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CDA Research Improves Soundness of Rotor Die Castings
Modeling Guides Process Control to Avoid Defects It is not unusual for die castings to display some degree of porosity due to the rapid and turbulent flow of metal into the mold cavity. For most products, small amounts of porosity are tolerable from the standpoint of mechanical behavior, but the situation is quite different in the case of motor rotors. Here, porosity control is particularly important because even tiny voids can diminish the advantages offered by copper's high volumetric electrical conductivity. Large voids, which generally appear in the rotor's end rings, are also detrimental since they can make the rotor difficult to balance. Early casting trials conducted by CDA found only 2%-3% porosity, chiefly in the ejector (far) end ring and virtually none in the conductor bars. A commercial manufacturer now in serial production (see "French Die Caster Offers Copper Motor Rotors," Update: Copper Motor Rotor, Vol. 4, No. 1), has achieved even better results, with castings routinely exhibiting 99.5% of full density. However, larger rotors subsequently cast by CDA exhibited as much as 25% porosity in one ejector end ring and about 8%-10% in others. This was judged to be unacceptable, and CDA accordingly initiated a combined analytical-experimental program to determine the origin of the defects and develop steps to prevent them. Walkington Engineering, a Wisconsin-based organization specializing in computational fluid dynamics, performed the analytical work, modeling the flow and solidification of copper within the mold. Simulation is faster and less costly than performing a lengthy series of casting trials. It can also suggest optimum process conditions, which can then be tested in a relatively small number of shots. 
The trial rotor casting, Figure 1, consisted of the squirrel cage with its end rings and conductor bars, plus a system of gates and runners. The part is edge-gated with four approximately semicircular gates on each side of the vertical runner. Several other gating systems were modeled, but none showed
significant advantages. The modeling program examined two important casting parameters: variations in the plunger position-speed profile (the shot profile) and the casting temperature.
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