Reinforced thermoplastic improves Mercedes suspension component

Owners of new Mercedes E 500 Class luxury cars will ride more comfortably on Vibracoustic "Airmatic DC" pneumatic suspension, which now features a composite subassembly as a replacement for the central rear axle's key component, a pneumatic piston. A metal version of the piston was designed by Vibracoustic Hamburg GmbH to replaced traditional coil springs. Connected to a mini-compressor onboard the automobile, the Vibracoustic system automatically adjusts air pressure within the piston cylinder (pictured at far left) to maintain the same ride and handling characteristics, regardless of the passenger/cargo load.

The metal parts, however, were expensive and contributed excessively to vehicle weight. The composite replacements, developed by Quadrant Creative Moulding & Systems (formerly ERTA CMS, Tielt, Belgium) in close cooperation with Vibracoustic, had to form a reliable connection between the piston components. Quadrant CMS designed a piston in two parts, a main air cylinder with compressor hose connector and a cap, initially testing prototypes molded from PBTP, a grade of thermoplastic polyester. Accelerated aging tests, however, showed that PBTP was vulnerable to hydrolysis (deterioration due to water absorption). For the production part, Quadrant used a 30 percent glass-reinforced black nylon compounded by CP Polymertechnik GmbH (Ritterhude, Germany, select 238), which not only delivered the desired stiffness and toughness, but better resisted hydrolysis, as well. In addition, the assembled nylon piston performed better than the PBTP version in acoustical vibration damping, also a system design goal. The parts are injection molded in hardened, chrome-plated steel tools, at pressures ranging from 400 to 500 kg/cm2 (5,690 to 7110 psi). Part wall thickness averages just 3.5 mil (0.0035 inches or 0.889 mm), but is stiffened with extra ribs. The cap (lower right in photo) and cylinder are threaded and form an air-tight seal by means of a 0-ring seated in the joint. The nylon assembly passed leak testing with helium, followed by testing of the entire compression system at maximum loads, says Quadrant's Ivan Meersman.

Though primarily motivated by price, the shift to glass-reinforced nylon enabled consolidation of the previous multi-part metal piston assembly. "It gave us greater design freedom and reduced weight in addition to its clear cost advantage," says Meersman.