Silicone Injection Molding: The Complete LSR Guide

Silicone injection molding — in production terms, liquid silicone rubber (LSR) injection molding — is how industry makes soft, heat-resistant, biocompatible parts by the millions: medical seals, baby-bottle nipples, gaskets, phone seals, keyboard membranes, automotive connectors, and silicone overmolded grips. Unlike solid silicone rubber (which is compression or transfer molded from pre-cut sheets), LSR is a two-component liquid that is pumped, mixed 1:1, injected into a hot mold, and vulcanized in under 60 seconds for thin-wall parts.

The result is a material that feels like rubber but is molded with the precision and cycle economics of plastic injection molding — with one crucial difference: the mold runs on a cold runner system and the cavity is hot, a thermal split that only LSR machines manage. This guide is the complete engineering reference for silicone injection molding: how it works, what it can and cannot do, medical and food-grade compliance, mold design, tolerances, cost and lead-time reality, overmolding, defects, and how to buy it without surprises.

Molditquick runs LSR molding alongside our automated injection presses, with dedicated dust-controlled silicone production, cold-runner tooling delivered through an audited tooling partner network (precision wire EDM and CNC), and quality systems covering IATF 16949 and ISO 9001 in a 10,000 m² facility with 280 people.

Table of Contents

1. What is silicone injection molding?
2. LSR vs solid silicone rubber vs TPE
3. LSR properties: why liquid silicone wins
4. How LSR injection molding works: mixing, metering, vulcanization
5. The cold runner mold: the heart of the process
6. Process parameters: temperature, pressure, cure time
7. Medical and food-grade silicone: compliance that matters
8. Tolerances and dimensional behavior of LSR parts
9. Silicone overmolding: bonding LSR to plastic and metal
10. Cost and lead times
11. Applications: medical, baby products, automotive, electronics
12. Common silicone molding defects and fixes
13. DFM checklist for LSR parts
14. Choosing a silicone injection molding manufacturer
15. FAQ

What is silicone injection molding?

“Can you injection mold silicone?” — yes, and the answer has two flavors:

1. Liquid silicone rubber (LSR) injection molding — the modern standard. Two liquid components (Part A: platinum-catalyzed base; Part B: crosslinker) are metered, mixed, injected cold into a closed mold, and cured by heat. Cycle times of 30–90 seconds are typical for thin-wall parts; cure takes under 60 seconds for most geometries ( LSR guide).
2. Solid silicone rubber injection molding — high-consistency rubber (HCR) is screw-plasticized and injected into a hot mold, then cured. Slower, more manual, and used for thick, high-durometer parts.

When buyers search “injection molding silicone,” “liquid silicone rubber injection molding service,” or “silicone molding services,” they almost always mean LSR — it is faster, more precise, more automated, and cleaner than HCR. LSR is the material of medical devices, baby products, and precision seals; HCR remains for specialized high-durometer and thick-section work.

The process chain: metering unit → static mixer → cold runner → hot cavity → vulcanized part. The injection unit and runner stay cold (10–30 °C) so the reactive liquid does not cure early; the cavity is heated (typically 120–180 °C) so the crosslinking reaction fires on contact. That hot/cold boundary inside the mold is what makes LSR tooling a specialist craft.

LSR vs solid silicone rubber vs TPE

Criterion	LSR (liquid silicone)	HCR (solid silicone)	TPE (thermoplastic elastomer)
Form at injection	Two liquid components	Sheet/dough, screw-fed	Thermoplastic pellets
Curing	Heat-activated crosslink (platinum)	Peroxide or platinum cure	None (melt/solidify)
Cycle time	30–90 s typical	Minutes (cure-time bound)	20–60 s
Temperature range	−60 to +250 °C continuous	−60 to +250 °C	−40 to +120 °C typical
Chemical/sterilization resistance	Excellent (autoclave, E-beam, gamma)	Excellent	Limited
Tear strength	Good (high-tear grades exist)	Excellent	Good
Precision	Excellent (molded, not cut)	Fair (flash trimming)	Excellent
Surface finish	Smooth, mold-faithful	Flash lines need trimming	Smooth
Cost per part	Medium-high	Medium (labor-heavy)	Low-medium
Best use	Medical, seals, baby, precision	Large/thick parts, high durometer	Cost-driven grips, gaskets, phone cases

The practical rule: need 260 °C-class heat resistance, sterilization, biocompatibility, or sealing under duress → LSR or HCR. Need soft-touch ergonomics at the lowest cost, no extreme temperature, and recyclability → TPE (see our TPU injection molding guide and the injection molding material comparison).

