Defect characterization：The surface of the zinc alloy precision casting is made up of small blister, which can be found, polished or processed after being exposed, sprayed or electroplated. Causes: It is mainly the stoma and contraction mechanism, the stoma is usually round, and the contraction most is irregular shape. 

1.Causes of shrinkage: a: In the solidification process of molten metal, shrinkage cavity is produced due to shrinkage of the volume or final solidification. b: The uneven thickness of the casting or casting parts overheat, causing a certain part to solidify slowly, and the surface of the shrinkage cavity. Due to the existence of pores and shrinkage cavity, the hole may enter the water when the die casting is processed, when the paint and plating are baked, the gas in the hole expands; Or the water in the hole changes steam and expands, resulting in blistering on the surface of the castings.

2.The causes of stomata: a: The alloy liquid is high in gas and precipitates out when solidified, when the gas in the cavity, the gas produced by the coating, and the solidified gas of the alloy, the air hole formed in the precision casting is finally left when the mold exhaust is bad. b: In the process of filling and solidification, the molten metal can produce holes in the surface of the casting or the inside of the casting. c: The gas that comes out of the paint.