Simple Solutions That Work! Issue 13

28 DAVID C. SCHMIDT Vice President FINITE SOLUTIONS, INC. ARTICLE TAKEAWAYS: • Computer simulations make good rigging design fast, thorough and highly accurate • Learn about the 5 steps in the design process PREDICTING AND ELIMINATING DEFECTS IN INVESTMENT CASTINGS USING COMPUTER SIMULATION INTRODUCTION Computer simulation makes it possible to synthesize elements of good rigging design into a general method that is fast, thorough and highly accurate. And, because of the automation involved, this tool allows new foundry engineers to effectively design casting process methods. The design process consists of these steps: • Simulation of the ‘Naked’ Casting • Gate Sizing and Feeding Design • Rigging Geometry Creation • Verification via CFD/Solidification Simulation ‘NAKED’ SIMULATION The first step in the rigging process is to run a simulation of the part ‘naked’; without any rigging system. Simulation shows the effects of part geometry on the overall solidification. Filling analysis is typically not done, providing extremely rapid results, and can point out preferred gate locations which promote directional solidification. All that is required for the initial simulation is a casting model, normally provided by the customer in the STL file format, and basic process details such as casting alloy, shell material/ thickness, pouring temperature and shell pre-heat temperature. Our example of a commercial part is an impeller casting. Unrigged simulation results are shown in Figure 1 . Once the unrigged simulation is complete, solidification data is converted to thermal modulus information, and feeding zones are determined. In this case, two zones are predicted; one on the top and one on the bottom of the casting. By plotting the higher modulus areas, we can find the preferred gate attachment points. The feeding zones and last points to freeze on each zone are shown in Figures 2 and 3 . Figure 1: STL model of an impeller casting. ‘Naked’ simulation results, without filling.