Published 2026-09-04 · 3 min read
Seven STL export mistakes that ruin your print
A slicer does not see a nice model. It sees a pile of triangles. If those triangles do not close a volume perfectly, the program has to guess what is inside and what is outside — and guessing goes badly. Here is what usually breaks, and how to fix it.
1. The model is not watertight
A good STL describes a closed volume: every edge is shared by exactly two triangles. When one triangle is missing there is a hole, and the slicer no longer knows where material ends. You get missing walls, or infill in places that should be solid.
Holes usually come from open surfaces: you drew a plan, extruded half of it, or deleted a face to look inside and forgot about it.
- Never export surfaces — only solids.
- If your editor reports the volume, check it: zero or negative volume means an open shell or flipped normals.
- Real boolean operations (not grouping) always leave a closed solid.
2. Flipped normals
Every triangle has an outward face. When one is turned around, the slicer thinks inside is outside and produces strange layers, often on just part of the model.
This happens most after mirroring on one axis: a plain mirror reverses the winding of every face. Serious editors fix it automatically; simple ones do not.
3. Overlapping faces from grouping
Push two cubes together and group them, and two coincident faces stay inside. The slicer reads them as an internal wall — and prints it, wasting material and time.
The fix is a real boolean union, which deletes interior geometry, not a group that merely holds parts together.
4. Wrong scale
STL carries no units. A file drawn in centimetres arrives in the slicer read as millimetres, and the part comes out ten times too small.
Model directly in millimetres. If a file comes from someone else, measure a known dimension before printing — a 30-second check that saves hours.
5. Too few or too many triangles
A sphere with 20 segments is visibly faceted. The same sphere at 512 segments produces a file of tens of megabytes that the slicer chews on for a minute with no visible gain: printer resolution is 0.1–0.2 mm anyway.
Practical rule: raise the segment count until edges stop showing at the real size of the part, then stop.
6. Walls below the minimum thickness
A 0.4 mm wall with a 0.4 mm nozzle is borderline: if the slicer rounds down, the wall disappears from the print entirely. Below 0.8 mm nothing is safe.
Check thin walls before export, not after the part comes out perforated.
7. The part does not sit on the bed
A model floating 3 mm above the zero plane arrives in the slicer floating. Some slicers drop it automatically, others add support all the way up.
In GrafitGo objects sit down on the floor when you create them and when you change their parameters, precisely so this surprise never happens. At export you see the volume and estimated PLA mass — if the mass looks absurd, the model is wrong, not the printer.
Which is better, STL or 3MF?
3MF keeps units, colours and fewer rounding errors. STL stays the most compatible: every slicer opens it. If your slicer supports 3MF, use it; otherwise binary STL, not ASCII.
How do I repair an STL someone sent me?
Modern slicers have automatic repair that closes small holes. For bigger problems, rebuild the part parametrically — often faster than patching triangles.
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