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 todays 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
todays 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 cleaners
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 filters
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.