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LSR Silicone (Liquid Silicone Rubber)

Elastomer/TPE $$$

Liquid silicone rubber. Two-component (A/B) platinum-cured silicone elastomer injection-molded as a 1:1 mixed liquid at room temperature into hot tooling (160–210°C), where it vulcanizes in seconds to a fully crosslinked thermoset elastomer. Wide service temperature range (-55°C to +200°C continuous, briefly higher), exceptional biocompatibility (USP Class VI implant-grade available), excellent weatherability, transparent and pigmentable. **The dominant material for medical implants, infant products, food-contact seals, and high-volume precision elastomer parts** where the platinum-cure chemistry (vs peroxide-cure HCR silicone) enables thin-wall, intricate geometry with zero post-cure shrinkage. Not a thermoplastic — once cured, LSR cannot be reflowed or recycled.

Service °C
180–200°C continuous (356–392°F); Wacker LR 3003/90 rated -55 to +210°C
Tensile
6–12 MPa (870–1,740 psi); Wacker LR 3078/60 reports 8.9 MPa
Density
1.10–1.25 g/cm³ (typically 1.12–1.15; filled grades higher)
Cost
$$$
$12.00/lb
Trade names: Elastosil LR (Wacker — a dominant commercial LSR family)Silastic LSR (Dow Silicones / Dow Corning legacy)SilForce LSR (Momentive)LIM (Liquid Injection Molding — generic process term)Bluestar Silicones LSR (now Elkem Silicones)Shin-Etsu KE-2000 LSR seriesTrelleborg LSRNuSil MED LSR series (medical implant focus)

Liquid silicone rubber. Two-component (A/B) platinum-cured silicone elastomer injection-molded as a 1:1 mixed liquid at room temperature into hot tooling (160–210°C), where it vulcanizes in seconds to a fully crosslinked thermoset elastomer. Wide service temperature range (-55°C to +200°C continuous, briefly higher), exceptional biocompatibility (USP Class VI implant-grade available), excellent weatherability, transparent and pigmentable. **The dominant material for medical implants, infant products, food-contact seals, and high-volume precision elastomer parts** where the platinum-cure chemistry (vs peroxide-cure HCR silicone) enables thin-wall, intricate geometry with zero post-cure shrinkage. Not a thermoplastic — once cured, LSR cannot be reflowed or recycled.

Properties

Mechanical
Mechanical properties for LSR Silicone (Liquid Silicone Rubber)
Tensile6–12 MPa (870–1,740 psi); Wacker LR 3078/60 reports 8.9 MPa
Yield1–4 MPa (typical elastomer)
Elongation300–800% (Wacker LR 3078/60 reports 460%; higher elongation in softer grades)
Modulus1–20 MPa across the soft-to-hard LSR hardness range
HardnessShore A 5–80 (some grades exceed Shore A 90); narrow ±3 hardness tolerances available
Poisson's ratio0.49
Thermal
Thermal properties for LSR Silicone (Liquid Silicone Rubber)
Continuous max180–200°C continuous (356–392°F); Wacker LR 3003/90 rated -55 to +210°C
Short-term max~250°C short-term with appropriate heat stabilizers
Min service-55°C continuous (-67°F); some grades to -60°C — **flexibility is retained at cryogenic temperatures**
Conductivity0.17–0.22 W/m·K (thermally conductive grades up to 2 W/m·K with metal filler)
CTE200–300 × 10⁻⁶/°C — very high, typical of silicone elastomers
Specific heat1500 J/kg·K
Plastic-specific
Crystallinityamorphous
Tg-127°C — extraordinarily low; silicone backbone is flexible at cryogenic temperatures
HDTN/A — LSR is a thermoset elastomer with no meaningful HDT
UL RTI~180°C (UL RTI, electrical)
Moisture (sat)0.5–1.0% at saturation (slightly absorbent but stable)
Water (24hr)0.2–0.6% (24hr immersion)
LOI26%
UL94V-0
µ (friction)0.45

Variants (7)

General-Purpose LSR (e.g. Elastosil LR 3003 series) lsr-general
Trade: Elastosil LR 3003/50, Elastosil LR 3003/70, Elastosil LR 3003/90, Silastic LSR 9000, SilForce LR-3050

General-purpose injection-grade LSR. Wide hardness range (Shore A 30–90). Transparent natural. Food-contact compliant after appropriate post-cure. The default LSR for non-medical commercial production.

