Silicone (LSR) Injection Molding Guide

What silicone (LSR) injection molding actually is

Liquid silicone rubber (LSR) is not a thermoplastic, and molding it is not thermoplastic injection molding with a different material. LSR arrives as two liquid components — a vinyl-functional base polymer and a platinum catalyst / cross-linker package — that are metered in a fixed ratio, blended in a static mixer, and pumped into a hot steel mold. Inside the mold the material cross-links into a solid elastomer. There is no pellet melting, no screw plasticizing in the conventional sense, no sprue to regrind, and no crystallization to manage. Instead, the whole process revolves around three numbers: dosing accuracy, cure time, and the flash that appears wherever the mold does not close perfectly.

That distinction matters more in 2026, not less. LSR is the material of choice for medical wearables (ECG electrodes, hearing-aid seals, insulin-pump diaphragms), food-contact and baby-care parts (nipples, bottle seals, gaskets), EV and electronics seals, and LED optics. These are thin-wall, high-precision, high-cleanliness parts where a thermoplastic alternative would fail on service temperature, chemical resistance, or the soft-touch requirement. This guide is written from the buyer-engineer side: how the process actually runs, what the tool demands, where flash comes from, what tolerances are honest to demand, and how the cost is built. We run LSR in-house at our Dongguan plant — [OUR PLANT] — so the numbers below are the ones we quote against.

The Snapshot

LSR is dosed 1:1 by an A/B pump (typical 10–60 g/s shot rate) and cured at mold temperatures of 140–200 °C in 10–120 s; thin-wall parts typically cure within 60 s.
Real program: a medical ECG button in medical-grade LSR, held to ±0.03 mm, running 300K units/year, with first parts in 7 weeks on a 420-stainless rapid tool.
Shrinkage runs 2.0–3.0 % — roughly 3× a thermoplastic’s 0.4–0.8 % — so the mold is cut oversize and the cavity steel is held to ±0.02 mm.
Flash is the #1 defect: LSR’s low viscosity (2,000–100,000 mPa·s depending on shear) leaks through any parting-line gap above 0.01 mm.
Steel tools are mandatory — 420 stainless or H13; aluminum softens and drifts under LSR’s sustained 170–200 °C cure heat.
[OUR PLANT]: we run LSR on Sodick machines (18+3) under ISO 13485 (medical), ISO 9001 and IATF 16949, across a 10,000 m² / 280-person facility.

How the LSR process flow works
LSR molding machines and dosing units
Tooling: why LSR needs steel, not aluminum
Venting and flash control
Material and process windows
Tolerances, shrinkage and metrology
Post-cure and secondary operations
Cost structure of an LSR program
Real LSR programs from our floor
Common LSR molding mistakes
How to audit an LSR molder
Frequently Asked Questions
Sources
Start your LSR program

How the LSR process flow works

Unlike thermoplastic molding, LSR runs as a closed dosing loop — material goes in as liquid, comes out as a cured elastomer, and nothing is recycled:

Metering — A (vinyl-terminated polymer) and B (platinum catalyst + cross-linker) are pumped at a fixed 1:1 ratio (some grades use 10:1 systems). A ratio error of even 0.5 % shifts the cure and leaves uncured, tacky zones.
Static mixing — the two streams pass through a 12–24 element static mixer; insufficient mixing produces streaky, under-cured regions that fail tear and compression-set tests.
Injection — the blended liquid is pushed into the hot cavity at low pressure (30–120 bar), filling 0.1–0.2 mm walls the way water fills a mold.
Cure — the mold at 140–200 °C (production windows typically 170–200 °C) cross-links the rubber in 10–120 s; thick sections take the long end of that window.
Demold — the cured part is ejected hot; there is no cooling phase to speak of, which shortens the cycle versus thermoplastics of similar wall thickness.

A typical small LSR part cycles in 30–90 s, and the cure step — not injection — is the rate limiter. Mixed LSR has a finite pot life at shop temperature (commonly hours rather than days, a typical published datasheet value), so machines idle with purged lines and material is metered just-in-time from the barrel.

LSR vs. liquid injection molding (LIM). The terms are often used interchangeably; LIM is the machine-side name for the same two-component process. The key difference from solid-silicone compression molding is that LIM fills the cavity under pressure, which is what enables thin walls, tight tolerance and automation.

LSR molding machines and dosing units

Machine selection for LSR is different from thermoplastic: the screw that plasticizes pellets is replaced by a metering pump that feeds the static mixer, and the barrel must be cooled, not heated.

