Automotive lightweighting applications: The road ahead
By Don Rosato
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Note: This is the fourth and final article of a four-part series covering (1) automotive lightweighting trends, (2) material advances, (3) process technologies and (4) applications.
Lightweighting measures are expected to be applied to every car model launched in the coming years. The average car contains 15 percent of its total weight — or roughly 400 pounds — of plastics, with its use in automotive manufacturing accelerating. As the automotive industry gears up to meet new emissions regulations, lightweight, structural engineering plastics will play a critical role.
The trend to replace metal with plastics is also allowing integration of functionality often not possible in metal versions. Innovative materials combined with new production methods and reinforcing structures will play an important role in reducing vehicle weight. They will be the driver in such applications as lightweight car roofs and fuel-efficient wheel rim technology.
The Mercedes SLK Vario is equipped with an electrohydraulic hardtop that collapses automatically into the trunk to transform the car into a convertible in less than 20 seconds. Mercedes-Benz is offering a choice of three variants of the lightweight-construction Vario-roof for the new SLK namely:
The polycarbonate/acrylonitrile butadiene styrene (PC/ABS) roof module frame exhibits high-energy absorption in crash situations. It is made from Bayer's Bayblend T85 XF. The PC/ABS blend retains break resistance even at temperatures well below freezing. With a temperature of deflection under load of 127 degrees Celsius (0.45 MPa, ISO 75-1, -2), the unreinforced material is able to satisfy the high thermal requirements placed on horizontal bodywork parts. Other strengths include high chemical and stress-cracking resistance, plus 10-15 percent better flowability compared with the standard material. To provide drivers with a panoramic view even with the top up, the modular roof system is optionally sold with a transparent roof.
The polycarbonate (PC) panoramic roof element manufactured by Webasto is produced in a two-component injection-compression molding process. It is made of Bayer's Makrolon AG 2677 polycarbonate. It is 4 kg or 40 percent lighter than a glass equivalent. Other benefits of PC as a glazing material are:
Lightweight wheels not only have a mass-reduction effect that reduces emissions and fuel consumption but also further improves driving performance and efficiency by reducing unsprang weight. Less rotational inertia is required to accelerate and decelerate the wheels. Additionally, less mass in the wheel allows for mass reduction in adjacent, unsprang components such as brakes, steering and suspension. Wheel rim aerodynamics also impacts fuel efficiency. Wheel openings are needed to help cool brakes, but at high speeds they interrupt the flow of air around the vehicle.
The world's first production ready thermoplastic wheel rim was designed for Daimler AG's Smart Forvision concept car. While thermoset rims are being used in racing and for short production runs, compared to thermoplastic rims, which can be injection molded, they are much more complex and more expensive to manufacture, making them unsuitable for high-volume car production.
This all-plastic wheel rim design is made of long glass fiber-reinforced BASF Ultramid Structure and weighs only 6 kg. BASF's Ultramid Structure is a composite plastic reinforced with long fibers that can withstand high loads. The use of these rims, which are more than 30 percent lighter than a standard-production aluminum wheel, reduces total vehicle weight by 12 kg.
The long-fiber reinforcement makes the material especially strong. BASF Ultramid Structure can also be used as an alternative to metal in other applications requiring high energy absorption such as engine mounts, battery holders, seat structures, crash absorbers and other structural components. The rim was designed with the aid of BASF's proprietary Ultrasim software tool, which allowed the position and shape of the rim ribs to be optimized.
Of keen interest also is the new patented eVOLVE hybrid-composite wheel technology from Lacks Wheel Trim Systems, which balances weight reduction and optimized aerodynamics and shows a dramatic highway fuel economy improvement of 1.1 miles per gallon. The technology was developed for the Ford Focus as an initial proof of design. The hybrid-composite wheel builds on an aluminum structure that incorporates high density foam and a PC/ABS top coat to reshape the wheel exterior improving rim aerodynamics. The final wheel is 4.5 pounds lighter than a traditional steel wheel.
Dr. Donald V. "Don" Rosato serves as president of PlastiSource, Inc. a prototype manufacturing, technology development and marketing advisory firm located in Concord, Mass., and is the author of the Vol 1 & 2 "Plastics Technology Handbook".