I created an object (gear with hole) to print with my 3D printer. I don't know the reason but when I want to print it, the hole of gear disappears.
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You have geometry made out of a few separate closed volumes that are overlapping:
That is definitely not a good idea. You should have one continuous volume per object. I can only guess how your printing process works, however the printer will probably need a continuous volume so the software is probably trying to automatically guess what it is using Boolean operations that do not work well with overlapping geometry that is also present in the model. You should simply model the object as a single continuous volume that has one continuous surface that does not overlap with itself. You should not have any overlapping geometry(vertices, edges or faces), there should also be no holes or non-manifold geometry in your models if you wish to avoid problems when 3d printing. In some cases printing software may fix the issues automatically, but obviously, if you wish the process to work every time as expected, you should ensure the geometry is tidy and suitable for printing in the first place.