The importance of hardness meters

Examining the role that durometers play when measuring the hardness of rubbers and plastics.

Rubber and other elastomeric materials can vary significantly in their hardness. ‘Soft’ materials are generally more easily flexed and more compressible and so can, for example, provide more shock absorption and cushioning.

Due to the fact that material hardness is a key characteristic of an elastomer and one which can be related to important material properties, it is beneficial to be able to both accurately specify a material hardness for solings, and to measure the hardness of supplied product. Hardness measurement is important for both product design approval and for quality control monitoring.

Measurement meters

SATRA manufactures a range of hand-held durometers – both analogue and digital – which can also be bench-mounted. These versatile and quality instruments are ideal for checking the hardness of a wide range of elastomers and plastics.

Each versatile and easy-to-use durometer available from SATRA utilises the Shore method of hardness measurement and features a spring-loaded indentor which project 2.5mm through a hole in the flat foot of the instrument. In use, the flat foot is pressed down squarely on the surface of the material to be measured. If the material is very soft, it will offer little resistance to the indentor pin, which will be pressed into the equipment body only very lightly against the instrument spring. By contrast, testing a hard material will result in the pin being pressed back further into the durometer against the instrument spring. The durometer converts the deflection of the pin into a linear readout from 0 to 100, where 0 is very soft (no pin movement) and 100 is very hard (pin pressed back flush with the base of the instrument foot).

To give discrimination across a wide spectrum of hardness, the Shore system uses a number of overlapping ranges of hardness scales (for example, scales designated ‘Shore A’, ’B’, ’C’, ’D’, ’DO’ and ’OO’). Traditionally, separate instruments are provided for each scale range which includes different indentor diameters and tip geometry, in addition to the rating of the indentor spring for each scale range.

By contrast, SATRA durometers have a patented interchangeable module allowing a wide and useful range of the Shore harnesses scales to be measured on one instrument. Each interchangeable module supplied is calibrated to the durometer head. The only exception to this interchangeability is the softest Shore OO range which, due to its very low indentor loads, needs to be prepared as a separate item. While the instrument is hand-held, SATRA also provides a stand into which it can be mounted. This assists the presentation of the foot squarely to the sample and can improve repeatability and accuracy. However, the value of using a stand will depend on the hardness measurement tasks which need to be conducted. When using a stand, weights (supplied by SATRA appropriate to the meter) are required, which provide a controlled force to press the foot against the material being tested.

There are some important operational aspects to be considered when specifying or reporting hardness values of elastomers and plastics. When testing the hardness of elastomeric materials, the hardness reading is time-dependent. Readings tend to reduce the longer that the indentor is held in place, due to a ‘visco-elastic’ creep effect (that is, a time-dependent increase in strain when the material is subjected to a constant stress). A choice of reading times is available, requiring agreement between the supplier and customer, after which they are stated in specifications or test results.

The hardness of elastomers and plastics can be temperature-dependent, with higher temperatures leading to a lower hardness. Therefore, adhering to standard test temperatures is important to maintain the accuracy of the hardness results.

Another source of errors is sample preparation. Mistakes can occur if the sample to be measured for hardness is not parallel sided and flat, or is not of sufficient thickness. An uneven contact with the surface on a thin unsupported sample could lead to inaccuracies in readings.

We can calibrate durometers against a range of SATRA and international standards.

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