Getting a clear focus with plastic materials

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AACHEN, GERMANY (Sept. 5, 10 a.m. ET) — Plastics have been used in car headlamps for decades, replacing other materials as design and technical demands have increased. Matthias Kalwa, process technology manager at headlamp producer Hella, presented headlamp developments in an IKV plastics processing institute colloquium workshop in Aachen, Germany, in March.

The 1984 Volkswagen Golf II had an aluminized and lacquered steel reflector and a glass cover, but the reflector of the 1993 Golf III was made of an aluminized and lacquered bulk molding compound (BMC).

From 1990, headlamp covers changed from glass to simple coated clear polycarbonate or two-colour polycarbonate versions. These were more expensive but lower in weight, providing greater design freedom.

The first low profile beam projection headlamps were made with a glass lens, but newer designs have a thick plastic lens that has to withstand temperatures above 160°C. Multiple LED versions can require 9-10 lenses.

Kalwa said compact low profile beam headlamps have short focal length lenses, and this creates an injection molding challenge for the thick lenses with their precision micro-optic surface features. But he said there is also further potential to reduce overall system costs by integrating more plastic parts.

Also in March, at the VDI plastics in automotive engineering conference in Mannheim, Germany, Volkswagen technical development director Ulrich Hackenberg talked of 40 percent weight saving and greater design freedom with micro-structured plastic lenses compared with glass ones. However, there can be cost disadvantages, he said.

Hackenberg said reducing the Touran's rear foglamp housing internal height from 60mm in 2003 to 20mm in 2011 resulted in the internal temperature rising from 103°C to 185°C. The extra heat was conducted away with a metal sheet. VW wants to eliminate the metal sheet and its extra weight by using either higher temperature or heat conductive plastics.

In Aachen, IKV researcher Julian Schild described in-mold plasma coating (IMPC), involving a backmolded transfer film with a 3-5µm plasma polymeric layer to provide scratch resistance. This eliminates secondary processes to apply a scratch resistant coating.

The polymeric layer adhesive strength is low so the film can be removed. High surface energy is used at the interface with the plastic melt to ensure the plasma polymeric layer adheres firmly to the molding. Schild said silica-like plasma systems contain no organic substances, so have better scratch performance than hard coat lacquer.

A PP transfer film made in a hydrogen (60 percent) and oxygen (40 percent) plasma process gave full coating transfer and satisfactory bond stability after 48 hours. Higher hydrogen levels resulted in some coating cracks or even crumbling.

Care is needed to prevent damage to the film by melt shear forces, so future work will look at molding process optimization with injection-compression molding and the use of other transfer films.

IKV trials have shown centering errors of 50µm in a two-cavity mold and above 100µm in a single cavity mold when molding biconvex lenses, but a lower error of 10-15µm is required. The lenses were molded in Evonik's Plexiglas 7N grade of PMMA on an Engel e-Motion 440/150 tie-bar-less machine.

Paul Walach described a method to reduce centering errors by injection-compression molding the lenses with an adjustment unit integrated in the fixed mold half. The unit is fitted with temperature controlled thermal expansion bolts, reducing centering errors to around 11µm by allowing a lateral shift of the optical insert.

IKV did this work as a plastic lens molding partner in the SFB/TR4 trans-regional research project.

At the VDI conference, Stephan Berlitz, manager of innovation and development for lighting/electronics at Audi in Ingolstadt, Germany, said Audi is involved with Hella, Evonik and Darmstadt University in a three-year LiSiLED project which will end in August 2013. The project is aimed at "realisation of highly integrated optics for LED headlamps" that should save 3kg weight per car by using plastics to replace glass optics.

Stefan Majocco, responsible for plastic technologies at Porsche in Weissach, Germany, listed the plastics used in the Porsche 911 Carrera's main headlamp: high temperature PC, PEI, 30 percent glass fiber reinforced PBT, 40 percent talc filled PP (housing) and transparent PC with hard coating and interior black edge for the headlamp cover.

Close contour cooling is used for the injection-compression molded headlamp cover, due to material accumulation in the complex cover. Water-cooled copper pins provide effective cooling and avoid sink marks at the narrow, sharp angled extremity.

