durometers

A durometer is a device used to measure the hardness of materials, especially polymers, elastomers, foams, and some coatings. It gauges how resistant a material surface is to indentation under a standardized force, providing a numerical hardness value on established scales such as Shore A, Shore D, or Shore 00. Durometers are widely used in quality control and material selection to ensure products meet specified performance requirements.

Principle and scales: The instrument applies a fixed preload to a calibrated indenter against the material surface and, after a dwell period, records the indentation. The hardness value corresponds to the depth of indentation on a scale defined by the indenter geometry and force. Shore A is used for softer elastomers and flexible plastics; Shore D for harder plastics; Shore 00 for very soft materials. Each scale uses a different indenter and preset force, as defined in international standards.

Procedure and use: Measurements are taken on flat, smooth, representative areas of a specimen under controlled conditions, typically at a specified temperature and humidity. Multiple readings are usually recorded and averaged. Readings range from 0 (very soft) to 100 (very hard), though the exact interpretation depends on the scale and material.

Standards and calibration: ASTM D2240 is the primary standard in many industries for durometer hardness testing; ISO standards cover similar methods for rubber and plastics. Regular calibration against reference blocks and periodic maintenance ensure accuracy and reproducibility.

Limitations and considerations: Hardness readings reflect surface properties and can be influenced by surface finish, temperature, and material heterogeneity. Results may not directly translate to bulk mechanical behavior, so durometer testing is often complemented by other hardness or mechanical tests.