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I am very new to Blender without any 3D design background. I am designing a solid object as shown in Figure 1 and Figure 2. Figure 1 Figure 2 However, when I export the object to STL file and view it by Preview in Mac OS, the object appears to have an unwanted hollow (Figure 3). Another object that I design in the same project has no problem Figure 3 Therefore, could you please help me to solve this problem? Thank you very much.

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I guess your Mac Viewer shows only faces, oriented to camera, while Blender (by default) shows both sides

For viewing what is the back side and what is front, you should turn face orientation overlay

enter image description here

You can recalculate face orientations using Shift+N, but your object should be manifold.

You can also use Alt+N to flip orientation face by face

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  • $\begingroup$ Thank you for your answer. Does your answer mean the Mac viewer (Preview) is showing the STL wrongly and my STL object is actually solid? $\endgroup$ – Quang Dong Dec 31 '20 at 11:43
  • $\begingroup$ Well, it is a tricky question. It is not an error that the Mac viewer shows polygons only from one side, it is a standard optimization method. On the one side STL is solid, and the Mac viewer shows these polygons, but on another side (when they are blocked by other parts of the model). On the other side STL is not solid, it is non-manifold, because on the borders of back-faced polygons and front-faced polygons normals of faces turns 180 degrees. So it is not wrong or bad, it is how it has to be. You should turn all faces in a correct way, so the Mac viewer can show the model properly. $\endgroup$ – Crantisz Dec 31 '20 at 11:53
  • $\begingroup$ I have tried to recalculate face orientations and it works. Thank you for your help. To be honest, I still don't fully understand the role of face orientation, but the problem is solved for now. Thanks again. $\endgroup$ – Quang Dong Jan 1 at 3:39
  • $\begingroup$ en.m.wikipedia.org/wiki/Back-face_culling $\endgroup$ – Crantisz Jan 1 at 9:41

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