” shaped solidification front. This effect is described by the balance of pressure forces:
$$P_{\text{metal}} \approx P_{\text{gas gap}} + P_{\text{capillary}}$$
Where $P_{\text{metal}}$ is the ferrostatic pressure, $P_{\text{gas gap}}$ is the pressure in the gas gap, and $P_{\text{capillary}}$ is the negative capillary pressure that aids the infiltration of liquid into the coating. A high capillary pressure (which is a direct result of good wettability) reduces the gas gap pressure and improves the overall filling process.
Optimal Coating Formulation
Based on the orthogonal analysis, variance analysis, and production trial validation, I have formulated an optimized coating for the reduction of carbon-related sand foundry defects. The recommended composition (by mass fraction of the refractory aggregate) is as follows:
- Refractory Aggregate: 100% Magnesium Olivine Powder (with a particle size distribution of 140-200 mesh)
- Inorganic Binder 1: 4% Lithium-based Bentonite
- Inorganic Binder 2: 3% Kaolin Clay
- Inorganic Binder 3: 2% Silica Sol (added after mixing)
- Organic Binder 1: 1.2% PVA
- Organic Binder 2: 0.4% Binder B
- Organic Binder 3: 2.0% Guilin No. 5
- Suspending Agent: 0.6% CMC
- Surfactant: 0.1% Non-ionic Surfactant
- Carrier: Water (added to achieve a suitable viscosity for application)
This optimized coating formulation was found to possess excellent suspension stability, application properties, high permeability, and, most critically, superior wettability for the liquid decomposition products of the foam pattern. The use of this coating has been proven to be an effective countermeasure against the formation of carbon deposits, wrinkles, and carbon pickup in ferrous castings, thereby significantly reducing the incidence of these common sand foundry defects.
Conclusions
This research provides a comprehensive investigation into the mechanisms of carbon-related sand foundry defects in the lost foam casting process and offers a novel, practical solution. Based on my experimental and theoretical analysis, I have drawn the following conclusions:
- Carbon black generation: During pouring, carbon black can form through two primary pathways in the thermal decomposition of polystyrene. The first is the initial depolymerization step, where the volatile products have a higher H/C ratio, leaving behind a carbon-rich residue. The second is the deep cracking of these volatile products at temperatures above 600°C, leading to the formation of carbon black and hydrogen.
- The role of wettability: The wettability of the coating by liquid polystyrene was identified as a critical, previously underestimated factor. A coating with excellent wettability facilitates the rapid removal of the liquid pyrolysis products from the mold cavity through capillary action. This eliminates the source of carbon black, resulting in lower gas gap pressure, reduced heat loss, and a cleaner casting surface. Conversely, a coating with poor wettability traps the liquid polystyrene, which then decomposes in-situ, generating carbon black and leading to defects like surface carbon deposits, wrinkles, and carbon pickup.
- Relationship between wettability and permeability: The research established a positive correlation between wettability and permeability. Generally, a coating with high wettability also exhibited high permeability, and vice-versa. However, permeability alone is not sufficient to guarantee defect-free castings. A coating with high permeability but poor wettability was shown to be ineffective in eliminating carbon defects, underscoring the independent and crucial role of wettability.
- Aggregate particle size is paramount: Among all variables studied, the particle size of the refractory aggregate had the most significant effect on both wettability and permeability. The use of coarser, more spherical aggregates (in the 140-200 mesh range) in combination with suitable binders and suspending agents significantly enhances the performance of the coating.
- Validated coating formulation: A novel coating formulation was developed and validated under production conditions. This coating, incorporating magnesium olivine aggregate, a mixed binder system, and a composite suspending agent, was proven to be highly effective in reducing carbon-related sand foundry defects across a range of ferrous alloys, including high-chromium cast iron, heat-resistant steel, and ductile iron, consistently producing castings of superior quality with minimal carbon defects.
In summary, the control of coating wettability is as critical as, and complementary to, controlling its permeability. The findings of this research challenge the traditional focus on permeability alone and offer a more complete understanding of the complex interactions that govern the formation of carbon-related defects in lost foam casting. The developed coating provides a robust and practical solution to this persistent problem, paving the way for the wider production of high-integrity castings and a more widespread and successful application of this promising manufacturing technology.