Effect of Die casting Coatings on Surface Quality of Die castings

1. Separation of air mold and carbon film
Graphite powder coatings have good peelability in cast iron castings. Graphite produces a carbon monoxide gas film and a bright carbon film during casting, solidification, and cooling to prevent metal liquid from interacting with the sand mold or core surface and preventing sand sticking. By mixing the waste engine oil and the graphite powder on the surface of the wet cavity, and relying on the reduced gas film and the carbon film produced by the high casting, a thin-walled cast iron casting having an abnormally smooth surface can be obtained. temperature.
2. Separation of slag
After casting, a slag layer is formed at the interface between the coating and the casting, which has a high viscosity, adheres to the refractory aggregate, and the like, thereby hindering the penetration of the molten metal. The slag layer reacts with the metal liquid without silicate, and does not wet with the metal liquid or is substantially not wetted with the metal liquid, and the effect is best. During the cooling process, the shrinkage coefficient of the slag shell and the cast wire is greatly different, and a large shear stress is generated at the interface between the slag shell and the casting, so that the coating shell is automatically peeled off. The theory of using slag layers to prevent sand adhesion is called "slag isolation theory." The coating is made of readily reactive metals and metal oxides that undergo a thermal reaction to produce a new refractory oxide that prevents the penetration of metal liquids.
3. Oxidation
Theoretically, whether the casting adheres to the sand layer depends on whether there is a critical iron oxide layer thickness between the casting and the coating, and its thickness is about 100 μm. In order to prevent chemical sand adhesion, the oxide thickness should be increased. When the critical thickness is exceeded, the coating or sintered layer of sand and metal oxide is easily peeled off from the casting. The limestone sand type has good anti-sticking effect in steel casting: CaCO3=CaO+CO2, which leads to a strong oxidizing atmosphere in the mold cavity and exacerbates the surface oxidation of the steel casting.

The coating with good application effect in the steel casting sand mold is easy to stick sand or even serious when applied in the steel slab sand mold, which can be explained by the easy formation of the iron oxide layer on the surface of the steel casting. Cast iron has a high carbon content, and carbon is oxidized in front of cast iron, so it is difficult to form an iron oxide layer having a critical thickness in cast iron. Therefore, the "oxidation" theory does not apply to the anti-stick coating of cast iron parts.

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