Rinsing Blind Holes Prior to Dying

Q. I have problems with leaching from blind holes in black anodized parts. The front plates I am running have side holes and top holes that are mostly M2 and M3 in diameter. I could not find a way to rinse out the sulphuric acid after the anodizing tank just before the dying tank. We are rinsing them in two cold-rinse tanks before the dye by dipping the spline with the front plates about 50 times in each tank. Do you have other suggestions? E.S.

A. Rinsing blind holes prior to dying is a tough and common problem with anodized parts. There are a few techniques that may be used to thoroughly rinse the acid out of blind holes and other recesses, but the practicality of some methods depends on the size of tanks you have in the anodizing line. With smaller tanks in a hand-rack line or a small hoist line, it is usually easier to use special rinsing techniques to get the job done. With large tanks, some of the “hands-on” techniques may be more difficult.

The best way I know of to get all of the acid out of blind holes is to rinse by hand with a water hose and nozzle, like a garden hose. Using the spray nozzle, you can make the effort to individually rinse out all the holes under pressure with clean water. Do this once and then dip the load or rack in the rinse tank. Then rinse with the spray nozzle one more time and go to the dye tank. After this second spray nozzle rinse, do not go back into a rinse tank. The water from the hose is cleaner than anything that is in a rinse tank, and the chances of getting a good dye job without bleed-outs is increased.

Another way to help rinse the acid out of blind holes and other folds and crevices in the parts is to alternate back and forth between a hot-rinse tank and a cold-rinse tank. Of course, you can always use compressed air to blow out each hole or fold, but really clean compressed air is required for this. Otherwise there is a risk of contaminating the parts with oil from the compressed air.