size of the casting, number of cores and other factors. T e impact of part design

complexity, increasing the number of cores and com- plexity of core geometries were analyzed with respect to production cost of mold making using traditional and 3-D sand printing.

Case Study 1: Train Air Brake

T e part geometries in

these case studies are deriva- tive designs of actual castings. Each case study starts with a solid casting, and cores are sequentially added until all the desired cores are in- cluded. T is methodology maintains the constant bounding box while gradually increasing the part complexity with the growing number of cores. T e fi rst case study involves the

Figure 6. This graph depicts the total costs (tooling and fabrica- tion) for Case Study 1 where conventional patternmaking costs are shown for 30, 100 and 1,000 units.

train air brake casting shown in Figure 3. Using conventional processes, the design and assembly of eight cores are required. Beginning with a solid part, cores were added sequentially until the fi nal number of cores (eight) was reached. T e corresponding design attributes described in the complexity factor equations are shown in Table 2. In conventional pattern making, a tooling cost is associated with pattern and corebox fabrication. T e relation- ship between tooling costs per set of mold and the corresponding complexity factor is shown in Figure 4. Figure 5

shows the relationship between fabrica- tion costs for both conventional pattern making and 3D sand printing at diff er- ent levels of complexity for Case Study 1. For conventional pattern making production costs, the fabrication cost proportionally increases with increas- ing complexity: as cores are added, the cost in labor to assemble cores, cost of materials (i.e., sand, glue) and scrap costs all increase. It was observed that lower levels of

complexity lead to higher fabrication cost in 3-D sand printing than conventional mold manufacturing approach. In the case of the part design with a complex- ity greater than 56, the fabrication cost of 3-D sand printing was lower than conventional pattern making. 3-D sand printing provides a unique advantage

here by consolidating cores into single core. T is results in lower labor and scrap costs with higher numbers of cores. Figure 6 incorporates both tooling and fabrica- tion costs as a function of part design complexity. For conventional manufacturing, cost curves for quantities of 30, 100 and 1,000 were included to show that the costs of patterns and core boxes were amortized across the production volume. For a production volume below 30 castings, 3-D sand print- ing is more aff ordable than conventional pattern making even in the case of no cores.

In other words, the breakeven point is the lowest level of complexity for this family of castings at this quantity. However, for quantities greater than

30 castings, it depends on the level of part design complexity. As quantity increases, the breakeven point shifts to increasing levels of complexity. For production quantities of 1,000

castings, the tooling cost per mold/ set is so low that fabrication costs signifi cantly dominate and cost/com- plexity behavior is almost identical to the fabrication costs.

Case Study #2: Turbocharger In a second case study involving a

turbocharger, cores were sequentially added starting with a solid casting un- til the incorporation of all three cores (Fig. 7). However, in the case of this part design, the core geometries were diff erent for each sub-case, wherein the fi rst core is added in the shape of a cube and subsequently the cubic core is replaced by two cylindrical cores. Finally, the cylinders are replaced by the three actual cores. T e relationship between tooling

set and complexity factor is presented in Figure 8. Figure 9 presents the rela- tionship between fabrication costs for both conventional pattern making and 3-D sand printing at diff erent levels of complexity for the turbocharger. T e conventional patternmaking

Figure 7. Shown is the turbocharger casting and its core geometry in Case Study 2. 28 | METAL CASTING DESIGN & PURCHASING | Nov/Dec 2016

production costs increased as a func- tion of complexity; however, a drastic

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