Improving optical lenses with new LSR grade, surface modification

Düsseldorf, Germany — Liquid silicone rubber is a key material used in new adaptive driving beam headlights, which automatically shine less light on occupied areas of the road and more light on unoccupied areas.

At the recent Silicone Elastomers World Summit in Düsseldorf, a materials supplier and a molding specialist gave a presentation on how to mold the complex LSR parts needed for the ADB matrix lenses.

Hannes Rieger, head of research and development at Oftering, Austria-based Elmet Elastomere Produktions und Dienstleistungs GmbH, a mold maker and LSR mixing and dosing equipment producer, prepared the presentation, which was given by Francois De Buyl, R&D and technical service and development manager at Midland, Mich.-based Dow Silicones.

LSR matrix lenses with integrated optical lens and light guide functions were first produced by Lippstadt, Germany-based Hella GmbH & Co. KG on the 2016 Mercedes-Benz E-Class car and later on the Porsche Panamera, molded by Nuremberg, Germany-based Optoflux GmbH with 84 light guides in three rows.

Meanwhile, advances in light control have enabled still fairly demanding, if not quite so complex, ADB matrix lens/lightguide parts with less than 84-pixel lightguide systems. These mostly two-row integrated high-beam and low-beam ADB lens/lightguide parts increasingly used as LSR optics have spread to other vehicles, with various examples seen in live demonstrations by injection molding machinery producers at various plastics industry trade fairs.

Such ADB optics now consist of 10-24 lightguides of 6-24 millimeters in length with four or more facets and with 2-15 mm thick out-coupling lens sections. Lightguide draft angles vary between 0.5º and 10º or more, Rieger said, pointing to stress-free demolding of such complex parts being facilitated with integrated side grip elements and use of vacuum end-of-arm tooling (EOAT) for part removal from the mold.

More than 10 years of silicone ADB lens technology has resulted in Dow Silastic MOS lenses used in more than 4 million vehicles globally, in more than 30 models, with more than 10 Tier 1 automotive supplier approvals, via a network of more than 10 molders.

ADB headlights remain most widespread in Europe, but they are used around the world, except in the United States, where the National Highway Traffic Safety Administration has approved them but hurdles are faced due to very strict lighting requirements.

The first ADB matrix lenses were molded in Dow Silastic MS-1002, but improved performance comes from a new generation Dow MS 5002 grade of LSR. This involves a much higher Part A component viscosity and slightly lower Part B viscosity, with mixed viscosity after 48 hours being much lower at 30,000 millipascals per second compared with 65,000 mPa.s for MS-1002.

The new material is more easily and precisely mixed and molded thanks to the lower viscosity, and the post-cured properties are very close in terms of Shore A hardness and tensile strength. Yet elongation at break is significantly higher at 96 percent instead of 80 percent.

Dow says MS-5002 enables enhanced cross-linking through a range of molding temperatures due to platinum catalyst and inhibitor concentration optimization, as well as "careful design and selection of the cross-linking agent," with the structure of the Si-H siloxane cross-linker oligomer controlling the rate of cure as a function of temperature.

Molding a two-row 16-lightguide ADB lens in a Dow in-house mold, developed in collaboration with Fischlham, Austria-based ACH Solution Hefner Molds GmbH, on an Engel eMac injection molding machine, showed by measuring relative light intensity, unacceptable mold fouling on lightguides from the inside surfaces of the mold inserts after 1,800-2,500 shots with MS-1001. But this occurred only after 7,000-10,000 shots with MS-5002, showing potential for four to five times higher productivity.

Trials made on an Engel Victory 330/120 Tech machine showed there was 50 percent cure time reduction caused by shear-induced heating when increasing injection velocity from 5 cubic centimeters per second at 130º C to 110 cm3/s at 180º C. But slightly lower velocity is suggested, as molding at 180º C makes it difficult to produce perfect optical parts.

Attention was paid to fast cure time, as the combined thickness of the lightguides and the integrated out-coupling lens sections are larger than with the first ADB lenses, amounting together to 24.5 mm in the center. "This leads to challenging cycle times for mass production by liquid injection molding," Dow says.

Another aspect of LSR ADB matrix optical lenses and LSR lenses in general was addressed in a presentation of Andreas Schäfert, director business development medical devices at Esslingen, Germany-based molder and mold maker Wilhelm Weber GmbH & Co. KG.

Schäfert spoke about surface modification of injection molded LSR optics. He started by describing an LSR disadvantage in terms of sticky surfaces that tend to collect dust and other contamination that is hard to remove, as well as "a sticky uncomfortable feeling on the skin."

Weber is well acquainted with ADB matrix lens issues, as it has been producing LSR matrix lenses for Hella headlights on the Audi A8 car with 32 integrated light guides in two rows.

Weber has worked together with the Bremen, Germany-based PLATO department for plasma technology and surfaces at Fraunhofer IFAM institute for production technology and advanced materials on the use of vacuum-ultraviolet (VUV) radiation in wavelength of 100-200 nm to modify LSR surfaces to make them so smooth and nonsticky that they no longer attract contamination.

VUV radiation breaks some CH3 carbon bonds of the top molecule layers, resulting in O and O3 radical formation between the VUV source and the LSR, which bond with other parts of the silicone molecule to form, depending on VUV dosage, a 2-50 μm thick SiO2 glasslike molecular structure. The coating is sufficiently thin to not cause any significant change in light beam paths.

The patented OpSiLIGHT or SilMoLight modification process works with an incoherent 172 nm Xenon-Excimer source faster than with an 185 nm low-pressure mercury lamp, but the latter gives a better result due to thicker modified layers. Schäfert talked about VUV system treatment times of between 20 seconds and 5-10 minutes.

Schäfert also showed an LED circuit board with 98 LEDs that had been overmolded with LSR lenses. A test with half the circuit board untreated and the other half VUV-treated showed how 0.3 mm long nylon fibers could, after shaking, be easily blown off the treated half by compressed air, while the nylon dust remained on the untreated half. Similar tests have also been made with dust applied to LSR matrix lenses, with microscope images clearly showing all dust removed by blowing VUV-treated lenses but retained on untreated lenses.