While an important aspect of 3d modelling for 3d printing , checking and fixing flipped normals in your 3d model may not be an immediately thing to check out and do. This can result in models that would fail or or would materialized in an unexpected fashion when 3d printed.
Luckily, these are quite easily checked for and fixed in Rhino3D. Here’s a run-through of that scenario and process
Here’s a simple low poly model in Rhino3D which looks pretty typical. But when you bring it into a slicer program, you may see something like this:
One of the front facing surfaces of the piece seems to be missing/invisible! Flipped normals are the culprit here! Normals represent the directionality of a surface. Unfortunately, for that particular problematic surface, it was inconsistent with the other surfaces which make up the model.
Using the dir and flip commands
In Rhino, one can check the directions of the normal by using the dir command. This will show white arrows indicating the normals of each surface. Evidently, they are inconsistent on the front facing surface here.
Here the dir command was used on just 2 of the surfaces to show more clearly the issue at hand. Thankfully the fix for this is as simple as running a flip command on the offending surface, or to just click on it when the dir comamnd is showing the arrows. Further, it is always also good practice to join all the surfaces into a closed polysurface when bringing it out for 3d printing and typically that would help to make all the normal directions consistent.
Once the fix has been applied, the 3d model can be sliced as expected, as the corrected model on the right shows:
Changing the Backface Color
Now, it’s rather tedious to have to check for normals actively and only catch it during the slicing process. So thankfully, there is a setting in Rhino3D which will allow for the quick spotting of any flipped surface surfaces.
This is the “Color Backfaces” option in the “Display” panel on the left. Setting the further option of the color to be something like a loud, neon green make any problematic surfaces stand out more too! So, it should be easy to fix as soon as it happens.