Small and medium-sized casting manufacturers will explain the phenomenon of sand sticking to aluminum castings.

　　Small and medium-sized casting manufacturers will explain the phenomenon of sand sticking to aluminum castings.

　　Small and Medium-Sized Casting Manufacturers: Sand Adhesion Issues in Aluminum Castings

　　Aluminum castings with sand adhesion not only affect the appearance quality of the parts but can also lead to their scrapping. Therefore, sufficient attention must be paid to this issue in casting processes.

　　Small and Medium-Sized Casting Manufacturers: Sand Adhesion Phenomenon

　　There are two types of sand adhesion: chemical adhesion and mechanical adhesion. The adhesive layer in the former case generally does not contain metallic iron.

　　Small and Medium-Sized Casting Manufacturers: Mechanical Sand Adhesion

　　Metallic molten material penetrates the gaps between sand grains in the mold or core, bonding to the casting surface after the sand has sintered. This can form either a thin layer—or even a layer several millimeters thick. In some cases, the molten metal may permeate the entire cross-section of the core, leading to blockages within the cavity. Such adhering sand is often impossible to remove, forcing the castings to be scrapped.

　　Small and Medium-Sized Casting Manufacturers: Chemical Sand Adhesion

　　The strong adhesive iron silicate slag, formed by the interaction of metal oxides generated from chemical reactions in the molten metal with the molding materials, often occurs at the gate and thick-walled sections of castings—particularly when using thin sand molds or cores in conjunction with relatively thick castings.

　　Small and Medium-Sized Casting Manufacturers: Causes of Sand Adhesion

　　1. Under sufficient pressure, the molten metal penetrates into the gaps between sand grains due to the high hydrostatic head of the molten metal—specifically, the pouring height of the casting and the pressure generated by the gating system. If this pressure exceeds the resistance caused by capillary action within the sand grain interstices—that is, the dead-body capillary pressure \( P_{\text{cap}} = \frac{\gamma \cos \theta}{r} \)—where \( \gamma \) represents the surface tension of the molten metal, \( \theta \) is the wetting angle of the metal in the capillary tube, and \( r \) is the radius of the capillary tube—a phenomenon known as mechanical sand adhesion occurs. When the hydrostatic head surpasses 500 mm, and if the molding sand used is excessively coarse, mechanical sand adhesion is likely to develop unless a coating is applied. As the formula indicates, the larger the sand grain size (i.e., the coarser the sand), the more prone the casting process becomes to mechanical sand adhesion.

　　2. Dynamic pressure caused by the flow of molten metal within the mold,

　　3. The "explosion" or "backdraft" of the mold refers to the dynamic pressure caused when flammable gases released during mold casting mix with air and are ignited by the hot molten metal.

　　4. Once mechanical sand sticking begins, even if the pressure decreases, the penetration of molten metal will continue until the front edge of the penetrating molten metal solidifies. In other words, when the temperature of the molten metal drops below the solidus temperature, the penetration can come to a halt.

　　5. Common causes of chemical sand adhesion include low refractoriness and sintering points in wet-process and core-making raw materials, as well as impurities in the quartz sand. Additionally, insufficient use—or improper application—of coal powder or its substitutes in coatings, combined with excessively high pouring temperatures, can contribute to the issue. Other factors, such as slag entering the mold during pouring, may also lead to inadequate casting conditions.

　　Small and medium-sized casting manufacturers: What causes uneven casting surfaces?

　　Cast surfaces often exhibit unevenness due to two main reasons: first, inconsistent compactness in the mold, causing uneven movement of the mold walls during pouring; second, an improperly designed injection system that fails to adequately compensate for shrinkage, resulting in sink marks or shrinkage cavities. Therefore, it’s essential to first analyze the underlying causes before implementing targeted corrective measures.