Zinc Alloy Die Casting blistering defect causes and solutions
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 buckles, various metal buckle, etc., and therefore require a higher quality of casting surface. At the same time requires a good surface treatment performance. The most common defect of zinc alloy die castings surface blistering. Defect characterization: die casting surface protruding vesicles, die casting out of found exposed after polishing or processing, fuel injection or after plating.
1, holes caused by: stomatal and contraction mechanisms, stomata tend to be circular, while the contraction of the majority of irregular shape. (1) pores causes: a liquid metal in the mold filling, solidification process gas into the casting surface or internal holes. b paint volatile gas intrusion. c alloy liquid containing gas is too high, precipitation during solidification. When the cavity in the gas, paint, volatile gases, solidification precipitation of the gas, eventually leaving the stomata formation in the casting mold venting is poor. (2) shrinkage causes: a liquid metal solidification process, because the volume is reduced or last solidified parts are not liquid metal feeding, and the formation of shrinkage. b uneven thickness of the casting or casting local overheating, resulting in a certain part of the slow solidification, the volume contraction recess formed on the surface. Due to the presence of pores and shrinkage, so that the die-casting during the surface treatment, holes may enter the water, when the painting and plating after baking, the gas thermal expansion of the holes; or holes in the water becomes steam, volume expansion, resulting in the casting surface blistering.
2, intergranular corrosion caused by: harmful impurities in the zinc alloy composition: lead, cadmium, tin will gather in the grain at the junction leading to intergranular corrosion, metal matrix due to intergranular corrosion and broken, and the plating accelerated the scourge, by the crystal intergranular corrosion of parts to swell and coating the top, killing the casting surface blistering. Intergranular corrosion in a damp environment to make the casting deformation, cracking, or even broken.
3, cracks caused by: watermarks, cold shut lines, thermal cracks. 1) watermarks, cold shuts pattern: the contact wall of the liquid metal in the filling process, the first liquid metal into the premature solidification into the liquid metal can not and has solidified metal layer fusion as one formed in the casting surface docking stack pattern, strip defects. Waterlines are generally shallow in the casting surface; cold every pattern that could spill into the internal casting. 2) hot cracking: when the solidification of castings of uneven thickness, stress; b prematurely to the top of the metal strong enough; c top out Shouli Bu d is too high mold temperature coarse grains; e harmful impurities exist . The above factors are likely to produce cracks. When the existence of watermarks, die casting cold shut lines, heat cracks, plating solution will penetrate into the cracks in, converted to steam during baking, pressure from the top plating layer formed blister.
Solution to the defective:
1 the control hole: The key is to reduce the amount of gas mixed with the castings, the ideal flow of metal into the cavity shall be continuously accelerated by the nozzle after shunt cone and runners to form a smooth and consistent with the direction of metal flow, using a conical flow channel design, pouring flow should be continuously accelerated by the nozzle inside the gate gradually reduced to reach this goal. In the filling system, the mixed gas is the formation of pores, due to turbulence and metal liquid mixture from the liquid metal from the casting system into the cavity of the analog die-casting process it is clear that the sharp change in the runner-bit and incremental runner cross-sectional area, will make the metal flow turbulence while the volume gas stable liquid metal only help into the overflow tank and exhaust ducts, and expel gas from the runner and cavity-mode outside.
All parts for shrinkage: For die-casting solidification process as much as possible at the same time distribute heat evenly, while solidification. Reasonable outlet design, thickness and location of the inner gate, mold design, mold temperature control and cooling, to avoid shrinkage produce. For intergranular corrosion phenomenon: the content of harmful impurities in the control of alloy raw materials, especially Pb <0.003%. Note that the waste of impurity elements.
3, waterlines, cold shut lines: can improve mold temperature, increase the speed of the inside gate, or in the cold every district to increase the overflow tank to reduce the presence of cold shut lines.
4, for the hot cracking: die casting thickness not sharp changes to reduce stress; the casting process parameters to be adjusted; lower mold temperature.