Hella uses LSR for first time in serial production headlamps

Liquid silicone rubber is a new player in automotive headlamps. The story of how it got there really started a couple of years ago.

Back to a presentation at the VDI Association of German Engineers' Plastics in Automotive Engineering Congress in Mannheim, Germany, in March 2015.

At that meeting, Tilman Maucher, lighting development project manager at automotive lighting producer Hella KgaA Hueck & Co. in Lippstadt, Germany, showed how polymethylmethacrylate (PMMA) and polycarbonate (PC) were being used as amorphous thermoplastic materials for injection molded headlamp lenses.

Replacing glass with thermoplastics resulted in less weight, greater functional integration and lower cost.

While heat within headlamps had been rising as designers moved toward lower-profile headlamps, introduction of high-power LED light sources meant that there was less heat going forward toward the front of the headlamp compared to more traditional halogen or xenon HID lamps.

Less heat going forward meant more opportunities for thermoplastic lenses. Maucher said the first plastic lens approval was in 1993, on a path leading to Hella's first full-LED headlamp, with a polycarbonate secondary lens used in series production on the Cadillac Escalde Platinum in 2008. A plastic secondary lens was used less than two years later on the Audi A8.

But there were headaches with thermoplastics, as Maucher said in his 2015 VDI presentation.

"If the properties of PMMA and PC which are relevant to application in the headlamp are compared, it becomes evident that neither material emerges as a favorite. Although both materials have clear strengths, they do also have weaknesses which must be accepted or compensated for."

A window of opportunity was about to open for LSR, as Maucher added: "It remains to be seen whether in future the transparent and injection-moldable liquid silicone advertised by various manufacturers will find a place in selected optical applications in the headlamp."

By the time of the 2017 International Injection Molding Conference in Aachen, Germany, LSR was in use in headlamps on the 2016 Mercedes-Benz E-Class car.

LSR is used for the primary optical lens and light guide system that is front-mounted in the high-heat environment in close proximity to the 84 LED semiconductors made by Osram Opto Semiconductors GmbH in Regensburg, Germany. Mercedes said only 24 LEDs had been used previously on the CLS car.

Maucher said at IIMC 2017 that the E-Class application in 2016 is "the first time a transparent silicone has been used in a Hella serial production headlamp."

The LEDs are mounted in three rows on a circuit board, which is integrated with the control unit in a heat sink. LSR is injection molded to form the complex single-part primary lens, consisting of 84 thin light guides in three rows feeding light from encapsulated LEDs to the integrated primary lens part of the single-piece molding. The light then travels to the less heat-exposed secondary optics lens in PMMA and PC.

Aside from higher weight, glass was completely out of the question for such a complex primary optics lens with integrated light guides. Thermoplastics would have difficulty in demolding the part with its strong undercuts due to light guides having different angles, while this is no problem for LSR, due to its high flexibility.

LSR World has separately established that mold maker Leonhardt e.K. in Hochdorf, Germany, produced the mold insert for the E-Class Multibeam headlamp's LSR combined lens and light guides part.

Leonhardt talks of each filigree detail of the insert needing to be precisely formed and the insert polished in an erosion and manual polishing process to a mirror finish with surface roughness (Ra) down to 0.05 µm or below.

Erosion polishing gets the level down to 0.07 µm, with subsequent manual polishing reaching the 0.05 µm level. This high level is also achieved by Leonhardt in some medical optical lens and reflector applications and is much more precise than the company's 0.5 µm standard roughness level.

Maucher said thermoplastics were excluded for the E-Class Multibeam primary optics on account of lower heat and UV resistance than LSR. Maucher also spoke favorably of LSR on account of lower yellowness, and transparency being higher than these thermoplastics and apparently even higher than with glass. Despite 145° C Vicat softening temperature, Maucher says PC is limited in practice to around 120° C, beyond which it yellows significantly, unlike LSR with its 150° C continued use temperature.

Maucher observed that while LSR transparency is close to that of PMMA, refractive index at 1.41 is lower than for PMMA (1.5), PC (1.6) and glass (1.5), with allowance needed for values varying with temperature in polymer lenses. With a low Abbé number of 30, PC has notably poorer birefringence, leading to much more color breakout in light beams than PMMA (59), glass (57) and LSR (50), Maucher revealed.

He described transparent LSR as being "quite new," since it has only been available for "a few years." Maucher referred to four suppliers of transparent optical-grade LSR: Momentive Performance Materials GmbH, Dow Corning Corporation, Shin-Etsu Chemical Co. and Wacker Chemie AG.

Dow Corning supplies Shore A 72 hardness MS-1002 Moldable Silicone to Hella for the E-Class integrated lens and light guide molding, as a material "with the required combination of hardness and elongation."

Examples of the LED lenses shown by Maucher at IIMC 2017 were mounted in the frame used to hold and fix them in the car, and this could be seen as injection molded in another high-heat material, namely polyphenylene sulfide (PPS), a logical decision on account of the frame's exposure to high heat in close proximity to the LEDs.

Hella has been working with IKV, Momentive Performance Materials and Austrian mold maker Elastomere Produktions- und Dienstleistungs-GmbH in Oftering, Austria.

The project, supported by the DFG German Research Foundation, aims at replacing epoxide or polyurethane resin encapsulation primary optics, on conventional wire-bonded or surface mounted device (SMD) LEDs with LSR. This involves an integrated solution in which the LED should be overmolded with LSR, while mounted on its circuit board, to provide both encapsulated primary and front-mounted primary lens functions.

Maucher said the integrated solution reduces the amount of light lost around the periphery of the light beam according to the "Lambert cosine radiation law" and increases light capture "to increase the efficiency of the overall optical system."

Another development list item is metallizing LSR lens surfaces to see which effect it could have on light beams, Maucher added.

Maucher said that "after first headlamps have been equipped with optical elements made of [LSR], others will follow."

The Multibeam LED headlamp won a 2016 Red Dot Product Design award for Daimler. And upon receiving a Daimler supplier award in March 2017 for the headlamp, Hella stated that aside from production in Lippstadt, it also produces it in "other versions" in Slovakia and at a joint venture in China.

LSR can benefit along with a continuing shift to LED headlamp systems. Hella's marketing data expects LED share expected to increase in Europe from 4 percent in 2013/14 to 19 percent in 2018/19.

Usage in the NAFTA region will move from 1 percent to 11 percent over the period, and China from less than 1 percent to 4 percent.

German leadership in new LED technology that has driven LED penetration in Europe faster than expected, Maucher said. He added, "LED technology will replace HID in the medium to long term."