When it comes to materials like rubber, plastic, or foam, one of the most important characteristics to measure is hardness. However, hardness means different things depending on the material. For flexible materials used to make gaskets and washers, durometer is used.
Understanding Durometer
Interestingly, durometer indicates more than just hardness. It also refers to the instrument that is used to measure it. Technically speaking, durometer indicates how resistant a particular material is to indenting or deforming when under pressure. Simply put, durometer lets you know how hard or soft the material feels.
Durometer is usually measured using a durometer gauge, which has a small spring-loaded indenter. When the gauge is pressed into the material’s surface, it returns a reading on a scale, typically from 0 to 100.
More About Durometer Scales
There are several durometer scales, but the two most commonly used are Shore A and Shore D. Shore A is used for softer materials such as rubber, elastomers, and flexible plastics. Shore D is used for harder plastics and thermoplastics. Curiously, these are named after Albert Ferdinand Shore, (1876-1936). He was an American inventor who helped develop the concept of durometer and the measurement scales a century ago.
A low durometer such as 30 means that the material is soft and flexible. A rubber band is a good example. A high durometer such as 90 means that the material is hard and rigid. A skateboard wheel is a great example of a component with a high durometer. A car tire typically has a Shore A durometer around 60-70.
Why Durometer Matters for Gaskets and Seals
Generally speaking, durometer measurements are crucial in engineering, manufacturing, and product design for all types of products. Choosing the right hardness will affect how the part performs, wears, and interacts with other materials.
It is a particularly critical consideration for gaskets and washers. Why? Because it determines the most important factor: sealing effectiveness. Keep in mind that in order for gaskets and washers to fill surface irregularities, they must be soft enough to deform. While doing this usually produces a tight seal, they must also remain firm enough to resist deforming and of course, maintain pressure. Lower durometer seals will perform well in static applications that require conformability. Good examples of this include HVAC flanges or valve cover gaskets. Higher durometer seals are needed in more demanding applications where they must resist extrusion under high pressure. Uses include pump housing flanges, high pressure pipe flanges and engine gaskets.
Final Thoughts
Durometer is a technical term that you don’t usually hear every day. In reality, it is an everyday measurement behind countless products from phone cases to shoe soles. It is particularly important for gaskets and washers, since it determines the most important factor: the ability to create a reliable seal. Ultimately, understanding durometer guides engineers and designers to choose the right material for the job. The one that balances flexibility, strength, and reliability.