Question: In the course of cost optimization, we have been turning over every stone in the company several times for some time now. Your reports - such as the energy-saving series - have already enabled us to achieve a lot. Now we have arrived at the topic of "layer thickness distribution". Apart from small optimizations to the electrolyte, we no longer see any great potential for drum products. The situation is different with our rack systems. Although we are familiar with the basic principles, there is disagreement as to which measures are effective and which are not worth the investment. Can you please give us some general pointers here?
Answer: The issue of coating thickness distribution (macroscattering) is very complex and must be considered on a case-by-case basis. There are process technologies in which this can be optimized very well, for example in strip electroplating or the pad process - especially using robot technology.
Different techniques have become established for rack goods, some of which are more and some less complex and may cause further problems. Your concerns regarding effectiveness could be extended to process problems, as there is always the risk of a process being optimized to the point of failure. This can take on very different dimensions and, as mentioned, varies from case to case. The most common problems occur when the proven composition of the electrolytes is "tinkered with", resulting in a deterioration of properties that are not immediately noticeable. Properties such as opacity, gloss level or roughness can be determined quickly, while hardness, coefficient of friction or corrosion resistance must be checked more closely.
This means that all of the following methods are theoretical in nature. They can work perfectly in your systems, but they can also cause new problems.
Suspension and positioning
When suspending parts from a plating rack position, there are a few principles to follow in order to achieve optimum metal deposition results:
- Equal height position: ensure that all parts are the same relative distance from the surface of the electrolyte. This applies in particular to hooks on which larger parts are attached to the product carrier.
- Secure hold: Ensure that the parts are securely fastened to the rack position to prevent movement, deformation or current interruption during the electroplating process.
- No contact: Ensure that no parts lie directly on top of each other or touch each other, as this will lead to unwanted contact between the parts.
- Equal alignment: Ensure that all parts are aligned in the same direction to achieve a uniform coating thickness distribution.
- Equal spacing: Ensure that the parts are the same distance apart.
Shields and screens
As more current naturally flows on the outside of the racks than on the inside, you can equip the racks with shields. In most cases, this can be achieved relatively easily using plastic plates. Depending on the panel and construction, these can deform over time, so they need to be maintained regularly to achieve a uniform result.
Bogies
Depending on the part geometry, so-called bogies can be used. They are mainly used for cylinders. The ends are often also shielded. Rocker bogies are a modified version. Here the parts are only rotated back and forth slightly.
Anode distribution
A common cause of poor layer thickness distribution is anode distribution. For example, plates that come off unevenly on the anode rod and are checked less frequently. This problem is also known to occur with filled baskets that are not evenly or insufficiently filled in places.
First ensure that the anodes are filled regularly and cleaned if necessary. It is not uncommon for dirt to accumulate at the bottom of anode bags and cause the anodes to fade. As with the cathode, further optimization can be achieved by targeted blanking. As a rule, anodes are arranged in such a way that there is more in the middle than on the outside.
Internal anodes
The use of internal anodes is particularly well known in the hard chrome sector. The presence of an internal anode can significantly improve the layer thickness distribution on the surface of the workpiece. The expense is generally only worthwhile in extreme applications, such as the hard chrome plating described above. In other processes, it is only used if the geometry of the part does not allow for sufficient spreading and all other options have been exhausted, for example in the case of pipes or deep blind holes.
Electrolyte selection
In general, you should opt for electrolytes that are known for their good throwing power. Depending on the metal, quite different electrolytes are sometimes used, but alkaline electrolytes and/or electrolytes with a high proportion of complexing agents are generally used. The better the throwing power of an electrolyte, the better the metal distribution over the entire electroplated surface. As the amount of metal deposited depends on the current density according to Faraday's 1st law, the scattering ability depends on the current density distribution. A distinction is made between primary and secondary current density distribution.
The primary current density distribution is related to the geometric shape of a part. The secondary current density distribution is more important than the primary one in electroplating. It is caused by the usually very complicated polarization processes, which redistribute the current and generally make the layer thickness distribution more uniform.
Generally speaking, polarizing measures can be helpful in achieving a homogeneous layer thickness distribution. However, it is important that these measures are carefully planned and executed, as they could also have side effects, especially in alloy depositions where polarizing measures have a more positive effect on the more negative potential.
Another side effect of polarization can be a higher hydrogen evolution in the high current density range. This can lead to burns, as is well known in nickel plating. There is also a risk of hydrogen embrittlement in hardened steels.
Flow technology
Intelligent flow technology, i.e. convection, can also improve the layer thickness distribution. To achieve this, the parts in the middle of the rack must be subjected to a better flow than those on the outside. However, care must be taken to ensure that the flow is not too strong, as otherwise lighter parts may detach from the frame. With some processes, the appearance can also change significantly. For example, a higher flow can make the parts in the middle of the rack much shinier or duller.
Temperature
Regulate the temperature of the electrolyte to optimize the metal deposition process. A temperature that is too high or too low can lead to the formation of an uneven coating. Uneven temperature distribution in the electrolyte is also a disadvantage. For hot processes and solid parts, it is often necessary to preheat the parts in hot water before the electrolyte so that the temperature in the electrolyte does not drop.
Addition of additives
Add certain substances (additives) to the electrolyte that can improve the metal deposition process. These additives can control the metal distribution very well, e.g. by having an inhibiting effect in the high current density range. At this point at the latest, however, you should definitely consult the supplier of the electrolyte.
Literature
[1] Practical electroplating; T.W. Jelinek; 7th edition; Eugen G. Leuze Verlag; ISBN 978-3-87480-277-2
[2] Comparison of the properties of various materials; Prof. Dr. Dr. Günther Hartwig; 1st edition; Eugen G. Leuze Verlag; ISBN 978-3-87480-243-7
[3] Solving environmental problems through surface technology; Peter Winkel; 1st edition; Eugen G. Leuze Verlag; ISBN 978-3-87480-236-9
[4] Die galvanische Verchromung; G.A. Lausmann and J.N. Unruh; 2nd edition; Eugen G. Leuze Verlag; ISBN 3-87480-216-7
