Causes analysis and improving methods of undercasting die-casting products
Posted on : March 22, 2022 By GREFEE
Zinc alloys electroplating is no longer a new knowledge to electroplating technology. Zinc-aluminum die casting parts mainly use zinc for manufacturing, and there is a very compact layer on the surface of this type of component. The inside of it is the porosity structure. Meanwhile, it is also an active amphoteric metal. Using proper treatment methods and electroplating process can ensure the electroplating layer of zinc alloys has good adhesion and make it a qualified product.
It is necessary to strictly control each section of the process to ensure good zinc alloys electroplating. As the domestic economy boosts, China pays much more attention to environmental protection. As a high energy consumption and high pollution industry, its cost is increasing due to the impact of the Covid pandemic. The development of new technology is quite essential, and it is the duty of all electroplating professionals.
1. Most zinc alloys are die casting parts. Due to the inconstant cooling temperature after die casting, it is easy to have segregation on the surface. The castings have to be pre-heated (also can de-stressing) and check if there are any bubbles after baking by high temperature. If there is, then we cannot polish or electroplate. The high-temperature baking treatment is required to practice in the 150℃ oven and maintain the temperature for over half an hour till there are no blisters, which indicates that the part is qualified. Usually, many blank die casting makers fail to see it; however, this has been a necessary procedure to bake in other countries. Because this process not only eliminates the blisters on blank which resulted from air trapping but also prevents the risks of part scrapping due to the blusters after electroplating or electrophoresis.
2. Die casting parts have the characteristics of compact outer layers and loose inner walls, so the design of die casting mold and die casting technique is essential. When designing a product, there are some factors that we need to consider: is it easy to polish and hang up, will the cylinder fall off in electroplating, or will the potion be hidden in the blind holes. Next, the plane workpiece should be designed into a slight cambered surface with a few degrees smaller if it does not assemble into a 90 degrees vertical plane (only a few degrees smaller virtually still looks like a plane, but this is quite beneficial to the follow-up， like polish, electroplating, packaging, etc). However, it is very likely to have surface defects, like blisters, poor die casting, water stains, cracks, shrinkage cavities, pitting, spots, etc. It is only an accusation to deceive themselves by only asking polishers to polish off these defects. The result is not only time wasted but also money.
3. Zinc die casting alloys are consist of zinc, aluminum, copper, and magnesium. Zinc constitutes 95%~96%, and aluminum accounts for 3.5%~4.3%. The remaining are copper and magnesium. Zinc and aluminum are active amphoteric metals with poor chemical stability. They are easy to be oxidized and eroded in the air. So it is not suggested to store the zinc alloys blank for long after die casting. Besides, if the product gets in touch with water or rain before the electroplating, it shall be electroplated immediately, no more than 12 hours at least. If it cannot be electroplated immediately, it needs to dry and be re-polished, or there would be pitting and blistering.
4. Zinc and aluminum die casting parts must choose a good mold release agent. Do not use heavy oil, like vaseline or industrial greases working as a mold release agent, or the base cooper cannot do plating due to the oil in the cavity or places that cannot be polished. Another thing is the selection of a water milling brightener in water milling. Improper selection will lead to an organic layer that is hard to remove on the surface after polishing. This layer could cause bubbles even it is just slightly baking. So, we have to remove the wax by soaking it in concentrated sulfuric acid and then use Lejiang LJ-d009 stripping powder to remove this layer thoroughly.
5.As die casting parts have a complex structure and many screw holes and blind holes, this part is likely to have burrs after plating. The best way to deal with it is to plug the holes with corresponding screws. For those blind holes which do not have screws, we can clean them with ultra-waves, which would not pollute the plating bath due to the potion.
6. For complex-shaped ones, we should pay attention to the venting when hanging up, especially the zinc alloy components. We cannot do plating on pre-plating the copper layer due to poor venting, which would lead to burrs due to the corrosion in acid-cooper.
7. For zinc alloys with higher quality requirements, we would hang up and remove the wax by thermal desorption ultrasound, then remove the oil by cathode electrolysis to make the part ready for the pre-plating after activation. It is worth noticing that oil cleaned by the cathode electrolysis should use circulating filtration with the metal complexing agent to prevent the metal from being deposited on the part surface, causing dimness. The acid concentration in the activation tank should not be too high to keep the part its original color. If it is possible, we can use activated carbon circulating filtration. There are slight bubbles during activation. So, it won’t be pitting.
8. Zinc alloys should be better impact plating by 0.5~1.0 m in cyanide pre-plating copper with high cyanide and low copper and then thicken it in cyanide pre-plating cooper with low cyanide and high cooper. As the decomposition of sodium cyanide will produce free alkali, we should prevent the PH from increasing. Equation： N aCN +H O =N aO H +H CN ↑
9. Selection of the alkali copper power supply:
Whether zinc alloys parts can be electroplated successfully or not depends on the alkali cooper. Thus, we should control the process carefully and pay attention to the alkali cooper power supply. Shipu Yuan, an electroplate expert, introduced that cyanide pre-plating cooper uses a power supply with a high ripple coefficient and gap or periodic commutation. This is more impact than the copper layer is from a low ripple DC power supply since it has a higher semi-bright range and anti-impurity capability.
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