The whole conversion makes sense once one table is in your head: every question about OBJ to STL is answered by which layer of the file you care about. Here is that table, then the step-by-step, then the three traps that cause almost every failed print blamed on the format.
What each format keeps
| Layer | OBJ | STL |
|---|---|---|
| Triangles | Yes | Yes |
| UVs | Yes | No |
| Materials and colour | Yes, with the MTL file | No |
| Unit system | No - convention only | No - millimetres assumed |
| Multiple parts | Groups | Loose shells in one file |
| Slicer-native | Usually converted first | Always |
The conversion, step by step
01 - Import and measure. File, Import, Wavefront OBJ - then read the object dimensions in the sidebar before anything else. You are looking for real-world numbers: a tabletop miniature lives around 28 to 32 mm tall, a desk prop in the centimetres. If the numbers are not in that league, fix the scale now, not in the slicer.
02 - Set the scene units. In Scene Properties, choose the unit system and unit scale that match the numbers you just read, and keep them for the whole session - switching units mid-project is how 1000x errors sneak in.
03 - Repair what a printer cannot forgive. Holes in the surface, flipped normals, zero-area faces and paper-thin shells. The 3D-Print Toolbox finds all of them; the fixes are quick when the mesh is simple and worth scripting when it is not.
04 - Export. File, Export, STL. Tick Selection Only, keep Scale at 1.0 if the scene units are already honest, and name the file like the print queue depends on it - because it will.
The three traps
Scale. STL has no unit field; slicers assume millimetres. A model that exports 0.03 units tall arrives as a speck, and one exported in metres arrives as a building. This is the number one STL-is-broken complaint, and it is never the format - always the scene.
Non-manifold geometry. A printer prints a watertight solid, and a hole in the surface is a hole in the print. Look for boundary edges, internal faces and vertices floating between shells; the toolbox lists them so you can fill, bridge or delete with intent.
Paper-thin shells. AI OBJs often arrive as open shells - beautiful in the viewport, unprintable on the bed. Solidify gives thickness; check the wall against the nozzle or resin detail limit before committing to a print run.
Before you slice: a thirty-second audit
Open the 3D-Print Toolbox in the N panel and press Check All. The report should come back clean - or every item on it explained. Non-manifold edges get filled or bridged, thin walls get thickened, overhangs deserve a second look before supports. Thirty seconds here saves a failed six-hour print.
When 3MF is the better container
STL is the lingua franca and it is not going anywhere, but two situations deserve 3MF: multicolour prints, where 3MF carries the colour while STL cannot, and multi-part kits, where a single 3MF keeps every part exactly where it snapped together. Export both when a client is the audience; export 3MF when only a printer is.
When the OBJ comes from an AI generator, expect the same three repairs every time: density that needs a decimate, shells that need solidify, and a scale that needs a reality check against a ruler. None of it is hard - it is the tax on the shortcut, and paying it in Blender takes minutes.