Information on the problems and treatment methods of zinc alloy buttons and accessories in the production process.

Recently, the surface of some alloy buttons in the company was smooth before electroplating, but after electroplating, it was found that some alloy buttons had blisters on their surfaces, which seriously affected the appearance and quality of buttons. The master of the company's die casting department thought of many ways to solve this problem. Symptoms of surface defects: there are protruding bubbles on the surface of alloy buttons. Bubbles may appear after die casting, polishing, oil spraying or electroplating. The reasons can be analyzed from the following aspects: 1. Causes of stomata: mainly stomata and contraction mechanism, stomata are often round, while contraction is mostly irregular. (1) Causes of blowholes: A During the solidification process of liquid metal mold filling, blowholes are generated on the surface or inside of the casting due to gas invasion. B the gas volatilized from the paint invades. The gas content of C alloy liquid is too high, and it precipitates during solidification. When the gas in the mold cavity, the gas volatilized from the paint and the gas precipitated from the solidification of the alloy are not discharged smoothly in the mold, they will eventually remain in the pores formed in the casting. (2) Causes of shrinkage cavity: A In the solidification process of molten metal, shrinkage cavity occurs due to volume reduction or the final solidification position cannot be supplemented by molten metal. B castings with uneven thickness or local overheating will cause a certain part to solidify slowly, and when the volume shrinks, a concave position will be formed on the surface. Due to the existence of pores and shrinkage cavities, water may enter the holes of die casting during surface treatment, and the gas in the holes will expand when heated during painting and baking after electroplating. Or the water in the hole will turn into steam and expand in volume, thus foaming the surface of the casting. 2. Causes of intergranular corrosion: harmful impurities in zinc alloy components: lead, cadmium and tin will gather at grain boundaries to cause intergranular corrosion, and the metal matrix will break due to intergranular corrosion. Electroplating will accelerate this scourge, and the parts affected by intergranular corrosion will expand and lift up the coating, resulting in blistering on the casting surface. Especially in humid environment, intergranular corrosion will make castings deform, crack and even break. 3. Causes of cracking: water ripple, cold cracking and hot cracking. Water ripple and cold parting: During the mold filling process, the molten metal first enters the mold wall and solidifies prematurely, and then enters the mold wall and cannot fuse with the solidified metal layer into a whole, forming overlapping ripples at the butt joint of the casting surface, and the water ripple with strip defects generally appears in the shallow layer of the casting surface; However, cold ribs may penetrate into the casting. Thermal crack: when the thickness of casting A is uneven, stress will be generated during solidification; B premature pop-up, metal strength is not enough; C is forced unevenly when it is ejected, and the high die temperature of D makes the grain coarse; There are harmful impurities. All the above factors may cause cracks. When there are waterlines, cold lines and hot cracks in die casting, the solution will penetrate into the cracks during electroplating, and will be converted into steam during baking, and the electroplated layer will be pushed up by air pressure to form bubbles. Of course, when problems arise, we must solve them. What can I do to avoid bubbles after alloy buttons are made into finished products? The key to control blowholes is to reduce the amount of gas mixed into castings. The ideal metal flow should enter the cavity at a constant speed from the nozzle through the shunt cone and the runner, forming a smooth and consistent metal flow. The cone runner design can achieve this goal, that is, the pouring flow rate should be gradually reduced from the nozzle to the inward gate at a uniform speed. In the filling system, the mixed gas forms pores due to turbulent mixing with liquid metal. It can be clearly seen from the study of the simulated die casting process of liquid metal entering the cavity from the gating system that the sharp transition position and the increasing cross-sectional area of the runner will make the liquid metal flow turbulent and entrain gas, and the stable liquid metal will be beneficial for the gas to enter the overflow groove and exhaust groove from the runner and cavity and be discharged from the mold. For shrinkage cavity: all parts during die casting and solidification should be uniformly radiated and solidified at the same time. Shrinkage can be avoided by reasonable nozzle design, 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.