1. The foam model is decomposed into a large amount of gas  the residue can  be discharged in time. The dry sand with foam  coating layer is desiccant. Under the high temperature of the liquid alloy, a large amount of hydrogen  oxygen are poured into the casting, which is the main reason for the formation of pores.

2. Poor deoxidation of molten steel, unclean slag removal in the hearth, furnace  ladle, too short sedation time, poor slag retaining in the pouring process,  unreasonable pouring process cause slag holes.

3. Due to the unreasonable design of the gating system, the filling speed of the metal liquid is greater than that of the foam gasification  the gas discharge rate. The filling front will cause the gasification residue to be encapsulated in the metal liquid again to form a decomposed hole in the inner wall.

4. Pouring temperature is low, filling front metal liquid can  make the foam fully vaporized, the undecomposed residual material is  enough to float to riser  solidified in the casting to form pores.

5. The position of the ingot runner is unreasonable,  the dead zone is formed when filling. Due to the gas pressure in the cavity, the gasification residue is accumulated at the dead end to form pores. The section area of the runner is too large, so that the filling speed is larger than that of the foam gasification. The foam is devoured  the gas is partially dissolved in the alloy,  the gas can  be discharged to form pores.

6. The permeability of the coating is poor  the negative pressure is insufficient,  the permeability of the filling sand is poor, so the gas  residue in the mold cavity cannot be discharged in time,  the pores are formed under the filling pressure.

7. The pouring speed is too slow to fill the sprue cup, expose the sprue, draw in the air, inhale the slag,  form the entrainment pores  slag holes.

8. The capacity of sprue cup is too small, the metal liquid forms eddy current, invades the air  generates air hole.

9. The connection between sprue cup  sprue  pouring system is  well sealed, especially the connection between sprue  sprue cup is  well sealed. Under the action of negative pressure, sand inclusion  air hole are easy to form. This phenomenon can be calculated  explained by Bernoulli equation.

10. Molding sand particle size is too fine, dust content is high, permeability is poor, negative pressure degree distortion caused by internal blockage of negative pressure pipe, so that the negative pressure value around the cavity is far lower than the indicated negative pressure,  the gasification material can  be discharged  the coating in time to form pores  wrinkles.