橡楚编辑部 8/14/2026 47 min read
Light-transmitting liquid silicone rubber (LSR) parts for car lights are core components of automotive lighting systems. With excellent light transmittance, weather resistance, high and low temperature resistance, and yellowing resistance, they have gradually replaced traditional PC and PMMA materials as the preferred light-transmitting components for high-end car lights. This article systematically introduces their performance advantages, key production process points, quality control standards and practical application scenarios in different types of car lights, providing technical references for relevant manufacturers and purchasers.
The global automotive lighting industry is undergoing a transformative shift driven by electrification, autonomous driving, and advanced driver-assistance system (ADAS) integration. Traditional light-transmitting materials such as polycarbonate (PC) and polymethyl methacrylate (PMMA) are increasingly unable to meet the complex performance requirements of next-generation car lights, which demand higher thermal stability, UV resistance, design flexibility, and optical consistency. Light-transmitting liquid silicone rubber (LSR) has emerged as a disruptive solution, with the global market for light-transmitting LSR car light parts projected to grow at a CAGR of 18.2% from 2024 to 2030, reaching $427 million by the end of the forecast period, according to a 2024 report by the Automotive Lighting Research Association.
Unlike thermoplastics, light-transmitting LSR is a two-part, platinum-catalyzed elastomer that cures via cross-linking at elevated temperatures, retaining its elastomeric properties across a wide operating temperature range. Its unique combination of optical clarity, mechanical resilience, and chemical inertness makes it ideal for a wide range of car light applications, including secondary optical lenses, light guides, seal-integrated optical bezels, and ADAS LiDAR sensor windows for smart headlights. This article provides a comprehensive technical analysis of light-transmitting LSR parts for car lights, covering material performance characteristics, precision manufacturing processes, application cases, and quality control frameworks, to offer actionable insights for automotive lighting engineers and supply chain stakeholders.
The suitability of light-transmitting LSR for car light parts stems from its tailored material properties, which address key pain points of conventional thermoplastic optical materials. Below is a comparative analysis of its core performance metrics relative to PC and PMMA, the two most widely used optical materials in existing automotive lighting systems.
Optical performance is the primary evaluation criterion for light-transmitting car light parts, as it directly impacts luminous efficiency, beam pattern compliance, and driver visibility. Standard optical-grade light-transmitting LSR for automotive applications has a visible light transmittance of 92–94% across the 380–780 nm wavelength range, matching or exceeding the 89–91% of PC and 92–93% of PMMA. Its refractive index ranges from 1.41 to 1.53, adjustable via formula modification to match the refractive index of adjacent optical components (such as LED chips or primary PC lenses) to minimize Fresnel reflection losses, which can reduce overall optical system efficiency by 3–5% when mismatched.
A critical advantage of light-transmitting LSR over thermoplastics is its long-term optical stability under harsh automotive operating conditions. Table 1 summarizes the optical degradation performance of three materials after 1000 hours of accelerated aging tests per the ISO 4892-3 standard for automotive exterior components:
As shown in the table, light-transmitting LSR exhibits almost no yellowing or haze increase after UV exposure, as its siloxane backbone has a bond energy of 452 kJ/mol, higher than the 344 kJ/mol energy of UV radiation at 350 nm, preventing molecular chain breakage and chromophore formation. This stability ensures that car light luminous flux remains within 5% of its initial value over the 15-year service life of a vehicle, eliminating the need for frequent headlight alignment or replacement to meet regulatory requirements such as ECE R112 and FMVSS 108.
Additionally, light-transmitting LSR has a very low birefringence of <10 nm/cm, compared to 50–200 nm/cm for injection-molded PC, which eliminates optical distortion in high-precision applications such as matrix headlight pixel lenses and LiDAR sensor windows. This property is particularly critical for ADAS-integrated headlights, where distorted light output can lead to incorrect sensor readings and reduced system reliability.
Car light assemblies operate in extreme temperature conditions, ranging from -40°C in cold climate regions to 150°C near high-power LED light sources, with rapid temperature fluctuations during vehicle startup and shutdown. Light-transmitting LSR has a continuous operating temperature range of -60°C to 200°C, with no brittleness at low temperatures or softening at high temperatures, unlike PC which has a glass transition temperature (Tg) of 140–150°C and PMMA which has a Tg of 105°C. For high-power matrix headlights with LED power densities up to 12 W/cm², LSR optical components can maintain dimensional stability within ±0.02% at operating temperatures up to 180°C, ensuring consistent beam pattern alignment.
The mechanical properties of light-transmitting LSR also address key reliability challenges for car light parts. It has a Shore A hardness of 30–80, adjustable via formula modification, allowing it to be engineered for both rigid optical lenses and flexible seal-integrated optical components. Its elongation at break ranges from 100% to 600%, with a tear strength of 15–35 kN/m, making it highly resistant to impact and vibration during vehicle operation. In contrast, PC has an elongation at break of 10–20% and PMMA is inherently brittle, with an elongation at break of 2–5%, making them prone to cracking under mechanical stress or thermal cycling.
