Saturn bumper beam integrates technology

This January, at the North American International Automotive Show in Detroit, Saturn unveiled its mid-1997 Coupe. In addition to styling and underhood innovations, the new vehicle contains an injection molded solitary rear bumper beam that replaces a previous 19-piece aluminum and expanded PP foam bumper system.

While consolidating parts is a major step forward, the real news centers around the beam's 5-mph impact performance, according to Saturn's Phil Minaudo. As a member of the design team that also involved material supplier GE Plastics and molder Nascote Industries, Minaudo believes this is the only injection molded rear bumper in North America that both exceeds the required FMVSS barrier impact standard and meets a 5-mph pole impact test. "Our division's philosophy is to continually aim at reducing repair costs, and a high-performance single beam helps us achieve that goal," he says.

Goal-Oriented Design

Both technology and team engineering are key elements of the development process for this breakthrough molded application. The project began when Saturn's design team set goals for a rear bumper redesign. "We wanted lower mass, low-temperature impact, fewer parts, easier assembly, and recyclability," Minaudo recalls.During the evaluation process, GE Plastics approached the Saturn team with a proposal for an injection molded beam that could be fitted with a flexible TPO fascia. Saturn engineers liked the idea, and began design work for a shockless beam with integrated fascia support and molded-in towers that crush on impact to absorb energy. (Saturn applied for a patent on this energy management system.)

Team members selected a PC/PBT alloy (Xenoy 1102) that GE developed for the application. Once the initial design was finalized, IGES files were transferred to analysts at GE for structural analysis. "We built a thin-shell finite-element model," recounts GE's Janet Rawson, bumper engineering program leader, "then performed nonlinear FEA for both the pole impact and barrier load cases. The analyses revealed high deflections during pole impact and high load transfer during barrier impact, so ribs were added or moved and wall thicknesses modified to minimize both events."

After Saturn incorporated these changes, a final CAD model was analyzed again at GE using both structural analysis and mold filling simulation. This final step helped designers select gating that optimized flow patterns, balanced the last areas to fill, and moved knit lines away from high-stress areas.

Prototype Testing

Until this point, not a single part had been tested. "We wanted to ensure that the design was completely optimized before any tooling was built to cut down on trial and error," Rawson notes. The team then built prototype tools and molded sample parts for testing. Mark Abraham at Gilbar Test & Engineering performed three different forms of dynamic impact testing in which the sample bumpers were mounted to a rigid, reinforced test cart. All tests, including center pendulum, flat barrier, and pole, took place at an impact speed of 5 mph.

How well did actual physical testing correlate with results from the many computer-aided analyses? Rawson found that a fully constrained boundary condition during analysis gave the most accurate reading. Using this condition, predicted peak loads and total energy for all three types of impacts closely reflected actual results. However, she reports that planned additional research should improve the predictive capability by developing a better failure model at high strain rates.

Intelligent Production

When all of the results were in, production tooling was cut. The mold included a valve-gated manifold. This allowed Nascote to vary timing and sequence of individual gates. Mold design also included porous metal inserts for the molded-in towers to eliminate any gas traps.

Nascote began production molding of the bumper beams in May of last year, using a 3175-ton press and a 6.6-kg shot. Before production began, however, manufacturing engineers evaluated more than 30 processing scenarios using Design of Experiments. Each setup varied mold temperature, melt temperature, injection speed, and other parameters, as well as gate sequencing.

To test bumpers resulting from each scenario, Nascote used an existing hydraulic impact tester that recorded load, energy, and displacement over time. Samples were molded, chilled at Ð20C, then mounted and impacted. (According to Minaudo, Nascote was the only molder with this capability, a key element in winning the business.) Tested bumpers that offered the best performance were sent to Gilbar for additional testing. In this manner, Nascote identified the ideal processing setup. During production, technicians still test one bumper per shift for impact performance.

Design Advantages

Aside from turning a hybrid aluminum/expanded PP assembly into a one-piece injection molded part, the new bumper design achieves several other goals of the Saturn team. Upper and lower fascia supports are integrated, for one. Mass was reduced by 1.5 kg. Once the TPO fascia is removed, the beam is completely recyclable with no extra steps necessary. Assembly time and labor were reduced as well.

What about the bottom line? Saturn invested less for the single injection molding tool, because the part provides a total energy management system. Even with another all-plastic bumper system, it would have had to invest in tooling for both a beam and a separate energy absorber.