Dosing unit — a two-component piston pump that meters A and B at the mix ratio with closed-loop ratio monitoring. Shot size is set by pump volume, so the machine is sized to part weight, not to clamp tonnage alone.
Barrel temperature — the LSR barrel runs at 20–40 °C (cooled) to prevent premature cure before the material reaches the mold; the mold is the only hot zone.
Clamp and injection — injection pressures of 30–120 bar mean clamp needs are far lower than for thermoplastics of the same projected area; the fill pattern, not the pressure, is the challenge.
Vertical machines — preferred for insert molding and overmolding (an LSR seal overmolded onto a metal or plastic substrate), because the insert can be placed in the lower mold half.
Cold-deck (cold-runner) systems — keep the runner at barrel temperature so the material stays liquid; a valve-gated cold deck recovers the runner automatically in high-volume runs.
[OUR PLANT] runs Sodick injection molding machines (18+3) across the plant, with LSR programs allocated to machines fitted with cooled barrels, dosing pumps and closed-loop cure-temperature control.

The practical consequence: when you read an LSR quote, the machine-hour rate reflects dosing-unit and temperature-control complexity, not clamp size. A 100-tonne thermoplastic machine is not the right comparison.

Tooling: why LSR needs steel, not aluminum

LSR cures hot and stays hot. Aluminum molds — perfectly fine for thermoplastic rapid tooling at 40–120 °C mold temperatures — degrade under LSR’s sustained 140–200 °C cure heat: the cavity loses dimension, the surface breaks down, and the tool dies early. So the rule for LSR is steel at every tier:

Mold steel: 420 stainless (corrosion-resistant, the medical default) or H13 (hardened, long-run). Cavity tolerance held to ±0.02 mm.
Parting line: ground and fitted to ≤0.01 mm gap; what looks like a hairline on the tool becomes a flash fin on every part.
Venting: LSR displaces air it cannot compress; vents at the parting line and ejector pins must be cut to 0.005–0.01 mm depth so air escapes but liquid does not.
Cold runner: the runner stays cool (20–40 °C) until the gates; valve-gated cold decks eliminate runner waste and are standard for medical and high-volume work.
Texture and polish: the steel surface transfers directly to the part; SPI finishes from A-2 (fine) to D (coarse texture) are achievable on steel, but the polish budget should be set before quoting, because mirror finishes on a 420-stainless cavity add tool time.

Even the “rapid” tier is steel. The ECG button program used a 420-stainless rapid tool with first parts in 7 weeks — faster than a thermoplastic program might suggest, because the tool is simpler (no complex cooling circuit to the same degree), not because the material was downgraded. For context on mold classes, Xometry’s published ladder runs Class 105 (prototype) to Class 101 (high-volume production) (https://www.xometry.com/capabilities/injection-molding/); for LSR, the prototype rung is steel regardless.

Venting and flash control

LSR’s low viscosity is its blessing and its curse. It fills 0.1–0.2 mm walls that thermoplastics would short-fill, but it also finds every gap. Flash at the parting line is the #1 cosmetic and functional defect in LSR molding, and it is controlled in the tool, not by the operator.

Risk	Cause	Control
Flash fin	Parting-line gap > 0.01 mm	Grind/fit line to ≤ 0.01 mm
Air trap / burn	No vent path	Vents at parting line + ejectors, 0.005–0.01 mm
Short fill	Under-dosing or ratio drift	Verify A/B ratio ± 0.5 % continuously
Uncured tack	Cure < 10 s on thick wall	Hold 170–200 °C, extend cure
Streaks	Poor mixing	12–24 element static mixer

Two venting rules worth internalizing: (1) vent depth is set by the material’s viscosity — LSR vents run 0.005–0.01 mm, roughly one-tenth of what a PP tool would use; (2) the last-filled region of the cavity determines vent placement, so a mold-flow review before steel cutting is cheap insurance. On the ECG button program, the parting line and vents were cut and verified before the production run — that is why ±0.03 mm held across 300K units/year instead of being corrected after the first batch.

Material and process windows

LSR grades span very soft to firm, and the grade sets hardness, cure behavior and shrinkage simultaneously.

Property	Typical LSR range	Notes
Hardness	Shore A 10–70	Medical buttons ~ Shore A 40–60
Cure temperature	140–200 °C (typical 170–200 °C)	Mold temperature, not barrel
Cure time	10–120 s	Thin walls cure within ~60 s
Shrinkage	2.0–3.0 %	Mold cut oversize
Viscosity	2,000–100,000 mPa·s	Low → thin walls, flash risk
Service temperature	-50 to 200 °C	Stable across thermal cycle
Tensile strength	~6–11 MPa (typical published)	General-purpose platinum-cured grades
Tear strength	~15–50 N/mm (typical published)	High-tear grades survive demold abuse

Medical-grade LSR is platinum-cured — no peroxide residues, biocompatible, autoclave-tolerant — which is why the ECG wearable button runs in it. Food-contact grades follow the same cure chemistry and are typically manufactured to FDA / EU food-contact norms (verify the specific grade certificate for your market). Two chemistry notes: peroxide-cured LSR exists for cost-sensitive industrial parts but leaves cure by-products, and the platinum cure can be poisoned by certain contaminants (sulfur, amines, tin) — another reason clean material handling matters. The cure windows and mechanicals above are typical published datasheet ranges; exact values come from the grade datasheet and should be locked during DFM. Durometer selection drives the rest of the spec — here is the practical map:

Hardness band	Typical use	Example parts
Shore A 10–20	Very soft, skin-contact	Baby-care pads, sensor covers
Shore A 30–50	General seals and buttons	ECG button, gaskets, diaphragms
Shore A 50–70	Firm seals and structural soft parts	Valve bodies, bumpers, dampers

A grade that is too soft for the application tears at demold or in service; one that is too firm cracks under repeated flexing. Lock the hardness against the datasheet’s tensile, tear and compression-set columns, not just the Shore value, before the tool is cut.

Tolerances, shrinkage and metrology

LSR shrinks 2.0–3.0 %, roughly triple a thermoplastic’s 0.4–0.8 %, and that changes the mold math from the start:

Mold (cavity) tolerance: ±0.02 mm on the steel, cut oversize by the shrink factor so the cooled part lands in spec.
Part tolerance: ±0.03 mm achieved on the medical ECG button; ±0.1–0.2 mm is the honest general band for non-critical LSR features.
Shrink direction: LSR shrinks more isotropically than filled thermoplastics, so round features stay round — a real plus for seals and gaskets.
Post-cure: some grades take a +1–4 h @ 150–200 °C post-bake to finish cross-linking; if the spec demands full mechanicals, design the schedule for it.

Metrology is where tolerance claims are proven or buried. First-article and in-process checks on an LSR program should include a CMM for critical dimensions, a Shore A durometer for hardness (the property that maps directly to grade), and height gauges for flatness-critical features — the QA equipment set we run is the same class as the CMM / height gauges / moisture analyzers / pressure gauges / color controllers FirstMold publishes for its precision programs (https://firstmold.com/pbt-injection-molding/). For automotive and medical releases, plan Cpk ≥ 1.33 on critical dimensions and a written first-article report; that is the standard that makes a ±0.03 mm claim auditable. We hold ±0.02 mm on controlled dimensions as our plant precision capability — [OUR PLANT] — and publish measurement records with every program.

Post-cure and secondary operations

Most LSR parts need little after the press — one of the process’s cost advantages — but the small amount of secondary work is specific:

Deflash — the parting-line fin is thin (< 0.1 mm); hand or cryogenic deflash removes it in seconds. Cryogenic deflash (tumbling at low temperature) is the volume option for gaskets and seals.
Post-cure bake — +1–4 h @ 150–200 °C for full mechanicals, compression set and off-gassing requirements; medical and food-contact programs typically demand it.
Surface treatment — plasma or primer treatment where LSR is bonded or overmolded onto thermoplastics; LSR-to-metal bonds are handled in the tool with mechanical interlocks.
Marking and decoration — laser marking and pad printing are the usual routes; LSR accepts silicone-compatible inks.
Overmolding and insert molding — LSR overmolded onto PC / PPSU housings or metal inserts is the fastest-growing application family; see our overmolding service and insert molding service for design rules.
Cleaning and packaging — medical and food-contact parts are often molded, baked, cleaned and packed in controlled areas; a 300K units/year medical run budgets deflash, bake and packaging per part.

Plan these into the cycle cost from the start. The part that “just needs molding” on the quote sheet usually needs deflash, bake and inspection on the production floor — all of which show up in the per-part price.

Cost structure of an LSR program

A general injection-molding cost split — raw material 40–60 %, processing 20–35 %, mold 15–25 %, post-processing 5–20 % — is published by FirstMold for plastics (https://firstmold.com/materials/injection-molding-materials/). LSR shifts that split in two ways:

Material share runs high. LSR is priced well above commodity thermoplastics, so even though only the molded part plus runner losses is consumed, the material line dominates per-part cost. Runner waste hurts more than it would in PP — one more reason valve-gated cold decks pay for themselves.
Tooling is steel, so it is not cheap. A single-cavity 420-stainless LSR tool costs more than a comparable aluminum thermoplastic tool, but the tool is simpler than a heavily cooled thermoplastic production mold. For context on mold-cost bands, HLH Rapid publishes $3,000–$6,000 for simple molds and $7,000+ for complex steel or multi-cavity molds, with total projects running $10,000–$100,000 (https://www.hlhrapid.com/capabilities/injection-molding/).

Cycle economics favor thin walls twice over: thinner walls cure faster (shorter cycle) and consume less material. A 0.5 mm-wall gasket that cures in about 30 s on an 8-cavity tool out-produces a 2 mm-wall part on a single cavity by an order of magnitude. When a per-part price looks high, check the cure time and the cavity count — that is where the money actually is.

Real LSR programs from our floor

Medical ECG button (LSR) — medical-grade platinum-cured LSR, ±0.03 mm on critical features, 300K units/year, first parts in 7 weeks on a 420-stainless rapid tool. The venting and parting-line strategy was locked before production, and the program has run without a flash-rework loop.
LSR seals and gaskets (food-contact) — Shore A 40–60 grades, 0.1–0.2 mm wall sections, deflash plus post-cure on every batch; ±0.1 mm general tolerance band.
Overmolded LSR components — LSR overmolded onto thermoplastic and metal substrates for consumer and industrial soft-touch and sealing applications, run on vertical machines with insert handling.

The pattern across all of them: steel tooling at every tier, vent verification before production, and metrology records that make the tolerance claim auditable. If your part needs the same discipline, start with a DFM review.

Common LSR molding mistakes

Aluminum tool → dimension drift and surface loss under 140–200 °C cure heat; use steel (420 / H13) at every tier.
Loose parting line → flash on every part; the fin then needs deflash, and a bad flash line on a seal is a leak path, not just a cosmetic issue.
A/B ratio drift → uncured tack or over-cure brittleness; monitor the pump ratio continuously, not only at startup.
No vent path → burned, trapped-air voids on the last-filled feature; the last-filled region decides vent placement.
Ignoring 2–3 % shrink → mold cut to nominal and every part lands undersize; cut the cavity oversize by the grade’s shrink.
Wrong grade → an over-firm part cracks at demold, an over-soft part tears in service; lock Shore A and mechanicals against the application.
Skipping post-cure → compression set and mechanicals never reach the datasheet; the bake is part of the spec, not an option.
Designing the runner for thermoplastics → LSR needs cold-deck geometry, not hot-runner geometry; LSR cures in a hot runner and never reaches the cavity.

How to audit an LSR molder

Before you hand a medical or high-volume LSR program to a supplier, run this checklist:

Certifications — ISO 13485 for medical, IATF 16949 for automotive, ISO 9001 baseline; [OUR PLANT] holds all three. FirstMold has held ISO 13485 since 2012 with 300+ MedTech programs (https://firstmold.com/industries/medical/), a reasonable bar for a serious LSR partner.
Dosing and ratio monitoring — does the shop have closed-loop ratio control and calibration records for the dosing pump? This is the #1 quality lever.
Venting plan — can they show vent depth, placement and verification for a part like yours? Ask for a venting review inside the DFM.
Steel tooling — confirm 420 stainless / H13 for LSR; walk away from any quote that proposes aluminum for LSR regardless of volume.
Metrology — CMM availability, Shore A hardness testing, first-article reports and Cpk data from past programs.
Cleanliness — material handling, purging discipline and, for medical, controlled or clean-room molding areas; the platinum cure is sensitive to contamination.
Program management — weekly progress reports and mold photos during tooling, and a written cure-window validation at T1 — the same practice FirstMold cites for its Mexico operation’s 96 % first-trial target (https://firstmold.com/services/mold-and-molding-mexico/).

Frequently Asked Questions

1. Can LSR be injection molded like a thermoplastic? The machine looks similar but the physics is different: LSR is dosed 1:1, mixed statically, injected at low pressure (30–120 bar) into a hot mold where it cures by cross-linking (140–200 °C), not by freezing. There is no regrind and no cooling phase.

2. Why can’t LSR run in an aluminum mold? Aluminum softens and drifts under sustained 140–200 °C cure heat. Steel — 420 stainless or H13 — is mandatory at every volume tier, even for prototype tools.

3. What tolerance can LSR parts hold? ±0.03 mm on critical features is real (our medical ECG button holds it across 300K units/year); ±0.1–0.2 mm is the general band for non-critical features. The cavity steel is held to ±0.02 mm and cut oversize for 2.0–3.0 % shrink.

4. How long does LSR take to cure? 10–120 s depending on wall thickness and grade; thin-wall parts (0.5 mm range) typically cure within 60 s. Cure time, not injection, sets the cycle.

5. What is LSR shrinkage and why is it so high? 2.0–3.0 %, about triple a thermoplastic’s 0.4–0.8 %. The mold is cut oversize by the shrink factor, and softer grades trend toward the top of the band.

6. LSR or TPE — which should I choose? Choose LSR when the part sees heat (service to 200 °C), autoclaving, chemicals, or needs long-term sealing and biocompatibility; choose TPE when cost, colorability and simple overmolding matter more. Both overmold well; LSR demands the steel-tool and dosing discipline described above.

7. Is LSR food-safe and medical-safe? Platinum-cured LSR grades are available with FDA / EU food-contact and biocompatibility (e.g. USP Class VI) certifications; verify the specific grade certificate for your market. Platinum cure leaves no peroxide by-products.

8. Does LSR need drying before molding? No. LSR is not hygroscopic — there is no drying step, one of the process’s cost advantages over hygroscopic thermoplastics.

9. Can LSR be overmolded onto plastic or metal? Yes. LSR overmolds onto thermoplastics (with primer or plasma treatment) and metal inserts, typically on vertical machines; see our overmolding service for the design rules.

10. How much does LSR tooling cost? LSR tooling is steel, so it tracks steel production tooling rather than aluminum rapid tooling — HLH Rapid’s published bands of $3,000–6,000 for simple molds and $7,000+ for complex multi-cavity molds are a reasonable market reference (https://www.hlhrapid.com/capabilities/injection-molding/).

11. How thin can an LSR wall be? 0.1–0.2 mm walls are routinely filled — far thinner than most thermoplastics — which is why LSR is the material for micro-seals and diaphragm parts.

12. How fast can I get first LSR parts? On a steel rapid tool, about 7 weeks (our ECG button program); a hardened production tool typically runs 8–12 weeks. FirstMold’s aerospace claim of 72-hour prototypes and a 15-day mold guarantee shows how fast the top of the market moves (https://firstmold.com/industries/aerospace/).

13. What hardness range does LSR cover? Shore A 10–70, from very soft skin-contact pads to firm industrial seals; most medical and consumer parts sit at Shore A 40–60.

Sources

Xometry — Injection Molding (mold classes Class 105–101, tolerances, T1 lead times): https://www.xometry.com/capabilities/injection-molding/
HLH Rapid — Injection Molding (mold cost bands): https://www.hlhrapid.com/capabilities/injection-molding/
FirstMold — Medical industry capabilities (medical tolerance ±0.0254 mm, ISO 13485 since 2012, 300+ MedTech programs): https://firstmold.com/industries/medical/
FirstMold — PBT injection molding (QA equipment: CMM, height gauges, moisture analyzers, pressure gauges, color controllers): https://firstmold.com/pbt-injection-molding/
FirstMold — Injection molding materials (cost structure split): https://firstmold.com/materials/injection-molding-materials/
FirstMold — Mold & molding in Mexico (96 % first-trial target, weekly progress reporting): https://firstmold.com/services/mold-and-molding-mexico/
FirstMold — Aerospace (72-hour prototypes, 15-day mold guarantee): https://firstmold.com/industries/aerospace/
Protolabs — Injection molding service (certifications ISO 9001 / ISO 13485 / AS9100 / ITAR; parts in as fast as 1 day): https://www.protolabs.com/services/injection-molding/
LSR processing windows (cure temperature/time, viscosity, shrinkage) and tensile/tear ranges: typical published values from LSR supplier datasheets (Elkem Silicones, Shin-Etsu, Momentive); exact values vary by grade.

Start your LSR program

Send the part geometry, target hardness (Shore A 10–70) and whether the program needs medical or food-contact grade. We return a steel-tool plan (420 or H13), a venting and parting-line spec, and a cure window — held to ±0.03 mm on real programs — with first parts in 7 weeks via rapid tooling.