Quality Filtration and Agitation Equals Quality Plating
How to obtain and maintain quality plating using filtration and agitation techniques...
Was there ever really “quality” plating? In some cases, yes. One can find examples of plated automotive bright work and appliance or plumbing parts done in the 1950s and 1960s that still look good today.
Was the plating good because of heavy deposits and a lot of polishing and buffing? Was it due to proprietary solutions or laboratory technicians who sniffed and analyzed the process and were able to make it work right? It may have been that way; however, we also know about failed plating.
Why did the shiny fenders on our bicycles or the bathroom faucets or those nuts, bolts or screws end up rusty? Were buyers who demanded low-priced products that looked good at fault? Could manufacturers “get by” by offering marginal quality because the price was right? Perhaps, but today’s consumers are demanding higher and higher levels of quality.
How can high-quality plating be achieved? Better chemicals? Pure raw materials and anodes? Better proprietary additives? This certainly is a starting point.
Is a more uniform electrolyte or electroless solution the answer? Obviously so. Whatever the requirements, solutions must be maintained day-by-day, hour-by-hour and minute-by-minute. Statistical quality control dictates the need to know the conditions of plating solutions at the precise moment that plating will commence and know that an acceptable condition will be maintained during the entire time the part is in the tank. Platers need to know that the parts per million of insolubles (dirt particles) are maintained at the lowest possible level to ensure quality results.
Those involved in highly sophisticated plating applications, such as computer memory disks, seek the ultimate in quality. They cannot tolerate co-deposition of solids or accumulation of organic impurities. All platers can benefit from understanding their techniques and dedication to clean solutions.
Some have described the various methods of filtration and carbon purification as “necessary evils.” This certainly is not the case with today’s improved equipment. Unattended filtration with minimum media changes is possible with very little solutions loss or labor required.
Gone for the most part are layers of sludge, carbonates and super-saturated brighteners on the bottom of alkaline cyanide zinc tanks or murky solutions of copper, nickel, silver or cadmium. Instead, plating is and can be done in solutions often clean enough to read the denomination of coins on the bottom of the tank.
Rather than address increased solids-holding capacity and increased flow rate, this article will stress the advantages of preventing particles from getting into the plating tank in the first place.
Start with the Cleaner
Special attention to the cleaning cycle is perhaps the best place to start. Even plastic parts that appear to be clean may have silicone mold release on the surface. Therefore, the proper cleaner with vigorous agitation in what was formerly a static tank may be appropriate. Filtering the cleaner with an appropriate coarse media will maximize solids-holding capacity and lengthen the cleaner’s active service life. Should a layer of oil develop, decanting or skimming when the solution is not being agitated can remove it. Additional oil may be removed with coalescing media that will separate the non-dissolved oil from the aqueous cleaner. A pre-filter may be required to keep the coalescing element free of solids.
Subsequent electrocleaning solutions followed by various rinses can also be clarified in this way by adding a chamber of carbon to adsorb oil. Manufacturing process oils should never reach your plating solution. As a final precaution, pre-rinses may require ion exchange to pick up soluble salts. Reverse osmosis may be required when troublesome salts are present in recycled rinse water.
A skimmer on the pump at each tank in the pretreatment cycle will minimize carryover of surface contaminants to the next tank.
Anodes and Air
Filtering the cleaner has probably prevented 50-60% of solids and other impurities from getting into the plating tank. What else can be done to prevent solution contamination? Anode quality, makeup water and chemicals should all be considered. Even the air that passes over the tank to an exhaust vent may be dropping solids. It is also possible that air used for agitation contains insoluble particles that can get into the tank. The air might also carry vapors from other process operations. These can be absorbed into the plating solution with the help of wetting agents.
Pumped/Eductor agitation is another method of agitating the plating solutions that uses high-flow centrifugal pumps to draw solution from the tank and re-deliver it through a sparger system similar to that used for air agitation. Eductors strategically placed along the horizontal pipe direct plating solution across the bottom of a tank or up a cylinder or into difficult-to-plate, low-current-density areas. Each eductor creates, without additional horsepower, up to four times the actual pumped liquid delivered to its orifice.
This agitation method has a number of advantages:
- Eliminates vapors introduced into the plating solution
- Eliminates uncontrolled temperature changes
- Eliminates air bubbles entering the suction line of centrifugal pumps that could cause them to cavitate and lose prime
- Minimizes brightener breakdown due to oxidation
- Eliminates salt crystal formation in the holes of the dispersion piping
With pumped/eductor agitation, the plating solution is totally self-contained, where minimal solids or vapors can get into the solution and temperature is controlled more easily. Now the plating solution should be filtered to remove any particles that slipped by the barriers and add the necessary carbon to remove any brightener breakdown. However, the amount of carbon will be greatly reduced and less usable brightener will be adsorbed.
Reducing Filter Media Cost
These are the steps you can take to reduce filter media consumption:
- Pre-filter as much as possible with preventive barriers (as pointed out previously), plus carbon adsorption, if required, in a separate chamber.
- Use high flow rates with coarsest possible media to achieve maximum dirt-holding capacity. For example: three-micron instead of one-micron or 30-micron instead of 15, but not 100-micron instead of one-micron. Increase filter media so that flow rate per cartridge or square foot will reduce media consumption by 55%. In other words, 12 cartridges instead of three with the same pump will consume 50% less filter media annually (See Table I).
Use a pump and eductors to minimize solids introduction to the bath.
Choosing a Filter
The choice of a filter to achieve the final clarification will depend on a number of factors: how much carryover of particles occurs on the product to be plated; or the amount of insolubles introduced from tainted anodes; the atmosphere; chemicals; or any other source.
