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When Plastic Meets Rubber, You Get Big Savings
Ultrasonic welding makes a one-piece part
Laurie Toupin, Contributing Editor -- Design News, January 13, 2003
Detroit — What do you get when you drop rubber, two metal inserts, and a plastic piston into a single mold?
Warn Industries got itself a stronger, high-tolerance diaphragm which saved the company $200,000 in equipment costs.
This annular actuator is the next generation Wheelend Disconnect. A vacuum force creates pressure against the diaphragm to create a linear motion that engages and disengages the four wheel drive (4WD) on the 2003 Ford Expedition / Lincoln Navigator. "All the driver has to do is turn a dial on the instrument panel," says John McCalla, director of OEM Design Engineering at Warn, a manufacturer of equipment for 4WD vehicals.
The original diaphragm, an independent rubber and metal part, connected with a plastic piston and shift fork via a system of pins. Both the diaphragm and piston were formed with six holes around their circumference. These connected to a plastic shift fork, which sported a matching set of six pins. "We'd have to clock and align everything using a laser guided alignment system similar to those used in missiles," says McCalla, "then weld them together."
The process was very challenging. "The complicated assembly fixture was temperamental to say the least," McCalla continues.
When Ford asked for an increase in volume, Warn began to investigate ways to simplify the process.
Enter Wisconsin-based Trostel, Warn's diaphragm supplier. In a massive team effort, Trostel engineers developed a way to eliminate the pin assembly altogether.
"We made the plastic piston part out of the mold," says Bernie Stritzke, director of methods for Trostel. Stritzke's team developed a process to drop all three inserts into the mold at the same time and mold rubber around them.
Mechanical Bond
The two metal parts, used to make the diaphragm more rigid, are coated in adhesive which bonds to the rubber. The piston however, could not be chemically bonded. If bonded, the sidewalls of the piston, would not allow the diaphragm to roll (travel) and actuate the 4WD mechanism.
So Stritzke designed a mechanical bond. "This consists of little rubber beads that sit on top of the round piston that actually seal as we ultrasonically weld the shift fork," says Stritzke. "The beads mechanically seal with the shift fork so there is no leak in the diaphragm."
Because the piston is molded into the diaphragm, the shift fork no longer has to be orientated. Technicians can just drop it on top and weld it. What use to take 20 to 30 seconds per part now takes less than five.
"The system also has created a much stronger assembly." says Mike Szott, technical account manager for Trostel. "The old design utilized six welded pins to hold the fork and piston together. This new design allows for stronger full-perimeter weld."
The process has been so simplified, that Trostel supplies the complete part for Warn, who can concentrate on their actuator.
"This is a pretty unique part and process," says McCalla. "I've never seen anyone do this before." Due to this unusual design, "Warn gains a higher quality component, and Ford gets improved 4x4 system reliability."
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