Liquid Silicone Rubber (LSR) Injection Molding

LSR delivers elastic, temperature-stable, and biocompatible parts that thermoplastics cannot. We run cold-runner LSR tooling for flash-free seals, gaskets, and medical components — and overmold silicone onto plastic and metal for sealed, soft-touch interfaces.

LSR Capabilities

Platinum-cure silicone, documented process windows, and clean-controlled molding.

Liquid Silicone Rubber (LSR) Molding

Two-component platinum-cure silicone injected into heated molds — flash-free, biocompatible parts for seals, gaskets and medical components.

Cold-runner tooling (zero flash)
40–70 Shore A range
Medical & food-grade LSR
Automated deflashing

LSR Overmolding

Silicone molded over a plastic or metal substrate for sealed, grippy, or soft-touch interfaces.

TPU/PC + LSR bonds
IP-rated sealing
Ergonomic grips
Wearable & device skins

Medical-Grade LSR

ISO 10993 and USP Class VI capable silicones for single-use, diagnostic, and skin-contact devices.

Lot traceability
Clean-controlled molding
Sterilization-ready
Validation support

LSR vs TPU & Thermoplastic Elastomers

Why specifiers pick liquid silicone over TPE/TPU.

Property	LSR	TPU / TPE
Temperature range	−50 to 200 °C	−40 to 120 °C
Biocompatibility	ISO 10993 / USP VI	Limited grades
Flash	Cold-runner: near zero	Parting-line flash
Steam/autoclave	Excellent	Degrades

What Makes LSR Different from Thermoplastics

Liquid silicone rubber is a thermoset — it cures, not cools — which is why it keeps its properties where plastics soften.

Temperature resistance from −50 to 200 °C — the widest elastomer window we mold, surviving autoclave, steam and under-hood heat.

Biocompatible: ISO 10993 and USP Class VI capable grades for skin-contact and implant-adjacent devices, with lot traceability.

Excellent transparency, tear resistance and elasticity — the properties First Mold cites for medical and baby products are the same ones we run in our dust-controlled LSR cell.

Food-grade and BPA-free options (FDA 21 CFR / LFGB) for kitchenware, baby and beverage contact parts.

Chemical and weather resistance — seals and gaskets keep sealing in oils, solvents and UV without hardening.

Near-zero flash with cold-runner tooling — material waste and deflashing labor are both minimal.

From Two-Component Silicone to Cured Part

LSR is a platinum-cure system: part A and part B mix, inject and vulcanize inside the heated mold.

Metering & mixing

The two liquid components are metered 1:1 and mixed in a static mixer on the machine. The mix stays liquid at room temperature, so the whole feed system runs cold.

Injection through a cold runner

The mixed silicone is injected at low pressure through a cold-runner manifold — the material only cures where it must, keeping waste to a minimum and flash near zero.

Vulcanization in the heated mold

The mold is held at 120–180 °C. The platinum catalyst cross-links the silicone; cure time scales with wall thickness, from seconds on thin seals to a couple of minutes on thick sections.

Cooling and ejection

The part is ejected hot but dimensionally stable. Mold design must allow demolding without tearing the soft, elastic part.

Deflashing & optional post-cure

Automated deflashing removes any vestigial flash. Post-cure can be run where full physical property development or medical validation requires it.

LSR Grades and Overmold Substrates

We run the hardness range, medical/food grades and substrate combinations that cover most LSR programs.

Shore hardness 40–70 A standard range (10–80 A available on request) for grips, seals, gaskets and membranesMedical-grade LSR — ISO 10993 / USP Class VI capable, sterilization-ready, with lot traceability under ISO 13485Food-grade LSR — BPA-free, FDA 21 CFR / LFGB compliant for baby and kitchenware contactPC + LSR overmolds — sealed electronics and medical components; IATF 16949 / LFGB compliance pathsPPS + VMQ (silicone) overmolds — high-temperature connectors and sensors; SAE J2460 / AMS 3217Optical / translucent LSR — light pipes, lenses and covers with molded-in clarity

LSR Tooling vs Thermoplastic Tooling

Runner systemLSR uses a cold runner so the material stays liquid until the cavity; thermoplastics use hot runners that keep the melt at temperature. Cold-runner LSR waste is minimal.

Mold temperatureLSR molds are HEATED to 120–180 °C to drive vulcanization; thermoplastic molds are cooled to 20–80 °C to freeze the melt. Different steel and thermal design entirely.

ShrinkageLSR shrinks more than most thermoplastics and must be compensated in the tool — we model the part's wall geometry against the grade's shrink curve before cutting steel.

Flash controlCold runner plus tight shut-off gives near-zero flash; thermoplastic parts accept parting-line flash that LSR customers usually will not.

Surface & releaseLSR cavities are mirror-polished and often Teflon/plasma coated for release; zero mold misalignment is expected — the same standard First Mold holds for its silicone molds.

Steel & ventingHardened steel with generous venting to let the low-viscosity liquid escape; thin-wall detail is limited by material flow, not by cooling geometry.

Medical, Wearable and Food-Contact Applications

Medical seals & diaphragms

Drug-delivery, diagnostic and surgical seals that must survive autoclave cycles — ISO 13485 lots with full traceability.

Baby products

Pacifiers, teats and feeding accessories in food-grade, BPA-free LSR — soft, transparent, tear-resistant.

Wearables & device skins

Skin-contact bands, buttons and IP-rated seals overmolded on PC electronics cores.

Automotive connectors & sensors

High-temperature silicone seals for connectors and sensor bodies, held to IATF 16949 process control.

Kitchen & food contact

Gaskets, spatulas and baking components in FDA 21 CFR / LFGB compliant LSR.

Optical components

Translucent light guides and covers with molded-in clarity and uniform transmission.

LSR Molding FAQs

✓What hardness range can you mold? Our standard window is 40–70 Shore A, with 10–80 A available — the right hardness is set by sealing force and tactile requirements.
✓How long is an LSR cycle? Cure time scales with wall thickness — seconds for thin seals, a couple of minutes for thick sections. Cycle planning happens in DFM against your geometry.
✓Can LSR be overmolded onto plastic or metal? Yes — PC+LSR and PPS+VMQ are standard combinations we run, plus metal inserts for sealed electronic assemblies.
✓Do you support medical validation? Yes — ISO 13485 process control, ISO 10993 / USP Class VI capable grades, lot traceability and sterilization-ready handling.
✓What tooling lead time should I plan for? The same in-house track as plastic tooling: rapid tools 7–12 days, production tools 25–30 days (45 for automotive-grade).
✓Is flash really eliminated? Cold-runner tooling plus tight shut-off gets flash down to near zero — that is why LSR seals and gaskets are molded, not trimmed.

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