Optically Clear Liquid Silicone Rubber (LSR) for LED Lenses and Optical Parts: Light Transmittance, Injection Molding Process, and Equivalent Commercial Grade Guide
LED lighting engineers keep running into the same three problems: lenses that turn yellow after a few hundred hours of blue-pump exposure, hazy parts that scatter light off-spec, and procurement teams stuck on a discontinued or overpriced grade with no obvious substitute. This guide breaks down what optically clear LSR actually delivers on the optical bench, how to mold it without bubbles or haze, where it beats epoxy and PC/PMMA, and how to write an RFQ that lands on the right equivalent to ELASTOSIL LR 3003, LR 3162, SILASTIC MS-1002, OP993, or TC-5130. It is written for LED lighting design engineers, Tier-1/2 automotive lighting buyers, optical product designers, and procurement teams sourcing a qualified replacement.
What Optically Clear LSR Delivers: Optical and Mechanical Property Profile
Optically clear LSR is a two-part, addition-cure (platinum-catalyzed) liquid silicone rubber that crosslinks into a transparent elastomer. The cured material transmits more than 90-95% of visible light at 2 mm thickness, shows haze below 1-2%, and carries a refractive index of roughly nD 1.40-1.43, which makes it a comfortable match for many glass-and-LED stack-ups. Hardness is tunable from about Shore A 20 to 80 depending on grade, so the same chemistry family can deliver a soft gasket-grade lens or a rigid cover.
Mechanically, post-cured clear LSR typically reaches 6-11 MPa tensile (ASTM D412), 150-500% elongation, 8-25 kN/m tear strength (ASTM D624 Die B), and a compression set of 20-30% after 22 h at 150-175 C (ASTM D395 B). Service temperature spans roughly -50 C to +200 C continuous, which is where the comparison with acrylics and epoxy starts to get interesting.
| Property | Typical Value | Test Method / Notes |
|---|---|---|
| Light transmittance (400-800 nm, 2 mm) | > 90-95% | ASTM D1003; blue 450 nm transmittance > 92% |
| Haze | < 1-2% | ASTM D1003 |
| Refractive index nD | 1.40-1.43 | Abbe refractometer, 25 C |
| Hardness | Shore A 20-80 | ASTM D2240; grade-dependent |
| Tensile strength | 6-11 MPa | ASTM D412, post-cured |
| Elongation at break | 150-500% | ASTM D412, post-cured |
| Tear strength | 8-25 kN/m | ASTM D624 Die B |
| Compression set (22 h / 150-175 C) | <= 20-30% | ASTM D395 B |
| Service temperature | -50 C to +200 C continuous | Grade dependent; higher peaks possible short term |
Molding Optically Clear LSR: Process Steps and Process Parameters
Optical clarity is won or lost in the molding cell, not in the chemistry. Follow this sequence from material to inspected part:
- Store A and B components at 20-25 C, sealed, and verify lot certificates for transmittance and viscosity before kitting.
- Dose 1:1 by volume through a static mixer; the mixed stream is degassed in-line or under vacuum to pull entrained air before it reaches the cavity.
- Inject into a mirror-polished, cold-runner LIM mold (no-runner systems preferred to avoid shear-induced haze and to eliminate scrap regrind).
- Hold at mold temperature 160-200 C until cure is complete – typically a few seconds to about one minute depending on wall thickness (0.5-5 mm).
- Demold with shrinkage compensation of about 2-3% built into the cavity; visually inspect under collimated light for bubbles, inclusions, and weld lines.
- Post-cure in a circulating-air oven at 150-200 C for 1-4 hours to drive residual volatiles and lock in long-term clarity.
- Run final QA: transmittance and haze on a coupon from each batch, plus Shore A on a tab.
| Parameter | Typical Setting | Why It Matters |
|---|---|---|
| Mix ratio A:B | 1:1 by volume | Off-ratio causes tacky or hazy parts |
| Mold finish | SPI A1 mirror polish | Surface roughness is printed into the optics |
| Mold temperature | 160-200 C | Below 160 C risks under-cure and haze |
| Cure time | Few seconds to ~60 s | Set by wall thickness (0.5-5 mm typical) |
| Demold shrinkage | 2-3% | Compensate in cavity machining |
| Post-cure | 150-200 C, 1-4 h, air oven | Removes volatiles, prevents yellowing |
| Regrind | None | Crosslinked LSR cannot be reprocessed |
Two contamination points ruin optical-grade LSR more often than bad material: sulfur, amine, or tin residues (from room air, gloves, or PVC-insulated wires) will poison the platinum catalyst and leave parts tacky or cloudy, and dust or fibers in the molding room will print straight into the lens. A cleanroom-class cell and platinum-only cure (no peroxide) are non-negotiable for visible-light optics.
