Selecting the correct rubber hardness, or durometer, is one of the most important decisions in designing a rubber part. Whether you’re developing a seal, suspension bushing, vibration isolator, or molded gasket, the hardness of the elastomer affects how the part compresses, seals, flexes, absorbs vibration, and ultimately survives long-term use.

This guide from Primo Rubber Co. breaks down what Shore A hardness is, how it’s measured, how it affects performance, and how to choose the right durometer for your application.

What Is Rubber Hardness (Shore A)?

Rubber hardness is measured using the Shore A scale, which ranges approximately from:

  • 0 Shore A — very soft, gel-like
  • 20–40 Shore A — soft, flexible rubber
  • 50–70 Shore A — medium hardness (typical for many seals, gaskets, bushings)
  • 80–95 Shore A — very firm, rigid elastomers

The test uses a device called a durometer that presses a probe (an indenter) into the rubber surface under a standardized force. The depth of the indentation determines the hardness value.

Why Durometer Matters

Rubber hardness directly affects:

1. Compression and Sealing Performance

Soft rubber (20–50 Shore A) seals easily but can extrude or deform under pressure.
Hard rubber (70–90 Shore A) resists deformation but requires more force to compress.

2. Vibration and Shock Isolation

Lower durometers absorb more vibration.
Higher durometers transmit more vibration but provide greater control and precision.

3. Abrasion and Durability

Harder compounds often resist abrasion better, but some soft rubbers (like natural rubber) are also highly abrasion-resistant depending on the formulation.

4. Assembly and Fit

Soft rubber conforms to surface irregularities.
Hard rubber maintains geometry and tight tolerances better.

How Durometer Is Tested

Shore A hardness testing follows ASTM D2240. Key details include:

  • A spring-loaded indenter presses into the rubber
  • The test is done quickly to avoid creep behavior
  • Parts should be at standard temperature and thickness
  • Multiple readings are typically averaged

Note that thin parts or complex geometries can produce misleading readings because they may flex during testing.

Typical Durometer Ranges by Application

Here are general guidelines; actual needs depend on geometry, compression, and loading.

20–40 Shore A

  • Soft gaskets
  • Conformal seals
  • Medical device membranes
  • Flexible diaphragms

50–60 Shore A

  • General-purpose seals
  • Electrical enclosures
  • Automotive weatherstripping
  • Pump seals

65–75 Shore A

  • Suspension bushings
  • Industrial mounts
  • Valve seals
  • Rubber rollers

80–95 Shore A

  • Heavy-duty isolators
  • High-pressure seals
  • Structural bumpers
  • Hard wheels or rollers

How to Choose the Right Durometer

Here are the most important factors to consider:

1. What forces will the part experience?

Higher forces may require a harder rubber to avoid permanent deformation.

2. How much compression or deflection is acceptable?

Soft rubbers compress easily; hard rubbers require more load.

3. Is vibration isolation important?

Go softer for greater damping, unless precision is needed.

4. Will the part face abrasion or wear?

Harder rubbers often resist abrasion better, but not always—compound choice matters.

5. What tolerances are required?

Harder rubbers can hold tighter dimensional tolerances.

6. Is chemical resistance a major factor?

Hardness alone doesn’t determine chemical resistance; material type matters too.

7. What is the part thickness?

Thin parts behave “softer” during assembly; you may need a higher durometer to compensate.

Additional Considerations

Durometer Drift Over Time

Rubber tends to harden slightly as it ages due to:

  • Heat exposure
  • Oxidation
  • UV and ozone
  • Chemical exposure

Compounds can be formulated to slow this effect.

Use of Dual-Durometer Designs

Parts can be molded with two hardness levels:

  • Soft exterior for compliance
  • Hard interior for structural support

This is common in suspension bushings and vibration mounts.

Summary: Picking the Right Hardness

Requirement Recommended Durometer
Soft seal / easy compression 30–50 Shore A
General-purpose seal 50–60 Shore A
Controlled movement (bushings/mounts) 65–75 Shore A
High precision or structural load 80–95 Shore A

Need Help Choosing the Right Durometer?

Primo Rubber Co. specializes in injection molding custom rubber components. Contact Primo Rubber Co. today to discuss your project and find the ideal material and design solution for your rubber parts.