One layer of refractory coating hung on the outer surface of the foam plastic model is one of the three key technological factors in the lost foam casting process. External coating is the key factor to reduce surface defects and prevent other defects.
The foam plastic model is buried in the sand box. Although the material in the sand box has been wrapped around the foam model, the shape of the material is almost spherical. In fact, the contact between the materials is also a point contact rather than a surface contact, which results in many small gaps between the materials. In addition to the effect of vacuum, the liquid metal is easy to enter into the gap of the mold material, resulting in the phenomenon of fleshy products, resulting in casting failure. Therefore, usually before the expendable pattern is cast, a refractory coating is coated on the exterior surface of the foam mold. The effect of this is:
(1) At room temperature, it can effectively improve the strength and stiffness of the plastic model, which can prevent the model from deformation or damage by dry sand in the process of transportation between processes and sand filling vibration, so as to ensure the stability of the cavity.
(2) During the pouring of molten metal, the refractory coating can effectively separate the molten metal from the molding sand, so as to effectively prevent the molten metal from penetrating into the molding sand and having unnecessary chemical reaction with it, so as to effectively prevent the defects such as sand sticking and sand holes in the castings, and greatly reduce the occurrence of mold collapse.
(3) the external coating can adsorb or pass through the decomposition residue of foam plastic mold, so that the decomposed solids and residues of foam plastics can be effectively discharged in the process of casting, so as to prevent carbon defects, blowholes and other defects in metal castings.