Medical-Implant LSR (USP Class VI) lsr-medical-class-vi
Trade: NuSil MED-4750, NuSil MED-4860, Elastosil LR 3003 US (medical), Silastic Q7-4750, Silbione LSR4730

Implant-grade LSR. ISO 10993, USP Class VI, long-term implant documentation. Used for cardiac leads, neurological devices, drug-delivery implants, contraceptive devices, breast implant envelopes, and surgical components. Significant cost premium and tighter quality controls than general-purpose LSR.

Self-Bonding LSR (overmolding-grade) lsr-self-bonding
Trade: Elastosil LR 3078/50, Elastosil LR 3078/60, Elastosil LR 3078/70

Self-adhesive LSR that bonds directly to PC, PSU, PESU, PPSU, and several engineering plastics during the overmold cycle **without primer or surface treatment**. Used for two-shot overmolded medical device housings, sensor seals, automotive electrical connectors.

High-Tear-Resistance LSR lsr-high-tear
Trade: Elastosil LR 3040/60, Silastic Q7-2245

High-tear-resistance LSR for diaphragms, bellows, and parts with cuts or stressed corners. Better tear propagation resistance than standard LR 3003.

Optical LSR (LED encapsulation, lenses) lsr-optical
Trade: Wacker LUMISIL LR series, Dow MS-1002, Momentive InvisiSil

Optical-grade LSR with stable refractive index (1.41), high transparency, low yellowing, and UV stability. Used for LED primary and secondary optics, solar panel encapsulation, and precision optical components. Cure shrinkage minimized for dimensional optical stability.

Conductive LSR (EMI shielding, electrical) lsr-conductive
Trade: Elastosil LR 3162, Silastic Q3-3559

Electrically conductive LSR (carbon-black or silver-filled). Volume resistivity 10⁰–10⁶ Ω·cm depending on filler. Used for EMI shielding gaskets, conductive contacts in keypads and switches, electrically conductive medical electrodes.

Thermally Conductive LSR lsr-thermally-conductive
Trade: Wacker Silgel TC, Dow Sylgard TC-series

Thermally conductive LSR (1–3 W/m·K) for LED thermal pads, battery thermal interface, and electronics thermal management. Electrically insulating while thermally conductive.

Processing

Machinability: poor
Chip: Cured LSR is a soft thermoset elastomer — conventional machining is impractical. The material deforms under tool pressure and tears rather than cutting cleanly.
Gumming: Moderate to high. Silicone's natural lubricity prevents adhesion to tools but the workpiece deforms under cutter pressure.
Finish: Limited by elastomer compliance. Mold surface finish is replicated directly to the part; post-mold finishing is rarely useful.
Tooling: Not a machinable material. Cured LSR sheets can be **waterjet-cut** or **die-cut** for gaskets and seals. Cryo-grinding produces powder for compounding. Post-mold trim is typically deflashing by cold-tumbling (LN2 embrittles the flash without damaging the bulk part).
LSR is fundamentally a **molded-only** material. The two-component A/B liquid is metered through a static mixer into hot tooling (160–210°C) where it cures in 15–60 seconds. **The dominant process is automated liquid injection molding** (LIM) on dedicated LSR presses with cold-runner systems (Mold-Masters, Hasco, Männer). Post-mold operations are limited: deflashing, post-cure (4 hr at 200°C for medical/food-contact extractables removal), printing, plasma surface activation if bonding is required. Bonding cured silicone to itself or to other substrates requires specific primers (e.g. Wacker PRIMER G 790) or self-bonding LSR grades (Wacker LR 3078) that adhere directly to PC, PSU, PESU, and PPSU during the overmolding cycle.
Process compatibility
injection molding excellent
extrusion fair
cnc machining poor
fdm not-applicable
sla dlp good
sls not-applicable
mjf not-applicable
thermoforming not-applicable
blow molding not-applicable
compression molding good

