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Copper Rotors Cast Vertically
(continued) 
After the shot is completed, the clamping force is relieved, and the piston is lowered. At this point, the solidified biscuit is locked to the piston by dovetails on the piston’s face, Figure 2. As the piston recedes, the biscuit breaks cleanly away from the casting, thereby greatly simplifying post-casting clean-up operations. The lower die assembly is then withdrawn sideways, and the piston is raised to expose the dovetails. This allows the biscuit to be slid off for recycling. The piston is then once again withdrawn vertically, readying the machine for the next shot. Inert Advantages 
“We like to think of our process as a bridge between low-pressure, or gravity die casting, and the conventional high-pressure method,” says John Lawson, technical and business director for THT’s motor division. “We realized that our vertical casting method offers a number of advantages for axially symmetric products like motor rotors,” says Lawson. “Our founder and president at the time, Ted Thieman, had years of experience casting aluminum rotors our way, and he had his own ideas about how to make the process work with copper.” According to Lawson, the most important technical advantages of the vertical process stem from the way liquid metal flows into the die. Most important is that, rather than making several 90-degree turns as it does during horizontal casting, metal flows essentially in one direction (up) from the shot-sleeve cavity to the die. Lawson believes this feature, plus the relatively short travel distance involved and the multiple gates arranged completely around the lower end ring, sharply reduce the chance of developing porosity in the casting. Other advantages of vertical casting cited by Lawson include smaller machine footprints, lower clamping pressures and much faster die change-outs compared with equivalent-sized horizontal equipment. What about die life? CDA’s research found that achieving die life in the thousands of cycles requires the use of nickel-based superalloys in critical die areas, along with preheating the entire die assembly to temperatures around 1200 F (650 C). However, the simplified arrangement of components in the vertical process does away with the need for some of the superalloy inserts used by CDA, explains Rick Kamm, THT’s cofounder, who heads new technology development for THT Presses. He says, “Most of the damage we saw was in the gates, which are relatively thin and crack readily. We initially got around that problem by simply replacing the gate plate periodically. After awhile, we began making the gates as inserts to the gate plate, which works just as well and cost less.” MORE >> |
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