Inside Manufacturing: A Reinforced Thermoplastic Car Hood?
Low-density GMT-cored sandwich construction and novel inductive mold-heating strategy are a viable option for horizontal body panels.
By Peggy Malnati, Contributing Writer | February 2007
Since the mid-’80s, glass-reinforced, injection-molded thermoplastics have seen limited use in vertical exterior auto body panels — primarily in fenders and doors, notably on General Motors’ Saturn lines. But conventional wisdom has held that reinforced thermoplastics lack sufficient stiffness to prevent creep across large, unsupported horizontal surfaces, such as hoods, deck lids and roofs, in thicknesses practical for the application. Thermoplastics’ high coefficient of thermal expansion (CTE) can make them a fit-and-finish nightmare when affixed to or placed adjacent to metal components. Additionally, few of these materials have the thermal stability to withstand the high temperatures inside bake ovens used in automakers’ paint lines. For these reasons, automakers looking to reduce weight and add design features to conventional steel panels have had to opt either for more costly aluminum or thermoset composites.
Source: GM
A new composite sandwich construction has enabled GE/Azdel engineers to produce the hood as well as the door panels used on the General Motors Volt, a hybrid gas/electric concept car that made its debut at the recent NAIA Show in Detroit, Mich. (See “Related Content,” at left.)
Among thermosets, sheet molding compound (SMC) has been used for both horizontal and vertical body panels since the 1950s. Although its benefits include low CTE, high stiffness, reasonably efficient production via compression molding and online paintability, critics claim that SMC is relatively heavy, requires labor-intensive prep to achieve a Class A finish and has minimal elongation, making it prone to brittle failure during crashes. Carbon fiber-reinforced composites, primarily hand-layed prepregs, have gained ground in high-end sports cars and supercars, but the production methods are too slow and labor intensive for high-volume production.
GE Plastics (Pittsfield, Mass.) and Azdel Inc. (Forest, Va.) — the latter, GE’s 20-year joint venture with PPG Industries (Pittsburgh, Pa.) — have been working to develop a product for horizontal body panels since Azdel’s inception. Their latest collaboration, now several years into development, is called high-performance thermoplastic composite (HPPC). The unique, reinforced thermoplastic sandwich construction offers molders of horizontal body panels not only lower specific gravity at a given modulus (hence, lower part mass) and greater ductility and impact than metals or thermoset composites, but also lower tooling costs and faster cycle times, the latter thanks to an inductive mold-heating technology developed by RocTool (Le Bourget du Lac, France).
Trends Drive Change
Several trends favor adoption of a thermoplastic option by OEMs. For 20 years, average annual production volume per vehicle nameplate has consistently decreased, thanks mainly to tougher global competition and a proliferation of OEMs in developing countries. Consumer demand for new features and OEM need for greater differentiation in a crowded marketplace has shortened nameplate lifecycles (before model facelift or phase out). This shifts the economies of scale in favor of plastics because horizontal body panels are the largest exterior vehicle parts and, hence, have the most expensive tooling. When vehicle builds fall below 50,000 units annually, there simply is not enough time to amortize the cost of the multimillion-dollar tooling required for metal stamping before the next changeover. Moreover, increasing fuel prices have forced automakers to put their vehicles on crash “weight-out” programs, particularly for large parts like hoods. Finally, new regulations in Europe and Japan are calling for designs that, in the event of a car/pedestrian collision, prevent the pedestrian’s legs from “submarining” under the front end of the vehicle and, instead, propel the pedestrian up and onto the hood. Thermoplastics present intriguing energy-management possibilities in front-end and hood impact scenarios.
“The market demand for lightweight, horizontal body panels is significant and it’s growing,” explains Derek Buckmaster, global market director – body panels and glazing at GE Plastics – Automotive. “We’ve been working to develop a suitable product that meets application needs and is commercially viable.”
The GE/Azdel team started with a list of target part properties: modulus similar to steel; CTE and mass similar to aluminum; impact resistance/energy management capabilities greater than either; and a Class A surface that would need no postmold prep and could be painted online or offline. The cost was to be comparable to current solutions (aluminum, SMC) with tooling costs appropriate for low-to-moderate production volumes. According to Buckmaster, “The toughest challenge has been to develop a lightweight material that is cost-effective at automotive production volumes.”
Source: RocTool
A charge in place with the mold in open position within RocTool’s Cage System inductive mold heating module. Note that the inductors are fitted with quick disconnects to facilitate mold opening and closing.
Several common material/process combinations have been evaluated and rejected. Extruded single- and multilayer-sheet products (3-mm/0.12-inch wallstock) — the latter featuring layers of unreinforced and chopped-fiber materials to facilitate thermoforming — and injection-moldable pelletized resins with 2-mm to 12-mm (0.10-inch to 0.47-inch) chopped-glass reinforcement were eliminated because stiffness was too low and CTE was too high. Conventional GMT blanks for compression molding were considered and even prototyped. Although parts were stiff enough, and Class A finish could be had via inmold decoration, they failed to meet mass targets. The team also considered Azdel’s low-density GMT (LD-GMT), trademarked Superlite, with long (13-mm/0.5-inch) glass fiber reinforcement that provided better stiffness and impact strength. Its high glass content moderated CTE and reduced mass, yet the material could be compression molded to form parts with the complexity of a hood. However, Superlite alone lacked sufficient mechanicals for a horizontal panel application.
The GE/Azdel team then conceived the idea of using Superlite as the core of a sandwich construction, with tough, thin, continuous fiber-reinforced skins. While the company’s unidirectional GMTs (mat-reinforced GMTs with additional uni reinforcement along one axis) were ruled out as skin material for weight reasons, the team evaluated unidirectional tapes and wovens combined with engineering thermoplastic resin and formed by solvent, slurry, powder and melt processes. The commingling of glass and thermoplastic fibers and film stacking also were assessed. Unidirectional tape offered the best balance of mechanical/aesthetic properties and cost. Part stiffness improved significantly without adding much mass or thickness, particularly when stacked two-up at 0° and 90° orientations on both the top and the bottom of the sandwich, with skin layers approximately 0.4-mm/0.02-inch thick. That said, Azdel’s Jesse Hipwell, commercial technology leader and an HPPC team member, emphasizes that sandwich construction offers the potential for great versatility. “We expect to be able to tailor the properties of the material by changing the number and types of layers,” he explains. The thickness and number of layers, the reinforcement orientation and type (woven, nonwoven, braid), fiber type (e.g., glass, carbon, aramid) and/or the matrices can be varied to suit the application. The choice of resin, for example, depends on the application. However, skin and core matrices must be from the same resin family to eliminate the need for tie layers — a restriction that facilitates recyclability and good cost control.