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Additives
medical grade (n/a — controlled production) — Implant-grade LSR (NuSil MED series, Wacker 3003 medical, Silastic Q7-series). Long-term implant documentation. Used for cardiac leads, neurological electrodes, contraceptive implants, drug-delivery devices, joint and tissue prostheses.
silicone lubricant (inherent — silicone itself) — LSR's silicone chemistry inherently provides slight surface lubricity and non-stick character. Not strictly an additive — the polymer itself.
thermally conductive (40–60% boron nitride, aluminum oxide, or specialty thermal fillers) — Thermally conductive LSR (Wacker Silgel TC, Dow Sylgard TC, Momentive InvisiSil TC) for LED thermal management gaskets, electronics thermal interface, battery thermal management.
antimicrobial (0.5–5% silver-based antimicrobial) — Silver-zeolite or silver-glass antimicrobial LSR for healthcare applications and infection-control products.
uv stabilizer (0.5–2%) — UV-stabilized LSR for outdoor automotive, solar, and outdoor consumer products. LSR's inherent UV resistance is excellent; stabilizers extend extreme-exposure life.

Chemical resistance

acids good Resistant to dilute acids at room temperature. Strong hot acids attack the siloxane backbone over time.
bases good Tolerates dilute bases. Concentrated hot caustic causes hydrolysis of the siloxane backbone.
aliphatic Solvents poor Significant swelling in mineral oils, hexane, kerosene, fuels. LSR is not the right elastomer for hydrocarbon contact. Use FKM/Viton, HNBR, or fluorosilicone.
aromatic Solvents poor Significant swelling in toluene, xylene, benzene.
fuels Oils poor LSR swells substantially in fuels and oils. **Not for fuel contact** — fluorocarbon elastomers are the right choice.
hot Water Steam excellent Excellent hydrolytic stability. Steam autoclave compatible for hundreds to thousands of cycles. The dominant elastomer for reusable medical and dental device sealing.
alcohols excellent Resistant to ethanol, IPA, methanol — common medical disinfectants.
LSR's chemical-resistance profile is excellent for aqueous, alcoholic, and polar service; poor for hydrocarbon and aromatic service. The selection logic versus other elastomers is largely driven by this — silicone where biocompatibility, thermal range, and steam tolerance matter; fluorocarbon (FKM) where fuels and oils are the service environment.
⚠ Stress-cracking agents
Aliphatic hydrocarbons (gasoline, diesel, mineral oil) — swellingAromatic hydrocarbons (toluene, xylene) — significant swellingChlorinated solvents — swellingSulfur compounds at elevated temperature (catalyst poisoning during cure)Concentrated mineral acids and bases at extreme conditions

Regulatory

✓ FDA grade
✓ NSF 51
✓ NSF 61
✓ USP Class VI
✓ RoHS
✓ REACH
✓ EU 10/2011
V-0 UL94 best

LSR's regulatory profile is exceptional. **USP Class VI implant- grade LSR (NuSil MED, Wacker Silpuran/3003 medical, Silastic Q7-series)** is commercially available with documentation for long-term implant applications. ISO 10993-5 (cytotoxicity), -10 (sensitization), -11 (pyrogenicity), and implant-test compliance is standard for medical LSR. FDA 21 CFR 177.2600 food contact after appropriate post-cure. NSF/ANSI 51/61 for food equipment and drinking water. **Inherent UL94 V-0 or V-1** depending on grade. EU 10/2011, BfR XV (German food contact). Withstands all common medical sterilization methods (steam autoclave, ETO, gamma, vaporized hydrogen peroxide) without significant degradation across hundreds to thousands of cycles.

Notes & applications

Overview

LSR (liquid silicone rubber) is the platinum-cured, injection-moldable member of the silicone elastomer family. Where HCR (high-consistency silicone rubber) is supplied as a millable gum stock and processed like rubber on traditional compression presses, LSR is supplied as two-component liquids (A and B) that meter through a static mixer into hot tooling and cure in seconds. The result is an injection- molding process that runs like thermoplastic IM but produces parts with the elastomeric, thermal, and biocompatibility properties of vulcanized silicone rubber.

