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3DAI
Free · runs in your browser · nothing uploaded

STL Repair: Fix Broken STL Files for 3D Printing

Slicers complain when a model has holes, non-manifold edges or inside-out triangles, and the print comes out with missing walls or odd gaps. Drop your STL here: the tool finds the problems, fixes them automatically and shows the result before and after. Download a watertight STL ready for Cura, PrusaSlicer, Bambu Studio or any other slicer. Your file never leaves your device.

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How to use it

  1. 1

    Drop the STL

    Binary or ASCII .stl up to 100 MB. The check starts right away, inside your browser.

  2. 2

    Read the report

    Open edges (holes), non-manifold edges, flipped triangles and separate parts are counted before and after the repair.

  3. 3

    Compare

    Switch the preview to Before to see the hole rims in red, then back to After to check the fix.

  4. 4

    Download

    Save the repaired binary STL and slice it as usual. The model keeps its size and position.

What the repair does

Each step is the same kind of fix that desktop tools apply automatically, run in a fixed order so every file gets the same treatment.

Weld pointsCorners that sit at the same spot (within a millionth of the model size) become one shared point, so neighboring triangles connect.
Duplicates and sliversTriangles that appear twice, or have no area, are removed.
NormalsNeighboring triangles are turned to agree, and each closed part is turned so its outside faces out. An inner cavity of a hollow part keeps facing inward.
Non-manifold edgesWhere three or more triangles share one edge, or two parts touch at a single point, the sheets are separated so each edge belongs to exactly two triangles.
HolesEvery open rim is traced and closed with new triangles laid on the plane that fits the hole best.
Loose specksOptional: tiny floating parts, common in scans and AI models, can be removed.

Why STL files break

An STL is only a list of triangles, with no record of which ones should join up. Exports from CAD and sculpting apps, 3D scans, AI model generators and format conversions often leave small gaps, doubled walls or triangles facing the wrong way. A slicer needs a closed, watertight surface to know what is inside the part, so these errors turn into missing layers, stray lines or a refusal to slice.

Good to know

  • Self-intersections (a surface that passes through itself, or two parts that overlap) are not detected or fixed. Most slicers handle overlapping parts; if yours does not, Blender or Meshmixer can remesh them.
  • Holes are closed with flat or gently curved patches. A large missing area, such as half of a scanned head, is closed but not re-sculpted.
  • Files up to 100 MB (about 2 million triangles). Models over about 1 million triangles can take a minute or more, and some phones run out of memory on them.
  • STL has no units. The tool keeps your numbers exactly as they are; most slicers read them as millimeters.

Want a full 3D model instead?

Repair fixes a model you already have. To make a new one, our AI generators build a printable 3D model from a photo, a sketch or a sentence, with an STL download. They use credits; this tool does not.

Image to STL →A full 3D object from one photo.Sketch to STL →Draw it, get a printable model.Text to STL →Describe it in a sentence.

Frequently Asked Questions

Yes. It has no limit on files, needs no account and uses no credits.
No. The repair runs inside your browser, in a background worker on your own device. Nothing is sent to our servers and closing the tab discards everything.
A manifold mesh is a closed skin where every edge is shared by exactly two triangles. A non-manifold edge has three or more triangles on it, often where two parts touch along a line or a wall was doubled. Slicers cannot tell what is inside such a model, so the repair splits those edges apart.
Close every hole, join triangles that should share an edge, remove doubled triangles and turn every triangle to face outward. This tool does all four and reports "watertight" when no open, non-manifold or flipped edges remain.
In most cases, yes: holes, open edges, flipped normals and non-manifold edges are the usual causes. If the report still shows problem edges after the repair, or your slicer reports self-intersections, the model needs remeshing in a desktop tool.
Not in any way you can see. Points that already sit at almost the same spot (within a millionth of the model size) are merged, and where two parts touch at an edge or a point, the copies are moved apart by about a hundred-thousandth of the model size. New triangles are only added inside holes.
This page reads STL. Convert another format to STL first with our free converters, then repair it here.