Almost every grade on this site carries a Shore A number. It is the single most-specified and least-understood property in rubber, so here is what it means in practice.
What the number is
Shore A is a durometer scale. A spring-loaded indenter is pressed into the rubber and the resistance is read from 0 to 100. Higher is harder. It measures resistance to indentation — nothing else. It does not tell you tensile strength, tear resistance, temperature range or chemical resistance, and you cannot infer any of those from it.
A separate scale, Shore C or Shore 00, is used for sponge and foam, because Shore A cannot read anything that soft meaningfully. That is why our sponge rubber and sponge silicone are quoted in Shore C.
The bands, in things you have held
- 30–40 Shore A — a rubber band, a soft silicone baking mat. Conforms to almost anything. Tears easily.
- 50–60 Shore A — a pencil eraser, a bicycle inner tube. The most common gasket range.
- 70–80 Shore A — a car tyre tread, a shoe heel. Firm, holds its shape under load.
- 90 Shore A — a shopping trolley wheel. Nearly rigid. Will not conform to an uneven surface.
How to choose
Sealing an uneven or low-load flange
Go soft: 40–50 Shore A, or sponge. A hard gasket needs high bolt load to seal, and if you cannot deliver that load it will leak no matter how good the material is.
Sealing a machined flange with real bolt load
60–70 Shore A. Hard enough to resist extrusion out of the joint, soft enough to take up the surface finish.
Wear and impact
Counter-intuitively, softer is often better. Soft natural rubber absorbs impact energy and springs back; hard rubber transmits it and chips. For chute liners and skirting, 40–60 Shore A natural rubber outlasts an 80 Shore A compound.
Anything under sustained compression
70–80 Shore A, and pay attention to compression set rather than hardness. A soft grade will take a permanent squash.
Two things worth knowing
Tolerance is ±5 points. A "60 Shore A" sheet is anywhere from 55 to 65. If your design only works in a 3-point window, the design is the problem.
Hardness moves with temperature. Every elastomer gets harder as it cools. A 50 Shore A gasket at −30 °C behaves like a much harder one, which is exactly why cold-weather seals leak.
Still not sure?
This is what samples are for. A silicone sample pack gives you 40, 50, 60, 70 and 80 Shore A side by side, and five minutes with them in your hand settles the question faster than any datasheet.