At NEDC, one of the most common requests we get is for an ultra soft gasket that can seal at very low compression pressures. Whether it is a medical device housing with thin plastic walls, an optical assembly that cannot tolerate high closure force, or a portable electronics enclosure that needs to survive drop tests, the softest gasket material is often the difference between a seal that works and one that leaks. In this post, I will walk through the main ultra-soft gasket materials we cut, how they compare, and when to choose each one. For a broader overview, see our soft gaskets pillar page.
What makes a gasket material ultra-soft?
When we talk about ultra-soft gasket materials, we are talking about materials that compress at very low pressures and have very low durometer values. An ultra soft gasket is typically one that seals at pressures below 5 psi, which is well below what a standard solid rubber gasket requires. On the Shore 00 scale, which is used for the softest elastomers, ultra-soft gasket materials typically fall in the 5 to 30 range. For context, a standard silicone sponge might measure Shore 00 20-40, while a PORON 4701-30 foam measures in the single digits on that scale.
Another way to think about softness is compression force deflection, or CFD. CFD measures how much force is required to compress a material to a given percentage of its thickness. Ultra-soft gasket materials typically have CFD values of 2 psi or lower at 25% compression. Some of the softest materials we cut, like crushed EPDM foam, compress at 0-2 psi.
Under ASTM D1056, the cellular elastomer specification, softness is graded from 0 (softest) to 5 (firmest). Grade 0 materials are the softest, and Grades 0 and 1 are what we typically consider ultra-soft.
PORON urethane foam: the compliant foam gasket standard
PORON urethane foam is one of the most widely used compliant foam gasket materials in the electronics and industrial markets. The PORON 4701 series includes grades from 4701-30 (the softest, at 0.08 g/cc density) through 4701-60 (firmer, at 0.13 g/cc).
What sets PORON apart is its compression set resistance. PORON 4701 materials recover their thickness after compression better than almost any other foam we cut, which makes them ideal for enclosure gaskets that are opened and closed repeatedly. PORON also excels at dust sealing because its microcellular structure conforms tightly to mating surfaces at low pressures. If your application involves a portable device with a gasket that gets compressed and released hundreds of times, PORON is usually the first material I recommend. See our low compression set gaskets page for more on compression set performance.
Silicone sponge: soft, closed-cell, and versatile
Silicone sponge is a closed-cell elastomer that offers a wider temperature range than PORON, typically -67F to +500F. We cut a range of silicone sponge materials including Rogers R10480S, the HT-800 series (HT-800, HT-820, HT-840, HT-870), and RS-800 series. These materials have Shore 00 durometer values in the 20-40 range, which puts them in the ultra-soft to soft category.
Because silicone sponge is closed-cell, it provides better water and fluid splash resistance than open-cell foam materials. It also has excellent UV and ozone resistance, which makes it a good choice for outdoor enclosures and equipment that will see long-term sun exposure. For more detail, see our silicone sponge gaskets page.
Silicone foam: the softest silicone option
If you need the absolute softest silicone material, silicone foam is softer than silicone sponge. Products like BF-1000 and BF-2000 have Shore 00 durometer values around 10 or lower, making them among the softest gasket materials available. Saint Gobain F12 is another ultra-soft silicone foam we cut, and Rogers HT-870 is a silicone foam that bridges the gap between sponge and foam in terms of softness.
The trade-off with silicone foam is cell structure. Some silicone foams are fully open-cell, which means they are excellent for dust and air sealing but will not seal against water the way a closed-cell sponge will. Others are partially closed-cell. If your application needs both ultra-soft compression and water resistance, we need to select the specific grade carefully.
Crushed EPDM foam: the softest of all
For the absolute softest gasket material we cut, crushed EPDM foam is the answer. Crushed EPDM can be manufactured to Grade 0 per ASTM D1056, the softest grade in the specification. It compresses at 0-2 psi, which means it seals under almost no pressure at all. We use crushed EPDM in applications where the mating surfaces are extremely fragile or uneven, and where the closure force is minimal. It also has excellent UV and ozone resistance, which makes it a good outdoor weather sealing material. For outdoor or environmentally demanding applications, you may also want to consider fungus resistant gaskets if biological growth is a concern.
When to choose each material
Here is a quick decision guide based on the applications we see most often:
- Choose PORON urethane foam when compression set resistance and dust sealing are the priority, especially in portable electronics and enclosures with repeated open-close cycles.
- Choose silicone sponge when you need a wide temperature range, UV resistance, or closed-cell water splash resistance in addition to soft compression.
- Choose silicone foam (BF-1000, BF-2000) when you need the softest possible silicone material and water sealing is not the primary concern, or when you specifically need an open-cell material for air or dust sealing.
- Choose crushed EPDM foam when you need the absolute softest material available and the application involves outdoor weather exposure where UV and ozone resistance matter.
If you are not sure which material fits your application, super soft silicone rubber gaskets are another option worth considering.
More information on ultra-soft gasket materials
If you have questions about which ultra-soft gasket material is right for your application, or if you need help specifying the right durometer, CFD, or ASTM D1056 grade, contact us at sales@nedc.com. We cut all of these materials daily and can provide samples, data sheets, and design recommendations based on your specific enclosure and sealing requirements.
FAQ
What is the softest gasket material available?
The softest gasket materials we cut at NEDC are crushed EPDM foam (Grade 0 per ASTM D1056, compressing at 0-2 psi), BF-2000 silicone foam (Shore 00 durometer around 10 or lower), and PORON 4701-30 urethane foam. Crushed EPDM is typically the softest overall, but BF-2000 is the softest silicone option and PORON is the softest option with the best compression set resistance.
What durometer is considered ultra-soft for a gasket?
For gasket materials, ultra-soft typically means a Shore 00 durometer of 30 or below. The Shore 00 scale is used for the softest elastomers and foams. Standard silicone sponge measures around Shore 00 20-40, while ultra-soft materials like BF-1000 silicone foam and PORON 4701-30 measure in the single digits to low teens on the Shore 00 scale.
What is a compliant foam gasket and when do I need one?
A compliant foam gasket is a gasket made from a cellular foam that deforms easily under low pressure to conform to mating surfaces. You need a compliant foam gasket when your enclosure has thin or fragile mating surfaces, uneven flanges, low closure force, or requires the gasket to fill gaps without high bolt load. PORON urethane foam, silicone sponge, silicone foam, and crushed EPDM foam are all compliant foam gasket materials.
PORON or silicone sponge: which should I choose?
Choose PORON urethane foam when compression set resistance and dust sealing are the priority, especially in portable electronics with repeated open-close cycles. Choose silicone sponge when you need a wider temperature range (-67F to +500F), UV and ozone resistance, or closed-cell water splash resistance. If you need the softest material possible and water resistance is not critical, silicone foam like BF-1000 is softer than both.