1. The poor air permeability of the coating may result in insufficient negative pressure, and the poor air permeability of the filling sand, which cannot timely discharge the gas and residue in the mold cavity.
2. The pouring speed is too slow, unable to fill the sprue cup, exposing the sprue, entraining air, and inhaling slag, forming air pockets and slag holes.
3. Many gases and residues generated from the gasification and differentiation of foam model cannot be discharged from the mold in time, and foam and coating layer are not dry enough to fill dry sand.
4. Because of the unreasonable planning of the pouring system, the filling speed of the metal liquid is greater than the foam gasification concession and the gas exhaust speed, which forms the differentiation pores of the black smoke on the inner wall by trapping the gasification residues in the metal liquid at the filling front and then gasification again.
5. The sealing at the connection between the sprue cup, the sprue, and the pouring system is not good, especially the connection between the sprue and the sprue cup is not well sealed.
6. The particle size of the molding sand is too fine, the dust content is high, the permeability is poor, and the internal blockage of the negative pressure pipeline causes distortion of the negative pressure degree.
7. Poor deoxidation of molten steel, incomplete slag removal in the furnace, furnace, and ladle, inadequate slag blocking during the pouring process, and unreasonable pouring technology form slag holes.
8. The pouring temperature is low, the molten metal at the filling front cannot fully gasify the foam, and the residual material cannot float to the riser and condense in the casting to form pores.
