METALS vs. PLASTICS: And the winner is?

DN Staff

October 9, 1995

13 Min Read
METALS vs. PLASTICS: And the winner is?

Steel still makes up 55% of the overall weight of an average car. The steel industry maintains that vehicles made of this durable material also are safer than those fashioned of other materials, a strong selling point. But they are not necessarily lighter, another a key marketing tool in today's weight-conscious, fuel-efficient economy.

Aluminum parts account for only 7% of the car's overall weight. However, that proportion has nearly doubled over the last 15 years. The major reason for the surge centers on the material's weight, not its cost advantage.

And, while both materials battle to increase their visibility in the automotive arena, plastics and composite makers invite the car makers to experience what they say their materials can do more effectively than their metal counterparts--consolidate components, provide greater flexibility in molding complex components, and enable engineers to get designs to market quicker. As a result, plastics' success stories continue to grow in both interior and exterior applications, particularly in under-the-hood and structural components.

Steel fights back. Steel doesn't plan to take a back seat to its competitors, however. It invited automakers to form a partnership eight years ago to tackle a wide variety of issues, such as making steel light, but still strong enough to resist dents. Such measures have had an impact on the automakers.

"Steel has changed more in the last five years than in the last 25 years," says Russ Stroud, a Chrysler purchasing executive. "You can get more strength with lighter kinds of steel. That's how steel mills are countering their competition."

The Auto/Steel Partnership consists of a consortium of sheet steel suppliers, Ford, General Motors, and Chrysler. Some of this effort showed up in the design of the 1994 Ford Windstar. The front-wheel-drive minivan contains three times more high-strength-steel parts than Ford's previous-generation, rear-wheel-drive Aerostar. In fact, about 60% of the Windstar's 165 body-structure (body-in-white) components are made of high-strength-steel stampings.

Ford achieved weight savings using several types of the high-strength material. For the three biggest stampings--the two front-door and sliding side-door outer panels--Ford chose electrogalvanized, bake-hardenable (BH) steel for its dent resistance and good formability characteristics. According to Truman W. Owens, an advanced technology process engineer at Ford, his company explored the use of plastic panels for the door, but chose zinc-coated, high-strength steel "because it is lighter than plastic and provides significant cost savings."

Ford also used BH steel on several unexposed structural body parts. Some examples: door reinforcements, body-side hinge reinforcements, D-pillar reinforcements, and roof headers.

Overall, Owens estimates that Ford saved between 5 and 10% in weight by combining high-strength steel with existing technologies throughout the Windstar's body-in-white. This resulted in fuel economy gains for the minivan.

But the story doesn't end here. The consortium has begun work on designing the lightest possible steel-bodied passenger car to help automakers reduce weight even more. For this project, 31 steel producers from Europe, the Far East, Australia, and North America are participating in the research. Porsche Engineering Services, Inc., a U.S. affiliate of the German automaker Porsche AG, will direct the UltraLight Steel Auto Body (ULSAB) endeavor.

Phase I of the study will provide data on designing the structure through the use of computer-aided engineering, finite-element-analysis models, simulation of crash worthiness, and analysis of cost and manufacturing feasibility. The study follows similar work completed in 1993 by Porsche Engineering on behalf of the American Iron and Steel Institute and Ford. That program showed how new steel technologies and "holistic" design techniques could reduce the body weight of a typical four-door, mid-sized sedan by at least 20%, and at a cost savings. Phase I results should be available early this fall.

"You may see demonstration hardware as a result of the Phase I work in a couple of years," says Ron Hughes, manager of technical affairs at Rouge Steel Co., Dearborn, MI. "It probably won't appear on the road for about five years," adds Hughes, who also chairs the AISI weight-reduction committee. However, Hughes notes, if you remove the door panel on a 1996 Ford Taurus you can take a closer look at the use of ultra-high-strength steel in the door beams.

Aluminum's counterattack. Not to be outdone, aluminum makers have mounted a counterattack. Last year, for example, Volkswagen introduced the aluminum-framed Audi A8, a luxury sedan built in partnership with Alcoa in Germany. Alcoa even spent millions of dollars to construct a plant where the car frames are made.

However, many still have questions about safety when it comes to aluminum versus steel. That might not be a factor in the long haul. "A smart designer can accommodate for differences in aluminum and steel," according to Tom Hollowell of the National Highway Traffic Safety Administration. Although his agency has not conducted crash tests on aluminum cars, Hollowell has followed the issue. The Audi A8, he notes, uses crash tubes designed to collapse and absorb energy.