The defining LSR properties:

  • Temperature range -55°C to +200°C continuous, briefly higher with heat stabilizers. Wider than any commercial elastomer except fluorosilicone and FKM.
  • Biocompatibility — implant-grade USP Class VI LSR is the go-to elastomer for long-term medical implants (cardiac leads, neurological devices, drug-delivery, joint prostheses).
  • Steam-sterilization compatible for hundreds to thousands of autoclave cycles.
  • Inherent UV, ozone, and weatherability — far better than any organic elastomer; LSR survives outdoor service for years without measurable degradation.
  • Optical transparency — natural LSR is water-clear; optical grades stable enough for LED primary optics.
  • UL94 V-0 inherent in most grades without flame retardants.
  • Wide hardness range (Shore A 5–80) with narrow tolerances (±3 Shore A typical).

The compromises:

  • Cost — $12+/lb for general-purpose, $40+/lb for medical implant-grade. Significantly more expensive than TPV or TPE-S.
  • Not melt-processable — once cured, LSR cannot be reflowed, recycled, or repaired. It’s a thermoset.
  • Poor fuel and oil resistance — swells significantly in hydrocarbons. Use FKM/Viton for fuel-contact service.
  • Bonding requires surface activation or self-bonding grades — cured silicone is hydrophobic and low-energy.

LSR vs HCR vs RTV silicone — the silicone family

Property LSR HCR RTV
Form supplied A/B liquid Gum stock A/B paste or RTV-1
Cure mechanism Platinum (hot) Peroxide or platinum Tin (RTV-2) or moisture (RTV-1)
Process Injection molding Compression / transfer Cast / dispense
Cycle time 15–60 sec Minutes Hours to days
Volume scale High (millions) Medium Low to medium
Hardness Shore A 5–80 Shore A 20–80 Shore A 10–60 typical
Medical implant USP Class VI Class VI grades exist Limited
Cost vs LSR baseline -10–20% -30 to +50%

LSR is the high-volume, high-precision silicone — the chosen material when production is in millions of parts and geometry is intricate. HCR (high-consistency rubber) is the lower-volume, larger-section alternative — gaskets, hoses, large-diameter tubing. RTV (room-temperature-vulcanizing) is for casting, prototyping, and field-applied sealants.

Platinum cure — LSR’s defining chemistry

LSR uses platinum (Pt) catalyst addition cure — the A component contains the Pt catalyst, the B component contains the crosslinker. Mixed 1:1 and exposed to heat, the platinum catalyzes hydrosilylation between Si-H groups and vinyl-functional silicone chains, producing crosslinked elastomer with zero byproducts and zero post-cure shrinkage (vs peroxide-cured HCR which produces small amounts of benzoic acid and similar byproducts).

Practical consequences:

  • Cleaner cure — no extractables from byproducts, hence the biocompatibility profile that makes LSR the medical implant standard
  • Dimensional precision — no shrinkage beyond thermal contraction on cooling, supporting tight-tolerance optical and medical applications
  • Catalyst poisoning is a real process risk — sulfur, amine, tin, and phosphorus compounds poison platinum. Storage containers must be resealed; tooling must be isolated from rubber, latex gloves, and incompatible adhesives. Failed cures appear as soft, tacky, uncured surfaces.

Self-bonding LSR — the overmolding breakthrough

Standard LSR does not bond to thermoplastic substrates without priming or surface activation. Self-bonding LSR grades (Wacker LR 3078, Silastic LSR-V series) chemically bond during the overmold cycle to:

  • Polycarbonate (PC)
  • Polysulfones (PSU, PESU, PPSU)
  • PA / nylon
  • Some PBT and PC/ABS blends

The bonding chemistry is proprietary but effectively involves reactive end groups in the LSR formulation that form covalent bonds to the thermoplastic surface during cure. Bond strengths typically exceed the cohesive strength of the LSR — failure is cohesive within the silicone, not at the interface.

The practical impact is economical two-shot overmolded medical device assemblies — rigid sterilizable polysulfone or PEEK housing co-molded with self-bonding LSR seals and grips, without a separate primer step. This is the standard construction for modern surgical instruments, drug-delivery devices, and reusable medical hardware.

