Since the casting filters are placed at the bottom of the casting pouring cups, sprue runner, or gate of the sand mold, the molten metal injected into the cavity from the casting pouring cups will stop instantaneously when passing through, so that the inclusions with lower density, mainly slag, are removed from the casting pouring cup. The molten metal floats up and separates out. These inclusions also produce deep adsorption when passing through regular or irregular meshes. This phenomenon can continue to purify the molten metal before the ceramic pouring cups are full.
Surface filtration is a metal liquid purification feature common to all filters. When molten metal passes through the screen, no floating or floating inclusions, such as slag with a relatively large surface area, are captured by the surface of the screen. Moreover, the captured inclusions will form finer filter holes with the filter screen, and continue to capture subsequent inclusions, thereby enhancing the filtering effect. But at the same time of surface filtration, with the decrease of pouring temperature, the increase of molten metal viscosity, and the prolongation of pouring time, the screen will be blocked to varying degrees, thus worsening the pouring situation.
Internal filtration is a unique function of the molten metal purification function of the ceramic foam filter, which uses the channels of the three-dimensional structure to adsorb and block inclusions. At the same time, the solid ceramic filter has a similar “congruent correspondence” relationship with the inclusions, which can adsorb the inclusions. When the molten metal passes through the internal tortuous channel, the inclusions that have not been filtered by the surface will repeatedly contact and collide with the skeleton of the filter screen with a large surface area, so as to be removed by adhesion. At the same time, since the main SiC on the surface of the skeleton of the filter screen is oxidized to a very thin glassy silica during the baking process, this layer of silica is softened by the heat of the molten metal during filtration, which further adsorbs impurities. It makes it difficult for impurities that collide with the skeleton of the filter to escape, resulting in the function of internal filtering.