Shore Hardness

A durometer presses an indenter of fixed shape into the surface with a calibrated spring. If the indenter does not penetrate at all the reading is 100; if it sinks fully it reads 0. ASTM D2240 defines twelve durometer types — A, B, C, D, DO, E, M, O, OO, OOO, OOO-S and R — and applies to rubber, thermoplastic elastomers, cellular materials, gels and some plastics. ISO 48-4 covers rubber with the A, D, AO and AM scales, the AO scale being intended for soft and cellular rubber. ISO 868 covers plastics and ebonite with types A and D only.

The scales are not interchangeable and there is no formula that converts one to another. Each uses a different indenter shape and spring force, so the same foam gives a different number on each scale. A good rule from durometer practice: if a reading on one scale goes above about 90, move to the next harder scale; if it falls below about 20, move to a softer one. For foam that usually means Shore 00 for soft PU, Shore C or Shore A for EVA and cross-linked PE, and Shore A or D for solid rubber and plastics.

Foam needs care in testing. The specimen should be at least about 6 mm thick, and a thinner sheet can be stacked to reach that thickness, although a single piece is preferred because the layers compress against each other. The reading also depends on how long the indenter dwells, so a specification should state the scale, the dwell time and the thickness tested. A skinned EVA sheet and the same sheet skived matte can give different readings, because the skin is denser than the core.

Watch for Asker C. Footwear and EVA suppliers in Asia often quote hardness in Asker C, a Japanese scale (SRIS 0101, JIS K 7312) designed for sponge and foam. It is a different instrument from Shore C, and a figure of 40 on one is not 40 on the other. When a supplier writes "40 Shore" without a letter, ask which scale and which instrument was used.

For packaging foam, hardness is a supporting figure rather than the main specification. Balaji specifies EVA by density — 60, 80, 100 and 120 kg/m³ — and PU by density from 16 to 40 kg/m³, because density is what governs load-bearing and cushioning on the cushion curve. Within one foam type a denser grade generally reads harder, but the relationship varies with cell structure and formulation, so it should be measured on the actual material rather than read off a chart. Specify a Shore range where the part must feel a certain way, seal against a surface or match an approved sample; specify density and compression set where it must protect a product.