Know what to use next.
Start with one simple cut. You can isolate difficult areas, draw more detailed seams and adjust joints once the basic flow feels familiar.
- Open a model or try the demo egg. Check its size. If the app reports a problem with the solid, repair it before cutting.
- Choose Plane in Cut Tool. This is a simple way to try splitting the egg. Pick an axis and slide the plane to where you want the split.
- Choose your joints. For a plain split, use no built-in pins by setting Count to 0. Add joints when you want the pieces to locate together.
- Press Cut. Inspect both pieces. Use Back to editing if the cut needs moving.
- Export the parts. Choose the output you need. Save a project as well if you want to continue editing later.
The Mask tool is optional. You can cut the whole model directly. Opening this guide from the app leaves your work in its original tab.
Load, size & repair
Load and size
Open an STL or 3MF, or drop a supported model file into the viewport. Size X, Y and Z are in millimetres. Changing one dimension scales the others proportionally, so the model keeps its shape.
Reduce for cutting makes a lighter working copy for faster cuts and previews. Light preserves more detail; Strong removes more; Balanced sits between them. The original is retained for export. Use full detail discards the lighter copy.
Inspect & repair
| Action | What it does |
|---|---|
| Inspect | Checks the working piece for holes, flipped or duplicate faces and separate shells. |
| Repair — safe only | Cleans problems such as duplicate faces and faces inside out without deliberately changing the shape. It cannot close every hole. |
| Advanced — removes and adds faces | Attempts more substantial repairs, including holes and problematic joined surfaces. It can change the shape where it patches; inspect the result. |
| Separate the loose pieces | Makes separate objects from disconnected lumps in the current piece. |
Read the result of a repair: it may still report that the model is not a closed solid. A successful repair updates the working model; you should not need to export and reload it.
Start over returns to the loaded model or the latest repaired checkpoint, clearing paint, seams and cuts on the working piece. Finished parts already set aside remain. Start this seam again only resets the seam you are drawing.
Mask tool & paint
Use Mask tool → Paint when other parts of the model get in your way. Use Cut Tool → Paint to cut when you want to select an entire section to separate. Both entry points use the current paint selection; switching between them does not create another independent selection.
- Press Paint in the Mask tool, then paint the area you want to reach. Turn the model to paint its back too.
- Press Isolate the painted part. Everything outside that area is hidden from view and from brush hits. The cut still uses the whole model.
- Use Choose a cut tool to return to the tool chooser. Your paint is preserved. Use Clear paint if you want to start a fresh selection inside this area.
Show the whole model restores the hidden geometry and keeps your paint. Isolation changes visibility, not the model’s shape.
| Paint option | When to use it |
|---|---|
| Surface | Paints along connected faces and stops at sharp folds. Use it for following a surface without selecting every nearby layer. |
| Sphere | Paints faces within a 3D radius, including nearby disconnected layers such as clothing over a body. Hidden isolated faces remain untouched. |
| Paint size | Changes the brush radius. The displayed size is in millimetres. |
| Shift + paint | Removes faces from the selection. |
| Preview painted boundary | Shows the selection’s boundary without cutting. Orange means the preview is on. Click again to hide it; the button returns to grey. Undo also hides a visible preview before undoing paint. |
| Repair touching boundary | Appears when a boundary is found to touch itself. Adds a small amount of paint around the problem. It does not generally smooth a jagged outline. Undo restores the previous selection. |
| Bucket fill | Paint a band right round the join, then press Bucket fill to fill everything the band closes off, like a paint bucket: the hand below a cuff, the foot below a boot strap. The largest side, the rest of the model, is left alone. |
| Snap paint to edges | Moves the edge of your paint onto the nearest rim, step or groove within reach. Where there is no clear edge, it stays as you painted it. A quick way to a clean seam: paint a band, Bucket fill, touch up with the brush, then Snap paint to edges. |
| Clear paint | Removes the paint selection so you can begin again. |
Choose a cut tool
A seam is the line around the model where the pieces will separate. Pick the tool that makes that line easiest to place.
