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

E. Almaghariz, B. Conner, L. Lenner, R. Gullapalli, G. Manogharan,
B. Lamoncha, M. Fang
Click here to see this story as it appears in the April 2017
issue of Modern Casting.

Three-dimensional sand printing provides a means to fabricate molds and cores without the need to make patterns and coreboxes. Metalcasters and end-users could benefit from learning more about when to use this evolving advanced technology over conventional patternmaking.

To know more, researchers examined 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 casting process involve tooling associated with mold making. This 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, elimination of sharp corners, and undercuts resulting in higher draft angles leading to increased fabrication costs. This 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 pattern removal prior to pour during sand casting. Often this leads to nonfunctional part design modification and/or additional processing steps after casting.

Additive manufacturing in the form of 3-D sand printing is complementary 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 freedom to produce complex designs not otherwise feasible or affordable using the traditional approach.

Tooling and Fabrication Cost
Downstream operations after fabrication of the corebox are independent of traditional and 3-D sand printing. The 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 treatment, 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.

Among several cost factors in sand casting, two major cost components are the tooling and fabrication costs which involve a variety of operations to produce the mold and cores and subsequent inspection. The unit cost of a corebox depends on the number of cores, cavity geometry/size, mold and core sizes and production volume for that specific part design (i.e., number of castings per design). In the case of traditional mold making, additional operations are required with multiple cores including the assembly of the cores, bonding of cores and inspection.

Several studies have identified the relationship between complex part designs which require multiple cores and its impact on tooling cost in traditional approaches to fabricate a corebox. In conventional manufacturing of sand molds, the production cost is influenced directly by part complexity because of the need for multiple operations, special tools, skilled labor, significant tool wear and lower productivity. Another analysis showed it was evident the cost for machining tooling was relatively higher for complex part designs with similar geometric volume.

Components of tooling costs include two main components: pattern and coreboxes. Tooling cost is influenced by pattern material, part size, desired accuracy and part complexity. Hence, it can be concluded that tooling cost for complex part geometry with larger part size and greater accuracy will be significantly higher than traditional manufacturing of tooling (for similar mold-core material).

The tooling cost in metalcasting facilities and pattern shops usually is amortized over the number of castings produced and is a critical factor that increases unit cost during low production volume. Tooling cost is a fixed initial cost in traditional mold making and this negatively impacts the number of part designs that can be produced economically. This is especially true for low quantity production that would occur during product development. The motivation of this study was to develop a model based on part design complexity, production volume and tooling-fabrication costs of coreboxes. The developed model can be applied to evaluate the economic feasibility of traditional sand casting methods and 3-D sand printing for varied combinations of part designs and production volume.

The Complexity Factor
The methodology employed in this study involves:

The creation of CAD models for each casting design for evaluation.

Quantification of part design complexity in cast parts using a criterion adopted from a prior study.

Estimation of fabrication costs associated with conventionally produced molds and cores and 3-D sand printing for varied production volume.

Analysis of fabrication costs as a function of part complexity factor values.

Estimation of break-even costs between traditional and 3-D sand printing to determine levels of part complexity where 3-D sand printing is more cost-effective.

Examination of the effects of changing the costs of 3-D sand printing.

The criterion for measuring part design complexity used in this study was adopted from a prior study focused on quantification of part complexity of cast parts for traditional processes. The tooling cost is influenced by tool design and complexity which is dictated by the part design complexity. For example, the mold for a complex part design such as a train air brake may require multiple cores. Alternatively, a simpler casting might be a solid uniform cross-sectioned part without the need for a single core. Designers and tool makers observed that the tooling cost depends on the number of cores, volume and surface area of part, core volume, draw depth (i.e., the depth of tooling) and variation in section thickness.

For a given part design, the cost of moldmaking for both pattern and 3-D sand printing was conducted. For conventional pattern making, tooling costs were generated using an Internet-based cost generator. The bounding box of the part, the number of cores and the number of part features are required to generate the tooling costs. Fabrication costs of molds and cores were estimated by industry quotation method based on the size of the casting, number of cores and other factors.

The impact of part design complexity, increasing the number of cores and complexity 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
The part geometries in these case studies are derivative designs of actual castings. Each case study starts with a solid casting, and cores are sequentially added until all the desired cores are included. This methodology maintains the constant bounding box while gradually increasing the part complexity with the growing number of cores.

The first case study involves the train air brake casting. Using conventional processes, the design and assembly of eight cores are required. Beginning with a solid part, cores were added sequentially until the final number of cores  was reached.

In conventional pattern making, a tooling cost is associated with pattern and corebox fabrication. The relationship between tooling costs per set of mold and the corresponding complexity factor shows the relationship between fabrication costs for both conventional pattern making and 3-D sand printing at different levels of complexity for Case Study 1. For conventional pattern making production costs, the fabrication cost proportionally increases with increasing 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 complexity 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. This results in lower labor and scrap costs with higher numbers of cores.

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 printing is more affordable 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 significantly dominate and cost/complexity 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 until the incorporation of all three cores. However, in the case of this part design, the core geometries were different for each sub-case, wherein the first 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.

The conventional patternmaking production costs increased as a function of complexity; however, a drastic increase occurred between one cube-shaped core and two cylindrical cores. For 3-D sand printing, it also was observed that at lower levels of complexity the fabrication cost was higher than that of conventional manufacturing. Unlike the previous case study, the 3-D sand printing cost does not ‘‘level out’’ because the volume of the cores is significantly increased due to the cylinders and the final core geometry.

For complexity factor values greater than 51, the fabrication cost of 3-D sand printing is lower than conventional mold making. As the final three core geometries were approached, the 3-D sand printed cores were consolidated into a single core providing a cost advantage over conventional moldmaking.

For a production volume of less than 26 castings, 3-D sand printing is more affordable than conventional patternmaking even in the case of casting without any cores.

However, for production volume greater than 26 castings, it depends on the level of part-core complexity. As seen in Case Study 1, the breakeven point shifts to increasing levels of complexity as the quantity increases. In the case of 1,000 castings, as observed in Case Study 1, the tooling cost per mold/set is significantly lower since fabrication costs is more significant.

Future Work
In order to accelerate the adoption of emerging technology such as 3-D sand printing in the metalcasting industry, this study recommends future work to examine the combinations of conventional patternmaking and 3-D sand printing for a single casting. For example, the economics of using conventional patterns for molds and 3-D sand printing for complex cores could be explored. Further, economics and fabrication time associated with using alternative additive manufacturing technologies for patternmaking such as material extrusion (also known as fused deposition modeling) could be explored.

This study assumed the 3-D sand molds and cores printing provided an equivalent surface finish and sand performance with traditional pattern making for mold and core manufacturing. However, an extension to this study would focus on incorporating additional factors to incorporate such attributes. Thus, evaluation of such factors can be achieved by measuring surface finish and testing of physical and mechanical properties. This work will give additional evaluation criteria for both approaches along with estimated cost.

Finally, incorporating these results into a CAD–CAM software system would be immediately beneficial. The end user should be able to plug in the geometric attributes of the castings and input cost parameters such as materials, consumables, labor, depreciation and other costs for both patternmaking and 3-D sand printing.   

This article is a summary of a manuscript published in the International Journal of Metalcasting. For more information on the manuscript, contact the AFS technical department at 800-537-4237.

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