Color-Converting Plastic Cuts LED Costs

The automotive industry historically drives materials innovation, ranging from the composite Corvette in 1953 to last year’s electric Chevy Volt concept car. And while most car makers and their Tier Ones have been slumping economically, the technology pipeline remains red hot.

One of the most spectacular examples comes from Delphi, the former GM parts business that hopes to emerge from bankruptcy protection early this year. Delphi engineers developed color-converting plastics using fluorescing dyes and proprietary light-scattering additives in molded plastic buttons to replace phosphors in custom light-emitting diodes (LEDs). The new technology enhances the visual effect for car passengers, reduces the number of LEDs required by 80 percent and cuts costs for OEMs by 66 percent.

“Many U.S. and Japanese automakers have historically chosen colors that were easily created by subtractive filtering of incandescent lamps,” explains Michael E. Fye, a color scientist at Delphi who co-invented the technology. High failure rates of incandescent backlighting and high cost LED-based approaches spurred an effort for alternatives.

“Our initial approach was to create a plastic that absorbs some of the blue LED energy and fluoresces (or re-emits) some energy at a longer wavelength,” says Fye.  “The combination of the fluorescent component and some of the original blue that is transmitted through the fluorescent creates the desired unsaturated color.”

The main button is made from fluorescent plastic and the body of the button is molded with an opaque plastic in a two-shot molding process using 200-ton Engel rotary platen two-shot machines. The body of the button contains the light. The button is then painted with a white translucent paint followed by an opaque black paint. A neodymium YAG laser (1064nm) removes the black paint in areas where backlight graphics will be displayed.

“During the daytime, the graphic appears white,” says Fye.  “At nighttime the graphic illuminates the backlighting color. A somewhat unique nighttime appearance is achieved. Most illuminated graphics are a subtractive filter in front of a remote light source.  The fluorescent graphic is more of a self-emissive element giving a very crisp, pleasing appearance.”

PCB light pipes

Another key technology development reduced the number of LEDs required. Ray Lippman and Gail Sylvester at the Delphi Lighting Development Center in Flint, MI, created lightpipes that are direct line-of-sight designs. The goal: the light rays would have no more than one or two reflected changes of direction between the LED source and the illuminated graphic area. The Delphi product design team let the scientists create holes in the circuit board to project light through the board into the areas requiring illumination. Key players in taking the invention to a high-volume application were Lee Ols, product leader, and Don Greer, lead product engineer.

There were many materials development partners. RTP  custom compounded the plastics in facilities in Indiana, Minnesota, Texas and China. The key resin is a high-flow polycarbonate from Bayer that provides the widest process window to control the color.  Several years of testing were required to find and develop dyes that could tolerate the molding temperatures and then withstand long exposures to high energy blue light. BASF supplied the dyes. The tooling was developed by Kno-Mar Tool,

The color technology is now used on several other components. “On every new program we weigh the cost, quality, performance benefits of all the backlighting technologies we have in our toolbox and choose the best fit,” comments Fye.    Delphi is in the process of licensing the invention through the Delphi Technologies Group.

The invention was awarded the grand prize late last year by the Society of Plastics Engineers in its annual automotive design competition.

The big drive in automotive is to reduce weight, usually by replacing complex metal assemblies with lighter, simpler injection molded parts.

Another example of innovation is the electronic throttle control (ETC) module in the Chrysler Pacifica crossover vehicle. It’s the first plastic ETC housing and replaces machined cast aluminum at weight savings of 28 percent and cost savings of 18 percent. The new throttle, made from polyester-based bulk molding compound, cuts the potential for ice freeze-up and potential throttle blade stick. A special zero-shrink BMC grade from BMC, Inc. was used. Molding the product for Bosch is German-based Christophery.

Also a winner in the SPE competition, the application shows the resilience of bulk molding compounds, which faced heavy competition from thermoplastics in the 1980s and 1990s. The material’s favorable price relationship to high-end thermoplastics and physical properties is keeping it a strong player in the automotive game.

“Although not exploited until recent years, BMC renders engineering latitude that is unique to many other thermoset and thermoplastic materials,” comments Len Nunnery, director of sales and marketing for BMC, Inc., West Chicago, IL. New advances in BMC include metallization processes that allow a thin coating of stainless steel, copper and brass on molded parts.

Laser welding assist

Another fascinating automotive development—with a strong assembly angle—was on display at K 2007 in Düsseldorf, Germany.

Tooling costs to make plastic bumpers and fascia can be reduced by a factor of five using a thermoformable sheet, which for the first time can now be laser welded. Fittings can be welded that allow attachment to cars. “In the past, it was impossible to fasten components to the thermoformed sheet in a cost-effective way,” comments Thilo Stier, innovation manager for A. Schulman, a plastic compounder based in Akron, OH. The main reason is that the visible surface of the sheet would show sink marks on the cosmetic surface.

The solution is use of a black laser-absorbing layer to concentrate the heat zone on the area that is being welded. A fitting can be welded in two to three seconds, Stier says.

The new technology could put Invasion five-layer extruded thermoformable sheet on a fast track. Schulman introduced its Invision sheet product in 2006 and formed A. Schulman Invision, Inc., a wholly owned subsidiary. Schulman is building a new Invision plant  in Findlay, OH. Sheet can be extruded in thicknesses ranging from 0.4 mm to 12 mm. Properties include good impact strength, UV stability and Class A surface, according to Stier.

The five layers are a protetive film, a clear ionomer layer, a color-matched inonomer layer, an adhesive tie layer and the backing material that provides structural support. The precolored ionomer layer, covered with a clear protective sheet, means that no painting is required. “Because you can eliminate the high capital cost of painting, this means that much smaller companies, such as processors can get in the business of supplying Class A parts to automotive OEMs,” Barry A. Rhodes, chief operating officer of A. Schulman of North America, told Design News in an interview.

R&D outlays at Schulman increased from $4.8 million to $7.8 million in its last three fiscal years, primarily to support development of Invision.

A big assist to the Invision development goes to a German assembly company called LPKF Laser and Electronics, which developed a new hybrid laser welding process. Another application shown at K 2007 was the first fully hybrid welded taillight. The method combines laser light with infrared radiation, and is primarily designed for the cost-effective assembly of large plastic components.

Auto supplier SL Corp. of South Korea had conducted several tests with conventional welding procedures for the welding of their taillights in the last months. It found the hybrid welding was best in terms of appearance and strength of the joint lines at the highest throughput, said Frank Brunnecker, who heads the LPKF Laser Plastics Welding division in Erlangen, Germany.

In an important way, the slumping economic fortunes of much of the global car industry are playing a big role in the rush of innovation. Technology is focused on enhancements that boost value, while also cutting costs.