Straight line — from A to B
Drag across the model, or click at A and then at B. The line runs straight between the two points, and A and B can be on the model or beside it, which suits hard-surface parts. The line wraps round the back of the model to close the seam. Drag any point on it to move it; right-click a point to delete it. Turn the model and check the back: on anything with a limb behind it, the wrapped half can cross a different part than you intended.
Band — an adjustable loop
Draw a loop, then drag the orange line to reposition it. Click to pin a point; right-click a pin to release it. Smoothing controls how much a dragged stretch eases out afterwards; 0 leaves the drag as placed. Normal dragging follows the held surface. Alt-drag places a point on the visible surface beneath the pointer.
Snap to edges moves the band or straight line onto the nearest rim, step or groove within reach, such as the edge of a sleeve. Where there is no clear edge, the line stays where you put it.
Rope — draw around the surface
Draw what you can see, release the mouse, turn the model, then continue from the loose end. Close the rope at its starting anchor. Use this when you want to control the path around the back instead of letting Straight line wrap it for you. Drag handles to adjust the contour.
Paint to cut — select the whole section
Paint the entire region you want to separate, including its back. Preview the boundary, correct gaps or overspill, then press Cut. Painting a narrow stripe can produce two boundaries; paint the whole intended section for a cap-like split.
Plane — a straight cut
Pick X, Y, Z or View to orient the plane. Use Position and the handles to move it, and the tilt control to rotate it. If it crosses several separate areas, you can pick the crossing rings you intend to cut. Cut through everything returns to the full plane cut.
Multi-plane — specify the number of cuts
Enter the number of cuts across X, Y and Z. The planes start evenly spaced; click a plane to adjust it. Three cuts along one axis make four sections. Nothing is cut until you press Cut.
Auto multi-plane — fit your printer
Choose a printer preset or enter the build width, depth and height in millimetres. Keep clear at each edge reserves space around the bed and below the height limit. The proposed planes are editable; check the fit report after moving them. The cuts panel selects and positions individual planes.
Cut face & seam controls
| Option | What changes |
|---|---|
| Follows the seam | The separating surface follows the contour you drew. |
| Keeps the drawn step | Keeps a short level region around the seam to form a step rather than a thin edge. |
| Flat plane — replaces the seam | Uses a flat separating surface instead of keeping the drawn contour’s shape. |
| Start this seam again | Clears the seam in hand. Seams already put aside remain. |
| Keep this seam, draw another | When offered, puts a seam aside so multiple seams can be cut together. A kept seam can be brought back for editing. |
The app shows options relevant to the selected cut tool. A preview is a proposal; Cut performs the split.
Joints & fit
Joints help the printed pieces line up when assembled. Available controls depend on the cut and joint type.
Built-in pins — attached
A pin is part of one half and fits a socket in the other. Count sets how many to use; 0 gives a plain split. Pin thickness and length control its size. Clearance controls the gap around the pin.
Loose tenons — printed separately
Separate connectors fit sockets in both pieces. Per joint chooses the number of connectors; Auto uses the available face area. Size can be automatic or fixed. A round Shape allows rotation around a single connector; other shapes resist it. Fit changes the gap: increase it if the connector needs forcing, decrease it if it rattles.
Magnet sockets — removable assembly
Makes recesses for magnets. Choose a preset or enter the magnet’s measured diameter and thickness. Advanced fit provides extra room around the magnet and extra depth below the face. Check these against the actual magnets and your printing process.
Joint numbers
Number the joints engraves matching numbers on mating faces that received connectors. Depth controls the engraving depth. Useful when assembling many pieces.
Inspect joint positions after cutting. Where handles are offered, move pins along the cut face, adjust their length, or tilt the pins together. Reset the joints returns to the generated placement.
Inlay & push-fit cuts
Two more ways to cut along a seam drawn with Band, Rope, Straight line or Paint to cut. Their buttons sit beside Cut. Both are Pro tools.
