When castings are made by the investment casting or permanent mold process, a considerable amount of heat is lost from the surface of the hot shell or die in the form of radiation. The amount of radiant heat loss varies according to whether various surfaces face the ambient surroundings or other hot shell or die surfaces. When facing other shell or die surfaces, the temperature difference is smaller and the amount of radiant heat loss is less.
This variation in radiant heat loss can be simulated by a process of applying “View Factor” calculations to the mesh. The View Factor Calculation takes into account the visibility of all shell or die surfaces to all other shell or die surfaces as well as the surrounding environment, and adjusts the conditions at each surface accordingly.
View Factor Calculations are applied to a mesh, AFTER a model has been meshed.
As an example, consider the following model of an investment casting with two castings on a tree:
The first step in the View Factor Calculation is to select Model…Materials List and click on the HT Coefficients tab. The value that you place in the External HT Coefficient box will be the ‘high’ value used for radiation heat transfer. This number is related to the temperature of the shell or die, and is typically about 5-7 BTU/hr-sq ft-F for permanent mold dies and 10-20 for investment shells. Values for use can be calculated using the HTC Calculator utility.
Once the External HTC is set, mesh the model by clicking on Model…Create Mesh. Meshing this example casting with a shell mold results in the following investment shell:
Now, having created the shell by meshing, we can apply the View Factor Calculation to take into account the variation in radiation heat exchange around the surface of the shell.
To do this, we first click on the Mesh icon on the project tree to highlight the mesh. Then, on the menu bar at the top of the window, select Mesh and then View Factor Calculation. The calculation will be performed and displayed as shown on the next page:
In this view, the dark areas on the shell are losing heat most rapidly due to a high rate of radiant heat exchange with the surroundings. The lighter areas are those that see mainly other portions of the hot shell, and those areas are losing heat more slowly as they exchange radiant heat with those other portions of the shell.
After having performed the View Factor Calculation, you can go on to the next step and run a simulation. The View Factor adjustments are now built into the mesh.
By the way, multiple View Factor Calculations will not change the mesh. So, if you can’t remember whether you did the calculation or not, select it again to be on the safe side.
In the same way, the View Factor Calculation can be applied to a permanent mold casting. The view factors are applied to every surface in contact with ambient conditions, so it doesn’t matter if the die/shell is created as a part of the model, or by meshing.
In general, View Factor Calculations are of limited use in sand casting simulations and would not be applied to such a model, due to the low temperatures on the outside of the mold during solidification. A possible exception would be the shell molding process, where the mold thickness can be quite small compared to part thickness.