Last updated: 2026-09-01 · 10 min read

What Is LSR Injection Molding — and How Does the Process Differ from Thermoplastic Tooling?

Liquid silicone rubber (LSR) injection molding is a thermoset process that cures a two-part platinum-catalyzed silicone compound inside a heated mold cavity at 160–200 °C. Unlike thermoplastic injection molding — where molten resin is injected into a cold mold and solidifies by cooling — LSR is injected cold (20–30 °C compound temperature) into a hot mold and cross-links irreversibly.
The process differences affect every aspect of tooling design: gate geometry, venting depth, parting-line sealing, ejector strategy, and mold-temperature control. A thermoplastic mold cannot be repurposed for LSR without fundamental redesign.

Cycle time — The total time for one injection molding cycle: injection + packing + cooling + mold open/close + ejection. Typical range: 15–60 seconds. Cycle time determines production rate and cost per part — reducing cycle by 5 seconds saves 10–15% annual production cost. See also related guide.

Key Facts

  • LSR is a thermoset, not a thermoplastic. Once cured, it cannot be re-melted, re-ground, or recycled. The material is supplied as a two-part system (A: vinyl-terminated PDMS + platinum catalyst; B: cross-linker + inhibitor) mixed at a 1:1 ratio at the injection barrel.
  • The mold is heated to 160–200 °C (the opposite of thermoplastic tooling, which is cooled). Uniform mold temperature across the cavity surface is critical — a 10 °C gradient produces uneven cure, resulting in tacky spots or over-cured brittle zones.
  • LSR viscosity is 100–1,000× lower than thermoplastic melt viscosity. The material flows through extremely thin sections (0.1 mm) and fills micro-features that thermoplastic resin cannot . The low viscosity also means that parting-line sealing, vent design, and ejector-pin clearances must be tighter to prevent flash.
  • Cycle times range from 15 to 90 seconds depending on wall thickness (cure time is proportional to the square of the wall thickness). A 2 mm wall cures in approximately 20 seconds; a 5 mm wall requires approximately 60 seconds.
  • LSR parts are used in medical devices (catheter tips, gaskets, valves), infant-care products (nipples, pacifiers), automotive seals and gaskets, and electronic encapsulation (connector seals, LED lenses). The material is inherently biocompatible, UV-stable, and resistant to temperatures from −60 °C to +250 °C.

    • Cycle time — The total time for one injection molding cycle: injection + packing + cooling + mold open/close + ejection. Typical range: 15–60 seconds. Cycle time determines production rate and cost per part — reducing cycle by 5 seconds saves 10–15% annual production cost. The related guide covers adjacent topics.

    How Does the LSR Injection Process Work?

    The LSR injection process follows a sequence that is fundamentally different from thermoplastic molding. The two-part compound (A and B) is stored in separate drums at room temperature. A metering pump delivers both components at a precise 1:1 ratio into a static mixer, where they are blended homogeneously. The mixed compound enters the injection barrel at 20–30 °C — cold, not heated. The barrel does not have heater bands; it is water-cooled to prevent premature cure (scorch) in the barrel.

    The cold compound is injected through a cold runner system into the heated mold cavity (160–200 °C). Upon contact with the hot cavity surface, the silicone begins to cross-link. The cure time depends on the wall thickness and the mold temperature. After curing, the mold opens and the part is ejected — typically with ejector pins, but sometimes with air ejection or stripper plates for delicate geometries.

    1

    Metering and mixing

    Two-component pump delivers A and B at 1:1 ratio through a static mixer. The compound enters the barrel cold (20–30 °C). The pot life of the mixed compound is 24–72 hours at room temperature — long enough that the barrel does not require purging between shifts.

    2

    Cold injection

    The barrel is water-cooled to prevent scorch. Injection pressure is lower than thermoplastic molding (30–100 bar vs 700–2,000 bar) because the LSR compound has extremely low viscosity. Injection speed must be controlled to avoid turbulence and air entrapment.

