Hardness is the single specification that changes how a silicone product feels, seals and survives. It also causes more sample rejections than any other line on a drawing — usually because it was specified as a single number with no tolerance, or judged by hand instead of by durometer.
Quick answer
Pick hardness from function: 10–20A for gel-like cushioning, 30–40A for soft consumer touchpoints and infant products, 50–60A for general-purpose parts and most seals, 70–80A for structural, wear-resistant or pressure-bearing components. Specify it as a value plus ±5 Shore A.
The Shore A scale in practice
| Shore A | Feels like | Typical applications |
|---|---|---|
| 10–20A | Gel, very soft and sticky to the touch | Cushions, prosthetic pads, vibration damping |
| 30–40A | Soft, squeezable, elastic band | Teethers, facial cleansing brushes, soft grips, breast patches |
| 50–60A | Pencil eraser | Kitchenware, spatulas, seals, phone cases, keypads |
| 70–80A | Firm tyre tread | Industrial gaskets, bushings, high-pressure seals, wear parts |
1. What hardness changes downstream
- Sealing. Softer material conforms to imperfect flanges at low closing force; harder material resists extrusion under pressure.
- Tear strength. Soft compounds tear more easily, so thin walls and sharp internal corners become risky below ~30A.
- Compression set. Higher hardness generally recovers better after long compression, which matters for gaskets that must stay sealed for years.
- Demolding and trimming. Very soft parts stretch and distort during demolding and are hard to trim cleanly, which raises unit cost.
- Perceived quality. On consumer touchpoints, 30–45A reads as premium; above 60A reads as industrial.
2. How hardness is measured — and misread
Durometer readings follow ASTM D2240 / ISO 7619: a minimum 6 mm thickness, a flat surface, and readings taken at a defined instant after contact. Measuring a curved or thin part gives artificially low values, and stacking thin parts is not a valid substitute. If your part cannot be measured properly, agree with your supplier that hardness is verified on a molded button from the same batch.
3. Specify it properly
Write on the drawing: "Silicone rubber, platinum cured, 50 ±5 Shore A per ASTM D2240, verified on molded test button from production batch." That single line eliminates most hardness disputes.
4. Practical selection tips
- Order a hardness sample set (30A / 50A / 70A) in your actual geometry before finalising — hand feel differs enormously with wall thickness.
- If a part must both seal and hold shape, consider two hardnesses in one assembly rather than compromising in the middle.
- Remember that colour loading and filler content shift hardness slightly; approve the final coloured compound, not the natural one.
- For grip surfaces, texture often solves what a hardness change cannot.
Related reading: designing silicone seals and gaskets.
Working with Tongsheng
Dongguan Tongsheng Electronic Technology Co., Ltd. has manufactured silicone rubber molds and silicone products since 2014. We run in-house CNC tooling, compression and liquid silicone injection molding, and a testing lab, from a 2,000+ sqm ISO 9001:2015 certified factory in Humen Town, Dongguan, with 100+ staff and 15 engineers. Food-grade (FDA / LFGB), medical-grade and industrial-grade compounds, Shore A 10–80, Pantone and RAL color matching.
Send 2D/3D drawings (STP, IGS, PDF) and we quote within 24 hours — request a quote, email coraycn@gmail.com, or message us on WhatsApp.