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Simulation Keeps Design Real and Optomized

Jiten Shah

This burner box was converted from a five-piece welded assembly into a one-piece casting resulting in significant manufacturing time savings––but the conversion also reduced a bottleneck in production and scaled production for higher capacity. 

Additionally, with design optimization, enhanced heat transfer and improved manufacturability were achieved. The use of extensive CAE (Computer Aided Engineering) coupled with advance casting process simulations are very valuable tools for such a casting conversion with performance optimization. Overall, the implementation of reusable dunnage helped to limit environmental waste for better sustainability.

The 3D printed sand toolingless process was used to shorten the product development cycle significantly for the prototype as well as for the low-volume 60 piece production for form, fit, and functionality verification and validation––before production tooling was built.

A burner box is combustion chamber and a heat exchanger for the HVAC system. Welded fins lack the close connection with the main body. But a casting, which is a one-piece, continuous configuration, allows a better heat transfer path, which is further enhanced with cast fin features. They increase the apparent cooling areas and are designed to orient in the direction of the pattern pull with generous drafts.

One of the first critical decisions a casting design engineer has to make is the parting plane and orientation of the casting shape.   Ideally, if a design is created to eliminate the need for any core, it enhances dimensional repeatability at lower cost as core-making and setting are the additional steps over molding. 

Modern casting process simulation tools are very effective to validate various casting orientations, rigging designs, and process parameters to ensure the desired quality, microstructure, and properties are achieved before the design is finalized and tooling is made, thereby eliminating the traditional pour and pray process.   

The key to achieving realistic design optimization using CAE tools is the availability and use of accurate material properties, such as strain life fatigue. AFS has developed extensive engineering properties for various grades of ductile iron sand casting alloys––this data is made available to design engineers through a web portal  at: www.AFSCADS.com. These properties contain pedigree information such as the section thickness, type of molding/casting process, typical chemical composition and microstructure, backed with a link to the reference research report available in the virtual library of AFS.

The size (diameter, height with reference to the wall section thickness) and location of holed features and their desired positional tolerance influence the design engineer’s decision whether to make them as-cast or to cast the part as a solid and drill the holes later as a part of the machining. If the cored holes are too small, the sand will get fused and will make the subsequent machining difficult.  Sometimes, the decision is also influenced with the directional solidification intended to achieve the desired soundness.

It is required to provide generous radii to extending features such as cooling fins and ribs as opposed to sharp edges and fillets at all the junctions––L, T, X ,and Y for smoother transitions; the overall effect is greatly reduced stress concentration associated with sharp changes in the cross sections for longer fatigue life, better moldability, better flow pattern of the liquid metal, and overall cleaner castings.