3D Print Model Splitting

This skill turns a single complex sculpted STL into multiple Bambu-ready, physically assemblable parts.

Core rule

Do not confuse printable with assemblable.

• MeshFix/slicers can make parts watertight and printable.
• They do not guarantee that interfaces have clearance.
• The validated workflow succeeds because it treats part interfaces as the core problem.

Scope boundaries

Good fit:

• figurines, statues, toys, decorative sculpts, and similar triangle-mesh STL files;
• human-guided semantic part boundaries;
• glue-fit or lightly finished assemblies;
• Bambu-ready 3MF/STL handoffs.

Poor fit unless explicitly requested:

• fully automatic semantic recognition;
• precision mechanical snap-fits;
• automatic locator pins/magnets without human-confirmed anchors;
• CAD-grade tolerance guarantees.

Dependencies

Common tools/libraries:

• Blender for STL import, face/material annotation, rendering, and optional interference checks;
• Python packages as needed by scripts: trimesh, numpy, manifold3d, pymeshfix, Pillow;
• Bambu Studio or another slicer for final slicing/print review;
• project-local folders for generated STL/3MF/Blend/PNG outputs.

Do not write generated model outputs into the skill directory.

Progressive references

Read only what the task needs:

• references/sop.md — generic workflow from STL to assemblable 3MF.
• references/clearance-lessons.md — Boolean/MeshFix/clearance failure modes.
• references/blender-material-annotation.md — human Blender annotation instructions.
• references/versioning.md — project versioning and status conventions.
• references/case-study-v13.md — optional validated example for a 5-part figurine.

Standard workflow

1. Prepare annotation file

   • Create a Blender .blend from the input STL.
   • Add material slots for intended parts.
   • Use a project-local output folder.

2. Human material-face annotation

   • Human selects triangle faces in Blender Edit Mode.
   • Assign each face to a material slot.
   • Material assignment is geometry metadata (polygon.material_index), not texture paint.

3. Split by material and lock a baseline

   • Use material indices to export parts.
   • Clean tiny islands, cap open boundaries, export STL, render preview.
   • Lock a baseline that is only “split + capped”, with no clearance/pins/experimental Boolean.
   • Never overwrite the baseline.

4. Repair only before clearance

   • Use MeshFix when needed to make baseline parts watertight before clearance.
   • Treat MeshFix as a printability repair, not an assembly solution.

5. Normalize winding before Boolean

   • Ensure each STL has outward / positive-volume winding.
   • If volume is negative, invert before expanding cutters.
   • Wrong winding can reverse the clearance direction.

6. Apply interface clearance

   • Decide the real remaining assembly interfaces.
   • Define a responsibility table: which part subtracts which expanded neighbor.
   • Use robust manifold Boolean difference for interface allowance.
   • Choose clearance based on printer, scale, material, and finishing expectations.

7. Do not MeshFix after clearance

   • After clearance, use only tiny manifold simplify/sliver cleanup.
   • Strong MeshFix can fill intentional clearance cavities and recreate interference.

8. Validate the deliverable, not only source STL

   • Export the final 3MF/STL set.
   • Re-read 3MF objects and validate watertight/non-manifold properties.
   • Run pairwise interference checks on adjacent parts.

9. Slicer and physical test

   • Use Bambu Studio or another slicer for import/slicing/support/print preview.
   • Do not treat “no overlap warning” as assembly proof.
   • Physical assembly test is the final gate.

Interface-clearance hard rules

• Start from an un-clearanced baseline when changing the part plan.
• Do not directly merge parts that already had clearance cut between them; this leaves internal cavities/seams.
• Keep clearance work localized to real assembly interfaces.
• If assembly is tight, create a new version and increase clearance locally; do not repair a failed branch blindly.

Deliverables checklist

For a final handoff, include:

• final Bambu 3MF or slicer-ready project;
• final STL parts;
• preview/contact sheet;
• version note;
• validation summary;
• 3MF object validation report;
• pairwise interference report;
• notes on failed/disallowed routes.

Script organization

Scripts in scripts/ are reusable helpers with parameterized paths and part definitions. Keep new scripts project-agnostic: no user-specific absolute paths, no private names, and no hardcoded part schema unless the script clearly accepts a config/CLI override.