    3

    Hot-cavity cure

    The mold is heated to 160–200 °C using electric cartridge heaters or oil-circulation channels. Cure time is proportional to wall-thickness squared: t_cure ≈ k × d², where k is a material-specific constant (typically 8–15 s/mm²). A 2 mm wall cures in 20 seconds; a 5 mm wall cures in 60–90 seconds.

    4

    Demolding and ejection

    LSR parts are flexible and can be stretched during ejection without damage. Ejector-pin placement must account for the part’s elasticity — a pin on a thin section can push through the cured silicone if the force is concentrated. Wide-area ejector pads or stripper plates are preferred for LSR parts.

    How Does LSR Tooling Differ from Thermoplastic Tooling?

    Design Element Thermoplastic Mold LSR Mold
    Mold temperature 20–120 °C (cooled) 160–200 °C (heated)
    Temperature control water cooling channels cartridge heaters / oil channels
    Gate type sub-gate, pin-point, edge open-tip or valve-gate (cold runner)
    Vent depth 0.02–0.05 mm 0.005–0.02 mm
    Parting-line seal standard shut-off precision shut-off (low-viscosity flash)
    Ejector system round pins wide pads / stripper / air poppet
    Shrinkage 0.3–2.0 % 1.5–3.5 %
    Mold steel P20, 718H, S136 hardened steel (HRC 50+) — LSR is abrasive

    The most significant difference is the vent depth.
    LSR has a viscosity of 10–100 Pa·s during injection (compared to 100–10,000 Pa·s for thermoplastic melt). The low viscosity means LSR can flash through vents that would be safe for thermoplastic resin. Vent depths for LSR are typically 0.005–0.02 mm — shallow enough to prevent flash but deep enough to allow air escape. This requires higher-precision machining and tighter parting-line shut-off tolerances than standard thermoplastic tooling.

    The mold steel must also be selected for the operating temperature. LSR molds run at 160–200 °C continuously, which means the steel must retain hardness and dimensional stability at elevated temperature. P20 (pre-hardened to HRC 28–32) is not suitable for LSR molds because it softens above 150 °C. Hardened tool steels such as S136 (HRC 50–52) or H13 (HRC 48–52) are standard for LSR cavities.
    The higher steel cost and the additional machining precision contribute to LSR molds costing 20–40 % more than equivalent thermoplastic molds.

    What Are the Cold-Runner Options for LSR?

    LSR processing uses cold-runner systems to prevent premature cure in the runner network before the material reaches the cavity. There are three cold-runner configurations:

  • Open-tip cold runner — the simplest design. The nozzle tip sits in a cooled manifold block and the compound flows through an open orifice into the cavity gate. The runner is cooled by water circulation to maintain the compound at 20–30 °C. After cure, the runner is removed with the part and discarded (LSR cannot be recycled). Open-tip is used for large parts and multi-cavity molds where runner volume is acceptable.
  • Valve-gate cold runner — a pneumatically or hydraulically actuated pin opens to allow flow and closes when the injection is complete. The valve gate eliminates the runner entirely: the pin seals at the cavity surface, and there is no runner to remove. Valve-gate systems reduce material waste (significant for medical-grade LSR at $50–150/kg) and eliminate the secondary operation of runner trimming. The trade-off is higher tooling cost and maintenance complexity.
  • Direct-tip cold runner — a single-tip system for single-cavity molds or hot-tip applications where the gate vestige must be minimal. The tip is a small, individually cooled nozzle that sits directly at the cavity gate. Direct-tip systems minimize runner volume and are used for small parts (under 5 g) where even a small runner represents a significant percentage of the shot weight.

    • — Arthur, Tooling Engineer at aCatMould: the cold-runner choice drives both the per-part cost and the secondary operations. A valve-gate system adds $3,000–8,000 to the mold cost but eliminates runner trimming and reduces material waste by 15–30 %. For a medical LSR part running 500,000 shots/year, the valve-gate system pays for itself within the first year through material savings alone.

