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Metal Casting Technologies : June 2006
Filling analysis is automatically followed up with solidification analysis, using a combined thermal and volumetric calculation. This technique not only predicts poor directional solidification, but provides the most accurate analysis of macro-shrinkage due to volumetric feeding from the risers. The figure above shows the progression of solidification and the figure on the right, volumetric feeding of the risers. The 'piping' of the risers is displayed by plotting where the metal has lost 10% or more of it's density due to volumetric feeding. The two figures opposite show sectioned views of the casting near the parting line. The figure on the left shows the progression of solidification, based on the Critical Fraction Solid (CFS) Time, which is a measure of the end of volumetric feeding. Notice that there are two bright spots in the casting. These areas were caused by the addition of feeders in the areas and the mould filling dynamics. The figure on the right shows two indications of very fine porosity in the web sections. Since these areas are not in the sidewalls of the part, they are acceptable for this type of commercial casting. In many cases, the design portion of the analysis can be done in an hour or less. Verification simulations, using full CFD analysis, can be done typically in about two hours to overnight. Simulation times will vary depending on such things as computer processor speed and available memory, casting complexity and materials cast. In general, thinner walled castings require more computation time, and materials with higher thermal conductivities, such as aluminium and copper, will also take longer to simulate, all things being equal. CONCLUSION Casting simulation software has gradually evolved from a problem detection or verification tool to an integrated part of the design method process. Simulation is no longer used to check a gating and risering system, but to actually be the driving force for the design of a rigging system. Even complex geometries can be successfully rigged in a short period of time using such tools. The use of actual simulation results directly in the methoding process produces a more accurate result than manual techniques, and does it in a much shorter time period. This integrated approach reduces overall costs and reduces lead times. A valve body casting case study was presented to provide a detailed example of how such a tool is applied to commercial castings. ● Solidification progression on the rigged casting. Volumetric feeding of the risers. Low level porosity in web sections. Progression of CFS Time. METAL Casting Technologies June 2006 39