{"id":6701,"date":"2026-11-09T14:19:11","date_gmt":"2026-11-09T13:19:11","guid":{"rendered":"https:\/\/www.alsace-techniques-etiquetage.fr\/?p=6701"},"modified":"2026-11-09T14:19:03","modified_gmt":"2026-11-09T13:19:03","slug":"compression-set-foam-gasket","status":"publish","type":"post","link":"https:\/\/www.alsace-techniques-etiquetage.fr\/en\/converting\/compression-set-foam-gasket\/","title":{"rendered":"Compression set: why foam gaskets end up leaking"},"content":{"rendered":"
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Compression set is the share of its thickness that a foam fails to recover after being compressed for a long time. A gasket that loses thickness no longer presses hard enough on the mating surfaces, and it ends up leaking. It is listed on the datasheet, but values can only be compared when the test methods match, and it is controlled through the choice of foam and the compression ratio.<\/p>\n<\/blockquote>\n

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Choose the right foam for your gasket<\/a><\/p>\n

What is compression set?<\/h2>\n

Compression set is the difference between the original thickness of a foam and the thickness it recovers after being compressed for a set time at a set temperature. It is expressed as a percentage and often shortened to C-set.<\/p>\n

Rogers<\/a><\/span>, the maker of PORON and BISCO foams, gives the example of a 1-inch specimen that only recovers to 0.75 inch: its compression set is 25%. On a 10 mm gasket, that means a gasket that only comes back to 7.5 mm.<\/p>\n

For a sealing gasket, it is one of the properties that determine service life. The gasket seals because it pushes constantly against both surfaces. When it loses thickness, that pressure drops, and so does the seal.<\/p>\n

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How is compression set measured?<\/h2>\n

The principle never changes: measure the specimen, compress it between two plates down to a spacer, hold it for a set time at a set temperature, release it, and measure it again after a recovery period. The test conditions, however, depend on the standard.<\/p>\n

\n\n\n\n\n\n\n\n\n
Main compression set test methods<\/caption>\n
Standard<\/th>\nMaterials covered<\/th>\nCompression<\/th>\nTime and temperature<\/th>\nCalculation basis<\/th>\n<\/tr>\n<\/thead>\n
ISO 1856, method A<\/td>\nFlexible latex and polyurethane foams thicker than 2 mm<\/td>\n50% or 75% (\u00b1 4%) of thickness<\/td>\n22 h at 70 \u00b0C (158 \u00b0F)<\/td>\nOriginal thickness<\/td>\n<\/tr>\n
ASTM D3574 (as described by Rogers)<\/td>\nPolyurethane foams, including PORON<\/td>\n50%<\/td>\n22 h at 70 \u00b0C (158 \u00b0F), 30 min recovery<\/td>\nOriginal thickness<\/td>\n<\/tr>\n
ASTM D1056 (as described by Rogers)<\/td>\nSilicone foams and sponges<\/td>\n50%<\/td>\n22 h, temperature depends on the material (Rogers cites 100 \u00b0C \/ 212 \u00b0F for silicone); 30 min or 24 h recovery depending on the grade<\/td>\nActual deflection (original thickness minus spacer)<\/td>\n<\/tr>\n
ASTM D395, method B<\/td>\nElastomers<\/td>\n25%<\/td>\n22 h or 70 h at a specified temperature, 30 min recovery<\/td>\nActual deflection (original thickness minus spacer)<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n

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Sources: excerpt of the ISO 1856:2018<\/a><\/span> standard, Rogers guide to compression set<\/a><\/span> and Intertek page on ASTM D395<\/a><\/span>.<\/p>\n

\"Compression<\/a>
The two formulas do not give the same percentage for the same specimen. Click to enlarge.<\/figcaption><\/figure>\n

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Why two foam datasheets are not always comparable<\/h2>\n

Two foams listed at 3% and 6% are not necessarily different. Three parameters change the figure.<\/p>\n