Shore hardness — durometer — is a single number describing how much a rubber or elastomer resists a standardized indenter pressing into it. Higher number, firmer material. It is the first spec you give a supplier for any soft part, and it is also the most commonly misused, because hardness alone does not tell you how a part will feel, seal, grip, or last. This guide covers what the number means, what values suit which applications, and what you have to specify alongside it.

What the Shore Scales Actually Measure

A durometer gauge presses a specified indenter into the material under a specified force and reads the resistance on a 0-to-100 scale. There are several scales, and mixing them up is a classic specification error:

  • Shore A — the scale for flexible rubbers and elastomers. Covers everything from very soft gels through tires. Most soft parts in consumer products are specified in Shore A.
  • Shore D — for rigid and semi-rigid plastics and hard elastomers. Used where Shore A tops out.
  • Shore OO / OOO — for very soft foams, gels, and sponge materials that read near zero on Shore A.

The scales overlap at their extremes, and the overlap is nonlinear. Roughly, the high end of Shore A corresponds to the low end of Shore D. Always write the scale with the number: "70 Shore A," never just "70."

Tolerance matters as well. Commercial rubber is typically held to about plus or minus five points, so a "50 Shore A" part can arrive anywhere from the mid-40s to the mid-50s. If your design only works within a couple of points, you need a tighter agreed tolerance and you should expect to pay for it.

Typical Durometer Ranges by Application

Useful starting points, not laws:

  • Very soft, roughly 10–30 Shore A: gel cushions, soft-touch grips, skin-contact pads, squeeze bulbs, anti-vibration mounts for light loads. Feels like a soft eraser or softer.
  • Soft, roughly 30–50 Shore A: most static O-rings and gaskets, keypad membranes, flexible bellows, sealing lips, wearable straps. This is the workhorse band for sealing.
  • Medium, roughly 50–70 Shore A: overmolded handles and grips, wheels for light equipment, tubing, protective bumpers, feet. Firm enough to hold shape, soft enough to feel compliant.
  • Firm, roughly 70–90 Shore A: drive belts, load-bearing bushings, dynamic seals, heavy-duty wheels, structural boots. Little compliance but excellent abrasion resistance.
  • Rigid, Shore D territory: hard rollers, rigid TPU housings, semi-structural elastomeric parts.

For sealing specifically, hardness interacts with compression. A softer compound seals a rough or slightly warped surface at lower closing force — valuable when a plastic housing does not sit perfectly flat. A firmer compound resists extrusion under pressure and holds up in dynamic service. If sealing is what you are after, the gland geometry and the compression set of the compound matter more than the durometer, and it all feeds into whatever IP rating you have promised.

What the Number Does Not Tell You

Two materials at the same durometer can behave completely differently. Hardness is silent about:

  • Tear strength and elongation. A 50 Shore A silicone and a 50 Shore A TPE tear at very different loads.
  • Compression set. Whether the part springs back after being squeezed for a year — the property that decides whether a gasket still seals after long service.
  • Resilience and damping. Two equally firm materials can bounce or absorb energy quite differently, which matters enormously for vibration isolation and impact pads.
  • Temperature behavior. Most elastomers stiffen in cold and soften in heat. A grip that feels perfect at room temperature can feel like a rock in a winter warehouse.
  • Chemical and oil resistance. Entirely a function of the polymer family — silicone, EPDM, nitrile, TPU, TPV — not the hardness.
  • Coefficient of friction and grip feel. Surface texture and tackiness dominate perceived grip. A textured firm material often grips better than a smooth soft one.
  • Compatibility with the substrate. An overmolded soft layer only stays attached if the elastomer family bonds to the rigid substrate — see overmolding and two-shot molding.

The Most Common Mistake

Choosing durometer by squeezing one sample plaque between finger and thumb. Perceived softness depends heavily on the part's geometry: wall thickness, rib structure, curvature, and how it is supported. A 60 Shore A part with a thin unsupported wall feels much softer than a 40 Shore A part in a thick solid section. Teams routinely pick a hardness from a flat sample chip, mold the real part, and find it feels nothing like what they chose.

The fix is simple: evaluate hardness in the actual geometry, at the actual wall thickness, with the actual support behind it.

A Practical Selection Process

  1. Start from function, not feel. Write down what the part must do — seal at a given closing force, damp a given frequency, resist a given abrasion, be gripped by a wet hand. Function narrows the range fast.
  2. Choose the polymer family first. Temperature range, chemical exposure, UV, and any skin or fluid contact requirements decide silicone versus TPE versus TPU versus a thermoset rubber. Only then pick a hardness within that family.
  3. Get a durometer ladder. Ask the supplier for the same compound in three or four hardnesses spanning your range.
  4. Print or machine test parts in real geometry. Flexible 3D printing gets you close for feel studies — see TPU 3D printing for flexible parts — though printed parts will not match molded properties for load-bearing use.
  5. Test with real users in the real environment. Cold hands, gloves, wet surfaces, and long sessions all change the verdict. This is squarely ergonomics territory.
  6. Specify fully on the drawing. Compound designation, hardness with scale and tolerance, color, cure system, and any required certifications.

Two categories need extra care. Anything contacting skin over time, or contacting food or a patient, brings material qualification requirements — for medical devices that means biocompatibility testing, and a supplier's willingness to provide documentation should be part of your selection criteria. Applicable requirements depend on your specific product and market, so treat this as engineering guidance rather than regulatory advice. Second, if the soft part is molded against a rigid one, hardness selection is part of the broader question of how to choose plastic for your product across the whole assembly.

Projects House selects and specifies elastomers as part of mechanical and industrial design work — seals, grips, overmolds, and damping components, with the supplier documentation to back them. If you have a soft part that feels wrong, leaks, or hardens in the field, describe it through our contact form and we will help you get the specification right.