Hard edges. Sharp corners. Tight radii. Deep pockets. On the original Ford Mustang, design cues such as these were executed in sheet metal, and they became the stuff of legend.
For the 21st century version of this iconic vehicle – including the 2006 convertible – Mustang designers wanted to evoke the spirit of the 1960s classics as strongly as possible. But they would need to do it with injection-molded front and rear bumper fascias. And these would be molded on presses at Auto Alliance that were limited in capacity to less than 3,000 tons of clamp force.
Bruce Kleiber, Ford’s product engineer for Mustang bumper systems, explains the conflicting requirements: “With injection pressures limited, we would need either a thicker fascia, which would compromise the design details, or a higher flow material with sufficient strength to handle items like the attachment of the park/turn lamps.”
The product development group at Solvay Engineered Polymers, Inc., proposed using SEQUEL® 1440 HF thermoplastic polyolefin, with a melt-flow rate more than 25% higher than the SEQUEL 1440 grade that had been successful in the previous Mustang. “A series of mold-flow analyses,” says Kleiber, “proved that we could maintain a nominal 2.7-mm wall thickness on all the fascias – except 3.0 mm on the rear GT due to the lowered ground effect. And the parts would be light enough to keep the vehicle in the preferred weight class. The stylists could have their cues, and the parts could be molded on existing equipment at pressures low enough to avoid flashing.
Balance of properties
This would be the first large-scale program to launch in SEQUEL 1440 HF polyolefin. But the balance of properties was ideal for the Mustang fascias: a combination of high stiffness, tensile strength, impact resistance, and improved melt-flow rate.
The rear fascias, in particular, are very tall, and the surfaces that wrap around the side of the car to meet the wheel wells are very long, so improving the flow rate was critical to achieving complete mold fill.
5-mph bumper impact rating
The impact resistance of the high-flow material was one key to the 5-mph bumper system rating on the new Mustang. Designers had moved the rear license-bracket location from the deck lid on previous models to a deep recess in the center of the rear fascia.
Preliminary part testing led to improvement in the ductile behavior of the SEQUEL 1440 HF material in order to address this specific area. In subsequent testing, even this sensitive location would survive the low-temperature impacts that are part of certification to the FMVSS 219 impact standard.
Designing material for a specific application
“Because we compound our materials,” points out Rick Flynn, who was responsible for advanced quality planning on this program at Solvay Engineered Polymers, “we can address each of the properties separately. So we could ask Ford to determine the priorities of the application. If this were a reactor-grade TPO, we would have to ask the customer what compromises they would be willing to accept between stiffness and elasticity, or between dimensional stability and impact resistance.”
In the end, the product team working on the Mustang fascias developed a material that met the conflicting requirements of design and production. And legions of Mustang faithful are expected to nod in approval.