    What Design Rules Apply to LSR Parts?

    LSR part-design rules overlap with thermoplastic rules in some areas (uniform wall thickness, generous radii, draft angles) but diverge in others (flash control, ejector strategy, undercuts).

    Design Rule LSR Requirement Reason
    Wall thickness 0.3–6 mm (uniform) cure time ∝ d² — thick sections cure slowly and may trap volatiles
    Draft angle 1–3° LSR grips cavity surface; draft reduces demolding force
    Undercuts permitted (elastic demolding) LSR elongation at break is 200–700 %; undercuts up to 10 % of diameter can be stripped without side-actions
    Parting-line location on a flat surface, not on a radius flash at a radius is visible and difficult to trim; flash at a flat edge is a straight-line trim
    Radii min 0.3 mm internal, 0.5 mm external sharp corners create stress concentration and flash risk
    Shrinkage allowance 2.0–3.0 % (typical) LSR shrinks more than most thermoplastics; the cavity must be oversized accordingly

    The undercut rule is a significant advantage over thermoplastic molding. Because cured LSR has elongation at break of 200–700 %, shallow undercuts can be stripped from the mold without side-action mechanisms. A snap-fit hook with a 0.5 mm undercut on a 10 mm diameter feature can be ejected by stretching the part over the core — something impossible with a rigid thermoplastic part. This eliminates side-actions and reduces mold complexity (and cost) for parts with simple undercuts.

    What Are the Common LSR Processing Defects?

  • Flash — caused by insufficient parting-line seal, vent depth too large, or injection pressure too high. LSR’s low viscosity (10–100 Pa·s) means it flows into gaps that thermoplastic melt cannot penetrate. Flash on LSR parts is thin (0.01–0.05 mm), difficult to trim without damaging the part, and visible on cosmetic surfaces.
  • Incomplete cure (tacky surface) — caused by mold temperature too low, cure time too short, or uneven heating. A tacky surface indicates that the cross-linking reaction has not reached completion at the cavity surface. The fix is to increase mold temperature or extend cure time, not to change the compound formulation.
  • Air traps and voids — caused by turbulent injection, insufficient venting, or air entrapped during metering. Air bubbles in the barrel (from improper degassing of the A/B components) produce voids in the cured part. The fix is to ensure proper degassing of the compound drums and to use slow, laminar injection speed.
  • Short shot — caused by insufficient injection pressure, compound viscosity too high (expired or improperly stored compound), or gate size too small. LSR compound shelf life is 6–12 months at room temperature; expired compound has higher viscosity and may not fill thin sections.
  • Burn marks — caused by trapped air auto-igniting at the high mold temperature (160–200 °C). The fix is to improve venting, not to reduce mold temperature (which would extend cure time and reduce output).

    • Frequently Asked Questions

    Can LSR be overmolded onto a thermoplastic substrate?

    Yes — LSR-to-thermoplastic overmolding is a two-shot process where the thermoplastic substrate is molded first (and cooled), then the LSR is injected over it in a second cavity. The substrate must withstand the LSR cure temperature (160–200 °C) without deformation. PBT, PPS, PEEK, and high-temperature nylon are suitable substrates; ABS, PP, and standard nylon soften or deform at LSR cure temperatures. Adhesion between the LSR and the substrate must be tested — some grades bond chemically, others require a primer or mechanical interlock.

    Is LSR the same as HTV (high-temperature vulcanizing) silicone?

    No. Both are silicone rubbers, but they differ in molecular weight and processing. HTV silicone has a high molecular weight (gum-like consistency) and is processed by compression molding or extrusion. LSR has a low molecular weight (liquid consistency) and is processed by injection molding. LSR offers faster cycle times, better dimensional control, and the ability to fill thin walls and complex geometries that HTV cannot achieve.
    HTV is used for large, simple shapes (gaskets, O-rings, tubing); LSR is used for precision parts (medical devices, electronic seals, micro-components).

