Silicone Injection Molding: The Complete LSR Guide
Manufacturing Guide
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 (东莞国宏精密) runs LSR molding alongside our 21 Sodick injection machines, with dedicated dust-controlled silicone production, in-house tooling (wire EDM 9+4 machines, CNC), and quality systems covering IATF 16949, ISO 13485, and ISO 9001 in a 10,000 m² facility with 280 people.
Table of Contents
- What is silicone injection molding?
- LSR vs solid silicone rubber vs TPE
- LSR properties: why liquid silicone wins
- How LSR injection molding works: mixing, metering, vulcanization
- The cold runner mold: the heart of the process
- Process parameters: temperature, pressure, cure time
- Medical and food-grade silicone: compliance that matters
- Tolerances and dimensional behavior of LSR parts
- Silicone overmolding: bonding LSR to plastic and metal
- Cost and lead times
- Applications: medical, baby products, automotive, electronics
- Common silicone molding defects and fixes
- DFM checklist for LSR parts
- Choosing a silicone injection molding manufacturer
- FAQ
What is silicone injection molding?
“Can you injection mold silicone?” — yes, and the answer has two flavors:
- 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 (Kemal, LSR guide).
- 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 (Kemal, 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:
- 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.
- 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.
- 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.
- 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.
- 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 (RJC Mold, Xometry).
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) | (Kemal, 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 (First Mold 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 grade’s datasheet plus the molder’s 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 | (Kemal, LSR guide) |
| General LSR parts | ±0.1–0.2 mm | (Fictiv, LSR guide) |
| Large, flexible parts | ±0.25 mm and above | Soft parts flex more than they hold (Kemal) |
| Platform machining tolerance | ±0.003 in (0.08 mm) + ±0.002 in/in | (Protolabs) |
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.
Cost and lead times
Cost drivers for silicone injection molding:
- Material — LSR costs more per kilo than most thermoplastics; high-cavitation molds and runner recycling offset it.
- Tooling — cold-runner molds with valves, heated cavities, and high cavitation are specialist builds; expect higher tool cost than an equivalent plastic tool.
- Cavitation — seals and gaskets are typically 8–64 cavities; the more cavities, the lower the per-part cost and the higher the tool cost.
- Cure time — thick parts (2–3 min cure) cost more per part than thin seals (<60 s).
- 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 (First Mold, RJC Mold); production tooling 4–6 weeks standard, complex/multi-cavity 6–12 weeks (Zetar); platform rapid production ~7 days after tool validation, no MOQ (Protolabs).
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 in-house tooling shop builds the cold-runner molds in the same facility 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 (First Mold 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 (First Mold 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:
- Cold-runner LSR experience — ask for photos of their molds and a sample part history; a plastic molder “trying LSR” is a risk.
- Cavitation and automation — can they run 16+ cavity molds with automatic demolding?
- Cleanroom + certifications — ISO 13485 for medical, cleanroom-class production, ISO 9001 baseline; IATF 16949 for automotive.
- Material relationships — do they stock certified medical/food LSR grades with COAs?
- Testing — hardness, tear, compression set, dimensional reports; do they own the equipment?
- DFM quality — does the quote include gate, vent, and bonding review?
- References — ask for a seal or gasket program reference and verify lead times.
Molditquick qualifies on all seven: dedicated LSR line, in-house cold-runner tooling, dust-controlled workshop, IATF 16949 / ISO 13485 / ISO 9001, and CMM + elastomer testing in-house.
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 (Kemal).
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 (Kemal).
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 ISO 13485 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 (First Mold 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 (Kemal, Fictiv).
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 (Kemal, Fictiv).
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 in-house 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 (Protolabs). 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.