I developed an alcohol-based coating system specifically for sand mould casting when external chills are placed in the mould cavity. In my work, the central problem is not simply the refractory nature of the coating. The real difficulty is that the same coating must adhere to two very different surfaces: a porous resin-bonded sand mould and a dense metallic chill. In sand mould casting with external chills, the coating must wet, bond, dry, and later detach in a controlled way. If the coating is too cohesive on the chill, it becomes difficult to clean. If it is too permeable or too thin, molten iron can penetrate the interface and cause sand adhesion, metal penetration, or chill fusion. If it is too volatile, gas defects appear. Therefore, I treated the coating as an interfacial engineering material rather than as a simple refractory slurry.
My objective was to create an alcohol-based coating for sand mould casting that combines fast drying, high suspension stability, low gas evolution, strong adhesion to both sand and chill surfaces, and easy stripping after solidification. I used a bauxite–white corundum composite refractory aggregate, a lithium-based bentonite suspension agent modified by a two-step intercalation–grafting route, and a phenolic resin–polyvinyl butyral composite binder in industrial ethanol. I then used an orthogonal experiment to identify the effects of aggregate grading, suspension agent content, and binder content. I combined range analysis with weighted normalization to select the best formulation. Finally, I validated the coating in production-scale sand mould casting with external chills.
1. Role of Coatings in Sand Mould Casting with External Chills
External chills are metallic inserts placed in a sand mould to accelerate local solidification. They are widely used in heavy-section castings, especially large thin-wall grey iron parts, because they reduce shrinkage porosity and adjust the thermal gradient. In sand mould casting, the chill changes the local heat-transfer path. Heat leaves the liquid metal through the chill, then through the chill–coating interface, then through the coating, and finally into the mould. The coating is therefore a thermal and chemical barrier. It must survive liquid metal impingement, thermal shock, and mechanical erosion while still allowing heat extraction at a useful rate.
I found that the most common failures in sand mould casting with external chills can be grouped into four categories:
| Failure mode | Origin at the interface | Consequence | Coating requirement |
|---|---|---|---|
| Sand adhesion | Molten metal penetrates the sand surface or reacts with binder residues | Rough surface, high cleaning cost | Dense refractory layer, strong sand adhesion, low permeability |
| Metal penetration | Coating porosity or insufficient thickness | Mechanical interlocking of iron and mould | Optimized particle packing, adequate coating thickness |
| Chill fusion | Iron reacts with or welds to the chill surface | Chill loss, difficult separation | Stable barrier, controlled wettability, easy stripping |
| Gas defects | Binder or solvent decomposition | Porosity, blowholes, surface pits | Low gas evolution, fast solvent removal |
The thermal resistance of the coating can be expressed in a simplified one-dimensional form. If the coating thickness is \(h\), the effective thermal conductivity is \(k_c\), and the interfacial contact resistance is \(R_i\), then the total resistance between the chill and the mould is
$$R_{total}=R_i+\frac{h}{k_c}$$
This equation shows why I could not simply make the coating very thick. A thicker coating improves the barrier function but also increases thermal resistance. In sand mould casting with external chills, excessive thermal resistance reduces the chilling effect. Therefore, the optimum coating must be thin enough for heat extraction and dense enough to prevent penetration. I aimed for a dry coating thickness between 0.5 mm and 1.2 mm, with a preferred production thickness near 0.7 mm.
The adhesion between the coating and a surface can be described by the work of adhesion:
$$W_a=\gamma_s+\gamma_l-\gamma_{sl}$$
Here, \(\gamma_s\) is the surface energy of the substrate, \(\gamma_l\) is the surface tension of the coating liquid, and \(\gamma_{sl}\) is the interfacial energy. For sand mould casting, the sand substrate is rough and porous, so mechanical interlocking contributes strongly. For the chill, the surface is smooth and metallic, so chemical compatibility and wetting become more important. This difference is the reason I designed a hybrid binder system rather than a single binder.
2. Design Principles for the Alcohol-Based Coating
I established six design principles for the coating used in sand mould casting with external chills.
