Simple Solutions That Work! Issue 18

DAVID C. SCHMIDT Vice President Finite Solutions, Inc. ARTICLE TAKEAWAYS: • Simulation of unrigged castings drives gating and riser design • Flow simulation predicts/prevents filling-related defects • Computational Fluid Dynamics (CFD) improves solidification accuracy INTRODUCTION Simulation is the process of predicting, on a computer, what happens when metal is poured into a mold and the metal cools and solidifies. By simulating this process, we hope to predict potential defects in the casting and redesign the process to eliminate these defects, BEFORE making actual castings. Using Simulation to Rig Castings and Prevent Defects THE DESIGN PROCESS Typically, the design process begins with receipt of part specifications from the customer. Traditionally this involved a paper drawing, however, nowadays most part geometry is contained a 3D CAD file which facilitates the use of computer simulation. The first task of the foundry engineer is to decide on a basic process design for the casting, i.e., in what orientation the part will be cast, how it is to be gated and risered, and how many castings are to be placed in a single mold or tree. Simulation can be immediately helpful even at this early stage of design. We have created a design process that starts with running a “naked” simulation of the part as received from the customer, completely surrounded by mold material and without gates or risers. This unrigged simulation typically runs in just a few minutes and allows the part to be viewed from a thermal standpoint, showing the progression of “natural” solidification and the location of thermal centers in the casting. Figure 1. In many cases this analysis will determine the orientation of the casting in the mold; contact points for risers become obvious, and the best orientation of the casting to accommodate those contact points can be decided immediately. Gating and Riser Design Wizards are built into the simulation software, so that location, number and size of risers and suggested size and shape of gating components can be calculated more or less automatically to establish an initial rigging design for the casting, Figure 2. Once the initial design is developed, it is necessary to prove out and fine-tune the design by running complete flow and solidification analysis. This is because design rules are general in nature and cannot take into account all of the dynamics that will occur within a complex casting/gating system. It is usually necessary to construct a 3D model of the casting with the complete proposed rigging system for simulation. The simulation process occurs in two phases: Flow of the liquid metal as it enters and fills the mold cavity, and the subsequent cooling and solidification of the metal along with formation of macro- and micro-porosity defects. FLOW MODELING Flow modeling is an integral part of the casting simulation process. Flow modeling allows flow-related defects such as misrun and oxide (dross) formation due to excessive velocity to be predicted and reduced or eliminated through design changes, prior to production of the casting. Flow modeling is used to evaluate the gating design to insure the desired delivery of metal in the casting cavity. In addition, flow modeling provides the most accurate initial temperature field for modeling 44