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Fundamental causes for real (mechanical/physical) penetration are metallostatic pressure, dynamic pressure during casting, and crystallization pressure during solidification.
Sand molds display a certain pore system that corresponds to their packing density. At the boundary between the metal and the mold material, there is a balance between the metallostatic pressure, the capillary forces of the mold material, the wettability and the surface tension of the metal. When the melt meets the grains of the mold surface during casting, it can penetrate into the pores of the mold surface under the effect of the metallostatic pressure until a balance between the interfacial tension at the mold surface and the penetration pressure (critical pressure value at which the melt penetrates through the topmost layer of grains) has been reached. As a result of this, the surface of the cast piece is rough.
The interfacial tension, which counteracts the penetration pressure, is influenced by the capillary forces of the mold material (primarily by the porosity), by the wettability of the mold surface and by the surface tension. The interfacial tension of casting materials on an Fe-C basis reaches relatively high values. The extent of this tension is influenced primarily by the chemical composition of the main alloying elements and the presence of surface-active elements, such as bismuth, lead, phosphorous, silicon and others. Adding cerium, sodium and zircon also increases the interfacial tension.
The melt wets the sand structure of the mold surface and displays it with varying intensity (roughness, wetting depth). The larger the penetration pressure, the grain radius of the mold material, the radius of its pores and the density of the grains are, and the smaller the interfacial tensions are, the higher the values are that are reached by the wetting depth. The wetting angle and therefore the wettability of a mold surface can be significantly influenced by the formation of a layer of lustrous carbon.
The size of the pore radius of a compacted mold part depends primarily on the grain structure (grain size, grain particle size distribution), the additions (proportion of binder, sediments), the intensity of compaction (packing density), and the sintering behavior of the mold material.
We use the term penetration when the roughness depth is larger than or equal to the radius of the grains of the mold material. This means that it is only possible to draw a line between roughness and penetration under consideration of the grain size of the mold material.