Shore hardness is the single most specified property in rubber and elastomer engineering — yet it is also one of the most commonly mis-measured. Incorrect hardness data leads to wrong compound selection, sealing failures, and assembly interference issues. This step-by-step guide explains how to measure Shore hardness correctly and what the values mean for your application.
Shore A vs Shore D Scale
Shore A (ASTM D2240 Type A): indenter is a truncated cone with 35° included angle. Used for soft to medium rubbers and elastomers. Range: 0 (completely soft) to 100 (rigid). Shore D: indenter is a sharp 30° cone. Used for hard rubbers, hard plastics, and elastomers above Shore A 90. Rule: above Shore 90A, switch to Shore D measurement.
Measurement Procedure
1) Condition specimen at 23±2°C for minimum 3h before testing. 2) Specimen minimum thickness: 6mm (stack thin samples to meet this). 3) Apply durometer perpendicular to flat specimen surface. 4) Apply full contact load within 1 second. 5) Read value within 1 second (for instantaneous reading) or 15 seconds (for delayed reading — specified in contract). Take 5 readings across specimen; discard outliers; report mean ± SD.
Common Measurement Errors
Most common errors: measuring on convex surfaces (gives falsely high readings); insufficient specimen thickness (gives falsely high readings); slow contact application (melt relaxation occurs before reading); cold specimens (gives falsely high readings — always condition at 23°C). Blue Diamond internal standard: all hardness reports include specimen temperature, reading time (15s), and number of measurements.
Hardness vs Application Guide
Shore 20A–40A: very soft silicone gaskets, gel pads, comfort foam skins. Shore 40A–60A: standard sealing gaskets, medical silicone, general rubber parts. Shore 60A–80A: automotive seals, EPDM weatherstrips, industrial O-rings. Shore 80A–90A: roller coverings, drive belts, anti-vibration mounts. Shore 90A+/Shore 30D–60D: hard rubber, semi-rigid plastics, gear pads.