Inlay cut — a piece to print in another colour
Draw the outline of an eye, a badge or a panel, then press Inlay cut. The shape is pressed into the model, and the piece it removes is kept as an inlay to print separately and press back in.
| Inlay setting | What it does |
|---|---|
| Depth | How thick the inlay is. The floor of the recess follows the surface this far under it. |
| Fit | The gap all round the inlay, so it still presses into the recess once printed. The recess is widened; the inlay stays the shape you drew. |
| Flat floor | A flat floor at the lowest point of the outline, instead of one that follows the surface. The inlay prints flat on the bed. |
Push-fit cut — parts that push together, no pins
Press Push-fit cut instead of Cut. The end of one part becomes a tapered plug that pushes into a socket in the other: good for an arm into a sleeve or a head into a collar.
| Push-fit setting | What it does |
|---|---|
| Plug length | How far the plug reaches past the cut into the other part. |
| Wall | How far in from the seam the side of the plug is. This sets how thick the rim of the socket is. |
| Tip width | How wide the end of the plug is compared with its root. Narrower goes in more easily and prints without supports. |
| Fit | The gap all round the plug in its socket, so it still pushes in once printed. |
| Lower the cut line | Moves the whole cut line down the part from where you drew or painted it, keeping its shape. Use it when a thin sliver is left at a rim. Press Push-fit cut again to see the difference. |
| Plug on the other part | Swaps which part gets the plug and which gets the socket. |
Review & continue
| Action | What it does |
|---|---|
| Separation | Spreads the pieces apart for inspection. It changes the view, not the exported geometry. |
| Close up / Explode | Switches between assembled and separated views. |
| Back to editing | Returns to the editable model with the seam retained so you can adjust and cut again. |
| Save both parts | Saves both halves as parts in the Objects panel. Pick either one there to cut it again. |
Check the back of each piece and the joints before exporting. The Objects panel lets you select parts; names and colours help distinguish them as the job grows.
Clear all, in the trail above the model, takes everything off so you can load a different model. Click it twice; it cannot be undone.
Save & export
Projects preserve editable work. Give the project a name and save it in the browser, or use Save to a file for a project file you can keep elsewhere. Open a file in Projects restores that project. Exported STL files are geometry, not editable project backups.
| Export choice | Use it for |
|---|---|
| This model — STL | The current model as one STL. |
| Reduced working copy — STL | The lighter model, when a reduced copy exists. |
| Both halves / Part A / Part B | The result of the current cut, together in a 3MF or as individual STLs. |
| Every part — one 3MF | All the pieces collected in one file. |
| Every part + assembly guide — ZIP | A bundle of parts with assembly information. |
Export choices change with the current model and cut result. Choose the contents first, then export. Every export of cut parts includes Made-with-ModelSplitter.pdf, a one-page note that goes with the files.
Sculpt, move & shapes
Sculpt — change the model surface
Smooth softens the surface, Inflate pushes it out and Flatten presses it toward a plane. Size controls the brush area; Strength controls how strongly it acts. Hold Shift to reverse the brush. Each stroke can be undone.
Mask paints areas to hold still during sculpting. Shift removes protection; Clear the mask releases all protected areas.
Move — position an object
Select an object, then use Move, Turn or Size to display its handles. Centre it on the model aligns its centre with the working model. Done — deselect removes the handles so you can draw again.
Shapes — add material or make a cutting object
Add a Cube, Ball, Tube, Cone or Ring, move it into place, then use Boolean. Choose Sharp, Chamfer (a bevel) or Round edges. Edge controls the size of the bevel or rounding. Where edge selection is offered, choose individual edges or use Back to every edge. Finish with Done with this shape.
Drop onto surface: press it, then click the model, and the shape lands there facing into the surface, ready for Boolean or for Carve and keep.
Boolean — combine two solid objects
Select two objects. Union joins them. Difference removes object 2 from object 1. Intersect keeps their overlapping volume. Carve and keep carves object 2’s shape out of object 1 and keeps object 2 as its own piece: a socket, and the insert that fits it, such as an eye to print in another colour. Swap ① ② reverses their order; this matters for Difference and Carve and keep. Both objects need to be suitable closed solids.
Add a base — a separate stand that pins on
Pins are made by a cut, so the base is joined on first and then cut off again with pins.
- Add a Tube for a round base or a Cube for a square one, and size it.