First, fast drying. The solvent must be volatile enough to burn off quickly after application. Industrial ethanol is suitable because it has a low boiling point, low toxicity relative to many alternatives, and good compatibility with phenolic resins. However, ethanol also creates a low-viscosity continuous phase, so the suspension agent must build a strong thixotropic network.
Second, high suspension stability. Refractory particles such as bauxite and white corundum have high density. Without a proper suspension agent, they settle quickly. I measured the 24 h suspension rate as a key indicator. I targeted a suspension rate of at least 93% for sand mould casting use.
Third, low gas evolution. Organic binders decompose at high temperature. The coating must not generate excessive gas. I targeted a gas evolution below 20 mL/g according to the relevant sand mould casting coating standard.
Fourth, balanced adhesion. The coating must adhere to the sand mould and to the chill. If adhesion to the chill is too high, the chill cannot be recovered. If adhesion to the sand is too low, the coating can peel during handling or be washed away by molten metal.
Fifth, high abrasion resistance. After drying, the coating must survive mould closing, transport, and dust removal. I measured mass loss under a standard abrasion test.
Sixth, controlled stripping. After casting, the coating must release from the chill without leaving a fused layer. This is critical for chill reuse and for reducing cleaning time.
I summarized the target properties in Table 2.
| Property | Target for sand mould casting with external chills | Reason |
|---|---|---|
| Condition viscosity | 5.5–12 s | Good brushability and levelling |
| 24 h suspension rate | ≥93% | Storage stability and uniform application |
| Gas evolution | <20 mL/g | Reduce blowholes and surface pores |
| Coating thickness | 0.5–1.2 mm | Barrier without excessive thermal resistance |
| Abrasion mass loss | <0.5 g | Survive handling and mould closing |
| Brushability grade | Class I preferred | Uniform coverage on sand and chill |
| Surface roughness after casting | Ra ≤ 12.5 μm | Reduce cleaning and improve quality |
| Sand adhesion area | <2% | Prevent sand burn-on |
3. Raw Materials and Functions
I selected raw materials based on refractory performance, alcohol compatibility, and behaviour in sand mould casting. The refractory aggregate was a mixture of bauxite and white corundum. Bauxite provides high refractoriness and moderate thermal expansion. White corundum provides hardness, chemical resistance, and dimensional stability. Together, they form a composite aggregate that resists iron oxide attack and thermal shock.
The suspension agent was based on calcium bentonite. Natural calcium bentonite has poor suspension performance in alcohol because its interlayer spacing is limited and its surface chemistry is not compatible with ethanol. I therefore converted it to a lithium-modified form, then intercalated it with organic modifiers, and finally grafted silane and other functional groups onto the layers. This two-step route is described in detail later.
The binder system was a mixture of thermosetting phenolic resin and polyvinyl butyral (PVB). Phenolic resin provides high-temperature bonding, char formation, and network strength. PVB improves flexibility, adhesion to metal, and film formation. The mass ratio of phenolic resin to PVB was fixed at 2.5:1 based on preliminary trials. Industrial ethanol was the solvent.
| Component | Role in sand mould casting coating | Key property |
|---|---|---|
| Bauxite, 250 mesh | Coarse refractory framework | High refractoriness, gas permeability |
| Bauxite, 400 mesh | Fine filler | Densification, surface smoothness |
| White corundum, 250 mesh | Hard refractory phase | Abrasion resistance, chemical stability |
| Modified bentonite | Suspension agent | Thixotropy, particle suspension |
| Phenolic resin | High-temperature binder | Char strength, network formation |
| PVB | Flexible binder | Adhesion to metal, film flexibility |
| Industrial ethanol | Solvent | Fast drying, low residue |
4. Modification of the Bentonite Suspension Agent
I used a “intercalation–grafting” two-step method. In the first step, I treated calcium bentonite with lithium carbonate to promote lithium exchange. In the second step, I intercalated the lithium-treated bentonite with organic modifiers, including a quaternary ammonium salt, a thiazole-based compound, and sodium carboxymethyl cellulose. In the third step, I grafted a silane coupling agent and a fluorinated hydrazide derivative onto the intercalated layers.
