Thermoplastic Elastomers, or Thermoplastic Rubber compounds, as they're sometimes called, are a class of materials that behave a lot like rubber, but process like plastic. TPEs consist of hard and soft phase systems in the form of blends, alloys, or copolymers.
What Are Thermoplastic Elastomers (TPEs)?
Why Shore Hardness Matters for Soft TPE Materials
TPE materials are often used in applications requiring a "soft touch", like a grip on a toothbrush handle, or in applications where flexibility is desired. TPEs can even be used to absorb energy in acoustic or vibrational dampening applications. TPEs are quite unique materials, because depending on the recipe, they can take the form of a super-soft gel or a hard plastic like polypropylene. It is no wonder why "Hardness" is an important property that is measured and specified when talking TPEs.
What is Shore Hardness and How is it Measured?
The Shore Hardness scales were created to provide a common reference point when comparing different materials. Shore Hardness is measured using a durometer gauge, which uses a spring loaded steel rod to compress the surface of the TPE sample and provide a reading. A reading of 0 Shore describes the maximum possible indentation of the rod into the specimen, and 100 Shore indicates almost no indentation al all or a very high resistance to indentation.
Shore Hardness Scales Explained (Shore OO, A, and D)
There are different Shore Hardness scales for measuring the hardness of different materials. For each scale there is either a different probe/indenter shape as well as a specific spring force. The Shore OO Scale measures extremely soft materials like gels (think of gel insoles). The Shore A Scale measures a wide range of material types; from very soft and flexible to semi-rigid plastics with almost no flexibility at all. The Shore D Scale measures very hard rubbers, semi-rigid and rigid plastics (PVC pipe, for example). As you can see from the graphic, there is overlap on the different scales. For example a material with a Shore hardness of 95A is also a Shore 50D. TPEs can range from 10 Shore 00 up to 72 Shore D, spanning all three hardness scales, however, the most commonly used is the Shore A scale.
How Shore Hardness Relates to Other Material Properties
Shore hardness is often used as an indication of flexibility or stiffness, which is called flexural modulus. Flex Modulus measures resistance to bending, whereas hardness measures resistance to indentation. However, within a TPE family there exists a direct relationship between these properties; as hardness increases, so does flexural modulus. The tensile strength of a TPE material, which measures it's resistance to breaking under stress, is also directly related to hardness within a TPE family. One property that varies inversely with hardness is coefficient of friction. Softer TPEs usually contain more oil, resulting in higher surface tack, in general.
Want to Learn More?
Do you have any questions about measuring hardness or about TPEs in general?
Frequently Asked Questions:
Shore hardness scales differ based on the softness or hardness range of the material being measured. Shore OO is used for very soft, gel-like materials such as foams and soft elastomers. Shore A measures flexible rubber and soft thermoplastic elastomers (TPEs), while Shore D is used for harder elastomers and rigid or semi-rigid plastics.
Shore A is the most commonly used hardness scale for many TPE materials, particularly those with a rubber-like feel and moderate flexibility. Very soft TPE compounds, such as gel-like materials used in applications like footwear insoles, are often measured on the Shore OO scale, while harder TPEs may be evaluated on the lower end of the Shore D scale depending on formulation and end-use requirements.
Shore A and Shore D use different durometers and cannot be directly converted, but they overlap in the mid-range of hardness. Shore A measures softer, more flexible materials, while Shore D measures harder, stiffer materials. Comparing Shore A vs Shore D values helps engineers understand transitions between elastomeric and plastic-like behavior.
A Shore 95A hardness indicates a very firm elastomer or TPE that is still flexible but offers high resistance to indentation. Materials at 95A Shore hardness are often used in applications requiring durability, abrasion resistance, and structural support while maintaining some elasticity.
Shore hardness values are approximate, not absolute. Results can vary based on test conditions such as sample thickness, temperature, surface finish, and dwell time. Shore hardness charts are best used for relative comparison, not as precise material specifications.
Shore hardness helps designers and engineers quickly compare materials based on feel, flexibility, and stiffness, making it a critical screening tool during material selection. It influences performance factors such as grip, sealing ability, abrasion resistance, and user experience across rubber, TPE, TPU, and plastic applications.
A durometer is the instrument used to measure Shore hardness by determining a material’s resistance to indentation. Different durometers are designed for different Shore scales, such as Shore A or Shore D, ensuring accurate measurement across soft elastomers, TPEs, and harder plastics.
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