How to split an STL into parts in Blender (and keep every piece printable)
Quick answer: import the STL (File ‣ Import ‣ STL), go into Edit Mode and select all. Run Mesh ‣ Bisect, and in the operator panel tick Fill so the cut faces are closed. Then Clear Outer to keep one half; undo and Clear Inner for the other (or separate with P ‣ Selection). Export each with File ‣ Export ‣ STL, Selection Only. Bisect is one flat plane per cut and makes no connectors. For pegs, you add cylinders with a Boolean modifier by hand. If you need "fits my printer, with connectors, all pieces watertight", a splitter that plans cuts from the printer size does it in one step.
- Blender is free and precise, and Bisect with Fill produces clean, closed pieces. The cost is time: every cut, connector and export is manual.
- The three mistakes that produce unprintable pieces: Bisect without Fill (open shells), a Boolean on a mesh that was not manifold to begin with, and exporting without Selection Only (every piece contains every other piece).
- Blender 4.2 and later: install 3D Print Toolbox from Edit ‣ Preferences ‣ Get Extensions for the manifold check and one-click export.
When Blender is the right tool
Blender is the right choice when the cut must be somewhere specific — along a sculpted fold, through a hollow bust at an angle, around a feature you want to keep in one piece — or when you want to edit the pieces afterwards. It is the wrong choice for the routine job of "this is 320 mm tall and my printer does 250": you would be placing planes by hand, typing peg cylinders, running Booleans and exporting six times, when a tool built for that reads the printer size and does it in one pass. Most people end up using both: Blender for the one awkward cut, a splitter for the grid.
Step by step: split an STL in Blender 4.x
- Import. File ‣ Import ‣ STL (.stl). Leave Validate Mesh on; it repairs corrupt data on the way in. The model arrives at its native size in metres — a 200 mm part shows as 200 units, which is fine for printing as long as you export at the same scale.
- Check it is watertight before you cut. Open the N-panel ‣ 3D Print Toolbox (Blender 4.2+: Edit ‣ Preferences ‣ Get Extensions, search "3D Print Toolbox", install) and click Check All. "Non-manifold edges: 0" is what you want. If it is not zero, Make Manifold often fixes it; a Bisect or Boolean on an open mesh produces open pieces.
- Place the cut. Tab into Edit Mode, A to select all, then Mesh ‣ Bisect (or Toolbar ‣ Knife ‣ Bisect). Drag a line across the model in the viewport — the first cut is only to create the operator; precision comes next.
- Set the plane numerically. In the Adjust Last Operation panel (bottom-left, or F9) type the Plane Point and Plane Normal: for a horizontal cut 120 mm up, point (0, 0, 120) and normal (0, 0, 1). Hold Ctrl while dragging to snap the line to 15° steps if you prefer dragging.
- Tick Fill. This closes the cut with a cap face; without it both halves are open shells that most slicers will refuse or print hollow. Blender fills the cut "with materials, UV maps and colour attributes based on the surrounding geometry", so painted colour survives.
- Keep one side. Tick Clear Outer and you are left with the lower piece. Press F to flip which side counts as inner if it kept the wrong half. (Alternative: leave both, then select the faces of one half and press P ‣ Selection to make it a separate object — handy when you want both pieces in the scene.)
- Make the other piece. Ctrl+Z back to the uncut mesh, repeat the Bisect with the same point and normal (the panel remembers them), tick Fill and Clear Inner. Rename the two objects so you can tell them apart in the exporter.
- More cuts. Each extra plane is another Bisect on the piece it crosses. A 2 × 2 × 2 grid is seven Bisects, each done twice (one per side), each with its own Fill. Keep a note of plane values; the pieces must share them exactly or the caps will not mate.
- Connectors (optional). Add ‣ Mesh ‣ Cylinder (e.g. radius 2.6 mm, depth 17 mm for a 5 mm peg with 0.1 mm clearance) placed so it straddles the cut. On each piece add a Boolean modifier, Difference, with the cylinder as object, Solver Exact, and Apply it. Print the peg itself as a separate 5 mm cylinder, 16 mm long. Two per face stops rotation.
