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Using a Complexity Factor to Calculate Cost Benefits of 3-D Sand Printing

A part complexity-based cost metric has been developed to analyze decisions



hree-dimensional sand printing provides a means to fabricate molds and cores without the need to make

patterns and coreboxes. Metalcast- ers and end-users could benefit from learning more about when to use this evolving advanced technology over conventional patternmaking. To know more, researchers exam- ined the cost of molds and cores as a function of part design complexity quantified by a complexity factor. Two case studies illustrate how

the complexity of the castings is systematically varied by changing the geometry and number of cores. Tooling costs and fabrication costs are estimated for both 3-D sand printing and conventional patternmaking to calculate the break-even points for the two methods. Integral aspects of every sand cast- ing process involve tooling associated

with mold making. Tis includes the fabrication of patterns used to make the molds and the production of coreboxes to make cores. Some of the major limitations in moldmaking using traditional techniques include constraints such as limitations on minimum wall thickness, elimina- tion of sharp corners, and undercuts resulting in higher draft angles leading to increased fabrication costs. Tis is further amplified in the case of tooling for parts with higher complexity. For example, an expensive core and/ or set of cores are required for parts with complex internal geometry such as an engine block. In some cases, part design modification is required to facilitate pat- tern removal prior to pour during sand casting. Often this leads to nonfunctional part design modification and/or addi- tional processing steps after casting. Additive manufacturing in the

form of 3-D sand printing is comple-

Table 1. Geometric Attributes Used as Inputs for the Complexity Factor Model Part dimensions (length, width, height) Bounding box volume Part volume

Surface area of part Number of cores Volume of core

Thickness of part, min and max Draw depth

30 | MODERN CASTING April 2017

L, W, H Vb Vp Ap Nc

Vc, i Tmin Dd

and Tmax

mentary to the traditional approach of moldmaking in sand casting. 3-D sand printers can directly print a mold from computer-aided design (CAD) models of the desired part design in a matter of a few hours without the need for patterns or coreboxes. 3-D sand printing provides unique

advantages in moldmaking such as significantly reduced leadtime and flexibility without the need for tooling. It also offers additional geometric free- dom to produce complex designs not otherwise feasible or affordable using the traditional approach.

Tooling and Fabrication Cost Te sequential processing steps of

conventional sand casting are outlined in Figure 1. Downstream operations after fabrication of the corebox are independent of traditional and 3-D sand printing. Te primary scope of this study is associated with decision-making in the tooling of molds and cores and the fabrication of coreboxes prior to pouring. It is assumed part design complexity

will have minimal or no influence on the cost per casting in postfabrication operations including pour, shakeout and secondary operations such as heat treat- ment, machining and inspections. However, it should be noted

the consolidation of required cores (through 3-D sand printing) could substantially eliminate or mitigate flash that would generate additional finishing or inspection.

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