Will the particles be slimy as in an alkaline zinc bath, which would blind off the flow through a surface filter media? Or gritty, and therefore, easy to filter from an acid copper tank? Or will they contain precipitated iron from plating steel in an acid or zinc or nickel bath?
A quick evaluation will at least help begin the process. Choose 15-75 micron retention for the slimy zinc or precipitated iron and denser for most other baths. Depth-type filter media provides for this range of particle retention. Otherwise, if surface media is employed, then an extended area must be considered to match the solids-holding capacity to maintain good flow rates.
Note that flow rates across media will somewhat change the percentage or efficiency of retention because of the different levels of velocity per square meter of surface or per cartridge. Increasing the amount of filter media reduces velocity. Reducing velocity across the filter media will pay big dividends by reducing the actual amount of cartridges expended or frequency of surface cleaning.
Extending the life of filter media requires matching particle retention ability of the media to the range of solids present in the liquid. Unfortunately, we usually do not know the percentage of particles of each size, so we must rely on past experience. If necessary, coarser or more dense media can be substituted to achieve the desired results.
Having sufficient solids capacity is the main requirement of a filter, so that the pressure drop across the media is minimal over the time between servicing. This is one factor in favor of depth-type cartridges, because psi drop is usually low over 85% of their life, whereas surface media follows a straight-line increase in pressure drop.
When pressure increases across the media, flow decreases (based on the assumption that virtually all pumps used with plating solution filters are centrifugal). A reduction in flow is critical to the filter’s ability to remove particles from the plating tank, because recirculatory filtration is used on a reservoir (the plating tank) instead of in-line clarification, as might be the case of a filter on incoming water lines.
Recirculation has other benefits. Suppose a certain type of filter media stops most of the solids, but not all. Thereafter, a second, third or fourth pass through the filter may produce the desired result.
For instance, if a filter media with 90% retention efficiency of five-micron particles is used, it also removes some lower percentage of finer particles, perhaps 50% of three-micron particles. If the porosity of the media did not change, you could expect to pick up an additional 50% of the three-micron particles on the second pass, now leaving 25% of what you started with. With constant recirculation, it is possible that essentially all three-micron particles could be retained in the media. But it must be pointed out that this clarification only applies to the solution that passes through the filter, which is why turnover rates are so important.
There is the effect of the increased density caused by the collected particles on the media, which may speed up or increase the percentage of retention. Or their presence may hinder the flow and slow down the turnover rates. This would suggest that too-dense media might have been used.
Filter media with a broad range of porosities lends itself to recirculation applications. Consider the possibility of using coarse media instead of fine or two grades of media on the same tank.
A significant benefit of using less dense media to achieve the desired particle retention is the increased solids-holding capacity offered by coarser media. Compared to fine media, coarse media may provide up to five times the solids retention before flow is reduced to an unacceptable rate. The media is then replaced with coarse media, and recirculation commences until all the liquid is clarified.
Will it work? Yes, it has worked for years: swimming pools, hydraulic and lubricating systems, plating and other types of finishing processes usually do not have a “dirty” tank and a “clean” tank. They rely on continuous recirculation filtration to get the desired results. The difference is that these applications allow for a limited amount of some solids to be present until removed. The presence of solids could not be tolerated in finished products such as soft drinks, food oils and syrups or chemicals, hence the need to either do a good job of filtering the first time or recirculate until the desired clarity is achieved. We are aware of many examples of success with coarse media. For instance, 30-micron cartridges will keep hydraulic oil looking like new, will change a neglected swimming pool from green to clear overnight and turn slimy, oily alkaline zinc solution from milky to clear. It all depends on the number of passes, which dictate the flow rate required.
For instance, a 1,000 galbatch that is transferred at 10 gal/minute will take one hour and 45-min, but to turn the tank over ten times to achieve 100% contact with the filter, a 160 gal/minute pump is required. With one-hour turnover recirculation, a “dirty” tank becomes clean with the solids in the filter.
Take this approach one step further and keep in mind that high quality plating is the number-one objective. Ten turnovers per hour might come close to having the entire solution pass through the filter at least once. You are plating every time parts enter the solutions. Do you need ten times turnover per minute? Probably not, but the original intent was to filter out the particles to achieve high-quality plating. So you do have to consider the turnover rate that will achieve your objective.
If organic decomposition is a problem, then separate carbon treatment is required. Some platers still use powdered carbon, citing a need for fast adsorption of organic impurities either in a batch process or with the carbon coated on the surface of the filter. However, if uniform purification is necessary, gradual, consistent adsorption downstream of the filter works well, offering some significant advantages that contribute to solution clarification and desired levels of plating quality.
A separate carbon chamber allows the filter to achieve maximum solids holding capacity and maintains a low level of organic impurities without the mess of handling the dusty, black powdered carbon. Other benefits include a reduction of manual evaluations in the laboratory and, when troubleshooting is required, it is comforting to know that bath contaminants are not the cause.
Statistical quality control will monitor results attainable from increased filtration and separate carbon purification or indicate the further need to increase the same until the ultimate quality goals are achieved.
Work backwards through the processing sequence to create a filtration program that will provide clean solutions and high quality deposits. Start with the pre-plate cleaning and rinsing steps and look for ways to prevent solids and oils from getting to the plating tank in the first place. Move forward to the plating solutions, recognizing the effect flow rate (turnover) will have on getting the solids to the filter. Consider the benefits of two-stage coarse filter media for the extra solids holding capacity it can provide. Consider “airless” eductor solution agitation. When the program is established and operating smoothly, fewer laboratory personnel will be involved in problem solving. Instead, they will have more time to work on other aspects of your total quality assurance program.