Q. What is the Low Current Density method?
A. It’s a Type II aluminum anodizing method that rely on low current, usually between 3 to 6 Amp per square foot. Giving the low current being applied to the anodizing tank, a smaller power supply can be used and most of the optimal acid tank parameter can easily be kept within range. This method has been pushed up to 12 Amps per square foot with good success with sufficient tank agitation and cooling.

Q. How long should I anodize for?
A. The recommend anodizimg time is 2hr (120min) when using the 720 rules at 6 Amp per square feet, but we had good luck with anodizing time of 1.5hr (90min). A longer anodizing time will build a thicker anodized layer; it will provide better dye uptake with a richer color since the dye sink in easier when the pores are deep. Note that the anodized layer cannot build more than .001″ deep pores, so over anodizing will slowly etch the part.

Q. Can I use tap water for my dye and cleaning tank?
A. It is strongly advised to NOT use tap water anywhere in your anodizing tank. Distilled water is obviously the best but can be expensive in the long run; we use a RO system with great result and recommend it to everyone. 

Q. What’s the difference between Constant Current (C.C.) and Constant Voltage (C.V.) method?
A. As the name implies, constant current is when you use a fixed current (Amp) setting for anodizing, and constant voltage is when using a fixed voltage. We will be exclusively using the Constant Current method for the low current density Type II anodizing that we are doing. Most power supply will let you run either of these mode.

Q. What Acid concentration should I use?
A. A mix of 50/50 distilled water and sulfuric acid bought from an auto part store for car battery refill is recommended. Also remember AAA: Always Add Acid to water. A small amount of water in a high concentration of sulfuric acid can make a reaction; a small amount of cencentrated acid added to water won’t.

Q. How do I know if my acid concentration is good?
A. The best way to check if your acid concentration is on point is to anodize a part that you can easily calculate the surface area (a square or rectangle part). Using the 720 rules you should be really close to 15v if you push the recommended current calculated from your surface area. If the voltage is lower, add acid until you get 15v (making the tank concentration more conductive). On the other end if your voltage is really high, add water until you get closer to 15vdc. Acid hardly evaporate, so once you get your ratio good, just top off your tank with water every now and then.

Q. What cathode material and size should I use?
A. Both aluminum and lead can be used for the cathode, but not all aluminum cathode are equal. A 6061T6 aluminum cathode will get etched away by the acid over time and you end up with a lot of dissolved aluminum in the tank, which end up skewing the conductivity of the tank. You also need to take out the 6061T6 aluminum cathode out of the tank between each batch. A much better approach is to use 6063T6 aluminum; it won’t get dissolved and can stay in the acid tank. A lead sheet can also stay in your tank between batch, but it will also slowly add lead waste in your tank which can be problematic in some case. We personally use a 1/16″ thick 12″X36″ lead sheet in our tank.

As for sizing, the rule of thumb is 3:1 anode to cathode ratio. If you have a 20 amp power supply able to anodize 480 square inches at 6 Amp per square feet, you need a cathode of about 160 square inches. A lot of ratio and combination WILL work, but we are always trying to get the best result and consistency.

Q. My color are dull and / or leaching out in the sealer tank
A. If color is leaching out of the part the anodic layer is not able to fully absorb dye. This is usually caused by a bad acid concentration, insufficient tank agitation causing hot-spot or bad temperature control.

When the ratio is too high the parts get etched faster than the anodized layer is getting build up. Common result are dull looking color (red turns pink or really pale, black looks gray and have no shine in it) and the parts looks burnt.

If you have insufficient tank agitation the anodizing process will create hot spot around the parts and these will show up as dull and flat spot on the part. 

Improper cooling of the tank can also yield the same result; hot acid etch the parts faster than it’s able to build the anodic layer.  Always try to keep your acid tank between 20~22*C (68~72*F) from start to finish. Frozen water bottle can be used to keep the acid tank cool, keep a couple handy and get to learn how many bottle are needed for an entire batch at different Amp level. More Amp pushed into the tank will always build up more heat.