The structural change can be followed by X-ray diffraction. According to Bragg’s law,
$$2d\sin\theta=n\lambda$$
where \(d\) is the interlayer spacing, \(\theta\) is the diffraction angle, \(n\) is an integer, and \(\lambda\) is the wavelength. In my measurements, the \(d_{001}\) spacing of the natural calcium bentonite was about 1.26 nm. After lithium exchange, it decreased slightly to about 1.22 nm. After intercalation with the organic modifiers, it increased to about 3.12 nm. After grafting, it remained near 3.09 nm. This increase confirms that the organic species entered the interlayer gallery and expanded the layered structure.
The swelling index also changed significantly. The natural bentonite had a swelling index of about 5.8 mL/g. After modification, it increased to about 20.1 mL/g. This improvement indicates weaker interlayer forces, better delamination, and stronger solvent uptake. In alcohol-based coatings for sand mould casting, this means the modified bentonite can form a more effective card-house network.
| Stage | d001 spacing / nm | Swelling index / (mL·g-1) | Expected effect |
|---|---|---|---|
| Natural calcium bentonite | 1.26 | 5.8 | Poor alcohol suspension |
| Lithium-exchanged bentonite | 1.22 | — | Start of interlayer modification |
| Intercalated bentonite | 3.12 | — | Expanded gallery, improved dispersion |
| Grafted modified bentonite | 3.09 | 20.1 | Stable thixotropic network in ethanol |
The thixotropic behaviour of the suspension agent can be approximated by a Herschel–Bulkley model:
$$\tau=\tau_0+K\dot{\gamma}^n$$
where \(\tau\) is shear stress, \(\tau_0\) is yield stress, \(K\) is consistency, \(\dot{\gamma}\) is shear rate, and \(n\) is the flow index. The modified bentonite increases \(\tau_0\) and \(K\) at low shear while allowing \(n\) to remain close to 0.8–1.0. This gives the coating a high yield stress when at rest, so refractory particles do not settle, but it still flows under brushing. This balance is essential in sand mould casting with external chills because the coating must cover vertical chill faces without running down.
5. Coating Preparation
I prepared the coating in a specific sequence to avoid particle agglomeration and binder precipitation. First, I dry-mixed the bauxite and white corundum in a wheel mixer. Then I added the pre-prepared modified bentonite slurry, followed by the phenolic resin solution and the PVB solution. I mixed the paste for 2.5–3 h. After that, I discharged the paste and diluted it with additional ethanol until the density reached \(1.60 \pm 0.02\) g/cm³.
The coating density was calculated as
$$\rho_c=\frac{m_c}{V_c}$$
where \(m_c\) is the coating mass and \(V_c\) is the coating volume. I controlled density because it affects viscosity, suspension, and coating thickness. If the density is too low, the coating is thin and prone to penetration. If the density is too high, brushing becomes difficult and the coating may sag on vertical surfaces.
The coating thickness after drying can be estimated from the wet mass and the coated area:
$$h=\frac{m_w-m_d}{A\rho_c}$$
where \(m_w\) is the wet coating mass, \(m_d\) is the dry coating mass, \(A\) is the coated area, and \(\rho_c\) is the dry coating density. In production, I used this relation to calibrate brushing and to maintain a dry thickness near 0.7 mm.
6. Experimental Methods
I prepared sand specimens from furan resin-bonded sand. The specimens were cylindrical, 50 mm in diameter and 55 mm in height, and were hardened according to the sand process. I also prepared metallic chill specimens with dimensions of 100 mm × 50 mm × 20 mm. The chill material was the same as that used in production.
I measured condition viscosity using a standard flow cup. I measured the 24 h suspension rate by allowing a coating sample to stand in a graduated cylinder and recording the volume of the settled layer. I measured gas evolution by heating a known mass of coating at high temperature and collecting the released gas. I measured abrasion resistance by subjecting the dried coating to a standard abrasion test and recording mass loss.
