When specifying gasket material, along with factors like strength, working temperature range and chemical resistance, it’s important to consider hardness. Hardness determines how well a material fits against uneven surfaces, with softer materials performing better.
The hardness of rubbers and other elastomeric materials is specified primarily in terms of durometer or Shore number. Here’s an introduction to this rubber hardness scale.
Measurement by Indentation
Hardness is generally measured by pressing a point into a sample of material. Measuring the size, depth, or both, of the resulting indentation indicates the hardness.
For materials softer than metals, hardness is measured with durometer. This uses a calibrated spring to push a conical foot into the material. The foot penetrates further into softer material with depth inversely proportional to hardness.
A procedure for durometer testing is given in ASTM D2250. This standard covers factors like test duration, material temperature, material thickness, and minimum distance of the indenter from an edge.
Rubber Hardness Scales
The readout from a durometer is a dimensionless number on a scale from 0 to 100. A 0 shows the indenter went through the material while a reading of 100 means it left no mark at all.
As the hardness of rubbers and other elastomers covers a wide range – think latex gloves to golf balls – hardness values are reported using one of three scales. Developed by Albert Shore in the 1920s, these scales are identified as 00, A and D.
Originally there were other scales, but these are the only ones used today, and only Shore A and D are relevant for gasket material. The scales overlap so for example, the hardness of a particular rubber could be either 75 Shore A or 50 Shore D.
Specifying Rubber Gasket Material Hardness
Softer rubbers and elastomeric materials are preferred for sealing applications, with Shore A the rubber hardness scale used most often. Common gasket materials like NBR, EPDM and silicone are produced in a wide range of hardnesses: most are available from 30 to 80 Shore A. If you need assistance with hardness specification, Hennig Gasket and Seals can help.
