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The reason and solutions for blistering defects of die-casting parts such as zinc alloy cufflinks and tie clips
Causes and solutions for blistering defects of zinc alloy die-casting parts: Zinc alloy die-casting parts are widely used in various decorative aspects, such as furniture fittings, architectural decoration, bathroom fittings, lighting parts, toys, tie clips,zinc alloy cufflinks ,cellphone holder , belt buckles, various metal ornaments. Buckle, etc., therefore, the surface quality of the casting is required to be high, and good surface treatment performance is required. The most common defect in zinc alloy die castings is surface blistering. Characterization of defects: There are protruding vesicles on the surface of die-casting parts, which are found after die-casting, exposed after polishing or processing, appearing after injection or plating.there is three point to cause those creacted .
1, the hole caused: mainly the pores and contraction mechanism, the pores are often round, and the shrinkage is mostly irregular. (1) Causes of pores: a. During the filling and solidification of the molten metal, holes are formed on the surface or inside of the casting due to gas intrusion. b The gas volatilized by the paint invades. c alloy liquid contains too much gas and precipitates during solidification. When the gas in the cavity, the gas volatilized by the paint, and the gas which is solidified and precipitated by the alloy, when the mold is poorly exhausted, the pores formed in the casting are finally left. (2) Causes of shrinkage cavities: a During the solidification process of the molten metal, shrinkage holes are generated due to shrinkage of the volume or the fact that the molten metal is not replenished at the final solidified portion. b The casting or casting with uneven thickness is locally overheated, causing a certain part to solidify slowly, and the surface is concave when the volume is contracted. Due to the existence of pores and shrinkage cavities, the holes may enter the water during the surface treatment of the die-casting part. When the baking is performed after painting and electroplating, the gas in the holes is thermally expanded; or the water in the holes may become vapor and expand in volume. This causes the surface of the casting to foam.
2, intergranular corrosion caused: harmful impurities in the composition of zinc alloy: lead, cadmium, tin will accumulate at the grain boundary to cause intergranular corrosion, metal matrix broken due to intergranular corrosion, and electroplating accelerates this scourge, accepting crystal The inter-corroded portion expands to push the coating up, causing the surface of the casting to foam. Especially in the humid environment, intergranular corrosion will deform, crack and even break the casting.
3, caused by cracks: water lines, cold insulation, hot cracks. 1) Water grain and cold grain pattern: During the filling process, the metal liquid contact wall that enters firstly solidifies prematurely, and then the molten metal cannot be fused with the solidified metal layer, forming a stack at the surface of the casting. Strips, strip defects. Water marks are generally shallow on the surface of the casting; cold welts may penetrate into the interior of the casting. 2) Thermal crack: a when the thickness of the casting is uneven, the stress is generated during the solidification process; b is prematurely ejected, and the strength of the metal is insufficient; when the c is pulled out, the mold temperature is too high, so that the crystal grain is coarse; . All of the above factors may cause cracks. When the die casting has water marks, cold lines
and hot cracks, the solution will penetrate into the crack during electroplating, be converted into steam during baking, and the pressure will rise up to form a foaming layer.