Another key advantage is LSR’s low compression set of <10% after 22 hours at 125°C per ASTM D395 Method B, which allows it to function as both an optical component and a seal. This eliminates the need for separate rubber gaskets in optical assemblies, reducing part count by 30–40% and lowering the risk of water ingress, a leading cause of car light failure which accounts for 22% of all automotive lighting warranty claims according to 2023 J.D. Power data. Light-transmitting LSR is also highly resistant to automotive fluids, including engine oil, transmission fluid, road salt, and car wash detergents, with no swelling or optical degradation after 1000 hours of immersion per ISO 1817 testing.
The transition to smart, ADAS-integrated headlights requires optical components with increasingly complex geometries, including micro-lens arrays, free-form surfaces, and integrated fluidic channels for active thermal management. Light-transmitting LSR’s low viscosity (10,000–1,000,000 cP before curing) allows it to replicate micro-features with high precision during injection molding, with achievable feature sizes as small as 5 μm and surface roughness as low as Ra 0.02 μm, eliminating the need for post-processing polishing required for precision thermoplastic lenses.
Unlike thermoplastics, which have high shrinkage rates (0.5–0.8% for PC, 0.2–0.6% for PMMA) that vary with wall thickness, LSR has a uniform linear shrinkage rate of 0.1–0.3%, allowing for the production of complex, multi-thickness parts with tight dimensional tolerances of ±0.005 mm for features smaller than 10 mm, and ±0.05 mm per 100 mm for larger parts. This is critical for micro-lens arrays used in matrix headlights, which may have over 100 individual lens elements per assembly, each requiring dimensional accuracy to within 0.01 mm to ensure correct beam shaping and glare reduction for oncoming drivers.
Light-transmitting LSR also enables overmolding onto a wide range of substrates, including aluminum heat sinks, PC lamp housings, and copper circuit boards, with a bonding strength of >3 N/mm when using appropriate surface treatments. This allows for the integration of optical, structural, and thermal management functions into a single component, reducing assembly time by up to 60% and lowering overall system weight by 25–35% compared to conventional multi-part thermoplastic assemblies. For example, a typical adaptive driving beam (ADB) headlight assembly using overmolded LSR lenses weighs 1.2 kg, compared to 1.8 kg for an equivalent PC-based assembly, contributing to improved electric vehicle range.
The performance of light-transmitting LSR car light parts is highly dependent on precise control of the manufacturing process, as even minor contaminants or process inconsistencies can lead to optical defects or reduced mechanical reliability. Below is a detailed breakdown of the key manufacturing stages and process control requirements.
The first stage of LSR part manufacturing is the preparation and mixing of the two-part LSR material, which consists of a vinyl-terminated polydimethylsiloxane (PDMS) base (Part A) and a methylhydrosiloxane cross-linking agent with platinum catalyst (Part B). For automotive optical applications, the mixing ratio of Part A to Part B is strictly 1:1 by weight, with a permissible tolerance of ±0.5% to ensure consistent curing and optical properties.
To prevent contamination that can cause haze or discoloration, the entire material handling system must be enclosed in a Class 100,000 cleanroom environment, with all material contact components made of 316L stainless steel or PTFE to avoid leaching of foreign particles. The material is de-aired under vacuum (<10 mbar) before mixing to eliminate air bubbles, which can cause internal scattering and reduce luminous efficiency by up to 10% if present in the final part. Table 2 outlines the key process parameters for material preparation:
For colored or diffused light-transmitting LSR parts (such as rear light guides with controlled light scattering), additives such as diffuse particles (PMMA or silicone microspheres) or color pigments are mixed into the LSR base at this stage, with a dosing accuracy of ±0.01% by weight to ensure consistent optical properties across production batches.
Injection molding is the most widely used manufacturing process for mass production of light-transmitting LSR car light parts, as it offers high repeatability and low per-part cost for volumes above 10,000 units. The molding process uses a cold runner system to keep the LSR material in a liquid state before injection, with the mold cavity heated to 150–190°C to initiate cross-linking.
Key process parameters for optical LSR molding are tightly controlled to avoid defects such as weld lines, air traps, and under-curing. The injection speed ranges from 10 to 50 mm/s, with an injection pressure of 50–150 bar, adjusted based on part complexity and wall thickness. For micro-lens arrays with feature sizes below 50 μm, a high-speed injection system with a response time of <10 ms is used to ensure complete filling of micro-features before curing begins. The curing time varies from 30 to 120 seconds, depending on part thickness (1 mm per 30 seconds of curing time as a baseline), with a mold temperature uniformity of ±1°C across the cavity to ensure consistent cross-linking and prevent residual stress.
