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The Elastomerics Blog by Stockwell Elastomerics

Friday, November 27, 2015

ASTM D6576 and Closed Cell Silicone Sponge

Silicone gaskets group, Stockwell Elastomerics
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
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 hasket 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 comments

Thursday, October 29, 2015

MIL-DTL-83528/4 EMI Flange Gaskets

MIL-DTL-83528/4 is a standardized family of EMI flange gaskets. The dimensions and materials are well defined by the MIL spec. Stockwell Elastomerics manufactures standard and custom EMI gaskets from Type B, QPL listed conductive rubber. These gaskets are capable of attenuating EMI noise as well as sealing out environmental conditions.

MIL-DTL-83528E Type B Material:
Silver-plated, aluminum-filled silicone capable of 100 dB of plane wave shielding effectiveness at 10 GHz with a continuous use temperature range of -55°C to +160°C.

Example PIN: M83528/004B001
M83528 - indicates the EMI / RFI family of conductive materials
004 – indicates the flange style
B – indicates the specific material type (B indicates Ag/Al filled silicone)
001 – indicates the size and exact dimensions of the conductive flange gasket

How are MIL-DTL-83528E Type B Gaskets made?
Stockwell Elastomerics manufactures EMI gaskets from a range of processes including, compression molding, waterjet cutting, tool-less flash cutting, and die cutting. These capabilities allow Stockwell Elastomerics to mold Type B EMI sheets and cut MIL-DTL-83528/4 gaskets with very little waste.

MIL-G-83528 Gasket Materials and Non-Mil Options
MIL-DTL-83528/4 points to Mil-G-83528 which defines material type and performance requirements. The mil spec can be restrictive from a  design perspective - for example, hardness is limited to 65 durometer or expensive conductive particles (silver) are required. For EMI applications not requiring MIL-G-83528 EMI materials, Stockwell offers non-mil spec EMI materials that have been third party tested per Mil-G-83528 for attenuation performance. Materials include softer EMI rubber for lower compression force, EMI gasket material with passivated silver/aluminum for better corrosion resistance and nickel graphite filled conductive rubber to reduce cost.

Domestic EMI Gasket Manufacturer
Stockwell Elastomerics is an ITAR registered, domestic EMI gasket manufacturer. All EMI gaskets, both molded EMI gaskets and cut EMI gaskets are produced at Stockwell Elastomerics' Philadelphia, Pennsylvania location.

For more information about M83528/004B EMI gaskets or custom EMI gaskets, please contact Stockwell Elastomerics at 1-800-523-0123 or by email at

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posted by Steve Hughes at 4:41 PM comments

Thursday, October 15, 2015

Green Heat Conductive Rubber Pad

Green Conductive Rubber
Green Heat Conductive Rubber
Heat conductive rubber, also referred to as green heat conductive rubber, is a unique closed cell silicone sponge that has very fine ceramic particles mixed in the polymer to enhance its thermal properties.  This expanded, conductive rubber helps distribute pressure and protect delicate materials in the heat press.  Heat conductive rubber press pad material is visually identified by its light green color and fine fabric finish.

Green Heat Conductive Rubber
Width:  36"
Thicknesses:  0.035", 0.062", 0.125", 0.188" & 0.250"
Length: Continuous  (0.25” available in 36” x 36” sheets only)

Stockwell Elastomerics has been a long time supplier of green heat conductive rubber to, sublimation printers, composite laminators, heat press applications and thermal management for OEMs.  Stockwell maintains inventory of green heat conducting rubber, Teflon (PTFE) sheet, smooth high temperature foam, reinforced thermal rubber and various other materials used in heat press applications and composite laminations.  

Heat press materials offer by Stockwell Elastomerics
Sublimation Pads
Press Pad Materials
  • Green heat conductive rubber pad
  • Non-stick Sheet (for overlaying)
  • Insulating silicone sponge sheet (slight texture surface)
  • Insulating silicone f
    oam sheet (smooth surface)

For more information about heat press pads, thermally conductive green rubber, green heat conductive rubber pad material, please call 1-800-523-0123 or email

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posted by Steve Hughes at 2:25 PM comments

Thursday, September 03, 2015

What is foam compression set?

