In 2016, the Mercedes-Benz E-Class came with amazing
new headlamps, each containing 84 LED chips in three rows to produce low beam, high beam, and high-resolution
New development directions with 4 to 10 kilopixels are steadily building toward megapixel resolution, and this technology is for the foreseeable future state-of-the-art in production ADB resolution.
For the primary optics, this setup results in a very hot environment and a high radiation load. This calls for new materials, since standard thermoplastic resins cannot withstand these high temperatures, while glass optics cannot be produced efficiently with the very fragile and thin optical light guides needed to achieve compact optics and headlamp packaging.
To address this need, Optoflux have developed a method to manufacture primary optics in LSR (Liquid Silicone Rubber), which provides very good transparency at operating temperature up to 150 °C. It’s UV-stable, and can be manufactured with high precision.
The main challenges for this application were to minimise the light loss of each LED—a tough challenge for accuracy of the incoupling area, the transparency of the material, and the surface finish; and to prevent crosstalk between adjacent LEDs, all while attaining and maintaining robust tolerance to high radiation loads. More, to avoid gaps between the beam components provided by different LEDs, the shape of the lightguide has significant undercuts which pose a challenge for demoulding during serial production. So Optoflux chose to pursue development with LSR.
Tests had to be conducted with different materials until a stable moulding process compatible with serial production was achieved. One of the big challenges for optical silicone is the tendency to deposit films on the mold, resulting in optical degradation. So emphasis had to be put on a sophisticated cleaning program using different chemical agents and process steps, and to avoid mechanical cleaning steps of the optical inserts as much as possible. Results were fruitful; today a constant optical performance over the complete production cycle can be provided. Nevertheless, further work on the silicone resin and process is ongoing, to extend cleaning intervals and tooling insert service life.
The quality of the injection tools and inserts have a crucial impact on the performance of the optics. Due to the very low viscosity of the LSR, every gap larger than 4 µm will result in flash (unintended protrusions of material) with a bad effect on the optics. To avoid such gaps, the optical inserts for the light guide cannot be made of multiple parts, but must be eroded and polished in one piece. For the surface finish of nozzles, manual polishing is unavoidable. However, very much experience is required to polish up to 100 nozzles with diameter less than 1 mm and depth of about 20 mm without rounding the edges of the light guides. To evaluate the achieved surface quality, both surface roughness measurements and optical transmission measurements are performed for a final approval of the inserts. Of course, in the tooling and manufacturing processes alike utmost care must be taken in handling the inserts, since repair is very difficult and expensive (if it’s possible at all).
LSR injection molding differs significantly from
conventional molding with thermoplastic resins. The solidification of LSR is a
result of a thermally initiated polymerisation. Immediately before the
injection process, the two components of the LSR are mixed, and the liquid is
then injected in the hot mold, the heat of which activates and accelerates the
After 1-2 minutes of reaction time, depending on the type of LSR, the polymerisation is complete and the part can be ejected. Due to the soft structure, the parts cannot be grasped by a simple gripper, but must be picked across a larger area to remove them reliably from the tool. A camera-controlled 100% check of the ejection is conducted, to avoid the risk of eventually leaving torn silicone fragments in the mould due to defects or improperly-ejected parts.The length of all nozzles is measured, so that missing or torn-off nozzles are detected and the molding process is stopped, to avoid damage on the inserts.
Several camera-based 100% inspection tests are done afterwards, to detect flashes or surface defects.
For a stable and trouble-free production, the control and stability of the material and process parameters is crucial. Deviations result in faster degradation of the inserts, so shorter cleaning cycles are needed, resulting in lower output, higher maintenance efforts.
After acceptance of the part by the inspection system, the next step is a tempering process to remove incompletely-polymerised volatile silicone monomers. This causes a shrinkage of 1–3%, leaving the optical parts at their final size. For the part design it is important to make sure the correct shrink factor is evaluated before manufacturing the optical inserts. The tempering process parameters (duration, temperature) depend on the type of LSR material and are in the order of 4 to 20h at about 140°C – 180°C.
In summary, after several years of development and optimisation, a serial process for manufacturing silicone optical components has been developed and successfully implemented. The optical performance of the produced parts is outstanding, fulfilling all optical and environmental requirements. Optoflux launched serial production in 2014, and since then they’ve produced more than 5 million primary optics for matrix beams. Ongoing tasks will be the development of more cost-effective production methods to increase output and reduce production cost.