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3D printing tutorial

Split an STL by color and add alignment joints

Need to print a model bigger than your build plate, or in two materials? In Forma3D you can paint the faces of a mesh to mark where it should separate, then generate matching peg-and-socket joints in the same pass.

Tutorial updated October 11, 2026 · iOS 17 or later · Screenshots show Forma3D running on Mac. The same tools are available on iPhone and iPad; layouts may differ.

The arch in Forma3D painted in two colors before separation: red faces on the left, green on the right The same arch after separation, laid out as individual parts on the Forma3D grid
On this page
  1. Paint the split regions
  2. Check the painted mesh
  3. Bake the transform
  4. Choose the separation settings
  5. Separate and inspect the joints
  6. Export each part

Steps

  1. Open Paint and mark the split line

    Select the arch in Object mode and open the Paint tool. Paint one side of the model — here the left half goes red. Every face that should end up in that part needs the same color; faces left unpainted stay in the default region.

    The white arch model in Forma3D with the Paint tool open, ready to color the left half

  2. Paint the second region

    Paint the other half green so the whole surface is covered by exactly two color regions meeting at a clean boundary. These colors are only labels for the separation — they don't have to match your filament choices, and the "Target parts" count in the next panel groups small stray islands into the surrounding region, so a speck of wrong color won't spawn an unwanted extra part.

    The arch fully painted, left half red and right half green, seen in Object mode

  3. Bake the current transform

    If you scaled or rotated the object after importing it, open Transform and press Bake. Bake writes the current scale and rotation into the mesh's vertices and resets the node's transform, so the joints are generated on the real geometry rather than a scaled node. The visible shape doesn't change.

    Transform panel open in Forma3D with the Bake rotation button highlighted

  4. Configure the Separate panel

    Open Separate and set the mode to By paint. For this arch the settings are: Target parts 2, joint Peg box (the square tongue-and-groove), Peg on Higher color, Tolerance 1.0% (a percentage of the joint size, not millimeters), and Sharp edges on. "Peg on" picks which painted color region carries the peg — Higher color or Lower color — so flip it if you want the peg on the other half. Press Re-separate to refresh the preview while tuning, and Separate to apply.

    Forma3D Separate panel showing By paint mode, Target parts 2, Peg box joint, Tolerance 1.0 percent and Sharp edges on Magnified Separate panel: By paint mode, Target parts 2, Peg box joint, Peg on Higher color, Tolerance 1.0 percent, Sharp edges, Re-separate and Separate buttons

  5. Separate and inspect the joint

    Press Separate. The Objects list now shows two parts instead of one. Move the pieces apart slightly to check the joint: one part carries a square peg, the other the matching socket. The parts in this screenshot were moved apart by hand to show the interface — nothing here guarantees a printable fit, so verify the joint on your printer before committing to a long print.

    The arch separated into two objects with a square peg and socket joint, both listed in the Objects panel Close-up of the square peg on the right part and the matching socket on the left part

  6. Export each part

    Select a part, open Export → Export STL, and choose Selected objects so only that piece is written. Repeat with a different file name for the second half. STL carries no color; export 3MF instead if you want the face colors preserved. In your slicer, check orientation, seam placement, wall thickness around the joint, and whether the peg needs supports — a small test print of just the joint region is a cheap sanity check.

Good to know

How is "By paint" different from a line split?

A line cut slices the model along a straight screen-space plane. "By paint" follows the face regions you painted, so the boundary can run anywhere across the surface — useful when the split shouldn't be straight.

Is there a faster way for a straight split?

Yes — no painting needed for a single straight cut. Switch to Edit mode, select the edge component, tap one edge where the cut should go, then press Loop Select → Select Loop to select the full ring. Press Split and choose "Split by Edges" for a clean cut into two objects, or "Separate with Joint" to open the same joint workflow covered here.

Is the tolerance value in millimeters?

No — Tolerance is a percentage relative to the joint's size, not an absolute millimeter gap. 1.0% is a starting point; every printer and material fits differently, so test the joint before printing the full part.

Can I split into more than two parts?

Yes — paint as many regions as you need. Target parts controls how the painted regions are grouped into final objects, which matters when a color appears as several disconnected islands.

Related guides

Edit STL files

All the mesh-editing tools in Forma3D — selection, Boolean, cut, repair, and export.

Multi-color 3MF

Keep face colors through the export for multi-material slicer workflows.

How to Paint for 3D Printing

Paint faces, then clean up color islands and merge similar colors — the paint-cleanup companion to splitting by color.

Get Forma3D on the App Store

Frequently asked questions

How do I split a large STL for a smaller print bed?

Open the STL in Forma3D, then either use the cut tools, select an edge loop and Split by Edges, or paint regions and Separate with joints — then export each piece.

Do split parts need connectors?

Usually yes. Add alignment joints or plan for glue surfaces; Forma3D's Constructor workflow supports print-fit joints and tolerance settings for reassembly.

Can I split an STL on iPhone without a computer?

Yes. Import, cut, separate, and export all run on-device — no desktop slicer or editor is needed.