I graded brushability by applying two consecutive brush coats to both sand and chill specimens. After 2 min of standing, I observed the vertical surface for running, brush marks, and thickness uniformity. The grading system is shown in Table 4.
| Grade | Sand surface | Chill surface | Thickness |
|---|---|---|---|
| Class I | Uniform, no brush marks | Uniform, no brush marks | 0.5–1.2 mm |
| Class II | Basically uniform, slight brush marks | Basically uniform, slight brush marks | 0.5–1.2 mm |
| Class III | Uneven or too thin/thick | Uneven or too thin/thick | <0.5 mm or >1.3 mm |
The 24 h suspension rate was calculated as
$$S_{24}=\frac{V_s}{V_0}\times100\%$$
where \(V_s\) is the volume of the stable suspension after 24 h and \(V_0\) is the initial volume. The gas evolution was calculated as
$$G=\frac{V_g}{m_c}$$
where \(V_g\) is the volume of gas released under standard conditions and \(m_c\) is the mass of the coating sample.
7. Orthogonal Experiment Design
I selected three factors for the orthogonal experiment: aggregate grading, suspension agent content, and binder content. The aggregate grading was expressed as the mass ratio of 250 mesh bauxite to 400 mesh bauxite. The suspension agent content and binder content were expressed as mass percentages based on the total refractory aggregate. I used an L9(3³) orthogonal array. The factor levels are shown in Table 5.
| Level | A: m(250 mesh bauxite):m(400 mesh bauxite) | B: suspension agent / wt% | C: binder / wt% |
|---|---|---|---|
| 1 | 60:20 | 3 | 2 |
| 2 | 53:27 | 4 | 4 |
| 3 | 40:40 | 5 | 6 |
The full orthogonal array is shown in Table 6. I measured condition viscosity, 24 h suspension rate, gas evolution, abrasion mass loss, and brushability grade. For data analysis, I converted brushability grades into numerical values: Class III = 3, Class II = 2, and Class I = 1. Lower values are better for brushability, viscosity, gas evolution, and abrasion loss. Higher values are better for suspension rate.
| Run | A | B | C | Condition viscosity / s | 24 h suspension / % | Gas evolution / (mL·g-1) | Abrasion loss / g | Brushability grade |
|---|---|---|---|---|---|---|---|---|
| 1 | 1 | 1 | 1 | 5.0 | 88 | 11.0 | 0.067 | III |
| 2 | 1 | 2 | 2 | 6.5 | 94 | 13.4 | 0.056 | I |
| 3 | 1 | 3 | 3 | 7.3 | 91 | 15.9 | 0.052 | II |
| 4 | 2 | 1 | 2 | 6.2 | 90 | 13.8 | 0.063 | I |
| 5 | 2 | 2 | 3 | 8.3 | 93 | 14.2 | 0.049 | III |
| 6 | 2 | 3 | 1 | 5.9 | 95 | 12.6 | 0.069 | I |
| 7 | 3 | 1 | 3 | 8.0 | 89 | 13.5 | 0.050 | III |
| 8 | 3 | 2 | 1 | 6.0 | 94 | 12.0 | 0.070 | II |
| 9 | 3 | 3 | 2 | 7.1 | 93 | 13.2 | 0.058 | II |
8. Range Analysis
I used range analysis to determine the influence order of each factor. For each factor and each level, I calculated the average response \(k_i\). The range \(R\) is
$$R=\max(k_1,k_2,k_3)-\min(k_1,k_2,k_3)$$
A larger \(R\) means a stronger effect on that property. The results for viscosity are shown in Table 7.
| Factor | k1 | k2 | k3 | R | Influence order |
|---|---|---|---|---|---|
| A: aggregate grading | 6.27 | 6.80 | 7.03 | 0.76 | C > A > B |
| B: suspension agent | 6.40 | 6.93 | 6.77 | 0.53 | — |
| C: binder | 5.63 | 6.60 | 7.87 | 2.24 | — |
The binder content had the strongest effect on condition viscosity. This is because the phenolic resin–PVB system forms micro-gels in ethanol. As binder content increases, the continuous phase becomes more structured and viscous. The aggregate grading also matters because finer particles increase the specific surface area and the amount of free solvent absorbed on particle surfaces. The best viscosity was obtained with A1B1C1, but this combination had poor suspension and brushability in other tests.
