Prep a model in Blender

---
name: prep-a-model-in-blender
description: >-
  Prepare a model in Blender for 3D printing: real-world scale in millimetres,
  applied transforms, a closed and consistently oriented mesh, wall thickness
  the nozzle can print, and a clean export to 3MF or STL that is verified by
  re-importing it. Use after a model exists (made, downloaded or scanned) and
  before it goes to a slicer.
title: Prep a model in Blender
date: 2026-09-24
author: Benchy
tags: [3d-printing, blender, mesh, export, skill]
license: MIT
metadata:
  version: "0.1.0"
  schemaVersion: 2
  surface: "Blender 4.2+ · no account needed"
---

# Prep a model in Blender

A slicer can repair a little. It cannot fix a model at the wrong scale, a part
made of overlapping shells, or a wall thinner than the nozzle. This skill gets
a model into a state where the slicer's job is only slicing.

## Requirements

Blender 4.2 or later, run either by the user or by the agent through Blender's
command line (`blender --background --python script.py`) or an authorised
Blender connection. If none is available, say so and stop; do not describe
checks you could not run.

Work on a copy. Keep the user's original file untouched and save the prepared
model under a new name.

## Steps

1. **Set units.** Scene → Units: Metric, Unit Scale 1.0, Length in
   millimetres. A model imported from another tool is often 1000× too small or
   25.4× off (inches); compare its bounding box with the dimensions in the
   plan, and rescale only with the user's confirmation.
2. **Apply transforms.** Select the printable objects and apply rotation and
   scale (Object → Apply → Rotation & Scale). Unapplied scale is the most
   common reason an export arrives at the wrong size.
3. **Make one closed mesh per part.**
   - Merge by distance to remove duplicate vertices.
   - Recalculate normals outside.
   - Find non-manifold geometry (Select → All by Trait → Non Manifold) and fix
     holes, loose edges and internal faces.
   - Where separate shells overlap, join them with a Boolean union, or keep them
     as separate parts on purpose.
   - The 3D-Print Toolbox extension reports these checks in one panel. Use it
     where installed, and report its numbers.
4. **Check thickness.** The thinnest wall should be at least two nozzle widths
   (0.8 mm for a 0.4 mm nozzle). The toolbox's thickness check marks thinner
   areas. List them rather than silently thickening features the user
   designed.
5. **Orient for the plate.** Put the face named in the plan's print orientation
   on the XY plane at Z = 0. The slicer can re-orient, but the file should
   arrive in the intended position.
6. **Export.**
   - Prefer 3MF: it carries units and, in slicers that read it, multiple parts
     as one project.
   - Export STL when the user's tools need it; STL has no units, so state
     "millimetres" beside the file.
   - Export selected objects only, with modifiers applied.
7. **Verify the export.** Re-import the exported file into an empty scene and
   measure its bounding box against the plan. Report the three dimensions. An
   export that was not re-imported is unverified.

## Report

Return the prepared `.blend`, the print file, and a short list: scale and
bounding box, non-manifold count before and after, thin areas found, the
orientation used, and anything left for the user to decide. Report only the
checks you ran.

Keep the original, the prepared `.blend` and the print file together: in
Benchy Box they are the _native source_ and the _print file_ of the same
packet, and World reads those roles unchanged when the model is published.

## What this skill does not do

It does not redesign the model, add supports (the slicer does that), or
declare a model printable. A clean mesh is necessary for a good print, not
sufficient.