Durometer Scales

Durometers that measure the hardness of rubber and plastic are available in a range of scales from foam rubber to hard plastic. The Shore A, B, C, D, E scales are technical standards described by ASTM D2240, Standard Test Method for Rubber Property. Asker C scale is described by the Japanese Standards Association, JIS K 7312. A durometer can calibrated to meet these standards and is verified by an calibration certificate traceable to NIST or ISO/IEC 17025 calibration certificate.

Choose a scale that is appropriate for the material you are testing.
The durometer dial reads from 0 to 100 points.
Readings less than 10 or above 90 should be avoided.

1. If more than 90 points in A scale, use D scale.
2. If less than 10 points in D scale, use A scale.
3. Use other scales to obtain a reading closer to 50 points.

Samples

The surface of the sample must be flat, clean and smooth. (Maximum allowable surface roughness is ±0.001″). The sample size must be at least 0.75″ x 1.75″ and thicker than 0.25″ (6 mm).

If the sample is thinner than 0.25″ (6 mm), several layers of the same material can be stacked on top of each other. Make sure there is no air between the layers (Do not glue layers together).

Measurements from samples that do not conform to the above requirements may be taken, but test results should be used only as a reference.

Testing

Take the average of five measurements.
Select different locations on the sample for each measurement, at least 0.25″ away from any previous measurement. Keep your measurements at least 0.5″ from all edges.
Maintain sample temperature at 23° C, ± 2° (73.4° F) for an hour previous to testing.

1. Place the sample on a flat, hard, horizontal surface.

2. Hold the durometer between both hands over the sample so that the indentor touches it.

3. Push down perpendicularly until the presser foot makes firm contact with the sample.

4. Take a reading.

Creep

Measurements may tend to creep back if read at different times after testing. For example, you may get a slightly different reading if you read the result immediately versus 30 seconds later. It is recommended to keep this aspect of measuring consistent.

Storage

If you are using a durometer that has been stored for a while, press the indentor on a surface about 20 times before using it.

To minimize force and speed variables in testing use a constant load stand.

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 athletic shoe manufacturers.

Hardness testing methods vary depending on the type of durometer being used, but the samples being tested must all share certain characteristics. Samples should be at least 6 mm (1/4 inch) thick, flat, and parallel (Thin samples are stacked together).

If using a handheld durometer, pressure is applied gradually with both hands. However, constant load systems should allow the durometer to descend at 3.2 mm/sec (1/8 inch/sec).  In both situations, it is important to apply pressure smoothly and without shock load. When repeated measurements are being made on the same samples, test points should be separated by 6 mm (1/4 inch) and 12 mm (1/2 inch) from the edge of the sample.

Hoto Instruments specializes in durometers, constant load stands and motorized, fully-featured auto durometers for scales A, B, C, D, DO, and E for rubber and plastic.