Lost Foam Simulation Helps Reduce Lost Foam Defects.

In the last year, Mercury Marine (Brunswick Corp.), Fond du Lac, Wisconsin, has made dramatic strides in its ability to understand the lost foam casting process using computational fluid dynamics (CFD) software that allows engineers to model metal flow in the mold on the computer screen. CFD involves the solution of the governing equations for fluid flow and heat transfer at hundreds of thousands of discrete points on a computational grid in the flow domain. When properly validated, an analysis allows engineers to look inside the mold and determine the exact position of the flow front at any point in time, as well as the temperature and pressure of the metal at any point in the mold. This is far more information than can be obtained from plant trials and usually allows engineers to understand where problems might arise. The geometry of the model representing the mold can be changed quickly on the computer and re-analyzed to determine the effect of the change.

Mercury Marine first utilized this software several years ago when it approached Flow Simulation Services, Albuquerque, New Mexico, because it was having difficulty with folds in an engine block. The foundry was hoping to simulate the casting's metal filling and solidification processes as the first step to eliminating defects.

Numerical modeling of the lost foam process is a challenging task since fluid flow is strongly coupled to the heat transfer between metal and foam. To help address these problems, the service provider used Flow-3D software from Flow Science, Inc., Los Alamos, New Mexico, which has a model designed to accurately simulate lost foam casting. Mercury Marine engineers provided the geometry for the 3.0-liter engine block and the metal feed system, including sprue, runners and gate attachments, in the form of surface models in .STL formatted files. Mercury Marine also provided thermophysical information on the properties of the foam pattern, Flow Simulation Services created a model with 900,000 cells that included the pattern and the mold.

The model took 4 days to run and the results predicted the location of the folds and whether they were sufficient to cause a defect. The results accurately predicted the defects that have been periodically occurring in the real-world casting process at Mercury Marine.