Why can rack plating be done well but barrel plating is difficult to do well under the same conditions?
author: YITECH
2025-02-26
Many people must have had such an experience. The same plating solution and the same plated parts can be plated well by rack plating, but barrel plating is not well or is difficult to do well. Why? In fact, this is inseparable from the particularity of barrel plating.
1. The influence of the mixing cycle
Barrel plating will inevitably be affected by the mixing cycle of parts.
The mixing cycle refers to the time required for the parts to turn from the inner layer to the surface layer and then from the surface layer back to the inner layer during barrel plating. The rack-plated parts are packaged separately and there is no influence of the mixing cycle.
Therefore, for some cases where the substrate of the plated parts and the plating solution are "incompatible", rack plating generally has no problem, while barrel plating often does not plate well or is difficult to do well.
It can be said that the mixing cycle is the main factor affecting the coating quality or plating difficulty of barrel and rack plating.
For example, NdFeB material has strong chemical activity. If a simple salt plating solution (such as acid zinc plating, sulfate nickel plating, etc.) is used for plating, there are risks such as substrate surface oxidation (which affects the bonding strength), substrate corrosion, and plating solution pollution.
This is a situation where the substrate and the plating solution are "antagonistic". Despite this, there are basically no obstacles to NdFeB rack plating. After the electroplating starts, the current is directly and unobstructedly applied to the plated part. The plating speed is greater than the surface oxidation speed, and the coating quality is guaranteed.
It can be said that NdFeB rack plating is not much different from ordinary parts rack plating.
But this is not the case with NdFeB barrel plating. Using a simple salt plating solution, due to the existence of a mixing cycle, when the part is located in the inner layer, the electrochemical reaction basically stops and the substrate surface oxidizes.
Therefore, in order to ensure the quality of the coating, NdFeB barrel plating will take a series of measures, such as (when pre-plating or direct plating) try to use solutions with fast plating speed, strictly control the size of the barrel, increase the water permeability of the barrel, uninterrupted operation between processes, and high current shock, which correspondingly increases the difficulty of NdFeB barrel plating.
It is also the same reason that it is more difficult to directly plate cyanide-free alkaline copper on steel or zinc alloy parts than to plate them on the rack. That is, barrel plating is more likely to cause "displacement plating" than rack plating due to the existence of mixed cycles, and there is a greater risk of bonding between the coating and the substrate.
Therefore, when barrel plating cyanide-free alkaline copper on steel or zinc alloy parts, it is best to plate the bottom nickel first, not too thick, a thin layer is enough, and the difficulty is greatly reduced. Rack plating (may) not be required.
Barrel plating of acid copper is difficult and is not widely used in production. The reason may not be the acid copper process itself, but mainly because it has high requirements for the bottom plating layer. If the bottom plating layer is not done well, it will have a greater impact on the subsequent copper plating.
Roll plating of acid copper requires that the bottom plating layer must reach a certain thickness, uniformity, depth, etc., so as to play a good "isolation" role between the substrate and the acid copper solution.
Otherwise, when the parts are located in the inner layer during the roll plating process, the substrate may be subject to a greater risk of corrosion from the acid copper solution because the electrochemical reaction is basically stopped.
Roll plating of acid copper is also a typical example that is greatly affected by the mixing cycle. However, rack plating of acid copper basically does not make deliberate requirements for the bottom plating layer.
NdFeB roll plating of nickel-copper-nickel also has such a problem.
NdFeB roll plating of bottom nickel is also to "isolate" the substrate from the subsequent coke copper solution, otherwise, due to the strong chemical activity of the NdFeB substrate, it will inevitably suffer from oxidative corrosion of the coke copper solution.
Therefore, the bottom nickel layer of NdFeB also requires a certain thickness, uniformity, depth, etc. However, if the bottom nickel layer is thick, it will affect the thermal demagnetization of the magnet, which is a contradiction. You can consider plating coke copper twice or using a better-performing cyanide-free alkali copper process. It is difficult to improve product quality by sticking to the rules.
A small part with a high zinc content is plated with bright nickel without base copper. Hanging plating can obtain a qualified coating, while barrel plating will result in a black coating, and a certain amount of bubbles will be generated on the surface of the part during the barrel plating process.
