natural solidification simulation was run. Natural solidification illustrates how the casting would solidify if it were filled with aluminum at a uni- form temperature of 1,292F (700C). Te natural solidification provides crit- ical information such as which parts of the casting will solidify first and which will solidify last. Figure 3 shows what parts of the thermal command center will solidify last. Te engineer used this information to determine the orientation in which to cast the part, as well as locations to feed the casting with in-gates and risers. After an initial gating system was
designed, the mold filling was simu- lated. Tis analysis provided invalu- able information regarding the metal temperature and velocity during the
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The cross-over point for any casting is the quantity at which the cost to produce by tooling is equivalent to the cost of manufacture by 3-D printed sand.
filling process. Te part was cast using a low pressure sand casting process in which the sand mold was filled from below. In this process, nitrogen gas is used to pressurize the aluminum in the furnace and force it up a ceramic tube into the mold. Low pressure casting ensures tranquil fill and prevents oxi-
dation of the aluminum by eliminating turbulence during the filling process. It also was possible to examine the predicted solid fraction to ensure the metal was not going to solidify with cold shuts prior to filling the part. Te gating system for the thermal
command center was adjusted more than 10 times over a one-week period and the analyses re-run until the engineers were convinced a satisfactory gating system had been designed. The sand molds were printed using silica sand and furan binder. The machine had a build volume of 59 x 27.6 x 27.6 in. (1,500mm x 700mm x 700mm) and was able to print the complete mold and inter- nal core during one build cycle that took approximately 30 hours. Figure 4 depicts the three printed sand components.
Te subsequent de-powdering
clean-up process took another six hours before the mold and single internal core were ready to assemble. The total timescale from receipt of 3-D data, through gating design and extensive fill and solidification analysis, 3-D printing of molds and casting was 15 days. The casting was heat treated to the T6 condition and inspected prior to machining. A white light scan of the casting compared it to the machined part model, and x-ray inspection revealed no visible defects. It also was leak checked at 30 psi under water with no leaks.
Analysis of Cross-Over Point for Tooling
Fig. 4. The core package was comprised of three printed sand pieces: drag core (a), single internal core (b, c), and cope core (d). The internal core assembled into the drag core also is shown.
38 | METAL CASTING DESIGN & PURCHASING | Sept/Oct 2016
Te cross-over point for any casting is defined by the quantity at which the cost of manufacture by tooling is
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