Guia completo para moldagem por injeção de borracha de silicone líquido (LSR)

Updated June 2026 · Reviewed by the Guangdong Engelhardt Rubber & Plastic Technology technical team

Liquid silicone rubber injection molding is a thermoset process that forces a chilled, two-part platinum-cured silicone into a hot steel cavity, where it cures into a flexible, durable part in seconds. It’s the one silicone process that runs almost like a thermoplastic machine yet behaves chemically in reverse, and that single inversion shape everything about its tooling, defects, and economics. This guide explains how the LSR process actually works, how it differs from solid-rubber and thermoplastic molding, what its tooling and defects demand, and how to brief or vet a molder.

LSR sits inside the broader family of rubber injection molding processes, but it’s the only one that arrives as a pumpable liquid. That difference is why a shop can automate it lights-out, and why its mold has to fight flash at a scale no thermoplastic ever sees.

What Liquid Silicone Rubber Is, and Why It Molds “Backwards”

Liquid silicone rubber is a high-purity, two-part silicone elastomer with an inorganic siloxane backbone, alternating silicon and oxygen atoms carrying methyl and vinyl side groups. One component carries a platinum catalyst; the other carries a cross-linker (a methyl-hydrogen siloxane) plus an inhibitor. Mixed 1:1 and exposed to heat, the platinum drive an addition cure that permanently cross-links the chains. Unlike a thermoplastic, which melts and re-solidifies through a physical change, this is an irreversible chemical reaction, the part can’t be re-melted or reprocessed. As an LSR material, its defining material properties are flexibility, biocompatibility, and thermal stability, which carry through to every molded part it produces. A quick selection rule: if the part is a seal, membrane, or skin-contact surface, reach for a softer 20–40 Shore A grade; if it must resist wear or hold a structural edge, move to 60–70 Shore A, then confirm the choice with a sample, because durometer trades off against tear strength and sealing force.

Can silicone rubber be injection molded?

Yes, but only the liquid form. LSR’s low viscosity lets metering pumps push it through pipes and a static mixer, so it can be injection molded on automated equipment. Solid, gum-like high-consistency rubber (HCR) can’t flow this way and is instead processed by rubber compression molding or transfer molding. According to the encyclopedia entry on LSR injection molding, the cured material resists extreme temperatures, has excellent electrical insulation, and offers a hardness range of 5 to 80 Shore A.

That inversion is why LSR equipment chills the material right up to the gate and only lets it see heat inside the cavity. It’s also why platinum cure matters: platinum-cured silicone leaves no peroxide residue and delivers better tensile and tear strength and clarity than peroxide-cured silicone systems, one of the defining characteristics of LSR among silicone materials, and exactly why medical and food-contact liquid silicone rubber applications standardize on it. For a deeper material-versus-material breakdown, see our comparison of silicone versus conventional rubber.

How the LSR Injection Molding Process Works, Step by Step

LSR injection molding, also called silicone injection molding when described by material, is a continuous, metered, thermoset sequence rather than the melt-and-freeze loop of plastics. Unlike a standard silicone molding setup that works solid gum, the injection molding machine here handles a pumpable liquid. Here’s the full chain from drum to demold:

1. Metering, two metering pumps draw the base and catalyst from their pails or 55-gallon drums and hold them in a constant 1:1 ratio. A color pigment can be injected here.
2. Mixing, a static mixer blends the two streams into a homogeneous compound just before injection, because the cure clock start the instant the components meet.
3. Chill and hold, mixed material is kept cool in the metering section and cold runner so it stays liquid until the cavity.
4. Injection, on an LSR injection molding machine, a shut-off nozzle open and the screw acts as a piston, driving the compound through the cold runner into the heated mold cavities. A positive shut-off valve stops the screw from being pushed back and overfilling the barrel.
5. Curing, inside the cavity (≈150–210 °C) the platinum addition reaction cross-links the silicone in roughly 20–60 seconds, depending on wall thickness.
6. Demolding, the mold open and a robot or chute removes the parts. An optional post-cure in an oven (commonly around 175 °C) drives off volatiles and optimizes tensile, modulus, and durometer for critical parts.

How do you mold liquid silicone rubber in practice?

In practice the molder tunes three levers, injection pressure, vulcanization temperature, and vulcanization time, against part geometry. The LSR tool is housed in an LSR-specific injection molding press built for precise shot control, which is what separates liquid silicone injection molding from a converted plastic injection molding press. A landmark process-design study in Medical Plastics & Biomaterials documents injection pressures of roughly 200–1,200 psi (most applications run 300–700 psi) and vulcanization temperatures of 250–375 °F, the heart of the curing process. Note the distinction many guides blur: those are injection pressures into the cavity. The much higher 500–5,000 psi figure you’ll see elsewhere is the supply/metering pressure used to pump high-durometer material to the machine. Confusing the two leads engineers to badly over-spec a press.

