How to make an STL file for 3D printing
STL is the format every slicer accepts, and making one is the easy part — the file will export whether or not it will print. What matters is the handful of checks between finishing a model and exporting it, which is where almost every failed print is decided.
The short answer
Model or sculpt the shape, remesh it so the surface is watertight, thicken anything below about 2 mm at final scale, decimate if the triangle count is unwieldy, then export STL. The critical property is watertightness — a single closed surface with no holes or self-intersections — because a slicer works by asking whether a point is inside or outside the model.
Step by step
- 1
Make the shape
Sculpt organic subjects — miniatures, busts, creatures, terrain, ornaments — with brushes, since these are shapes you judge by eye. Use parametric CAD instead for mechanical parts with real tolerances. Ignore print preparation entirely while you work; it is a finishing step, not a constraint to model under.
- 2
Remesh to make it watertight
Run a voxel remesh. This rebuilds the whole surface as one continuous closed shell, eliminating holes, gaps and self-intersecting faces in a single operation. This is the step that decides whether the file prints, and it is why sculpts that look perfect on screen still fail in a slicer.
- 3
Thicken anything fragile
Build up or inflate any part thinner than roughly 2 mm at final print scale — fingers, blades, antennae, tails, thin ornament. If it looks delicate on screen it will snap in your hand, however successfully it printed.
- 4
Check the detail scale
Detail below about 0.4 mm at final size will not resolve on a filament printer. Fine pores and hairline scratches are wasted effort unless you are printing in resin, which is exactly why tabletop miniatures are resin-printed.
- 5
Decimate, then export STL
Slicers slow badly on multi-million-triangle meshes and cannot reproduce that detail anyway. Decimate to hold the silhouette at a manageable count, then export STL. Remember that STL fuses everything into one solid — it has no concept of separate objects.
What STL actually stores, and what it doesn't
An STL file is a list of triangles. That is genuinely all it is: each triangle's three corners and the direction it faces, repeated until the surface is described. There is no colour, no material, no texture, no UV data, no object names and no hierarchy.
This is why STL is both universal and limited. Every slicer on earth reads it because there is almost nothing to misinterpret, and it is useless for anything except geometry. If you want to keep separate named parts, colour or materials, export OBJ or GLB alongside it and treat the STL as the print-only output.
It also explains a common surprise: exporting a multi-object model to STL fuses everything into a single solid. That is correct behaviour for printing — the printer builds one physical object — but it means STL is a one-way door. Keep your source file.
Why watertight matters so much
A slicer converts your model into printer instructions by repeatedly asking a simple question: at this point in space, am I inside the object or outside it? If the surface is a single closed shell, the question always has an answer. If there is a hole, a gap, or a face that passes through another face, it does not — and the slicer will either refuse the file, silently produce nonsense, or print something with missing walls.
Meshes become non-watertight in ordinary ways: joining two shapes that overlap, pulling a form so far that the surface folds through itself, deleting geometry and leaving an opening. None of these are visible in a render, which is why the problem is usually discovered at the slicer.
A voxel remesh solves all of them at once by rebuilding the surface from the volume rather than repairing the existing geometry, which is why it is a more reliable fix than any mesh-repair tool.
Frequently asked questions
Can I make an STL file online?
Yes. Mold sculpts, remeshes and exports STL in the browser with nothing to install, on any machine with WebGPU including Chromebooks and tablets. Sculpting and remeshing are free; downloading the STL is part of a paid plan.
Why does my STL fail in the slicer?
Almost always because the mesh is not watertight — there is a hole, a gap, or geometry passing through itself. A voxel remesh rebuilds the surface as one closed shell and fixes the whole class of problem in one operation. The next most common cause is parts too thin to print.
How many triangles should an STL have?
Enough to hold the silhouette smoothly and no more. Multi-million-triangle files slow slicers dramatically while the printer reproduces far less detail than that. Decimating to something in the low hundreds of thousands is usually invisible in the print and much faster to work with.
STL or OBJ for 3D printing?
STL, in almost every case — it is universal, unambiguous, and stores exactly what a printer needs. OBJ is the better choice when you want to keep separate named objects or carry the model into other software, so exporting both is a reasonable habit.
Can I 3D print a sculpt straight out of a sculpting tool?
Usually not straight out — sculpts are dense and frequently not watertight. Remesh, thicken fragile parts and decimate first. Those three steps take a couple of minutes and prevent nearly every avoidable print failure.
Sculpt it, remesh it watertight and export STL — all in a browser tab.