Hendrick Motorsports recently became the first NASCAR organization to form its own steel body parts with the help of an advanced, castable polyurethane specially designed for rapid production of metal-forming dies. Long recognized for its award-winning NASCAR race teams, the Charlotte, NC firm is also well-known as a technological leader in the design and production of the race cars used by its teams.

Supplied by the RenShape® Solutions Group of Huntsman Advanced Materials, Auburn Hills, MI, RenCast™ 6470 polyurethane supports fast production of complete die sets for forming as many as 300 replacement parts. Car sections produced on the polyurethane dies range from relatively small A posts and fenders to large C posts comprising the main support and full side of the race vehicle.

The same RenCast polyurethane is used by European auto manufacturers, including Mercedes and Volvo, to form body panels. The filled RenCast 6470 polyurethane system is easy to handle and pour and, once cured, exhibits a Shore hardness of 91D, ultimate compressive strength of 13,900 psi, and ultimate tensile strength of 4,700 psi.

"Before producing the polyurethane dies and then making our own parts, we hand-formed each replacement panel for our cars," says Ralph Brawley, manager in Hendrick's Metal Shaping Dept. "The ability to quickly cast durable short-run metalforming dies with the Huntsman polyurethane has been a great asset in saving time and streamlining the manufacture of multiple car panels.

"In our business, we must remain flexible," he continues. "Body panel designs change continually and we have to be ready to change with them. If kirksite dies were used, we'd have to melt down the tooling and start over with every alteration. Polyurethanes allow us to simply mill out the die section being modified, recast the surfaces, and we're back in production."

Background

Hendrick Motorsports was founded in 1984 when the company fielded a single entry in the prestigious NASCAR Winston Cup Series and earned three poles and three victories that first season. Today, the organization is a multimillion dollar enterprise headquartered at an enormous 62-acre complex in Charlotte. At the facility, Hendrick technicians use state-of-the-art stereolithography modeling machines, CNC machining equipment, digital coordinate measuring machines, and compression molding presses for in-house design and manufacture of engines, chassis, and most recently, body panels for its Monte Carlo race cars. During 2004, Hendrick is sponsoring five full-time NASCAR teams that will take part in the Nextel and Busch Series races.

Die Building

Hendrick technicians begin by building a master pattern, using an original part that has been modified with body putty to produce the desired new dimensions. Surfaces are sanded and primed, yielding super-smooth patterns for forming high-quality steel panels without marks or rough areas. Next, technicians place the completed pattern in a wooden box, add wood or metal runoff panels for the subsequent casting of the draw ring, seal edges with silicone, and release surfaces with paste wax. Polyurethane is then mixed, degassed, and dispensed from special handling equipment that was custom built at the Metal Shop.

"We typically pour between 300 and 1,000 lb of material," Brawley reports. "Because the polyurethane is highly filled with RenShape Solutions' DT 082 sand-like material, exotherm and shrinkage haven't become problems even when pouring as much as 1,000 lb of polyurethane at once. However, we have noted some erosion on large die surfaces and we believe that the heat created on large pours may have caused sheet wax to sag, producing the surface defect. In the future, we may limit the size of a single pour to 400 lb to minimize exotherm."

After a die has been cast and cured overnight at room temperature, it is removed from the box and the back surface is machined flat, using a 6 in. fly cutter. The die is then placed in a new box and sheet wax is added to the surface to represent the thickness of the 22-gauge AK draw-quality steel that will be used to form car panels. (Horizontal areas are waxed with 0.030 in. thick sheet wax while wax thickness on vertical surfaces is twice the metal thickness.) When waxing is finished, the punch is poured, cured, and machined flat.

To complete the die set, a new box is built around the polyurethane die and all surfaces are coated with release agent. Then, 0.25 in. thick sheet wax is added to the sides of the die to provide for movement of the draw ring during stamping. The ring is cast to a minimum of 5 in. thick and then cured. Total time for building the die set, including all three pieces, is about seven days for most parts.

Part Forming

To stamp panels, the die set is installed in Hendrick's press and oil is applied to surfaces to help the metal stretch. A steel blank is placed on the punch, which is then pushed up into the die to form the part.

"Our RenCast polyurethane dies are strong enough to withstand the 400 tons of pressure used to form large panels like C posts. It's also suitable for deep 3 in. draws," Brawley says. (Hendrick technicians use nitrogen cylinders on deep-draw projects so that metal is slowly and tightly formed against the die without wrinkling or tearing.) Brawley concludes, "In-house production of metalforming dies provides us with a whole new opportunity to reduce turnaround time on replacement panels for our fleet of award-winning race cars."