In simple terms, compression set is a measure of how much a material does NOT rebound.  This is expressed in a percentage, for gasketing, the lower % the better (to ensure "apples and apples" confirm test temperature).

Why do we care about compression set?
Compression set, sometimes called C-set, is an indicator of how a material behaves under certain conditions.  The compression set test for foams and sponges include a 50% compression at various temperatures.  Room temperature 23°C, intermediate 70°C, and high (100°C for silicone).  Since heat is an “accelerator” of compression set, an engineer would roughly relate these temperatures to their application.  A gasket material  that takes a C-set at elevated temperature will quite often stop functioning as a gasket over time.  This would be metered against the expected life of the product, whether there is re-compression, level of sealing (e.g. NEMA 4X vs NEMA 3R), preventive maintenance, cost, and most important the cost of a leak/equipment failure due to a leak.

Below is a graph produced by Rogers Corporation, this shows compression set values of different gasket materials at different temperatures, silicone is tested at a higher temperature (100°C) since it is considered a high temp gasket material and resist the effects of heat.

 Compression Set of Foams

Example application:  
A couple years ago Stockwell had a customer who had remote monitoring equipment in the desert;  the only way to access the enclosures for this equipment was by helicopter.  The gasket needed to seal under desert conditions; temperature cycling, rain sealing, dust sealing, and not be affected by ice. The cost to access and repair the equipment would have been enormous.  Stockwell’s Applications engineers recommended a closed cell silicone foam material that could withstand high temperatures without taking a compression set.  Had the application been for a piece of lab equipment, a very different material may have been selected.

Rubber Compression Set in summary:
Compression set is just one data point that follows an ASTM test procedure in order to help engineers distinguish the performance properties of various materials on an even playing field.  There are many other factors and key properties to sealing successfully such as stress relaxation and cell structure that need to be considered for the overall best gasket material choice.  Selecting a commercially available material and how to use it can be daunting, Stockwell’s Applications Engineers are available to discuss your project.

For Applications Engineering assistance or more information on low compression set rubber, sponge and foam, please contact Stockwell Elastomerics at 1-800-523-0123 or email

Additional information on Creep, Compression Set, and Stress Relaxation

posted by Steve Hughes at 10:41 AM comments

Thursday, August 06, 2015

New Poron Webpage

Stockwell Poron
Poron Urethane Foam - Stockwell Elastomerics
As a manufacturer of gaskets and pads made from high performance materials, Stockwell Elastomerics is a proud supplier of Poron urethane foams.  Poron is manufactured by Rogers Corporation and brought to market through a select network of converters who have been educated on the use of Poron as well as understand the value it brings to customers.  Stockwell Elastomerics is a Rogers Preferred Converter and maintains inventory of the industrial Poron foam grades.

Stockwell has a newly updated webpage to help designers and engineers understand and select a Poron grade that best fits their applications.  And as always, Stockwell’s Applications Engineers are available to answer questions about Poron foam and other material options that may be suitable for your project.


Poron foam uses:
  • Poron enclosure gasket
  • Poron dust gaskets
  • Cleanroom gaskets
  • Poron cushion pads
  • Poron touchscreen gasket
  • Poron battery cushion pads
  • Gap filling foam for electronics
  • Poron shock absorbing pad
  • Poron foam gaskets - light water exposure
  • Light leak gaskets

For more information about Poron urethane foam or other high performance gasket materials, please contact Stockwell Elastomerics at 1-800-523-0123 or email

posted by Steve Hughes at 1:49 PM comments

Monday, July 13, 2015

Double Sided Tape for Gaskets

Poron with 3M 200MP
Stockwell Elastomerics is a custom gasket manufacturer who specializes in high performance materials.  A part of a gasket that is often overlooked is the adhesive backing, we hear it called a peel and stick backing, sticky backed gasket, adhesive gasket or some version of these. What many customers don’t realize is the adhesive backing, in most cases, are double sided tape.  As a 3M Select and Adchem converter, Stockwell maintains inventory of master logs of double sided tape. These adhesives, usually referred to as pressure sensitive tapes (PSAs), are applied to most of Stockwell’s materials (including silicone rubber).  Most master logs are self-wound; this means when unwound, one side of the adhesive is exposed.  The exposed side is bonded to the rubber material; the non-exposed side has liner (usually paper or plastic) that is peeled of when the gasket is installed.

