About zinc alloy electroplating

May. 05, 2022   |   153 views

Solid zinc alloy electroplating is no longer a new electroplating process.Zinc aluminum die casting is a kind of die casting parts with zinc as the main component.There is a very dense surface layer on the surface of this part, and inside it is an evacuated porous structure and active amphoteric metal.Therefore, only by adopting appropriate pretreatment methods and electroplating process can we ensure that the electroplating layer on zinc alloy has good adhesion and meet the requirements of qualified products.

To do a good job in zinc alloy electroplating, we must control every link. With the rapid development of China’s economy, the state has gradually strengthened the remediation of environmental pollution. As a electroplating industry with high energy consumption and high pollution, it is facing the continuous increase of production costs. Therefore, the development and improvement of new technologies and processes is particularly important, which requires the joint efforts of our electroplating people.

1. The majority of zinc alloy products are die-casting. Due to the uneven temperature during cooling after die-casting, the surface is prone to segregation. The die-casting parts must be heat baked (or stress relieved). After high-temperature baking, check whether the products have bubbling. If so, they cannot be transferred to polishing or electroplating process.The hot and high-temperature baking treatment requires that the workpiece should be kept in an oven at 150 ℃ for more than 30 minutes, and it is qualified if there is no bubble. Our blank die-casting factory often fails to do this, but this baking has become a necessary process in foreign countries. In this way, it can first eliminate whether the blank is wrapped with air and has dark bubbles, so as to prevent the hidden danger of foaming after electroplating or electrophoresis and then scrapping, resulting in heavy losses.

2. Die castings often have the characteristics of dense outer surface and loose inner wall, so the design of die-casting die and die-casting process are the key.Product design should consider the following sequence: whether it is good to polish, whether it is good to hang, whether the cylinder will fall off during electroplating, and whether there will be liquid medicine hidden in the blind hole?In addition, for plane workpieces, if the assembly does not require a 90 degree vertical plane, design a micro arc surface with a few degrees as small as possible, and do not make a 90 degree vertical plane (the micro arc surface with a few degrees is still a plane in sensory sense, but this little change can greatly improve the quality of subsequent polishing, electroplating, packaging, etc.).The mold, die-casting is not well controlled, there will be: dark bubbles, casting dissatisfaction, water stains, cracks, shrinkage cavities, pits, spots and other surface defects, and one-sided emphasis on requiring polishing workers to polish these defects is just a kind of cover up joke, and the result is life and money.

3. Zinc alloy die castings (zznal) are composed of zinc, aluminum, copper and magnesium, of which zinc is 95% ~ 96%, aluminum is 3.5% ~ 4.3%, and the rest are copper and magnesium. Because zinc and aluminum are very active amphoteric metals with poor chemical stability, they are easy to corrode and oxidize in the air, so zinc alloy product blank die casting should not be stored for too long.In addition, before electroplating, the product cannot be parked for more than 12 hours after touching water. If it touches water or rain, it must be electroplated immediately. If it cannot be electroplated immediately, it must be dried and polished again, otherwise the product will have pitting and blistering after plating.

4. A good release agent must be selected for zinc and aluminum die-casting parts, and heavy oil such as industrial grease and vaseline must not be used as release agent, otherwise the inner cavity of parts or places where grinding is not in place will not be plated due to unclean degreasing;In addition, the water mill brightener selected during water mill polishing is also very important. If it is not selected properly, a layer of organic film that is difficult to remove will be formed on the surface after polishing. This film will “bubble” if it is not removed, and it must be soaked in hot concentrated sulfuric acid to remove the wax, and then use film removal powder to completely remove this film.

5. Due to the complex structure of die-casting parts, many parts have screw holes and blind holes. This kind of parts are easy to produce burrs after plating. The best way is to plug the eyes with corresponding screws. It is easier to clean the blind holes without screws with ultrasonic, and it will not pollute the plating bath with liquid medicine.

6. For complex shapes, special attention should be paid to “exhaust” during hanging, especially for zinc alloy parts. Due to poor exhaust, the pre plated copper layer cannot be plated, resulting in burrs due to corrosion in acid copper.

7. Generally, zinc alloy parts with high quality requirements are mostly directly hung up, dewaxed and degreased by thermal de ultrasonic, and then activated into pre plating after cathodic electrolytic degreasing. It should be noted that cathodic electrolytic degreasing should adopt circular filtration, and an appropriate amount of metal complexing agent must be added to prevent metal from depositing on the surface of parts and causing flower.The acid concentration in the activation tank should not be too high, and the parts should maintain their natural color. If possible, activated carbon can be used for circulating filtration. There are slight bubbles during activation, so it is not easy to produce pitting.

8. For zinc alloy parts, it is best to conduct 0.5 ~ 1.0 min impact plating in high cyanide and low copper cyanide pre plating copper, and then thicken in low cyanide and high copper cyanide pre plating copper. Due to the decomposition of sodium cyanide, free alkali will be produced, especially to prevent the pH from being too high.Reaction formula: N aCN +H O =N aO H +H CN ↑

.9. Selection of alkali copper power supply: the key to the success of zinc alloy parts electroplating lies in alkali copper.While controlling the process, we should pay more attention to alkali copper power supply.According to Mr. Yuan Shipu, an old expert in electroplating industry, cyanide pre plating copper adopts the power supply with high ripple coefficient and gap or periodic commutation, which has more detailed copper layer and larger semi bright range than the low ripple DC power supply that people usually pursue, and can greatly improve the anti impurity ability.