current density

[fourkbmx]

what is the procedure to calculate the CD for using in the 720 rule, or should i use any number from 3-6, and what is the importance on this number does it makes a difference on the final result or the low CD number is compensated by higher amps?

[luminous]

Any number from 3 to 6 is fine. The main advantage of the higher numbers is speed. To achieve a given coating thickness, a CD of 6 amps will only take half as long as a CD of 3 amps.
If you've got the time, there's nothing wrong with the lower CD. You should get similar results at either end of the range as long as you adjust the *time* accordingly.
According to the 720 rule, for a 1 mil coating a CD of 6A/SqFt gives 720 / 6 = 120 minutes. A CD of 3A/SqFt gives 720 / 3 = 240 minutes. For a coating thickness of 0.75mil you would use 3/4 of the time indicated by the 720 rule.

The main disadvantages of the higher CD's are:
1) In order to push a higher CD, the power supply also needs to have a higher maximum output voltage than is required with lower CD's.
With a CD of 4 amps/SqFt you can probably get away with a power supply that only does 12 volts max. But with a CD of 6A/SqFt you may need 15-16 volts or more to get the desired current. So the maximum VOLTAGE of the power supply limits your maximum CD, and the maximum CURRENT of the supply limits the maximum square footage of your work.

2) Higher CD's put more heat into the ano tank, so you may also need an improved tank cooling system to keep the temps in the "happy zone" and avoid dissolution problems.



steve

[sswee]

Just to reiterate what luminous said.
A CD is chosen for speed (run time) and pore size.
To choose what CD will work best for you, you'll have to consider several things.
The first thing to look at is the PS capability. From Fibergeek's experiments, you'll need 2.5x the CD plus 2V to 5V for a max voltage. I've used this equation since I started and found it to be valid. IE. to run a 6A CD, you would need 15V minimum 18V preferred available from the PS.
The next thing to consider is the tank size and the size of part. The higher the CD or SA, the more heat generated. If the tank cannot absorb and distribute the heat generated by a higher CD or SA, you'll combat dissolution. Tank temp is directly correlated to voltage demand. To push a set amount of current takes more voltage at colder temps than it does at the warmer temps.
What worked for me was to start out at a lower CD and work my way up a little at a time. I made notes on the variables so I could track which changes had what effects. IE. How much a change in CD effected tank temp with the other variables the same, the tank temp effect when CD, SA and other variables were the same. It helps to run similar parts and only make one change at a time so you can tell what is causing what.
Keep at it and it'll all fall into place.
SS