Start with the number nobody specifies
Drawings get approved on geometry and seals fail on material. A cross-section can be dimensioned to two decimals and still leak within a year if the compound loses its recovery. The datasheet is where that outcome is decided, and it is usually the only document in the packet that nobody reads closely. Five properties carry most of the risk.
Durometer is a band, not a number
Hardness is measured as Shore A under ASTM D2240. A compound quoted as a single figure is still supplied across a band, because cure state, thickness and test temperature all move the reading. Specify a range with a tolerance rather than a point value, and remember that harder is not automatically better: a harder section needs more closing force to fill the same groove, and a softer one creeps more under a constant load.
Tensile strength and elongation
Tensile strength and elongation at break come from ASTM D412. Tensile is the stress at failure, elongation is how far the material stretched to get there. For a static seal, tensile is rarely the governing number - a gasket is not being pulled to failure in service. Elongation matters more in practice, because it is what lets a profile stretch over a corner or through a tight bend during assembly without splitting.
Tear strength is the assembly number
ASTM D624 measures resistance to a growing cut. It is the property that decides whether a thin web survives being pulled into a retainer, or whether a nick at a cut end turns into a split along the length. If a profile has thin sections, sharp internal corners or a lip that has to be worked into place, tear strength deserves a place in the specification even though it rarely appears in one.
Compression set is the one that ends service life
ASTM D395 compresses a sample by a fixed percentage, holds it at a stated temperature for a stated time, then measures how much of its original thickness it failed to recover. That residual deformation is what turns a seal into a spacer. Two figures are only comparable if the temperature and duration match, so always ask what conditions the number was measured under - a compression set quoted without them cannot be compared with anything.
Temperature ratings are three numbers wearing one label
A temperature range on a datasheet usually blends a brittle point at the low end, a continuous service figure in the middle and a short peak at the top. Peak figures mean little without a duration. Silicone is chosen less because it survives heat than because it keeps its elasticity there while other elastomers stiffen, and because it stays workable at the cold end where many of them crack. Ask which of the three a quoted number is.
Solid and sponge do not share a scale
Sponge and solid silicone are both called silicone and behave differently in a joint. Sponge is specified partly by density, which sets how much force it takes to compress it, so a low-density section closes a door or lid with far less effort than a solid section of the same size. When the closing force matters, compare density and compression deflection rather than durometer alone.
Certificates have a scope, and the scope is the point
Food contact, flame rating and restricted substance compliance are certified per compound, and often per colour, thickness and end use. A letter covering one grade in one colour does not extend to the rest of the range. Ask for the certificate with its scope, issuing lab and date, and confirm the grade on the certificate is the grade being quoted - this is the step that prevents a compliance problem showing up at goods-in. Our current commercial terms and documentation questions are answered on the FAQ page at /faq/.
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