Hardness is the first number in every soft-part enquiry and the most misunderstood. A buyer writes Shore 50 A expecting a specific feel and receives a part that is close enough to pass on the bench and wrong enough to be returned from the field. The scale is not precise in the way a dimension is precise, and writing a good hardness specification means knowing what the number does and does not control.
What the Shore A scale measures
The Shore A durometer presses a standard indenter into the material with a defined spring force and reads the penetration. It is a point measurement of resistance to indentation, taken on a flat sample of a stated thickness, and it is sensitive to all three of those conditions. A thin sample reads harder than a thick one of the same material. A surface that has been handled reads differently from one that has not. The number is reproducible when the test conditions are stated, and misleading when they are not.
| Shore A | Feel | Typical parts |
|---|---|---|
| 0 to 10 | Gel-like, conforms to any surface | Soft pads, cushioning inserts |
| 10 to 30 | Very soft, squeezable, elastic | Pure-TPE soft parts, novelties |
| 30 to 50 | Soft but holds shape | Grips, overmoulds, seals |
| 50 to 70 | Firm rubber feel | Bumpers, gaskets, tyres |
| 70 to 90 | Hard rubber, minimal give | Structural elastomer parts |
Choosing between Shore A and Shore D
The A scale runs out around 90, and materials above that move to the D scale, which uses a sharper indenter and a higher force. There is no exact conversion between the scales, and the overlap region around 90 A and 40 D is where mis-specification happens: a part specified 90 A and quoted 40 D is not necessarily wrong, but the two numbers were measured on different scales and the buyer needs the moulder to state which instrument was used. For parts above 90 A, specify Shore D from the start and quote both scales on the first article so the programme has a common reference.
Writing the specification
A hardness specification that a moulder can quote has four elements: the scale, the target, the tolerance, and the test conditions. The tolerance is plus or minus 3 to 5 points on the A scale for most programmes; tighter than plus or minus 3 is achievable but the compound cost rises and the lot-to-lot control becomes the dominant quality issue. The test conditions state the sample thickness and the conditioning, because a part measured at 2 mm and one measured at 6 mm do not agree, and the disagreement lands on the buyer's bench as a rejected lot.
Hardness and the rest of the specification
Hardness does not travel alone. The same compound at the same hardness can differ in tear strength, compression set and colour, which is why a hardness-only specification is not a material specification. The useful sequence is to specify hardness for feel, then the performance property that the part actually needs, then the compliance declaration, and to hold a physical master that carries all three. Programmes that change hardness mid-season re-verify the colour, because the same pigment load reads differently at 10 A than at 40 A, and a colour approved at one hardness is not approved at another.
Related reading
Our custom soft TPE moulding programme covers the pure-TPE family, and the surface finish programme covers texture and finish on silicone parts. The platinum silicone vs TPE comparison covers the material choice behind the number.
Related product categories
The materials and part types discussed above are grouped in our sourcing directory:
