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Mill ve. "Highly Cosmetic" finish

Is there a published standard for Mill finish and another for Cosmetic finish in extruded aluminum?

Lou LaCourse
Employee - Franklin, New Hampshire, USA
2007

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2007

Mill finish isn't precisely defined; it will vary with alloy, process and heat treatment. Some alloys aren't heat treated, and some softer ones can even be cold extruded. Heat treatment discoloration is allowed; however, porosity from excessive solutionizing temperature isn't (not visible at 500x magnification). Heat treatment scale is greatly reduced when solutionizing and quenching steps are incorporated into the extrusion process, as often done for 6000-series alloys, see ASTM B221, para. 9.3 or B807.

ANSI H35.2 includes surface roughness limits. E.g., Table 10.7 Surface Roughness -- Wire Rod, Bar and Profiles gives allowable depth of 'conditions' (including die lines and handling marks). For thicknesses up through 0.063" the max. depth is 0.0015", for 0.064-0.125" the max depth is 0.002", for 0.125-0.188" the max depth is 0.0025", and so on. However, a footnote states that these tolerances are for average profiles; "wider tolerances may be required for some profiles, and closer tolerances may be possible for others." Curiously, these are the exact roughnesses given in Designing with Aluminum Extrusions published by Reynolds Metals Co. in 1952.

Greater roughnesses are allowed, by 1.5-1.67x, for hollow extruded tubes (more difficult to make). Also, finer surfaces are typically required for architectural aluminum (usually Al 6063) and the bright alloys used for automotive trim; these may be customer standards. One can also buy 'cold-finished' bar, rod or wire per ASTM B211, but that is loosely defined as "improved surface finish and dimensional tolerances."

The Aluminum Extruders Council has a lot of information available, much of it free: http://www.aec.org/
The Aluminum Association has a lot of information available, some of it free: http://www.aluminum.org/

Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.