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The Elastomerics Blog by Stockwell Elastomerics

Friday, November 27, 2015

ASTM D6576 and Closed Cell Silicone Sponge

Military Specification Mil-R-6130C established requirements for cellular rubber used for sponge gaskets and cushioning pads in defense equipment in the 1980’s and 1990’s. Both closed cell (Type 2) and open cell (Type 1) sponges were covered in this specification. Grades A and B were typically met by neoprenes and Grades B and C were met by silicones. There were three (3) firmness conditions: Soft, Medium and Firm. Soft condition called out 2 to 7 psi Compression Force Deflection (CFD), Medium called out over 7 to 14 psi CFD and Firm specified over 14 to 21 CFD. CFD is basically defined as the amount of force needed to deflect one square inch of sponge by 25% per ASTM D1056.

In the 1990’s a committee was convened to develop the ASTM D6576 Standard for Flexible Cellular Rubber materials. ASTM D6576 was derived from ASTM D1056, which lists six (6) categories of sponge firmness. The intermediate firmness ranges are narrow and difficult to control in full production batches of closed cell sponge. A subset of the products covered by this standard are closed cell silicone sponge products. Apparently one of the leading producers of closed cell silicone sponge was not represented on the ASTM committee, and these six compression force deflection, or firmness ranges were published with the standard on April 10, 2000.

Many of us in the industry did not anticipate the cancellation of Mil-R-6130C by the US Department of Defense on March 19, 2001. Mil-R-6130C was replaced by ASTM D6576. Why is ASTM D6576 an issue? As one example, Mil-R-6130C condition soft formerly required 2 to 7 psi CFD; now ASTM D6576 condition soft requires a tighter 2 to 5 psi CFD. Producing and certifying to these tighter firmness ranges has been difficult for the silicone sponge manufacturers. Therefore, they frequently request an exception to this specific property.

Comparative Firmness Conditions

ASTM D6576
Deflection Categories

Deflection Categories

Stockwell Elastomerics
Silicone Sponge

CFD Values
CFD Values
St Gobain
CFD Values
Super Soft
0 to 2 psi
2 to 7 psi
2 to 7 psi
2 to 5 psi
5 to 9 psi
7 to 14 psi
6 to 14 psi
9 to 13 psi
6 to 14 psi
13 to 17 psi
14 to 21 psi
R10490 Fluorosilicone
9 to 18 psi
17 to 25 psi
12 to 20 psi

Design Considerations:

  • If ASTM D6576 is a requirement on your new silicone sponge gasket designs – your engineering team should confirm whether the narrow compression force deflection range of the ASTM D6576 standard is necessary. We can provide certifications for ASTM D6576 for silicone sponge materials taking an exception to the narrow compression force deflection range.
  • If ASTM D6576 is called out on a requisition for an existing gasket drawing requiring closed cell silicone sponge for a Government Prime contractor, and vendor certifications and / or test reports are needed, expect a waiver on the compression force deflection range.
  • If the gasket drawing is a legacy design and Mil-R-6130C is requested, let us know. Even though the specification is officially expired, with adequate lead time a material batch can be tested and qualified to meet this specification with a certificate of conformance.
  • Closed cell silicone sponge is produced like baking a cake. Instead of dough; a thin preform of catalyzed silicone gum is heated and allowed to expand. Controlling the density, thickness and compression force deflection of the cured silicone sponge may require more leeway than ASTM D6576 allows.

For more information and assistance with ASTM D6576 and closed cell silicone sponge, please call 1-800-523-0123 or email

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posted by Bill Stockwell at 2:37 PM


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