DUROMETER: WHAT IT IS FOR, HOW IT WORKS, PARTS, TYPES - PHYSICAL - 2025

The durometer is a device used to measure the hardness of a material. However, the word durometer is also used to designate the degree of hardness of a material. It should be understood that the use of the word durometer, in this case, refers to the fact that the stated value of hardness was obtained through the use of equipment called a durometer. For example, the pencil eraser has a Shore A 40 durometer (hardness), which means it is medium soft.

The word durometer is used specifically to designate an international standard for measuring the hardness of rubber, plastics, and many non-metallic materials. Hardness is measured on a Shore A or D scale.

Digital hardness tester with its respective case. Source: Miladfarhani / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)

Shore hardness is measured in a durometer, in which an indenter is dropped onto the surface of a body, and its rebound is then measured. Albert Ferdinand Shore developed his apparatus for measuring hardness in the 1920s.

Hardness, on the other hand, is a measure of the physical alteration that a material can experience when subjected to the action of penetration, abrasion or scratching. The less the physical alteration of a material, the greater its hardness.

What is the durometer for?

The hardness of pencil erasers is measured using a durometer. Source: Pexels.

The durometer used with elastic materials, or also called the Shore durometer tester, is a device that, as its name indicates, is used to measure the hardness of an elastomer or rubber material; Or rather, measure the sealing capacity of a piece.

The higher numbers on the scale indicate that the elastic material has a greater resistance to indentation or penetration of the piece of the durometer that fulfills this function. Therefore, the greater its hardness. But the term durometer is also used to indicate the hardness of a material.

Thus, the greater the durometer of an elastic part, the greater its hardness. For example, the rubber bands used to hold, have a durometer around 20 A. While, pencil erasers, have a durometer of 40 A, and the wheels of shopping carts, a durometer 95 A.

This indicates that market cart wheel rubbers are harder than pencil erasers and band rubbers.

How does the durometer work?

Place the piece whose hardness is to be determined on a horizontal surface. Depending on the material to be examined, the elastic piece must have a minimum thickness. In addition, it has to be flat to guarantee an accurate measurement of the hardness of the piece.

The measurements made repeatedly on the same point of the piece, produces an attenuation of the durometer readings. Therefore, a distance of at least 6 mm is recommended between successive hardness readings.

Also depending on the type of durometer, one way or another is used to apply a force on a graduated spring that is compressed and transmits a force on a part, to which it is attached (indenter) and will penetrate the examined sample.

The hardness measurement is carried out on an indicator that has a scale from 0 to 100, with 100 corresponding to the maximum hardness. That is, when the indenter does not penetrate the part on which its hardness is being measured.

The force should be applied as quickly as possible, and the hardness reading is made for a duration of 15 seconds ± 1. Therefore, a stopwatch should be used while the force is being applied.

Parts

Presser foot

It is located at the bottom of the durometer and is flat and perpendicular to the indenter. The presser foot makes contact with the material to be examined and ensures proper interaction of the durometer with it.

Identer

The indenter is perpendicular to the presser foot and protrudes from it. It is a piece whose lower end penetrates the material to be examined and whose shape varies depending on its type.

The indenter is attached to a calibrated spring that compresses when force is applied to it and transmits this force to the indenter for its penetration into the material under study.

The force applied to the spring depends on the type of material being studied. It can be applied manually, or using a lever for it.

Indicator

An indicator that indicates the measurement of the hardness of the elastic material on a scale between 0 and 100. Its shape can vary depending on the requirements of its use.

Durometer types

Type A

It is applied in rubbers and elastomers. The indenter has a 30º conical shape and the tip is truncated. A force of 821 gf is applied to the spring.

Type B

It is applied to harder elastomers, plastics, paper and fibrous materials. It is recommended to use over the hardness value 93 A. The indenter has a 30º conical shape. A force of 821 gf is applied to the spring.

Type C

Medium hard elastomers and plastics. Useful for not leaving a mark on the surface of the material examined. The indenter has a 30º conical shape and ends in a truncated point. A force of 4,533 gf is applied to the spring.

Type D

It is applied on hard rubbers and plastics such as thermoplastics, floors and bowling balls. The indenter ends in a 30º conical shape. A force of 4,533 gf is applied to the spring.

Type E

The indenter ends in a spherical shape with a radius of 2.5 mm. A force of 821 gf is applied to the spring.

Type M

Applies to materials as thin as 0.050 inch. The indenter ends in a 30º conical shape. A force of 78.0 gf is applied to the spring.

Type O

It is applied on very soft elastomers, textile windings and soft granular materials. It is recommended to use below the hardness value corresponding to 20 A. The indenter ends in a ball point of 1.2 mm radius. A force of 821 gf is applied to the spring.

OO type

It is applied in light foams and spongy rubber gels. The indenter ends in a 1.2 mm radius ball point. A force of 113.3 gf is applied to the spring.

Type OOO

It is applied in ultra-soft gels and spongy rubber. The indenter ends in a 6.35mm radius ball point. A force of 113.3 gf is applied to the spring.

DO type

It is applied on dense granular material and bovine textile. The indenter ends in a 1.2 mm radius ball point. A force of 4,533 gf is applied to the spring.

OOOS type

The indenter ends in a 10.7 mm radius ball point. A force of 197.0 gf is applied to the spring.

Examples of the hardness of materials

Finally, four examples of materials whose hardness has been measured with a durometer will be listed:

-Gummy jelly candies, 10 Shore OO hardness (extra soft)

-Rubber bands, 62 Shore OO hardness. Shore A 20 (soft)

-Shoe heel, hardness Shore A 80, Shore D 30 (hard)

-Plastic shell, Shore D hardness 80 (Extra hard)

References

1. Smooth-On. (2020). Durometer Shore Hardness Scale. Recovered from: smooth-on.com
2. Wikipedia. (2020). Shore durometer. Recovered from: en.wikipedia.org
3. Lee Hofmann. (2016, November 10). What is Durometer? Recovered from: urethaneusa.com
4. Rothkopf. (January 02, 2018). The Importance of Durometer. Recovered from: rothkopf.com
5. Rex Gauge Company. (2020). What is a Durometer. Recovered from: durometer.com
6. Asker. (sf). What is durometer? Recovered from: asker.co.jp