① Because the preparatory work in the early stage of production is not in place, the theoretical negative pressure value of the system is designed relatively low, resulting in sand sticking defects of castings.

② Due to the low temperature of the molten metal, the casting of the workpiece is not complete. After the casting product is subjected to external force, there will be cracks or fracture, and there are oxide inclusions at the fracture, which are not fused together. This is the reason for the cold shut (as shown in the figure).

③ The shape of slag hole formed by metal oxide of casting material and charge slag and slag of refractory material is irregular. From the appearance color, the slag hole caused by slag is black, while the charge slag hole is white. At the same time, the hot melt of refractory material is used in ladle, which shows black slag on the surface of casting, which is the cause of slag inclusion (as shown in the figure).