George, thanks for the tips!
And great looking chrome parts! I will have to do some chrome myself, but that’s further down the road on this project.
On to my issue, regarding your points:
1. Anode-cathode separation: the part being plated is 4.5” from each anode. (9” dia. bucket, part placed in the middle). This is per Caswells instructions, range 3-9”, and using Caswells supplied bucket (also see my “discoveries” further down).
2. Agitation: I’m using a submersible, magnetic rotor pump. It’s rated at 2.5 gpm. In my 1.5 gal bath, it produces a lot of agitation (possibly too much). It’s clearly keeping the part, and anodes, bubble-free.
3) Fizzing: If I see NO fizzing, I get a very poor plate. W/slight fizzing (barely noticeable), I get a so-so plate. W/some fizzing, I get a decent plate, but still not satisfied w/overall appearance. There is some unevenness of texture & reflectivity. W/strong fizzing, the part burns.
All these states of “fizzing” are directly proportional to current & voltage.
4) CC/CV power supply: perhaps these things are too “smart” for this application.
If I set it for 1.5v CV, and let current do what it will, then current comes in way too high, @ 2-2.5 amps for 12 sq-in piece = @200mA/sq-in. The part burns. If I crank current control to begin limiting, voltage drops accordingly.
If I set it for 300mA CC, and let voltage do what it will, then it comes in way too low, @ 0.2-0.25v. The part plates poorly.
Both of these parameters are entirely consistent w/ohms law, V=IR. Given a fixed resistance (the electrolyte is actually variable, but still w/in a limited range), if you increase voltage, then current MUST increase. Conversely, if you decrease current, voltage MUST decrease too.
And that’s the problem. There is NO way to get BOTH voltage and current to meet Caswells specification, EXCEPT by changing the resistance in the electrolyte.
In further experimenting I’ve done since my original post, based upon tips and comments provided by users on other forums, I’ve made a few discoveries that are useful. The tips I received from other forums were to IGNORE voltage, and just set the current density as needed. To a man, they all said use 100mA/sq-in minimum, and this is in keeping w/info I have found at commercial & industrial plating system suppliers.
1. Anode-cathode separation: during one test, I accidentally knocked one anode out of position ( it moved closer to the cathode), and noticed a corresponding voltage drop. So I fiddled w/the positioning, and found that by moving both anodes to one side of the tank, and the cathode to the other (@ 8” separation). I gained @ 35% in voltage! So I ran the next few tests in this configuration (increase was 0.4 -> 0.55v in one test, 1.0 -> 1.4v in another test), rotating the part every few minutes to get full coverage. The results were better, but still not optimal IMO. I’m trying to find a long/skinny rectangular tank to further experiment with.
2. Electrolyte resistance: In 9 separate tests at different current densities, I noted that electrolyte resistance (as calculated from measured V & I), was inversely proportional to voltage and current. (resistance was higher for lower voltage/current & vice-versa).
see tables & pictures at: <
http://www.hogheaven.com/hobby/plati...c/cctest1.html>
I had noted this non-linearity before, but had not quantified it until these tests. I have also noted that electrolyte resistance varies w/total immersed area, for any given current density. I have not yet quantified that data. Hope to do so one day.
With this variability in electrolyte resistance, it makes it very difficult to “calculate” needed VI using only V=IR. I am looking for, but have not yet found, a simple explanation of this relationship. I have found a few sites with higher math equations, but they involve derivatives and integrals and such which are beyond my simple mind.
3. current density: I tested following the other tips I received, and ran tests at current densities from 25-125 mA/sq-in. (results at the above link). I didn’t quite “ignore” voltage, but simply let it come in at whatever it wanted to be, and recorded that data. I got the best results at 75mA/sq-in., but since this also produced a higher voltage, it’s impossible to say which is the controlling factor.
4. zinc brightener: the first 5 pieces I ran in electrolyte w/o brightener (90% of my needs are for un-brightened parts). While the higher current density pieces did plate better than prior tests, I still didn’t quite like the way they looked, and wanted to see the results w/brightener. So I ran 4 more pieces w/brightener added.
Surprise! Voltage at EVERY current density was UP! I had not anticipated this, but it does point out that electrolyte conductivity CAN be manipulated chemically. So I still have some hope of finding the magic concoction that will allow me to do so.
5. electrolyte temperature: I should have anticipated this, but simply forgot about it while planning more tests. In all my prior testing, ambient air temps were at 90-95º, and electrolyte solution usually 80-85º. Well for the last week it has been cold & rainy here, w/overnight temp @55º, and daytime @ 65º. The first test I ran I notice higher voltage than before, and realized that temp was different. Measured bath temp was 65º! So I have decided to keep the electrolyte as cool as possible from now on.
With all these “discoveries”, I am able to plate much better now, but still not convinced that it’s the best it can be, particularly the brightened parts.
Unfortunately, without any technical rebuttal or other explanation so far, I still have to conclude that either:
a. Caswells specified electrical operating parameters are way off base, or
b. the supplied electrolyte is completely out of balance to allow achieving these parameters
Coming from an electronics background, I still have to believe that BOTH voltage and current play an important role (I can’t just ignore voltage), and they must be within their specified operating ranges to get the best results.
So, I will continue to experiment and see what happens.
Sean