The range analysis for the 24 h suspension rate is shown in Table 8.
| Factor | k1 | k2 | k3 | R | Influence order |
|---|---|---|---|---|---|
| A: aggregate grading | 91.0 | 92.7 | 92.0 | 1.7 | B > A > C |
| B: suspension agent | 89.0 | 93.7 | 93.0 | 4.7 | — |
| C: binder | 92.3 | 92.3 | 91.0 | 1.3 | — |
The suspension agent content was the dominant factor. The modified bentonite forms a card-house network that supports dense refractory particles. The best suspension was achieved at B2 or B3. The aggregate grading also had a moderate effect because a broader particle-size distribution reduces settling velocity. The binder had the smallest effect, although high binder content slightly reduced suspension because it increased the viscosity of the continuous phase and may have caused partial flocculation.
The gas evolution results are shown in Table 9.
| Factor | k1 | k2 | k3 | R | Influence order |
|---|---|---|---|---|---|
| A: aggregate grading | 13.43 | 13.53 | 12.90 | 0.63 | C > B > A |
| B: suspension agent | 12.77 | 13.20 | 13.90 | 1.13 | — |
| C: binder | 11.87 | 13.47 | 14.53 | 2.66 | — |
The binder content had the largest effect on gas evolution. Phenolic resin and PVB decompose at high temperature and release carbon monoxide, carbon dioxide, and light hydrocarbons. Therefore, increasing binder content increases gas evolution. The suspension agent also contributes to gas evolution because organic modifiers on the bentonite decompose. The aggregate grading has a smaller effect, but finer aggregates can reduce gas permeability and make gas release more difficult. For low gas evolution, the best combination was A3B1C1.
The abrasion resistance results are shown in Table 10.
| Factor | k1 | k2 | k3 | R | Influence order |
|---|---|---|---|---|---|
| A: aggregate grading | 0.0583 | 0.0603 | 0.0593 | 0.0020 | C > A > B |
| B: suspension agent | 0.0600 | 0.0583 | 0.0597 | 0.0017 | — |
| C: binder | 0.0687 | 0.0590 | 0.0503 | 0.0184 | — |
The binder content was again the strongest factor. Increasing binder content from 2% to 6% reduced abrasion mass loss from about 0.0687 g to about 0.0503 g. The reason is that the resin–PVB network forms a continuous ceramic–organic skeleton after drying. The aggregate grading also matters because a well-graded mixture forms a denser packing. The best abrasion resistance was achieved with A1B2C3.
The brushability results are shown in Table 11.
| Factor | k1 | k2 | k3 | R | Influence order |
|---|---|---|---|---|---|
| A: aggregate grading | 2.00 | 1.67 | 2.33 | 0.66 | C > A ≈ B |
| B: suspension agent | 2.33 | 2.00 | 1.67 | 0.66 | — |
| C: binder | 2.00 | 1.33 | 2.67 | 1.34 | — |
Brushability is a complex property. It depends on viscosity, yield stress, and adhesion to both sand and chill. The binder content had the strongest effect because it controls the rheology of the continuous phase. At 2% binder, the coating was too thin on the chill. At 6% binder, the coating was too viscous and left brush marks. The best brushability was achieved at C2, with a moderate binder content. The suspension agent also affected brushability: too little suspension agent caused sagging, while too much caused a high yield stress and poor levelling.
9. Weighted Normalization and Optimal Formulation
Because the five properties have different units and different directions, I used weighted normalization to select the optimal formulation. I assigned weights based on the needs of sand mould casting with external chills:
| Property | Weight / % | Direction |
|---|---|---|
| Condition viscosity | 10 | Lower is better |
| 24 h suspension rate | 25 | Higher is better |
| Gas evolution | 20 | Lower is better |
| Abrasion resistance | 10 | Lower mass loss is better |
| Brushability | 35 | Lower grade number is better |
For a positive indicator, the normalized value is
$$Z_{ij}=\frac{x_{ij}-x_{j,\min}}{x_{j,\max}-x_{j,\min}}$$
For a negative indicator, the normalized value is
$$Z_{ij}=\frac{x_{j,\max}-x_{ij}}{x_{j,\max}-x_{j,\min}}$$
The weighted score for run \(i\) is
$$S_i=\sum_{j=1}^{5}w_jZ_{ij}$$
where \(w_j\) is the weight of property \(j\). The normalized scores are shown in Table 13.
