Hard Anodize Close Tolerance Processing

Is there anything taboo about removing a part during hard anodizing to mic features to hit close tolerance features? Is removing parts from the tank mid process harmful to the anodize coating or affect the parts negatively when they are placed back into the tank? Assume quality racking where electrical contact will not be lost as it is removed and handled for measurement. Strictly questioning the very aspect of removal and exposure to atmosphere outside the tank and then reinsertion to continue plating until a desired thickness is achieved.

hard anodizing is done at around -2 degrees C, you will have to take into account the thermal shrinkage if you want to do it that way, in case the parts need to be dyed, you might run into problems if you let them dry off during these measurements

what is normally done is anodize slightly oversize (for OD) and lap/grind to needed size, this also helps with surface roughness, because the hard anodizing surface is not exactly smooth, even if the part was polished to mirror prior to anodizing, for instance, hydraulic valves are hard anodized oversize, then ground to size at the same time getting the smooth surface and roundness

I personally did some tests on small batches of small round parts, first thing was to map time/vs thickness graph, and then, using that graph, I could get individual parts to exact size even if they were +/- few microns different from each other before anodizing, not something that would be done in production process, too fussy, but doable, that time/thickness relation is very repeatable if the bath, voltages, amperage is reasonably well controlled

Parts have tons of varying features. Low quantity high dollar parts have to be correct the first time no time to document a process and then nail successive pieces by repeating off previous data. Just need to hit a a bore size on one hole for instance or a thickness or a certain feature. No dye. Hard anodize natural / clear. Part doesn't have to be completely dried off. Just a quick pull and mic or pin check. Does a pull out and reinsertion as a I described there throw up any red flags as detrimental to the overall process and quality of your coating?


I realize a lot of stuff with thicknesses probably can be controlled pretty accurately with proper equipment and knowledgeable staff. I am interested in the above method if a case arises where I go to a plater that may not be typically accustomed to holding tight tolerances through process and equipment monitoring.

when I said to do trial runs I didn't mean the actual finished parts, make dummy parts with similar surface area and couple special features you're going to inspect and use the same hanging method to avoid another variable

in theory, there are few things that can happen, anodizing layer is an insulator for the current that flows inside the bath, so there is some voltage drop across it, meaning - some heat will build up in the layer, by continuously stopping and restarting the process you quite definitely will get some sort of effect there, might not be detrimental, there is a technique called "pulse anodizing" used in the TypeII process, to grow the oxide layer at a faster rate, and it follows this pattern, the idea is to apply larger than usual current to the part for short time (tens of seconds) followed by short rest period - then high current again, rest etc. - the rest period is there to deal with the excess heat on the surface of the part, this extra heat would normally cause the built up layer to dissolve back into acid, but if done right, a thicker layer can be formed in less amount of time compared to constant current process

so in theory, if you stop/restart frequently, you will get some other thickness of the layer than in a continuous process, I just haven't heard anyone doing it, that is not to say it isn't done, there are things anodizers will not share

but the main issue is the temperature change you will get, first it will affect the reading you will get with your measuring instrument, as I said, hard anodizing is done in -2C, that is good 25 degrees difference to normal room temperature, on a 10mm solid length of material that would amount to 0,005mm, so you'd have to at least verify that with some test pieces, and every time you remove the part from cold acid and start to rinse it, measure it, put it back, there will be some expansion/contraction going on, that may lead to stress buildup in the oxide layer itself, might be a problem, might not be, as I said, normally you would run a few test pieces, verify the repeatability of the process and then run your expensive part, if you need to do the another batch of parts couple months later, you have to verify again just to be sure, acid concentration and dissolved aluminum content in it affects the process, normally shops will control these variables, but still there can be some differences, I've even red about problems caused by air buildup in the acid over continuous use, when the bath is ran batch after batch without extended rest periods, later in the such a day the resistance of the bath goes up, so that will affect the result you will get

My parts are 41 inches in diameter overall. They are rings and don't weight much. Duplicating a dummy part to test run is more troublesome than most parts I would typically have. I do not like the idea of removing the part in order to check thickness mid process. What about the possibility of a witness samples that could be placed near the top of the racking so the racking could be lifted enough to remove the witness samples. Would that provide any reliable relation to what the part is experiencing? I assume the answer would be that the witness sample thickness and the way they are connected to the whole rack would dictate how accurately they represent the part.

I have done things in the past where witness samples were used on separate racking to monitor a close tolerance hole. They had no relation to the part other than a process was run and data collected. Then the process was duplicated and the witness samples monitored and compared to prior results. I assume you could devise a witness sample that would represent the part better. Still the best thing would be to run one setup and go from there since there are so many variables.

