Hardness & Softness
The property of hardness is easily recognised, but in design it must be specified to achieve a given objective.
Solid rubbers range from 20° to 98° Shore A, where 20° is extremely soft like foam and 98° is as hard as bakelite or nylon. As a reference, the ball of the human thumb is 25°, a Staedtler white rubber eraser 55° and a bath plug 95° Shore A.
Designers use rubber in its whole range of hardnesses and each application has to be individually considered. Once a mould has been produced, it is relatively easy to make the same part in other colours and hardnesses to suit different functions.
Whatever the hardness required, it may still be necessary for a rubber component to deform in order to seal against an uneven surface or to resist abrasion.
All rubber types can be compounded to cover most of the range of hardnesses.
Examples of Components
Hardness is required in a part designed to grip paper rolls. It must resist abrasion and not distort in operation. Conversely, rubber suckers used to lift paper sacks have to be very soft to conform to the rough and porous surface.