Zinc alloy die casting is widely used in various decorative aspects, such as furniture accessories, building decoration, bathroom accessories, lighting parts, toys, tie clips, belt buckles, all kinds of metal buckle, etc., so the surface quality of casting requirements are high, and the requirements of good surface treatment performance. The most common defect of zinc alloy die casting is surface bubbling.
Defect characterization: die-casting surface has protruding vesicles, die-casting is found, polishing or processing after exposure, spray or electroplating after appearance.
The reasons causing：
1.Holes cause: mainly stomata and contractile mechanism, stomata are often round, and most contractile are irregular.
(1) Causes of pores: in the process of filling and solidification of liquid a metal, holes are generated on the surface or inside of the casting due to gas intrusion. b coating gas invasion. The gas content of c alloy liquid is too high and precipitates during solidification.
When the gas in the cavity, the gas from the coating, the gas precipitated from the alloy solidification, when the mold exhaust is poor, the final remaining in the casting formed pores.
(2) The cause of shrinkage hole: during the solidification process of a metal liquid, shrinkage hole is generated due to the shrinkage of the volume or the final solidification site can not be fed by the metal liquid. b The uneven thickness of the casting or the partial overheating of the casting, resulting in slow solidification of a part and concave surface formation when the volume shrinks. Due to the existence of pores and shrinkage holes, the holes may enter the water when the die casting is surface treated. When the paint and electroplating are baked, the gas in the holes is heated and expands. Or the water in the hole will become vapor and the volume will expand, thus leading to bubbling on the surface of the casting.
2.Intergranular corrosion causes:
Harmful impurities in zinc alloy components: lead, cadmium, tin will gather at the grain junction leading to intergranular corrosion, the metal matrix is broken due to intergranular corrosion, and electroplating accelerates this evil, the intergranular corrosion part will expand and the coating will be jacked up, causing the surface of the casting blistered. Especially in humid environment, intergranular corrosion will cause casting deformation, cracking and even breakage.
3 cracks caused by: water lines, cold lines, hot cracks.
Water lines and cold lines: During the filling process of liquid metal, the contact wall of liquid metal entering first solidifies prematurely, and then the liquid metal entering cannot fuse with the solidified metal layer, forming overlaps on the surface of the casting, resulting in strip defects. The water grain is generally in the shallow layer of the casting surface; The cold gap grain may penetrate into the casting.
Thermal crack:
（a）when the thickness of the casting is uneven, the solidification process produces stress;
（b）premature ejection, insufficient metal strength;
(c) uneven force during ejecting;
(d) Excessive mold temperature makes grains coarse; e Harmful impurities are present.
All the above factors may produce cracks.
When there are water lines, cold lines and hot cracks in the die casting, the solution will penetrate into the cracks during electroplating, and be converted into steam when baking, and the electric coating will be jacked up by air pressure to form bubbles.
Solution to defects:
Control air hole, the key is to reduce the amount of gas in mixed with the casting, the ideal metal flow should be accelerated by the nozzle after cone and sprue into the mold, forming a smooth and metal flow in the same direction, the taper runner design, namely water flow should be accelerated to from the nozzle to the gate to reduce gradually, can achieve this goal. In the filling system, the gas is mixed with the metal liquid to form pores. From the study of the simulated die-casting process in which the metal liquid enters the cavity from the casting system, it is obvious that the sharp transition position in the runner and the increasing cross-sectional area of the runner will make the metal liquid flow turbulent and gas entrapment. Smooth metal liquid is conducive to the gas from the runner and cavity into the overflow tank and exhaust tank, out of the mold.
For shrinkage hole: to make every part of the die casting solidification process as far as possible at the same time uniform heat dissipation, solidification. Shrinkage hole can be avoided by reasonable nozzle design, inner gate thickness and location, mold design, mold temperature control and cooling. For intergranular corrosion, it is mainly to control the content of harmful impurities in alloy raw materials, especially lead <. 0.003%. Pay attention to the impurities brought by the waste.
For water lines and cold lines, the mold temperature can be increased, the speed of the inner gate can be increased, or the overflow tank can be increased in the cold zone to reduce the appearance of cold lines.
For hot cracks: die casting thickness should not change sharply to reduce stress; Related die casting process parameters for adjustment; Lower mold temperature.
