Making a 3D face mask without making it a full head

[eniye9]

I am trying to make the back of a 3D face mask solid. I have imported the STL file but the actual face is fine. I need a way to make the back of the face the same sso that when I 3D print the face it does not fill up behind the head and it remains exactly like a mask

[TheCADWhisperer · Accepted solution]

I would use free Autodesk MeshMixer for that.