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How to solve zinc alloy electroplating blistering?

by:JunQi     2021-08-29
Zinc alloy die castings are currently widely used in various decorations, such as tie clips, belt buckles, various metal buckles, etc. Therefore, the surface quality of the castings is required to be high, and good surface treatment performance is required. The most common defect of zinc alloy die castings is surface blistering. The bulging of the casting surface in the following cases is called blistering, which is the external manifestation of defects in the casting. (1) Found after die casting. (2) Revealed after polishing or processing. (3) Appears after painting or electroplating. (4) Found after being placed for a period of time. Causes 1. Pores: Mainly caused by pores and contraction mechanism. The pores are often round, and the contraction is mostly irregular. Causes of pores: a. In the process of filling and solidification of molten metal, pores are formed on the surface or inside of the casting due to the intrusion of gas. b. The intrusion of volatilized gas from paint. c The gas content of the alloy liquid is too high and precipitates during solidification. When the gas in the cavity, the gas volatilized by the coating, and the gas precipitated by the solidification of the alloy, when the mold exhaust is poor, the pores formed in the casting are finally left. Causes of shrinkage cavity: a. In the process of liquid metal solidification, shrinkage cavity occurs due to the reduction in volume or the failure of the final solidified part to be fed by the liquid metal. b. Castings with uneven thickness or local overheating of castings cause a certain part to solidify slowly, and the surface will form a recess when the volume shrinks. Due to the existence of pores and shrinkage holes, the holes may enter water during surface treatment of die castings. When painting and electroplating are carried out after baking, the gas in the holes will be heated and expanded; or the water in the holes will become steam and expand in volume. This results in blistering on the surface of the casting. 2. Intergranular corrosion: harmful impurities in zinc alloy components: lead, cadmium, and tin will gather at the grain boundary to cause intergranular corrosion. The metal matrix is u200bu200bbroken due to intergranular corrosion, and electroplating accelerates this scourge. The intermittently corroded parts will expand and lift up the coating, causing blistering on the surface of the casting. Especially in a humid environment, intergranular corrosion can cause the casting to deform, crack, or even break. 3. Crack caused: water pattern, cold barrier pattern, hot crack. Water pattern and cold barrier pattern: During the filling process, the molten metal that enters first contacts the mold wall and solidifies prematurely, and then enters the molten metal and cannot be fused with the solidified metal layer. Moiré patterns are formed at the butt joints on the surface of the casting. Strip defects appear, see Figure 2. The water mark is generally in the shallow layer on the surface of the casting; while the cold barrier may penetrate into the inside of the casting. Hot cracking: a. When the thickness of the casting is uneven, stress is generated during solidification; b is ejected prematurely, and the metal strength is not enough; c, the force is uneven when ejected; d is too high mold temperature to make the grain coarse; e harmful impurities exist. When there are water marks, cold barrier marks, and hot cracks in the die casting, the solution will penetrate into the cracks during electroplating, and it will be converted into vapor during baking, and the pressure will lift the electroplated layer to form blisters. The key to solving the defects is to reduce the amount of gas mixed into the casting. The ideal metal flow should continue to accelerate from the nozzle through the split cone and runner into the cavity to form a smooth and consistent metal flow. The cone is used. The shape of the runner is designed, that is, the flow should be gradually reduced from the nozzle to the gate at an accelerating rate, which can achieve this goal. In the filling system, the mixed gas forms pores due to the mixing of turbulent flow and the liquid phase of the metal. From the study of the simulated die-casting process in which the molten metal enters the cavity from the casting system, it is obvious that the sharp transition position and the incremental casting in the runner The cross-sectional area of u200bu200bthe channel will cause the molten metal flow to appear turbulent and entrain gas, and the stable molten metal is conducive to the gas entering the overflow groove and the exhaust groove from the runner and the cavity, and is discharged out of the mold. For shrinkage cavity: make all parts of the die-casting solidification process at the same time as possible to uniformly dissipate heat and solidify at the same time. Reasonable nozzle design, inner gate thickness and position, mold design, mold temperature control and cooling can be used to avoid shrinkage. For the phenomenon of intergranular corrosion: mainly to control the content of harmful impurities in the alloy raw materials, especially lead <0.003%. Pay attention to the impurity elements brought by the waste. For water lines and cold partition lines, increase the mold temperature, increase the speed of the inner gate, or increase the overflow groove in the cold partition area to reduce the appearance of cold partition lines. For hot cracks: do not change the thickness of the die-casting sharply to reduce stress; adjust the relevant die-casting process parameters; reduce the mold temperature. Reasons for blistering of zinc alloy electroplating: The surface treatment before electroplating is the main cause of blistering of zinc alloy. Because zinc alloy is an amphoteric metal, it is easy to react with acids and alkalis. Generally, there are degreasing and pickling before electroplating. Two processes; degreasing is generally an alkaline room temperature degreasing agent, and the pickling solution is generally a mixture of three acids (HCL\H2SO4\HNO3). Degreasing is the main cause of zinc alloy electroplating blistering: general degreasing It is soaked and stirred for a long time with a heated room temperature degreasing agent, and the chemical reaction time is longer. Generally, the pickling time is short, and this foaming phenomenon is not easy to occur. (Because the pickling uses a proportion of strong acid, there are It is very corrosive, so the general zinc alloy does not remove the oil during electroplating, and it can be electroplated with sulfuric acid activation.) The electroplating process of zinc alloy: due to the dielectric potential of zinc alloy metal, zinc alloy cannot be directly plated on the surface For silver, nickel, gold, etc. plating, a thicker intermediate copper layer is required (generally about 5 microns for copper). However, the loose structure of zinc alloy after die-casting will also cause blistering, which is not die-casting. Qualified. Generally, the defective rate of this phenomenon will not be too high, which requires everyone to analyze the process in which the blistering phenomenon occurs.
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