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Mold Finish - NTMA Standard, converting NTMA to Ra, RMS for silicone parts

Hello,

My name is Nick. I have a question regarding surface finish for molds and associated silicone parts that come from those molds. I have a print that calls out an NTMA surface finish for a mold and also calls out an NTMA finish for the silicone parts that come out of that mold. I am familiar with Ra and RMS surface finish and can't find any correlation between NTMA and Ra or RMS. Is there a conversion formula or table that goes between NTMA and Ra or RMS that you as finishers know of? Thank you for your time.

Nicolas Welche
product development engineer - Santa Barbara, California
2007

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I don't have a direct answer.
The National Tooling and Machining Association has several publications on moldmaking. 'Introduction to Moldmaking' includes "surface finishes for molds; fabrication materials for molds; mold problems and solutions; mold repair techniques-welding; glossary of terms and tables-conversion."
They also have a Mold Finish Specimen Kit: "A book-sized molded case that opens to reveal 29 different surfaces produced by various finishing processes." www.ntma.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.
2007

I agree with Ken. The best (and perhaps the only) conversion you will get is from their surface comparator and your own eyes. Inner surfaces of molds normally do not lend themselves to profilometer reading (designed for flat surfaces), so that's why, in the plastic industry we all use visual comparators mostly for the metallic surfaces but for the molded parts as well.

Guillermo Marrufo
Monterrey, NL, Mexico
2007

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Thanks for the response. I have the kit from NTMA, and as you mentioned it appears that visual comparison is the only option.

Nick

Nicolas Welche
product development engineer - Santa Barbara, California
2007

Where can I purchase the NTMA roughness standards?

Don Cohen
metrology - Livonia, Michigan
March 15, 2010

Hi, Don. It appears that you can buy this direct from www.ntma.org. I currently see it listed under store>publications>mold finish specimens kit.

Good luck.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
March 16, 2010