Durometer, or hardness, is a material property that describes a material’s tendency to resist localized deformation or indentation. The Shore A hardness (durometer) scale is one of many durometer scales used to measure material hardness. Shore A durometers range from 0 to 100 — the higher the durometer value, the harder the material. The Shore A scale is often used in the polymer industry to aid in material selection, ensure consistent quality products, and easily compare the hardness of materials. However, it’s important to note that the hardness values of two different scales, such as Shore D vs. Shore A, cannot be compared since measured hardness values are relative to other materials using the same scale.
The Shore A durometer scale is commonly used for soft to medium-soft materials such as vulcanized and natural rubber, TPEs (thermoplastic elastomers), flexible polyacrylics and thermosets, leathers, wax, and felt. This article will define the Shore A hardness scale, its use in industry, its benefits, and its limitations.
What Is the Shore A Hardness Scale?
The Shore A hardness scale is a method used to characterize how resistant materials are to localized deformation or indentation. It is one of many scales used to measure the durometer (or hardness) of a material, typically of polymeric materials. The Shore A scale measures the hardness of a variety of materials — from soft rubbers, TPEs (thermoplastic elastomers), and silicones to semirigid plastics.
Hardness values determined using the Shore A scale are relative to the hardness of other materials that have been measured using the same scale. An empirical relationship between the depth of the indentation made in one material compared to a standard of 0 to 100% of the possible indentation depth, is used to report an empirical number known as Shore A hardness.
How Is the Shore A Hardness Scale Used in Industry?
The Shore A hardness scale is widely used in the polymer and plastics industry to characterize the hardness of various materials. Manufacturers use durometer testing as a quality check for manufactured products. For example, silicone hardness or rubber hardness can easily be measured and evaluated against manufacturing standards. Additionally, designers use the Shore A hardness scale to compare potential materials that may be suitable for an intended application.For more information, see our guide on What is Durometer.
What Types of Materials Can Be Tested Using the Shore A Hardness Scale?
Nonmetallic materials such as thermoplastics, thermosets, TPEs (thermoplastic elastomers), silicones, and more are the types of materials tested using the Shore A hardness scale. Shore A durometers range from 0-100, where soft materials like rubber bands have a hardness of approximately 20 A and harder materials like golf balls have a hardness of approximately 90 A. Therefore, nonmetallic materials harder than golf balls, such as certain types of wood, bone, and hard hats are incompatible with the Shore A scale.
What Are the Benefits of Using the Shore A Hardness Scale?
The benefits of using the Shore A hardness scale are listed below:
- Allows manufacturers to determine which material is sufficiently hard (or soft) for a particular application.
- Enables easy comparison of material hardness when using the same scale.
- Gives manufacturers a tool to perform statistical process control, thus ensuring consistent quality products.
What Are the Limitations of the Shore A Hardness Scale?
The limitations of using the Shore A hardness scale are listed below:
- Values cannot be accurately compared to hardness values obtained using other scales.
- Is only suitable for measuring non-metallic materials such as thermoplastics, thermosets, TPEs (thermoplastic elastomers), silicones, cellular materials, and waxes.
How Is the Shore A Hardness Scale Determined?
The Shore A hardness scale is determined by measuring the hardness of different materials and comparing their hardness values using the same hardness scale. The hardness of materials measured using the Shore A scale is relative to the hardness of other materials using the same scale. Hardness values are obtained by the amount of indentation observed when a particular indentation force is applied. Softer materials will have deeper indentations while harder materials will have shallower indentations. For example, a shopping cart caster with a Shore hardness of 90 A is harder than a gel insole with a hardness of 30 A. With enough materials measured using the same scale, the scale is determined by plotting the hardness of each material.
How Does Temperature Affect Shore A Hardness Measurements?
Most materials measurable by the Shore A hardness test will give lower hardness readings at higher temperatures, and higher hardness readings at lower temperatures. Suppliers of elastomeric products such as O-rings design their products to a specific Shore A hardness (e.g., Shore A 70), and report the temperature range in which the material retains an appropriate durometer level (neither too stiff nor too soft) to perform its function.
What Is the Range of Hardness Values on the Shore A Hardness Scale?
The Shore A hardness scale ranges from 0 to 100. Harder materials have higher Shore A values than softer materials. For instance, a rubber band has a hardness of approximately 20 A while a shoe sole has a hardness of approximately 70 A.
What Are Examples of Materials That Use the Shore A Hardness Scale?
The types of applications that use the Shore A hardness scale are shown in Table 1 below:
