LSR’S BROAD HARDNESS SPECTRUM: TAILORED FOR PERFORMANCE
While today’s popular non-stick bakeware products manufactured out of Liquid Silicone Rubber (LSR) are flexible to the touch, they maintain a certain degree of resistance to deformation. While ear buds or in-ear headphones, on the other hand, require a close and comfortable fit with a flexibility that enables it to match the shape of the inner ear for a more natural feel.
Depending on the desired application of LSR, the silicone molding can be adaptable. The hardness can be varied between 5-90 (Hardness, Shore A) by changing the cross-linker ratio. The hardness is measured as a function of penetration depth of an indenter. The indentation hardness is inversely related to the penetration (the higher the value, the softer the material), and therefore dependent on the elastic modulus and the viscoelastic behavior of the material. This makes silicone molding ideal for a wide range of impressions and applications.
There are twelve different types of hardness measurement devices (durometers) specified in ASTM D2240 or DIN 53 505: Types A, B, C, D, DO, E, M, O, OO, OOO, OOO-S and R. The letter implies a specific indenter geometry and applied force which has to be selected according to the maximum allowed penetration depth. When using silicone molding for LSR, a Type A durometer is recommended in which the indenter should be loaded with a 1 kg weight. The hardness needs to be read within 1 second after the maximum indentation depth is reached.
 ASTM D 2240: Standard Test Methods for Rubber Property – Durometer Hardness; ASTM International, 2010
 Campo, E.A.: Industrial Polymers; Hanser Gardner Publications, Cincinnati, 2007
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