A durometer is a device used to measure the hardness of a wide variety of materials, from soft rubbers and polymers to hard plastics. Hardness measurements are used for quality control or comparison purposes. Durometers with varying levels of sophistication and accuracy may be used depending upon the application. Analog hand-held durometers with or without peak indicators are used for basic testing. They can be mounted to a durometer operating stand to eliminate load and speed testing variables. Motorized auto loading durometer test stands offer digital speed control, and temperature-controlled chambers.
The basis of a durometer consists of combinations of different indenter shapes and forces applied. Industry standards dictate the geometry and design of different durometer types, also known as durometer scales. In the United States for example, ASTM D-2240 outlines Shore durometer scales, environmental requirements, and sample sizes used in hardness testing. The appropriate durometer scale must be chosen for the material being tested. Two common types include Shore A and Shore D. Shore A is typically used for normal rubber and soft plastics while Shore D is used for hard rubber and plastics. Asker C is another common scale used by shoe manufacturers.
Hardness testing methods differ depending on the scale of durometer being used, but samples being tested must share certain characteristics. Samples should be at least 6 mm (1/4 inch) thick, flat, and parallel. Thin samples should be stacked together.
If using a handheld durometer, pressure should be applied gradually with two hands holding the durometer. However, constant load systems should limit the durometer to descend at 3.2 mm/sec (1/8 inch/sec). In both situations, it is important to apply pressure evenly and without shock load. When repeated measurements are made on the same sample, test points should be separated by 6 mm (1/4 inch). All test points should be atleast 12 mm (1/2 inch) from the edge of the sample.