This is because the surface of the part is affected by the mixing cycle during barrel plating, and the surface of the part is corroded by the bright nickel solution, which affects the quality of the coating.
During rack plating, the parts are separately packaged and hung, and the mixing cycle can be regarded as zero. Then, after the current on the surface of the part is turned on, it will be plated in the shortest time. At this time, the corrosion degree of the part is the smallest, so a qualified coating can be obtained.
2. Influence of current density
Because barrel plating is restricted by instantaneous current density, the upper limit of the given average current density is not easy to increase. Therefore, a current density that is smaller or much smaller than that of rack plating is often used. The low current density brings more troubles to barrel plating than to rack plating.
For example, impurity metals with positive potential in the plating solution (such as copper, lead, etc.) are easy to deposit before the main metal ions when the current is small.
The current used in barrel plating is relatively small, and impurities such as copper and lead are very easy to deposit, thus affecting the quality of the coating. When the potassium chloride barrel zinc plating solution contains lead ions, the coating of the low current area of the part is not bright or unqualified, or only the high current area of the part is bright when it leaves the tank, and it disappears after light or passivation.
When there are copper ions in the plating solution, the brightness of the coating of the part leaving the tank is acceptable, and it turns black after light or passivation. However, under the same circumstances, the impact of rack plating of copper, lead and other ions will be much smaller.
This is because the current used in rack plating is relatively large. At this time, the coating deposition is mainly zinc with a more negative potential, while the probability of copper, lead and other ions with a more positive potential depositing at the cathode is reduced, which reduces the impact on the coating. In addition, since the current of barrel plating is relatively small, the current in the low current area of the part is even smaller, so the deep concave parts of complex parts are often of poor quality or unqualified (that is, the deep plating ability is not good).
For example, there is basically no obstacle to the galvanizing of an oil barrel cover by rack plating, but the black or dark gray coating is obtained in the two "concave" on the barrel surface by barrel plating.
In the case of galvanizing an internal thread expansion bolt, rack plating can coat all the threads, while barrel plating cannot. The difference in current density causes the difference in coating quality between barrel plating and rack plating.
Increasing the current used for barrel plating is the key to reducing this difference. To increase the current used for barrel plating, it is necessary to reduce the instantaneous current density at the hole or increase the upper limit of the instantaneous current density.
3. The influence of the drum opening
The drum opening hinders the conductive ions inside the solution from running to the cathode, resulting in a lower concentration of conductive ions in the drum. Therefore, the coating uniformity of the barrel-plated parts will be worse than that of the rack plating, and it is difficult for general barrel plating to meet the quality requirements of high-precision parts.
In addition, when barrel plating alloys, due to the shielding effect of the drum opening, the plating solution components (especially the alloy ratio) in the solution inside and outside the drum may be quite different, so the plated products may be quite different in terms of coating alloy composition and appearance.
For example, the nickel content in the barrel-plated zinc-nickel alloy coating may not reach the required (or expected) content (the corrosion resistance of the coating is compromised), and the appearance of the barrel-plated imitation gold coating is not good, and sometimes "color correction" treatment is performed.
However, rack-plated alloys rarely have such problems. Improving the drum opening to increase the drum's water permeability is the key to solving or improving the problem.
Therefore, many times, rack plating can be done well while barrel plating may not be done well (or it is difficult to plate well). This requires finding out the reasons for poor or difficult plating according to the situation, and then prescribing the right remedy to solve or improve the problem.
DongGuan YiTai Electronic Technologies Co.,Ltd
DongGuan YiTai Electronic Technologies Co.,Ltd is a manufacturing service provider located in Dongguan, China.
YiTai specialized in CNC turning machining and sheet metal fabrication mainly. As a member of Hung Mou Group, we focus on the overseas marketing development. And based on our parent company’s manufacturing capability and resources, YiTai also expended machining services such as die casting, injection molding, aluminum profile extrusion, 3D printing, which are committed to providing customers with one-stop purchasing services and experience.
CNC MACHINING , CNC MILLING , CNC TURNING , SHEETMETAL , FASTENER , OTHERS
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