Rubber Gaskets

The hardness of elastomeric gasket materials is measured with a durometer. Knowing how this device is used helps in interpreting specifications and selecting gasket material.

Durometer Construction

Durometers come in two forms, analog and digital. Analog durometers look like the traditional stopwatch with a single hand that sweeps around the dial. This dial is mounted on a flat foot, from which protrudes a pin. The pin is spring-loaded, so when the foot is pressed against the gasket material the pin moves up into the body of the durometer. The harder the material, the more the pin moves into the body. Or to put it another way, softer materials let the pin press in deeper.

The dial is marked from zero to 100. These numbers have no units but are related to the spring load and the size and shape of the head of the pin, more properly called the ‘indenter.’

Shore Hardness

Spring strength and indenter geometry are specified in ASTM standard D2240. This fixes every aspect of rubber hardness testing, including the size of the ‘presser foot’, sample preparation, the duration for which the indentor is pressed into the material, and calculation and presentation of results.

Rubber and rubber-like materials can vary enormously in hardness, so ASTM D2240 defines a number of different scales. Each scale has its own indenter form and spring load. Gasket materials are typically measured on the Shore A scale. The ‘A’ indenter is a pin of 1.27mm (0.050”) diameter, tapered at 35 degrees to finish as a truncated cone with a flat area of 0.79mm (0.031”) diameter. At a reading of 100 (no indentation,) the spring force will be 8.05 Newtons.

Determining the Hardness Number

According to ASTM D2240, the test specimen should be at least 6.0mm (0.24”) thick. Hardness is calculated as the mean or median of five measurements taken at least 12.0mm (0.48”) from any edge.

A Comparative Measure

Being dimensionless, the Shore A number tells you little about the properties of an individual material. Its real value is as a standardized test method, allowing comparison of alternative materials for elastomeric gaskets.

The question of how hard a gasket should be comes up quite often. For an answer we need to look at what the gasket actually does.

Gasket function

The job of every gasket is to fill an uneven gap between two surfaces, forming a barrier that stops fluid moving to where it shouldn’t be. Larger gaps and more uneven surfaces need a softer gasket. For example, a gasket between two parallel machined pipe flanges can be hard, resisting loads as the joint faces are tightened together. In contrast, the gasket sealing an electrical enclosure needs to be softer and compress more because the enclosure door will tend to bend as it’s latched.

So a general rule is that a gasket should be as soft as possible in order to fill the gap between two surfaces. At the same time it must be strong enough to resist the lateral forces acting on it.

For elastomeric gasket materials two parameters define hardness: Shore hardness and compression force deflection (CFD.) Here’s what these two terms mean.

Shore hardness

Hardness in this context is a measure of how well a material resists a permanent indentation. The hardness of rubber and elastomeric materials is measured on a durometer and reported as a “Shore A” number. Very soft materials like a rubber band will be around 20, a pencil eraser is between 30 to 40, and car tires measure 60 to 70 Shore A.

Compression force deflection

CFD measures firmness and is defined in ASTM standard D1056 as the force needed to reduce the material in thickness by 25%. According to this standard materials are given a grade correlating to their firmness. Grade 0 material needs less than 2 psi to reduce its thickness by 25%, so is very soft. At the other end of the spectrum a grade 5 material needs at least 17 psi to achieve the same compression. A gasket material that compresses easily accommodates variation in the gap between two surfaces without needing more closing force than can be applied by the clamps or latches.