For foundries, the quality of castings is their core competitiveness. Alcohol-based coatings are key to improving the surface quality of castings; mastering their formulation technology is like holding the key to unlocking high-quality castings. Today, we'll delve into the secrets of alcohol-based coating formulation.

I. About Alcohol-Based Coatings

Alcohol-based coatings typically use industrial alcohol, isopropanol, or other alcohols as the main solvent or dispersion medium, and are made by uniformly dispersing refractory aggregates, binders, suspending agents, and other functional additives within them.

Its greatest advantage is its rapid drying after ignition, forming a robust coating on the sand mold surface. This not only significantly improves the surface quality of castings but also effectively prevents defects such as sand adhesion and inclusions. Our practical case studies demonstrate that the scientific use of alcohol-based coatings can reduce casting scrap rate by 10%–20% and improve surface roughness by 20%–30%.

In a typical formulation, refractory aggregates account for 50%–70%. Refractory aggregates directly determine the refractoriness, chemical stability, and anti-sand adhesion properties of the coating. Common aggregates include: zircon powder for cast steel and high-end cast iron; brown fused alumina and white fused alumina for alloy steel; and quartz powder, graphite powder, and chromite powder for cast iron and non-ferrous metals.

However, it is important to note that the particle size distribution of the aggregate is crucial. More coarse particles result in better coating permeability but a rougher surface; more fine particles result in a dense and smooth coating but are prone to cracking. Therefore, it is usually necessary to mix two or more aggregates with different mesh sizes.

II. Regarding the carrier solvent:

The carrier solvent accounts for approximately 20%–35%, generally using industrial alcohol with a concentration ≥95%. Excessive moisture content will severely affect the coating's suspension, brushability, and ignition drying effect. Isopropanol dries quickly and has good performance, but its cost is relatively high. The binder accounts for 1.5%–3.5%, and is divided into high-temperature and low-temperature types, respectively ensuring the coating's bonding strength at high temperatures and its immediate strength after ignition.

Suspension agents are used to prevent the sedimentation of solid particles; the most commonly used suspending agent is organobentonite. It can form a stable gel network in alcohol. Lithium-based bentonite requires activation treatment; PVB has both suspending and binding functions. Additives include wetting agents, defoamers, and preservatives, further improving the workability and storage stability of the coating.

III. The formulation process of alcohol-based coatings is particularly critical.

1. Raw materials must be strictly inspected, equipment cleaned, and open flames strictly prohibited in the operating area. Ventilation must be maintained, and operators must wear respirators, gloves, and other protective equipment.

2. Pour all the alcohol into the mixing tank. While stirring at high speed (>1000 rpm), slowly and evenly sprinkle in the organobentonite for 20–40 minutes until a uniform, viscous colloid is formed.

3. Switch to medium speed (500–800 rpm) and slowly add refractory aggregates, binders, and other powdered materials in sequence, preventing clumping and dust generation.

4. After adding all the powder, increase the speed to >1200 rpm and stir at high speed for 1–2 hours. For a higher quality coating, it can be further ground using a colloid mill or roller mill to make the coating finer.

5. Seal and mature the coating for 12–24 hours. After its properties stabilize, test the viscosity, Baumé degree, and suspension properties. If necessary, fine-tune with alcohol or pre-prepared bentonite slurry.

IV. Key Technical Control Points in the Formulation of Alcohol-Based Coatings

Viscosity/Density: Affects coating effect and coating thickness; typically, the Baume degree should be controlled between 40–70 Be°.

Suspension: A good coating should remain uniform for a long time, with minimal sedimentation and a loose texture.

Brushing/Leveling: The coating should be easy to brush, not self-brushed, and have good leveling properties.

Ignition and Drying: Requires rapid and uniform drying, without deflagration or cracking.

Moisture Resistance: Prevents moisture absorption and softening; this can be improved by adding PVB or resin.

Furthermore, it is crucial to note that alcohol-based coatings are flammable and explosive. The entire formulation, storage, and use process must be strictly fireproof and explosion-proof. The environment must be well-ventilated, equipment properly grounded, and storage areas must be far from heat sources and equipped with explosion-proof electrical equipment.

In conclusion, using good materials, scientific formulation, strictly following the production process, ensuring adequate stirring and curing time, relying on performance testing to guide production, and always prioritizing safety are key. Mastering these points will undoubtedly give you greater confidence and a more stable path to improving casting quality.