Mold design is a critical factor in optical LSR part quality. The mold cavity surface is polished to a mirror finish of Ra 0.01 μm, with core pins and inserts made of hardened S136 stainless steel to prevent wear and ensure long-term surface quality. The mold is vented with channels of 0.005–0.01 mm depth to allow air to escape during injection, preventing air traps that cause visible defects. For overmolded LSR parts, the substrate is pre-heated to 80–100°C before insertion into the mold to improve bonding strength and reduce residual stress at the interface.
Unlike thermoplastic optical parts, which require extensive post-processing such as polishing, coating, and edge trimming, light-transmitting LSR parts require minimal secondary operations due to the high replication accuracy of the molding process. However, some specialized operations are required to meet automotive performance requirements.
The first post-molding step is deflashing, where excess flash from the mold parting line is removed. For optical LSR parts, this is done via laser trimming with a 355 nm UV laser, which has a cutting accuracy of ±0.01 mm and does not leave any rough edges or thermal damage to the optical surface, unlike mechanical trimming which can cause micro-cracks and haze.
For parts requiring enhanced scratch resistance (such as external LiDAR windows), a thin (2–5 μm) hard coating of silica or polysilazane is applied via plasma-enhanced chemical vapor deposition (PECVD), increasing the surface pencil hardness from 2H to 6H, matching the scratch resistance of PC while retaining the LSR’s impact resistance. For parts requiring anti-reflective (AR) properties, a multi-layer AR coating can be applied, reducing surface reflectance from 4% per surface to <0.5% across the visible light range, improving overall system efficiency by 7–8%.
A final post-curing step is performed for all automotive LSR parts, where parts are heated to 200°C for 2–4 hours in a nitrogen atmosphere to complete cross-linking, remove any residual low-molecular-weight siloxanes, and reduce compression set to <10%. This step is critical to prevent fogging of the car light interior, which can occur if volatile siloxanes outgas from the LSR part and condense on the outer lens surface. Post-curing also improves the long-term thermal stability of the LSR, reducing yellowing over the vehicle’s service life.
Light-transmitting LSR is currently used in a wide range of mass-produced car light applications, from entry-level passenger vehicle rear lights to high-end smart matrix headlights and ADAS sensor windows. The following case studies illustrate its practical performance benefits and validation requirements.
Adaptive Driving Beam (ADB) matrix headlights are one of the fastest-growing applications for light-transmitting LSR, as they require high-precision optical components with excellent thermal stability. A leading German luxury automaker’s 2024 electric SUV uses LSR micro-lens arrays for its 128-pixel ADB headlight system, replacing the previous generation’s PC lenses to improve performance and reduce cost.
Each LSR micro-lens array consists of 128 individual 1.2 mm diameter aspherical lenses, molded into a 50 mm x 20 mm component with a total thickness of 3 mm. The LSR material used has a refractive index of 1.51, matched to the LED emitter’s primary lens to minimize reflection losses, resulting in a 12% increase in overall system luminous efficiency compared to the PC-based design, from 82 lm/W to 92 lm/W. During thermal cycling testing from -40°C to 150°C for 1000 cycles, the LSR lens array maintained dimensional stability within ±0.01 mm, with no change in beam pattern alignment, while the previous PC design showed a 0.08 mm dimensional shift that required active temperature compensation in the headlight control unit.
The LSR lens array is overmolded onto an aluminum heat sink, eliminating the need for adhesive bonding and reducing thermal resistance between the lens and heat sink by 40%, allowing the LED emitters to operate at a 15% higher power density without exceeding temperature limits. The overall assembly cost was reduced by 28% compared to the PC-based design, due to reduced part count and lower assembly labor. The system passed all regulatory requirements, including ECE R112 beam pattern compliance and 15-year UV aging resistance, with no measurable yellowing or haze increase after accelerated aging testing.
Rear combination lights, including brake lights, turn signals, and position lights, are another high-volume application for light-transmitting LSR, particularly for electric vehicles where weight reduction and reliability are critical. A leading Chinese EV manufacturer’s 2023 compact EV uses integrated LSR light guide and seal components for its full-width rear light bar, replacing a traditional assembly consisting of PMMA light guides and separate EPDM rubber seals.
The 1200 mm long LSR light guide has a complex cross-section with integrated prismatic features to distribute light uniformly across the entire length of the rear light, with a luminous uniformity of >90% across the output surface, exceeding the 80% requirement for automotive rear lights. The integrated seal flange on the edge of the light guide has a Shore A hardness of 40, providing a watertight seal between the light guide and the PC rear lamp housing, eliminating the need for a separate rubber gasket and reducing part count by 2 parts per vehicle.
The LSR light guide is manufactured via two-component injection molding, with a high-hardness (Shore A 70) LSR for the light guide core and a low-hardness (Shore A 40) LSR for the seal flange, produced in a single molding cycle with no secondary assembly required. The overall assembly weight was reduced by 32% compared to the previous PMMA/EPDM design, from 0.85 kg to 0.58 kg, contributing to a 0.3 km improvement in electric vehicle range. The component passed IP6K9K water and dust ingress testing, with no water intrusion after high-pressure, high-temperature car wash testing, and passed 150