For 3D printing a solid object has to be seen by the slicing program as a “manifold” object. Manifold means the geometry has no open or mis-matched edges. Keep in mind that an STL file is really a collection of connected triangles (in other words, it is a mesh) , and if any of the triangles don’t match up with adjoining edges and/or corners, the resulting STL file will not be manifold.
If the object is not manifold the STL file is considered to have errors (like cracks, holes, mis-matched edges) and, depending on the slicer, it either will not be sliced at all, or if it is sliced the resulting GCode will not print properly.
The way slicers work is to fill the inside of a solid object with what is called “Infill”. Infill is meant to “fill up” the empty space inside a solid object so it can have a top printed that won’t collapse due to no support underneath. Here’s a screenshot of my slicer showing what’s been printed at a height of 3.2 mm of a “solid” cylinder that is 50 mm in diameter and 50 mm high.
The dark red cross-hatch material is the infill; it is the so-called “grid” pattern set at 20% infill. That means only 20% of the open volume will be filled with printed material. Here’s what the finished print will look like:
The top surface will be 100% solid and the finished print will look like a solid cylinder, but the inside volume will be 80% empty space. This is done to minimize both print time and the amount of filament used to complete the print. Of course if a truly solid final print is needed you specify 100% infill.
The key to success is to present your slicer with only Solid Breps from GH/Rhino. In some cases these are not easy to make because of tiny tolerance issues with Rhino geometry. Sometimes I’ve been able to fix these kinds of problems with either the GH Solid Union command or Rhino’s Solid/Union command (these might be the same thing), but usually, if the resulting BRep is not closed, I have to use an external program that is designed to fix problematic geometry. The one I find most effective and easy to use the the Windows 3D Builder program. It uses the same algorithms as the well-known Netfabb program.
Finally, if your finished geometry looks good on screen but is not manifold, and if a program like 3D Builder can’t fix it (yes, this really does happen) , it means you’ve got some sort of mistake in the way you constructed you geometry. Mistakes like this can be hard to find, but they can be found and corrected. Fortunately GH makes it easy to try different approaches, or, as I’ve done a few times, start over completely and re-build the geometry from scratch.