Our foundry is a specialized automotive casting plant producing nearly 10,000 tons of castings annually, all using green sand molding (sand casting foundry). For many years, the surface quality of our castings was poor, and we struggled to find a solution. When the state invested in importing the Steyr series heavy-duty trucks from Austria, the requirements for casting appearance became extremely stringent—especially for the front and rear brake drums. Overseas producers used high-pressure molding and advanced cleaning equipment to meet these demands. Based on our decades of experience, we selected and formulated a high-performance alcohol-based coating and retained our original molding process. Through extensive trials, the surface roughness and dimensional accuracy of the brake drums achieved the German DIN and technical standards, fully replacing imported products. This article elaborates on the selection of materials for alcohol-based coatings, their application in our sand casting foundry, and the superior results obtained.
Composition of Alcohol-Based Coatings and Process Requirements
The selection of refractory aggregates must consider the casting material, wall thickness, mold type, and potential defects. Commonly used aggregates include corundum, zircon sand, quartz powder, and graphite. For our gray iron brake drums (casting weight ~XX kg, main wall thickness ~XX mm, pouring temperature 1420–1460°C, mold hardness ~85–90, core hardness ~70–80), we chose a mixture of amorphous graphite and flake graphite as the refractory aggregate. Extensive practice has proven that this graphite-based aggregate is ideal for alcohol-based coatings used in gray iron sand casting foundry.
As a green sand casting foundry, we required a fast-drying coating to shorten the production cycle. Alcohol-based coatings typically use solvents like methanol, ethanol, or isopropanol. The solvent selection depends on evaporation rate, allowable airborne concentration, ignition characteristics, and cost. Abroad, isopropanol is popular due to its favorable ignition properties, but it is expensive and has a strong odor. In China, ethanol (>95%) is commonly used but has high polarity, low surface tension, deep penetration into the mold, and difficulty in ignition. Methanol is toxic and unsuitable alone. We combined the advantages of ethanol and isopropanol by mixing them, using ethanol as the primary diluent. The blend provides a balanced evaporation rate and ease of ignition.
Binders in coatings must provide both room-temperature strength and high-temperature erosion resistance. For room-temperature strength, many Chinese foundries use phenolic resin, furan resin, allyl alcohol resin, or alkyd resin. For high-temperature binding, a suitable solution had been lacking internationally. We developed a composite material as the high-temperature binder, which performed excellently in our sand casting foundry.
Suspending agents are critical to prevent sedimentation. In China, high-quality suspending agents for alcohol-based coatings are scarce. The Shenyang Electromechanical College and Shenyang Tractor Factory jointly developed three types: Type A based on bentonite paste, Type B based on lithium-modified bentonite, and Type C based on organically modified bentonite. We adopted a modified version of Type B for our coating.
The following table summarizes the typical formulation we used for the alcohol-based coating in our sand casting foundry.
| Component | Material(s) Selected | Function | Typical Weight Percentage (%) |
|---|---|---|---|
| Refractory aggregate | Amorphous graphite + flake graphite | Prevent metal penetration and provide thermal insulation | 55–65 |
| Solvent | Ethanol (95%) + isopropanol (7:3 ratio) | Carrier for drying; also aids ignition | 25–35 |
| Room-temperature binder | Phenolic resin (solid content ~50%) | Provide strength after drying | 3–5 |
| High-temperature binder | Composite inorganic-organic material | Maintain integrity at pouring temperature | 2–4 |
| Suspending agent | Lithium-modified bentonite-based paste | Prevent sedimentation, improve stability | 1–3 |
| Additives | Surfactant, defoamer (trace) | Improve wetting and reduce bubbles | <1 |
Coating Application Process in Sand Casting Foundry
The coating application procedure in our sand casting foundry followed strict steps to ensure consistent quality:
- Mold preparation: Before brushing, we used a soft brush to remove loose sand from the mold and core surfaces.
- Mixing: The coating was stirred thoroughly before use and kept agitated during application.
- Application: After brushing, the coating was immediately ignited to dry the layer. This step is crucial; otherwise, the coating strength would be reduced.
- Coating thickness: The dried coating thickness was controlled to 0.3–0.5 mm for general areas, and the surface roughness of the coating itself was maintained at Ra 6.3–12.5 µm.
- Drying time: After ignition, in the dry season we allowed ~1 hour of air drying; in the wet season, the time was shortened to avoid moisture reabsorption. Then the molds were closed and poured.
The coating serves several important functions in our sand casting foundry:
- Improves surface roughness: Without coating, the mold surface roughness was Ra ~25–50 µm. After coating, it improved to Ra 6.3–12.5 µm, representing an enhancement of about two ISO grades.
- Prevents mechanical and chemical sand adhesion: The graphite barrier reduces metal penetration and chemical reactions.
- Protects the mold: Reduces sand erosion and inclusions.
- Enhances mold surface strength and refractoriness: Contributes to better dimensional accuracy of castings.
Application to Brake Drums and Results
We adopted the commercial SD-Ⅲ type coating produced by a coating factory in Shandong. Its technical specifications are shown in Table 2.
| Parameter | Value |
|---|---|
| Specific gravity (g/cm³) | 1.45–1.55 |
| Viscosity (Ford cup, seconds) | 25–35 |
| Gas evolution (mL/g) | ≤ 12 |
| Suspension stability (24 h, %) | ≥ 95 |
| Penetration depth (number of sand grains) | 2–3 |
| Rheological behavior | Plastic flow with thixotropy |
| Shelf life (months) | 6 |
The suspension stability can be expressed mathematically as:
$$ S = \frac{h_0 – h_{24}}{h_0} \times 100\% $$
where \( h_0 \) is the initial height of the coating column and \( h_{24} \) is the height of clear liquid after 24 hours. Our measured S ≥ 95% indicates excellent resistance to sedimentation.
To integrate the coating into our production line, we modified several process parameters in our sand casting foundry:
Sand System Modifications
The original sand formulation and properties are listed in Table 3, along with the modified version after introducing the coating.
| Parameter | Original System | Modified System |
|---|---|---|
| New sand (AFS 50–60) addition | 10% | 10% (coarser AFS 45–55) |
| Return sand | 90% | 90% |
| Sodium bentonite | 6% | 6% |
| Coal dust | 5% | 0% (eliminated) |
| Heavy oil | 0.5% | 0% (eliminated) |
| Green compressive strength (MPa) | 0.10–0.12 | 0.12–0.14 |
| Permeability | 100–120 | > 130 |
| Moisture (%) | 3.8–4.2 | 3.5–3.8 |
The modified system not only eliminated coal dust and heavy oil (reducing smoke and improving the work environment) but also enhanced sand properties to better withstand coating brushing without mold damage.
Pouring System Adjustments
After applying the coating, the mold permeability decreases. To compensate, we increased the sprue cross-sectional area by 15–20% and shortened the pouring time. Additionally, we added 4–6 vent holes (diameter 6–8 mm) on the top surface of the casting to facilitate gas escape. The pouring temperature was maintained at 1420–1460°C as before.
Post-Ignition Drying
Immediately after igniting the coating, we used a blowtorch to further dry the coating surface. This step removed any residual moisture that could condense in the sand layer. However, we waited about 30–60 minutes after torching before closing and pouring, to reduce the moisture-saturated zone that can cause expansion defects.
Coating Thickness Control
For the brake drum wall thickness (15–20 mm), the optimal dried coating thickness was 0.3–0.5 mm. If too thin, sand adhesion occurred; if too thick, the coating became difficult to brush and prone to cracking. We monitored thickness using a wet-film gauge and by weighing.
The following table summarizes the improvements in casting quality after implementing the coating in our sand casting foundry.
| Parameter | Before Coating | After Coating |
|---|---|---|
| Surface roughness Ra (µm) | 25–50 | 6.3–12.5 |
| Sand adhesion defect rate (%) | 15–20 | < 2 |
| Inclusion/scab defect rate (%) | 8–12 | < 3 |
| Mold hardness required | ≥ 90 | ≥ 85 (easier to achieve) |
| Cleaning time per casting (min) | 8–10 | 3–5 |
| Overall cost per casting (RMB) | Reference | Reduced by ~12% |
The surface roughness improvement can be expressed by the reduction ratio:
$$ \Delta Ra = \frac{Ra_{before} – Ra_{after}}{Ra_{before}} \times 100\% $$
With Ra values of 37.5 µm (midpoint of 25–50) and 9.4 µm (midpoint of 6.3–12.5), the reduction is about 75%.
Additionally, the elimination of coal dust and heavy oil in the sand system reduced the gas evolution during pouring. The total gas from the coating plus sand can be estimated as:
$$ V_{gas} = V_{coating} + V_{sand} $$
where \( V_{coating} \) from Table 2 is ≤ 12 mL/g, and the sand without coal dust has a much lower gas evolution (~5 mL/g compared to ~15 mL/g previously). This reduction minimized the risk of gas-related porosity.
Conclusion
The successful application of an alcohol-based graphite coating in our sand casting foundry for Steyr brake drums has demonstrated remarkable benefits:
- Surface roughness improved by approximately 2 ISO grades, meeting German DIN standards.
- Defects such as sand adhesion, scabs, and inclusions were drastically reduced.
- The sand system no longer required coal dust or heavy oil, improving the working environment and reducing cost.
- Cleaning time and energy consumption decreased, leading to an overall cost reduction of about 12%.
We have established a comprehensive process standard for coating selection, preparation, application, and post-treatment. This experience confirms that with proper formulation and process control, alcohol-based coatings can elevate the quality of green sand castings to levels comparable to high-pressure molding, even in a conventional sand casting foundry.
Keywords: sand casting foundry, alcohol-based coating, brake drum, surface roughness, graphite refractory, suspension stability.