One layer of refractory coating hung on the outer surface of the foam plastic model is one of the three key technological factors in theprocess. External coating is the key factor to reduce surface defects and prevent other defects of lost foam castings.
The foam plastic model is buried in the sand box. Although the shape of the sand mold in the lost foam casting can be wrapped around the foam plastic 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, resulting in many small gaps between the materials. In addition to the effect of vacuum pumping, liquid metal is very easy to enter the mold gap, resulting in fleshy phenomenon of lost foam castings, 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 purpose of this is:
① At room temperature, the strength and stiffness of the plastic model can be effectively improved, 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 lost foam casting molding cavity.
② During liquid metal pouring, the refractory coating can effectively separate the liquid metal from the EPC molding sand, so as to effectively prevent the liquid metal from infiltrating into the molding sand and causing unnecessary chemical reaction with it, so as to effectively prevent the defects such as sand sticking and sand holes of product castings, and greatly reduce the box collapse of lost foam casting.
③ The external coating can adsorb or pass through the decomposition residue of foam plastic mold, so that the decomposed debris and residues of foam plastics can be effectively discharged in the process of lost foam casting, and the defects such as carbon defects and blowholes can be prevented.