Dying Anodized Parts With Small Holes


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Q. We send our 6061 aluminum bars to an outside anodizer for Type II anodize and black dye. These parts have several very small diameter, blind holes, both threaded and non-threaded. When we get these parts back from the anodizer, the tapped holes will have bleed-out areas of white streaks, and sometimes the holes are filled with a white powder. The anodizer says this is caused by machining lubricant and chips left in the tapped holes. Our shop and quality people insist the parts go out clean.


We’re not sure what to do. Is there a cleaning process that we should do before sending these parts to the finisher? Currently the shop just uses picks to pull out the large chips and air hose to do the final clean out. M.O.
A. This is a good question about a condition that occurs rather frequently. Small, tapped, blind holes can be a problem. Due to surface tension of the solutions, these parts can be very difficult to rinse. Chemicals,  or even dirty rinse water, can be easily trapped in the holes if they’re not completely rinsed out, causing the dye in and around the hole to “bleed out.” In the presence of anything but the correct pH environment (in the hole), the dye won’t be absorbed by the anodic coating. 
In a threaded hole there’s more surface area for solutions to cling to and, hence, it’s more difficult to rinse thoroughly. Even if the holes have no machining debris, they can be difficult to rinse thoroughly unless some extraordinary measures are taken by the anodizer. 
There are a few methods than can be used effectively to make sure the holes are clean prior to anodizing. These include blowing out the holes with compressed air before the parts are racked, picking each hole with a stick or wire and cleaning each hole with a cotton swab soaked in solvent. As the parts are processed, rinsing procedures are very important. Racks should be rinsed up and down in clean rinsewater several times after each chemical process step. Rinse tanks should have thorough air agitation. It helps if the parts can be spray-rinsed by hand with a hose above each rinse tank to make sure only clean water remains in the holes. The most critical places to do this are after the etch rinse and after the anodize rinse step. If there’s any caustic chemical left in the holes when the parts are immersed in the anodizing tank, the reaction of strong base and strong acid can cause a hard, sticky goo to form in the hole—nearly impossible to get out. Only when all holes are absolutely clean will they dye successfully without bleeding. It sounds like a lot of trouble and it is. Unfortunately, I know of no alternatives.
The best procedure might be to make sure the holes are absolutely clean before sending them to the anodizer. The sooner the machining lubricants and chips are cleaned out, the easier. If you’re certain all holes are clean before you send them to the anodizer (perhaps you personally inspect 100%), then you can be certain that if there are bleed-outs, it’s the anodizer’s problem. Also, it might be  good for them to inspect all parts upon receipt. If the anodizer says the parts are received dirty, then it’s to his advantage to call this out before parts are processed. It is legitimate for the anodizer to charge more if a lot of special attention is required on the anodizing line after one or more process steps.