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The effective methods to prevent bubbles are: reducing the gas content in the metal liquid, increasing the air permeability of sand mold, and adding an air riser at the top of the mold cavity.

The surface of the casting is adhered to a layer of sand which is difficult to remove. It not only affects the appearance of casting, but also increases the workload of casting cleaning and cutting, and even affects the life of the machine. The method to prevent the formation of sand is to add coal powder in molding sand, and to brush the surface of the cast mold with anti-sand paint.

(1) Observe the surface shape of casting defects. If the surface is uneven, very rough, and dark gray, the hole with irregular shape is shrinkage cavity. (2) If the location of the hole is at the final solidification thickening of the casting, or at the hot spot where the two walls intersect, and is located in the middle or upper part of the section, it is a shrinkage cavity. (3) The most concentrated hole defects on the thick and large section of steel castings are shrinkage or air shrinkage.

In order to prevent sand expansion, the strength of sand mold and the rigidity of sand box should be increased, and the pressing force or fastening force when closing the box should be increased. In addition, the pouring temperature should be reduced to make the surface of molten metal crust earlier, so as to reduce the pressure of molten metal on the mold.

Sand inclusion is a kind of groove and scar defect formed on the surface of castings, which is very easy to produce in wet mold casting of heavy plate castings. Avoiding large plane structure can effectively prevent sand inclusion.