The rocker cover, injection molded of DuPont Minlon mineral-filled nylon 66, includes a series of integrated baffles to separate oil from the positive crankcase ventilation air supply. The rocker cover incorporates the housing for the PCV valve, an emission control device; has a molded-in air make-up nipple; provides heat-staked inserts for mounting the ignition coil; and includes two heat-staked baffle plates.

A thermoplastic rocker for DaimlerChrysler's Neon integrates engine components and reduces weight.

The cover, which weighs 2.8 pounds, is about 30 percent lighter than the part it replaced. The rocker cover has a wall thickness of 2.5 mm.

Key performance challenges for the rocker cover were the ability to meet thermal and mechanical requirements and to resist warpage. "We have to keep the part as flat as possible, to apply uniform forces in assembling the rocker cover to the engine block," said Gian Luigi Molteni, product development manager of DuPont Automotive. The main issue was to avoid leakage, which can jeopardize the function of the engine, he noted.

The part, and the seal between the rocker cover and engine block, had to be designed to resist stress relaxation due to long-term exposure to underhood temperatures and mechanical forces when the cover is bolted down. The mineral reinforcement of the nylon reduced shrinkage and warpage, Molteni said. DuPont used finite element modeling to optimize the cross section of the silicone seal, which is 4.5 mm in diameter and is mounted in a groove along the flange of the rocker cover.

The engine also incorporates an integrated oil pan gasket carrier, made of glass-filled DuPont Zytel nylon 66, which improves sealing efficiency and reduces assembly time by eliminating manufacturing steps, said Molteni.

Although thermoplastic rocker covers are relatively new to vehicles manufactured in North America, they have a track record in Europe, said Molteni. One example that illustrates weight reduction and integration possibilities is a glass-reinforced Zytel nylon 66 rocker cover designed for the six-cylinder diesel engines on the Mercedes-Benz 1998 Atego, Vario, and Unimog light trucks. The injection molded rocker cover and integrated oil separator allowed a one-shot production process that reduced part count and assembly time. According to DuPont, the cover weighs 40 percent less than the aluminum cover it replaced, and is quieter by 2 decibels.

The Toyota XL and XLS models have rocker panels coated with Avery Dennison Avloy paint film.

The formable paint film is made by casting wet paint films, first a clear coat and then a base coat, in a reverse roll coating process. On the top of the base coat, a layer of thermoplastic adhesive is applied. The cured paint films are laminated to a thermoplastic backing sheet with heat and pressure. The dry-paint film laminate is supplied in roll form to the molder and is thermoformed and trimmed into the shape of the finished part. This preform is inserted into the mold cavity before the plastic is injected, causing a melt bond to form that permanently integrates the dry-film laminate and the final molded product.

Avery Dennison said that Avloy paint film has gloss and distinctness of image characteristics comparable to those of conventional spray paints, and has superior adhesion and chip resistance. It has a smooth backing sheet that helps it achieve a "Class A" surface over filled plastics. The finish resists fading and the effects of chemicals.

A promising—and nontoxic—alternative is boric acid, which is also one of the most slippery solids around, according to researchers at Argonne National Laboratory in Argonne, Ill. Tests show that boric acid can provide friction coefficients as low as 0.02-0.05, which is one-fourth to one-sixth the value of other, more expensive solid lubricants, such as boron nitride and graphite. The exceptionally low friction of boric acid can keep aluminum and magnesium alloys from sticking or transferring to die or roll surfaces.

Boric acid is crystallized in layers of tightly bonded atoms, but the layers themselves are weakly bonded to each other. When stressed, the layers move over each other easily, so friction is low.

Surfaces of magnesium and aluminum develop a thin oxide layer. Conventional lubricants do not respond to the oxidized layer, so the surface under cold-forming does not deform uniformly, said Ali Erdemir, the research project's principal investigator. Boric acid has a tendency to react with the oxides on a micro scale, forming a strong bonded layer of the lubricant to the surface, he explained. "When boric acid is applied to a surface, it becomes extremely slippery. Aluminum sheets in a stamping operation can be stretched to their limit, without causing rupture or deformation," Erdemir said.

Boric acid dissolved in water can be sprayed on a surface. After the water evaporates, a thin layer of boric acid film remains, bonded to the oxidized layer. After the forming operation, the boric acid can be washed away with water, and even recycled, said Erdemir. He expects boric acid to remain lubricious in temperatures up to 200 or 250°C.

Today, due to a drop in price and more stable supply, magnesium use is on the upswing, expected to climb to 20,000 metric tons in 2001 and possibly to 45,000 metric tons or more by 2004, he said. Magnesium is attractive because it has only two-thirds the density of aluminum, and can be cast thinner—2 or even 1.7 mm wall thickness is feasible, compared to 3 or 4 mm for aluminum.

A Ford magnesium steering column machined keylock cylinder weighs only 0.75 lb.

One of the biggest magnesium applications at Ford is the instrument panel. The material will save 14 to 15 pounds over the current steel structure it will replace. Sweder said magnesium can be cast as one piece into a fairly complex shape and is rigid for its weight. Gauges can be snapped in easily, and the instrument panel can be cast in a way to help keep the electronics cooler, reducing the cost of wiring, he said. Ford specified magnesium for the panel on the 2001 Ford Explorer, and plans to use it in F-series trucks.

Pedals and pedal brackets are another area for magnesium. About a third of them are magnesium today; the remaining two-thirds are aluminum.

Magnesium is being investigated for car seat stanchions and pans, which are now commonly made of steel or aluminum. Magnesium is also finding its way into certain elements of sunroofs, such as cover plates.

Ford is studying the use of magnesium for the mounting bracket that holds the anti-lock braking system module. Magnesium would weigh about a pound less than aluminum.

Right now, Ford has several magnesium steering column assemblies, and about 70 percent of steering wheels are magnesium. A magnesium alloy developed for the Windstar program is about a pound lighter than an aluminum wheel, and has good energy absorption properties during crashes.

home | features | news update | marketplace | departments | about ME | back issues | ASME | site search

© 2000 by The American Society of Mechanical Engineers