Putting the Pedal to the Plastic

The use of plastic in cars and trucks has grown over 600 percent since the early ’70s and some followers of the industry have suggested that all viable substitutions of plastic for metal have been tapped out. Such an assumption, as a number of new applications demonstrates, is foolish.

The automotive market will continue to lead the way in growth of the number and types of new metal-to-plastic substitutions. In particular resin suppliers and processors have targeted four especially promising areas for displacement of metal with plastic: body panels, under-the-hood, accessories and truck beds. If significant market penetration continues in any or all of these areas, the rate of plastic growth in automotive could outpace even the impressive gains made over the past two decades.

NYLON ROCKER COVERS MAY ROLL

While the use of nylon as a replacement for die cast aluminum or magnesium in rocker covers has been common in Europe for some time, the application is virtually non-existent in North America. The reasons are mainly historical and economic, says Siegfried Bossecker-Konigs, technical manager, automotive, Rhodia Engineering Plastics. Aluminum has a proven track record in a high-heat application for a part whose key function is to properly seal against the cylinder head. In Europe, where gas prices are much higher, OEMs have been more open to experimenting with ways to reduce vehicle weight.

A nylon rocker cover costs 20 to 30 percent less to manufacture and is up to 50 percent lighter, in comparison to aluminum. These advantages, along with Rhodia’s considerable experience in helping OEMs and suppliers design nylon rocker covers in Europe, are the chief selling points for expanded conversions to thermoplastic in North America, says Bossecker-Konigs.

“There are two main factors involved in a successful conversion to nylon in this application,” Bossecker-Konigs notes. “Choosing the correct material, and getting the right design.”

Rhodia’s standard grade nylon 6/6 for rocker covers is Technyl A 218 MT15 V25, designating a resin blended with 15 percent mineral and 25 percent glass-fibre. Rhodia will customize grades to meet the material requirements of a given application. The glass fibre increases stiffness and reduces creep, while mineral filler provides the material with isotropic shrinkage, a property valued by molders because it shortens the time needed to design and build the mold. Creep and modulus of elasticity are especially critical to the design of a nylon rocker cover because, unlike aluminum, these properties change in the thermoplastic with increasing temperature, says Bossecker-Konigs. Rhodia uses the operating temperature of the engine, with a safety factor added, in software modeling exercises in order to simulate creep and other properties and optimize design.

Rhodia supplies nylon for about 15 rocker covers in production in Europe, and is also involved in about 10 to 15 development projects with European manufacturers. One of the challenges often encountered in converting to nylon is adapting the design of the rocker cover to a cylinder head which had originally been designed to accommodate a metal cover. As redesign of the cylinder head is generally not an option, a team approach involving the resin supplier, gasket supplier and manufacturer is the best way to come up with a viable design, says Bossecker-Konigs.

A rocker cover for the Mercedes Benz Actros truck has only two fixation points to the cylinder head. Because of this, Rhodia supplies the manufacturer with a grade of nylon containing 40 percent glass-fibre, which provided extra stiffness. A rocker cover for the Alfa Romeo Twinspark engine has a bolt-to-bolt distance of over 200 mm between fixation points. Additionally, the rocker cover must support four ignition coils weighing 500 grams each. Engineers designed a double-wall rocker cover with an overlap on the cylinder head. The double wall acts something like a u-beam to add stiffness along the length of the part, while the overlap protects the gasket from dirt which can lead to a leaky seal.

The possibility of part integration is also enhanced with nylon, notes Bossecker-Konigs. A rocker cover manufactured for the Citreon ZX and Xantia models incorporates an air filter.

Another under-the-hood part, a one-piece, self-adjusting bracket for an automotive air conditioning desiccant canister, is made of BASF’s Ultradur B-4300 G2, a glass-fibre reinforced thermoplastic PBT polyester. The one-piece plastic bracket replaces a multiple-piece metal bracket, which required mechanical tightening. The lightweight bracket attaches to a side wall under the hood and firmly holds the aluminum canister, preventing rattling. For this application Ultradur provides good creep resistance, high flexural modulus, and high temperature resistance. Ultradur also has good resistance to chemicals and oils typically found under-the-hood.

AWARD WINNING METAL REPLACEMENT

In what may be the highest volume-per-part use of plastic in a metal-replacement application to come down the pipe in a while, General Motors announced last year that it would use Bayer Corporation’s polyurethane high density, structural reaction injection molding (HD-SRIM) system to mold the cargo box for a full-size pick-up truck. The composite cargo box, available this fall on Chevrolet Silverado trucks, will be molded with the new Baydur 425 internal mold release (IMR) polyurethane HD-SRIM system. Working closely with GM, Bayer screened more than 200 polyurethane formulations before Baydur 425 was selected as the material with the best combination of physical properties, processability and cost effectiveness. GM looked at a number of competing resins, the most viable being a vinyl ester resin, but real world testing showed that polyurethane SRIM was more chip resistant, according to a GM spokesman.

