Specifying silicone rubber starts with the cure system and the Shore A window, not the brand name. HTV (high-temperature-vulcanising), LSR (liquid silicone rubber), and RTV-2 (room-temperature-vulcanising, two-part) sit on overlapping but distinct operating envelopes, with HTV/LSR service rated from roughly -60 °C to +250 °C and RTV-2 typically capping near 200 °C for tin-cured systems [S1].
For keypad and sealing duties, Shore A 30-70 is the dominant band; molding-grade RTV-2 is poured at 5-10 kg batches and is the workhorse for architectural ornament, resin-cast duplication, and concrete-stamp masters [S2][S4]. Selection in practice is a 3-axis decision: cure chemistry, mechanical band, and the end-use thermal/chemical envelope.
Cure systems compared: HTV vs LSR vs RTV-2
HTV silicone is supplied as a gum or compound, processed on conventional rubber equipment (extruders, compression and transfer presses) and post-cured at 200 °C / 4 h to drive off peroxide residues and stabilise compression set; tensile strength lands in the 7-10 MPa band with elongation at break 200-700 % [S1]. LSR is a two-part pumpable A+B system, injection-moulded at 120-180 °C with cycle times in the 10-60 s range, and routinely hits 4-9 MPa tensile with 200-600 % elongation. RTV-2 is pourable or brushable, mixed 1:1 or 10:1 by weight depending on condensation (tin) or addition (platinum) cure, demoulding in 4-12 h at 23 °C and full properties after 24-72 h [S2][S4].
Pick HTV for continuous-section extrusions (tubing, profiles, O-rings), LSR for high-volume injection parts with tight flash control (medical, infant-care, automotive connectors), and RTV-2 for mould-making, prototyping, casting, and field-applied seals. The same silicone rubber chemistry shows up across all three; only the processing envelope and the achievable tolerances change.
Hardness, tensile and elongation: matching the mechanical band
Specifying the wrong Shore A window is the single most common cause of silicone seal failures. Below Shore A 20, the material extrudes under gland pressure; above Shore A 70, the seal loses conformability on flange out-of-flatness typical of cast iron and FRP flanges. The general-purpose sweet spot is Shore A 40-60, with 4-8 MPa tensile and 200-500 % elongation [S1].
For dynamic keypads and perishable overlays, designers cluster at Shore A 50-70 with a 300-500 % elongation target so the web over the carbon pill survives 1 million+ actuations. Static O-rings and gaskets usually sit at Shore A 40-60 for compression-set resistance; below 40 the rubber creeps, above 70 the sealing load goes through the gland bolts instead of the elastomer. In the broader industrial rubber family, only silicone and FKM routinely cover the -40 °C to +200 °C continuous-service window; EPDM rubber beats silicone on steam and brake fluid but caps at roughly -50 °C / +150 °C and is not specified for silicone's typical dry-heat duty.
Temperature, chemical and aging envelope
Silicone's headline spec is the thermal band: continuous service from -60 °C to +200 °C is routine, with post-cured HTV/LSR compounds rated for short-term peaks at 250-300 °C, and specialist fluorosilicone (FVMQ) and phenyl-silicone pushing the floor below -100 °C at the cost of tensile strength [S1]. UV and ozone resistance are inherent to the Si-O-Si backbone; no antiozonant wax is needed, which is why silicone dominates outdoor insulator boots, automotive ignition seals, and aerospace pneumatic lines.
Chemical compatibility is narrower than the temperature window suggests. Silicone swells in aromatic and chlorinated solvents (toluene, xylene, trichloroethylene, methylene chloride), in concentrated acids, and in high-pressure steam above roughly 120 °C / 1.7 bar; it resists water, dilute acids and bases, alcohol, and polar solvents like acetone and MEK only in short-term exposure. For hydrocarbon and fuel service, switch to nitrile rubber (NBR) for oils or FKM/FKM-Viton for hot hydrocarbon; silicone is the wrong elastomer for petroleum immersion above 80 °C.
Application-to-grade mapping: keypad, mold, seal, food
Silicone rubber keypads are compression-moulded as a single Shore A 50-70 sheet with a carbon-pill web, then printed and laser-etched; the design intent is a 1.5-3.0 N actuation force and 0.5-1.0 mm travel, achieved through web geometry rather than hardness tuning [S3]. For mold-making, RTV-2 tin-cured systems are the workhorse for plaster, wax, and low-temperature resin casting, with mold life of 20-50 pulls; platinum-cured RTV-2 extends that to 100+ pulls and is required for food-contact and medical parts because no by-products are released [S2][S4].
Food-grade and medical-grade designations are not certifications you can buy off-spec; they require a documented compliance chain. FDA 21 CFR 177.2600 and EU Regulation 1935/2004 cover food-contact, USP Class VI and ISO 10993-5 cover cytotoxicity for medical, and the platinum-cure route plus post-cure (200 °C / 4 h) is the standard mitigation for extractables. A related reference frame is the FKM/FFKM aerospace-seal spec bands in FKM vs FFKM for aerospace seals, which pin a parallel set of cure, hardness and temperature criteria for higher-temperature hydrocarbon service.
Sourcing levers, MOQ and lead-time
RTV-2 silicone rubber for molding is widely quoted in the US$5-8/kg band at 25 kg/pail, with MOQ frequently "not specified" for trading-company listings and 200-500 kg for direct mill orders [S2][S4]. HTV compound and LSR A+B kits move in 20-25 kg packs, with mill-direct MOQ at 1,000-5,000 kg and 3-6 week lead times; trading houses will split a drum for 200-500 kg minimum at a 10-25 % price premium. Custom keypad tooling is a separate line item: a single-cavity steel mould runs US$3,000-8,000, with a 4-6 week build and amortised across 50,000-200,000 parts depending on web complexity [S3].
Spec traps at the sourcing stage: a "food-grade" label without an FDA 21 CFR 177.2600 reference is not food-grade, a "medical-grade" label without USP Class VI documentation is a marketing claim, and Shore A readings taken on uncured or under-cured plaques are unreliable by 3-5 points. Always request a 7-day cured test slab at 23 °C / 50 % RH and a press-cured datasheet for direct comparison.
Failure modes and inspection checkpoints
Three failure modes dominate silicone rubber in service. Compression-set creep: gland loads that exceed 25 % of the elastomer's strain capacity cause permanent set above 30 % after 70 h / 200 °C; the cure is post-cure at 200 °C / 4 h and a Shore A in the 40-60 band. Tear propagation: thin webs under 0.5 mm at dynamic keypads initiate tears at the carbon-pill shoulder; mitigation is a minimum 0.8 mm web thickness and a fillet radius of at least 0.3 mm at the web root. Catalytic poisoning: platinum-cured LSR and RTV-2 are poisoned by amines, sulphur, tin, and some condensation-cure RTV residues; mould contamination with these stops the addition cure, so segregation of tooling and incoming-material QA is a hard line. [S1]
For a comparative read on how these mechanical/thermal criteria line up against another process elastomer, the UHMWPE supplier map 2026 is a useful datapoint on commodity-elastomer sourcing economics, though the chemistry is unrelated.
Verifiable next nodes: request a 7-day-cured test slab from any shortlisted supplier and run Shore A and tensile at your own lab; require a post-cure certificate and an FDA/USP statement on letterhead before PO release; flag any LSR tool that was last used for a condensation-cure RTV for cross-contamination audit.