STL files: six errors that keep coming back

1. The scale is wrong

By far the commonest problem. The STL format carries no unit: it stores numbers, and the software decides whether they mean millimetres, centimetres, inches or metres.

Classic outcome: a part exported from software set to metres arrives a thousand times too small. Or the reverse — we have received a 295 mm hub which, badly exported, measured 295 metres.

The remedy is simple: before sending, note the main dimension of your part in millimetres and state it in your message. We compare, and if the discrepancy is a factor of 10, 25.4 or 1,000, the diagnosis is immediate.

2. The mesh is not watertight

An STL describes a closed surface made of triangles that all touch. When edges are left free, the surface has holes: the slicing software no longer knows what is inside and what is outside.

This happens often with scanned models, with files converted from older formats, and with parts assembled from several bodies that do not merge cleanly.

Our online quote detects the fault by counting edges: every edge must belong to exactly two triangles. We repair it in most cases, but a badly damaged mesh needs rebuilding in CAD, which is charged.

3. The walls are too thin

Below 0.8 mm the nozzle can no longer lay material cleanly: the wall becomes a series of broken strokes, or disappears from the sliced file altogether.

The classic trap is the stiffening rib drawn at 0.5 mm in CAD because «that is plenty». It will not come out. Allow 1.2 mm minimum for a wall that has to hold something, and 0.8 mm as an absolute floor for a purely cosmetic wall.

Our online analysis calculates the average wall thickness of the part and warns you when it drops below that threshold.

4. Overhangs exceed 45°

Molten plastic needs something to rest on. Beyond 45° from vertical, the material has nothing beneath it: supports have to be built, which cost material and time and leave marks.

Many overhangs can be removed upstream, with a draft angle, a chamfer under a horizontal hole, or simply by reorienting the part on the bed. We do that optimisation as a matter of course, but it works better when the part was drawn with printing in mind.

Our 3D viewer colours overhanging faces red: you see immediately where supports will be needed.

5. The normals are inverted

Every triangle has a direction: an outer face and an inner one. When some triangles are flipped, the software believes the inside is the outside, and fills the part the wrong way round.

The symptom is visual: in most viewers, flipped faces appear black or vanish. It is a common fault in files that have been converted repeatedly, and it corrects automatically in nearly every case.

6. The mesh is too coarse — or too fine

Too coarse and a cylinder becomes a polygon: you see the facets, and diameter dimensions are wrong. Too fine and the file reaches hundreds of megabytes with no visible gain, because the mesh precision far exceeds that of the machine.

The right export setting sits around 0.01 to 0.05 mm chord tolerance. A file of 5 to 50 MB is normal; beyond 100 MB there is usually a setting to revisit.

The simplest answer is often STEP

If your part comes from CAD software, send a STEP instead. It keeps the exact geometry rather than approximating it with triangles, it carries its units, and it lets us correct a dimension without starting over.

STL stays perfect for anything modelled for printing. For anything coming from engineering, STEP avoids half the problems on this page.

Also worth reading

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