Durometer is a measure of a rubber’s resistance to indentation. More commonly, however, it’s known as a measure of hardness. Modulus, another material property, is the stress or force that’s required to produce a strain. Higher-durometer rubber usually has a higher modulus, but that doesn’t mean the relationship between durometer and modulus is straightforward – especially in terms of stiffness.
Durometer, Modulus, and Stiffness
Durometer is an indirect measurement of stiffness, a term that describes the rigidity of a material. The opposite of stiffness is flexibility or pliability. Softer, more pliable elastomers like the rubber that’s used in pencil erasers have lower durometers. Harder, stiffer rubber like the materials used in shopping cart wheels have higher durometers.
Compared to durometer, modulus is a better measurement of stiffness. Yet stiffness and modulus aren’t precisely the same. Modulus is a property of the constituent material, such as EPDM or silicone rubber. Stiffness is a property of the rubber product’s structure, such as a round cord or square shape. This can add to the confusion, especially because stiffer elastomers generally have a higher modulus.
How to Specify Stiffer Rubber
Material scientists have time to master these distinctions, but what do engineers and product designers need to know? When you need rubber products that are stiffer, remember that durometer is a good first step but not the end of your journey. For critical applications, you’ll also need to specify a modulus range once you’ve selected a specific rubber compound.
For example, a 60-durometer rubber could have a wide range of modulus due to variations in the compound’s formulation. Durometer isn’t uniform across a rubber product either. In fact, it’s one of the most unrepeatable measurements in the rubber world. Especially with molded parts, it’s tough to get repeatable durometer readings because of geometry and operator variability.
Testing Challenges and Supplier Solutions
Rubber is tested for durometer according to ASTM D2240. An indentation is made in the center of a 1/2” x 1” diameter button or plied-up sheet. This indentation hardness is dependent on the modulus, but the story doesn’t end there. As the ASTM D2240 explains, “No simple relationship exists between indentation hardness determined by this test method and any fundamental property of the material tested”.
Do you have questions about durometer, modulus, stiffness, and product selection? By partnering with The Rubber Group, engineers and product designers can get the right rubber products for their specific applications. To learn more about our company and how we can help you, contact The Rubber Group.