Liquid Silicone Rubber (LSR) Injection Molding: Process, Properties, and When to Choose It Over TPE

How LSR Injection Molding Works — and Why It Is the Reverse of Standard Thermoplastic Molding

Standard injection molding melts a thermoplastic in a heated barrel and injects it into a cooled mold. LSR injection molding does the opposite. The two liquid components — Part A (the base polymer) and Part B (the platinum catalyst) — are stored separately in refrigerated tanks and pumped through a cold-deck manifold kept below 15–20°C to prevent premature vulcanization. They are combined in a 1:1 ratio by a static mixer immediately before injection into a heated mold cavity (typically 170–200°C), where the platinum catalyst triggers rapid crosslinking that locks the molecular network permanently.

Because LSR is a thermoset, curing is irreversible — unlike a thermoplastic, it cannot be remelted. The upside is exceptional stability: standard LSR tolerances of ±0.1 to ±0.2 mm are achievable with well-designed tooling, and shrinkage is predictable at 2–3%. Injection pressures range from 500 to 5,000 psi depending on durometer and part geometry — significantly lower than most engineering thermoplastics, which typically require 5,000–20,000 psi, reducing clamp tonnage requirements and tool wear.

Tooling for LSR demands sealed parting lines with extremely tight shut-off surfaces. Because LSR’s viscosity is much lower than molten thermoplastic, even a 0.01 mm parting-line gap can produce flash requiring manual trimming. SIMTEC Silicone Parts, one of North America’s leading LSR specialists, documents that tooling design quality — particularly shut-off geometry, cold manifold integration, and venting — is the single largest determinant of first-article success in LSR programs.

LSR vs. TPE vs. EPDM: A Direct Properties Comparison

Engineers evaluating flexible sealing or soft-touch materials most frequently compare LSR against thermoplastic elastomers (TPE) and ethylene propylene diene monomer (EPDM) rubber. The table below summarizes the key decision-relevant properties, drawing on data from SIMTEC, Fictiv, and Moraine Plastics.

LSR Applications by Industry

The flexibility and biocompatibility of LSR make it the material of choice in demanding application families across four major industries. The Fortune Business Insights LSR market report identifies medical devices as the fastest-growing end-use segment, commanding the largest revenue share in the global LSR market in 2025.

Medical devices represent LSR’s highest-value market. Drug delivery components (valve membranes, syringe tip caps, infusion line stoppers), respiratory mask seals, surgical instrument grips, wearable glucose sensor housings, and implant-adjacent components all rely on LSR’s USP Class VI and ISO 10993 biocompatibility. Medical-grade LSR does not leach plasticizers — a critical distinction versus PVC or certain TPE grades used in intravenous and drug-contact applications where extractable limits are regulated.

Automotive applications exploit LSR’s thermal endurance. Turbocharger hose seals (continuous 180°C duty), ECU connector gaskets in under-hood environments, spark-plug boots, and EV battery pack sealing membranes all require sustained temperature resistance and low compression set over multi-year service. Momentive’s automotive LSR grades are documented for continuous service to 200°C — a specification that eliminates every TPE option from consideration.

Consumer electronics and wearables increasingly specify LSR for smartwatch bands, waterproof earphone tips, foldable-device hinge covers, and soft-touch keypads. LSR’s UV and ozone stability prevents yellowing or cracking under multi-year outdoor exposure — a durability advantage that standard TPE straps without UV stabilizer packages cannot reliably match beyond three to five years of service.

When to Choose LSR — and When TPE Is the Better Engineering Call

LSR commands a tooling cost premium of 20–40% over equivalent TPE molds due to the cold-deck manifold system, precision parting-line shut-off surfaces, and heated mold temperature control circuits. That premium is justified by four specific engineering criteria. If any of the following apply to your design, LSR should be your starting material:

• • Sustained service above 130°C. No commercial TPE family maintains dimensional and mechanical integrity above roughly 130°C for more than short excursions. LSR holds its properties continuously to 200°C, with documented automotive grades in service at 200°C for the life of the vehicle.
• • FDA, USP Class VI, or ISO 10993 biocompatibility certification is required. Medical-grade LSR achieves all three standards. While select TPE grades satisfy FDA 21 CFR 177 requirements for indirect food contact, they do not achieve USP Class VI implantation standards across the full hardness range.
• • Compression-set specification below 15% after 22 hours at 175°C. SIMTEC uses this threshold to define LSR’s sealing advantage. TPE typically exhibits 30–60% compression set under those conditions — insufficient for long-life static seals in automotive powertrain or high-cycle valve applications.
• • Multi-year outdoor UV and ozone exposure. LSR’s silicone backbone is inherently UV-stable without additive packages. TPE grades degrade meaningfully after three to five years of direct sunlight and ozone exposure, particularly in outdoor wearables, automotive exterior trim, and solar energy sealing applications.

If none of these four criteria apply — if the part operates below 120°C, targets a consumer-grade non-medical application, runs indoors, and has no strict compression-set specification — TPE is the economically correct material. A well-designed TPE tool will cost 20–40% less, run on standard thermoplastic injection equipment, and deliver cycle times of 10–30 seconds versus LSR’s 15–45 seconds. The decision is not about which material is better; it is about which material is right for the specific duty cycle and regulatory environment of your part.