LSR properties: why liquid silicone wins

Property	Typical value	Why it matters
Service temperature	−60 °C to +250 °C continuous	Survives autoclave, engine bay, cryogenic seals
Hardness range	Shore A 10–80 (typical 20–70)	From soft nipple to firm gasket, in one chemistry
Tear resistance	10–50 kN/m (grade dependent)	Seals that peel, not tear
Elongation at break	200–700 %	Flexing, sealing, stretching applications
Biocompatibility	ISO 10993 / USP Class VI grades	Implant-adjacent and skin-contact devices
Food contact	FDA 21 CFR 177.2600 grades	Baby products, kitchenware, beverage seals
Chemical resistance	Excellent vs body fluids, mild chemicals	Medical and food contact
Electrical properties	High dielectric strength, stable	Connector seals, insulators
Compression set	Low at high temperature (grades differ)	Seals that stay sealed for years
Colorability	Full range incl. translucent	Branding, light guides, indicators

One property above all drives LSR adoption: it cures, it does not melt. Unlike TPE, an LSR seal that sits against a hot engine block or goes through 134 °C autoclave cycles does not creep or re-flow — it holds its sealing geometry for the life of the product.

How LSR injection molding works: mixing, metering, vulcanization

Two-component metering

LSR is supplied as Part A + Part B in a precise 1:1 ratio (some systems 10:1). A metering pump (gear or piston) delivers both components to a static mixer, which homogenizes them just before injection. Pitfalls that matter:

• Ratio drift = under-cure or over-cure; metering units are calibrated and verified on shift starts.
• Mixed LSR has a pot life (hours at room temperature); the process is designed so mixing happens continuously, minutes before injection — never batch-mixed like epoxy.
• Colorant can be added at the metering unit via a third pump for colored parts without pre-colored resin.

Injection and vulcanization

• The mixed liquid is injected through a cold runner (10–30 °C) into the hot cavity (120–180 °C).
• Heat triggers platinum-catalyzed crosslinking. Cure time scales with wall thickness: thin-wall parts cure in under 60 seconds; thick parts (5+ mm) take 2–3 minutes ( LSR guide).
• Because LSR shrinks little (~2–4 % linear for the molding grade band, mostly handled in mold design) and vulcanizes chemically, parts come out dimensionally faithful with no cooling warpage.
• Cycle economics: at 30–90 s cycles, a 16- or 32-cavity LSR mold produces millions of seals per year from one machine.

The cold runner mold: the heart of the process

LSR tooling is unlike plastic tooling in five ways:

1. Cold runner, hot cavity. The runner system is water-cooled (10–30 °C) so the liquid stays fluid; the cavity is heated (120–180 °C) so it cures. The thermal boundary — “cold deck” vs “hot cavity” — is engineered, insulated, and sealed.
2. No runner waste (with cold-runner valves). The cold runner can be valve-gated: after each shot, a needle valve cuts the runner off from the cavity, and the runner liquid is purged and recycled. Material yield approaches 100 % — important when LSR costs more per kilo than most plastics.
3. Cavity venting is aggressive. LSR outgasses during cure; trapped air causes the classic “bubbles” defect. Vents at 0.01–0.02 mm plus vacuum venting are common for medical parts.
4. Steel selection. P20/718H for prototypes; H13, S136/420SS, or hardened tool steel for production; cavity surfaces often get PVD/nitride or release-agent-free polish. High-cavitation tools (16–64 cavities) are the norm for seals.
5. No draft angle needed on soft parts. Soft LSR (Shore A <40) peels off the cavity; firmer grades need 0.5–2° draft and polished surfaces.

Mold-class expectations follow the same ladder as plastic: Class 105 prototype (<500 shots, 10–18 days) through Class 102/101 for multi-million-run production (## Process parameters: temperature, pressure, cure time

Parameter	Typical range	Notes
Metering ratio	1:1 (A:B)	Verified per shift; ratio drift = cure failure
Injection unit temperature	10–30 °C (cold)	Keeps liquid below reaction onset
Cold runner temperature	10–30 °C	Water-cooled manifold
Mold (cavity) temperature	120–180 °C	Platinum catalyst fires in this band
Injection pressure	50–200 bar (low!)	LSR is a low-viscosity liquid, not a melt
Cure time	< 60 s thin wall; 2–3 min thick (>5 mm)	( LSR guide)
Cycle time	30–120 s	Cure time is the long pole
Linear shrinkage	~2–4 % (handled in tool steel sizing)	Higher than plastics; mold design compensates
Part release	Automatic, no mold release (medical forbids it)	Surface finish + draft handle release

The surprising numbers for plastic molders: low injection pressure (LSR is water-thin) and high mold temperature (plastics use cold molds). Getting the cold/hot balance wrong produces pre-cured “snakes” in the runner or soft, under-cured parts at the far cavity.

Medical and food-grade silicone: compliance that matters

A large share of silicone injection molding work is medical silicone injection molding — and the compliance stack is real:

• Material: LSR grades certified to ISO 10993 (biocompatibility) and USP Class VI for device contact; FDA 21 CFR 177.2600 for food contact ( silicone page).
• Production environment: medical and baby-product programs run in dust-controlled (cleanroom-class) workshops — a stray fiber becomes a visual defect on a translucent nipple or a contamination event on a device seal.
• No mold release: mold releases contaminate surfaces and fail biocompatibility; the tool design itself must eject cleanly.
• Process validation: ISO 13485 demands validated cure parameters, lot traceability, and documented process controls — the same discipline we apply to every medical program.
• Testing: hardness, tear, elongation, compression set, and extractables verification on critical programs.

Buyers should ask for the gradeindustry sheet plus the molder cleanroom and ISO 13485 certificates — a “medical-grade silicone” claim without the system behind it is a marketing phrase, not a compliance statement.

Tolerances and dimensional behavior of LSR parts

Soft rubber does not hold the same numbers as rigid plastic — but modern LSR molding is far more precise than compression-molded rubber:

Part class	Realistic tolerance	Reference
Small, rigid features	±0.05–0.10 mm	( LSR guide)
General LSR parts	±0.1–0.2 mm	( LSR guide)
Large, flexible parts	±0.25 mm and above	Soft parts flex more than they hold
Platform machining tolerance	±0.003 in (0.08 mm) + ±0.002 in/in

Why tolerances are softer than plastic:

• Softness: Shore A 30–50 parts compress under measurement force; the same gauge reads differently shot to shot.
• Shrinkage: LSR linear shrinkage (~2–4 %) is absorbed by tool sizing, but geometry-dependent shrinkage still moves dimensions.
• Gating and flow: unfilled cavities, knit lines, and gate placement shift local dimensions.

The practical rule: tolerance critical sealing diameters and mating features on the drawing, and let the molder validate them with first articles — measuring soft parts needs the right fixtures and load, which is a supplier capability to check, not assume. Molditquick reports LSR dimensions with CMM and optical measurement plus hardness/tension testing per AQL.

Silicone overmolding: bonding LSR to plastic and metal

Silicone overmolding (LSR overmolding) bonds liquid silicone onto a rigid substrate — plastic, metal, or glass — in a second shot. Applications: smartphone seals on metal frames, medical device grips on ABS handles, baby-bottle grips, automotive connector seals, tool handles, waterproofing gaskets on electronics enclosures.

The bonding physics decide everything:

• Chemical bond on engineered plastics: PC, PA, PBT, PPS, and some ABS families form a true chemical bond with LSR when the substrate is clean, dry, and held at the right temperature. The bond is strong enough for sealed assemblies.
• Mechanical bond elsewhere: metals and low-energy plastics get mechanical interlocking — undercuts, grooves, or through-holes that the silicone flows into and locks around.
• Primers: specific plastic/metal families need primers (silane-based) applied to the substrate before overmolding.
• Process: the substrate is molded or machined, placed in the LSR tool, and the second shot cures around it. Multi-shot (2K) LSR machines automate this for high volume.

DFM rules: hold the substrate in the cavity so melt pressure does not shift it; gate LSR away from bond-critical faces; keep the LSR wall uniform (0.5–3 mm typical); design the bond area to avoid peel loading. Full material-compatibility tables and geometry rules are in our overmolding guide and two-shot vs overmolding guide.

For buyers who need a quote rather than the theory, our silicone injection molding service covers LSR and HCR parts, cold-runner tooling and cleanroom molding in one scope.

Cost and lead times

Cost drivers for silicone injection molding:

1. Material — LSR costs more per kilo than most thermoplastics; high-cavitation molds and runner recycling offset it.
2. Tooling — cold-runner molds with valves, heated cavities, and high cavitation are specialist builds; expect higher tool cost than an equivalent plastic tool.
3. Cavitation — seals and gaskets are typically 8–64 cavities; the more cavities, the lower the per-part cost and the higher the tool cost.
4. Cure time — thick parts (2–3 min cure) cost more per part than thin seals (<60 s).
5. Cleanroom/medical overhead — dust-controlled production and ISO 13485 documentation add cost that food/medical programs must budget.

Lead-time reference bands: prototype tooling 7–18 days (production tooling 4–6 weeks standard, complex/multi-cavity 6–12 weeks (platform rapid production ~7 days after tool validation, no MOQ.

Volume reality: LSR seals run in the millions per year (automotive and medical); prototype and low-volume runs start at hundreds of parts. Molditquick quotes per volume band — prototype, low-volume, and mass production on the same line — and our audited tooling partners build the cold-runner molds under the same quality system that molds with them, which shortens the DFM-to-SOP path and keeps mold revisions on one P&L.

Applications: medical, baby products, automotive, electronics

Medical — respiratory masks and seals, infusion pump tubing and seals, syringe plunger tips, catheter components, wound-drain bulbs, implant-adjacent seals, device grips. LSR is the default elastomer for sterilizable, biocompatible, long-life components ( silicone page).

Baby & maternal — bottle nipples, pacifiers, teethers, feeding spoons, breast-pump parts. Food-grade, BPA-free, sterilization-tolerant, and tear-resistant: LSR owns this category ( silicone page).

Automotive & EV — connector seals, grommets, valve seals, battery-pack sealing, headlamp gaskets, climate-control flaps. Service temperature to 250 °C and long compression-set life qualify LSR for under-hood and EV battery duty.

Electronics & consumer — phone and smartwatch seals, speaker/mic gaskets, keyboard membranes, camera lens rings, wearable straps, waterproof connector boots.

Industrial — O-rings, diaphragm seals, pump membranes, food-processing seals, valve seats, electrical insulators.

Common silicone molding defects and fixes

Defect	Appearance	Root cause	Fix
Bubbles / voids	Spherical voids in cured part	Trapped air, inadequate venting	Vacuum venting, deeper vents, slower fill
Under-cure / soft parts	Tacky, low hardness	Mold temp low, cure time short, ratio off	Raise cavity temp, extend cure, verify 1:1 ratio
Over-cure / scorched	Brittle, discolored, odor	Mold temp high, long residence	Lower temp, faster cycle, check metering
Flash / burrs	Thin fins at parting line	Low clamp, worn parting, overfill	Raise clamp force, recondition parting, reduce shot
Short shot / incomplete fill	Missing geometry at far end	Cold runner too cold, low pressure, blocked vent	Warm runner slightly, raise pressure, vent
Knit lines / flow marks	Visible seams in thin-wall parts	Split flow around cores	Re-gate, raise temp, vacuum assist
Pre-cure in runner (“snakes”)	Curly cured strands	Runner too warm, long residence	Cool runner to 10–20 °C, purge, check hot deck seal
Contamination spots	Specks or fibers	Cleanroom breach, dirty material	Enforce dust control, clean line, filtered material
Tear on demolding	Torn edges at ejectors/undercuts	Soft grade, sharp edges, aggressive ejection	Draft, larger ejectors, slower demold, high-tear grade
Sticky surface	Part clings to cavity	Over-cure, release failure	Verify cure, polish cavity, check grade (no mold release in medical)
Sink / voids at thick sections	Depression over thick rib	Cure shrinkage in thick mass	Reduce wall, move gate, higher pressure
Color streaks	Non-uniform color	Colorant ratio/flow issue	Calibrate color pump, improve mixing

The first diagnostic on any LSR defect is the temperature map of the tool — hot/cold imbalance explains most bubble, under-cure, and pre-cure problems.

DFM checklist for LSR parts

• Wall 0.5–3 mm typical; thick sections flagged (cure time and sink cost).
• Uniform wall; transitions gradual.
• Draft 0–2° (soft grades self-release; firm grades need draft).
• No sharp internal corners — radii reduce tear initiation.
• Gate placement away from cosmetic/sealing surfaces.
• Venting planned with the mold designer (vacuum venting for medical/baby).
• Substrate bond details for overmolding (chemical vs mechanical bond, primers).
• Tolerance callouts realistic for soft parts (see section 8).
• Grade specified: hardness (Shore A), tear, FDA/ISO 10993 requirement, color/translucency.
• Sterilization method stated (autoclave/E-beam/gamma) — it can drive grade choice.
• Cleanroom requirement stated on the RFQ.

Full checklist: DFM checklist guide.

Choosing a silicone injection molding manufacturer

Silicone molding is where “cheap supplier” stories end badly — under-cured seals fail in the field, and the defect is invisible until the product leaks. Screen suppliers on:

1. Cold-runner LSR experience — ask for photos of their molds and a sample part history; a plastic molder “trying LSR” is a risk.
2. Cavitation and automation — can they run 16+ cavity molds with automatic demolding?
3. Cleanroom + certifications — ISO 13485 for medical, cleanroom-class production, ISO 9001 baseline; IATF 16949 for automotive.
4. Material relationships — do they stock certified medical/food LSR grades with COAs?
5. Testing — hardness, tear, compression set, dimensional reports; do they own the equipment?
6. DFM quality — does the quote include gate, vent, and bonding review?
7. References — ask for a seal or gasket program reference and verify lead times.

Molditquick qualifies on all seven: dedicated LSR line, cold-runner tooling with wire EDM capability, dust-controlled workshop, IATF 16949 / ISO 9001, and CMM + elastomer testing run on our own metrology floor.

FAQ

1. Can you injection mold silicone? Yes — liquid silicone rubber (LSR) is injection molded with two-component metering, cold-runner molds, and hot cavities; thin-wall parts vulcanize in under 60 seconds.

2. What is LSR injection molding? Liquid silicone rubber injection molding: Part A + Part B mixed 1:1, injected cold into a runner system, cured by heat in the cavity at 120–180 °C into a finished elastomer part.

3. How long does silicone injection molding take? Thin-wall parts cure in under 60 seconds; thick parts (5+ mm) take 2–3 minutes. Total cycle is typically 30–120 seconds.

4. What temperature is silicone injection molding? Metering and runner: 10–30 °C (cold). Cavity: 120–180 °C (hot). The cold/hot split is the signature of LSR tooling.

5. Is silicone injection molding safe for medical use? Yes — medical-grade LSR is ISO 10993/USP Class VI biocompatible and sterilizable; medical programs require IATF 16949-level systems and cleanroom production, which Molditquick holds.

6. Is silicone injection molding food safe? Food-grade LSR meets FDA 21 CFR 177.2600 and is BPA-free — the standard for baby nipples, teethers, and kitchenware ( silicone page).

7. What is the shrinkage of silicone in injection molding? LSR linear shrinkage is roughly 2–4 %, compensated in tool steel sizing rather than left to chance; dimensional tolerances on molded parts run ±0.05–0.25 mm depending on size and softness .

8. LSR vs TPE — which should I use? LSR: −60 to +250 °C, sterilizable, biocompatible, permanent-set resistant — for medical, baby, automotive seals. TPE: cheaper, recyclable, thermoplastic — for cost-driven grips and gaskets in mild environments. See the material comparison.

9. What is silicone overmolding? Bonding LSR onto a plastic/metal substrate in a second shot — chemical bond on engineered plastics (PC, PA, PBT, PPS), mechanical lock or primers on metals and low-energy plastics. Details in our overmolding guide.

10. What tolerances can silicone injection molding hold? Small rigid features ±0.05–0.10 mm; general parts ±0.1–0.2 mm; large flexible parts ±0.25 mm and above .

11. Why is silicone injection molding expensive? Specialized two-component machinery, cold-runner tooling, high cavitation for economic seals, cleanroom requirements, and material cost — but per-part economics beat compression molding at volume.

12. How do you prevent bubbles in silicone injection molding? Vacuum venting, generous venting channels, and balanced fill; trapped air is the #1 bubble cause.

13. What is the difference between LSR and HCR silicone molding? LSR: liquid, two-part, injection molded, fast cycles, high precision. HCR: solid sheets, compression/transfer or screw-fed injection, slower, thicker/high-durometer parts.

14. Does silicone injection molding require a cold runner? Yes — the runner stays cold (10–30 °C) to prevent pre-cure; valve-gated cold runners also recycle runner material for near-100 % yield.

15. Where can I find silicone injection molding near me / manufacturers? Silicone injection molding manufacturers are global; the screening list in section 14 covers certifications, cleanroom, cavitation, and DFM quality. Molditquick quotes globally from China with tightly coupled tooling — send your model to our injection molding service or see the dedicated LSR service page.

16. What is the MOQ for silicone injection molding? No fixed MOQ — prototype runs start at hundreds of parts; high-cavitation tools make millions/year economical. See our low-volume injection molding service.

Ready to mold silicone? Send your model, durometer, and compliance requirement to our injection molding service — we return a DFM review with gate/vent/bonding recommendations and a costed quotation. Explore material options in the plastic material selection guide and the full materials library.