Audi carried over the aluminum theme in the all-new A4 sports sedan introduced late last year. It employed forged aluminum for the links in the new four-link front suspension of the front-wheel-drive vehicle. Aluminum adds the benefit of light weight to Audi's "virtual steering axis" that nullifies the undesirable effects of driving forces on the front wheels.

And, on the question of safety, evidence continues to mount that aluminum cars can be as safe as steel counterparts. Ford and Audi, for instance, say they are comfortable with crash tests they have conducted on aluminum-frame vehicles. "We tested them in rear and frontal collisions and side intrusions," says William C. Stuef, Ford's manager of vehicle design and production. "The goal was to be equivalent to steel. It was at least that in each case tested."

Aluminum companies also hope to move into other automotive areas once dominated by steel. Take the case of rotors for brake systems. Duralcan USA, a division of Cleveland's Alcan Aluminum Corp., has designed an aluminum-matrix composite for a new brake rotor that the company claims cuts the component's weight in half. The composite, which consists of 20% aluminum oxide or silicon carbide to 80% aluminum, also gives the rotors added wear resistance.

It's anticipated that at least one U.S. car will sport the new rotors in the 1997 model year, according to Floyd Busch, director of automotive marketing for San Diego-based Duralcan.

Magnesium's magnet. Like steel and aluminum, the automotive industry has shown a growing interest in magnesium and its alloys kindled by tighter safety and fuel-efficiency requirements, improved fabrication processes, and economics. Helping to fuel this interest is a magnesium extrusion process that can produce a variety of structural parts, according to Scott O. Shook, industry manager for Dow Magnesium and Fabricated Metals, Midland, MI.

The die-casting process injects the molten metal at a high velocity and pressure into the mold cavity for rapid solidification. Under proper conditions, the cavity fills completely before the metal solidifies. The high pressure imposed in the metal ensures complete cavity filling and excellent reproduction of intricate details of the part being molded.

Using this process, Dow worked with Ford, Lear Seating, and caster Spartan Light Metal Products to develop magnesium alloy seat stanchions for the Windstar minivan. Compared to steel, the magnesium stanchions saved 13 pounds per seat, says Spook. The Ford Cobra Mustang also employs a magnesium alloy bucket seat frame.

Design engineers find the die-cast magnesium alloy attractive for its weight savings, crash energy-absorbing qualities, production efficiencies, and corrosion resistance, Shook adds. Other magnesium components include: valve covers for Dodge, Ford, and Mercedes high-performance cars; Toyota steering wheels and various steering column parts; and transfer cases made by Dana.

Powder-metal maneuvers. Winners of the 1995 powder metal Part-of-the-Year Design Competition also show a diverse mix of new applications for automotive uses. For instance, the Grand Prize in the ferrous category went to Metal Powder Components, Inc., Coldwater, MI, and Eaton Corp.'s Truck Components, Kalamazoo, MI, for a five-level, nickel-steel part that serves as two halves of an inter-axle differential (IAD) case.

The IAD case is used in a heavy-duty, tandem-axle assembly on class 8 trucks. The two-pound halves are arc welded to form the case, which distributes input power between the front and rear axles in the tandem. The case must withstand rotary and thrust forces, while significantly reducing the cost and size of the assembly. It replaced a casting that required extensive machining.

Windfall Products, Inc., St. Marys, PA, walked off with top honors in the stainless steel category for a powder metal spacer and guide assembly. Used in an auto fuel-injection system, the spacer, with its molded-in wrought guide ring, reduced the two-piece assembly price by 25%. The part has a 43 HRB hardness, an elongation of 10%, an ultimate tensile strength of 47,000 psi, and a yield strength of 28,000 psi.

Expect to see more use of powder-metal parts in 1996 models, says Pete Johnson of the Metal Powder Industries Federation, Princeton, NJ. Chief among the growing uses for powder metals: engine bearing caps and connecting rods.

Plastics' progress. Even with these gains on the auto front by metal makers, plastics continue their onslaught onto the automakers' components menu. In a recent "Automotive Plastics Report," Market Search, Inc., Toledo, OH, identified five areas in which plastics suppliers will find strong opportunities over the next five years:

On-engine components: Intake manifold and rocker-arm covers, "are sweeping the industry," the report notes. It also predicts that the plastic intake manifold--providing improved engine performance, lighter weight, and cost reduction--is on its way towards near-complete displacement of metal.

  • Structural applications: Filament-wound springs have lost penetration at GM, but are proliferating to Ford, says Market Search. High-glass-content bumper beams in several thermoset and thermoplastic polymers are gaining penetration where major weight savings are required. Filament-wound drive shafts have found a significant niche. The next major expansion of structural plastics: the radiator support on the new Taurus/Sable models, because "it allows more economical modular assembly and will proliferate to other high-volume vehicles."