Variant guidance — which LSR grade

  • General-purpose LSR (LR 3003 series, Silastic 9000) — pick for technical applications, automotive sealing, consumer products, food contact. Post-cure for food/medical extractables compliance.
  • Medical-implant LSR (NuSil MED-4750 series, Q7-4750) — pick for USP Class VI long-term implant applications. Cardiac leads, neurological electrodes, drug-delivery devices, breast implant envelopes.
  • Self-bonding LSR (LR 3078, LSR-V series) — pick for two-shot overmolded medical devices with PC, PSU, PEEK, or PPSU rigid housings. Saves a primer step.
  • High-tear LSR (LR 3040 series) — pick for diaphragms, bellows, and parts with high tear propagation exposure.
  • Optical LSR (LUMISIL, MS-series, InvisiSil) — pick for LED encapsulation, secondary optics, and solar applications requiring optical clarity and UV stability.
  • Conductive LSR — pick for EMI shielding, conductive contacts, and electrode applications.
  • Thermally conductive LSR — pick for LED thermal management, battery thermal interfaces, and electronics cooling gaskets where electrical insulation is also required.

Failure modes worth designing around

Platinum catalyst poisoning is the dominant processing failure mode. Sulfur (rubber, latex gloves, some pigments), amines (some adhesives, sealants), and tin (RTV silicone equipment, some plumbing sealants) all poison platinum and prevent cure. Manifests as soft, sticky, partially-cured parts. Tooling must be dedicated and upstream materials qualified. Container reseal discipline is essential.

Tear propagation from notches — LSR’s bulk tear resistance is moderate but cracks propagate quickly from cuts and sharp internal corners under repeated flexing. Radius all stress concentrations. High-tear grades (LR 3040 series) for parts with notch exposure.

Compression set under sustained load — LSR recovers less completely than EPDM thermoset or HCR silicone but more than TPE-S. For long-term static sealing under high preload, specify low-CS LSR grades or consider HCR alternatives.

Hydrocarbon swelling — LSR is fundamentally wrong for fuel and oil contact. Fluorosilicone, FKM (Viton), or HNBR are the correct choices for these environments.

Surface contamination — cured LSR’s low surface energy attracts hydrocarbon and silicone oil contamination. Printing, bonding, and labeling require corona, plasma, or chemical activation.

Hot-air degradation above 220°C — silicone elastomer chemistry slowly oxidizes at elevated temperatures, losing elongation over thousands of hours. Heat stabilizers and fluorosilicone or fluorocarbon elastomers extend this ceiling.

Applications by industry

  • Medical devices — the dominant industry by value. Long-term implants (cardiac leads, neurological electrodes, drug-delivery, contraceptive devices, breast implant envelopes), surgical instrument seals and grips, sterilizable device components, catheter components, IV valves, dialysis fluid handling. Implant- grade LSR is the workhorse elastomer in modern medical hardware.
  • Consumer products — pacifiers, teething rings, infant feeding nipples, food-contact seals (coffee machine valves, water dispenser components), kitchen utensils (silicone spatulas, baking molds, ice trays), reusable straws and storage products.
  • Wearables — smartwatch and fitness tracker bands, hearing aid components, medical monitoring device skin-contact interfaces.
  • Automotive — ignition cable boots, sparkplug boots, sensor seals, headlight gaskets, underhood electrical seals, exhaust gas sensor seals. The wide -55°C to +200°C service range matches automotive thermal cycling.
  • LED lighting and optical — primary optics encapsulation, secondary lens optics, transparent diffuser elements. Optical LSR’s stability against UV yellowing makes it the preferred encapsulant over polyurethane and epoxy.
  • Aerospace — high-temperature electrical harness sealing, ozone-resistant outdoor cable jacketing, helicopter rotor blade sealing components.
  • Industrial — diaphragms, valves, bellows in pharmaceutical and food-processing equipment. Sensor housings in chemical processing where wide thermal range matters.
  • Hearing health — custom-molded hearing aid earmolds, in-ear monitor shells, audiologist-fitted custom devices. LSR’s biocompatibility and softness make it the standard.

Sources & standards

Standards: ASTM D2240 (Shore A hardness)ISO 37 (tensile)ISO 815 (compression set)ASTM D624 / ISO 34-1 (tear strength)FDA 21 CFR 177.2600 (silicone food contact)USP Class VI (medical implant)ISO 10993-5/10/11 (biocompatibility)NSF/ANSI 51 (food equipment)NSF/ANSI 61 (drinking water)UL 94 V-0 / V-1 (inherent)

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