Hardness is the most quoted — and most misused — spec in rubber parts. It matters because it drives sealing force, compression behavior and damping, yet it is only one of three levers alongside compound and geometry.
Shore A 20-40: soft seals, cushioning pads, overmolded grips. Low force to seal, excellent conformability, but higher compression set — avoid for high-pressure gaskets.
Shore A 50-70: the workhorse band — most O-rings (50-70A), general gaskets, dust seals, anti-vibration pads. Balanced sealing force and recovery.
Shore A 70-90: load-bearing mounts, bumpers, structural pads, keyed precision parts. Watch stress relaxation on long-term static loads.
Two practical notes. First, hardness tolerance: ±5 Shore A is the industry norm — critical applications should be specified with a tolerance band, not a single number. Second, silicone reads about 5 points softer in feel than natural rubber at the same measured hardness; damping behavior differs too. When in doubt, order samples at two hardness levels — sampling takes 7-12 working days at Rubbrother, and mold fees are refundable on volume orders.