Zinc alloy die casting is currently widely used in a variety of decorative aspects, such as furniture accessories, architectural decoration, bathroom accessories, lighting parts, toys, tie clips, belt buckle, all kinds of metal buckle, so the casting surface quality requirements are higher, While requiring a good surface treatment performance.
Defect characterization: the surface of the die cast with protruding vesicles, die-cast out to find, polished or processed after the show, after the injection or plating. cause:
1. Holes caused by: mainly stomatal and contraction mechanism, the stomata are often round, and the contraction is mostly irregular shape.
(1) causes of stomata: a metal liquid in the filling, solidification process, due to gas intrusion, resulting in the casting surface or inside the hole. b Dissolved gas from the paint. c alloy liquid gas content is too high, solidification precipitation.
When the gas in the cavity, the paint volatilizes the gas, the alloy solidification of the gas, in the mold exhaust bad, and ultimately stay in the castings formed in the pores.
(2) causes of shrinkage: a molten metal solidification process, due to the size of the final solidification of the parts can not get the liquid liquid shrinkage, and produce shrinkage. b uneven thickness of the castings or castings overheating, resulting in a part of the solidification slow, the volume shrinkage when the surface of the formation of concave. Due to the presence of pores and shrinkage, so that the casting surface in the surface treatment, the hole may enter the water, when painting and plating after baking, the hole heat expansion of the gas; or hole water will become steam, volume expansion, Thus causing the casting surface to blister.
2. intergranular corrosion caused:
Hazardous Impurities in Zinc Alloys: Lead, cadmium, and tin will accumulate at the grain boundary to cause intergranular corrosion. The metal matrix is broken by intergranular corrosion, and the plating accelerates the scourge. The intergranular corrosion will expand The top of the coating, resulting in casting surface blistering. Especially in the wet environment intergranular corrosion will cast deformation, cracking, or even broken.
3. Cracks caused: water lines, cold lines, hot cracks.
Water, cold stripes: metal liquid in the filling process, the first access to the metal liquid contact wall premature solidification, after the metal liquid can not be merged and has been solidified metal layer as a whole, in the casting surface forming a stack of overlapping, There is a stripe defect. The watermark is usually in the shallow surface of the casting; and cold stripes may penetrate into the interior of the casting.
Hot crack: a When the thickness of the casting uneven, solidification process to produce stress; b premature ejection, the metal strength is not enough; c top when the uneven force d too high mold temperature to grain thick; e harmful impurities exist.
The above factors are likely to produce cracks.
When the die casting there are water lines, cold lines, hot cracks, plating solution will penetrate into the cracks in the baking into steam, the pressure from the top of the plating layer to form a bubble.
Solution to defective programs:
Control the formation of stomata, the key is to reduce the amount of gas mixed into the casting, the ideal metal flow should be accelerated from the nozzle through the diversion cone and runner into the cavity to form a smooth and consistent metal flow, the use of tapered flow Design, that is, the flow should be continuously accelerated by the nozzle inward gate gradually reduced, can achieve this goal. In the filling system, the mixed gas is due to the turbulence mixed with the metal liquid to form pores, from the molten metal into the cavity by the casting system simulation of the process of casting, it is clear that the sharp transition in the runner and incremental pouring Road cross-sectional area, will make the metal liquid flow turbulence and rolling gas, smooth metal solution is conducive to gas from the runner and cavity into the overflow tank and exhaust slot, out of the mold outside.
For the shrinkage: to die during the solidification process as much as possible at the same time evenly heat, while solidification. Through a reasonable nozzle design, gate thickness and location, mold design, mold temperature control and cooling, to avoid shrinkage generation. For intergranular corrosion: the main control of alloy raw materials harmful impurities, especially lead <0.003%. Pay attention to the impurity elements brought by the waste.