As far as mid process checking and temperature change. The tight holes I am checking are very small (3/32). I also assume I could just test with an eddy tester. I know everything plates differently based on geometry. All I really would want on the first part with an eddy tester would just assurance we are in a reasonable ballpark. I realize where my close tolerance areas were may be different and that could be documented for next time. Same thing with the witness sample method. Even if they were an approximation as reassurance you were in the ballpark of thickness I'd be satisfied.

Talk to anodizers in your area. If they know their stuff, you should be able to tell. A witness part is a good idea. Sometimes it can make a difference where the part is located in the tank -i.e. multiple parts racked at different heights. If the circulation is good in the tank, that would help.
What kind of tolerance are you needling held? Guys I use can hold a total range of .0005 pretty easily if I do my part. They likely can do better, but that's as close as I've asked them to do in the past. Threads can be an issue, especially blind holes.
Good luck!

I would need .001 - .0015 total anodize thickness. I have through holes on the part that are .0934 - .0944 final tolerance. Really need to do a part and see how it all takes on really. I imagine those small holes will take on less plating. It is very likely I am able to get into a quality place which will be able to handle tolerances. My reason for questioning other methods like pulling the part out and checking would be if giving my needed turnaround time I was forced to rely on a plater that doesn't typically deal with close tolerance plating. Sometimes the best place to take it may be backed up and I'm just trying to look ahead at what options I may have to get a good part depending on where it may need to be taken.

My customer has moved to electroless nickel for much of their work which is great for even reliable plating. Hard anodize is awful. I haven't always had the best sources for anodize either which contributes to my distaste for it.

baran3 said:

As far as mid process checking and temperature change. The tight holes I am checking are very small (3/32). I also assume I could just test with an eddy tester. I know everything plates differently based on geometry. All I really would want on the first part with an eddy tester would just assurance we are in a reasonable ballpark. I realize where my close tolerance areas were may be different and that could be documented for next time. Same thing with the witness sample method. Even if they were an approximation as reassurance you were in the ballpark of thickness I'd be satisfied.

Click to expand...

In theory the witness sample should work, but it has to be the same stock of material for one, and racking has to be 'ideal' for both parts for them to behave the same way, what I mean by that is your large part may have surface area A, the small part might be A/10, and if you use, lets say 1 contact for the small part, you should use 10 contacts for the large part, to make sure the voltage potential on both parts is the same, if voltage is the same, then current density on both parts will be the same and that will ensure that both parts grow the oxide layer at the same rate

then there is the bath agitation aspect that Eaglemike mentioned, it must be sufficient so top and bottom portions of the tank are at the same temperature, if you want to test that, then you need to do the following, provide 2 sample parts to the anodizer, and ask them to rack one at the top and one near bottom of the rack in a batch of parts that will run the same current that your part(s) will, that way you will simulate the heat input into the tank and get an idea how top/bottom portions of the tank behave

regarding geometry and difference in plating, anodizing doesn't work like metal deposition plating does, where cathode is consumed to provide ions that then are deposited on the work piece, in that process, yes, geometry and cathode placement will affect plating thickness, in aluminum anodizing, the cathode is the acid, coating will grow evenly everywhere where the acid is in contact with the part - if the agitation in the tank is lacking, you might get temperature differential for one, second - small bubbles form on the surface of the part, which prevent acid from being in contact with the part thus retarding the oxide formation, those gas bubbles also can sometimes be trapped under a pocket feature of a part and prevent oxide formation, same thing happens in small holes - you can have acid in there at the beginning of the process, but as the gas forms inside that tiny hole, it displaces the acid and because of the geometry, the flow around the part can't flush that gas out - so the oxide on the outside of the part will be one thickness, while the hole may have almost no oxide thickness - happens mostly with blind holes, even worse if they are tapped, and even worse if the part has to then dyed black and the acid leaches out and eats the dye away... I often get to do parts with threaded blind holes right down to M1.6 size, I even try to rack them with those holes facing downwards, so gas pushes the acid out (customer doesn't request those threads plated), flush them by hand, and even then sometimes there are dye defects around the hole, always a lottery with those

so if your 3/32" holes are deep and blind and have to have the same oxide thickness as the outside of the part, you might be in trouble there...

anyway, I can't think of other issues you might run into, make a list, talk to the anodizers, try to talk with the operators, technical guys, not the sales people, those will always assure you that they have done thousands of parts like that with no problems etc.

p.s. noticed you said they are though holes, you might still want to "overplate" them and then run a diamond lap in there to get the final size for sure, little extra work for the peace of mind, might want to test that out with the test coupons mentioned earlier

Thanks for all the insight and detailed explanations of the anodize process. It's good to have an idea of the technical aspects of the process and I've been getting more and more knowledgeable as time goes on where I can hopefully have productive conversations with suppliers. Problems always come down to I don't have enough time on my end of the job and the suppliers are also busy where it's difficult to coordinate and arrive at really good processes for small quantities. I wish I had my own tanks, a ton of money, and endless time to study and test stuff on my jobs myself. Perhaps in the next life lol.