How can adding adhesive save cost?
1) Producing the part - in some cases, having an adhesive backing allows gaskets and pads to be produced on equipment that can run faster.  Usually due to the fact that the adhesive liner helps prevent the rubber material from stretching during processing.

2) Assembly throughput – for mid and high volume production, gasket or pads can be supplied kiss-cut on a roll.  Kiss-cut parts allow for easier and faster installation of the gaskets.  The time saved in throughput usually offsets the added cost of the adhesive and in most cases offers cost savings.

The best adhesive for gaskets
Call Stockwell’s Applications Engineers – it’s fairly cliché, but the devil is in the details.  Temperature, bond strength, surface energy, surface texture, chemical exposure, reinforcement, cost… are points to consider.  Stockwell’s engineers work closely with customers and  3M to ensure the proper adhesive is selected for each applications.

Types of gasket adhesive backings:

  • General purpose adhesive
  • Film supported adhesive
  • Low tack or repositionable adhesive
  • Low surface energy adhesive
  • Very high bond (VHB) adhesive
  • High temperature silicone adhesive
  • Thermally conductive adhesive
  • Electrically conductive adhesive

For more information about adhesives for gaskets and pads, please contact Stockwell Elastomerics at or call 1-800-523-0123.

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posted by Steve Hughes at 1:44 PM comments

Wednesday, June 17, 2015

Soft Silicone Rubber for Sealing to IP67 & IP68

IP67 is a rating often associated with ruggedized electronics, outdoor lighting fixtures, and other enclosures requiring a high level of sealing.  Stockwell Elastomerics manufactures custom rubber components for OEMs where reliable, long term IP, NEMA and UL sealing requirements are critical.
IP67 Gaskets

IP67 requires an enclosure or housing to be dust tight (6X) and protect against water ingress (X7) when submerged up to 1 meter for 30 minutes; for many electronic manufactures, this is considered a minimum requirement.  When a rugged device is designed the expectation is that it will be able to survive a longer submersion period and withstand a range of environmental conditions.  Taking into account temperature extremes, drop tests, accelerated aging, and other conditions, silicone rubber is one of the best materials for sealing to IP67.

Silicone rubber is a very versatile elastomer, it is readily available in soft, low durometers such 10, 20 on the Shore “A” scale, even softer closed cell silicone sponge is also available.  These soft silicone grades are preferred since they easily conform to enclosure variations and “absorb” tolerance stacks, sealing under lower clamp pressures.  These lower clamp forces are key for plastic and thin metal enclosures that can deform under too much load.  This deformation quite often creates unintended ingress points.

In addition to their low durometers, soft silicone rubber can be molded or cut from sheets.  Silicones are inherently UV and ozone resistant, and have wide operating temperatures.  On the low temperature side, silicone will remain compliant and functional below the glass transition point of other elastomers.  On the high end of the temperature range, silicone is very stable, this stability helps resist compression set and stress relaxation where other polymers would take a compression set or permanent deformation.

Key features of Silicone for IP67:

  • Soft, low durometers are standard
  • UV resistant
  • Ozone resistant
  • Wide operating temperature
  • Can be cut or molded
  • UL94 HB flame rated,20 duro sheet materials
  • EMI attenuation (conductive grades)

Sealing to IP67 and other IP, NEMA and UL50 standards can be difficult.  Stockwell Elastomerics’ Applications Engineers work closely with OEM design engineers to help select the best material for their enclosure project.

For more information on IP67gaskets, soft silicones or other enclosure gaskets, please contact Stockwell Elastomerics by email at or call 1-800-523-0123

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posted by Steve Hughes at 3:29 PM comments