“Liquid silicone rubber is slightly compressible, injection molding with this type of material is somewhat like molding a spring. Higher injection pressure results in lower part shrinkage, and higher vulcanization temperature causes greater shrinkage.”

One under-discussed shop trick: a 5–10 °F temperature difference between the two mold halves makes the part stick to the cooler side, which controls which half it demolds from. Push the variance too far, though, and you accelerate guide-pin wear. For the cross-process view, compare this loop with our breakdown of compression molding versus injection molding.

LSR vs HCR vs Conventional Rubber Injection Molding

A common, and costly, assumption is that LSR is “just liquid HCR.” It isn’t. The cure chemistry, the feed form, the automation, and the scrap economics all diverge across these molding methods. High consistency rubber (HCR) is a solid, gum-like silicone that’s often (but not always) peroxide cured; it’s slow to cure, less dimensionally repeatable, and frequently needs a post-cure. LSR’s pumpable, low-viscosity nature is what unlocks fully automated, near-flashless molding and lower material waste than working solid gum. It also differs from traditional thermoplastic injection molding: where plastic injection melts and re-freezes a polymer, LSR chemically cures molded silicone in place, so traditional injection molding intuitions about cooling and shrinkage don’t transfer directly. The LSR vs HCR vs Thermoplastic 3-Way Process Ladder below maps the practical trade-offs.

Here’s the decision rule that follows from the ladder: choose LSR when you need flexibility, biocompatibility, or extreme-temperature performance in silicone rubber parts you’ll run in volume; choose HCR for large, simpler cross-sections at lower volume; choose a thermoplastic when the part must be rigid and re-meltable. In short, how liquid silicone rubber injection molding works is fundamentally a curing problem, not a cooling one. For the silicone-specific cut of this question, see our deeper pieces on LSR vs HCR and LSR vs TPE.

LSR Tooling and Cold-Runner Mold Design

LSR tooling is where the “Two-Heat Rule” cashes out into steel. Because the material must stay cold until the cavity, the mold uses a cold runner (cold deck)a chilled manifold that injects directly into the part with little or no cured runner waste. Its cavity runs hot. A good cold runner is expensive compared with conventional hot-runner tooling, but it’s what makes near-zero runner scrap possible. The patent record reflects how much engineering goes into this: EP 1293323 B1 describes injecting LSR through a cold-runner system with backflow prevention, and KR 101983242 B1 covers a shut-off nozzle that stops leakage and overfill.

Mold steel is hardened stainless because LSR is mildly abrasive and the tool see heat cycling. Unlike plastic molds, LSR cavities generally don’t need a high-polish finish or draft angles, since the cured rubber releases without distortion. Venting is the make-or-break detail: Plastics Technology’s tooling primer stresses that LSR’s very low viscosity forces extremely tight, well-placed vents. Typical vent depth for LSR runs about 0.0003–0.0005 in.; a peripheral tear-tab vent may run 0.002–0.005 in.

For the dimensional side of this, our guide to injection molding tolerances and our LSR design guidelines go deeper on wall-thickness, gating, and undercut rules. In our own 3,500 m² mold workshop, we cut LSR cold-deck tooling on Makino and Roeders machines and validate shrink on first articles before release, because a mold sized to the wrong shrink number is a re-cut, not a tweak.

Common LSR Molding Defects and How to Prevent Them

LSR’s low viscosity is its gift and its curse: it fills micro-features beautifully and flashes into any gap just as easily. As one silicone moldmaker put it bluntly on a molding forum, “you won’t achieve flash-free parts with 3D-printed molds, silicone flashes at about .00015 in.” Real flash control is a steel-and-venting discipline, which is why machine builders patent hardware specifically to stop it; patent KR 101983242 B1 describes a shut-off nozzle whose whole job is to prevent the leakage and overfill that drive flash. The 5 LSR Defects Root-Cause Table maps the failures we see most often to their cause and fix.

What are the disadvantages of LSR?

Here are the honest limitations: LSR demands specialized, costly cold-runner tooling and an LSR-specific press; flash is an ever-present finishing risk that may require trimming; a post-cure step is often needed for critical mechanical or food/medical properties; and the material can’t be reprocessed. None of these are deal-breakers, but they explain why LSR rewards volume and a disciplined molder, and why a low-volume one-off can be cheaper in another process. In our on-site quality lab we screen first articles for flash, voids, and durometer before a tool is released to production, because the cheapest defect is the one caught before the run start.

Where LSR Injection Molding Wins, Applications by Industry

Among rubber products made by molding, LSR injection molding parts earn their place wherever a part must be flexible, biocompatible, electrically insulating, or stable across extreme temperatures. Its biocompatibility and food-contact credentials are not marketing claims, they are codified. U.S. FDA regulation 21 CFR 177.2600 explicitly lists silicone elastomers containing methyl and vinyl groups among rubbers permitted for repeated food contact, subject to extraction limits (no more than 20 mg/in² total extractives in water over the first 7 hours). For implantable and device contact, materials are screened to USP Class VI and qualified at the device level under ISO 10993.

