Maintaining electrical conductivity in an aluminum assembly (line card ejector)
Q. I am a mechanical designer new to the anodizing process but love the look of Mag flashlights. I have read several articles about the process. Our product is deployed in an office so will not be subject to a a harsh outdoor environment.
The mechanical component in question is a linecard ejector comprised of a lever and a fulcrum assembled together with a steel split-spring pin (ass'y is then mounted to an chemical filmed aluminum extrusion). Ejector needs to be highly cosmetic. Lever and fulcrum to be machined from extruded 6061-T6 aluminum. The hinge mount has 2 tapped holes which are used to secure ejector assy. to the faceplate with two 6-32 screws. The ejector assy. needs to provide an electrically conductive path. As such, the screw holes and pin holes need to be conductive.
I am thinking Type II, Class 2 anodizing (dark color) for the cosmetic appearance, color and hardness. Since anodized aluminum is non-conductive, is it reasonable to mask/plug the threads and holes? Will the exposed aluminum surfaces eventually (20 year life) corrode and inhibit electrical conductivity? Is there a secondary (primary) process I could specify to protect the non-anodized surfaces?
Would metal plating be a more cost effective choice? Which one(s) zinc, nickel, etc? Remember, it needs to look nice.
I could spec. the ejector mount electropolished/passivated SS but the lever itself will be aluminum. Can anyone provide a source for rule of thumb relative costs of plating processes. Better yet, discussion of relative TOTAL costs of material selection tied machinability and finishing process? E.g., Machining aluminum is cheap but then I need to mask/anodize vs. SS more $to machine but I would only need to electropolish.
For anodizing do I specify color. I am used to paint where I can refer to the Pantone color system. What do anodizers use? I'll stop here as I could go on an on. Thanks in advance for your attention.Richard Huang
- San Jose, California, USA
A. You can chem film the areas that you masked or plugged, either prior to anodizing or after anodizing. This will give you better protection then just bare aluminum. I can't say that it would protect for 20 years, but it will still allow you to have electrical conductivity.TED NOVOTNE
- MPLS, Minnesota, USA
A. Aluminum is a poor choice for consistent, long term conductive path. Look at the problems experienced with house wiring using aluminum wire for an example. In my opinion stainless would be a better choice. I do not know about the plating options.
Good luck,John Holroyd
- Elkhorn, Wisconsin
A. You have not noted what size your parts are, but with 6-32 threads it would seem they are small enough so that Electroless Nickel would be an economical option. Figure about $0.15 per surface square inch in production quantities.Pete Faxon
- Oxnard, California
A. I think his first indication of black anodize while masking the holes would be suitable based on the fact that the product in question is strictly internal use. Chromate conversion coatings will allow for a longer life compared to a hole left raw, but the cost:lifespan is not able to be determined logically or with even a remote guess that would be reasonable.
I'd suggest you go with your initial estimate, have someone quote the part(s) to you as black anodized with the holes being masked. For the simple fact that you do desire a nice cosmetic finish while still being suitable for your environment, I think your initial thoughts were right on the money.Matthew Stiltner
plating company - Toledo, Ohio
A. Hi. I guess it depends on exactly what is meant by "conductive", since this isn't a go/no-go term :-)
For what reasons does the assembly need to be electrically conductive to what kind of voltage & amperage? If the ejector lever is anodized, you don't seem to mean conductive for static electricity from the operator to the frame. If you mean that you don't want "floating grounds" hanging around on the metal frame parts, I don't think bare aluminum satisfactorily conducts 5 volt very low amperage signals. I would tend to disagree with Matthew, and suggest chromating (chem-filming) of all metal-to-metal contact surfaces as a minimum.
Ted Mooney, P.E. RET
Pine Beach, New Jersey
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