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Stainless Steel finishes/roughness

I work in the pharmaceutical field where much of our equipment/components are made of stainless steel. We require (or prefer) all surfaces that come in contact with product to be mirror finish or electropolished. Both finishes appear to be the same.

Lately, equipment suppliers refer to a roughness value and labeled Ra "X", e.g., Ra5, Ra20, RA.5, Ra<>X, etc. (I believe the unit of measure is microns.)

Situation: My company ordered SS316L bulk product containers and our User Requirements Specification (URS) specified the interior surfaces to be a mirror finish. During the Factory Acceptance Test, the certificate indicates the interior surfaces having an Ra<.5 value. When we referred to our URS specifying mirror finish, they said the mirror finish "would destroy the .5 micron finish".

Question: Are the terms "mirror, satin, brushed, high luster, etc." finishes designated by an Ra value? If so, then an Ra<05 microns must be a mirror finish?

Thanks,

Stephen Cycyk, Jr.
pharmaceuticals - Newark, Delaware
2006

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2006

The terms you refer to are descriptive of methods of producing finishes of different appearance, some of which will meet the specification you need to achieve.

See letter 9524 on this site, also visit: http://www.mel.nist.gov/div821/webdocs-13/surfcalib.htm for additional information.

Several of the finishes you describe could comply with the value you need, depending upon the grit used.

Ask your customer for a sample, and then investigate methods to match it.

Daryl Spindler, CEF
decorative nickel-chrome plating - Greenbrier, Tennessee

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To answer your question, no unless specified on the exact RA there are many different interpretations. I'm one of the best, I'd know. Mirror finish could either be buffed out using black compound which is a "dirty #8" otherwise using white buffing bar ⇦this on eBay or Amazon [affil links] will give you a lower Ra and if not good enough can always zip over it by buffing with blue rouge. Brushed finish, unless in the food or dairy industry, standard "brushed" is a 32 Ra or otherwise known as straightlining your piece with a 100 grit belt and a worn down coarse scotchbrite belt. That application could also be associated with a satin finish.

Brandt Hinton
- Elk River, Minnesota
2006

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2006

Brandt Hinton,

Can you provide me a reference source for reading that will provide me a description of finishes and roughness?

Thanks,

Stephen Cycyk, Jr.
pharmaceuticals - Newark, Delaware

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You might want check out www.nidi.org and get their definitions of stainless steel finishes. Publications are free. FYI. The "dirty #8" mirror finish I suspect is really a No. 7 mirror (mirror finish with polishing grit lines still visible). Hope this helps.

Michael Liu Taylor
specialty stainless steel distributor - Dallas, Texas
2006

Take a look at the technical archives of ISPE, the International Society for Pharmaceutical Engineering and search under stainless steel finishes. There you will find a full explanation of standard finishes that can be obtained via mechanical treatment, chemical treatment, electropolishing and passivation. There is also a table to compare the various ways that surface finishes can be described.

Louis Rugole
pharmaceutical contract manufacturing - Toronto, Ontario, Canada
February 2, 2009

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Ed. note: Those archives are apparently only available to ISPE members