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STL Files / / Understanding the 3D Printing File Type

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STL Files / / Understanding The 3D Printing File Type

60% of CAD users say STL file prep and repairs are the biggest challenges for engineers when preparing files for 3D printing.  You’re not alone.


Inherently, 3D CAD programs provide an invaluable benefit for engineers and designers to visualize their product in a 3-dimensional, virtual space.  However, exporting to an STL file format happens to be a major pain point for all users.  For example, low resolution files will have facets that may appear in a 3D printed model and those with very high resolutions will create an excessively large file that is difficult for 3D printing softwares to process.  In order to get a good quality 3D print, you need to start with a good STL.  How to create the goldilocks STL file that balances resolution, file size, accuracy and quality? 


STL files are made up of triangles which approximate a surface.  In order to 3D print an STL file, the mesh must be closed (watertight), meaning all edges of the triangle align with another and all of the normal directions are matching.  When exporting from your CAD package, it’s important to consider the different options that your software provides such as distance deviation or angle resolution.  In addition, every design program has a different method of converting files to STLs and some are more successful than others. Fathom has identified a list of potential STL errors and recommendations to proactively address these issues / /

CAD to STL : Export Considerations Potential STL Errors
Distance Deviation Inverted Normal
Angle Resolution Bad Edges
Edge Length Holes
Aspect Ratio Noise Shells
ACSII Versus Binary Multiple Shells
  1. Inverted Normal / / A triangle’s normal is flipped the wrong way which creates an open mesh.  Because of this, slicing software will be unable to create contours for printing. Simple STL editing software will enable you to flip the direction of the normal to correct the issues.

  2. Bad Edges / / When an edge of a triangle is not adjacent to another triangle edge.  Many times bad edges that are very close to one another may be stitched together to close the mesh. This requires STL repair and we recommend to re-model the areas where the boundary edges were detected in the native CAD software. Sometimes adding thickness or clearance where needed can help solve faulty edges.

  3. Holes / / Missing triangles from a region prevents a closed mesh. Sometimes this is an easy fix within the STL but that is not always the case. An edit or re-export of the original 3D CAD file may be necessary.

  4. Multiple Shells / / For the most part, it’s ideal for your STL to be made of one shell body for printing. Unwanted meshes and noise shells are typically created in error during an STL export. We recommend removing noise shells for cleaner contours. As always, solving any issues with your models in the native CAD software tends to have better results.

  5. Overlapping & Intersecting Triangles / / Overlapping and intersecting triangles may produce areas too small to print. Intersecting faces usually lead to failures since the software doesn’t recognize which areas are inside or outside of the model. We recommend combining the bodies into a single solid before exporting into an STL.

Quality in, quality out.  By addressing the most common mistakes and potential errors with STL file preparation, engineers and designers are able to bypass headaches and wasted time.  Even the most popular CAD packages will be frustrating when it comes to STL file export.  Like any language, this requires self learning and patience.  The expert team at Fathom has identified the most common pitfalls and guarantees that this STL Guideline document will save you time and ensure that your 3D printed parts accurately represent your idea.


Download the PDF to learn the rest of the guide—How to take your STL Files to Printed Parts

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