| Run | Weighted normalized score |
|---|---|
| A1B1C1 | 18.24 |
| A1B2C2 | 19.81 |
| A1B3C3 | 18.13 |
| A2B1C2 | 18.76 |
| A2B2C3 | 18.52 |
| A2B3C1 | 20.28 |
| A3B1C3 | 17.70 |
| A3B2C1 | 19.79 |
| A3B3C2 | 19.44 |
The highest score was obtained for A2B3C1. This formulation uses an aggregate grading of 53:27 (250 mesh bauxite to 400 mesh bauxite), 5% modified bentonite suspension agent, and 2% phenolic resin–PVB binder. The binder mass ratio remained 2.5:1. The optimized formulation is shown in Table 14.
| Component | Optimal amount |
|---|---|
| Bauxite, 250 mesh | 53 parts |
| Bauxite, 400 mesh | 27 parts |
| White corundum, 250 mesh | 20 parts |
| Modified bentonite suspension agent | 5 wt% based on refractory aggregate |
| Phenolic resin | 1.43 wt% based on refractory aggregate |
| PVB | 0.57 wt% based on refractory aggregate |
| Industrial ethanol | Balance to density 1.60 g/cm³ |
The optimized coating achieved a 24 h suspension rate of 95%, a gas evolution of 12.6 mL/g, a brushability grade of Class I, and excellent abrasion resistance. These values satisfy the relevant requirements for sand mould casting coatings: condition viscosity between 5.5 s and 12 s, 24 h suspension rate at or above 93%, gas evolution below 20 mL/g, and abrasion mass loss below 0.5 g.
10. Mechanism of Performance Improvement
I explain the performance improvement through four mechanisms: particle packing, thixotropic network formation, hybrid binder bridging, and controlled interfacial failure.
Particle packing. The aggregate mixture uses 250 mesh and 400 mesh bauxite with 250 mesh white corundum. The coarse particles form a load-bearing skeleton. The fine particles fill the voids between the coarse particles. The packing efficiency can be approximated as
$$\eta_{pack}=\frac{\rho_{bulk}}{\rho_{true}}$$
A higher packing efficiency reduces porosity and permeability. This is critical in sand mould casting because molten iron can penetrate pores larger than a critical size. I estimate that the optimized grading increases packing efficiency by 8–12% compared with a single-size aggregate. The finer 400 mesh bauxite also improves surface smoothness, which helps release after casting.
Thixotropic network formation. The modified bentonite forms a three-dimensional network in ethanol. The network has a high yield stress at rest, which prevents dense particles from settling. When the coating is brushed, the network breaks down and the viscosity decreases. After brushing, the network rebuilds, which prevents sagging on vertical chill surfaces. This behaviour is essential in sand mould casting with external chills because the chill faces are often vertical or inclined.
Hybrid binder bridging. Phenolic resin provides strong high-temperature bonding and char formation. PVB provides flexible chains that adhere to metal surfaces. When the coating dries, the two binders form an interpenetrating network. The phenolic resin increases cohesive strength, while PVB increases adhesive strength and reduces brittleness. The binder also bridges sand grains and refractory particles. This dual function is why the coating adheres well to both the sand mould and the chill.
Controlled interfacial failure. After casting, the coating must fail at the chill–coating interface rather than within the coating or at the sand–coating interface. I designed the binder system so that the coating forms a continuous film on the chill but does not react chemically with the chill surface. The char layer remains coherent but weak enough to be removed. This reduces cleaning time and improves chill recovery.
Thermal stress also affects coating integrity. The thermal stress in the coating can be estimated as
$$\sigma_t=\frac{E\alpha\Delta T}{1-\nu}$$
where \(E\) is the elastic modulus, \(\alpha\) is the thermal expansion coefficient, \(\Delta T\) is the temperature change, and \(\nu\) is Poisson’s ratio. A coating with high \(E\) and high \(\alpha\) is more likely to crack. The composite aggregate and flexible PVB phase reduce \(E\) and accommodate thermal mismatch. This is another reason the hybrid binder is superior to a single rigid binder.
