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Best Practices for Importing CAD Geometry into FLOW-3D

CAD geometry can be imported into FLOW-3D via a number of file formats.The STL (stereolithographic) format is the most robust and is generally recommended above all other supported formats.The STL format describes the surface of a solid object as a set of triangles. Each triangle is described by three vertices and a unit normal which specifies which side of the triangle represents the outward surface of the solid. A correct STL file must represent a solid object as a water-tight set of triangles and all of the unit normals must be pointing outward from the surface.

While most CAD software packages export STL files, these files may contain errors such as missing triangles, inverted unit normals, and zero area triangles. While FLOW-3D’s FAVOR™ method can generally "heal" surfaces with a few errors, missing triangles and reversed normals can create problems in the simulation such as pressure iteration failures. It is strongly recommended that STL files be checked for correctness before importing them into FLOW-3D.

Correcting STL Files


Two tools are provided with FLOW-3D installations on Windows that allow STL files to be checked for correctness and subsequently fixed if errors exist. MiniMagics, from Materialise Software, is provided with FLOW-3D for checking the correctness of STL files. Figure 1 shows an STL file with a missing triangle as displayed in MiniMagics. The missing triangle is shown in red and the message "This part contains errors!" is displayed.

Finding a bad edge in a STL file using Mini-Magics
Figure 1. .STL bad edge found in Mini-Magics.


A simple STL fixing tool called pyADMesh is provided with FLOW-3D to fix minor errors in STL files. pyADMesh is accessible from the FLOW-3D start menu. The missing triangle shown in Figure 1 can usually be fixed by pyADMesh but major issues such as large groups of missing triangles or inverted unit normals are difficult to fix. The best solution for these types of problems is to check the original CAD file for problems. When designing in CAD software, the best first step is to ensure that the part was designed using proper guidelines and best practices.

Binary vs ASCII Formats

STL files can usually be exported in either binary or ASCII format. The binary format creates significantly smaller files and are processed much faster than ASCII formatted files. The binary format is recommended for these reasons.

How to Export STL Files

Below is a list of some of the most common CAD packages with step-by-step procedures for exporting STL files from a number of popular CAD software packages:


  1. At the prompt type STLOUT
  2. Select objects
  3. Choose "Y" for Binary
  4. Choose Filename


  1. File > Export > Rapid Prototype File > OK
  2. Select Part to be Prototyped
  3. Select Prototyped Device > SLA500.dat > OK
  4. Set Facet Surface Smoothing to 150
  5. File > Export
  6. Choose STL


  1. File > Export > Model
  2. STL
  3. Set chord length to zero
  4. Set angle control to 1
  5. Select OK


  1. File > Save As
  2. Select File Type > STL
  3. Enter name for STL file
  4. Save
  5. Select Binary STL files

Solidworks (2007)

  1. File > Save As
  2. Select File Type > STL
  3. Options > Fine unless using General Moving Objects then Options > Coarse
  4. Save

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