Collimator lenses for LED headlamps

Polycarbonate provides a high-performance and cost-effective alternative to glass and PMMA

Light-emitting diodes (LEDs) are the light source of the future, especially those versions that emit white light. It is predicted that they will provide significant savings – for example, in street lamps, building illumination, advertising boards, automotive lighting technology and liquid-crystal displays – because of their low energy consumption and long service life. Polycarbonate is the ideal material for producing the focusing optics of these LEDs. Bayer MaterialScience AG therefore invested heavily in injection-molding equipment in order to develop this area of application for its Makrolon® polycarbonate. “We can now produce components for high-precision focusing optics from special, custom-made grades of Makrolon®. Moreover, we can measure the optics precisely and correlate simulation of the emitted light’s profile extremely accurately with its geometry,” explains Martin Döbler, expert in plastic optics in the Polycarbonates Business Unit at Bayer MaterialScience.

For example, the company worked together with Light Prescriptions Innovators (LPI), a leading developer, manufacturer and licenser of optical components for non-imaging optics, to produce a collimator lens made from Makrolon®. These kinds of lenses play a key role in LED automotive headlamp designs and are used to gather LED light with minimum losses and direct it onto the road in line with safety and lighting requirements. The “RXI” collimator lens designed by LPI for this application has free-form surfaces with non-symmetrical geometries and focuses the total light on its own, as opposed to traditional lens systems for this area of application, which consist of several components. “The RXI lens is extremely compact and has a highly complex geometry, often with extreme changes in wall thickness that are very close together. So it’s a particular challenge to produce this lens in polycarbonate by injection molding,” says Rainer Protte, Team Leader, Advanced Processing in the Polycarbonates Business Unit at Bayer MaterialScience.

To satisfy the high requirements, a two-cavity mold with a complex system of temperature control is used. This makes it possible to produce lenses using a double-layer injection-molding technique, in which a premolding is created in the first cavity and then overmolded in the second cavity to give the part its final shape. “This technique and the complex system of temperature control enabled us to produce lenses in exceptional surface quality, while also avoiding sink marks – which seriously impair the optical efficiency of the lenses – and minimizing cycle times,” says Protte.

In the field of LED focusing optics, polycarbonate offers several advantages over PMMA, which is also transparent. Thanks to its high heat resistance of up to 130 °C, polycarbonate copes reliably with the maximum operating temperatures of LEDs. It can also be used to produce thinner-walled lenses because it has a higher refractive index than PMMA. Its god toughness means guides, housing parts and fixing elements such as snap-fit hooks can easily be integrated into the lenses. This toughness also makes for robust, break-resistant lenses which do not damage easily in everyday use. Polycarbonates boast a good thermal conductivity, which reduces cooling and cycle times and therefore boosts productivity. This is particularly vital for optics with free-form surfaces because these typically have wall thicknesses of more than ten millimeters. As the square of the wall thickness is employed in the cooling time formula, very long cycle times have to be endured for these types of plastic optics.

Polycarbonate also offers advantages over glass. High-precision injection molding can be used to produce the fine lens details more cost-effectively than is possible with glass. What’s more, polycarbonate lenses are much lighter and do not have to be polished.