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The Elastomerics Blog by Stockwell Elastomerics

Friday, November 02, 2012

RMA Tolerance, Fixed and Closure Tolerances Explained

A common question recently came up again about the Rubber Manufacturers Association (RMA) tolerance table and what "Fixed" and "Closure" dimensions are. A way to think of this is a clam-shell mold (top and bottom halves).

The RMA Fixed dimensions refer to the dimensions that aremachined into the top and/or bottom plates. Below shows a 5.000" +/- .025" based on RMA "A2" Fixed dimension tolerance.

The RMA Closure dimension is the dimension created when the two halves come together (clam-shell is closed).  Below shows a rough example of two mold plate that are compressed in a press, 0.188" +/-0.008".

Stockwell Elastomerics does liquid injection molding (LIM) with liquid silicone rubber (LSR) and does compression molding with specialty silicone rubber materials such as electrically conductive silicone and fluorosilicone. Both compression molding and LIM are cold material going into a hot press. Parting line flash and other flash condition are removed with Stockwell Elastomerics' in-house cryogenic deflasher. Cryogenically deflashing saves greatly on part cost.

Helpful Links:
Stockwell Elastomerics Molding Compounds
• RMA Tolerance table
• Additional silicone molding info

For more information on silicone molding, LIM, liquid silicone rubber or other processesing methods such as die cutting and waterjet cutting, please contact Stockwell at 215-335-3005 or

posted by Steve Hughes at 12:24 PM


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