- Check again, then export. 3D Print Toolbox ‣ Check All on each piece (non-manifold edges must be 0 after the Boolean). File ‣ Export ‣ STL with Selection Only ticked and only that piece selected — or set Batch Mode to Object to write one STL per object in one go. Check Apply Modifiers if you left any unapplied.
The three ways this goes wrong
- Open pieces. Bisect without Fill, or a Boolean on a non-manifold input. Symptom: the slicer shows a hollow shell or reports errors. Fix: undo, repair the source with Make Manifold, cut again with Fill.
- Pieces that do not mate. Two halves cut at slightly different plane values (dragging twice instead of typing the numbers). Always type the Plane Point.
- Every file contains everything. Exporting without Selection Only. Each STL then holds all the pieces stacked at their original positions — the slicer prints the whole model again.
Blender vs a dedicated splitter
| Task | Blender 4.x | splicestl |
|---|---|---|
| Cut at an arbitrary angle or along a feature | Yes — Bisect at any plane, knife, Boolean with any shape | No — axis-aligned planes (seam-aware placement moves them along the axis) |
| Fit pieces to a printer automatically | No — you compute the plane positions | Yes — from 152 printer presets or a size |
| Multi-axis grid in one operation | No — one Bisect per plane per side | Yes |
| Connectors | By hand: cylinders + Boolean, per socket | Pegs, dovetail, tenon, finger, magnet, dowel, screw, your own STL; placed and keyed automatically |
| Watertight check | 3D Print Toolbox (extension) | Every piece verified by Manifold before export |
| Export | One STL per object (Batch Mode) or Selection Only per piece | One zip: pieces laid flat, connectors, plates, README, assembly sheet, labels |
| Edit the pieces afterwards | Yes, fully | Rotate / scale / mirror / hollow / simplify only |
| Cost, install | Free, ~300 MB download | Free, browser |
A sensible workflow for a big sculpt: do the one artistic cut in Blender (say, separate the head from the shoulders along the collar line), export the two bodies, then drop each into the splitter to grid them to your bed with pegs.
Frequently asked questions
How do I cut an STL in half in Blender?
Import it, Tab into Edit Mode, A to select all, Mesh ‣ Bisect, drag a line, then in the operator panel type the Plane Point and Normal, tick Fill and Clear Outer. Undo and repeat with Clear Inner for the other half. Export each with File ‣ Export ‣ STL, Selection Only.
Why are my Blender-cut pieces hollow or broken in the slicer?
Bisect was run without Fill, so the cut left an open edge loop, or the source mesh was not manifold before the cut. Check the source with the 3D Print Toolbox extension (Check All), use Make Manifold if needed, then cut with Fill ticked.
Can Blender add pegs or connectors when splitting a model?
Not automatically. You add a cylinder that straddles the cut and subtract it from each piece with a Boolean modifier (Difference, Exact solver), then print the peg separately slightly undersized. A splitter such as splicestl places and sizes the connectors for you, including keyed and screw joints.
Does Blender keep the model's colours through the cut?
Bisect's Fill option builds the cap from the surrounding geometry's materials, UV maps and colour attributes, so vertex colours and materials carry across. Note that the STL format itself carries no colour; export 3MF or OBJ if colour matters.
What Blender version does this need?
Bisect, Fill and the Boolean modifier have worked the same way for years; the steps here are for Blender 4.2 LTS and later, where the 3D Print Toolbox is installed from Get Extensions rather than the old Add-ons list, and the STL importer offers Validate Mesh.
Sources
- Blender 4.2 LTS Manual, Bisect — invocation (Mesh ‣ Bisect; Toolbar ‣ Knife ‣ Bisect), Plane Point / Plane Normal, Fill, Clear Inner / Clear Outer, Flip (F), Ctrl snapping. Read 13 Oct 2026.
- Blender 4.2 LTS Manual, STL import/export — Validate Mesh, scale and axis options. Read 13 Oct 2026.
- Blender Extensions, 3D Print Toolbox (v1.4.1, 5 Aug 2026) — Check All, Make Manifold, Hollow, quick export. Read 13 Oct 2026.
- Clever Creations, How to split a 3D model for printing (updated Mar 2024) — the common Bisect-only instructions this guide extends with Fill, manifold checks and connectors. Read 13 Oct 2026.
What changed — 13 October 2026: first published.