Here’s how that plays out on the floor. A diagnostic-device team once brought us a duckbill check valve that had been compression molded in HCR, long cycles, manual flash trimming, and cracking pressure that drifted lot to lot until parts failed inspection. Re-engineered for LSR injection molding in a 40 Shore A platinum grade, the same valve held its cracking pressure inside a tight band across a six-figure annual run, demolded automatically, and cleared USP Class VI. That’s the recurring pattern: when a flexible part has to hit a regulatory bar and a repeatable spec at volume, LSR is usually what keeps it in production instead of getting re-sourced.

Because LSR is naturally translucent and easily pigmented, the same process turn out colorful, custom molded products without secondary painting. Self-adhesive LSR grades extend the process into two-shot and silicone overmolding, bonding a soft silicone face directly onto a rigid plastic or metal insert without primers or secondary gluing, a soft button face over a nylon housing is the classic example.

What LSR Injection Molding Costs, and the 4-Gate Suitability Screen

LSR cost has two layers: a one-time tooling investment (the cold-deck mold, often the single largest line item and the lengthiest step) and a per-part price set by cavitation, material grade, durometer, cycle time, and any post-cure or secondary operations. Equipment choices matter too, all-electric presses, prized for cleanroom work because they avoid hydraulic oil, cost at least 25% more than a comparable hydraulic machine. Because tooling is front-loaded, LSR’s unit economics improve sharply with volume, it rewards high-volume production and longer production runs, which is why it’s rarely the cheapest route for a true one-off. Its flip side is precise molding of complex silicone parts at a per-piece cost that falls as quantities climb. Our guide to custom rubber molding cost breaks the drivers down further.

Is LSR injection molding suitable for low-volume production?

LSR low-volume runs can work, but with eyes open. Many medical and consumer programs use LSR during late-stage prototyping to produce near-production parts for clinical trials and pilot runs. Watch the tooling bill, though: because the cold-deck mold dominates upfront cost, low volumes carry a high per-part tooling burden. If you only need a handful of parts, ask your molder whether a simpler bridge tool or an alternative process serves the milestone better, then switch to a production LSR tool when volume justifies it.

How to Choose an LSR Injection Molding Partner

Choosing a molder matter more in LSR than in most processes, because flash control, cold-runner tooling, and ratio discipline are shop-floor skills, not catalog items. Use this checklist when you vet an LSR injection molding supplier:

• ✔ Quality systems. ISO 9001 at minimum; ISO 13485 for medical, IATF 16949 for automotive; FDA / LFGB / NSF for food and medical contact.
• ✔ In-house tooling. Can they cut and correct cold-deck molds themselves? Outsourced tooling slows every shrink re-cut.
• ✔ Automation & cleanroom. Lights-out cold-deck cells and, for medical, ISO-classified cleanrooms reduce contamination and cost at volume.
• ✔ On-site testing. A real metrology and material lab to verify durometer, dimensions, and extractables before release.
• ✔ Capacity headroom. Enough presses and mixing capacity to scale without re-sourcing mid-program.

For context on what that capacity looks like in practice: our plant runs around 700 injection machines alongside two 55-litre silicone mixing lines (about 3,000 tonnes of silicone throughput a year), a 3,500 m² in-house mold workshop turning out 500-plus mold sets annually, and an on-site chemical and physical testing lab, under ISO 9001, IATF 16949, FDA, LFGB, and NSF. That vertical integration is what lets a shop fix a flash or shrink problem at the tool instead of in the quote. To see where LSR fits among our wider LSR injection molding services, start there.

Industry Outlook, Where LSR Injection Molding Is Heading (2026 and Beyond)

For buyers in 2026, the decision that matters is not whether LSR will grow, it is who can serve where it is growing. Two demand drivers are pulling LSR injection capacity toward automated, certified shops. First, automotive electrification is multiplying the need for high-temperature sensor, connector, and busbar seals that survive under-hood and battery-pack environments, an application set LSR is purpose-built for, and the reason automotive already dominates liquid-injection-molding demand. Second, medical-device programs keep tightening biocompatibility and cleanliness expectations, which favors molders with cleanrooms, ISO 13485, and validated USP Class VI / ISO 10993 material flows.

Technologically, the trend is toward fully-automated cold-deck cells and on-machine cure-rate control, which lower unit cost at volume and widen the gap between automated and manual shops, the steady stream of cold-runner tooling patents (such as CN 218462821 U) tracks that automation push. Market analysts size the global LSR market at roughly US$3.6 billion in 2026 with high-single-digit annual growth through the early 2030s, useful as directional background only, not as a sourcing reason. Actionable takeaway: if you’re planning a 2026–2027 program in medical or EV components, weight your supplier shortlist toward shops that already run automated LSR cells with the right certifications, because that capacity is where the demand is concentrating. For the upstream view, our liquid silicone rubber molding guide covers material grades and DFM in more depth.

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