When designing custom rubber parts, durometer is one of the most important — and often overlooked — properties to get right. The durometer of a rubber compound determines how soft or hard it feels, how it compresses, and how it performs under load.

Choosing the wrong hardness can lead to seal failures, poor vibration control, or parts that simply don’t fit as intended. In this post, we’ll break down what durometer means, how it’s measured, and how it affects the performance of your rubber components.

What Is Durometer?

Durometer is a measure of a material’s hardness — specifically, its resistance to indentation. It’s usually measured on the Shore hardness scale, which was developed to evaluate the hardness of elastomers and plastics.

For most rubber materials, the Shore A scale is used. This scale ranges from:

  • 0 Shore A — extremely soft and flexible (like a rubber band)
  • 100 Shore A — very hard and rigid (similar to a shopping cart wheel)

The measurement is taken using a durometer gauge, which presses a small indenter into the material’s surface under a standardized force. The depth of indentation determines the hardness reading.

Why Durometer Matters

Rubber hardness plays a direct role in how a part performs under pressure, heat, and motion. A seal that’s too soft may compress too much and leak; one that’s too hard may fail to conform to its mating surface.

Here’s how durometer impacts key performance areas:

Property Soft Rubber (Lower Durometer) Hard Rubber (Higher Durometer)
Compression & Flexibility More flexible and easily deformed Stiffer and resists compression
Vibration Damping Better energy absorption Lower damping capacity
Abrasion Resistance Lower Higher
Tensile Strength Moderate Generally higher
Sealing Ability Excellent for uneven surfaces Best for smooth, tight tolerances

In short: soft rubbers seal better, while harder rubbers last longer under load.

Typical Durometer Ranges by Material

Each rubber material has its own hardness range and can be formulated to meet specific performance goals:

Material Typical Durometer Range (Shore A) Common Applications
Silicone 30–80 Seals, gaskets, medical devices
EPDM 40–90 Outdoor seals, automotive weatherstripping
Nitrile (Buna-N) 40–90 Oil-resistant gaskets, hydraulic seals
Neoprene 30–90 Shock mounts, industrial seals
FKM (Viton) 60–95 High-temperature chemical-resistant seals

Selecting the right hardness is a balancing act between flexibility, strength, and durability.

Design Considerations for Rubber Hardness

When specifying durometer for your part, consider:

  1. Compression Set Resistance — A harder compound may resist deformation over time, but can also increase sealing pressure.
  2. Operating Environment — Heat, chemicals, and exposure to oil can change a material’s hardness over its lifespan.
  3. Mating Surface — Softer rubber is ideal when sealing against rough or uneven surfaces.
  4. Part Geometry — Thicker sections can feel stiffer even at the same durometer rating, while thinner walls may require harder materials to prevent collapse.

Working with an experienced manufacturer can help you dial in the right hardness for your exact use case.

Choosing the Right Partner

Selecting the correct durometer isn’t just about picking a number — it’s about understanding how your part will perform in the real world. The best results come from collaboration between designers, engineers, and manufacturers.

How Primo Rubber Co. Can Help

At Primo Rubber Co., we help customers optimize durometer selection for performance, cost, and manufacturability. Our team has extensive experience molding rubber parts across a full range of hardness levels and materials — from soft silicone gaskets to rigid, high-strength bushings.

If you’re designing a new product or troubleshooting an existing one, we can help you choose and mold the right rubber compound for your needs.

Contact Primo Rubber Co. today to learn more about our custom rubber molding capabilities and material options.