Using Hardness Scales With TPEs
“What hardness do you want?” This question is almost always the first question that comes up in a discussion of a customer’s needs concerning a TPE product. Why is this the case? One key reason is that the hardness, or durometer, of a TPE material is one of the few properties of a plastic material that can actually be the subject of a sensory experience…namely, the sense of touch. In the world of TPEs, touch, or haptics, is one of the key criteria designers use when choosing to use a TPE.
What is the Shore Hardness Scale?
Hardness is defined as a material’s resistance to permanent indentation. In the 1920’s Albert Shore developed hardness scales and invented a piece of testing equipment that measured “hardness”, or as it is more technically referred to…durometer. Since Shore invented the testing device and developed the hardness scale, hardness has been referred to as “Shore hardness”.
Hardness in the TPE world is usually measured on the “A” scale from 0 Shore “A” to 90 Shore “A”. TPE materials harder than 90 Shore “A” are typically measured on the “D” scale. A 90 Shore “A” material is roughly equal to a 40 Shore “D” material. Styrenic Block Copolymer (SBCs) TPEs range in hardness from 0 on the Shore “A” scale to the upper ranges of the Shore “D” scale.
Other factors determining scale selection to ensure accuracy:
- Type of material
- Thickness of specimen
- Scale limitation
- Item location
While our sense of touch is not quite as exact as a Shore hardness tester, we can determine and sense, with subjectivity, how soft or hard a material is. The Shore hardness tester produces an objective hardness measurement by using a spring loaded metal indenter that pushes into the material at a controlled force and speed to penetration. A Shore hardness test confirms our less objective determination and, more importantly, allows designers to specify a hardness number on their part specification thereby ensuring product quality and consistency from one part to the next.
Shore hardness is sometimes confused with “stiffness” in the TPE world. While there is perhaps a loose correlation, they are not the same. Stiffness is measured objectively by a flexural modulus test. Stiffness is a great test for something such as tubing applications where the tube may have to bend around a radius or resist kinking. Because hardness is such a key term in the TPE world, many designers use hardness instead of stiffness to specify materials in applications where hardness may not be relevant. This leads to an inability to identify problems when dysfunctionality with a part is present.
Shore hardness is a key enabler in ergonomic enhancement or “soft-touch” applications because these types of applications rely heavily on hardness (or if you will…softness) to determine the optimal therapeutic and haptic effect that a designer is looking for when designing a soft touch product.
If you have any questions about the Shore Hardness scales or questions about TPEs in general, contact Star Thermoplastics by phone or email today.