The use of highly transparent liquid silicone rubber (LSR) for automotive lighting was presented in a paper at the VDI conference by Heiko Bayerl, marketing manager for elastomers at Momentive Performance Materials. The paper was prepared together with Oliver Franssen, LSR global marketing director.

Bayerl focused on highly transparent Silopren LSR 7070, a relatively hard (Shore A 70) and viscous LSR (A/B components 250/150 Pas). The material has 95 percent light transmission at 2mm, a refractive index of 1.41 at 589nm wavelength and no birefringence, as there is no molded-in stress.

Bayerl showed results of heat ageing tests at 150¡C up to 6,480 hours for the LSR 7070 material, three other LSR grades, COP (cyclo olefin polymer), PC and PMMI (polymethacrylmethylimide). The 95 percent light transmission was maintained for LSR, but was below 50 percent for PC, 55 percent for COP, and around 75-85 percent for two PMMI grades. The LSR samples were the only ones that did not yellow. PC performed less well than LSR 7070 in Xenon testing, in terms of light transmission, haze and yellowing.

Kunststoff Institut Lüdenscheid (KIMW) has worked in LSR lens development, such as a thick lens and reflector molding in LSR 7070 with wall thicknesses of 1.7-9mm. KIMW tests showed hardly any brightness change (grey scale) after heat ageing over 700h at 85¡C at 85 percent humidity.

Injection molding machinery producer Arburg, a leader in LSR processing, included a Fresnel lens molding application at its technology day's event earlier this year. A lower viscosity Momentive grade, Silopren LSR 7080-40K (A/B 150/50 PaS), was used to mold Fresnel lenses with 5.5g part weight in a single cavity Emde mold on an Allrounder 270S machine. The LSR was mixed and dosed by Reinhardt-Technik equipment.

Bayerl says low viscosity enabled precise and consistent replication of the Fresnel micro-structured surface. The lenses were designed by the Fraunhofer IPT production technology institute's 2008 spin-off company Innolite, based in Aachen, Germany.

Innolite's mold making specialist Rainer Klar, speaking at the 3rd Aachen Precision Days Conference in February, referred to Fresnel lenses used in large concentrator photovoltaic (CPV) arrays to concentrate sunrays onto solar cells.

Such "monolithic parquets", which comprise several hundred Fresnel lenses, have an area of 1,800mm x 1,500mm. Concentrator Optics, based in Coelbe, Germany, makes these by hot embossing Evonik's Plexiglas Solar 0Z023 PMMA sheet in a roll-to-roll process using a vacuum press.

In April, Evonik launched Plexiglas Solar Pre-Fabs, an off-the-shelf acrylic lens package, at the CPV-8 Conference in Toledo, Spain. They have up to 300 lenses formed with replication technology from 10x Technology in the US, and use a Fresnel-K"hler optical design from Light Prescriptions Innovators (LPI) in the US.

LPI says its Ventana "power train" system's primary optical element process "yields very sharp tooth and notch radii of <5µm". LPI started supplying Ventana prototyping kits in June.

The CPV Consortium 2010 report predicted 275MW of CPV capacity will be installed in 2012 and 1,500MW in 2015. Concentrator Optics says the solar Fresnel lens market should be worth €500m by 2015.

But Innolite's Rainer Klar said there is a market bottleneck, with more than 70 CPV module assemblers, more than 17 triple-junction cell producers, but just four Fresnel lens parquet specialists.

Innolite also works on micro-structured polymer lenses for LED headlamps, evaluating resulting light beam shapes (gradients). Klar says free-form optics with microstructures are the most challenging designs. The company has also produced molds and supplied PMMA lenses for the OptiLight project, involving street lighting presently on trial in Aachen streets.

At Arburg's event, a polycarbonate version of the same 9.5mm thick, 4.5g Innolite OptiLight project street lamp LED lenses were injection-compression molded in a two-cavity mold on an Allrounder 370A 600-170 Alldrive machine fitted with an Arburg Multilift robot.