Durometer describes the instrument used to measure hardness, as well as the material’s hardness. Durometer is measured by the depth of an indentation into the material under a standardized force. Softer material will allow a deeper indentation, while harder materials will allow the opposite.
Common scales used today in casting are Shore A and Shore D. Shore A materials are rubbers, while shore D materials are plastics. Durometers range in scales of 10 with acceptable tolerance of plus or minus 5 points. For example a shore 60 A material will have a lower acceptable limit of 55 A, and a higher acceptable limit of 65 A.
What does this mean for you? It is important to understand the look and feel of the various durometers so that you can determine which material is best for your application. In addition to having different feels, the materials will have different properties (i.e. shrink rates, demold time, gel time, etc.) that need to be considered before molding.