You have been told your file cannot be printed as is because it has inverse normals or geometry problems. What does that mean???
When using the term "Inverse Normals" in 3D printing, one is referring to the way a model was built. To better understand this, you have to remember that a 3D printable part has walls with two sides.
For instance, if you are printing a solid box, you will have 6 sides on the outside surface:
For the sake of this article, we are coloring all outer sides orange.
If we open the cube to see how it was built, we will see it also has inner sides. For the sake of this article, all inner sides will be yellow:
Now, if you had simple rectangles and were trying to build your cube by simply replicating the sides and forget to turn all the copies around, you could have the following issue:
Notice how one of the cube sides has the outer side (orange) inside and the inner side (yellow) in the outside. Well, that is an inverse normal and would give you the following cube:
Our printers would not be able to read the inner side, for which it would assume something is missing in the cube and the print will most likely fail or warp.
You may wonder "What if I want just one wall?" Well, if you want just one wall, because you are 3D printing, you would still need 6 sides to your part, so the same thing applies:
Always make sure all your outer sides are facing out and your inner sides facing in and remember that every object needs 6 sides... unless it is a sphere. :-)
*Please note that inverse normals are not the only geometry issues you may encounter. For other design no-to-dos, please visit our article "What are the Requirements for 3D Printing?"