Elastomer Hardness Selection
Durometer, also known as Shore durometer, is a method for measuring the hardness of materials, such as rubber. The measurement is standardized and measurement ranges from 0 to 100, but it is not tied to any specific unit of measurement. Essentially, durometer is a relative comparison of hardness between materials that have undergone testing using the same measurement scale, device, and standard. The Shore hardness system is a set of globally recognized measurement standards that utilize Shore durometers, which are instruments that follow consistent and standardized techniques for gauging hardness. The aim of these standards is to establish a universal and easily reproducible reference for comparing material hardness.
What are Shore A and Shore D?
Shore durometer numbers are commonly available in two different but interrelated scales, namely Shore A and Shore D, for the majority of rubber materials. Shore A durometers are utilized to measure the hardness of softer materials and exhibit a broad range that encompasses flexible rubbers and semi-rigid plastics with minimal flexibility. On the other hand, Shore D durometers are employed to measure the hardness of harder materials and can be used to evaluate the hardness of hard rubbers, semi-rigid plastics, and hard plastics.
What Actually is Hardness?
Material hardness refers to the ability of a material to resist penetration or permanent indentation. It is crucial to determine the hardness of a material before selecting it for any given application, especially in the case of plastics and rubbers.
In the evaluation of materials, hardness testing is conducted to determine the depth of indentation produced by a standardized pressure foot or indenter under a given force. This process is used across all material types.
What is Hardness Test Method?
Various standardized testing methods exist to measure rubber durometer, with ASTM D-2240 and ISO 868 being two of the most widely used methods. These methods are typically applicable to rubbers that fall within the 20 to 90 Shore A range.
ASTM D-2240 involves the use of a tool that penetrates the rubber under specified conditions to measure the indentation. This method is suitable for evaluating the indentation of vulcanized rubber, thermoplastic elastomers, elastomeric materials, gel materials, and some plastics.
ISO 868, on the other hand, measures the indentation resistance of plastics using two types, namely type A for softer materials and type D for harder plastics. This method enables the measurement of initial indentation or indentation after a specified period to understand how the material performs in certain environments.
It is crucial to recognize that a durometer scale solely measures hardness and does not predict abrasion resistance or wear rate. Therefore, additional testing is necessary to account for these factors.