How to Prepare an STL File (7 Errors That Stop a Print)

A 3D printer needs to know, for every point in space, whether it is inside the object or outside it. That sounds obvious, and it is exactly what a broken file fails to say. Here are the seven faults we actually see, in the order they cost people time.

1. Wrong units

By far the most common. STL files carry no unit information at all — only numbers. A model exported in inches arrives 25.4 times too large; one exported in metres arrives a thousand times too small. Always export in millimetres, and check the dimensions after upload: our quote tool shows the bounding box in millimetres immediately, which catches this before you pay for anything.

2. A surface, not a solid

Modelling tools aimed at visuals — SketchUp, Rhino, Blender used loosely — happily produce shells with no thickness and open edges. On screen they look like objects. To a slicer they are infinitely thin sheets with no inside. Every wall needs real thickness, and the model must enclose a volume.

3. Holes in the mesh

Also called non-manifold geometry: gaps, edges belonging to only one face, or faces that intersect each other. The slicer cannot tell inside from outside and either refuses the file or produces something strange. Most tools repair this automatically now, and the free repair options below fix the great majority.

4. Inverted normals

Each triangle has a front and a back, and the back is meant to face inward. When some are flipped, the slicer sees the object turned inside out in patches — usually printing as strange voids in an otherwise fine part.

5. Walls too thin to exist

A printer lays down lines of a fixed width — commonly 0.4 mm. A wall thinner than that cannot be printed at all, and one that is only barely thicker prints as a fragile single line. Keep walls at 1 mm or more; 1.6 mm and up for anything that will be handled.

6. Floating and disconnected pieces

Parts of the model that touch nothing, or connect only at a mathematically perfect point. These fall off during printing. Every piece needs a real overlapping join to the rest of the model.

7. Detail finer than the nozzle

Engraved text under about 4 mm tall, or embossed lines thinner than half a millimetre, will not survive. If text matters, make it larger, make it raised rather than engraved, and give it depth.

Free tools that fix most of this

• Microsoft 3D Builder (Windows, free) — open the file and it offers to repair; accept and re-save. Fixes a surprising share of everything above.
• Autodesk Netfabb's online repair service — upload, repair, download.
• Meshmixer — Analysis, then Inspector, which flags each fault and repairs them individually.
• Blender's 3D Print Toolbox add-on — free, thorough, and it explains what is wrong rather than silently changing it.

Exporting well in the first place

1. Model in millimetres from the start, so no conversion is needed.
2. Export as binary STL rather than ASCII — same geometry, a fraction of the file size.
3. Set the export tolerance fine enough that curves are smooth, but not so fine that a simple bracket becomes a 200 MB file. Around 0.01 mm deviation is a good default.
4. Combine everything into a single solid before export where you can.
5. Open the exported file again in a viewer before sending it. Half of all bad files are caught by simply looking at them.