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.