- Move it under the model so the feet sink a millimetre or two into it.
- Select the model and the base, and press Union. They are now one solid.
- Choose Plane, and place the plane about 0.5–1 mm above the base’s top or a little inside it, not exactly on the top face.
- Choose your joints and press Cut. The model and the base export as two parts that pin together.
Pins are sized from the cut face. If the feet are too small for pins, put the plane lower, inside the base, so the cut runs across the base’s full outline. To print the model and base as one piece, stop after Union.
Pose & erase
Both are Pro tools that change the model before you cut it.
Pose — bend an arm, turn a head
- Open Pose and press Paint. Paint the part that moves: the edge of the paint is the joint it bends at. Shift + drag takes paint off.
- Press Pose and turn the part with the handle.
- Press Done to keep the pose and take the paint off.
Joint softness sets how far either side of the joint the bend is shared out: low is a sharp hinge, high is a long smooth bend.
Erase — take off a part joined in several places
For something like glasses on a face. Open Erase, press Paint and paint the part to take off. Then choose:
- Erase the painted part deletes it and closes the holes it leaves in the model.
- Lift out the painted part takes it off as an object of its own, closing the holes in both.
Moulds
Make a printed box or shell round the model for casting in silicone. Both are Pro tools and add the mould as new objects.
| Mould | What it makes |
|---|---|
| Solid mould | A box with two sides and a lid, for a solid block of silicone round the model. Silicone round it sets how much silicone goes on every side; Wall sets the box’s thickness. |
| Jacket mould | A thin, even skin of silicone inside a printed two-part shell. It uses less silicone and peels off thin parts. The whole mould scales with the model. |
Air vents chooses how trapped air escapes: a vent or air catcher for every pocket, the main ones only, or none. Places left without are marked in red, to vent by hand. Open it up slides the halves apart on screen to see inside; it does not change the parts. The silicone and resin the mould takes are listed in the export’s assembly guide.
Install the app
Model Splitter can be installed on your computer, with its own icon and window, and it works without internet.
- In Chrome or Edge: press Install app in the app, or use the install icon in the address bar.
- Windows: get it from the Microsoft Store.
- Linux: get it from the Snap Store, or run sudo snap install model-splitter. This also works for Firefox users.
All three are on the download page. The installed app updates itself whenever it can reach the internet. Every 30 days it needs to go online once; until it does, exporting pauses, but cutting, saving projects and everything else carry on.
Mouse & keyboard
- Left-drag: use the active tool or its handles.
- Right-drag: rotate the view. Middle-drag: pan. Wheel: zoom.
- Shift + paint: deselect. During sculpting, Shift reverses the brush.
- Ctrl/Cmd + Z: undo. With a painted-boundary preview showing, the first undo hides that preview.
- Ctrl/Cmd + Shift + Z: redo. Redo can restore the selection rather than rerunning a cut; press Cut when ready.
- Axis widget: select a face to look from that direction.
- F or Reset view: put the camera back so the whole model is in view.
- Panel edges: drag the divider to resize a sidebar. Open and close panel headings to keep the tools you need in view.
When something goes wrong
Cut asks me to repair the model again
Read the repair result in Inspect & repair. Safe repair may leave holes that require Advanced repair; even Advanced repair cannot fix every model. If it reports a cuttable model but the next cut repeats the same repair request, report the exact message and the steps that trigger it.
The boundary looks wrong or runs around twice
Rotate the model and inspect the whole selection. Unpainted gaps, islands or a painted stripe can produce multiple boundaries. Add paint or use Shift to remove overspill. The preview button only shows or hides a guide; it does not change the selected faces or cut the model.
I cannot draw on the model
Check the selected tool and finish moving any selected object or shape. If an area is hidden, Show the whole model brings it back. Try a fresh stroke directly on the surface and check the brush size.
The cut does not separate the model
Check that the seam fully surrounds the intended section, including its back. On layered models, isolate the relevant area and keep the seam on the intended surface. Read the cut’s error message before changing settings; use the offered details when reporting a repeatable failure.
To report an issue, email hello@modelsplitter.com. Include the tool, exact message and sequence of actions.