Hardness is the first number most buyers quote when they request a liquid silicone rubber sample, and it is also the one most often chosen by habit rather than by calculation. Shore A 20 and Shore A 70 are the same chemistry, but they behave like different materials on the press, in the mould and in the hand.
For manufacturers, hardness decides three things at once: how the part feels, how it releases from the cavity, and how much load it can carry before taking a permanent set. Getting it wrong usually shows up late — as flash, as tearing at the gate, or as a customer complaint about a seal that leaks after two months in service.
The trade-off is not linear, and it is not intuitive. In the RUIHE 6250 standard food-grade range, moving from 20 to 70 Shore A raises tensile strength from 5 MPa to 9 MPa, but cuts elongation at break from 700% down to 350%. Tear strength moves the other way: 13 kN/m at 20 Shore A, up to 30 kN/m at 70 Shore A.
That is the whole selection problem in one line. Soft grades stretch and recover; hard grades resist cutting and compression. You rarely get both in the same shot.
Shore A 20–30: teats, valves, spoon heads and soft-touch grips. These need high elongation and clean demoulding rather than strength.
Shore A 40–50: the working range for general seals, gaskets and tubing, where compression set and tear have to be balanced.
Shore A 60–70: structural parts, connectors and grommets that are assembled by force or hold a thread.
Shore A 80: hard parts that still need elasticity, such as wear pads and load-bearing profiles.
Two grades at the same hardness can still perform completely differently. The 6250-HAB series runs the same 20–70 Shore A steps as the standard range but is formulated for higher clarity — around 82% light transmittance and 8% haze — so it can be used for visible parts. The 6450 series trades part of that clarity for colour consistency and higher strength.
Cure behaviour matters just as much. Standard grades should be post-cured at 200 °C for 2–4 hours to drive off volatiles before food-contact use, while non-post-cure (FV) grades reach their final properties straight out of the mould with VOC below 0.35%. If your line has no oven, hardness alone will not tell you which grade fits.
Working inside a single series keeps processing stable. The RUIHE 6250 series covers 20 to 80 Shore A on the same platinum-cure platform, with matched rheology and cure speed, so a mould designed for one hardness can usually run the neighbouring step with only minor parameter changes.
At GUANGZHOU RUIHE NEW MATERIAL TECHNOLOGY CO., LTD, we supply the 6250, 6350, 6450 and FV series with full TDS, MSDS, COA and FDA/LFGB documentation, and we adjust hardness, colour and cure system against a customer drawing.
