General-purpose silicone rubber covers Shore A 30 to A 70 and is the default choice for everyday gaskets, O-rings, tubing, and profiles where the working envelope stays between roughly -50°C and +200°C, with upper excursions above 100°C (212°F) tolerated without permanent compression-set damage [S1]. For most fabrication shops, the first decision is not the color or the durometer: it is the curing chemistry and the feed form, because those two parameters lock the molding method, the cycle time, and the unit cost [S5].
Three feed forms dominate general fabrication: solid HTV/HCR gum for compression, transfer, extrusion and calendering; liquid silicone rubber (LSR) for fully automated injection molding at 170-190°C; and RTV systems for low-temperature sealing, potting, and prototype tooling [S5]. Material selection typically starts with processing method, then durometer, then specialty properties (flame retardancy, FDA, medical grade, electrical insulation) [S1][S5].
HTV/HCR vs LSR vs RTV: feed form, cure, processing
HTV (high-temperature vulcanized) silicone, often called HCR or solid silicone, is the workhorse for compression molding, transfer molding, extrusion, and calendering, and is supplied as a putty-like gum that is heat-cured with peroxide or addition-cure systems at typical mold temperatures of 150-180°C [S5][S6]. HCR portfolios from medical-oriented suppliers cover a broad fabrication envelope, with hardness, tensile, and elongation adjustable per application [S3]. Compression molding remains the dominant fabrication method for heat-cured silicone because of its low tooling cost on simple geometries and short changeover [S6].
LSR is a two-component, low-viscosity liquid pumped through cold-runner systems into heated molds, with a typical processing window of 170-190°C mold temperature, 80-120 bar injection pressure, and 15-30 second cycle times for thin-wall parts [S4]. The viscosity advantage of LSR is what enables micro-features, flash-free production, and automated vision inspection at multi-million-part annual volumes [S4]. RTV (room-temperature-vulcanized) silicones cure without heated molding pressure and are typically reserved for sealing, bonding, encapsulation, potting, repair, and short-run mold-making rather than volume part production [S5].
Hardness, tensile, and the Shore A 10-80 envelope
General-purpose silicone spans Shore A 30 to A 70, which is the spec band most often quoted for everyday molded and extruded goods [S1]. The wider LSR envelope runs from Shore A 10 (very soft, gasket-like) to Shore A 80 (semi-rigid, keypads and valve seats), and that is the band an automated LSR cell is designed to hold within ±0.5°C mold-temperature stability [S4].
Beyond hardness, the spec sheet that matters for fabrication includes tensile strength (typically 5-10 MPa for general grades, higher for high-strength HCR), elongation at break (often 200-700%), tear strength, compression set (often 15-30% after 22h/175°C for general grades), specific gravity, and post-cure requirements [S1][S3]. For general fabrication, a single grade covering the full durometer window is rarely the best choice; the molding method and the part geometry should drive the grade [S1][S4].
Selection criteria: method, volume, geometry, compliance
Compression molding (HTV/HCR) is the right pick when part volume is modest (under roughly 10,000 parts/year), wall sections are 1.5 mm or thicker, tooling budget is constrained, and the part tolerates a small flash line and manual deflashing [S6]. Extrusion is the default for continuous profiles, tubing, and sheeting, with HCR compounds fed through a heated die and post-cured in-line or in batch ovens [S5].
LSR injection molding earns its premium tooling cost when production volume is high (typically tens of thousands to millions of parts), wall sections drop below 1.5 mm, micro-features are required, and the application is sensitive to flash or manual handling (medical, food-contact, infant care) [S4]. Standard LSR materials are general-purpose silicones with a balanced combination of flexibility, heat resistance, and processing speed, which is what makes them the default for new automated cells [S2]. For a broader material comparison in sealing applications, see the Silicone vs NBR vs FKM for Oil and Gas Seals: Spec Map reference.
Compliance, additives, and application-specific grades
Beyond base mechanicals, general fabrication often pulls in compliance requirements that drive the grade. Common asks include FDA food-contact compliance, USP Class VI or ISO 10993 biocompatibility for medical devices, flame retardancy (UL 94 V-0 rated grades exist for both HCR and LSR), and electrical-grade compounds with high dielectric strength and tracking resistance [S1][S3].
Additives and fillers shift properties predictably: fumed silica raises tear strength, carbon black provides conductivity for antistatic parts, and specialty pigments must be checked against curing chemistry because some pigments inhibit platinum-cure LSR [S3][S4]. For silicone-coated fiberglass fabrics used in industrial curtains, conveying, and insulation, a coated-fabric spec sheet typically lists base fabric weight, coating thickness, and operating temperature window as the three primary selection fields [S7].
Process control and quality gates for volume cells
Volume-grade LSR cells are specified around four hard numbers: mold temperature held to roughly ±0.5°C, material utilization at or above 99.2%, injection tolerances under 10 microns, and cycle times of 15-30 seconds for thin-wall parts [S4]. Automated metering, mixing, and dispensing of the two LSR components is what keeps the platinum-cure reaction reproducible batch to batch; manual or gravity-feed setups do not reach the same part-to-part consistency [S4].
On the HCR side, compression-molding process control is more forgiving but still benefits from temperature uniformity across the platens, controlled cure time, and a defined post-cure schedule (commonly 200°C / 4 h) to drive off peroxide by-products and stabilize compression set [S6]. Buyers comparing molding services should ask for evidence of process capability (Cpk data on critical dimensions), lot traceability, and the actual curing system in use (peroxide vs addition/platinum) [S1][S4].
Limits, failure modes, and when not to use silicone
Silicone is not the default for every rubber job. Tear strength of general-purpose silicone is lower than many organic elastomers, so high-pressure dynamic seals and abrasion-loaded applications often call for polyurethane or NBR instead [S1]. Chemical resistance to concentrated acids, hydrocarbons, and many solvents is also limited; FKM or EPDM typically outperform silicone in those media, and a side-by-side material map for sealing is in the Silicone vs NBR vs FKM for Oil and Gas Seals reference.
General-fabrication buyers should also remember that RTV is not a substitute for HTV or LSR in volume parts, and that over-specifying a medical-grade HCR into a non-medical application just inflates cost and lead time without a functional benefit [S3][S5].
Quick selection rule and the next spec to lock
The fastest general-fabrication decision: pick HCR/HTV for compression, extrusion, or calendering of parts in the Shore A 30-70 band on modest volumes and tight tooling budgets; pick LSR for automated, high-volume, micro-feature parts at 170-190°C with Shore A 10-80 capability; pick RTV only for sealing, potting, and prototype tooling [S1][S4][S5]. The next spec to lock on the data sheet is the cure system (peroxide vs platinum addition) and the post-cure schedule, because that single choice drives biocompatibility, odor, and compression-set performance more than any other [S3][S6]. Buyers ready to source should request a per-grade data sheet with tensile, elongation, tear, compression set, specific gravity, and the applicable compliance certificates (FDA, USP Class VI, UL 94, EN 45545-2) before issuing an RFQ.
For component-level specifications, see silicone rubber, epdm rubber, and industrial rubber.