    What is the minimum viable production volume for LSR injection molding?

    LSR molds cost 20–40 % more than equivalent thermoplastic molds due to the heated cavity, precision venting, and cold-runner system. The minimum viable volume depends on the part complexity and the material cost. For a simple gasket with a $15,000 mold and $80/kg LSR compound, breakeven is approximately 10,000–20,000 parts. For a complex medical part with a $40,000 valve-gate mold and $150/kg compound, breakeven is approximately 30,000–50,000 parts.

    Does LSR require post-curing?

    Some LSR grades require a post-cure (secondary vulcanization) in an oven at 200 °C for 2–4 hours to drive off low-molecular-weight siloxane byproducts and achieve full mechanical properties. Post-cure is mandatory for medical and food-contact applications (to meet extractable limits) and recommended for parts requiring maximum compression-set resistance. For general-purpose industrial seals and gaskets, the in-mold cure is sufficient.

    Can LSR parts be bonded to other materials after molding?

    LSR has a naturally low surface energy (similar to PTFE), which makes adhesive bonding difficult without surface treatment. Plasma treatment or corona treatment increases surface energy and enables bonding with epoxy, cyanoacrylate, and silicone adhesives. Alternatively, self-bonding LSR grades contain adhesion promoters that bond directly to specific substrates (glass, metal, certain thermoplastics) during the molding process without a separate adhesive step.

    What tolerances are achievable with LSR injection molding?

    LSR dimensional tolerances are wider than thermoplastic injection molding due to the higher shrinkage (2–3 %) and the sensitivity of cure shrinkage to mold-temperature variation. Typical achievable tolerances are ±0.1–0.3 mm for features under 25 mm and ±0.3–0.5 mm for features above 25 mm. Tighter tolerances (±0.05 mm) are achievable with process optimization and tighter mold-temperature control (±2 °C), but the cost increases due to additional process monitoring and cavity adjustments.

    Key Takeaways

    1. LSR injection molding is a thermoset process — cold compound injected into a hot mold, cured irreversibly. The tooling design is fundamentally different from thermoplastic molds: heated cavity, precision vents (0.005–0.02 mm), cold-runner system, hardened steel (HRC 50+).
    2. LSR mold cost is 20–40 % higher than an equivalent thermoplastic mold due to the heating system, precision machining, and cold-runner hardware. The additional cost is recovered at volumes above 10,000–50,000 parts depending on part complexity and material grade.
    3. The low viscosity of LSR (10–100 Pa·s) enables thin-wall fill (0.3 mm) and micro-feature replication but demands tighter parting-line seals and shallower vents than thermoplastic tooling. Flash is the most common defect and the most difficult to correct after T1.
    4. LSR’s flexibility (200–700 % elongation) allows shallow undercuts to be stripped without side-actions — a design advantage that reduces mold complexity for parts with simple snap-features and barb fittings.
    5. Material selection must match the end-use environment: LSR is inherently biocompatible and temperature-resistant (−60 to +250 °C), but the specific grade must be selected for the regulatory pathway (medical, food-contact, or industrial) and the bonding requirements of the assembly.

    An engineer specifying an LSR mold for the first time often underestimates the tooling differences from thermoplastic injection molding. The heated cavity, the cold-runner system, the precision venting, and the hardened steel all add cost — but they are non-negotiable requirements of the process. A quotation that presents the LSR tooling cost alongside a thermoplastic equivalent, with the cost drivers itemized, gives the customer the context to understand why the LSR mold costs more and where the cost is allocated.

    Sources
    Wacker Chemie — LSR processing guide
    Dow Performance Silicones — LSR design guide
    Plastics Technology — LSR molding fundamentals

    Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet