Chrome Plating a Ball
Q. I have a problem related with chroming a 40" ball. We put in 2mt X 2mt x 4mtr deep tank. The anodes are straight kept along four sides in straight fashion. The solution is good. There are about 22 long anodes of 2'dia. The fixturing is from ball stem about 150 mm dia with thich copper plates. The total current from rectifier is about 4000 amps @ of 1amp/sq. Temp of bath is 50 centigrade. We have a problem of coverage where by the bottom portion of ball small patch does not get chromed. Can you suggest the reasons?
A. Without looking at a photo or drawing of the ball, I'll have to assume that you are chrome plating somewhere in the neighborhood of 5,000 square inches of surface area (+/- 500 square inches)... if this is correct, you are chrome plating at less than 1 amp per square inch... This low current density would be extremely low in any circumstances, whether or not the bath was in good shape. Equally important in this matter, "current density" is directly influenced by the relationship (spacing) of the part surface to the anode surface.
With a straight anodes the majority of the DC current would be applied where the anode(s) and part are closest, and current would fade where the anode(s) and part are the furthest from each other.
A proper fitting anode (conforming) could be designed and built which would eliminate the low current areas and cover the part surface area equally. Such an anode would be a big help in this case but may not be practical, unless you have enough of a part quantity to make it worth the cost and effort.
A slightly "trickier" but much better solution might be to fixture the part in such a way that the ball can be rotated one or more times during the plating cycle so all surface areas have an equal opportunity to communicate with the anode...
A third suggestion would have been to lower the bath temperature slightly to increase throwing power but you are already quite low at 122F (50 centigrade).
If you need further assistance with rotating the part during plating, please let me know.
Q2. Thanks for your reply with all the possible combinations. You have really understood my problem. I tried this rotating method but it seems to disturb chroming in terms of current. Do you have any fixture mechanism whereby job can be rotated and the current not disturbed.
A2. Again I'm at a slight disadvantage without seeing the part, or knowing the weight and or seeing the plating set up, but the point I was making is that "motion" plating is a very efficient way to chrome plate, although almost no one does it anymore because it's very difficult and frankly there are better ways to chrome plate today using conforming anodes and careful spacing.
What I had in mind in my earlier email was several 10-20 degree movements of the ball early in the plating cycle, maybe 2-3 times every 10-15 minutes preceded first by cutting the voltage down to 2 volts, then after movement to a new angle, with part secured, resume plating by stepping up the voltage incrementally until your back at full plating speed. With care, you won't affect the chrome deposit and you would improve the part to anode coverage by a modest percentage.
It may be impractical, even impossible to move the part on a rigid fixture. An articulating or hinged fixture may be a better approach. Of course this is tricky and you must avoid contacting the plating surface with tooling or anodes.
The more I think about your situation, the more I think you would be money ahead to have an upper and lower lead cage made, maybe with a copper core design to support the weight and easily carry the high current. Such a cage would have a series of horizontal bars hanging from vertical bars/ hooks and spaced so that your existing vertical hanging lead rod side anodes could pass thru. The lower horizontal lead cage would address the bottom of the ball and could remain in the tank with all the vertical side lead anode bars.
After the part was placed into the tank surrounded on the sides and bottom with lead anodes, a series of upper lead crossbars could be lowered into position to cover the top of the ball. Now sides as well as top & bottom have anodes to communicate with. This would not require part movement. It would also lower the overall voltage because the top and bottom would not have to rob current from the side anodes. The entire cage would plate at a more even rate.
A final tip using the cage, If you made the upper and lower lead cages with their own separate DC (+) connections, you could switch off some or all of the long vertical side anodes and finish the plating cycle using only the upper and lower crossbar cages... concentrating all your power to the low current areas...?
When I'm not giving advice on the "ask the expert" column, my company designs and builds anodes and specialized tooling for hard chrome. If you still have questions, send me a drawing or some photos of the part and your fixtures, rack and hangers and I'll try to assist you further.