Clear LSR vs Epoxy vs PC/PMMA for LED Optics
Epoxy and acrylics are not wrong – they just win on different axes. Use this comparison to pick the right family before you start grading equivalents.
| Criterion | Optically Clear LSR | Epoxy (optical-grade) | PC / PMMA |
|---|---|---|---|
| UV / heat yellowing | Excellent – stable under blue pump and heat | Yellows over time, especially under UV/heat | PMMA yellows mildly; PC yellows more under UV |
| Continuous service temperature | -50 to +150-200 C | Up to ~120-130 C (Tg-limited) | PC ~115 C; PMMA ~90 C |
| Flexibility / sealing | Flexible elastomer, seals against housings | Rigid, brittle | Rigid, not a seal |
| Scratch / surface hardness | Lower – needs hard coat for touch surfaces | Higher than LSR | Highest of the three |
| Raw material cost tendency | Higher | Lowest | Low to mid |
| Typical use | LED lenses, light guides, headlamp DRL, sensor windows, encapsulants where visibility matters | Rigid structural optics, low-cost indoor lenses | Cover lenses, diffusers, reflectors |
For high-power LED primary and secondary optics, remote-phosphor lens designs, automotive exterior lighting under heat soak, and any outdoor IP-rated assembly, clear LSR is the safer long-term bet. For low-cost indoor lenses where the LED junction stays cool and the part never sees UV, epoxy or PMMA is still a defensible choice.
Equivalent Grade Guide: Matching ELASTOSIL LR 3003, LR 3162, SILASTIC MS-1002, OP993, TC-5130
Buyers searching for equivalents usually need to replace a grade that is discontinued, on allocation, or simply overpriced for the volume. A defensible equivalent matches Shore A hardness, A/B viscosity, tensile and elongation, transmittance and haze, cure profile, and post-cure schedule. Newsil supplies optical-grade LSR compounds engineered as functional equivalents to the grades below – request a datasheet and trial sample for qualification.
| Reference Grade | Typical Hardness (Shore A) | Typical Use | Key Specs to Match for an Equivalent |
|---|---|---|---|
| Wacker ELASTOSIL LR 3003 series | ~ 40-70 depending on variant | Optical molding, LED lenses, light guides | Platinum addition cure, transmittance > 90% at 2 mm, haze < 2%, matched cure profile and viscosity |
| Wacker ELASTOSIL LR 3162 | ~ 60-70 | Optical-grade molding, automotive lighting | High clarity, UV-stable, post-cure schedule 200 C / 2-4 h |
| Dow SILASTIC MS-1002 | ~ 40-50 | Optically clear moldable silicone for lenses and covers | Low viscosity for thin walls, refractive index ~ 1.41, platinum cure only |
| OP993 | ~ 50-60 | Clear LED encapsulant and lens molding | Water-clear, low yellowing, tensile and elongation matched to datasheet |
| TC-5130 | ~ 30-50 | Soft clear LSR for light guides and gasketing optics | Lower Shore A, high elongation (> 300%), low haze |
When you send an RFQ, include target transmittance and haze, wavelength range (visible or near-UV), refractive index if the part is optically coupled, hardness Shore A, platinum-only cure confirmation, water-clear vs diffused color, shrinkage compensation, surface finish, automotive certifications (IATF 16949 supplier chain), and annual volume with lens geometry. For broader context on the full liquid silicone rubber portfolio, see our liquid silicone rubber product line, and for a deeper dive into how volatile content drives haze and fogging in optical parts, read our low-volatility, oil-bleeding-free LSR technical note.
Conclusion
Optically clear LSR is the most UV- and heat-stable transparent elastomer available for LED and optical parts, and it can be molded in thin, complex geometries without the yellowing that limits epoxy or the temperature ceiling that limits PC/PMMA. If you have an existing commercial reference grade – ELASTOSIL LR 3003, LR 3162, SILASTIC MS-1002, OP993, or TC-5130 – send the datasheet plus your transmittance, haze, and hardness targets to our engineering team and we will line up a Newsil equivalent compound, process window, and post-cure schedule for qualification.
Frequently Asked Questions
What light transmittance and haze can I expect from optically clear LSR at typical LED lens thicknesses?
Post-cured optically clear LSR transmits more than 90-95% of visible light (400-800 nm) at 2 mm thickness, with haze below 1-2% (ASTM D1003). At the blue 450 nm pump wavelength most high-power LEDs use, transmittance typically stays above 92%, which keeps lumen depreciation in check.
What is the standard LSR injection molding process for clear LED lenses and how long does post-cure take?
Parts A and B are mixed 1:1 by volume through a static mixer and injected into a mirror-polished (SPI A1) cold-runner LIM mold held at 160-200 C, with cure cycles from a few seconds up to about one minute depending on wall thickness. Demold shrinkage is around 2-3%, and a post-cure of 1-4 hours at 150-200 C in a circulating-air oven is required to drive off volatiles and lock in optical stability.
How do I match an equivalent to ELASTOSIL LR 3003, LR 3162, SILASTIC MS-1002, OP993, or TC-5130?
Specify Shore A hardness, A/B viscosity, tensile and elongation, transmittance and haze at your wall thickness, cure profile, and post-cure schedule from the reference datasheet, then send it to your supplier for cross-matching. Newsil can supply functional equivalents in this grade family – share your target transmittance, haze, wavelength range, and annual volume with our engineering team to start a trial.
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