Inflatable models and carbon film isolation

Graphite powder coatings exhibit excellent peelability when applied to cast iron components.

Graphite forms a bright carbon film during the casting, solidification, and cooling processes, preventing interaction between the molten metal and the sand mold or core surface, thus avoiding sand adhesion. By applying a coating made from waste engine oil and graphite powder mixed together on the wet mold cavity surface, a smooth surface finish on thin-walled cast iron parts can be achieved through the reduction gas film and carbon film generated by the high casting temperature.

2. Slag Isolation

After casting, a slag layer forms at the interface between the paint coating and the casting, with a high viscosity that binds refractory aggregates together, blocking the penetration of the metal liquid. The slag layer performs better when it does not produce silicate chemical reactions with the metal liquid or is not wetted, or barely wetted, by the metal liquid. During cooling, the difference in thermal contraction coefficients between the slag shell layer and the casting metal results in significant shear stress at the interface between the slag shell and the casting, causing the paint shell to automatically peel off. This theory of utilizing the slag layer to prevent sand adhesion is termed the "slag isolation theory." Paints are formulated using metals and metal oxides that are easy to react, which, upon thermal reaction, generate a new refractory oxide that can prevent the penetration of the metal liquid.

Oxidation

The theory suggests that the adhesion of sand to castings depends on the presence of a critical thickness of ferric oxide layer between the casting and the coating, which is approximately 100μm thick. To prevent chemical adhesion, the thickness of the oxide should be increased; once it exceeds the critical thickness, the sintered layer of the coating or molding sand with the metal oxide is prone to peel off from the casting. Limestone sand molds, during the casting of steel, produce a strong oxidizing atmosphere within the mold cavity due to the following reaction: CaCO3 = CaO + CO2, which exacerbates the surface oxidation of the cast steel, thus making limestone sand an excellent anti-adhesion material.

The coating that performs well in sand mold applications for cast steel parts tends to easily stick to sand in cast iron parts, sometimes even severely. This can be explained by the fact that iron oxide layers are easily formed on the surface of cast steel. Cast iron has a high carbon content, and the carbon element oxidizes before the iron does, making it difficult to form an iron oxide layer with critical thickness during the casting of cast iron. Therefore, the "oxidation" theory is not applicable to anti-sticking coatings for cast iron parts.