What grades of aluminum work best for Hardcoat Anodizing?
A discussion started in 2005 but continuing through 20202005
Q. I am currently redesigning fixtures for a sealing process used in our production line. The process applies an evenly distributed load of about 100 lbs and a temperature of about 450 °F before rapid cooling the plates to 70 °F, process cycle time is about 1.5 minutes. Plate size is approx. 18 inch x 24 inch x .5 inch thick.
The design requires Hardcoat anodizing with Teflon (mill spec 8625 type 3 w/ Teflon) on all surfaces of the plate to reduce abrasion during the cycle. Surface finish spec is 32 or better (no mill marks).
What grades of aluminum are best for Hardcoat anodizing. These fixtures are in an abusive environment which means I may want to have them surface milled and recoated sometime in the future. What are the complications and precautions that I should be aware of if I choose to recoat these fixtures at a later time? Are there materials grades of aluminum that will be less influenced by this thermal cycling? Exceptional heat transfer is also a preferred.
Any suggestions or insight would be greatly appreciated.
- Lebanon, New Hampshire, USA
A. Your choices are narrowed down quite quickly - 2xxx series are poor for hardcoating due to their propensity to burn and also due to the fact that the resulting hardcoat is far worse from an abrasion resistance standpoint than nearly any other wrought alloy. Next 7xxx give nice coatings but as an aerospace, high strength alloy, it tends to be too pricey where other alloys will suffice. It comes down to 6061 or 6063 which is easily hardcoated, can be stripped, polished and re-hardcoated, etc. I can't imagine that one alloy over another would offer that much better heat transfer - they're all aluminum and transfer heat quite readily. Lastly, keep in mind that any surface finish will degrade upon hardcoat anodizing. To end up with a 32, you had better specify a 16 as machined! The amount of finish degradation is dependent on thickness of anodize desired - less if thinner.
Syracuse, New York
A. A responder in Ohio will dispute me, so I will quote Sheasby and Pinner who say that at the temperatures you mention the anodized coating will crack. You may want to re-apply the teflon after these cracks open up. 450 °F is not a good application for anodized aluminum. P & S quote,"Cracking is caused by the low coefficient of expansion of the anodic film which is approximately 0.2 times that of the metal." They go on to say that 2024 is the worst. Come back and let us know what happened.
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina
A. Hardcoat anodized aluminum is used successfully on millions of parts in lots of thermal cycle applications. These include; cookware, commercial pizza pans, and rubber and plastic molds. It is quite normal for hardcoat to micro-crack or craze. This does not appear to negatively effect it's wear and corrosion resistant properties.
Luke Engineering & Mfg. Co. Inc.
I have no choice but to agree with Chris, Robert -- because I use Calphalon hard anodized pans on the stovetop every day--and after more than a decade of use they are essentially as good as new. "Oven safe to 450 F."
Apparently "cracking" and "failing" are not the same thing. However, rapid repeated thermal cycling might be an issue; so, as Robert requests, please get back to us with what happens.
Ted Mooney, P.E.
finishing.com - Pine Beach, New Jersey
Striving to live Aloha
Hard Coat Anodizing - Cracks IssuesFebruary 21, 2020 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread
Q. I am researching Hard Coat Anodizing at the University for my dissertation, my experimental Set Up consist is similar to "Martin Hard Coat" Anodizing, I am Anodizing at 5 °C, sulfuric acid, 15 wt% sulfuric acid, for 60min, 3.5-4.5 A/dm^2; later I seal the parts in boiling water.
The coat under naked eye seems pretty good, but when I analyzed the part I found it has cracks. Does anyone have an insight in how to avoid these Cracks? Does anyone have experience in something similar ?
Student - Monterrey, NLE, Mexico
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