Rules of Thumb

Shore A Versus Shore D: The Important Differences You Need to Know

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When designing an o-ring for a specific application, material hardness plays an important role. Durometer, or Shore durometer, is a standardized device used to measure the hardness of polymers, elastomers, rubbers and plastics. When testing for hardness, a spring-loaded indenter is applied to penetrate through the specimen. The softer the material, the farther the indenter will penetrate. 

Shore hardness is expressed by a number ranging from 0 to 100 on either a Shore A or Shore D scale. The higher the number on the scale, the harder the material. The major differentiation between the Shore A and Shore D is that Shore A is specified to measure flexible rubbers while Shore D is specified for harder, rigid materials. However, the ranges do tend to overlap at higher levels. 

Measuring Shore A

Shore A hardness is measured based on ASTM D-2240 standards for cured rubber compounds, and ASTM D-1414 standards for finished o-rings. For ASTM D-2240 standards, the material thickness cannot be less than 0.24 inches. 

When it comes to measuring Shore A, typical rubber seals fall within a specific range:

  • 60 Shore A is softer, can stretch more easily and is better on rougher surfaces
  • 70 Shore A is standard in terms of hardness for sealing devices
  • 90 Shore A is very stiff and offers greater abrasion and extrusion resistance

ISO 48 standards are recommended when measuring hardness for softer materials, such as vulcanized rubber or thermoplastic. Softer Shore A materials are often used in delicate hardware applications or when minimum insertion force is required. 

Examples of Shore A materials include: 

  • Rubber bands
  • Automotive tire tread
  • Wheels of roller skates
  • Hydraulic o-rings

Measuring Shore D

The Shore D scale is used for harder materials such as plastics and ebonite. Similarly to Shore A, Shore D is measured based on ASTM D-2240 standards. However, Shore D is also measured according to ISO 868 standards, which can be divided into two types:

  • Type A is used for softer materials
  • Type D is used for harder materials

When durometer testing for Shore D, a hardened steel rod with a 30° conical point and 0.1 mm radius tip is used. This differs from Shore A durometer testing in which a truncated 35° cone with a 0.79 mm diameter is used. The difference in indenter sizes is based on the force required to penetrate the material. Shore A materials are softer and require less of a structured point to indent the material compared to Shore D materials. 

Examples of Shore D materials include:

  • Hard hats
  • Solid truck tires
  • Cast urethane plastic

Want to learn more about durometer testing for your o-ring? 

Ask an engineer today.