A file can open in a viewer and still be unusable for additive manufacturing. What matters is not the extension, it is whether the geometry closes, its scale, and what the surface implies.
Watertight: the non-negotiable
A watertight (manifold) mesh has every edge shared by exactly two faces. Slicers infer the interior from that surface. If the mesh leaks, the slicer either refuses to print or silently fills the hole some way it does not want you to learn about later.
- Non-manifold edges and duplicated faces are the usual culprits after boolean operations.
- Even 0.02 mm gaps can produce a part that looks right in preview and fails in the slicer.
Scale: the silent killer
STL stores nothing about units. A part drawn in inches can arrive 25.4 times too small, or too large, and print that way, because the slicer assumes millimeters. We report the bounding box we see and confirm it against your intent before anything is sliced.
Clearances and holes
Tight fits printed face-to-face will bind; that is physics, not a defect. We dimension holes and press fits for the material and use, and we flag when a design assumes metal tolerances.
The slicer can repair some things. It cannot invent intent.
Slicience editorial
So 'ready to print' in our queue means: closed mesh, millimeter scale confirmed, clearances reviewed against the material, and a chosen orientation with supports minimized. Everything else is a conversation worth having before plastic is spent.