If a model is not printing how you expected, it may be an issue with the .stl file itself. If the file was found online, it may be best to search for a similar design and try printing that.
If you are interested in learning more about using 3D modeling to fix issues, the resources in this section may help.
One way to check for potential problems with the .stl file is to compare the model in Cura's layer view, x-ray view, and solid view for abnormalities.
If a print looks okay when viewed in Cura's solid view, but parts (or, in the case below, the whole model) are not showing up in layer view, it may be that those parts are too thin to print. The printer can only print down to half the diameter of its nozzle. Since all nozzles on the LulzBot printers are .35mm, the smallest size possible is about .18mm. This is also the the size of the layer height for the high detail profile.
One fix is to scale the whole model to a larger size until all parts are large enough to show up in layer view.
If scaling the whole model up is not option, you may need to bring the file into a modeling program to edit the parts that need to be thicker.
This Sculpteo page defines non-manifold objects in the following way:
Image courtesy of Sculpteo
Non-Manifold also refers to objects, like the one below, which have holes in the mesh. Since the walls of the mesh do not have a defined thickness, the holes make it so the object cannot exist in the real world. To repair these holes in Meshmixer, check out this tutorial from Pinshape or this one from PrusaPrinters Blog.
Image courtesy of 3D Hubs
This LulzBot tutorial explains reversed normals in the following way:
"Every plane composing each shell of a 3D model has an intended inside and an outside, called a 'normal.' When a plane’s orientation is reversed in relation to those around it, it’s referred to as a 'reversed normal.' This becomes an issue when it occurs unintentionally within a model, and ranges from a tiny section of a shell, to intersecting shells of a complex multi-shell model, to an entire model with a single shell that is all reversed.
Reversed normals in the sphere on the side of the cylinder have caused inaccurate slicing that creates an unintended void."
To fix the issue in Fusion 360, follow this link.
To fix the issue in Meshmixer, watch this video.
Another possible issue happens when trying to load a very large file into Cura. This can cause Cura to give an error message of "failed to load .stl file" or to crash.
As explained by 3D Hub: "A mesh is 'over-refined' when the total number of triangles of the STL mesh is larger than required. This will not lead to any errors during 3D printing, but it will unnecessarily increase the size of the STL file, making it more difficult to handle. Usually, the tiny details that are represented by an over-refined mesh cannot be 3D printed, as they exceed the capabilities of most systems (in terms of accuracy and minimum feature size)."
To reduce the triangle count in Meshmixer, check out number four in this tutorial from Pinshape.
Image courtesy of Pinshape