11. Production Validation in Sand Mould Casting
I validated the optimized coating in a production foundry. The mould was a furan resin-bonded sand mould. The external chill exposure area was 25 cm × 15 cm per block, with a total chill surface area of about 1.2 m². The casting was a heavy machine tool bed made of HT300 grey iron. I applied two brush coats to a total dry thickness of about 0.7 mm. The coating was uniform and showed good brushability on both sand and chill surfaces.
The coating ignited and dried within about 5 s after application. The pouring temperature was 1,420 °C. After pouring and holding, the casting was cooled to 150–200 °C before shakeout. During cleaning, the coating stripped easily from the chill surface. The casting surface was smooth, and the cleaning time was significantly reduced. The chill blocks separated easily from the mould, which improved recovery and reduced chill replacement cost.
The before-and-after comparison is shown in Table 15.
| Item | Before improvement | After improvement |
|---|---|---|
| Surface roughness Ra / μm | 25.0 | 12.5 |
| Sand adhesion area / % | 8.5 | <2 |
| Cleaning time / min | 45 | 25 |
| Chill recovery rate / % | 65 | >90 |
| Gas pore defects (Φ > 2 mm) | Present | None |
The production results confirm that the coating solves the main problems of sand mould casting with external chills. The surface roughness decreased from 25.0 μm to 12.5 μm. The sand adhesion area decreased from 8.5% to less than 2%. The cleaning time decreased from 45 min to 25 min. The chill recovery rate increased from 65% to more than 90%. No obvious gas pores larger than 2 mm were observed, and no chill fusion occurred.
I also compared the optimized coating with the standard requirements. The results are shown in Table 16.
| Property | Standard requirement | Optimized coating |
|---|---|---|
| Condition viscosity / s | 5.5–12 | 5.9 |
| 24 h suspension rate / % | ≥93 | 95 |
| Gas evolution / (mL·g-1) | <20 | 12.6 |
| Abrasion mass loss / g | <0.5 | 0.069 |
| Brushability grade | Class I or II | Class I |
12. Discussion
The orthogonal experiment showed that no single factor controls all properties. Binder content is the dominant factor for viscosity, gas evolution, abrasion resistance, and brushability. Suspension agent content is the dominant factor for suspension stability. Aggregate grading has a moderate effect on all properties and a strong effect on packing density and surface finish. Therefore, the optimal formulation must balance these factors rather than maximize any one factor.
For sand mould casting with external chills, brushability is the most heavily weighted property because it determines whether the coating can be applied uniformly to both sand and metal. A coating that has excellent suspension but poor brushability will not cover the chill evenly. A coating that has excellent brushability but high gas evolution will produce gas defects. A coating that has excellent abrasion resistance but poor stripping will increase cleaning cost. The weighted normalization method allowed me to account for these trade-offs.
The modified bentonite is a key innovation. Natural bentonite does not disperse well in ethanol. By expanding the interlayer spacing and grafting organic groups, I improved its compatibility with ethanol and its ability to form a thixotropic network. The XRD and swelling results confirm the structural change. The production results confirm the practical benefit: the coating remains stable for 24 h and does not settle excessively.
The hybrid binder is another key innovation. Phenolic resin alone gives a rigid, brittle coating that may crack during cooling. PVB alone gives a flexible coating that may not have enough high-temperature strength. The combination provides both high-temperature strength and flexibility. This is important in sand mould casting because the coating experiences rapid heating, thermal expansion mismatch, and mechanical stress during shakeout.
The composite refractory aggregate is also important. Bauxite provides refractoriness and moderate thermal expansion. White corundum provides hardness and chemical resistance. The particle-size distribution provides dense packing. The fine 400 mesh bauxite fills voids and reduces surface roughness. The coarse 250 mesh particles provide permeability and resistance to erosion. This combination is well suited to the thermal and mechanical conditions of sand mould casting with external chills.
I also note that the coating density must be controlled carefully. If the density is too low, the coating is thin and porous. If the density is too high, the coating is difficult to brush and may sag. I maintained the density at \(1.60 \pm 0.02\) g/cm³, which gave a good balance between coverage and flow. The dry thickness of 0.7 mm was sufficient to prevent metal penetration but not so thick that it reduced the chilling effect.
13. Practical Guidelines for Sand Mould Casting
Based on my experiments and production trials, I recommend the following guidelines for applying alcohol-based coatings in sand mould casting with external chills.
| Step | Recommendation | Reason |
|---|---|---|
| Chill preparation | Clean the chill surface and remove rust or oil | Improves wetting and adhesion |
| Coating mixing | Mix for 2.5–3 h and control density to 1.60 g/cm³ | Ensures uniform suspension and brushability |
| Application | Apply two brush coats to 0.5–1.2 mm dry thickness | Balances barrier and heat transfer |
| Drying | Ignite or air-dry according to ethanol content | Removes solvent and prevents gas defects |
| Mould closing | Avoid mechanical damage to the coating | Maintains barrier integrity |
| Shakeout | Cool to 150–200 °C before shakeout | Improves coating stripping and chill recovery |
The coating should not be applied too thickly on the chill face. Excessive thickness increases thermal resistance and reduces the chilling effect. It also increases gas evolution and may cause spalling. The coating should not be applied too thinly either, because thin areas are vulnerable to metal penetration. The optimal thickness is a compromise between thermal and barrier requirements.
The suspension agent content should be high enough to prevent settling but not so high that it causes excessive viscosity or gas evolution. In my experiments, 5% modified bentonite gave the best balance. The binder content should be low enough to keep gas evolution under control but high enough to provide abrasion resistance and adhesion. In my experiments, 2% binder with a phenolic resin to PVB ratio of 2.5:1 gave the best overall performance.
14. Conclusions
I developed an alcohol-based coating for sand mould casting with external chills. The coating uses a bauxite–white corundum composite refractory aggregate, a modified bentonite suspension agent, and a phenolic resin–PVB hybrid binder in industrial ethanol. The bentonite was modified by a lithium exchange, intercalation, and grafting route. The interlayer spacing increased from about 1.26 nm to about 3.09 nm, and the swelling index increased from about 5.8 mL/g to about 20.1 mL/g. These changes improved dispersion and suspension in ethanol.
The orthogonal experiment showed that binder content is the dominant factor for viscosity, gas evolution, abrasion resistance, and brushability. Suspension agent content is the dominant factor for suspension stability. Aggregate grading has a moderate effect on all properties. The optimal formulation was A2B3C1: 53 parts 250 mesh bauxite, 27 parts 400 mesh bauxite, 20 parts 250 mesh white corundum, 5% modified bentonite suspension agent, and 2% phenolic resin–PVB binder with a mass ratio of 2.5:1.
The optimized coating achieved a 24 h suspension rate of 95%, a gas evolution of 12.6 mL/g, a brushability grade of Class I, and excellent abrasion resistance. In production validation on a heavy machine tool bed casting, the surface roughness decreased to Ra 12.5 μm, the sand adhesion area was less than 2%, no obvious gas pores larger than 2 mm formed, and no chill fusion occurred. The cleaning time decreased significantly, and the chill recovery rate increased to more than 90%.
The coating is therefore suitable for sand mould casting with external chills. It provides fast drying, high suspension stability, low gas evolution, strong adhesion to both sand and metal, and easy stripping. These properties address the main interfacial problems of sand mould casting with external chills: sand adhesion, metal penetration, chill fusion, gas defects, and difficult cleaning. The formulation can be adapted to other heavy castings where external chills are used to control solidification.
In future work, I would extend the approach to other chill materials and other casting alloys. I would also study the effect of coating thickness on the local cooling rate in sand mould casting with external chills. A coupled thermal–rheological model could help predict the optimal coating thickness for different casting sections. In addition, long-term chill reuse and coating aging should be evaluated under production conditions. The results of this study provide a practical basis for improving coating performance in sand mould casting with external chills.