A plastic/metal composite grill opening reinforcement (GOR) appearing on the 2000 Ford Focus won the Chassis Category of the 1999 awards competition conducted by the Automotive Division of the Society of the Plastics Engineers (SPE). The part, made for Ford Motor Co. by Visteon Automotive Systems, uses a patented hybrid technology from Bayer to produce a GOR with a 40 percent savings in weight over a metal part with equivalent performance.

The award-winning GOR consists of two metal stampings of hardened steel with a nominal thickness of 0.5 mm and Bayer’s heat-stabilized Durethan BKV 30H, 30 percent glass-filled polyamide 6 resin with a nominal thickness of 2.5 mm. Metal stampings are placed in the mold and the resin flows into and around them, mechanically locking to the metal and forming a single integrated unit. The composite part has a negligible stability difference from metal in a wide temperature range, therefore allowing it to be assembled on the car prior to e-coating. Part stiffness exceeds that of both plastic and metal competing technologies.

MOLDED-IN COLOR GOING GLOBAL

The molded-in color body panels which appear on the DaimlerChysler Micro Compact Car (MCC), or “Smart Car” recently won the top prize in the SPE Body Exterior category. The MCC’s body panels, molded from GE Plastics’ XENOY PC/PBT XD 1573 resin, were recognized as being the first ever molded-in color, thermoplastic body panels on a production vehicle. The panels, which are molded in red, white, yellow and black, are also the first interchangeable plastic body panels.

According to Venkatakrishnan Umamaheswaran, market development manager with GE, plastic body panels provide a 50 percent weight savings on a per panel basis in comparison to steel. The savings is especially significant considering exterior body and appearance panels can account for 30 to 35 percent of the total weight of the vehicle.

“The rationale for molded-in color technology is to reduce investment cost,” says Umamaheswaran. “An automotive assembly plant costs anywhere from $700 million to $1.2 billion. A paint shop represents about 50 percent of that cost and takes up about 50 percent of the floor space.” He notes molded-in color is especially attr
active at greenfield sites in foreign countries, as the technology offers away to substantially reduce investment costs in these new and uncertain markets.

The molded-in color XENOY resin can be run on traditional injection molding equipment but requires adjustments to the way in which the tool is designed and built, he says. Because the parts are not painted there has to be a Class A surface finish on the part as it comes out of the mold. Knit and flow lines need to be located in areas where they are not visible.

A luggage rack system developed for the Nissan Xterra was cited as a finalist for the award in the SPE Body Exterior category. The luggage rack is molded from GE’s GELOY PC/ASA alloy resin, which won out over both metal and nylon in the design stage. Tom Clinton, director of automotive components and materials, says the rack is the first of its kind on any SUV and is really a combination luggage compartment, roof rack and wind deflector. Clinton says the rack is molded in the same color as the body (gray), which helped the manufacturer realize a 61 percent cost reduction in comparison to painted nylon or metal. He notes that GELOY can be molded on standard injection molding equipment.

SUPERPOWER PLASTICS

A new circular power saw made by Milwaukee Electric Tool Corp. meets high standards for ruggedness and durability, and incorporates 12 components molded from three families of DuPont engineering polymers. Each saw has eight parts injection molded from DuPont Zytel nylon resins, three parts from Minlon mineral-reinforced nylon resin and one from Rynite PET thermoplastic polyester resin.

The combination of performance properties offered by the resins allowed designers to meet the objective of metal replacement. The saw’s motor hub, for example, is molded from Zytel 70G43L, a 43 percent glass-reinforced grade of nylon 6/6, and has an insert molded metal bearing. The hub replaces a machined aluminum hub and a bearing retained by a snap ring. The insert-molded design cuts hub costs by two-thirds, according to Jeff Zeiler, a senior product engineer with Milwaukee.

The saw’s motor housing is molded from a 35 percent glass-fibre reinforced grade of Rynite with an insert molded power circuit that eliminates the need to connect wiring during assembly. Connectors on a terminal board at the rear of the motor’s field plug into terminals in the housing.

“We’ve dropped saws numerous times from 6 to 9 ft. and they remained perfectly functional,” says Zeiler.

EMS-CHEMIE’s Grivory GV is an ultra-stiff glass-reinforced partially aromatic nylon which is suitable for metal alloy replacement, according to the company. With a flexural modulus of three million psi and flexural strength of 55,000 psi, the material is strong enough to be used for aerospace structural applications, but has found an increasing number of uses in markets such as automotive and recreation.

According to Dave Riffer, EMS-CHEMIE marketing manager for automotive, the Grivory resin’s aromatic group adds higher stiffness to the polymer chain. Additionally, Grivory GV has much better water absorption resistance than standard nylon 6 or 6/6 grades, which means mechanical properties such as stiffness and tensile strength are maintained even in high humidity molding environments, Riffer says.

Grivory GV can be supplied in the range of 20 to 60 percent glass-fibre, is UV stable and can be painted or used with molded-in color, according to Riffer. Current applications include exterior automotive mirror brackets, door handles and instrument panel and under-the-hood bracketry. Riffer says Grivory GV lends itself perfectly to bracket type applications because its light weight and high stiffness enable it meet the stringent noise, vibration and harshness (NVH) requirements for those applications. Grivory GV is also used in ski bindings, a flywheel for a small engine and household and hardware tooling.