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Convert to Solid

In one sentence: Close a Part shell or surface shape into a solid by stitching its boundary faces — requires the shell to be watertight.

Overview

Workbench: Part
Menu: Part → Convert to Solid
Shortcut: none

Takes a Part shell (an open or closed surface) and attempts to create a solid from it. Requires the input to be a closed shell — every edge must be shared by exactly two faces (no boundary edges / gaps).

Intuition

A shell is a collection of faces — like the walls of a building with no floor and no roof (or holes in the walls). A solid is a completely enclosed volume. Convert to Solid checks whether the shell's faces completely enclose a volume, and if so, declares it a solid that Part tools can work with for Boolean operations, mass properties, and PartDesign features.

This is the second step in the standard STL → solid repair pipeline:

STL file → Shape from Mesh → Convert to Solid → Refine Shape → usable solid

When to use it

  • After Shape from Mesh, to obtain a usable solid from a watertight mesh.
  • When a STEP or IGES import results in a shell rather than a solid.
  • When Shape Builder assembled a closed shell from faces.

When NOT to use it

  • Don't use it on open shells — if the mesh/shell has any gaps or boundary edges, the conversion will fail. Repair the mesh first.
  • Don't use it on meshes directly — convert mesh to shell with Shape from Mesh first, then use this tool.

Step-by-step

  1. Select the shell Part::Feature in the model tree.
  2. Choose Part → Convert to Solid.
  3. If the shell is closed, a solid Part::Feature appears.
  4. Verify with Part → Check Geometry.

Parameters

This tool has no task panel. The conversion is attempted immediately.

Common mistakes and pitfalls

Open shell — boundary edges exist

If the input shell has any gaps (boundary edges), Convert to Solid fails. Check the shape with Part → Check Geometry to locate boundary edges, then repair the mesh in the Mesh workbench (Mesh → Fill Holes) before retrying.

Self-intersecting mesh produces invalid solid

A shell with self-intersections will convert but produce an invalid solid that fails in Boolean operations. Fix self-intersections in the Mesh workbench first.

Shell with inverted normals acts as empty space

If the converted solid behaves incorrectly in Booleans (e.g., cutting produces no result), the shell normals may be inverted. Use Reverse Shapes to flip them before converting.

Python API

import FreeCAD as App
import Part

doc = App.newDocument()

# Create a closed shell and convert to solid
box_shape = Part.makeBox(10, 10, 10)
shell = box_shape.Shells[0]   # already a closed shell for a box

solid = Part.Solid(shell)
feat = doc.addObject("Part::Feature", "ConvertedSolid")
feat.Shape = solid
doc.recompute()

print(f"Shape type: {feat.Shape.ShapeType}")   # Solid
print(f"Volume: {feat.Shape.Volume:.2f} mm³")

See also