What do pencil erasers and hockey pucks have in common? Probably more than meets the eye. See, both of these are made of rubber, but you wouldn’t use a hockey puck to clean up your hastily written notes and a hockey puck made of pencil erasers probably couldn’t hope to reach anything near 100mph even on the strongest slapshot. Why? It all comes down to hardness. In this blog, we’ll take a closer look at durometer and investigate how it impacts gaskets and seals. 

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The first thing to know is that there is a difference between durometer and durometers. That may sound strange, but let’s explain: durometer is a hardness test which measures the depth of an indentation in the material created by pressure applied through a standardized pressure foot as a measure of compression. The depth of this compression is dependent on the hardness of the material, its properties, the shape of the pressure foot, and how long the test lasts. Harder rubber materials are more resistant to compression and permanent deformation after prolonged stresses at different temperatures. On the other hand, the instrument known as a durometer is used for testing the hardness of the plastic and rubber material. 

Different rubber materials and parts on white background

Different rubber materials have different hardness, which makes them suited for specific applications.

Durometer Scales

There are several scales of durometer used for materials with different properties, but the two most common scales, using different measurement systems: these are the ASTM D2240 type A and type D scales. The A scale is for softer materials and the D scale is harder ones.  

When it comes to gaskets and seals, rubber’s hardness is expressed on the Shore A scale. The higher the durometer, the harder the compound. When it comes to this scale, 70 is the standard: lower numbers equal softer materials and higher numbers represent harder ones. Softer compounds will stretch easier and seal better on rough surfaces. Low durometer gaskets (which are 10A, 20A, and 30A) also offer exceptional environmental sealing with relatively low closure force.

Solid gaskets in medium to high durometer range require higher closure force, so they are commonly used on metal and rigid enclosures. Harder compounds are known to have greater abrasion and extrusion resistance—which must always be considered in high pressure applications. However, there are three other properties you should be aware of when it comes to your gasket materials: 

  • Environmental: Environmental resistance supports longer service life. High and low temperatures especially have an impact on elastomers. Heat can affect the material properties and cold temperatures can cause the gaskets to leak. 
  • Chemical: Chemical resistance is critical, but also complicated. Rubber seals on fuel tanks, for example, must stand up to both diesel fuel and biodiesel blends. 
  • Tolerances: If a part is out of tolerance, problems occur. Engineers often understand part tolerances for metal parts, but may be less familiar with tolerances for rubber parts such as door and window seals. Which is why it’s particularly important to partner with experts in non-metallic materials. 

When it comes to gaskets and gasket materials, the term “soft gasket” is a relative one. For a heavy industrial application, a 40 durometer rubber may be considered a soft material, but in electronics, that same 40 durometer material would be considered hard. This is why, when it comes to your nonmetal seals and gaskets, it’s critical to partner with a team of experts who understand the durometer and other properties of rubber and other non-metal materials, and Breiner is staffed by such experts who want to see customers succeed.

Did you find this article helpful? If so, explore similar materials in our Nitrile Butadiene Rubber blog! 

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