Durometer: what it is for, how it works, parts and types

A 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 indicated hardness value 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 specifically used 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.

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

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

What is the durometer for?

The durometer used with elastic materials, or also called the Shore durometer, is a device that, as its name implies, is used to measure the hardness of an elastomer or rubber material; or rather, measure the sealing ability of a part.

Higher numbers on the scale indicate that the elastic material has a greater resistance to indentation or penetration by the part 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 larger the durometer of an elastic part, the greater its hardness. For example, the rubber bands they used to hold have a durometer around 20 A. While pencil erasers have a 40 A durometer and the wheels on the market, a 95 A durometer.

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This indicates that car tires on the market have a greater hardness than rubbers and treads.

How does the durometer work?

The part for which you want to determine the hardness is placed on a horizontal surface. Depending on the material to be examined, the elastic part must have a minimum thickness. Also, it must be flat to ensure an accurate measurement of the hardness of the part.

Measurements taken repeatedly at the same point on the part produce an attenuation of the durometer readings. Therefore, a distance of at least 6 mm between successive hardness readings is recommended.

Depending on the type of durometer, in one way or another it is used to apply a force to a graduated spring that compresses and transmits a force on a part to which it is connected (indenter) and will penetrate into the examined sample.

Hardness measurement is performed 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 in which its hardness is being measured.

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

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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 an adequate interaction of the durometer with it.

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

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

The force applied to the spring depends on the type of material being studied. Can be applied manually or using a lever.

Indicator

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

Types of Durometers

Type A

It is applied to rubbers and elastomers. The indenter has a 30° 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 above a hardness value of 93 A. The indenter has a conical shape of 30º. A force of 821 gf is applied to the spring.

Type C

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

Type D

It is applied to hard rubbers and plastics such as thermoplastics, floors and bowling balls. The indenter ends in a 30° 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  

It is applied on materials as thin as 0.050 inches. The indenter ends in a 30° conical shape. A force of 78.0 gf is applied to the spring.

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Type O

It is applied to 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 spherical tip with a radius of 1.2 mm. A force of 821 gf is applied to the spring.

Type OO

It is applied in light foams and soft rubber gels. The indenter ends in a spherical tip with a radius of 1.2 mm. A force of 113.3 gf is applied to the spring.

Type OOO

It is applied in ultra-soft gels and sponge rubber. The indenter ends in a spherical tip with a radius of 6.35 mm. A force of 113.3 gf is applied to the spring.

Type DO

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

Type OOOS

The indenter ends in a spherical tip with a radius of 10.7 mm. A force of 197.0 gf is applied to the spring.

Examples of hardness of materials

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

-Jelly, 10 Shore OO hardness (extra soft)

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

-Heel, hardness Shore A 80, Shore D 30 (hard)

-Plastic shell, Shore D 80 (extra hard)

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