30 Mar History of the Durometer Test
In almost every class I have taught, or seminar I have given about screenprinting, someone will ask a question regarding squeegee hardness and ask if they need to invest in a durometer. And, as an avid researcher, I often read technical papers on the screenprinting process in which the authors attempt to explain squeegee hardness by measuring with a durometer. And, seldom does anyone correctly refer to squeegee hardness, but squeegee durometer. Thus, it has become clear that the term durometer is misunderstood.
by Bill Hood, ASDPT Fellow
What is Durometer
Let’s set the record straight, durometer, like many other hardness tests, measures the depth of indentation. Depth is dependent on the hardness of the material and its viscoelastic properties. It is not a device! The device that measures the hardness of a squeegee is a Shore A Hardness Gauge. The Shore Hardness Gauge measures hardness in 1 of 12 scales with Shore-A and Shore-D being the most popular scales. We use the Shore-A scale to measure the hardness of a squeegee.
Let me start with the common misconception, which is the use of the term durometer. Most people use the term durometer incorrectly when speaking of squeegee hardness. Simplistic thinking has led to a number of devices being named for what they do, rather than what they are. Thus, in 1936, forty-one years after the test was introduced, the Shore Hardness Gauge began being referenced as a Durometer. If your thinking is that if a unit of measurement that contains the word meter must be a device, then how do you explain words like:
- Centimeter – a metric unit of length, equal to one-hundredth of a meter
- Diameter – a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints lie on the circle
- Nanometer – nanometer is a unit of length in the metric system, equal to one billionth of a meter
- Meter – meter is the base unit of length in the International System of Units
- Micrometer – A micrometer (also called a micron) is 1000 times smaller than a millimeter. 1 millimeter (mm) = 1000 micrometers (μm)
- Parameter – a numerical or other measurable factor forming one of a set that defines a system or sets the conditions of its operation
- Perimeter – a path that encompasses/surrounds a two-dimensional shape
The term durometer first appeared in published works in 1895 to describe the measurement of hardness. Durometer is from the Latin word, durus, meaning hardness and the Greek word metron, meaning measurement. Thus, durometer means “hardness measurement.” So, it would seem absurd to say “hardness measure measurement,” when we want to say “measurement of hardness”.
The measurement of the hardness of an object is a commonly employed mechanical test that has been in use in various forms since 1722. Our story of durometer starts with Albert Ferdinand Shore (1876-1936), an American metallurgist. Shore was the founder of the Shore Instrument and Manufacturing of New York City.
Initially, the Rockwell, Brinell and Vickers were a type of instruments used a “Load Indention” method, which measured the depth of penetration of a ball or diamond pyramid pressed into the surface under known loads. They were known as Load Indention Gauges. Inevitably these devices proved clumsy when applied to softer materials as they measured the indentation in situ. While the indentation is permanent in metal, it is transient in an elastomer.
In 1908, Albert Shore had patented a device that recorded the rebound height of a steel ball dropped into a tube on to the metal sample, and this instrument was called the Shore Scleroscope – it too was unsuitable for measuring rubber hardness, as it measured resilience.
Shore was a prolific inventor intent on creating an instrument that could measure hardness without damaging the surface and also be able to measure softer materials.
Finally, in 1915, Shore invented the first non-destructive device to measure hardness. Initially, the numbers on the dial referred to as degrees of hardness. In the 1930s, when someone finally realized that the numbers on the gauge should have a better name, it was decided to refer to them as degrees of Shore, in honor of Albert F. Shore. Thus, 50 degrees became 50 degrees Shore and finally was shortened to only 50 Shore and abbreviated as 50sh. And the device is appropriately known as a Shore Hardness Gauge or Shore Gauge.
Albert Shore was succeeded by his son Fred Shore, who was responsible for introducing a round style gauge in 1941. This device made the operation easier and increased the resolution from 5 degrees Shore to 1 degree Shore.
It is interesting to note that Shore never used the word durometer in any of his many patents on hardness gauges, and always spoke of “measuring hardness.” Nor did any of his devices use the term durometer in their marketing or branding during his lifetime. After the Shore family lost control of the company and it passed through several owners, the meter faceplate was altered to include the word durometer.
Unfortunately, that was where the misunderstanding began. The word durometer implied that the device measured the hardness of a material. But some unwittingly started referring erroneously to the device as a Durometer rather than Shore Hardness Gauge.
The Shore Company eventually became a part of The Instron Corporation in 1995. Indeed, the now generic name ‘Shore” belongs to the Instron Corporation, and they continue to erroneously refer to the Shore Hardness Gauge as a Durometer.
Durometer is a hardness test, measuring the depth of an indentation in the material created by a given force on a standardized point of a Hardness Gauge. The extent of which is dependent on the hardness of the material. This measurement is dependent on its viscoelastic properties, the shape of the standardized point, and the duration of the test.
ASTM D2240 durometers allow for a measurement of the initial hardness, or the indentation hardness after a given time. The basic test requires applying consistent force and measuring the hardness. If a timed hardness is needed, the force is applied for the necessary time and then read. The material under test should be a minimum of 6 mm (0.25 inches) thick.
Shore Hardness Gauge is an instrument typically used to measure hardness on polymeric, elastomeric, and rubber materials.
Shore refers to the degree of hardness results obtained and was named in honor of Albert Ferdinand Shore (1876-1936). There are several Shore scales for materials with different properties. The most common gauges are the ASTM D2240 type Shore-A and type Shore-D. The Shore-A gauge is for softer materials, while the Shore-D is for harder materials.
The ASTM D2240-00 testing standard details 12 gauge scales, depending on the material to be tested; types A, B, C, D, DO, E, M, O, OO, OOO, OOO-S, and R. Each gauge scale results in a value between 0 and 100, with higher values indicating a harder material.
It is essential to know that Shore is a dimensionless quantity that offers a comparative value within any particular scale. There is no simple relationship between a material’s Shore in one scale and its Shore on any other.
And, finally, you should never use Wikipedia as a resource for scientific study.