All About Durometer (Hardness)

What Is Durometer (Hardness)?

Durometer refers to the hardness itself, or a material’s ability to resist localized deformation. It’s often used to measure the hardness of polymers and other nonmetals. Durometer may also refer to Shore hardness or Shore durometer, named after Alfred Shore who developed the Shore hardness scale. The term durometer is also used to refer to the tool that measures the hardness of materials.

Durometer is a unitless value. That means a material’s hardness is relative to the measured hardness of other materials when using the same durometer scale. Hardness values range from 0–100. Materials with higher values are harder than materials with lower values. For example, a golf ball with a Shore hardness of 90A is harder than a pencil eraser with a Shore hardness of 40A. However, it’s important to only compare hardness values from the same scale. A hardness of 40 on the Shore A scale is not the same as a hardness of 40 on the Shore D scale. This is because hardness values are relative to materials on the same scale and no relationship exists between the values of different scales.

How To Conduct a Hardness Test

The steps below describe how to conduct a hardness test: 

1. Identify and attach the appropriate indenting foot to the durometer. The Shore hardness scale that should be used depends on the test specimen material, while the shape and size of the indenting foot depend on the Shore hardness scale.
2. Lay the test specimen on a hard, flat surface that is ¼” thick or greater.
3. Test the Shore hardness of the material by placing the durometer and its indenting foot against the surface of the test specimen.
4. Record the Shore hardness value.

The test method is the same no matter what Shore hardness scale is used.

What Is a Durometer Scale

A durometer scale is a classification that describes the hardness of different materials that are grouped together based on similar attributes such as rigidity and flexibility. Some hardness (durometer) scales, such as Shore A and Shore OO, are intended for softer materials. Others, like Shore D, are for harder materials. Each scale’s values range from 0 to 100, with materials becoming harder as their hardness value increases.

Different Types of Durometer Scales

There are several different types of hardness (durometer) scales that quantify the hardness of different materials. The ASTM D2240 is the standard measurement system used to measure the durometer of thermoplastic elastomers (TPEs), thermoset (vulcanized) rubbers, and elastomeric materials, among others. The 12 different hardness scales are described in the list below:

1. A

Shore A, or Type A, durometer (hardness) scale is used for soft and flexible materials such as: soft, vulcanized, and natural rubber, TPEs, flexible thermosets and polyacrylics, wax, leathers, and felt. Rubber bands have a hardness of about 20A while a tire tread has a hardness of about 70A. 

2. C

Type C scale is used for “medium-hard” rubbers, TPEs, and plastics and overlaps with the Type D scale. The Type C scale is intended for materials that have a hardness higher than 90B, but lower than 20D. For example, a watch band fabricated from a TPE can have a hardness of 20C, or about 12D.

3. D

Shore D, or Type D, scale is used for harder materials such as hard rubbers and rigid thermoplastics such as polycarbonate. For example, a skateboard wheel’s hardness is roughly 50D while a hard hat has a hardness of 80D. The Shore D scale is intended for materials that have a Shore A reading that is higher than 90A.

4. B

The Type B scale is intended for moderately hard rubbers, TPEs, fibrous materials, and paper products. This scale is intended for materials that have a hardness of over 90A, but lower than 20D. While Shore D is also used for materials over 90A, Shore B can be used to measure the hardness of materials on the lower end of the Shore D scale. For example, a caster on a shopping cart has a hardness of 76B or roughly 90A.

5. M

The Type M scale is used for irregularly shaped and thin rubbers, TPEs, and plastics. Examples include O-rings and gaskets. This scale is meant for materials that have a Shore A hardness between 20 and 85.

6. E

Durometer Type E is used to measure the hardness of soft rubbers, foamed elastomers, and medium-density textile windings such as wound threads and yarns.

7. O

The Type O scale is for soft rubbers, TPEs, and plastics and for medium-density textile windings. The scale is intended for materials that have a hardness below 20DO.

8. OO 

The Type OO scale is for exceptionally soft rubbers, TPEs, plastics, foams, sponges, animal tissue, and low-density textile windings. This scale is intended for materials that have a hardness below 20O.

9. DO 

Type DO scale is for materials that have a hardness above 90C. This includes “moderately hard” rubbers, TPEs, and dense textile windings. 

10. OOO

Type OOO is used to measure incredibly soft materials like suit foams, and open and closed-cell foams. 

11. OOO-S 

The Type OOO-S scale is for measuring viscoelastic polymer hardness. This includes objects like bicycle seats, chair cushions, and gel mattresses. 

12. R

Unlike the other scales, the R scale is not a Shore hardness, but rather a Rockwell R scale. The Rockwell hardness scale is similar to the Shore hardness scale but is often used for metals rather than plastics. The R scale is best for metals and hard TPEs, thermoplastic, and thermoset materials.

The three most commonly used scales are the A, D, and OO scales. While the boundaries of many of the durometer scales overlap (such as Shore B and Shore D which are both intended to be used for materials that have a hardness of over 90A), these three scales are comprehensive and can cover a wide range of materials with different hardnesses. It’s important to note, however, that the hardness values between different scales should not be compared, and hardness values between scales should not be converted.

Examples of Materials That Are Often Measured by a Durometer Scale

The list below describes some materials whose hardnesses are commonly measured by a durometer:

1. Vulcanized Rubber

Vulcanized rubbers are rubbers that have been hardened by chemical processes and heating. Vulcanization makes rubbers harder and more elastic than traditional rubbers. The hardness of vulcanized rubbers is often measured by a durometer so engineers can ascertain whether a particular material is ideal for an application. Vulcanized rubbers have hardness anywhere between 30A and 70A and are often used in tires, footwear, and gaskets.

2. Thermoplastic Elastomers

TPEs are used in several applications, from weather seals and shock absorbers to shoe soles and scuba flippers. Hardness (durometer) is measured for TPEs because TPEs are often used in highly abrasive environments in which both abrasion resistance and flexibility are desired. TPE hardness ranges from 20 Shore OO to 90A. 

3. Elastomeric Materials

Elastomeric materials are typically thermoset materials, such as foams, sponges, and rubbers that exhibit elastic or rubber-like properties. Elastomers are often used in adhesives, seals, and flexible parts in automobiles, consumer products, medical devices, and more. Durometer is often tested because these materials fulfill a wide range of applications in which flexibility is important. Durometer for elastomers most often ranges between 20A and 90A.

4. 3D-Printed Materials

3D-printer filaments and polymers can be used to print parts that satisfy a wide range of applications. Therefore, it’s important to know the hardness of these materials to ensure a successfully designed part. Hardness values for 3D-printed materials are often harder than TPEs and other elastomeric materials. Hardness values for 3D-printed materials range from 70A to 100A.

5. Cellular Materials

Cellular materials are porous materials that are considered either open-cell or closed-cell materials. Examples include sponges, foam, cork, and low-density winded textiles. Cellular materials are often used in lightweight construction, noise control, and crash energy absorption. Hence, hardness is obtained to determine the materials that are ideal for different applications. Hardness for cellular materials can range anywhere from 30–70 Shore OO.

6. Gel-Like Materials

Gel-like materials are materials that share the characteristics of both solids and liquids. Examples include silica gels, hydrogels, and gel insoles for shoes. Shore OO tests are used to measure the hardness of gel-like materials. Hardness values typically range from 10 Shore OO to 40 Shore OO.

Durometer (Hardness) and 3D Printing

Yes, the durometer is related to 3D printing. Durometer (hardness) is a property that is often referred to when determining the best 3D-printing materials for a particular application. The hardness of a 3D-printing material has a large impact on the successful implementation of a 3D-printed part.

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