  • Fuel tanks: A year ago, emissions rules were tightening, fuel formulations were being made more aggressive, and automotive OEMs were switching future vehicle plans from polyethylene gas tanks to steel. Now, the plastics industry has responded with coextruded lighter weight polyethylene/EVOH tanks that not only meet all emission requirements, but can be molded into shapes that save space.

  • Horizontal body panels: Where volumes are huge, and where low cost is the sole objective, steel will continue to be unassailable for hoods and decks, Market Search speculates. However, where light weight is required to meet fuel-economy/low-emissions objectives, the battle is between SMC (glass-fiber-reinforced polyester Sheet Molding Compound) and aluminum. A Market Search cost analysis indicates that SMC costs less per pound of weight saved than aluminum. And SMC retains this cost advantage even for production volumes above 250,000 a year.

  • Vertical body panels: Although fenders have been taking one step back for every two steps forward on the plastics front, and although two sets of door-gap-tolerances make doors an even more difficult application, plastics' progress is impressive and will continue. According to Market Search, motivating factors in plastics' favor are weight reduction, damage resistance, and lower-cost tooling.

Here are some prime examples of plastics' penetration as represented in 1995 and 1996 model-year cars. In the area of instrument panels, Ford selected DUREL(R) 3 electroluminescent (EL) lamps to backlight the new Integrated Control Panel (ICP) displays in the 1996 Ford Taurus and Mercury Sable. The display features the Ford JBL audio system and interior environmental controls. Unlike other types of instrument panel illumination, EL lamp systems provide soft, uniform light from any viewing angle, and do not generate heat.

The electroluminescent lamps, produced by the Durel Corp., Chandler, AZ, a joint-venture company of Rogers Corp. and 3M, are thin and flexible. They are produced by applying successive layers of proprietary inks and materials onto a polyester substrate.

A transparent, conductive coating, such as sputtered indium tin oxide, is applied to the polyester to act as a front electrode. Various inks that make up the phosphor dielectric and rear electrode layers are applied to the polyester substrate. After construction, the substrate is cut using specialized lasers, optically guided presses, or conventional tools. The process allows the light-emitting phosphors to be selectively applied to provide illumination only where needed. It results in reduced cost and lower power consumption, while allowing holes, cut-outs, and connectors to be located anywhere within the lamp.

In another instrument panel application, Ford chose an acrylonitrile butadiene styrene (ABS) resin for the structural air-conditioning (AC) duct on the 1995 Lincoln Continental. The component, molded from Cycolac(R) GHT4400 resin supplied by GE Plastics, Pittsfield, MA, is said to be the first commercial automotive application of a passive restraint structural instrument panel system.

When it comes to under-hood components, DuPont Automotive, Troy, MI, reports that applications of its Zytel(R) nylon 6.6 range from fuel rails and vapor canisters to carrier gaskets and power-steering reservoirs, some of which debut on the 1996 Ford Taurus and Mercury Sable.

"We're working with industry technology leaders on at least two major innovations: manifolds with integrated air-fuel modules, and nylon heat exchangers," says DuPont's J. Erik Fyrwald, Zytel business manager for the Americas.

Partners in design. A combination of steel and polymer can also be advantageous for automotive applications. Take the case of a new steel-and-polymer collapsible steering-shaft assembly for the 1995 325/330 CK Chevrolet and GMC trucks.

Use of Fortron(R) 1140L7 linear polyphenylene sulfide (PPS) supplied by the Advanced Material Group at Hoechst Celanese Corp., Summit, NJ, won out over such competing engineering polymers as nylon 4/6, polyphthalamide, and thermoplastic polyester for the application. Benefits cited for the PPS included: high strength, consistency in processing, and resistance to high heat, creep, and corrosive solvents.

The collapsible steering shaft provides positive steering control in an assembly that "gives way" in an accident. The GM design encases a central, solid-steel shaft within two hollow steel tubes of different diameters. A polymeric "shoe" made with Fortron holds the steel tubes rigidly in place under normal conditions, but predictably controls the the collapse of the assembly upon sufficient impact.

The assembly collapses in two stages. Initially, and only after a major impact, the polymer pins connecting the shoe with the steel outer tube shears, allowing the assembly's steel tubes to move within each other. This absorbs energy and leaves the steering system functional. Under less force, the inner steel tube continues to slide inside the outer, like a hand-held telescope. In all, the steering column will collapse over a distance of roughly 9 .50 inches.

So the war among the various materials continues, with an armistice encountered from time to time. Even if the battle cools down, engineers will come out the winners by having a greater selection of high-strength, lower-weight, cost-efficient materials on their materials menu.

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