Why you should use a CONSTANT CURRENT power supply

[mcaswell]

I am in the throes of rewriting/updating the manual, and I came across this old article by the late Fibergeek, developer of the LCD Anodizing System.

I felt it was worth posting here, as it could solve many peoples problems. It also pretty much shouts out not to use batter chargers and ordinary 'rectifiers'.

A CC power supply also greatly benefits anodizing. Since the anodize film is an insulator, the changes in electrical characteristics during the process are much larger than in plating. CC operation provides a uniform anodize pore structure all the way down to the base metal. This promotes even and deep dye penetration, and the consistent pore structure provides a better looking and stronger anodize coating than if the pores are distorted by changes in the current. For this reason Caswell Inc. recommends the use of CC for any and all anodizing applications.

[cprocess]

This is good information. Another benefit of using constant current is that it enables you to use the "Rule of 720" to calculate anodizing time to get a specific thickness. This rule states that it takes 720 amp minutes per square foot to produce one mil of oxide coating. In application, divide 720 by your current density (in amps per square foot). This will be the time required to produce 1 mil of oxide coating. For example, if I have 2 square feet of surface area to anodize, rectifier is set on 40 amps (= 20 amps per square foot), I divide 720 by 20 = 36 minutes to apply 1 mil of oxide coating.

[sswee]

If you are anodizing at 20 ASF, you are not anodizing with LCD. It opens up a large can of worms for newbies to try anodizing outside of LCD parameters.
SS

[SylantBill]

I have been using a 12 voly 50 amp car charger and having different results each time. I can't figure the 720 rule because a lot of my parts are screws and nuts and pieces under 1 sq inch. How does someone figure the 720 rule with these small parts.

[acidrain]

Originally Posted by SylantBill

I have been using a 12 voly 50 amp car charger and having different results each time. I can't figure the 720 rule because a lot of my parts are screws and nuts and pieces under 1 sq inch. How does someone figure the 720 rule with these small parts.

A current controlled (CC) power source is essential to good repeatable results.
Also, if you check the 720 rule calculator, you'll see that (depending on the current density you are using) 12 volts probably isn't enough.
At any rate, you'll need to accurately measure and hold the current (amps) that are going to the part (just let the volts do whatever they need to in order to maintain the amps).

Carefully measure the surface area of 1 part, and multiply by the number of parts in the batch. For smaller runs, it may be beneficial to add a sacrificial part. This will help even out any small measurement errors that are multiplied out by the number of small parts.

For example, you have 20 bolts that are .6 sq. in. each.
.6 x 20 = 12 sq. in.
Convert sq. in. to sq. ft... 12 sq. in. divided by 144 sq. in. (144 sq. in. in a sq. ft.) = .08 sq. ft. (rounded off).
Run these alone at 6 amps per sq. ft. current density = .48 amps for 120 minutes for 1mil.
This is a very small amount of current, and depending on your PS, may be impossible to control. Adding a sacrificial piece will increase the surface area, which will be easier to control:
Add a 6in x 6in x 1/8in piece of aluminum... 6 x 6 = 36. 36 x 2 (both sides) = 72. add the 1/8in edges... 24 x 1/8 = 3.
12 sq. in (20 bolts) + 72 + 3 = 87 sq. in.
87 divided by 144 = .6 sq. ft. (rounded off).
.6 x 6 amps per sq. ft. = 3.6 amps for 120 minutes for 1 mil.
3.6 amps is much easier to control than .48 amps.