Zinc alloy is widely used in sanitary ware, luggage, shoes and clothing accessories due to its convenient forming, strong plasticity, low cost and high processing efficiency. However, the manufacturing and production of zinc alloy products is a very difficult question. To produce a good product is not only the final plating; the painting must be done well. From material selection to design, every process such as molds is closely related.
1. The influence of zinc alloy material selection
Standard zinc alloy materials contain about 4% aluminum, 0.75% to 1.25% copper, 0.03% to 0.08% manganese, and the remaining 95% is the main component zinc. But sometimes our friends in the hardware factory will not pay attention to buying the materials that are returned to the furnace for many times. There is a kind of material that has too high lead content. (Some friends in the electroplating factory will find that some products are very white after the same pre-treatment, and some are batched. The secondary product is very black, and the release powder is not bleached.) Lead corrodes and breaks the boundary layer between the metal particles. During electroplating, the intercrystalline corrosion part expands and the plating layer is topped to produce blisters, so zinc should be controlled. The alloy composition, especially the harmful impurity lead, prevents the waste material from introducing impurities during the smelting operation.
2. Mold and product design influence
At the beginning of the design of zinc alloy products, it is necessary to consider the mold inlet, slag outlet and exhaust settings. Because the flow path of the workpiece for feeding and slagging is smooth and not entrapped, there is no water mark, no dark bubbles, which directly affects whether the subsequent electroplating is blistered, the qualified feeding and slagging molds are die-casting the workpiece, and the surface is smooth and white. No water stains. 2. Product design should not design a vertical plane as much as possible. It is best to design a small arc surface without affecting the appearance (a arc surface of a few degrees is often invisible to the naked eye but is good for all subsequent processes:
A. Face products, especially large planes, die castings are prone to scars
B. The fullness of the face product after polishing and electroplating is poor
3. Electroplating is water plating. Everyone knows that at the beginning of product mold opening and design, it is necessary to consider whether electroplating will contain water. Try not to design blind holes as much as possible, consider whether the potion is easy to pour water, and whether there is a slot for discharging potion in the combination of the workpiece, and ensure that there is enough clearance. Many finished watermark pits and white are related to this design.
Three, the influence of die casting process
Die casting should avoid blank blistering
(1) To control the generation of pores, the key is to reduce the amount of gas mixed into the die casting. The ideal metal flow velocity should be continuously accelerated. The nozzle enters the cavity through the split cone and runner to form a smooth and consistent metal flow. (So the tapered runner design is adopted, that is, the runner area is gradually reduced from the nozzle to the runner, which can achieve this goal). In the filling system, the mixed gas forms pores due to the mixing of turbulence 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 gating system, it is obvious that the sharp transition and increasing pouring in the runner The cross-sectional area of the channel will cause the molten metal flow to appear turbulent and entrain gas. The stable molten metal is conducive to the gas from the runner and cavity into the overflow groove and exhaust groove, and the air is discharged out of the mold.
(2) For water marks and cold barrier lines, increase the mold temperature, increase the flow rate of the alloy liquid in the inner runner, or increase the overflow groove in the cold barrier area to reduce the appearance of cold barrier lines. Increasing the mold temperature and increasing the flow speed of the alloy liquid in the runner can reduce the water mark.
(3) It is necessary to control not to open the mold prematurely and eject the casting. The mold temperature may be too high when the casting is ejected prematurely. Because there are a large number of bubbles inside the casting, when the molten metal solidification time is not enough, the strength is not established, and the mold is opened prematurely to eject the casting, the pressure bubble expands and the surface of the casting is raised and hot cracks occur.
(4) Mold loading must consider the tonnage and pressure of the mold loading machine. We personally experienced a 20-30 blistering event after zinc alloy electroplating. A friend from a hardware factory received a large order of several million, and 8 molds were opened. The pressure was insufficient and the exhaust was poor. No matter how the pre-plating treatment was solved, there would always be 20-30% blistering. Finally, 4 molds were blocked. Change to a mold to produce 4 pieces, and then plating will not cause any bubbles.
Fourth, the influence of the machining and polishing of the blank
Zinc alloy die-casting products have dense and smooth surface structure and low porosity, but the thickness of the surface dense layer is relatively thin, generally only 0.05~0.2mm. The inner layer is porous and loose structure. If we do not properly grasp the subsequent processing procedures, after damaging the dense layer on the surface, it will make the subsequent electroplating procedures difficult to do. Normally we are through two processes
1. Sanding, hemp wheel, cloth wheel light, here it is necessary to pay attention to the sand line not to be too thick, do not finish sanding the dense layer of the surface, and the polishing paste can not accumulate too much, which will cause difficulty in dewaxing after electroplating (most of the wax removal With chemical agents, in principle, the longer the wax is removed, the greater the risk of product quality). In addition, the product will flow to electroplating as quickly as possible after polishing. The longer the time, the harder the polishing wax will harden and the greater the processing difficulty
2. The surface of the rolled-polished workpiece is bright; many employees in the pickling process of the electroplating plant just pick it up casually, leading to the surface of the roller polishing agent is not cleaned, often blistering. In addition, the tumbler used in the tumbler polishing plant has a great relationship, and some surfactants in the tumbler polish are extremely difficult to wash off.
Five, the influence of electroplating process
1. Zinc is an amphoteric metal, the electrode potential is relatively negative, and it is more sensitive to acid and alkali, and is prone to chemical reactions leading to corrosion. A slight negligence in the pretreatment degreasing and activation process will cause the aluminum-rich phase or the zinc-rich phase to be preferentially dissolved, and uneven surface corrosion will cause pitting and blistering, which will affect the product quality. Therefore, the degreasing should not be too alkaline. Strong, such as sodium hydroxide and other strong bases should be controlled, and strong acid should not be used for activation
2. Before the product enters alkali copper (commonly known as copper bottom by hardware friends), there is still an oxide film (pickled film) on the surface of the workpiece before the plating tank. Therefore, it is very important to remove the film. It must be a copper-plated tank with a clean and fresh zinc surface. In the early years, the anti-staining salt can be used to remove the black wax. The environmental protection does not allow the discharge of anti-staining wastewater containing benzene rings. Membrane powder, instead of anti-dyeing salt, the effect is better than that of anti-dyeing salt, effectively solving the problem of blistering and pitting of smooth products, and COD emission meets international standards
3. Alkali copper plating tank has many organics, impurities, free cyanide is not in the range, test the composition of alkali copper tank to see if sodium cyanide is too low or sodium hydroxide is too high! If you add brightener, brightener is too high, alkali copper tank Cleanliness is very important. It is recommended to treat carbon every 3-5 days
4. The conductivity of the alkaline copper cylinder is also very important. Whether the anode dissolves normally and whether the anode copper plate is sufficient. This is very important. Some friends in the electroplating factory do not know this. The material can be dragged and added. Following this, the gains outweigh the losses.