# Hints and Tips: How to create a shell mold STL from a casting part design

Sometimes a metal casting engineer may have only a part and runner system for a particular casting design but actually needs to make the mold (or investment) for the part. The user can generate an STL file for the mold by using the thermal penetration depth capability in FLOW-3D v9.4. This saves the user time by not having to build a CAD model of the investment. Here are the necessary steps:

1. Import the part and runner into FLOW-3D and define it as a complement so that the surrounding region in the mesh is the solid.
 Image 1 - STL Part and Runner Image 2 - Mesh Overlay on Part

Image 1 shows an STL part with two runners to inject the liquid metal. Image 2 shows a computational mesh large enough to capture the part and allow for a thermal penetration layer around the cavity. Note that the runners extend through the mesh so the mold will be open at its extents.
1. Next, the thickness of the shell mold must be defined. For an investment mold where the desired thickness is known, this value is simply entered as the depth of the mold. If instead, the intention is to reduce computational times by only including that portion of the mold in which heat is conducted during the time of the simulation, then the thermal penetration distance into the mold must be evaluated from the physical properties of the material. When this distance is used for the mold thickness, FLOW-3D will allocate memory only for cells to that depth in the mold that encompasses the part. This reduction in active cells takes advantage of the unstructured memory allocation scheme in FLOW-3D to produce faster solution times.

 Image 3 - Full Energy Equation Image 4 - Maximum Thermal Penetration Depth

In order to be able to set the maximum penetration depth, the full energy equation needs to be activated under Heat Transfer on the Physics tab (Image 3). On the Meshing & Geometry tab set the Maximum Thermal Penetration Depth (Image 4).

1. Pre-process the simulation and load the prpgrf file in Analyze tab (Image 5).

Image 5 - Load prpgrf file
1. Choose 3D results. Select the iso-surface of "thermally active component volume" and render it (Image 6). Under the Display tab (Image 7), select Fluid Surface at the lower left corner and the Export to STL option should appear at the very bottom. Save it to a file and you have a shell mold in STL format. Note that the higher the mesh resolution, the more accurate the shape of the shell to be captured is.

Image 6 - Render Iso-surface contour "thermally active component volume"

Image 7 - Export rendered view to STL file

The shell mold generated in this example is shown in images 8 and 9.

Image 8 - Image of generated mold-investment

Image 9 - Clipped view of generated mold (investment)