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. Te sand molds were printed using
silica sand and furan binder. Te 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 internal core dur- ing one build cycle that took approxi- mately 30 hours. Figure 4 depicts the three printed sand components. Te subsequent de-powdering
Fig. 4. The core package was comprised of three printed sand pieces: drag core (a), singe linter- nal core (b, c), and cope core (d). The internal core assembled into the drag core also is shown.
clean-up process took another six hours before the mold and single internal core were ready to assemble. Te 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. Te 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
Te cross-over point for any casting is defined by the quantity at which the cost of manufacture by tooling is equivalent to the cost of manufacture by 3-D printed sand. Tis can be expressed in the number of parts. Fig- ure 5 shows the cross-over point for the thermal command center. Simi- larly, as shown in Table 1, it is possible to determine the lead time for both routes of manufacture. Te conventional way to manu-
Fig. 5. The cost comparison graph for the thermal command center shows that tooling becomes more cost-effective when 22 castings or more are produced.
40 | MODERN CASTING August 2016
facture a small number of prototype castings would be to make urethane soft tools by CNC machining blocks of urethane board. In this project the
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