Wrong geometry after slicing, why?

I have a file that looks like seen in the screenshot bellow (section cut to show profile). The selected area (marked in blue) is a slopped roof (like a tipi tent).

When I slice this file, Bambu Studio creates a flat bottom for the roof as seen here (layer in blue):

STL file can be downloaded here: https://file.io/KS7EnY6xbWpX

How come? Am I doing something wrong?

This may be due to the 2401 non manifold edges that the slicer says it has when I put it in, and I am trying to repair it but it is taking quite a bit.

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I was able to fix it and now it slices normally
UAP-AC-IW (1).3mf (1.1 MB)

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How did you fix it if you don’t mind me asking… I don’t get a message that there is an issue with the geometry of the file…

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It just gave me the normal popup message that there are non manifold edges.

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For those unfamiliar with the term “Non-Manifold,” let me explain what likely happened.

What is a manifold? Many people are unclear about the concept of a “manifold.” In 3D printing, a manifold simply refers to an “sealed enclosed space.” For a slicer to process a 3D model, the object must be watertight, meaning there are no holes or gaps in the geometry. This is because the slicer converts the 3D model into 2D layers for printing. If there’s a hole in the model, the slicer can’t compute the points correctly, resulting in the “Non-Manifold” error.

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The original model noted above was probably created in a CAD program. When exporting to the STL format, CAD systems often create a hollow shell rather than a solid object. To avoid this, consider exporting the model in a STEP file format instead, which can sometimes but not always preserve the solid geometry better. Some CAD programs like FreeCAD and Fusion360 have export or Mesh options which will ensure a solid model.

The “Non-Manifold” error occurs when the slicer encounters an issue in the mesh of triangles that form the 3D model. Essentially, there’s a missing or misaligned piece in the mesh, preventing the slicer from properly rendering the model. Many slicers, including Bambu Studio and Orca Slicer, rely on Windows’ built-in STL repair utilities to fix these issues. If you installed the Windows 3D Builder utility before it was discontinued in 2024, you can also use it to repair models.

In this case, the issue likely stems from a common problem during the export process: the solid model’s surfaces didn’t align perfectly, often due to rounding errors. One solution is to scale the model up in your CAD software—try increasing it by 10x or 100x. Then, import the scaled-up model into the slicer. The slicer might flag the model as too large, but you can use its scale function to shrink it back down to the correct size. This method works because larger models reduce the precision required during export, and the slicer is better equipped to handle the scaling and repair.

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This really helped me understand. Thank you very much for taking the time to share it! :pray: