Simple Solutions That Work! Issue 14

11 ADVANCED MANUFACTURING SOLUTIONS configure the system to lower the HTC on a casting surface when it solidifies. This is used to simulate the air gap that forms as the casting contracts and tries to pull away from the mold. HTCs and radiation view factor calculations can also be used to predict cooling on the outside of the die, in the foundry environment. Another major setup consideration is how to handle die cycling. Most dies are preheated before casting starts, but it still takes a number of cycles for the die temperatures to heat up to the operating conditions. In simulation, the number of warm-up cycles can be reduced by starting the die at a hotter temperature than normal, and let the die cool slightly to the operating temperature. For example, you may heat a die to 300°F in the foundry, but the overall operating temperature may be in the 600-700°F range. If you start the die at 800°F in a simulation, it may take, say, 5 cycles to cool to the operating temperature, rather than 15 cycles if the die had to warm up from 300°F. A final consideration is to speed up the ‘warm- up’ section of the simulation as much as possible by creating two meshes, one coarse and one fine. The coarse mesh is used for the warm-up phase, where the detailed progression of solidification is not important. With fewer nodes making up the mesh, the simulation progresses quite rapidly. If you use an 8:1 Fine:Coarse ratio, the warm-up phase of the simulation can run up to ten times faster. You can even use simplified filling analysis to speed this up even more. Once the die is at an operating condition, the temperature distribution from the coarse mesh is mapped into a fine mesh, detailed and accurate CFD filling analysis and solidification can be calculated for maximum accuracy, with minimum time spent. An example of the setup for simulation can be seen in Figure 8. All things considered; permanent mold casting is the most complex of the gravity filling casting processes to simulate. By paying specific attention to several model building and simulation setup techniques, you can produce simulation results that accurately predict what will happen on the shop floor. And those results can be produced in a very reasonable amount of time. Figure 7: Setup screen for a water-cooling channel Figure 8: Simulation Setup Using Coarse and Fine Meshes Contact: DAVID C. SCHMIDT dave@finitesolutions.com