3D Builder

3D Builder is a Windows app for preparing 3D models before printing. It opens STL, OBJ, PLY and 3MF files, shows the model on a virtual print bed and lets you scale and position it to exact millimeter dimensions. Microsoft dropped it from new Windows installs, so the package here is the last public build, version 2015.209.2327.4143.

Mesh repair is the feature most people install it for. Downloaded models often have holes, flipped faces or overlapping shells that make a slicer refuse the file. 3D Builder finds those errors on import and fixes them with one click, without asking you to understand the geometry.

The editing tools cover the common print-prep jobs. A movable cutting plane splits a large model so each half fits the print bed. Hollowing turns a solid object into a shell to save filament, and simplify reduces a heavy mesh to a polygon count your slicer can handle. Boolean merge, intersect and subtract combine several objects into one part, and the emboss tool stamps text or an image outline into a surface.

The app installs as an .appxbundle package and runs on 64-bit Windows 10 and 11 only. It needs no account and works offline. It is not a full CAD program: there are no sketches, constraints or parametric history, so for designing original parts you still want something like FreeCAD or Fusion.