Why does a part of the 3D model disappear when it is sent to print?

Question

3D

I made a model in Blender, exported it to STL, send it to special. ON, and there the interesting begins. At first, the model is displayed correctly, but after processing (when the software shows that it will actually print, honestly, I am not strong in the terminology of this area), part of the model is suddenly truncated, moreover, in rather unexpected places. How to be and what to fix?

Answer 1

Open Model - Splice Vertices/Remove Duplicates
Recalculate Normals