Slicer errors explained: what STL warnings mean and how to fix them

Checked on 1 October 2026. Warning texts are quoted from the slicers' own help pages and, where those paraphrase, from the text files the programs ship with (PrusaSlicer 2.9.6 and the current Bambu Studio source). Wording can change between versions.

Slicer warnings about an STL almost always mean the mesh is not a clean, closed solid: some edges belong to only one triangle (holes, open edges), some to three or more (non-manifold), some triangles face the wrong way (flipped normals), shells overlap (self-intersections), or part of the model starts in mid-air (floating parts, empty layers). The first four are mesh problems you fix in the file; the last is about orientation and supports. To see exactly which one you have, run the file through the STL analyzer, which counts open edges and bodies, then repair the mesh with STL Repair if it needs it.

The one rule behind most warnings

A printable mesh is a closed surface where every edge is shared by exactly two triangles, and every triangle's normal points outward. Slicers need that to decide what is inside the part and what is air. When the rule breaks, the slicer has to guess, and the guess shows up as a missing wall, a filled hole or a layer that disappears.

What each slicer says

The wording below is from the English interface; %d stands for a number.

PrusaSlicer

• Holes, open edges: "%d open edges", under a warning icon in the object list.
• Non-manifold and other errors: "Auto-repaired %d errors" or "Remaining errors".
• Flipped normals: "%d facets reversed".
• Floating parts, empty layers: "Floating object part" and "Empty layer between ... and ...".

Bambu Studio

• Holes, open edges: "Info: %d open edges".
• Non-manifold edges: "Error: %d non-manifold edges, %d non-manifold vertices".
• Flipped normals: "Error: reversed faces detected, which may affect rendering".
• Floating parts: "...has floating regions. Please re-orient the object or enable support generation."

UltiMaker Cura

• Holes, non-manifold edges, flipped normals: one "Model Errors" message, with the missing or extraneous surfaces highlighted on the model.
• Floating parts: we found no dedicated message in UltiMaker's documentation.

Prusa's knowledge base article on corrupted models explains that PrusaSlicer repairs models automatically to a certain degree and offers a further fix from the warning icon; the exact strings are in the PrusaSlicer translation template. Bambu Lab's Fix Model page says Bambu Studio shows a warning symbol in front of the object name and that its deeper repair uses a Windows service, so it is only available on Windows. Its strings are in the Bambu Studio translation template. UltiMaker's support article on model errors is blunt: Cura cannot fix loaded models for you, and slicing a non-manifold model may lead to unexpected print results.

Each problem, and what to do about it

Holes and open edges

Typical sources are a missing face in a modelling program, a scan with gaps, or an export that dropped tiny triangles. Small holes are closed by every auto-repair. Large openings, such as a missing base on a scan, need a decision about where the surface should be, which a repair tool can only guess.

Non-manifold edges

Three faces meeting on one edge usually come from two objects that touch along an edge, or an internal wall left inside a model. Repair separates those edges so each belongs to two triangles again.

Flipped normals

A triangle facing inward tells the slicer that the inside is outside in that spot. Bambu Studio's message mentions rendering; a whole shell with inverted normals can also be read as a hole rather than a solid. Most repairs reorient triangles to match their neighbours.

Self-intersections and overlapping shells

Two bodies pushed into each other without being joined are common in downloaded models and exported assemblies. Cura has a Union Overlapping Volumes setting that is on by default, as its mesh fixes article describes. The clean fix is a boolean union in the source model.

Floating parts and empty layers

Here the mesh can be perfect. A part that starts above the first layer, or a gap between two bodies, leaves layers with nothing to print. Rotate the model, enable supports, or cut the bottom flat in the slicer.

Stability alerts

PrusaSlicer can also warn about a "Thin fragile part" or "Low bed adhesion". These are not mesh defects either. Prusa's preferences article describes them as issues in the sliced object that supports or a brim can resolve, such as floating parts, unsupported extrusions and low bed adhesion. Repairing the STL will not make them go away; a brim, supports or a different orientation will.

How to fix an STL the slicer complains about

1. Read the message. Note whether it names open edges, non-manifold edges, reversed faces, or floating regions and empty layers. The last group is not a mesh problem.
2. Measure the file. Drop it into the STL analyzer. It reports whether the mesh is watertight, how many open edges it has and how many separate bodies it contains.
3. Repair the mesh. Upload it to STL Repair. It removes broken and duplicate triangles, separates non-manifold edges and closes holes, without moving any vertex, and lists what it changed.
4. Check again. Run the repaired file through the analyzer or reload it in your slicer. The warning should be gone.
5. Handle what repair cannot. Floating parts need orientation or supports; overlapping bodies need a union; a large missing area needs work in a modelling program.

The loose shard is still in the repaired file, because a separate small body is not an error to the mesh; it is something to delete in the slicer. If your slicer flags a mesh problem now, repair the STL online, confirm the result in the STL analyzer and load it again.

Limits of automatic repair

Repair closes openings by adding triangles across the existing boundary. That is right for small holes and wrong for a large missing surface, where the result is flat even if the original was curved. It also cannot tell which of two overlapping bodies you meant. If a model fails repeatedly, go back to the source: the non-manifold edges and holes tutorial covers fixing it where it was made. Broken meshes also cause trouble when you split an STL for a smaller bed or reduce its file size, so repair first in both cases.