Shore A is a measure of indentation resistance: how far a defined point sinks into the material under a defined load. It says nothing directly about tear strength, compression set or chemical resistance. Two compounds at the same 50 Shore A can behave very differently in the same groove.
That is why a durometer conversation usually starts with a question that sounds evasive: what is the part supposed to do?
The trade at the two ends
Softer compounds conform. They seal against a rough or imperfect surface, they release from a core with less force, and they are forgiving of a tolerance that drifts. The cost is that soft sections can tear at a thin neck, extrude into a gap under pressure, and pick up damage when a part is stripped off by hand.
Harder compounds resist. They hold a shape, take a thread, survive abrasion and resist extrusion. The cost is that they need a better surface to seal against, and they transfer more force into the assembly around them.
A working range, not a rule
| Band | Behaviour | Common use | Watch out for |
|---|---|---|---|
| 30–40 | Very conformable, low closing force | Delicate sealing, gentle masking | Tearing at thin necks, extrusion under pressure |
| 40–50 | Balanced, the usual starting point | Plug caps, liners, suction cups | Assuming it suits every groove depth |
| 50–60 | Firmer, better shape retention | Repeated-fit parts, food moulds | Higher demould force on deep draws |
| 60–70 | Stiff, abrasion resistant | Wear surfaces, structural pads | Poor conformance on uneven faces |
The bands overlap on purpose. A 45 Shore A compound with a good filler package can out-seal a poorly cured 35, and a 65 can feel like a 55 if the section is thin. Use the table to narrow the question, then test.
Three checks before the tool is cut
Geometry first
A 300-piece masking plug with a pull tab fails at the neck, not at the barrel. If the neck is thin, going up five points of hardness buys more than any geometry change that keeps the same wall.
Then the groove
For a sealing part, hardness and groove fill work together. A soft part in an over-filled groove extrudes; a hard part in an under-filled groove never touches the mating face. Both look fine on a drawing and both leak in a test rig.
Then temperature and time
Silicone keeps its flexibility across a wide range, which is why it gets specified for hot processes. But prolonged heat still raises compression set. If the part sits compressed at temperature for months, the compound and the cure matter as much as the number on the datasheet.
Changing hardness later
Hardness is one of the four light customizations that do not need a new tool. Colour, laser marking, durometer and packaging can all change on an existing mould, from 300 pieces per variant, adding three days to the standard lead time. It is worth running a small trial at two hardness values before committing a volume order — the second sample is cheap, a rejected shipment is not.
Tell us the failure mode and we will suggest a starting hardness.