Full definition
ASTM D2240 is the standard test method for measuring the hardness of elastomers and plastics through durometer hardness testing. This test is crucial in various industrial applications where the hardness of materials can significantly affect their performance and suitability for specific uses. The standard outlines several scales to accommodate different hardness ranges, with the Type A scale being the most prevalent for rubber materials. The Type A durometer utilizes a truncated cone indenter, measuring hardness on a scale from 0 to 100, and is designed to cover the range of 20 to 90 Shore A, which encompasses most common elastomers found in industrial applications. Types D, M, and O serve specialized needs, with Type D addressing harder rubber and rigid plastics, while Type M is tailored for micro-hardness testing of thin rubber specimens and Type O is used for softer sponge rubber.
The testing procedure is straightforward yet precise. A durometer must be pressed firmly and squarely against the flat surface of the rubber specimen, which must have a minimum thickness of 6 mm and a diameter of at least 12 mm. Readings should be taken after one second of contact, as specified by ASTM, although it is important to note that the DIN 53505 recommends a three-second reading, which may yield slightly different values. To ensure accuracy, at least five readings should be taken from locations that are at least 6 mm apart, and the average of these readings is recorded to the nearest whole number. Calibration of the durometer against standard test blocks is essential for maintaining measurement accuracy. The standard temperature for testing is specified as 23 ± 2°C. Given that hardness is one of the most frequently tested properties of rubber, it is included on nearly every rubber data sheet and is a key element of incoming inspections in quality control processes.
For manufacturers and quality control professionals, following the ASTM D2240 standard is vital, as it ensures that the hardness of the rubber products meets specified requirements. The importance of this standard also extends to product development and material selection, as knowing the hardness helps in evaluating the suitability of rubber components for various applications. Furthermore, ASTM D2240-15(2021) is aligned with international standards, being equivalent to ISO 48-4 for Shore methods and ISO 48-2 for the IRHD method, promoting consistency in testing across global markets.