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Zinc Alloy Die Casting now widely used in a variety of decorative aspects, such as furniture, accessories, architectural decoration, bathroom accessories, lighting parts, toys, tie clips, belt buckles, metal buckles, and therefore require a higher quality of the casting surface, at the same time requires good surface processing performance. The most common defects and zinc alloy die casting surface blistering.
Defect characterization: the surface of the die casting process small bubble, die out, polished or processed revealed, injection or after plating. Reason:
Holes caused by: porosity and contraction mechanism, the pores tend to be round, and the contraction mostly irregular in shape.
(1) the stomatal causes: a liquid metal filling, solidification process, since the gas invade, leading to the casting surface or internal holes. b paint out of the volatile gas intrusion. c alloy liquid gas content is too high, precipitated during solidification.
When the gas in the cavity, the coating volatilized gas solidification the precipitated gas, poor in the mold venting, and ultimately to stay in the pores formed in the casting.
(2) shrinkage causes: a liquid metal during solidification, the volume is reduced or last solidification parts are not liquid metal feeding and shrinkage. b uneven thickness castings or castings local overheating, resulting in a certain part of the slow solidification, the volume shrinkage and recesses formed on the surface. Due to the presence of pores and craters, so that the die member when performing the surface treatment, the holes may enter the water, when the baking after painting and plating, thermal expansion of the gas within the pores; or holes within the water becomes steam, the volume expansion, resulting in the casting surface blistering.
Intergranular corrosion caused by:
Zinc alloy composition harmful impurities: lead, cadmium, tin gathered in the grain at the junction leading to intergranular corrosion, broken metal matrix due to intergranular corrosion, electroplating accelerated this scourge, the expansion of the site of intergranular corrosion coating the top causing casting surface blistering. Especially in a damp environment intergranular corrosion make castings deformation, cracking, or even broken.
Cracks caused by: waterlines, cold shut lines, hot cracking.
Waterlines, cold shuts pattern: the liquid metal in the filling process, first into contact with the liquid metal wall premature solidification, after entering the liquid metal is not fusion and solidification of the metal layer as a whole, formed in the casting surface docking moiré, strip defects. Waterlines are generally in the casting surface shallow; the cold shuts pattern may penetrate into the internal casting.
Hot cracking: a casting thickness uneven solidification process to produce stress; B premature ejection metal strength is not enough; the uneven force d c the top of the high mold temperature coarse grains; e harmful impurities.
The above factors are likely to crack.
When the die casting exist waterlines, cold shut lines, hot cracking, the plating solution will penetrate into the cracks, converted to steam during baking, air pressure from the top plating layer formed foaming.
