KEMI Docs · 3D Printing Practice
Preparing STL Files for 3D Printing
Last updated: 2026-08-30
Most rejected print jobs — and most printed-but-won't-assemble parts — are preventable at the STL stage. Five checks, in the order we actually see problems in submitted files: units, manifold, tolerances, wall thickness, orientation.
1
Units — STL has none
The STL format stores bare numbers; millimetres is only a convention. Export in inches or metres and the recipient sees a model 25.4x smaller or 1000x larger — we have received a 5cm part that read as nine metres. The reliable habit: after export, open the file in a viewer and check the bounding-box dimensions.
2
Manifold — leaky models don't slice
Slicers cut the model into layers and decide inside vs outside. Open edges, zero-thickness faces and flipped normals break that decision — sections print hollow or solid in the wrong places. A defect-free model is called manifold (watertight). Files straight from solid CAD (e.g. Fusion 360) are usually fine; surface-modelled or mesh-edited files often are not. Auto-repair works well, but review the repaired shape once — repairs can guess wrong.
3
Fit tolerances — nominal dimensions never fit
FDM holes come out smaller and shafts larger than designed. Working starting points: loose fit 0.4mm, normal assembly 0.2-0.3mm, press fit 0.1-0.15mm of diametral clearance; screw clearance holes at nominal +0.4-0.5mm (M3 → 3.4-3.5mm). Printers and materials vary — print a small test piece for critical fits.
| Item | Recommended (FDM) | Notes |
|---|---|---|
| Minimum wall | 2× nozzle = 0.8mm; structural 1.2mm+ | thinner walls may be dropped by the slicer |
| Minimum hole | 2mm diameter | smaller: drill afterwards |
| Text/embossing | 0.4mm+ deep, 0.8mm+ stroke | engraved beats raised |
| Threads | design for heat-set inserts | tapping plastic fails with repeated use |
| Fits | 0.2-0.4mm diametral clearance | verify with a test piece |
4
Design with supports in mind
Overhangs beyond 45 degrees need support, and supported faces come out rough. Decide at design time which large flat face goes down, keep visible and mating faces away from supports, and replace bridges with 45-degree chamfers where you can — often supports disappear entirely.
5
Final checklist
- Bounding-box dimensions correct (mm).
- Part count clear — state if a multi-part file should be printed separately.
- Fit clearances designed in — identical shaft and hole dimensions mean redesign.
- No sub-0.8mm walls or tiny text.
- Material and finish stated — mechanical (PETG/ABS), cosmetic (PLA) or flexible (TPU).
FAQ
Can I send STEP instead of STL?
STEP is actually preferred — dimensions and true surfaces survive, so problems can be fixed. KEMI's quote system accepts STL, OBJ, 3MF, STEP and IGES.
How high a resolution do I need?
Enough that curves don't look faceted — an export tolerance of 0.01-0.05mm already exceeds print quality. Hundred-megabyte STLs add processing time, not quality.
Do I choose the print orientation?
Usually the printer does, for quality and strength. But note load direction (layers are weak in tension across them) and visible faces in your request notes if they matter.
See also
This article summarises the working rules KEMI uses in real design and fabrication work. For production, part datasheets and fab specifications take precedence.