Why is STL 3D model flipped normals when transfer?

STL 3D models often have flipped normals during transfer due to inconsistent export/import settings or software compatibility issues. Normals control a face’s "front" direction—if the source and target software use different coordinate systems (e.g., right-hand vs. left-hand) or normal orientation rules, the normals invert. This is common when moving models from CAD tools to 3D printing slicers. To fix it, re-export the STL with "consistent normals" enabled in the source software or use a free tool like Meshmixer to recalculate normals—this solves most issues easily.