Eau Claire, WI--Using a two-shot plastic molding process and a unique "living hinge" design, engineers have created an automotive duct door valve that's smaller, quieter, and easier to assemble.
Now being evaluated for use in a future GM vehicle, the new assembly opens and closes to allow passage of heated or cooled air through a car's HVAC system. It replaces a large plastic-and-metal version. Potentially, the new design could also serve in other automotive applications, because most cars use three or four such doors throughout their heating and air conditioning systems.
Unlike existing designs, the new assembly does not require a metal linkage for actuation. Instead, the device employs a scheme that allows it to collapse onto itself. This approach enables the duct door valve to rotate out of the way when the duct needs to be opened. As a result, it is lighter and more compact than older units that employ a separate actuation device.
Designed by engineers at Phillips Plastics, Phillips, WI, and Delphi Thermal Systems, Lockport, NY, the new duct door assembly consists of four hard, glass-filled-polypropylene parts joined by a rubber-like material known as Sanoprene, made by Monsanto Chemical Co. In the final configuration, the molded parts form a 4- x 9-inch door with a trapezoidal chamber on one side.
During normal operation, two steel coil springs inside the chamber maintain it at full volume--about 12 cu-inches. But when vacuum is applied, the chamber collapses to less than half that volume, overcoming the spring forces that hold it open. When this collapse occurs, all of the assembly's "living hinges" allow it to flex and, ultimately, move. As a result, the door rotates around its mounting axis and moves out of its normally closed position, allowing air to pass through the duct.
When vacuum is removed, the coil springs force the chamber open. The door then automatically returns to its normally closed position.
Key to the door's unique operating characteristics is a two-shot plastic injection molding process that endows it with the necessary combination of rigidity and flexibility. By employing glass-filled polypropylene with a Shore A durometer hardness of 110, engineers give the door and other key parts the strength of a conventional unit. By surrounding those parts with Santoprene (Shore A durometer hardness 55), they enable the chamber to bend and change shape. The so-called "living hinges"--named because their plastic fiber actually bends--eliminate the need for knuckles or other mechanisms that would complicate assembly.
"The living hinge mechanism takes the place of the linkage and all the other associated parts," explains Francis Peterson, Phillips engineer and designer of the new duct door.
Because Peterson worked closely with Phillips manufacturing engineer Todd Fox throughout the design of the new product, the duct door requires little in-factory assembly. Hard plastic parts are injection-molded first. Afterward, while the hard plastic is still warm, soft plastic is shot around it. In the process, a chemical bond forms between the parts. Two springs are then placed in the movable chamber, representing the only conventional assembly step.
The new design is a dramatic departure from conventional two-shot, plastic molded designs of the past. Until now, manufacturers typically reserved such designs for decorative parts, such as two-tone knobs and buttons. Rarely have two-shot molds been used to make moving, functional parts.
This duct door offers many advantages when compared to conventional versions, say Phillips engineers. Delphi Thermal Systems performed product testing and validation for GM, and showed that the new door was equiv-alent-to or better-than existing duct doors in terms of reliability and longevity. What's more, the design eliminates the popping and "klunking" of linkages during actuation, saves space and decreases weight.
Equally important, the new unit can easily be recycled. Because it employs no metal linkages, the duct door assembly doesn't require disassembly before recycling. Instead, the polypropylene and Santoprene can be ground up together. "Most of these devices are eventually trashed in landfills," Peterson says. "This one makes recycling an easier process."
Additional details...Contact Debbie Cervenka, Phillips Plastics, N4785 1150TH St., Box 185, Prescott, WI 54021, (715) 262-5451.