Simple Solutions That Work! Issue 13

MODELING THE RIGGING SYSTEM Gate and feeder bar calculations take only a few minutes to perform. Rigging components can be created in CAD or in the simulation software itself. Items that will be used for more than one casting, such as a standard size of pouring cup, can be created in a component format, and re-used as needed, saving considerable time in the model creation phase. If a library of gating components is developed and used, the entire rigging design process, from loading the unrigged model to having a fully rigged geometry ready for verification simulation, can be as short as 30 minutes or so. DESIGN VERIFICATION USING CFD AND SOLIDIFICATION ANALYSIS With the rigging system in place, a full Computational Fluid Dynamics (CFD) analysis is performed to predict and visualize mold filling. This also provides the most accurate temperature distribution in the casting and mold, which provides the best solidification analysis. In addition to temperature analysis, CFD can provide velocity information. It is important to keep metal stream velocities low during filling, to minimize chances for splashing and re-oxidation defects. 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 lack of volumetric feeding from the rigging system. 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, depending on 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 aluminum and copper, will also take longer to simulate, all things being equal. One of the things that feeding zone analysis does NOT tell us is the effect of metal flow. In this example, the foundry decided to invert the casting and gate on the top of the solid boss, hoping that the filling process would create 30 temperature gradients for directional solidification. The initial design is shown in Figure 6 . Unfortunately, filling did not have the desired effect, and there were isolated areas in each vane. The foundry then flipped the casting over and provided multiple gates into the top flange. The revised model and results are shown in Figure 7 . This example shows clearly why it is important to verify the rigging design with a full simulation, including fluid flow analysis. It is impossible for ‘rules of thumb’ to take into account all the variables and dynamics of a process as complicated as the filling and solidification of castings. However, those rules can help us get to a good rigging design much more quickly than by simple trial and error. Contact: DAVID C. SCHMIDT [email protected] Figure 6: Initial rigging design and Material Density plot, showing areas of poor feeding. Figure 7: Improved feeding by inverting the casting, adding multiple gates on the flange.