stl editing - Converting inner cavities to solid

  • Hi all! 🙂

    I hope there are some experts on stl editing here since I have an unusual question:
    I have an stl body which is a mold for casting and I want to extract the part that results if one casted material into that mold.
    I've already tried several methods like inverting the normals of the facets and the cutting away the outside bouandaries but the problem is that the mold comes from Slicer For Fusion 360 and thus consists of several layers with boundaries in between. If I cut away the otuside boundaries the boundaries between the layers still remain and I can see the inside model only from an orthogonal prespective.
    That means I have to somehow invert the mold so that only the inner cavity remains...

    To clarify the question a little more: I have a "normal" mold for metal casting as an stl file with all draft angles and stuff which is normally manufactured with sand and a wood model. But my goal is to slice this mold into 2D slices which can be cut with 2D toolpaths. The problem here of course is that all the angles orthogonal to the slice direction get lost (which is intended) but I want to visualize how the cast piece will look after that.
    Does anyone have an idea how to do that?

    Thanks in advance! 🙂

  • A bit unclear what you actually have as you say its an .stl and its from a slicer, doesn't compute.

    But if I assume you have an .stl file, which is a negative; convert it to a solid and use the combine tool to cut the now solid from a rectangular block of roughly the same size. However converting an .stl to a solid is not a good way to do edits and it will likely be painfully slow if it works at all.

    edit: autocarrot...

  • Just remembered the CNC kitchen guy recently published a video that is sort of opposite, but may help. Worth a watch in any case.

  • It is easy in OpenSCAD. You would so something like this:

    difference() {
        cube([ dimensions just a shade smaller than the mould ]);

    The result would be the part and could be exported as another STL. Caveats are OpenSCAD is very picky about the STLs it imports, they must be perfectly manifold with no degenerate triangles. You can't get at the STL dimensions or offset, to you would have to manually place the cube by trial and error.

  • Here's a way to move the stl to the origin:

  • @bearer "Slicer For Fusion 360" is not a slicer as used in 3D printing, instead it lets you cut 3D objects into several parts that can be manufactured by means of 2D cutting (laser, waterjet..). The slices therefore are several millimeters thick and not like 0,2 mm.
    And you are right about converting an stl to a solid; it is very slow (especially with a high triangle count). But I assume there are programs to edit stl files (like Meshmixer) which can do boolean operations but I didn't manage to find the right option..

    The video from CNCkitchen is very nice (I love this channel :D) but unfortunately doesn't really help my case since I have to work with stl files and not with nice parametric cad models (stl sucks by the way 😬 ).

    @nophead I will look into doing it with OpenSCAD. Thanks for the tip!

  • Moderator

    So you have CAM paths from fusion 360 to produce the part with a CNC? Or do you have an STL?

    Can you post the actual file you have to work with?

  • It'll be an stl, but looking a bit like a 3d print. this has some example pics:

  • @phaedrux I have an stl file. But I'm not sure if I can post it since it partly has to do with stuff in my university and I don't know if it's somehow confidential.. But @bearer posted a link where you can see the process very nicely (thanks for that btw 🙂 )

  • OpenSCAD difference is probably your best bet. If OpenSCAD won't load it, run it through one of the online repair tools. Even if you've never used OpenSCAD before... it is free, and this use is extremely easy.

    You said you can't post the STL on a forum. Or your not sure. I get that. If you are willing to email it to

    danal (dot) estes (at) gmail (dot) com

    I'll give it a shot, and send you the OpenSCAD program (as well as the resulting "fill" STL).

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