GM unveils Saturn: skin is half thermoplastic.

GM Unveils Saturn: Skin is Half Thermoplastic 

Ending many months of speculation, the Saturn Corp. subsidiary of General Motors Corp., based in Spring Hill, Tenn., allowed a public look at its first three models in late August, revealing their extensive use of injection molded thermoplastic body panels in a plastic/steel "hybrid" skin.

All vertical body panels and bumper fascia on the Saturn models are injection molded thermoplastics. The panels are bolted to the steel space-frame understructure, complementing the steel horizontal body panels, and making Saturn the first true high-volume vehicle platform making extensive use of TP body panels.

While there will be three different Saturn models introduced this year (a two-door sports coupe and the SL1 and SL2 four-door sedans), manufacturing engineers indicate that the facility has capability for producing two additional body styles in coming years.

Materials used for the major body panels include Noryl GTX 910 PPO/nylon alloy form GE Plastic for fenders and quarter panels; Pulse B250 PC/ABS from Dow Chemical for door panels, rocker panels and lower deck-lid panels; HiFax ETA-3095 TPO from Himont Inc. for upper and lower front bumper fascia; and an unnamed proprietary HiFax TPO grade for rear bumper fascia. On the two-door sports coupe and SL2 four-door model, the doors are split into interlocking upper and lower panels, providing two-tone paint flexibility for variations in body styling.

Saturn engineers revealed that there had been much development work aimed at using thermoplastic for the upper, horizontal portion of the deck lid. However, the eventual decision was to stick with steel on the horizontal surface, owing to insufficient stiffness and to read-through from the metal understructure on the thermoplastic panels. The engineers did say development work was continuing in this area, and hazarded a guess that Saturn's horizontal deck-lid panel could go to thermoplastic by the 1995 model year.

While weight savings is acknowledged as an inherent benefit in choosing thermoplastic over steel for the car's vertical panels, Saturn engineers attribute the selection mainly to four key considerations; impact resistance, design flexibility, corrosion resistance and cost savings from parts consolidation. Surface quality and recyclability also were the factors in picking thermoplastic over thermoset systems, they say.

MOLDED IN-HOUSE

Saturn molds all its own body panels. As we reported last month (p.75), the Spring Hill facility employs a total of 34 injection machines from Ube Industries to mold body panels and other vehicle components, plus one Cincinnati Milacron test machine. The body-panel processing area uses twelve 5000-ton presses and twelve 3150-tonners to fabricate thermoplastic doors, fenders, quarter panels and rocker panels. An additional 5000-ton Ube Machine is slated for delivery and installation next August, according to Jerry Gibbs, body systems business unit leader.

Vehicle Interior Systems (VIS), which is separate from the body panel area, operates 10 Ube injection molders (four 3150-tonners and six 2000-tonners). It's believed Saturn may install two more 3150-tonners in the interior systems area within the next 12 months.

The 34 presses are run with GE/Fanuc Series 6 Controllers, which use MAP (Manufacturing Automation Protocol) 3.0 as the communication network standard. The machines have 15-min quick-mold-change capability.

BLOW MOLDED PARTS, TOO

Besides the vertical body panels, there are many other thermoplastic applications designed into Saturn. Among these is a blow molded, unfilled HDPE gas tank made by Solve Automotive Inc., Troy, Mich. Solve uses Allied-Signal Inc.'s 50100 grade of HDPE for the 13.2-gallon tank, the same material used for the fuel tank of the Chevrolet Caprice Classic.

The rear seat ramps, backs and adjoining wings on the two- and four-door models are blow molded from talc-filled HDPE. The seat ramps incorporate molded-in ribbing designed as a safety feature, helping to prevent passengers from sliding forward in the event of a frontal collision.

The SL1 four-door model uses GTX 910 wheel covers. GE's Noryl 1265 is used for outside mirror housings, and GE's high-gloss Geloy 4001 ASA was chosen for the windshield cowl. The fuel filler door is molded from Du Pont's Minlon 12T mineral-filled nylon.

Instrument panels are mainly of polycarbonate, of which the structural carrier-panel assembly and upper cover are molded by Saturn's VIS group panel on 3150-ton presses.

The glove box and cluster hood come from outside vendors, molded from polycarbonate structural foam. A vendor also supplies the RIM urethane foam insulator pad for the instrument panel. Saturn produces the lower panels and knee bolsters from thermoformed vinyl skins over poured urethane foam. Substrates for instrument panels and door interiors are processed on four thermoformers in the VIS area, supplied by Kiefel Systems Inc., Succasunna, N.J., a new unit of Paul Kiefel GmbH of West Germany.

Moldmakers supplying tooling for the major plastic body panels for Saturn include Modern Tools Div. of Libby-Owens-Ford Co., Toledo, Ohio (doors); Paragon Die & Engineering Co., Grand Rapids, Mich. (bumper fascias); Corver Engineering Co., Detroit fenders); and Akromold Inc., Cuyahoga Falls, Ohio (quarter panels and rocker panels). Uddeholm Corp., Rolling Meadows, Ill., supplied the tool steel for body-panel and other molds.

DIMENSIONAL ACCURACY VITAL

In previous thermoplastic body-panel applications, dimensional stability was a key concern, and often a crucial stumbling block (see PT, Sept. '89, p. 84). Saturn engineers say they anticipated some problems with maintaining dimensional stability of thermoplastic panels and therefore designed wider margins between each panel. While these fit margins are slightly wider than on most compact cars, the engineers say it was "a worthwhile tradeoff," given the advantages derived from the polymer panels.

Plastic body panels are bolted to the space frame via a drill-and-pierce system, which registers key fixturing points and verifies dimensional accuracy. Plastic panels join the steel panels at the primer painting stage, but do not receive E-coat or nine-stage immersion phosphate cleaning as do the steel panels. The automated paint system, which uses over 50 robots from GMF Robotics Corp., includes one coat of primer, two coats of water-borne basecoat, and two coats of clearcoat.

Each metric space-frame body structure uses geometric dimensioning to hold key coordination points, maintaining a 1-mm cross-body tolerance. A laser-guided video inspection system from Perception Inc., Farmington Hills, Mich., inspects every space frame, and additional periodic checking is done with manual hard fixtures and coordinate measuring machines (CMMs), in order to ensure dimensional accuracy. Saturn makes use of CMMs from DEA Inc., Livonia, Mich., and LK Tool U.S.A. Inc., Tempe, Ariz., a unit of Cincinnati Milacron Co.

Jerry Gibbs, former plant manager of GM's Pontiac Fiero production, says there is little direct ancestry between that defunct vehicle platform and Saturn. While the two cars do share the space-frame design with hang-on plastic body panels, Gibbs says Saturn's space frame is many generations more advanced than the Fiero. Nearly all of the Fiero's body panels--horizontal and vertical--were thermoset plastics.

PHOTO : All bumper fascia and vertical body panels for Saturn are thermoplastic, with steel used

PHOTO : for horizontals. Bumper fascia is molded from Himont's HiFay TPO.

PHOTO : Plastic panels join steel panels at the primer painting stage, but do not receive E-Coat

PHOTO : or phosphate immersion. An automated painting cell, featuring over 50 GMF robots, applies

PHOTO : one coat of primer, two coats of water-borne basecoat, and two coats of clearcoat.

PHOTO : Engineers say thermoplastics were picked over steel and thermosets for the Saturn's

PHOTO : vertical body panels on the basis of impact/dent resistance, design flexibility, corrosion

PHOTO : resistance and parts consolidation.

PHOTO : The Spring Hill facility has 34 Ube injection molding machines, ranging in size from 2000

PHOTO : to 5000 tons. An additional 5000-tonner is on order, and two more 3150-tonners may be

PHOTO : booked during the next 12 months.